UK’s Leading Custom PCB and Electronic Board Manufacturer

Our mission at Rush PCB UK is to provide our customers in UK and Europe with a superior turn-key custom PCB and assembly solution. We have in place Quality Management Systems that rival those of large-scale manufacturers, and we can build PCB electronic assemblies for our customers that exceed the exacting standards they demand. At the same time, our commitment to personalized service makes us more responsive to our customer’s needs, enabling us to complete our electronic custom build quickly and accurately.

Our customers come from a wide range of industries—from semiconductor production equipment to electronic vehicle manufacturers to medical equipment manufacturers—and we cater to a wide range of technologies. We offer specialized prototyping services in addition to several other solutions, allowing us to meet the customers’ PCBA needs with the quickest turnaround in the industry. Our end-to-end custom circuit boards solutions consist of:

Engineering and Design
Electronic Circuit Board Manufacturing and Testing
Custom Prototyping

End-to-End Custom PCB Solutions

Whether our customers need accurate turn-key fabrication or the highest quality build of a rapid prototype, our expertise remains unparalleled. With more than two decades in the industry, we at Rush PCB UK know how best to build and assemble custom printed circuit boards, PCB electronic assemblies, and box builds that meet and exceed our customers’ needs.

We staff our engineering department with industry experts who fine-tune our manufacturing and testing processes for precision. We are certified for ISO 9001 and ISO 13485, and our products comply with UL 508A and IPC 610. All this allows us to claim the technical proficiency necessary for designing our customers’ electronic circuit boards.

Engineering and Design

Before we take up a project, our engineering team evaluates the data and documentation submitted by our customers. Our engineers specialize in DFM, DFA, and DFT, and apply these methodologies to ensure the project will proceed with the minimum potential issues. We review our customers’ designs for manufacturability and testability and work closely with the customer to resolve all potential issues. If necessary, we have the expertise to redesign the electronic circuit boards for our customers.

Electronic Circuit Board Manufacturing and Testing

At Rush PCB UK, we lead the industry in electronic circuit board manufacturing and assembly—we deliver the best results in the shortest possible time. We include expert engineering insights with our end-to-end custom circuit boards solutions, regardless of whether we are manufacturing PCB electronic boards for a production run or for a prototype.

Custom Prototyping

Our customers often require a prototype custom PCB for their proof-of-concept design, and usually, their requirement is urgent. To handle such custom prototyping projects, Rush PCB UK has an entirely separate team specializing in building prototypes. Our dedicated prototyping team uses a fast-tracked, limited run, quick turnaround manufacturing process to complete the custom PCB design in the shortest possible period.

For such fast custom PCB prototypes, we assign a product specialist to fast-track all the production steps. We immediately procure the parts, fabricate the custom PCB in-house, and the next day, the custom circuit board is ready for assembly.

In the meantime, out engineering staff starts work with the customer to resolve any potential design and manufacturing issues. Our product specialist evaluates the design throughout the entire manufacturing process, ensures completion of the board assembly, and ships to the customer as quickly as possible. This allows the customer to continue with their product development.

Conclusion

We, at Rush PCB UK, have a proven track record of working closely with our customers in UK and Europe as partners for rolling out new products into production and for providing them with proof-of-concept custom electronic circuit boards. Apart from our custom PCB manufacturing services, our fully staffed engineering department also works proactively with our customers in ensuring the successful manufacturing of a robust and repeatable design. We offer specialized prototyping services along with a host of other solutions. That enables the quickest turnaround to meet the PCB electronics needs of our customers.

What is Smart Manufacturing for PCBs?

The world over, manufacturers of electronic products continue to struggle to meet market demands. These are caused mainly by the requirement of rapid new product introduction to keep ahead of the competition, customization/personalization, expectations of quality, internet connectivity, and more. To meet these product demands and more, Rush PCB recommends using Smart Manufacturing for printed circuit boards (PCBs).

So far, electronic manufacturers have applied one or more solutions of applied digitalization to their processes during product development. These include:

PCB contract manufacturing through supplier collaboration
Advanced part and mold manufacturing through model-driven processes
Product optimization through integrated simulation and layout
Production ramp-up, process verification, virtual design, test management, and execution
Box builds and shop floor connections through manufacturing execution system
PCB assembly and test through integrated planning and management

With digitization, manufacturers can plan better and validate production alternatives faster. This helps to increase the performance and effectiveness of manufacturing operations. However, according to extensive studies, digitalization has not helped in creating the anticipated bottom-line impact.

Rush PCB recognizes that digitalization requires a new manufacturing/operating model, an integrated platform—one that unites all the domains necessary to engineer, manufacture, and deliver better quality PCBs. At Rush PCB, we call this Smart Manufacturing. For this, Rush PCB follows steps like:

Validating manufacturability of PCBs
Virtual design, simulation, and optimization of production processes
Managing manufacturing operations and materials
Using manufacturing data to generate business value

Validating Manufacturability of PCBs

Rush PCB uses Design for Manufacturing (DFM) analysis for fabrication, assembly, test, and reliability checks to asses issues affecting performance. We assess the PCB design and the placement of components for ease of manufacturing and assembly with the goal of making a better product at a lower cost. We do this by suggesting simplification, optimization, and refinement of the PCB design. For this we examine five principles during a DFM exercise:

Material
Design
Process
Environment
Compliance/Testing

Rush PCB recommends DFM in the early stages of the PCB design process, even before we begin the tooling process. Ideally, DFM requires the participation of all stakeholders, including designers, engineers, manufacturer, and material supplier. This cross-functional DFM exercise ensures optimization of the design.

Read About: Past and future trends in PCB design

Virtual Design, Simulation, and Optimization of Production Processes

At Rush PCB, we plan the fabrication and assembly processes to enable a smooth flow. The planning helps in preparing the process while identifying the impact of design changes on fabrication and assembly lines while delivering updated work instructions.

By validating processes, we visualize and analyze the entire assembly operation, thereby discovering issues related to human and machine assembly, while ensuring adherence to best practices.

The major advantage of the above exercise is a substantial improvement in capital investments planning and operating expenses prediction. This way of optimizing production leads to maximization of utilization and reduction of costs.

Managing Manufacturing Operations and Materials

At Rush PCB, we use materials management tools to ensure just-in-time delivery of materials to the fabrication and assembly line. This eliminates excess work-in-process, while improving inventory turnover.

Our comprehensive solution for electronic preproduction, production, and execution helps to manage data from all resources such as operators, tools, and machines to build complete traceability. We integrate this solution seamlessly with our product life-cycle management and enterprise resource planning systems.

Read About: PCB Routing Requirements

Using Manufacturing Data for Generating Business Value

All our manufacturing processes generate data, such as those on material consumed, process flow, quality, and more. This real-time, normalized manufacturing data helps us in driving intelligent, decision-making business analytics solutions, root-cause analysis, prediction of future performance, and cost and quality trends, thereby helping to improve our business value.

Advantages of Smart Manufacturing

With Smart Manufacturing, Rush PCB is on its way to eliminate disconnected systems, silos of information, and mounds of paper-based work instructions. This helps us to manage a continuous integrated work flow starting from design, to planning, to production, to delivery.

At every stage of PCB fabrication and assembly, a difference is eminently visible between Rush PCB’s Smart Manufacturing and the earlier piece-meal digitization strategy. Major differences include:

Designs are more reliable and manufacturable
Design, engineering, and manufacturing departments collaborate better
Data redundancy is lower
Shop floor planning mistakes are fewer
Manual data entry is less error-prone
Inventory and use of materials are more optimized
Best practices of manufacturing are better enforced
Work instructions are more accurate and up-to-date
Key performance indicator monitoring is through real-time data collection
Root-cause identification is faster
Higher product mix capability without loss of factory performance

To the customers of Rush PCB, its Smart Manufacturing strategy offers several advantages, namely:

Lower product development time, leading to faster time-to-market, with the advantage of frequent new product introduction. The quality improvement through DFM exercises improves the design for a longer life-cycle of the product.
With Smart Manufacturing, Rush PCB can quickly follow up with manufacturing, as soon as the design is complete. This allows for better product personalization and customization.
With more informed decision-making through manufacturing data, there is better visibility into analysis and manufacturing.
Smart Manufacturing at Rush PCB is improving the efficiency of our manufacturing processes and materials. This not only reduces overall manufacturing costs, it makes our PCBs more affordable to our customers.

For all your PCB requirements, please contact Rush PCB, or visit our website today.

PCB – Precision Glue Dosing

Rush PCB understands that high quality products require highly precise component placements on their printed circuit boards (PCBs). However, in traditional PCB assembly, there is typically a high amount of wastage because of fluctuations in glue viscosity, inconsistent height of the dispensing needle, and variable amounts of glue.

The effect of this variability is that assemblers often have to scrap expensive components, resulting in slowing production lines that hit the bottom line. Additionally, these variables also make the gluing process rather operator-intensive, further adding to the production time and costs.

Problems with Glue-Dosing

There can be several problems with glue-dosing. One of the most common issues is constant dripping of glue. Usually, there are three causes for this—very thin glue, large air pressure, large needle diameter.

Very thin glue may drip constantly, and the solution is to replace it with a thicker glue. A large air pressure may also force the glue to drop continuously, and this can be solved by reducing the pressure.

It is very important to use the correct diameter for the dispensing needle. If the diameter is too large, the glue can drip continuously. On the other hand, with a small diameter needle, the valve may start dripping once it is closed. This is because the small diameter needle causes a back pressure to build up. Rush PCB recommends using a tapered oblique needle to reduce back pressure.

Read About: How Does Prototyping Help?

Precision Glue-Dosing Solutions

To eliminate most of the above variables from their production environment, Rush PCB uses fully automated turnkey glue-dosing solutions. Driven by precise vision and motion systems, the turnkey solution is self-calibrating as it applies an identical amount and pattern of glue to every component. Not only does this process simplify the operator’s tasks, it also ensures repeatability, while reducing scrap. In turn, we at Rush PCB, transfer the cost reduction to the customer, resulting in lower quotes.

One of the biggest challenges during PCB assembly is applying microscopic amounts of epoxy glue efficiently in precise parallel lines. Irrespective of production cycles being short or long, there is a high possibility that the regular process of glue application may turn inconsistent or go wrong.

For instance, there are chances that the alignment or positioning of the surface mount devices (SMDs) might change slightly, there may be too much glue, the dispensing needle may be somewhat lower, higher, or offset from the previous PCB, there may be inadequate glue, the glue may be of a different viscosity, and so on.

To reduce the impact of the above variables, we at Rush PCB have installed an automated system in our production environment. This ensures a perfect alignment of the plate and glue-dispensing needle, while keeping them the same distance apart all the time.

The automated system consists of two high-resolution vision camera systems, and a mechanism for imparting precise motion, operating on smart software. The combination helps to keep the distance between the plate and the glue-dispensing needle to within ±5 microns on the three axes. Additionally, a volumetric glue-dispensing system ensures that only the required volume of glue dispenses to each part.

The system also has a recipe handler that helps with product changeovers. It allows the operator to reconfigure the system quickly by simply entering the new value such as glue type, speed, start/stop positions, and height.

Read About:Printed Circuit Boards and AI

Benefits of Precision Glue-Dosing

There are several advantages to Rush PCB when using an automated system for precision glue-dosing. As the entire system is fully automated, the process is entirely operator independent. Moreover, the system ensures a repeatable operation providing uniform product quality and takt time.

The turnkey glue-dosing system being self-calibrating, it does not require the operator to intervene periodically, thereby eliminating time-consuming stoppages. Moreover, the operator does not require training in calibrating the system.

The automatic dosing system dispenses glue volumetrically, ensuring application of precise amounts of glue.

Applications of Precision Fluid-Dosing Systems

Apart from attaching SMDs to PCBs for quality assembly, Rush PCB uses the precision glue-dosing system in other applications involving fluid dispensing. These involve applications such as:

Large IC Failure Protection with Underfill

While assembling large ICs such as PoP, BGA, and CSP, it is necessary to underfill them to prevent failure. Densely populated boards require tight keep-out zones with small, narrow fillets. A high degree of precision is necessary with applying underfill to these components to augment quality, speed, and productivity.

Adhesives for Thermal Management

Several components dissipate heat when operating, and require thermal management to keep them within safe operating temperatures. Most such components use heatsinks with heat-conducting adhesives in between the heat-producing IC and the heatsink. The heat-conducting adhesive helps to transfer the heat from the component to the heatsink. For proper application, the fluid-dosing system must dispense the heat-conducting adhesive in exact volumes and in a thin profile. The automated system helps in dispensing the adhesive in the right consumable combinations and speed.

Please contact Rush PCB for all your precision assembly requirements. Please visit the Rush PCB website, or call now for an instant quote.

Introduction to Semi-Flex Circuit Boards

Influence of IoT on PCBs

Polyimide is the typical base material for making flex printed circuit boards (PCBs). However, for some applications, OEMs find flex printed circuit boards (PCBs) made from Polyimide more expensive than their budgets allow. This may be because their application does not require the PCB to bend in a small angle, or bend repeatedly. In such cases, Rush PCB recommends the use of semi-flex boards.

Semi Flex Circuit Board

As the name suggests, a semi-flex board is partly flexible. It is possible to bend the semi-flex board only a limited number of times, typically about four to six times. It needs to be done before it develops a potential problem. Products with several circuit boards mounted at different angles to each other, without the requirement of ongoing flexibility, find semi-flex boards suitable. The OEM bends the circuit board only once during assembly, after which the board does not undergo any more flexing.

Semi-flex boards have all the advantages that a flex circuit offers. Just as in a flex circuit, there are no fragile wire interconnects or connectors in semi-flex boards. Over time, connector contacts can oxidize, leading to poor connections, compromising the reliability and safety of the product. Absence of connectors also implies a reduction in the number of solder joints, leading to an improvement in reliability. Flex and semi-flex boards simplify the production process, as there is no longer any requirement of manufacturing looms for special interconnect cables, and install them manually.

Semi-Flex Circuit Construction

Rather than use Polyimide, which is expensive, semi-flex circuits instead use a thin layer of standard FR4 material, with a lamination of copper layer on both its sides. Even without any special treatment, this thin FR4 layer can bend a few times without failure. The manufacturer uses four available copper layers for the rigid portion of the semi-flex board, with the flexible part consisting of the core or the thin FR4 layer with copper layers on both sides. Therefore, a two-layer circuit board forms the flexible connection between the two four-layer rigid circuit boards. The FR4 material defines the angle and radius that the flexible circuit can make.

For instance, the central core layer of the semi-flex PCB is a layer of FR4, with a thickness of 100 µm, and a layer of copper foil on its two sides with a thickness of 35 µm. This is how the manufacturer puts the flexible part in between, while continuing to build on the core, which continues through the entire circuit board. For the rigid parts, the manufacturer creates a double layer of prepreg on the inner two layers. The prepreg layer has a total thickness of 126 µm. A layer of FR4 material of 0.51 mm thickness and a layer of copper of 18 µm follows on the prepreg layer. Processing of the outer layers can follow the same methods as that for any ordinary four-layer circuit board. As the construction is symmetrical, the resulting semi-flex circuit board is stable and robust.

Limitations of Semi-Flex Circuits

An alternative less-robust method of construction process uses controlled depth routing or deep milling of specialized FR4 layers. This method helps to achieve a bending/flexing section within the traditional rigid FR4 construction. This is a typical inexpensive flex-to-fit solution for specific applications. It is suitable only for static operations, with the bend typically just for installation. The rigid sections may have up to 12 layers, with the bending performance going up to 50 bend cycles of 0°-90°-0°.

While designing the PCB, the designer must take into account the limitations of a semi-flex circuit:

Bend radius should not be less than 5 mm
Bend angle should not exceed 180°
Length of semi-flex part should not be less than 15.7 mm
Radius of corners should not be less than 1 mm
No holes, slots, or cut-outs should be present in the semi-flex part

Best Practices for Semi-Flex Circuits

Rush PCB recommends following some best practices for effective utilization of semi-flex circuits. For instance, designers should preferably plan copper layers of 35 µm thickness on both sides of the semi-flex part. They must surrounding the print traces with copper polygons.

According to Rush PCB UK, it is best to have a selective chemical Ni/Au (ENIG) final finish on the non-flexible part of the PCB. Manufacturers also offer alternative surface finishes such as HASL (SnPb), LF HASL (SnNiCu), OSP, Immersion Tin, Immersion Silver, and Electrolytic Gold. The designer can follow the same process for screen prints, slots, and cut-outs as a regular PCB requires, along with a green solder mask.

The designer can follow advanced production techniques such as peel-off mask, round edge plating, PTH on board edges, and copper up to the board edges. Some manufacturers offer options of carbon pads and gold edge connectors. As the semi-flex boards are less flexible as compared to flex circuits, manufacturers prefer to supply them in panels to avoid damage during transport and assembly.

Variants of Semi-Flex Circuits

Although the symmetrical configuration is most common for semi-flex circuits, manufacturers also make asymmetrical configurations according to demand and application.

For instance, it is possible to have an asymmetrical deep-milled semi-flex portion with a single layer or two layers of copper. Likewise, it is possible to have a built-up PCB with an asymmetrical configuration of the semi-flex portion with a single or two layers of copper.

Advantages of Semi-Flex Circuits

One of the major advantages of semi-flex PCBs over flexible circuits is the former is much cheaper. Although semi-flex circuits are only moderately more complex to manufacture than rigid circuits are. The main reasons for lower costs involve the use of FR4, rather than the more expensive Polyimide material.

In contrast to conventional flex and rigid-flex circuits, semi-flex circuits do not require the use of Polyimide foils. Therefore, the manufacturing process can omit the complex preparation of prepregs and cover foils. Additionally, the requirement for the usual tempering or annealing of the Polyimide material before beginning the soldering process is also not necessary for semi-flex circuits. Other advantages of a semi-flex circuit are:

Saves the cost of traditional connecting material between two rigid PCBs, i.e. cables or wires, crimps, connectors, binders, labels, including their individual cost of assembly
Improves the reliability offered by cables and connectors, especially as this is the most frequent cause of detect
Eases the assembly process and integration into boxes and housing
Applicable in cost-challenging applications, especially as replacement for rigid-flex
Applicable as angled connectors on large PCBs
Fits easily into boxes and housing
Possibility of semi-flex portions on both sides of the PCB
Reduces the product’s weight and volume
Allows designing in 3 dimensions
Easy to define characteristic impedance of the circuit systems on the printed circuit

Also Read: The Future of Flexible Circuits

IPC Standards for Semi-Flex Circuits

Semi-flex circuits follow the same IPC standards applicable to flex and rigid-flex boards. For designers and engineers, the most important among them are:

IPC 2223C
IPC-6013C
IPC-4202
IPC-4203
IPC-4204

The IPC 2223C is the Sectional Design Standard related to flex printed boards. It provides guidance on selecting adhesive materials as well as on connecting flex and rigid PCBs, providing specific and comprehensive tips for creating flex and rigid PCBs.

The IPC-6013C provides the Qualification and Performance Specifications in relation to flexible printed boards. It covers the qualification and performance requirements of single and multi-layered flexible and rigid-flex PCBs.

The IPC-4202 standard defines the base dielectrics that flexible printed Circuits can use.

The IPC-4203 standard defines the adhesive coated dielectric films and the flexible adhesive bonding films that flexible printed circuits can use as cover sheets.

The IPC-4204 standard defines the flexible metal-clad dielectrics that flexible printed circuits can use in their fabrication.

Summary

Rush PCB estimates semi-flex circuits to be approximately three times cheaper than an equivalent rigid-flex PCB combination. Compared to a system of rigid PCBs interconnected with cables, an equivalent semi-flex circuit can be up to 30% cheaper on the total cost.

New Configurations for Rigid-Flex PCBs

Rigid-flex PCBs comprise both rigid as well as flexible board materials, and they consist of layers of flexible substrates attached to rigid boards. With rigid-flex circuit boards, designers can have more options, while eliminating the need for expensive, unreliable wires connecting the rigid substrates.

Ensuring the integrity of the boards and successful manufacturing requires attention to detail. Rush PCB, the largest rigid-flex printed circuit board (PCB) manufacturer, manages four vital design parameters when configuring their PCBs and creating them.

Stack Management

The stackup for the PCB design needs an impeccable template for the finished product to work flawlessly. For this, designers must use appropriate software and create the rigid-flex layers of the PCB. Delays and issues related to PCB performance will surely result from improper communication with the fabricator, inefficient management, and flaws with the rigid-flex stackup.

Rush PCB recommends proper ECAD software and related tools for designing rigid-flex PCBs. The board design requires fine-tuning for region-specific stack up. The software must have board specific tools for altering the board outline, rather than making multiple complicated changes when designing the stackup.

Integration tools between ECAD and MCAD can save time when designing precise stackup for configuring rigid-flex boards. ECAD programs that support 3-D design is most suitable for rigid-flex design especially as it helps with verifying the placement and integrity of sensitive components and bends.

Ground Plane Management

The configuration for the rigid-flex PCB depends on whether it will be a flex-to-install or a dynamic-flex type. Flex-to-install PCBs need bending only once when the OEM places it in the product during assembly. On the other hand, a dynamic-flex PCB needs to bend and fold repeatedly whenever the device using it is active. Ground planes in a dynamic-flex PCB are under constant stress as it bends, and therefore, need more attention during design.

The key concern during the design of a dynamic-flex PCB is the integrity of the signals and that of the ground planes surrounding them. The designer must place emphasis on the reliability of the materials and substrates the PCB uses.

For instance, using solid copper ground planes in a dynamic-flex PCB may result in cracking and failure. Some designers prefer using annealed copper on the flexible parts. Others prefer reinforcement of Gold and Nickel plating on the ground planes. However, both the above increase the cost of the PCB while reducing flexibility.

Also Read: Material and Methods for High Quality Rigid-Flex PCBs

Rush PCB recommends using cross-hatched polygon rather than solid copper for ground planes. Although this does increase flexibility while reducing the possibility of cracking during repeated flexing, it impacts high-speed signals negatively. The recommendation is to add solid copper return paths below the high-speed traces. Making the return paths 5-10 times wider than the signal traces helps improve signal integrity.

Bend Management

Designers have several concerns when the rigid-flex PCB must bend. Apart from ground and power planes in the bend area, pads for SMDs and through holes are also at risk when present in the area of the PCB that will bend. Designers often resort to anchoring pads using additional coverlay and plating for through holes to strengthen the PCB.

Rush PCB recommends not using components or vias very close to bends in a rigid-flex PCB. When designing bends, the recommendations are to:

Reduce material thickness at bends
Avoid bends that are too tight
Avoid stretching of bend material

Mechanical stresses at the bend area will repeatedly stretch the outer layers while compressing the inner layers as the PCB bends. Unless the designer has properly configured the materials at the bend, the PCB is likely to fail prematurely.

Trace Management

Rush PCB UK recommends routing traces perpendicular to the bend during the design. In a rigid-flex design, the perpendicular routing helps reduce the stress on the traces when they bend. For double-sided flex design, the recommendation is to offset the traces. Staggering the traces on the top and the bottom layers makes the PCB stronger, enabling it to withstand repeated bending.

When bending traces, it is best to avoid a 90-degree bend, as the corners of the trace will be under greater stress than straight traces or curved traces. Therefore, using a gradual curve when bending the traces or using piece-wise linear curves reduces the chance of traces delaminating during bends.

Likewise, using tear-drop pads rather than round pads improves the reliability of rigid-flex boards significantly. For further copper adhesion to the flexible substrate, designers provide pads with anchor stubs.

For signal integrity, it is necessary the impedance of the high-speed signal path remains constant for the entire length of the trace. As the impedance may change as the track passes from the rigid to the flexible parts (and vice-versa) of the PCB, the ECAD tools should have the provision for altering the width of the trace to accommodate the change in impedance.

Rigid-Flex PCB Construction — Standard

Although manufacturers use several combinations and configurations for rigid-flex construction, some advanced constructions use odd layer count construction, varying flex layer counts, asymmetrical construction, air-gap flex layer constructions, blind and buried vias, multiple rigid area thicknesses, integrated ZIF constructions, shielded flex layers and more.

The standard method of constructing rigid-flex PCBs has the flex layers in the center of the construction with both the flex and rigid areas containing even layer counts. There is an even layer count in the thicker regions, and these are the rigid areas. The layer count may vary from four to eighteen layers or even more. The distinguishing feature of the standard design is that the construction sandwiches the flexible layers in its center.

Rigid-Flex PCB Construction — Odd Layer Counts

Among rigid board designs, layers with even counts are more common. However, designs with odd layer counts do exist, and have their own benefits. Similarly, rigid-flex boards may have an odd layer count.

For instance, it is possible to have a nine-layer rigid board along with three flexible circuit layers. Designs that necessitate two-sided shielding primarily use the odd layer construction. Stripline type impedance control mostly drives such requirements of two-sided shielding. The layers of flex consist of the ground + signal + ground construction, aimed at controlling RF and EMI. It is also possible to guide numerous interconnects between the rigid sections. This type of design offers a high amount of flexibility while reducing the cost.

The odd layer count construction is applicable to both the rigid and the flex parts of the PCB, but may also be independent of each other. For instance, it is possible that one side has an even layer count while the other end has an odd layer count, should the application demand such a design. The primary advantage provided by this configuration is an improved mechanical bend reliability due to improved flexibility, brought about by minimizing the flex area thickness.

Rush PCB recommends keeping bend reliability and constructions compliant with IPC 2223C. This will ensure all parts of the PCB have a high degree of reliability. To some extent, the odd layer-count configuration cuts down the cost of the design as it minimizes the final number of flex layers.

Rigid-Flex PCB Construction — Asymmetrical Design

The necessity for complex impedance usually drives asymmetric designs of rigid-flex PCBs. These designs require dielectric thickness to vary widely within the design. Constructions with asymmetrical design also improves reliability and manufacturability of the PCB by allowing a reduction of the aspect ratio of blind vias in the PCB.

However, asymmetric design may require fixtures to hold down the arrays during transportation for assembly. This is due to the introduction of warps and twists because of the unbalanced build of the PCB.

For instance, in an asymmetrical construction, the location of the flex layer may no longer be at the center of the design. Rather, the location of these layers may shift up towards the top or towards the bottom. In general, an asymmetric rigid-flex PCB design does not have any significant manufacturing concerns other than introduction of a few warps and twists.

Rigid-Flex PCB Construction — Varying Flex Layer Count

A varying flex layer count design is also common for rigid-flex PCB construction, where the layer count of the flex part varies between the rigid sections. As an example, if a PCB has three rigid sections, the second and third sections may have one or two flex layers joining them, with three or four layers joining the first to the second—this type of construction may have large variations in the configurations.

Rush PCB recommends using a construction with air-gaps in the flex layer for a varying flex layer count design to meet the IPC 2223C guidelines. Although the extra flex layers may run into the rigid section for routing, they may not surpass between the other sections. The reduction in the flex section, therefore, allows it greater flexibility and bend capability, which is the primary advantage of this design.

Rigid-Flex PCB Construction — Integrated ZIF Tail Construction

A ZIF tail integration within a rigid-flex construction is a common occurrence for the stackup. Such a construction eliminates the need for a flex circuit with a separate rigid section with a ZIF connector mounted on it. Therefore, the construction actually reduces the requirements of real-estate of the rigid area. The design also helps when the PCB is a high-density type with real-estate at a premium or a very thin design where it is not possible to incorporate the height of the ZIF connector.

The integrated ZIF tail construction actually improves the reliability of the PCB, as it eliminates the rigid section, the connector, and the associated interconnect points. The ZIF tail may require an additional stiffener using Polyimide, mainly to achieve the specified thickness requirements of the finger contact area of ZIF connector. If there are multiple layers in the flex part, the stiffener may not be necessary. However, more than four layers may make it difficult to achieve the ZIF thickness requirements. Rush PCB recommends limiting the integrated ZIF tail constructions to one and or two-layer flex configurations.

Rigid-Flex PCB Construction — Blind and Buried Vias

Rigid-flex PCB constructions may also utilize blind and buried vias, just as rigid circuit boards do. The necessity for using blind and buried vias arises from high-density applications, such as those for BGA and fine-pitch component mounting that may require via-in-pad designs. In case the blind vias require interconnects to the layers of the flexible circuits, it may be necessary for the designer to utilize an asymmetrical construction.

The number of sequential lamination cycles required by the design may limit the configuration. Typically, rigid-flex PCBs with multiple layers accommodate only a specific number of cycles of lamination, before the manufacturing methods and the dimensional tolerance of the materials prevent layers form registering effectively to one another. Again, just as in rigid PCBs, applications requiring via-in-pads can use capping and via fill.

Rigid-Flex PCB Construction — Air-Gap Construction

In rigid-flex PCBs with air-gap construction, manufacturers use isolated independent pairs of flex layers to substantially improve the flexibility. Rush PCB recommends constructions with two or more layers of flex to use this method, especially as using this method with four or more layers allows the design to conform to the guidelines of IPC 2223C.

Using air-gap construction precludes the use of any flexible adhesives within the rigid areas, ensuring the reliability of via structures within them. This method of construction also offers a very high degree of reliability and ensures a long-term operational life of the PCB.

Rush PCB recommends using coverlay on both sides of each flex pair, followed by a tiny air gap. All subsequent pairs follow a similar construction.

If the manufacturer constructed the design with bonding together of all layers, there would be significant issues of via reliability, and the flex section would have limited flexibility, to the extent of being almost non-flexible. Additionally, this construction would not meet IPC 2223C requirements.

Rigid-Flex PCB Construction — Multiple Rigid Area Thickness

Although a complex construction, some designs need rigid-flex PCB designs with multiple rigid areas of different thicknesses. Rush PCB recommends reviewing alternative options because of the manufacturing complexities that the design involves. In practice, manufacturers limit the construction to a maximum of two rigid area thicknesses, mainly due to the materials they require for the construction. The stackup is expensive, as the design is similar to manufacturing two boards at the same time.

In this design, one of the rigid sections may be considerably thicker than the other rigid section. Both sections may have plated hoes and vias. However, the manufacturer may have to limit the final thickness of the thinner section as there would be some commonality of materials between the two sections. For instance, the prepreg thickness on both rigid sections need to be the same. Presence of any extra core on one side would need mirroring it on the other side as well.

For instance, the manufacturer may start the construction as a four-layer stackup, encompassing layers two through five. They will run through all the manufacturing processes in sequence, leaving out creation of the outer profile. At this time, the manufacturer will have to start from the beginning again to add layers one and six, proceeding with all the manufacturing sequence once again, finally adding the outer profile. Unless the application demands following this process of multiple rigid area thickness, the process is an expensive one.

Rigid-Flex PCB Construction — Shielded Flex layers

Applications that require FR or EMI shielding use this method of construction, where manufacturers use specialized films in place of copper layers for shielding. Using these films saves on the expense of copper layers as well as allowing the flex construction to be thinner and hence improving its flexibility, along with an effective RF and EMI shielding.

Coverlays on the flex area can have selective multiple openings that expose the ground circuit. Electrically conductive adhesives on the films allow them to bond to the uncovered ground when laminated, thereby grounding the shield layers.

Summary

Combining the rigid PCB to the flex circuits technology allows adding a significant level of creativity, the overall reduction in packaging, and design integration. Rush PCB combines several specific constructions described above to create endless numbers of rigid-flex circuit configurations for their customers.

Toxic Materials and Safety Considerations During PCB Manufacturing

Rush PCB produces printed circuit boards (PCBs) as a platform for mounting electronic components such as semiconductor chips, resistors, and capacitors. Found virtually in all electronic products, PCBs have copper traces that provide the electrical interconnections between the components. With the proliferation of electronic gadgets and equipment, the once low-technology PCB has evolved into a high-technology product.

At Rush PCB, manufacturing printed circuit board is a highly complicated process, one requiring investments in several large equipment and involving over fifty process steps. We manufacture most of our high-speed miniature PCBs in clean-rooms, addressing the same concerns of health and safety as other microelectronic manufacturers do while manufacturing medical electronic equipment.

Apart from glycol-ethers, one of the major solvents the PCB industry commonly uses, large quantities of hazardous chemicals such as dimethyl formamide and formaldehyde are also in use. Lately, the PCB industry worldwide has made a serious effort to diminish the quantity of Lead it earlier used when manufacturing PCBs.

Waste Characteristics from PCB Manufacturing

There are typically three ways in which PCB manufacturing produces toxic materials waste:

Air Emissions
Effluents
Solid and Hazardous Waste

Air Emissions

Manufacturing PCBs produces potential air emissions from acids such as hydrochloric, sulfuric, nitric, phosphoric, and acetic; vapors from ammonia and chlorine; organic solvent vapors from acetone, isopropanol, trichloroethylene; petroleum distillates, xylene, acetate, and ozone depleting substances.

Assembling PCBs may produce air emissions such as from fumes of the soldering process, organic solvent vapors, including from organic acids, flux vapors, aldehydes and similar.

Although chlorofluorocarbons (CFCs) had been a preferred organic solvent for a variety of applications within the PCB manufacturing industry, most countries have now banned their use as these compounds exhibit a high potential for ozone depletion.

Effluents

Manufacturing PCBs produces a number of effluents rich in vinyl polymers, organic solvents, stannic oxide, and metals such as nickel, copper, iron, tin, chromium, palladium, lead, gold, cyanides, fluorides and fluoborates, sulfates, acids, and ammonia. Some metals may also form complex compounds with chelation agents.

Assembling PCBs may also produce effluents containing alkalis, acids, metals, fluxes, and organic solvents. Electroplating processes may produce effluents containing sulphates, cyanides, fluorides, and several metals.

Solid and Hazardous Waste

PCB manufacturing operations also produce solid waste that may occasionally be considered hazardous. Such solid waste may include inks, plating and hydroxide sludges, and scrap board material.

PCB assembly operations may also produce solid waste in the form of scrap boards, solder dross, rejected, broken, or discarded components, metals, and organic solvents. Assemblers may also treat some boards with brominated flame retardants, which may cause environmental risk when such boards are discarded in landfills.

As such, environmentalists consider all conventional electronics as hazardous in landfills, due to the presence of lead and other metal salts, specifically if a post-soldering operation has not cleaned them.

Apart from the above, PCB manufacturing may also produce sludges containing heavy metals. This is specifically the case where industries operate waste water treatment plants. Organic solvent waste residues may also be present requiring management and disposal.

Safely Considerations with Toxic Materials

PCB manufacturing and PCB assembly uses and produces many substances that are toxic enough to cause serious harm if they enter the body of individuals. Rush PCB recommends taking safety measures such as:

Working safely with toxic materials
Substituting with a less hazardous material where possible
Using good ventilation when working with toxic materials
Using warning signs and clear identification when storing toxic materials
Storing toxic material out of direct sunlight and in well ventilated spaces
Handling toxic materials safely
Disposing toxic material and waste safely
Wearing personal protective equipment

Good Housekeeping is Important

Preventing exposure to toxic material is very important during manufacture and assembly of PCBs. For this, Rush PCB suggests good housekeeping, as a clean and orderly workplace is safer for everyone. For instance, Rush PCB uses:

Appropriate spill control equipment and procedures to clean up any spills and build-up of toxic materials promptly and safely.
A pre-wetting technique and a vacuum cleaning equipped with a high-efficiency filter instead of dry sweeping of solid materials.
Proper disposal procedures for unlabeled and contaminated chemicals.
Waste containers compatible with the toxic material.
Properly labeled containers and storage for each toxic material.

Handling Toxic Materials Safely

All personnel who must handle the toxic materials need special training to do their jobs properly and safely. Rush PCB suggests the following:

Use only the smallest amount of chemical necessary for the job.
Never allow toxic vapor, dusts, mists, or gases to enter the workplace air—use proper exhaust techniques.
Avoid body exposure by wearing appropriate personal protective equipment for the eyes, respiration, and the skin.
Awareness of typical symptoms of poisoning and first aid procedures. Training should include immediate reporting to the supervisor any signs of illness or exposure.
Medical attention depending on the material, even when the exposure does not seem excessive.
Never allow returning unused or contaminated material to the original container.
Label containers legibly and inspect regularly for leakages or damage before handling.
Ensure container lids remain tightly closed when not in use.
Suitable emergency equipment for leaks, spills, and fires must readily be available.
Suitable emergency shower/eyewash stations must readily be available.
Open containers for transferring material using proper tools—prevent spillage.
Be careful when pouring toxic liquids—avoid spurting and splashing.
Never allow soldering, cutting, welding, or any other hot work on piping or containers unless they are free from all toxic vapors and liquid.
Maintain good housekeeping—do not allow accumulation of dust etc.

Disposal of Waste Toxic Materials

Safe disposal of waste toxic material is extremely important for the environment. Rush PCB suggests the following:

Always comply with local, provincial, and federal requirements when disposing toxic material. Requirements may vary depending on the jurisdiction.
Never allow flushing of toxic material down sewers or sanitary drains, as this method of disposal is illegal and unsafe practice.
Never allow mixing of hazardous waste material with regular garbage that is destined for a landfill.
Use only waste containers compatible with the waste material.
Properly and accurately label all waste containers.
Never allow mixing incompatible mixtures in a single waste container—avoid potential spills, fires, or explosions.
Never allow overfilling of liquid waste containers—allow filling only to about three-quarters capacity for vapors to expand. This reduces the potential for spills when moving overfilled containers.
Do not allow reusing empty containers as they may still be hazardous from the leftover toxic residue. Arrange for safe and proper decontamination before reusing the container.

Personal Cleanliness is Essential

Personal cleanliness is essential for those working with toxic materials. This provides protection not only for self but also for others (coworkers and family members). Rush PCB suggests the following:

Always wash hands before smoking, drinking, eating, or visiting the toilet.
Remove contaminated clothing and shoes, and wash them thoroughly in water before discarding or re-use.
Always store food and tobacco products in uncontaminated areas.
Make a conscious effort to not touch eyes or face with contaminated hands.
Never allow gum chewing when working with toxic materials.
Even when following all the above, it is necessary to wash thoroughly at the end of the workday.

Conclusion

At present, the world is facing a deluge from electronic waste or e-waste, comprising a multitude of components and materials, some toxic and hazardous, leading to an adverse impact on human health and the environment. Eminent electronic industries and PCB manufacturers such as Rush PCB are in the forefront of treating and disposing toxic and hazardous waste material they generate and using approved waste disposal and/or recycling operations.

Importance of PCB DFM Checks — Steps Involved

PCB assembly at Rush PCB involves several steps and processes, as we want to make sure in advance that our customers’ boards will not have any issues with manufacturing. Therefore, Rush PCB employs Design for Manufacturing or DFM checks to ensure we manufacture all PCB assemblies properly.

PCB DFM checks are totally cost-effective, as the system in use at Rush PCB is entirely automated for both DFM and DFA. Our system rapidly scans the manufacturing issues that may prohibit the PCB manufacturing process. Therefore, the application of this DFM/DFA check is beneficial to our customers, as it saves the PCB lead time while lowering the PCB cost.

It can become very expensive for the customer when they find errors in the fabrication or assembly process during the final stages of delivering the product to market. Anything may have gone wrong—PCB layout, material used, difference between prototype and original manufacturing files, and so on. By going through DFM checks it is possible to solve these problems before the Contract Manufacturer or CM starts fabricating and assembling their PCB.

Who Handles DFM?

Often, OEM design teams tend to look at DFM as something their CM should handle. In fact, most CMs perform DFM analysis prior to taking up production for identifying and fixing issues. This is a vulnerable step if the manufacturer does not share the changes they make with the design team and make the changes without a proper understanding of the requirements of the design and circuit performance. Any new revision by the design team will result in total chaos and probable failure of the PCB and finished devices.

Rush PCB recommends the design team performs their own DFM analysis prior to the prototyping stage to detect and weed out the issues, allowing them to incorporate the changes in the PCB design. This ensures the OEM lowers the cost by maintaining their design intent, and ensure the follow-on builds also work properly.

Involving the CM in the early stages of design works to the advantage of the OEMs. The CM, when reviewing the design, will match the parts on the board with their master component vendor inventory database, offering real-time recommendations on the availability of parts.

Design engineers at the CM will also review the design for its functional performance. They make recommendations for material the board should use and changes necessary at layer stackups for impedance control, suggesting routing changes for improvements. They may also suggest changes in component placement and routing for improving signal integrity.

Working closely at the early stages of the design is a win-win situation for both the OEM and the CM, resulting in smooth operation for the production of a successful and working PCB assembly, ready for market deployment.

Implementing DFM Checking Systems

It is easy to set up DFM checking systems as software packages are readily available. These packages work along with the CAD system for PCB design including schematic drawing and PCB track layout. The DFM system can detect errors that normally remain undetected during the reworking of the design. CAD systems often overlook issues such as acid traps, starved thermals, insufficient annular rings, slivers, and so on, but which can be disastrous during manufacturing. DFM software is usually equipped with fabrication analysis for detecting such issues.

Most DFM checking packages include easy-to-use PCB manufacturing analysis technology for identifying specific design issues that could be detrimental to PCB fabrication. DFM checking software, therefore, helps to reduce scrap, improve yield, while preventing setbacks from expensive time to market issues.

Major Issues Covered in DFM Checking

Major issues that DFM checking systems detect during design are:

Formation of Acid Traps
Possibility of sliver and island formation
Formation of solder bridges between pads
Paste mask openings for heat sinking
No solder connection or cold solder joints
Non-inclusion of test points
Proximity of copper to board edge
Optimization of drill size
Incorrect pad sizes
Component spacing
Component location and rotation

Formation of Acid Traps

Acute or odd angles of copper features on the PCB can form acid traps. These are areas where acid can remain trapped during the PCB etching process, and sometimes even cleaning chemicals are unable to clean them completely. The residual acid gradually erodes the nearby copper features, creating a smaller width of traces than intended, or even creating a discontinuity in the trace.

PCB designs with 4-5 mil traces are quite common today. Trapped acid can easily erode and open these thin traces. Designers can avoid such acid traps by not placing incoming traces at acute or odd angles to pads. Rush PCB advises keeping the angles of traces at 45 or 90 degrees to the pad, and the DFM software can flag non-conformances.

Possibility of Sliver and Island Formation

While designing, many plane layers can have slivers and islands or free-floating copper, which can create serious problems during etching. These freely floating copper specks can cause several issues as they find their way to other portions of the panel creating a short between closely spaced traces. Slivers may also cause noise and other interference as it is floating copper and may behave as an antenna does.

The only way to make sure there are no slivers or islands created is to check manually or let the DFM software detect them.

Also Read: How to Detect Circuit Board Faults?

Formation of Solder Bridges Between Pads

With designers using fine-pitch components more commonly than before, it is essential they also include solder mask in between adjacent pads. Excluding this essential solder mask can allow excess solder on pads to join during reflow, creating a solder bridge and an unwanted electrical short.

To get around this issue, designers need to check the alignment and spacing of solder mask from pads to neighboring shapes. They should also consult their CM for the minimum webbing space and alignment they allow in a design. The DFM system software can easily flag issues such as non-existing solder mask between adjacent pads, and if solder mask covers a pad.

Paste Mask Openings for Heat Sinking

A heat sink is essential for absorbing and dissipating heat from an electronic component. The thermal contact is usually through a metal base that attaches to a large copper surface on the PCB. For proper heat transfer, the electronic component is also soldered to the copper surface. However, a large opening in the paste mask allows deposition of a substantial amount of solder paste, allowing the component to float off the pads during reflow.

To prevent the above, designers must limit the amount of solder paste the paste mask can deposit on the copper pads. Rather than have a single large paste mask opening, the designer can break it up into several smaller openings, thereby preventing the component from floating away during the reflow process.

Rush PCB suggests designers should consult their CM for the proper size of the paste mask opening. Essentially, DFM systems check for proper paste mask openings.

No Solder Connection or Cold-Solder Joints

Vias placed within pads can create an issue, as the via can allow molten solder to flow down the opening and leave the pad with too little solder to form a strong joint. This forms a no solder connection or a cold-solder joint. The DFM software checks if the percentage of the via within the pad is within permissible limits or else flags it for plugging.

Non-Inclusion of Test Points

Inclusion of test points in the PCB during design allows testing the board after assembly and in the field. This allows an easy way to tell whether the board functions as intended. DFM systems allow checking test points for clearance from components, their pad size, placement along a grid, and so on, to allow a fixture to locate them easily.

Designers must Include test points in the PCB during the initial layout phase of the design. Adding test points later on could lead to creating noise and crosstalk in sensitive circuits. The primary requirement of test points is they are easily accessible and not hidden below components. Their spacing must also be adequately apart to allow test pins to access them.

Proximity of Copper to Board Edge

Manufacturers prefer making boards in panels, as it is cost effective to make multiple boards simultaneously. They separate the individual boards from the panel by various means such as shearing, breaking along a V-cut and other methods. Essentially, a minimum gap must exist between the edge and the copper surfaces on the board, to prevent the copper from damage when the fabricator separates the boards.

Another reason exists for maintaining a proper distance between the copper on the board and the board’s edge. Motorized transport must grip the boards properly during the various processes. For instance, the etching process requires application of electric current to the panel, and presence of copper too close to the edge of the board can create shorts.

Designers must consult their CM for various equipment they use and the spacing they require. The DFM system checks for and flags the issue if it finds the spacing inadequate.

Optimization of Drill Size

Designers often use several drill sizes for their boards. Changing drills during board manufacturing not only wastes time, it increases the expense for board fabrication. The DFM software checks for drill sizes in the board and flags the designer to consolidate them to an optimum number.

Incorrect Pad Size

Sometimes the designer may overlook a wider trace leading to different sized pads for an electronic component. While reflow, the different size of pads can lead to uneven heating of the component, resulting in the chip component lifting up on one of its ends, a condition known as tombstoning. DFM systems check for and flag instances of incorrect or mismatched pad sizes.

Component Spacing

Pick-n-place machines need a minimum gap between components to enable proper placement. The designer must consult their CM to know the minimum their machines need. While placing components during layout, if the designer places components too close, the CM will have a problem during assembly, as the pick-n-place machine may not function optimally. DFM systems check for this spacing and flags if it finds the spacing inadequate.

Component Location and Rotation

Placing smaller components very close to large ones may cause soldering problems during reflow. The larger components absorb more heat leaving the smaller components not hot enough for proper soldering. DFM systems predict this shadow effect and warn designers.

Conclusion

It is not only frustrating but also expensive for the customer to detect failure when getting ready to enter the market. According to Rush PCB, one of the ways of avoiding such failures is by planning for the future by designing for manufacturing. The availability of several good DFM software packages makes it easy to implement such checks and prevent failures from cropping up. Although it is always possible to manually identify and resolve such issues, the DFM software systems make checking independent of the human.

Key Steps in Designing Printed Circuit Board Layouts

Recent advances introduced by Rush PCB UK Ltd have had sweeping effects across the Printed Circuit Board (PCB) design industry. PCBs form the core of any electronic product today. They serve to hold the innumerable minuscule electronic components together, while allowing them to interconnect and function as the designer intended. Design and layout of PCBs is therefore, and important stage in the development of any product—often deciding the success or failure of the product.

Rush PCB UK Ltd has been in the forefront of this innovation, constantly evolving this technology. As a result, these designs have reached new heights in complexity and expectation, with PCB design rules and production processes evolving to achieve new layouts and capabilities. For instance, mass-produced PCBs commonly have very thin tracks and multiple layers. Even a few years ago, no one would have believed the feasibility of such designs. The availability of advanced PCB design software has also helped this progression tremendously.

However, such improved capabilities alone are not sufficient to effectively design printed board layouts. Experienced electronic engineers routinely struggle with creating layouts on a PCB or when designing a PCB following the best practices in the industry. Often, it becomes really difficult creating a quality board that will fully meet the customer’s need. As each design is different, balancing the functionality of a PCB while following the best design practices is an ever-involving process. To make the process easier, Rush PCB UK Ltd has outlined the process of designing PCBs and included some essential PCB design rules.

RushPCB UK

Determining the Basic Needs

Although requirements are broadly spelled out by the customer, the design engineer must translate these requirements into mechanical, electrical, and electronic terms. For instance, the design engineer must decide if the equipment requires a single PCB or several, and the functionality of each of the PCBs. Depending on the functionality and the interconnection between the PCBs, the design engineer will apportion a certain part of the total circuitry to that PCB.

Determining the Material for the PCB

The basic needs of the project also help in determining several important aspects, one of them being the materials of which the PCB should be made. For instance, the project may require the PCB to handle high frequencies, and therefore, the designer will use materials with controlled impedance. On the other hand, applications involving medical or automotive devices often require components that will fit into tiny spaces, thereby requiring a flexible PCB. Additionally, the final size of the PCB and its functionality often determine whether it will be made of a single layer or with multiple layers.

Based on the above needs and the functionality of the PCB, the designer will decide the initial schematic of the PCB, and hence the Bill of Materials or BOM.

Determining the Schematics and BOM

While the schematic determines the functions of the PCB, the BOM lists the components that the PCB requires, their source, and their footprints. Essentially, the schematic forms the blueprint engineers and manufacturers use during the development, production, and the assembly processes. The schematic and the BOM together contain details of the hardware for the PCB, material, components, and details of other materials the manufacturer will need during the production process. The designer must ensure the schematic refers to each component with a unique identifier, and they all have the proper footprints associated with them. Additionally, the information in the schematic must also tally with the BOM. In fact, designers typically use PCB design software to generate the schematic and the BOM.

The design engineer must ensure the schematic and the BOM contain the manufacturer’s part number for the components, and where possible, alternatives for each component, as alternatives help in speeding up the procurement process.

Determining the Net List and the Layout

After finalizing the schematic and the BOM, the designer has to generate the Net List. This is a list of interconnections of the components on the PCB. Each PCB will have its own Net List.

The Net List serves two purposes. The first is to generate the layout, and second is to allow the PCB manufacturer verify the interconnections when fabricating the PCB. The designer imports the Net List into another part of the PCB design software, which recalls all the footprints from its library and connects the pins of the components according to the Net List using a series of straight lines also called the Rats Nest. In the next step, the design engineer must follow some best practices for achieving an effective layout:

Implement the Appropriate Panel Size for the PCB
Implement the Proper Grid
Implement Design Rule Checks or DRC
Implement Design for Manufacturing or DFM

Implementing the appropriate panel size of the pcb is a vitally important step for the design engineer to follow, as the PCB has to fit into the space allocated in the equipment. Moreover, it also marks the areas on the PCB allocated for mechanical attachments, and hence, the designer has to be careful of not placing any electronic component in these areas.

Implementing a proper grid is essential for maintaining proper spacing between components and tracks, and the distance components must maintain from the edge of the PCB.

DRC is another essential requirement that a design engineer should start implementing right from the start of the design rather than check at the end of the design process. This allows addressing problems identified by DRC quickly, thereby minimizing requirement of massive changes at the end of the design process and saving time.

From the beginning of the design process, the design engineer should also be concerned about the manufacturability of the PCB. A discussion with the Contract Manufacturer during the design process helps to address manufacturing and assembly constraints right at the beginning, thereby avoiding requirements of changes at the end of the design process.

Determining Placement of Components

Placing each component at its designated spot on the PCB is tricky and depends on numerous factors and considerations. These include the overall PCB functions, thermal considerations, EMI/EMC considerations, and many more. However, design engineers typically place components following best practices order such as:

Major Connectors and Mechanical Attachments
Power Components
Precision and Sensitive Circuits
Critical Components
Others

During placement of components, the design engineer must also keep Design for Assembly or DFA requirements in view. For instance, if a connector requires manual operation, it must have space around it for finger insertion.

A requirement the design engineer must address at this stage is the position of appropriate test points to ease fault detection. Another requirement is the silkscreen should show clean and unambiguous marking for the components. This is a very important requirement as the information on the silkscreen is useful not only to the designer, but also to the manufacturer, assemblers, and testers. If possible, the design engineer should mark the functionality of the circuit, put test markers, and add component and connection placement directions. Manual assembly gains from silkscreen markings on both sides of the board, while simplifying the production process.

Design Review

Once the design engineer has satisfactorily placed all components, it is a good idea to conduct a design review along with a round of DFM and DFA analysis. Although this step may look unnecessary, it helps to locate mistakes and anomalies for timely correction.

Printed Circuit Board Routing

Most PCB design software have capability to auto-route traces connecting the components based on the Net List. The program does this by using the number of layers available for the connection and taking advantage of the available space by calculating the position of vias.

It is always a good idea to cross-check the interconnections after the computer has finished auto-routing the board. Most design software can produce a second Net List after routing, and the design engineer can verify the connections by comparing with the original Net List. Some areas may be difficult for the auto-router, and the design engineer may have to alter the layout to some extent to generate more space to allow proper routing in a dense environment.

Sometimes, the computer may produce results the design engineer may not approve. This is especially true for high-speed traces, where the designer may have to control the impedance for maximizing energy transfer.

DRC is an important parameter the design engineer must use during the routing process. It should place restrictions on the minimum spacing between traces and pads not only on the outermost layers but also on the inner layers as well. Trace width determines current carrying capacity, and the design engineer must allow wider traces where higher current is flowing.

Power and Ground Planes

One of the advantages of a stackup with multiple planes is to reduce the physical size of the PCB. Another advantage is to distribute power and ground lines as planes. PCB designers preferably dedicate one or more circuit layers for use as ground plane, and others as the power plane. Designers allocate the power and ground planes in pairs.

There are several advantages of using dedicated ground and power planes in multi-layered PCB. Planes have much lower inductance as compared to thinner traces, and act as low source resistance connection. They also act as shields on either side of the circuitry sandwiched in between the two. This helps to reduce EMI/EMC largely, while improving the operational efficiency. Being large tracts of copper, ground and power planes also act as efficient heat dissipators, especially as designers assign them to the top- and bottom-most layers of a PCB.

PCB Heat Management

Electronic components carrying current will generate heat. The extent of heat generated depends on several factors, which may include variation in copper thickness, the number of layers present, and absence of thermal paths. The design engineer must arrange to distribute this heat evenly, thereby preventing heat spots.

Design engineers follow several methods to manage heat in PCBs. One of them is to use the power and ground planes to the hot component to dissipate heat. As planes contain more copper, they dissipate heat better. Another method designers use for reducing heat generation is by using effective high-current routes and optimizing heat transfer. In some cases, designers also maximize the area they use for transferring heat. However, designers must do this early on in the design phase, and it can also affect the final size of the PCB.

Final Design Review

The final design review of the printed circuit board design must contain many other checks apart from the regular DRC check. At the very least, it should include among physical verification processes a layout-versus-schematic check, an electrical rule check, an XOR check, and an antenna check. Advanced PCB manufacturers such as Rush PCB UK Ltd use additional rules and checks for improving the yield, apart from basic checks manufacturers and designers typically employ.

For the design engineer, one of the good practices to follow is verifying the manufacturing parameters before submitting the PCB documents to the contract manufacturer. By personally generating and verifying the manufacturing parameters before submitting the final design of the PCB, the designer can save the OEM countless hours of confusion, misunderstanding, and losses. This is an important verifying step to expedite the manufacturing process, as it decreases the time necessary for correcting and rectifying the design before manufacturing can start.

Conclusion

According to Rush PCB UK Ltd, implementing the above key techniques and best practices leads to an effective and efficient method of designing Printed Circuit Board layouts. In fact, the process is further simplified by partnering with a contract manufacturer who works with the OEM to make the best and most cost-effective PCBs.

Material and Methods for High Quality Rigid-Flex PCBs

Contrary to the construction of standard PCBs with a metal or fiberglass base, flex PCBs consist of a flexible polymer core and a Polyimide film as a substrate. The advantage of Polyimide is it does not soften when heated, and stays flexible after the initial thermosetting. Unlike several types of thermosetting resins that become rigid after being heated, Polyimide remains flexible, and that makes Polyimide a superior material in flex and rigid-flex PCB construction. RUSH PCB UK Ltd uses upgraded Polyimide film that has good resistance to humidity and tearing.

Rigid-flex PCBs basically connect flex PCB materials to rigid PCB materials. This allows the PCB to bend in certain areas, and to stay rigid in others. Therefore, the board remains strong, but flexible at the same time. When designers want to transfer signals between the rigid and flex parts, they need to design the rigid-flex PCB. While the flexible part of the board resembles a regular flex circuit, the rigid sections may use materials that standard rigid PCBs use, such as fiberglass.

According to RUSH PCB UK Ltd, both the manufacturer and the OEM benefit when they involve in the conceptual design of Printed Circuit Boards (PCBs). Specifically, for flex and rigid-flex PCBs, it is necessary for both to understand the full project requirements and implications up front.

The Concept Design Phase

The project usually starts with discussions between the two engineering teams to develop a broad understanding on several factors such as:

Functionality of the rigid-flex PCB
Objectives of the project related to the rigid-flex PCB
Will the PCB have active components?
Number of interconnects on the PCB
Requirement of special signal capabilities such as impedance control, high current carrying traces, and other factors such as RF design and or shielding and protection requirements, operating temperature requirements, and similar
Shape and size of the PCB

Answers to above questions help to clarify the requirements, based on which, the manufacturer can then give their inputs to the OEM regarding rigid-flex technology best suited to their needs. This mostly relates to the material and methods for high quality rigid-flex PCBs. The first consideration is to decide whether the application really needs a flex-based solution.

While identifying the additional levels of functionality, it is also necessary to look into higher levels of integration between parts. By simplifying complex levels of integrations, it may be possible to reduce the cost of the PCB project.

Completing the design concept phase and determining that the application does require a flex and rigid-flex PCB, the designers must delve into deeper specific details of the design.

Specific Detail Requirements

Although there may be several detail requirements specific to a particular project, those most common to all designs are:

Minimum and maximum bend requirements
Will the bend be permanent or will it flex periodically?
Length of flex between bends
Requirement of stiffeners and type of stiffeners necessary

Based on the requirements in the concept design and specific details the manufacturer can suggest various suitable materials and methods for the high-quality flex PCB capable of meeting the quality and life requirements of the OEM.

Also Read: Five Reasons Why RushPCB is the Leading LED Board Manufacturer in UK

Materials and Methods of High-Quality Flex PCBs

In the last decade or so, RUSH PCB UK Ltd has taken the rigid-flex circuit design and fabrication to significant levels of evolvement. For instance, the rigid areas of our rigid-flex designs are capable of the same complexity and density as that of our HDI boards. For instance, just as for our HDI boards, the rigid areas of our rigid-flex designs can have the same fine lines/spacing, high operating temperatures, high layer counts, and compliance to RoHS standards.

In earlier methods of fabrication, manufacturers used several layers of adhesives while fabricating the rigid areas. However, the high coefficient of thermal expansion of adhesives led to a significant amount of stress on vias during thermal cycles that the boards undergo during the assembly cycles, and during operation. As a result, vias placed within the rigid areas would develop cracks in the copper plating.

The adhesives in the rigid-flex system may come from the copper clad flex laminate, the coverlay, and the material that bonds the rigid and flex layers. For solving the issue of reliability of vias, RUSH PCB UK Ltd has made necessary changes in the materials and methods of construction to eliminate or minimize the use of adhesives.

Adhesive-less Construction

Where earlier manufacturers used layers of copper bonded to the polyimide core with some acrylic type of adhesive, RUSH PCB UK Ltd uses adhesive-less laminates where the copper bonds directly to the polyimide core, eliminating the adhesive bond layers. Not only does this technique allow thinner PCB construction, it also allows for higher flexibility and vastly superior reliability.

Adhesive-less copper clad laminates have further advantages. They can operate at higher temperatures, their copper peel strength is higher, and they exert much reduced stress on vias due to lower Z-axis thermal expansion coefficients.

Similarly, earlier coverlay construction in rigid-flex design used full coverage types that covered the entire rigid area of the PCB. As the coverlay adhesive expanded, it would put vias and other PTH to severe expansion stress. RUSH PCB UK Ltd uses selective coverlay constructions that remain restricted to the exposed flex areas only, while extending to a maximum of 0.05 inches (1.27 mm) into the rigid areas. No via or PTH is placed in this interface area.

RUSH PCB UK Ltd also uses high temperature no-flow FR4 prepregs to laminate the rigid and flex layers together rather than layers of flex adhesive. The structure this technology achieves is dimensionally highly stable. In fact, this stability matches that of standard rigid PCBs.

High quality flex and rigid-flex circuits from RUSH PCB UK Ltd conform to IPC 2223C, the Sectional Design Standard for Flexible Printed Boards. This standard defines the minimization/elimination of the use of adhesives within rigid areas, use of adhesive-less substrates, and the use of partial/selective coverlay construction.

Conclusion

As the demand for portable electronic equipment grows, engineers are increasingly taxed with improving the capabilities of combining functionality with flexibility. This flexibility also takes the form of flexible circuits that fit where no other design solution can help. Along with a significant reduction of interconnects, and a substantially greater freedom of packaging geometry, the integrated hybrid of the rigid PCB and flex circuits allows designers to retain the precision, density, repeatability, and reliability of regular PCBs.

PCB Vias and Everything You Need to Know About Them

Looking at a complicated Printed Circuit Board (PCB) such as the motherboard of a computer, you are likely to find several tracks going nowhere, and terminated rather abruptly. However, a closer inspection, preferably with a magnifying glass, will reveal more details at the point of termination of the track. Most likely, you will see it ending in a small PCB pad, not much larger than the width of the track itself, with or without a hole in its center.

Fig.1: Tracks on a PCB

Fig.2: Close-Up of a Via

In reality, the track does not terminate, but rather continues to travel, albeit on a different layer, hidden under the outermost layer of the PCB. The pad at its end is actually a small pipe through the insulating material, electrically connecting the two parts of the track. In PCB terminology, such an arrangement that allows tracks to continue, but on a different layer, are known as a PCB vias.

Types of Via in PCB

Multilayer PCBs use different types of vias for various purposes. There might be through-hole vias, blind vias, and buried vias in the same PCB. Although the construction of all vias is same, their nomenclature depends on the layers of origin and termination.

Fig.3: Type of Vias

For instance, a via originating from the outermost layer, traveling through the board, and terminating at the other outermost layer is a through-hole via. In its passage through the layers of the board, it may or may not connect to intermediate layers, depending on the necessity of the electrical circuit.

A blind via originates from one of the outermost layers, but terminates on an intermediate layer, and therefore, is visible only on the originating layer. It may or may not connect to other layers in between.

A buried via is not visible from either of the outermost layers, as it originates in one of the inner layers and terminates in another inner layer, possibly connecting other layers in between.

Construction of a Via

By design, a via consists of two outer pads and a copper tube electrically joining them. The two outer pads reside on the originating and the terminating layers of the PCB, while antipads on all intermediate layers allow electrical isolation of the copper tube from the electrical circuits on these layers as it passes through.

Fig.4: Construction of a Via

While the two outer pads and antipads are part of the layout pattern a fabricator etches onto the PCB, an electrode position process forms the copper tube connecting the two. Although in regular multilayer PCBs, you may find through-hole vias, these are less likely in high density interconnect or HDI boards.

Difference Between Plated Through Hole and Via

The major difference between the two lies in their construction process. A fabricator can electroplate a through-hole only after assembling all the layers of a multilayer PCB, since a through-hole spans all the layers, while he can form a complete via, including electroplating it, when assembling each layer pair.

Another advantage with vias in multi layer PCBs is, the designer can either stack or stagger them to suit the requirements of the circuit layout, while he or she cannot do that with a through-hole. Therefore, vias help in increasing the layout density of a board, allowing the designer to reduce the size of and/or number of layers on a multilayer PCB.

Filling a Via

The designer may decide to fill the vias in the PCB during manufacturing. While blind vias require filling to avoid surface dimpling, some designers may specify an additional epoxy filling after lamination to maintain better surface flatness.

You can ask your PCB fabricator to fill the vias with either epoxy or metal epoxy. The choice between epoxy and metal epoxy is that the latter is conductive. Therefore, if you have designed the via with a thermal application in mind, for instance, to disperse heat from one side to the other, filling the via with metal epoxy will be a better choice as opposed to epoxy filling of the non-conductive type. As its barrel always has a layer of copper, a via always retains electrical continuity, regardless of whether the filling is conductive or not.

In highly dense PCBs, especially those with fine-pitch components such as BGA, fabricators fill PCB vias with epoxy and planarize them to make them flat. Flash plating over them makes them perfectly flat and suitable for mounting BGAs.

Other applications may need a Faraday shield on one side of a chip, which could double as a heat sink as well. Stitching the underside of the chip with vias is a standard practice, while filling them up with a conductive epoxy fill, helps in the heat conduction.

When concerned with EMI, you may use multiple vias in the region of a ground strap, filling them up to provide a conductive wall. An impedance-controlled structure may also benefit from closely spaced and filled vias on either side.

Special Vias

Although you will find warnings about placing vias within pads as these can siphon off solder paste while soldering, leaving the joint devoid of solder, you may not have much choice when designing with very closely pitched BGA packages. The space available around the pads may not be adequate for a dog bone, and the only option may be a via in the SMT pad, or partially in it.

You can get over the solder siphoning by having the via epoxy filled, flattened, and plated to encapsulate it. The other side of the PCB via may not be important enough and you can wall it off with a mask.

Sometimes, to attain very high routing densities, it may be necessary for the designer to use landless vias. The trace directly enters the hole without a PCB pad. As the vias do not have PCB pads, the designer can pack in more traces in between adjacent vias.

Stacked Vias and Laser drilling

Even after using blind and buried vias, you may still not have enough room for proper routing. In such circumstances, you may consider using laser drilled micro vias and or stacked vias. The two major benefits of laser drilling are extremely fine holes (sub 0.004”), and excellent registration. Both are obvious benefits for very dense parts.

Fig.5: Laser-Drilled Micro Via

Fig.6: Staggered and Stacked Micro Vias

Laser drilling does not pass through the layers, unlike that in mechanical drilling. It vaporizes the top copper layer, burns through the substrate dielectric layer beneath it, and stops when it touches the bottom copper layer. This accounts for the v-shaped pit as against a straight hole a mechanical drill-bit creates.

For stacked vias, designers place laser drilled vias directly on top of each other. However, designers typically use stacked vias only when board real estate is at a premium.

Vias and Signal Integrity

Just as people do, electrical signals too find it much easier to take a direct route when traveling from point A to point B. A via in the signal path forces the signal to take a detour, and the signal integrity suffers as a result.

For high-speed signals, there is also the challenge of via stubs. For instance, a via taking a trace from L1 to L3, may leave a stub down to L16. A high-speed signal will typically traverse all the way from L1 to L16 before reflecting to L3. This will attenuate the signal, as the effective electrical stub length will be almost double its mechanical length. Designers of thick boards take care of the problem by removing the unnecessary part of the barrel by back drilling. HDI boards do not face this problem, as they use laser drilled micro vias and stack them up to the desired layer.

Conclusion

The above types of via in PCB can increase density and bring down the cost in volume production. Laser drilled PCB vias can increase multilayer density and reduce layer count, without reducing the trace width or trace spacing.

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