pcb assembly

Integrity of the Signal in HDI Circuits

With rise-times of signals on the printed circuit boards (PCBs) continuing to drop, the age-old concerns related to signal integrity is always at the forefront of (PCB) Printed circuit board design. However, with the increasing quantities of printed circuits in high-density interconnect or HDI technology, there are some interesting new solutions.

Signal integrity analysis in PCBs has five major areas of concern:

  1. Reflection
  2. Cross-talk
  3. Simultaneous Switching
  4. Electromagnetic Interference (EMI)
  5. Interconnect Delays

Although HDI does offer improvements and alternatives for all the concerns above, it does not provide all the solutions. Signal integrity depends on the materials the PCB uses, and the materials the HDI technology uses, together with the PCB design rules and dimensional stack-up helps the electrical performance including signal integrity. Likewise, miniaturization of the PCB using the HDI technology is a major improvement for signal integrity.

HDI Benefits Signal Integrity

With new electronic components such as ball grid arrays and chip-scale packaging achieving widespread use, designers are creating PCBs with new fabrication technologies to accommodate parts with very fine pitches and small geometries. At the same time, clock speeds and signal bandwidths are becoming increasingly fast, and this is challenging system designers to reduce the effect of RFI and EMI on the performance of their products. Moreover, the constant demand for denser, smaller, faster, and lighter systems are compounding the problems with restrictions placed on cost targets.

With HDI incorporating microvia circuit interconnections, the products are able to utilize the smallest, newest, and fastest devices. With microvias, PCBs are able to cover decreasing cost targets, while meeting stringent RFI/EMI requirements, and maintaining HDI circuit signal integrity.

Advantages of Using Microvia Technology in HDI Circuits

Microvias are vias of diameter equal to or less than 150 microns or 6 mils. Designers and fabricators use them mostly as blind and buried vias to interconnect through one layer of dielectric within a multi-layer PCB. High-density PCB design benefits from the cost-effective fabrication of microvias.

Microvias offer several benefits from both a physical and an electrical standpoint. In comparison to their mechanically created counterparts, designers can create circuit systems with much better electrical performance and higher circuit densities, resulting in robust products that are lighter and smaller.

Along with reductions in board size, weight, thickness, and volume, come the benefits of lower costs and layer elimination. At the same time, microvias offer increased layout and wiring densities resulting in improved reliability.

However, the major benefits of microvias and higher density go to improving the electrical performance and signal integrity. This is mainly because the HDI technology and microvias offer ten times lower parasitic influence of through-hole PCB design, along with less reflections, fewer stubs, better noise margins, and less ground bounce effects.

Along with higher reliability achieved from the thin and balanced aspect ratio of microvias, the board has ground planes placed closer to the other layers. This results in lowering the surface distribution of capacitance, leading to a significant reduction in RFI/EMI.

HDI PCBs use thin dielectrics of high Tg and this offers improved thermal efficiencies. Not only does this reduce PCB thermal issues, it also helps the designer in streamlining thermal design PCB.

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Improved Electrical Performance of HDI Circuits

The designer can place more ground plane around components, as they implement via-in-pad with microvias. The increase in routabilty offers better RFI/EMI performance due to the decrease in ground return loops.

As HDI circuits offer smaller PCB design along with more closely spaced traces, this contributes to signal integrity improvements. This helps in many ways—noise reduction, EMI reduction, signal propagation improvement, and lowers attenuation.

The improved reliability of HDI circuits with the use microvias also helps in PCB thermal issues. Heat travels better through the thin dielectrics. Streamlining thermal design PCB helps remove heat to the thermal layers. Several manufacturers make complex enhanced tape BGAs of thin, laser-drilled polyimide films to take advantage of PCB design with HDI.

The physical design of the microvia helps in reducing switching noise. The reason for this decrease is due to decrease in inductance and capacitance of the via, since it has a smaller diameter and length.

Signal termination may not be necessary in HDI circuits as devices are very close together. Since the thickness of the layers is also small, the designer can utilize the backside of the interconnection effectively as well.

Just as the signal path is important in PCB design, so is the return path. Moreover, the return path also influences the resistance, capacitance, and inductance experienced by the signal. As the signal return current takes the path of minimum energy, or the least impedance, the low frequencies follow the path minimizing the current loop.

Miniaturization from using HDI technology provides interconnections with shorter lengths, meaning signals have to traverse shorter distances from origin to destination. Simply by lowering the dielectric constant of the HDI material system, the designer can allow a size reduction of 28%, and still maintain the specified cross-talk. In fact, with proper design, the reduction in cross-talk may reach even 50%.

Conclusion

HDI PCB design not only helps in improving the integrity of signals, but the presence of thin dielectric helps with the PCB thermal issues as well. In fact, HDI technology helps with all the five major areas of concern related to signal integrity.

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RUSHPCB UK

How to Detect Circuit Board Faults?

Printed circuit boards (PCBs) are increasing in complexity and diversity. With a wide array of applications, the only requirement common to all types of PCBs is they must function in accordance with their design parameters, without errors and failures. In short, PCBs must perform flawlessly.

Complex PCBs can have hundreds of components with thousands of solder connections and that gives innumerable opportunities for failure. The Printed circuit boards manufacturing industry makes sure all their PCBs meet the above challenge of flawless working through a battery of inspection and testing procedures to ensure the quality of their products.

Assemblers detect circuit board faults before assembly through various inspection methods. After assembly is over, they employ another set of inspection and test methods to solve PCB errors.

Evolution of PCB Inspection and Test Methods

Simple circuit boards with a handful of components needed only manual visual inspection (MVI) methods to ensure solder problems and placement errors were weeded out. With increasing complexity and growing production volumes, MVI systems were found to be inadequate, as humans soon grew tired, and could not be relied upon to carry out the task of inspection repeatedly for long hours. As a consequence, inspectors missed defects and faulty boards reached later stages where it was more expensive to solve PCB errors.

This brought up the next step in inspection systems—Automated Optical Inspection (AOI) methods—now a widely accepted inline process. Assemblers effectively use AOI to inspect PCBs before and after reflow soldering to check for a variety of possible faults. Now, even pick-and-place machines incorporate AOI capabilities, allowing them to check for misalignment and faulty component placements.

With the advancement of surface mount technology, components became smaller, and this increased the board complexity along with PCBs becoming double sided and even multi-layered. Additionally, introduction of fine-pitch SMDs and BGA packages brought out the limitations of AOI, forcing assemblers to implement even better inspection methods such as the Automated X-ray Inspection (AXI) systems.

After the assembly, PCBs are often tested for in-circuit components (ICT) and for functional testing (FCT). While ICT ensures the functioning of individual components on the board, FCT offers a final go or no-go decision for the entire PCB.

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Expected Faults in PCBs

Statistical data on PCBs shows the most common types of faults related to placement, soldering, and functionality. Among placement faults, components may be missing, wrong, wrong orientation, or misaligned. Among soldering faults, there may be dry or incomplete soldering, excess amounts of solder, solder bridging, and whiskers. Assemblers vary their inspection and testing methods depending on the type of defects they encounter and the effectiveness of the inspection methods.

Automated Optical Inspection

AOI methods inspect PCBs visually. The system usually employs still or video cameras to scan a well-lit board. There can be several variations, with the board being illuminated by different sources of light at various angles, and there may be more than one camera. Images from the camera are fed to into a computer, which builds a picture of the board and its contents. The memory of the computer holds the reference image of a golden board that has no faults. The computer compares the image the cameras have captured with the reference image and highlights the faults it detects.

With AOI systems, it is easy to detect faults such as open circuits, shorts, and dry solders. Moreover, it can detect missing and misaligned components. The biggest advantage with AOI is they can help solve PCB errors much better than human inspectors can, with greater accuracy, in less time, and without tiring. Therefore, manufacturers employ AOI systems inline at several points in the PCB manufacturing process.

3-D AOI systems are capable of measuring the height of components, and able to detect faults in areas that are sensitive to heights. However, they use visible light, which limits the functionality of AOI systems to line-of-sight. AOI systems are incapable of inspecting hidden connections such as under IC packages, especially BGAs.

Automated X-Ray Inspection

Chip scale packages (CSP) and Ball Grid Arrays (BGA) are special IC packages that have their connections under them. When mounted, the connections are hidden between the circuit board and the body of the IC, preventing them from being inspected visibly. Assemblers resort to AXI methods to inspect such hidden solder joints.

Printed boards are made of substrates and copper traces, and SMD components are soldered onto them. Materials usually absorb X-rays in proportion to their atomic weights. While materials containing heavier elements absorb more X-rays, those containing lighter elements allow X-rays to pass through without absorption. The PCB substrate and components are mostly made up of lighter elements, and the X-rays pass through them without being absorbed.

On the other hand, solder contains heavy elements such as indium, silver, bismuth, and tin, and these do not allow X-rays to pass through. Therefore, when inspecting a PCB assembly with X-rays, the solder joints show up with great clarity, while the traces and SMD packages are barely visible.

Therefore, AXI systems make it easy to detect and solve PCB errors such as soldering defects normally invisible to AOI systems. However, AXI systems are more expensive, and assemblers install them only if necessary.

PCB Assembly                                 Prototype

In-Circuit Testing (ICT)

Assemblers perform testing only after completing all PCB inspections and soldering the components. For ICT, it is necessary the designer has placed testing pads at critical points in the circuit when designing the PCB layout. Usually, the designer will place the test pads on a grid so a testing jig with spring-loaded pins can connect to the pads on the PCB. This test fixture is usually called a bed-of-nails.

During ICT, the test pins can check various components for shorts, opens, resistance, capacitance, and more, for determining any errors. Usually, such bed-of-nails is specific to a circuit board, and therefore inflexible and expensive. Moreover, with circuit density increasing continuously, bed-of-nails soon reaches its limits. Assemblers use another approach instead. While a simple fixture holds the board, a single probe or a few of them move to make contact at different points as necessary. Usually, software controls the probe movements, and this makes it easy to adapt the system to different boards.

Functional Circuit Testing (FCT)

Under FCT, the circuit board assembly is powered up while test equipment is connected to simulate the actual environment the board is expected to undergo in normal use. The functional tester is unique to the board under test, and its software program sequences through various test scenarios while collecting operational data from the devices on-board.

Depending on the extent of testing, the type of inputs required, and the expected outputs from the device under test, the FCT can vary in its complexity. However, it identifies functional defects in the PCB assembly, and helps to solve PCB errors.

Conclusion

Assemblers use different inspection and testing methods to solve PCB errors during manufacturing and assembly. With high volumes of production and circuit complexity, the automatic visual methods have mostly replaced manual visual methods of inspection. For complex PCBs such as those using BGAs and CSPs assemblers have to use X-rays to inspect invisible solder joints.

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Why RushPCB

Why RushPCB UK Is a Reliable PCB Manufacturing Company

Consumer demands and industry challenges are increasing tremendously towards lightweight products, miniaturisation, greater product design freedom, lower costs, more environmental friendly applications, and higher reliability. In all these aspects, flexible circuits from a trusted PCB manufacturer UK, RushPCB, are proving their worth.

Flexible Circuits from RushPCB UK

The flexible circuit technology offers a huge range of benefits and capabilities. Offered by the best PCB manufacturer UK, flexible circuits effectively eliminate wiring errors commonly associated with manual wiring harnesses, which simplifies assembly. As these circuits can flex, form, and bend to follow the contours of cabinets, they often eliminate several connectors, reducing component numbers, assembly effort, and time. All this goes to increase the product reliability.

RushPCB, a reliable PCB manufacturer UK, makes high-quality flex circuits that encourage 3-D packaging through their property of dynamic flexing. The circuits offer unmatched high speed and high frequency performance as they allow excellent control over transmission impedance, while offering lower impedance as compared to that offered by conventional wiring.

RushPCB offers flex circuits with dielectric substrates that are good conductors of heat. This improves heat dissipation, while flat conductors provide thinner circuits, leading to a huge improvement in airflow capabilities. Additionally, the compliant substrate minimises thermal mismatches.

The lightweight nature of flex circuits helps in reducing the weight of the product, which in turn, the OEMs can use for increasing their products’ packaging density, aesthetics, appearance, or for offering designs that are more integrated.

Advantages of Flexible Circuits from RushPCB UK

There are several benefits of using flexible circuits from the most trusted PCB manufacturer UK. RushPCB offers the thinnest dielectric substrates, as thin as 0.002 inches, and these reduce the package size and weight extensively—sometimes by as much as 75%—the weight reduction being especially attractive to the aerospace industry.

By using flexible circuits, OEMs can bring down their assembly costs. They achieve this in two ways—first, by reducing the number of assembly operations required, and second, by their ability to test the circuit before committing it to the final assembly. This comes from the highly reliable design of flexible circuits from the best PCB manufacturer UK, RushPCB, as their design offers an excellent means of reducing the number of levels of interaction required by the product.

Hand-built wire harnesses do ease the assembly process, but often introduce wiring errors that take up troubleshooting and repair time. Flexible circuits eliminate wiring errors entirely, as it is not possible to route them to points other than those already designated.

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SMT and Flexible Circuits Assembly

RushPCB, the best PCB manufacturer UK, offers flexible substrates, and uses the most advanced surface mount technology (SMT) components and reliable conductive lead-free solder pastes for mounting them. Flexible circuits from RushPCB come with highly compliant substrate material that effectively counteracts the effects of thermal stress, as SMT components are highly sensitive to thermal mismatch between the component material, mounting, and the substrate.

High Density Interconnect PCBs from RushPCB

For customers requiring even higher wiring density per unit area, highly trusted PCB manufacturer UK, RushPCB, offers the High Density Interconnect (HDI) PCB technology. HDI technology offers finer lines and spaces, smaller vias, capture pads, and higher connection pad densities than conventional PCB technology can. OEMs use HDI PCBs to reduce the weight and size of their products, while enhancing their electrical performance.

RushPCB makes HDI PCBs using microvia and buried via technology, along with sequentially placed lamination, insulation material, and conductor wiring layers for very high density of routing. Coming from the best PCB manufacturer UK, RushPCB, HDI PCBs are the best alternatives to expensive high layer-count standard laminates or sequentially laminated boards.

Signal Integrity in HDI PCBs

For high-speed boards, maintaining signal integrity is highly desirable. For this, the PCB has to possess excellent AC characteristics, such as high-frequency transmission capabilities, impedance control, and low radiation. Furthermore, stripline and microstrip transmission line characteristics necessitate a multi-layered design.

To maintain signal integrity, the insulating material in the PCB must have a low dielectric factor along with a low attenuation ratio. Unprecedented high-density is demanded by mounting and assembly methods for Direct Chip Attachment, Chip Scale Packaging, and Ball Grid Array packages. RushPCB achieves these using the microvias and buried via technology, which uses holes with diameters down to 150µm and even lower. Rather than use regular drill bits, RushPCB prefers to use highly accurate lasers for drilling such small-diameter holes.

Advantages of Using HDI PCBs

The HDI technology from RushPCB offers substantial advantages over regular PCBs—making products smaller and allowing high-speed and high-frequency operations possible. HDI offers compact boards that give better electrical performance and lowers the power consumption. Shorter connections mean better signal integrity and other performance improvements due to minimal stubs, closer ground planes, lower EMI/RFI, and distributed capacitances.

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RushPCB is Internationally Certified

OEMs, when selecting a consultancy for circuit board manufacturing, look for those with certification to international standards. Trusted PCB manufacturer UK, RushPCB, conforms to IPC-A-600, and the standard defines the acceptability of circuit boards for quality of workmanship and sets the comprehensive criteria for their acceptance.

That means RushPCB as PCB manufacturer UK produces quality products and identifies sources of non-conformance, if any, in their manufacturing processes. RushPCB conforms to the IPC-A-600 training and certification, and therefore, the manufacturing services reduces the risk of mounting expensive components on PCBs that are defective. This not only reduces scrap, but also facilitates better communication with OEMs.

As a trusted PCB manufacturer UK, RushPCB employs experienced engineers, purchasing professionals, and quality inspectors to define PCB requirements properly, specify requirements for purchasing, and to detect non-conformances. If you are looking for a reliable PCB manufacturing company, as a trusted PCB manufacturer UK, RushPCB will fulfil all your requirements.

 

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Rush PCB UK

Best PCB Layout Practices for 2018

One of the first steps that PCB design experts recommend when taking up a PCB layout design is to start with a high quality PCB design software. Make sure the software package comes bundled with a good library of component parts, and allows you to add to it. It should allow you to manage your layer structure easily, and to place and route a complex multilayer board design. Among the necessary features, it should come with a strong but flexible built-in DRC and allow you to conduct a DFM check. Overall, the design of the software package must be intuitive to enable a short learning curve.

Set up the Library for Multilayer Designs

Designing multilayer boards requires a different library configuration than necessary for single or even double layer boards. It is important to set up the following three areas for handling multilayer board design:

Pad Shapes: Designers differentiate the first pin of a through-hole IC with a differently shaped pad for easy orientation. However, this is necessary only on the topmost layer while on the inner layers all pads can retain the same shape. For libraries not set up for multilayer configuration, the pad shapes may have a mismatch.

Drawing Marks: Designers place different marks on various layers to identify them during fabrication and assembly. Therefore, when setting up the software package for multilayer boards, the designer must save the corresponding logos, tables, and views to the library. Additionally, standardizing them for the organization will avoid confusion.

Negative Planes:  When creating power and ground planes, multilayer PCB layouts use negative image plane layers. These layers require additional clearances around pad and footprint shapes for drilled holes. Therefore, pads and footprint shapes for multilayer design must contain these additional clearances for the negative planes. If you are not careful with these clearances, they will ultimately create shorts.

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source: flatworldsolutions

Understanding the Fabrication Shop Requirements

It is important for a PCB designer to work closely with a fabrication shop and understand their requirements, so that it is possible to fabricate the ultimate product without issues. Multilayer PCB designs offer several benefits over single and double-layer boards. Chief among them are space saving and increasing the design density. Multilayer boards also allow better control over signal integrity, but to achieve that it is necessary to make sure the fabrication shop is able to manufacture the multilayer design before you start.

Fabrications shops will have their own limitations based on their level of board technology. For instance, they may be set up to manufacture boards up to a certain layer count, or they may be able to make vias, traces, and spacing widths only to a certain dimension. Exceeding those limitations may mean looking for a better fabricator, thereby increasing fabrication costs, time, and effort, or not being able to get the board fabricated.

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Best Layout Practices for Basic Multilayer PCB Design

Reduce Crosstalk: It is important to guard against crosstalk from the beginning. Preferably, route the signals on adjacent layers so they are at 90° to each other—this helps to reduce broadside crosstalk problems.

Use Ground and Power Plane Layers: Distribute power and ground layers evenly throughout the stack. This will prevent ground loops, ground bounce, and help with creating microstrip structures for managing signal integrity.

Use Special Vias: Using special vias such as micro-vias, buried and blind vias opens up more routing channels for the designer. Check if the Printed Circuit Board CAD software allows using land-less vias and via-in-pad, as these are now becoming commonplace for packages such as BGA and other fine-pitch IC packages.

Use IPC-2223: Using a common point of reference makes it easier for both the designer and the fabricator. Communicating in a common language for documentation reduces errors and misunderstandings while avoiding expensive delays.

Use Modern File Formats: Rather than delivering files to your fabricator in Gerber format use a modern file format such as the ODB++ or one that meets IPC-2581 standards, as these formats identify specific layer types and the result is unambiguous documentation.

Best Layout Practices for Rigid-Flex PCB Design

No Corner Bending: Always place copper traces at right angles to the flexible circuit bend to avoid bending them at the corners. If this is unavoidable, use conical radius bends.

Curved Traces are better: 45° hard corners and right angle traces increase stress on copper traces when bending—using curved traces is a better option.

No Abruptly Changing Trace Widths: Any abrupt change in the width of a trace can weaken it. As a trace approaches a pad or via, prefer to use teardrop patterns to change its width gradually.

Using Hatched Polygons: Planes using solid copper pour in flex boards offer heavy stresses when bent. Using hatched polygons such as hexagons makes the plane more flexible.

Stagger Flex Traces: For traces running over one another in the same direction on either side of a layer creates uneven tension between the layers. Staggering the traces eliminates the stress.

Best Layout Practices for Industrial PCB Design

Designing for Industrial environments requires PCB designers to demonstrate not only functionality of the PCB, but also its reliability to work under harsh conditions. This is especially true for applications with expensive downtimes.

Use Proper Grounding:PCB layout for industrial applications must carefully segregate power ground, analog ground, and digital ground. This is essential for reliable performance of the PCB in harsh electrical environment. Connecting these various grounds to a suitable single point is also important.

Maintain Signal Integrity: Harsh electrical environments affect communication, analog, and digital both. This can be detrimental to the performance producing erroneous data. Although a proper selection of cables and other installations can offset this largely, PCB designers and manufacturer must follow sound design practices to maintain signal integrity.

Heat Management: Industrial environments can be very hot, and PCBs generating their own heat may easily cross their safe operating temperatures. Use thermal vias and other heat management techniques such as heat sinks to remove heat from the PCB and its enclosure.

Group Components: Prevent interference by grouping components based on their function in the circuit. For instance, analog circuits separated from their digital counterparts, and power circuits in their own area prevents coupling of interference. PCB design experts advise arranging the schematics into modules to plan better components placement.

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Design for Moisture Control: Some industrial environments may be moist and humid. Moisture buildup on the PCB could damage circuits and components. PCB design experts suggest designing the PCB layout with a view to applying a layer of conformal coating on the assembly. The PCB designer needs to factor in the additional heat buildup and the necessity to remove it. It may be necessary for the PCB design to incorporate an intelligent circuitry to detect humidity and turn on a heater integrated into the system.

Conclusion

Using a good CAD software package for PCB layout does not guarantee an optimum output. Although CAD software packages incorporate auto-routers to help the designer, it is best to review the output after auto-routing and rectify the quirks the software may have introduced.

HDI

Benefits of HDI Flex Circuits

Flexible circuits built with High Density Interconnect (HDI) technology offer significant design, layout, and constructions benefits over regular flexible circuits. HDI technology involves incorporation of microvias and fine features for achieving highly dense flex circuitry, and offers increased functionality with smaller form factors.  Use of HDI technology offers improved electrical performance, allows use of advanced integrated circuit packages, along with better reliability using thinner materials and microvias. Some advantages of HDI flex circuits are:

Working in Harsh Environments

Fabricators cover HDI flex circuits with Polyimide. Although this is a standard practice, other cover and base materials are also available to suit a broad range of harsh ambient conditions. Compared to regular circuits covered with soldermask, the Polyimide dielectric layer is flexible, and protects the circuit far beyond the capabilities of the brittle soldermask.

Repeatable Installation with Flexibility

Compared to ribbon cables or discrete wiring, an HDI flex circuit offers a repeatable routing path, which you can customize within your assembly. Not only does this give dependability where necessary, but also the longer lifespan of the HDI flex circuitry drastically reduces service calls.

Capability to Withstand High Vibration

Along with flexibility, the ductility and low mass of HDI circuits allows it to withstand high amounts of vibration much better than conventional circuits can, reducing the impact upon itself and its solder joints. The higher mass of regular circuits imposes additional stress upon itself, the components soldered on it, and its solder joints.

Working with Longer Duty Cycles

The design of HDI flex circuits allows them to be very thin, but adequately robust to withstand a high number of flexing cycles. In fact, HDI flex circuits are capable of flexing thousands to millions of cycles while carrying power and signal without a break.

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Packaging Options with HDI Flex Circuits

Designers can shape HDI flex circuits to fit where no other circuit can. As HDI circuits are a hybrid combination of an ordinary flex circuit and a bunch of wires, they exhibit the benefits of each and more. In reality, you get unrestricted freedom of packaging ability with HDI flex circuits, all the time retaining the repeatability and precision necessary. HDI flex circuits replace a few major components in equipment—the hard board, usually called the printed circuit board (PCB), and the connectors and wiring harness that bridge multiple PCBs. This offers several packaging options such as:

  • Lower Mass
  • Versatile Shaping
  • Stiffeners for Component Mounting
  • Vibration Resistance
  • Robust Connections
  • Repeatable Wire Routing
  • Faster Assembly Times
  • Reduction in Weight and Space

As the HDI flex circuit is made of thin material, it can often save up to 75% of the weight and space required by conventional circuit boards and wires. Designers feel compelled to adopt HDI flex circuit technology because they can form it into three-dimensional configurations. However, the flexibility often makes it difficult to mount large surface mount components on HDI circuits and engineers surmount the problem by selectively bonding stiffeners where required.

Some equipment have multiple boards interconnected with wire harnesses. Shock and vibration plays a large part in failure of these harnesses resulting in recurring costs. In most cases, a single HDI flex circuit can replace all the boards including their wire harnesses. As the HDI flex circuit is lighter, it is more resistant to the effects of shock and vibration, resulting in huge reductions to the recurring costs, Elimination of wire harnesses leads to lower routing errors, ultimately reducing test times, rework, and rejections.

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Moreover, HDI flex circuits also replace the connectors at each end of the wire harness. Flat foil connectors may have to replace some connectors. This is an advantage over the use of round wires, as flat conductors with their larger surface area dissipate heat better, and thereby, carry more current. Conductor patterns in HDI flex circuits have more uniform characteristics, leading to a better prediction and control over impedance, crosstalk, and noise.

Use of HDI flex circuits reduces several assembly processes such as color-coding and wrapping bundles of wire. In volume production, this not only reduces the chances of assembly rejects and in-service failures, it saves assembly time, and lowers the total installation costs.

Benefits to the Designers

Designers build up HDI flex circuits with microvias as this offers them several advantages. Drilled by lasers, microvias are extremely small, and their effective use opens up more space for routing. Combined with the use of thinner traces, this leads to high routing densities, effectively resulting in fewer layers.

HDI flex circuits present the only practical way for designers to mount multiple large BGA packages with less than 0.8 mm pitch. They also offer the lowest cost for high-density boards with high control over power and signal integrity with appropriate stackup definitions.

Processes requiring Restriction of Hazardous Substances (RoHS) do well to use HDI flex circuits, as newer materials are available that offer higher performance with lower costs. This is an advantage over conventional boards, as these newer materials are not suitable for sequential or standard laminations.

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Summary

HDI flex circuits are the best alternatives to expensive, high layer count sequential or standard laminated boards. Smaller HDI features are the only way to effectively breakout and route multiple instances of high pin-count and finer pin-pitch component devices on a single board. With all the above features and advantages, handheld consumer electronics is currently committed to using HDI flex circuits.

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Secrets of High Speed Printed Circuit Boards

In our fast-paced world, OEMs need to churn out new electronic devices very quickly to remain in the forefront of the market. For this, they require rapid PCB prototype services, which allow them to test their new designs thoroughly. Once they are ready to enter the market, OEMs need to tie up with a fast PCB production partner to fulfill the marketing demands. If the design requires high-speed printed circuit boards, the design house cannot afford the time to make trial and error, but must optimize the design on the first try. This ensures a smooth quickturn PCB production process. Therefore, the designer must start designing the board with assembly in mind.

Design for Assembly

Whatever be the type of PCB involved—rigid, flex, rigid flex, high density interconnect (HDI), or conventional—the bare boards will require assembly with additional components, before they are useful. Usually, the assembled PCB fits within a product or application, and overlooking this aspect of the assembly during design may ultimately lead to significant complications.

High Speed operation of PCBs requires the designer achieve the following:

  • Minimizing noise generation from the on-board power network
  • Minimizing cross-talk between traces
  • Reducing simultaneous switching noise
  • Proper impedance matching
  • Proper signal line termination
  • Reducing the effects of ground bounce

Board Material and Transmission Line Design

The dielectric construction material of the PCB is a major contributor to the amount of noise and cross talk the fast switching signals generate. A high frequency signal traveling along a long trace on the PCB could be affected seriously if the loss tangent of the dielectric material is high, resulting in high absorption and attenuation at high frequencies.

The modeling and effect of transmission lines also affects the signal performance and its noise separation. In general, any circuit trace on the PCB will have its characteristic impedance. This depends on the trace width, thickness, the dielectric constant of the PCB and the separation between the trace and its reference plane. Designers can route circuit traces on a PCB in two ways—in a microstrip transmission line layout or a stripline transmission line layout.

In a microstrip layout, the designer routes the circuit traces on an outside layer with a reference plane below it. The characteristic impedance of a circuit trace in a microstrip layout is inversely proportional to the trace width, and is directly proportional to the separation from the reference plane.

In a stripline layout, the designer routes the circuit traces on an inside layer of a multi-layer PCB, with two reference planes on either side. Here again, the characteristic impedance is inversely proportional to the width of the trace, and directly proportional to the separation from the reference planes. However, the rate of change with trace separation from the reference planes is much slower in a stripline layout as compared to that with s microstrip layout.

Designers for rapid PCB prototype services must be able to predict the characteristic impedance of their design if they are to get their design right the first time. Understanding the nuances of transmission lines helps with fast PCB production.

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Minimizing Cross-Talk between Traces

While designing a high speed PCB, designers must take steps to reduce cross talk between neighboring signal lines, even when following either the microstrip or the stripline layout. Designers follow certain thumb rules to minimize the cross talk:

  • Utilize as much space between signal lines as the routing restrictions allow
  • Place the transmission line as close as possible to the ground reference plane
  • Use differential routing techniques for critical nets—match the length to the gyrations of each trace
  • Route single-ended signals on different layers to be orthogonal to each other

Routing two or more single-ended traces in parallel with not enough spacing will increase the cross talk between them. Therefore, designers prefer to minimize the parallel run, often routing them with short parallel sections, minimizing long, coupled sections between various nets.

Maintaining Signal Integrity

For high-speed boards, it is very important that the signal maintains its integrity, that is, it is able to keep its amplitude, and shape as it travels from its source to its destination. Signals may be single-ended, such as clocks, or may be differential, which are very important for high-speed design. For traces carrying single-ended signals, designers follow design rules such as:

  • Keeping traces straight as far as possible, and using arc shaped bends rather than right-angled bends where necessary
  • Not using multiple signal layers
  • Not using vias in the traces—they cause reflections and impedance change
  • Using the microstrip or the stripline transmission line layout
  • Minimizing reflection by terminating the signal properly

Designers follow additional rules for differential signals:

  • Minimize crosstalk between two differential pairs with properly spacing them
  • Maintain proper spacing to minimize reflection noise
  • Maintaining constant spacing for the entire length of the traces
  • Maintaining the same length of the traces as this minimizes phase and skew differences
  • Not using vias in the traces—they cause reflections and impedance change

Effective Filtering and Grounding

Conducted noise from the power supply can hinder the functioning of a high speed Printed Circuit Board. Since a power supply may deliver noise of high as well as low frequencies, designers minimize this problem by effectively filtering the noise at the points where the power lines enter the PCB,

An electrolytic capacitor across the power lines can filter the ripple and low frequency noise, while a non-resonant surface ferrite bead will block most of the high frequencies. Since the ferrite bead will be in series with the supply lines, its rating needs to be adequate to handle the current entering the PCB. Designers also keep provision for a decoupling capacitor very close to each IC on the board, to smoothen out very short duration current surges.

Effective power distribution throughout the PCB is extremely important for printed circuit boards operating at high speeds. For doing this, designers often use power planes or a power bus network. Power planes on a multi-layer PCB comprise two or more copper layers carrying power to the devices—typically, the VCC and GND lines. By making the power planes as large as the entire board, the designers ensure the DC resistance is as low as possible.

This offers multiple advantages to high speed boards—high current source and sink capability, shielding, and noise protection to the signals. For two-layer PCBs, designers often use the power bus network, which has two or more wide copper traces for carrying power to the devices. Although the DC resistance of the power bus is high compared to power planes, they are less expensive.

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As high-speed digital devices operate simultaneously, their fast switching times may cause a board-level phenomenon known as ground bounce. This is a very difficult condition to predict, as several factors may influence to the occurrence, such as number of switching outputs, socket inductance, and load capacitance. Designers follow a number of broad guidelines to reduce the effects of ground bounce:

  • Placing vias adjacent to a capacitor pad, and connecting them with wide, short traces
  • Using wide, short traces to connect power pins to power planes or decoupling capacitors
  • Using individual links to connect each ground pin to the ground plane, no daisy-chaining
  • Adding decoupling capacitors for each IC and each power pin
  • Placing decoupling capacitors very close to the IC
  • Properly terminating the outputs to prevent reflections
  • Buffering loads to limit the load capacitance
  • Eliminating sockets as far as possible
  • Distributing switching outputs evenly throughout the board
  • Placing ground plane next to switching pins
  • Using pull down resistors rather than using pull up resistors
  • Using multi-layer PCBs with separate VCC and GND planes
  • Placing power and ground planes next to each other to reduce the total inductance
  • Minimizing the lead capacitance by using surface mount devices
  • Using capacitors with low effective series resistance

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Conclusion

Rush PCB UK recommends designers follow the above design guidelines for delivering rapid PCB prototype services to satisfy customers. However, please note that all other general guidelines for PCB design are also important and designers should follow them meticulously for fast PCB production.

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What are Optical Printed Circuit Boards?

As copper reaches its speed limit, engineers look at optics to replace copper for very high speed signals. Engineers also envisage replacing copper links between servers, routers, and switches with active optical cables. Already silicon chips are available with some optical components inside. The next phase is for optics inside printed circuit boards (PCBs).

Why Optical Systems in PCBs

Electro-optical printed circuit boards combine optical and copper paths on the same board. While the copper paths distribute power and low-speed data, the optical paths handle the high-speed signals. This segregation has several advantages. At high frequencies, signal integrity suffers due to skin effect, crosstalk, and skew when passing through copper systems. Optical systems do not have those issues, while also presenting greater channel density than copper does. Moreover, as optical signals do not need signal conditioning and equalization, optical systems consume lower power than do electrical signals. Additionally, optical systems can reduce the surface area of a PCB by 20% and the number of layers on the board by 50%.

Optical Technology for PCBs

Designers and manufacturers are migrating optical technology to the backplane and connectors. Although optical technology has been around in the form of SFP and QSFP interfaces for some time now, engineers are now developing optical backplane connectors and optical backplanes. These also include optical transceivers at their connecting edges. Now, it is increasingly possible to have optics appear within a board, rather than limit its presence at the edges. Therefore, optics is now moving closer to the electrical signal source. That means the processor, fiber optic patch cords, and waveguides can now be found on the PCB.

Manufacturers have been successful in developing optical backplane connectors and included a technique to align small waveguides to onboard transceivers. The future challenge is to develop on-board waveguides so that performance is guaranteed even if there are tight bends in the board.

Manufacturing Optical PCBs

Engineers use photolithography and film processing techniques to fabricate the flexible optical waveguides that will be able to move light around components onboard. According to technical information available, waveguides in the build will need walls at least 100 µm thick, and a bend radius less than 5 mm. These dimensions would allow designers to place the waveguide within connectors. This will also let light travel between a line-card and a backplane, without the necessity to convert it to an electrical signal.

PCB Manufacturers usually follow two different techniques when constructing the waveguides—non-contact mask lithography and direct laser writing. In non-contact mask lithography, spin coating applies the material to the substrate. However, as this process is more applicable to semiconductor manufacturing, lithography is better suited for small areas, and cannot be scaled up to handle large areas. Engineers use a process of draw-down coating for large areas, along with a doctor blade.

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However, engineers faced two problems with the above process. One, the waveguide material would curl up, requiring 170 g of force to flatten. Second, there was the difficulty of the waveguide adhering to the substrate. Adhesion to the substrate is important so the waveguide would not crack during mechanical process such as cutting the wafer or the substrate board.

It is important to have waveguides that do not attenuate the light too much as it travels through. Optical power measurements made with laser diodes as a source and a photo detector as the receiver indicate onboard waveguides introduce optical losses ranging from 0.046-0.050 dB/cm, even when the waveguides were bent to form two or three loops. Some signal loss is customary from wall roughness within the waveguide as well.

Optical Interconnects on PCBs

Onboard optical interconnects on PCBs can handle very high data rates and offer larger numbers of data channels than other electrical interconnections do. Moreover, as optical signal transmission is impervious to electromagnetic interference or EMI, it is suitable for mixed signal systems such as data acquisition and signal processing where sensor applications need high accuracy of analog electronics.

Optical waveguides on PCBs require not only low attenuation, but also a reliable manufacturing process for the optical layer. In an optical PCB, the fabrication steps and material properties of the waveguides need to be compatible with the manufacturing and assembly techniques prevalent with the PCB industry.

Apart from the optical path in an optical interconnection system, there must be coupling elements that can couple optical signals into and out of the waveguides. Moreover, common pick-and-place machines must be capable of suitably and automatically mounting these coupling elements without any active alignment between the optical waveguide and the coupling element. Use of structured polymer foils help in this integration.

Main issues of using polymers are their thermal and mechanical stability against the process conditions during PCB fabrication. Additionally, close coupling tolerances and imperfect positioning of waveguides within the PCB, mounting coupling elements often require active alignment. Engineers circumvent such problems in an optical PCB by using standard multimode glass fibers integrated within the layer stack. As glass fibers are highly stable both thermally as well as mechanically, PCB manufacturers can easily follow their proven processing steps for embedding the fibers into multilayer PCBs.

Moreover, the geometrical accuracy of glass fibers, apart from offering very low optical attenuation, is also very important for coupling methods. Engineers can passively align active optoelectronic components at the stubs of the fiber—the PCB has cutouts to make them accessible. A specific micromechanical alignment structure makes this passive alignment possible when combined with the optoelectronic chips—making mirrors and lenses unnecessary for coupling to the waveguides.

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Optical Coupling Elements

For using coupling elements on the PCB, they must be compatible with the assembly and soldering processes manufacturers use. Primarily, the alignment structure should be able to withstand the temperatures involved. Precision molding in silicon molds can achieve this. Manufacturers typically use a temperature of 180°C and duration of 90 minutes under a pressure of up to 15 bar for the lamination process when manufacturing multilayer boards. Soldering processes expose the board to temperatures exceeding 250°C. Optical waveguide polymers often show discoloring or decomposition at such temperatures. Engineers find glass fibers to be a suitable substance.

Glass fibers remain optically stable without any damage at the above temperatures. Additionally, being mechanically strong, glass fibers offer very low attenuation and exhibit very tight tolerances for their diameter. Rather than fixing the fibers on top of a readily processed conventional PCB, engineers embed them completely into the layer stack of optical printed circuit boards, between the top and bottom layers of the PCB using standard material such as FR4.

Summary

As against waveguides made from polymer foils, embedded glass fibers allow engineers to automatically align the optoelectronic transmitter and receiver components due to the accuracy of their contours. That makes it easy to develop optoelectronic coupling elements onboard, as they can align positively on the fiber using an advanced microstructure and achieve low coupling losses without requiring active position optimization.

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How Stable are the Dimensions of Flexible Circuits

The difference between a rigid printed circuit board and a flexible circuit lies in the dielectric material sets manufacturers use for fabricating them. Most rigid printed circuits are made from glass epoxy, whereas the material of choice for a majority of flexible circuits is Polyimide. Development of several versions of polyimide enables tailoring the material to meet specialty requirements such as in solar arrays, for space applications, and for other unusual environments.

Although it is possible to form glass epoxy in very thin constructions and even bend it for simple applications, polymer films are most suitable for continuous twisting, flexing, and multi-planar folding. Films of polyimide withstand numerous bending cycles without suffering any degradation of their mechanical and electrical properties. Therefore, polyimide films perform reliably in applications where bend cycles of over a million are common. The inherent flexibility of polyimide films offers the electronic packager a wealth of design options. However, a disadvantage of polyimide films is their material dimensional stability is inferior to that of glass epoxy materials.

Dimensional Stability

According to manufacturers, the dimensional stability of polyimide films depends on the residual stresses the manufacturing processes place in the film and its normal coefficient of thermal expansion.

However, the measure of stability represents only the effect of the film alone. The nature of stability grows more complex as the fabricator exposes the film to elevated temperatures and pressures for attaching the copper layers through processing to create an adhesive-less laminate, or through an adhesive lamination cycle. However, the process of creating a laminate and subsequently fabricating a circuit involves two different processing effects, and during each of these fabricating processes, the flexible substrate undergoes dimensional changes.

It is not easy to predict these changes. Raw material variation from batch to batch may cause dimensional changes to vary slightly. Changes also depend on the method of construction and processing conditions, as thin materials are likely to be less stable. Other contributing factors can be the percentage of copper etched, density of copper electroplating, ambient humidity, and material thickness.

Small dimensional changes in the circuitry panel is inevitable as it undergoes processing and exposure to a variety of etching, electroplating, pressures, temperatures, and chemistries. For instance, etch shrink is the stress etching copper releases, but fabricators mistakenly use it as a catch phrase for representing all the dimensional changes that a flexible circuit undergoes during processing.

Fabricators consider compensating for the above changes when setting up the part number for a new flexible circuit. However, accurate prediction of these feature movements requires empirical data from parts they actually produce.

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Effects of Material Instability

Lack of stability in the film material is manifest in violation of the minimal annular ring requirement, and in extreme cases, cause full breakout of the hole-to-pad alignment. Another possibility is in the misalignment of the coverlay. For a predicable material change, the operator can adjust either the conductor layout or the drill pattern to re-center the plated-through-hole in the pad.

Dealing with Dimensional Changes

Fabricators deal with dimensional changes by limiting the panel size, and this works very well for cases where the tolerances are extremely tight. In small panel sizes, the effects of dimensional instability issues on registration and alignment are lower, and the handling damages are at a minimum. However, smaller panel sizes may be less efficient for processing as against those for larger panels, since in a circuit factory several costs are based on panel size.

Compensating for Dimensional Changes

It is possible to achieve cost-effective production with suitable panel sizes while compensating for dimensional changes. Fabricators can adopt the following methods to adjust for dimensional changes occurring during circuit fabrication:

Applying Scaling Factors

Where the dimensional changes of the material are predictable, fabricators can apply scaling factors to tooling or secondary layers. The in-process measurements for a given lot can allow fabricators to use scaling factors based on dynamic calculations. For instance, the measured scaling factor of a panel may form the basis of creation of its solder paste stencil. Another instance may be of a final drilling program compensated dimensionally for a multilayer circuit.

Applying Software Compensations

Alignment systems using software controlled operations can use optical fiducials to detect dimensional shifts and compensate for them. Such fabrication machines measure these targets present on the outside corners of the panel and perform a dimensional analysis. Proper alignment is then a process of applying the necessary X, Y, and theta corrections.

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Processing Sub-Panels

Fabricators often divide the panel into smaller arrays for handling dimensional changes. They do this usually after creating the circuit image. As the processing is on subsets of the panel, fabricators effectively gain some of the advantages of small panel alignment, but without compromising the cost advantages of processing a large panel.

Fabricators typically use optical targets on smaller subset panels to compensate for stencil registration commonly. They also use hard tool dies to cut smaller pieces at a time from a multi-piece panel.

Summary

Dimensional change is the primary difference between rigid and flexible circuitry, and this requires compensation. Even though material change in flexible circuitry is typically less than one tenth of one percent, it accumulates over a dimension of several units, and can be of a significant nature. For a flexible circuit, this compensation for the expected change becomes a critical part related to panelization. This also serves to balance maximizing process efficiencies and maintaining dimensional tolerances and accuracy.

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

LED PCBs and assemblies have unique requirements that only an eminent LED PCB board manufacturer understands. One of the leading LED PCB board manufacturers in the UK, RushPCB has the technical expertise to manufacture up to 32 layers of PCBs in small and bulk quantities for local and global supplies. With several hundreds of satisfied customers all over UK and around the globe, there are several reasons why you can safely entrust your LED PCBs to RushPCB. Five of them are:

  1. RushPCB Understands LED PCB Principles

LED Manufacturing Companies in UK face two major areas of concern related to LED PCBs—thermal management and spillover light. Thermal management means heat generated from high power LEDs mounted on PCBs must be effectively removed and vented to prevent damage to the LEDs. For better heat conduction, manufacturers use metal core printed circuit boards or MCPCBs. Although this allows the heat from the LEDs to pass through the prepreg to the metal core, the issue can be a big challenge. RushPCB uses excellent metal core substrates from Univaco, Arlon, Bergquist, and Thermagon to dissipate the excess heat from the LEDs very effectively.

LEDs do not have reflectors, and the light spilling over from its rear and sides is generally wasted. LED PCB board manufacturers use a reflective white mask on the PCB surface so that the spillover light emerges from the front. It is necessary that the white-reflective mask not change color when heated during reflow or in regular use. RushPCB uses special quality material for the white mask that retains its thickness and reflective property under all assembly and operative conditions.

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  1. RushPCB Understands the PCB Bonding Process

MCPCBs require a different bonding process from conventional PCBs because the prepreg must bond to a metal core. There are two important aspects here—the thickness of the prepreg and the bonding process itself. As the prepreg is also the insulation between the metal core and the copper tracks, it must be of suitable thickness to withstand the voltages involved.

At the same time, the prepreg must also be thin enough to allow effective heat transfer from the LED to the metal core. RushPCB uses prepregs of optimum thickness to allow very good transfer of heat, yet offer good electrical insulation. A special technique by RushPCB ensures the bonding between the prepreg and the metal core does not allow any air bubbles between them, as these air bubbles can impede heat transfer.

  1. RushPCB Offers Excellent Surface Finish

Although LED PCBs and assemblies have a metal core to enhance thermal management, there is a layer of etched copper tracks on top just as conventional PCBs do. The white mask covers most of the copper tracks leaving only the solderable pads exposed. Unless protected by surface finish, the exposed copper pads can oxidize and tarnish, making the PCB unsolderable.

RushPCB offers several types of surface finishes that protect the exposed copper surface. Depending on the customer’s requirement, these can be leaded solder, lead-free solder, Electroless nickel immersion gold, Immersion silver, Immersion tin, or Organic surface protectants.

  1. RushPCB Offer the Best Laminates

Although most LED PCBs and assemblies use single layer MCPCBs, some applications call for double or multilayer MCPCBs as well. For such multilayer MCPCBs, RushPCB uses special laminate material that offer the best balance of cost and performance. They use different laminate materials from Japan, China, Korea, and Taiwan, which are not only efficient but also meet emerging trend requirements.

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  1. RushPCB Offers the Best Balance of Quality, Value, and Cost

Eminent LED manufacturing companies in UK source their LED PCBs and assemblies from RushPCB UK as they offer the best balance of quality, value, and cost in the market. With more than 15 years of experience, LED PCB board manufacturer RushPCB has the capability to turn our clients’ requirements and ideas into reality. Whether your LED PCBs are to be only assembled or designed from scratch, our seasoned engineers can take up design, assembly, and testing of LED PCBs keeping your requirements in focus.