rush pcb uk

PCB Assembly Compatibility with In-Circuit Testing

PCB Assembly Compatibility with In-Circuit Testing

RUSH PCB UK Ltd wants all its clients to be assured that we ship all products from our production facility only after testing them to be in perfect working condition. For this we have a variety of strict quality control procedures at each stage of our PCB assembly process. This includes not only standard services such as visual inspection and automated optical inspection, but also advanced test procedures such as X-ray inspection, and functional circuit testing or FCT. Although each method has its own advantages, for the most meticulous testing method, RUSH PCB UK Ltd recommends In-Circuit Testing or ICT.

In Circuit Testing (ICT)

Working at component levels, ICT allows localizing issues that may be present on the board under test. For instance, ICT can point to a specific device as the cause of the problem. With ICT, it is possible to test the individual voltage and current levels on the PCB, while including a step-by-step program execution.

The above helps in troubleshooting complex boards where the board is still a prototype and the design is not totally verified. Boards not passing the test may need reworking at component level to potentially save the batch. When this happens, our test engineers generate a Design for Assembly (DFA) recommendation to the client.

At RUSH PCB, we work closely with our clients and provide them flexible services tailored to their individual requirements. We have an engineering team to review the specific test requirements for a project, recommend the necessary equipment, and develop the test workflow. We even design test jigs if necessary. We have equipment to handle any type of project.

One of the advantages of using ICT is its speed of test. For instance, a few seconds is all it takes to test a complicated board. Therefore, projects involving large volumes of PCBs benefit exclusively from ICT. Apart from the speed of testing, detection of faults at component levels makes the diagnosis process faster and at the same time, does not involve a skilled operator.

However, for the ICT to be effective and accurate, the process requires a dedicated test fixture and a program. In addition, the PCB design must also allow the test machine and fixture to interface properly with the assembly. To maximize the test coverage and find the maximum number of potential faults, our clients must consider some points when designing the layout for their PCB assemblies. This not only reduces the redesign steps necessary at the prototype stage, it also helps in producing boards that perform right at the first attempt.

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Making PCB Assembly Compatible with ICT

Test Pads—have a test pad on each electrical network on the PCB, including on unused IC pins. It should be possible to connect to the test pad via a spring-actuated test pin in the test fixture. For through-hole technology, the test pin can engage the component leg on the solder side.

It is usual to place 0.05-inch (1.27 mm) diameter test pads on a 0.1-inch (2.54 mm) grid, with the test pads spaced 0.1 inch (2.54 mm) from any component, and 0.125 inches (3.18 mm) away from the edge of the PCB. The above dimensions allow using long-lasting standard test pins. RUSH PCB UK Ltd does not recommend using test pads of reduced diameters, as thinner test pins are generally more expensive, requiring more frequent replacements.

Probing—place all test pads ideally on the solder side of the PCB to allow the test pins on the jig to access them from the bottom side. While it is possible to place test pads on the top, the construction of the test jig will become more complicated and expensive as it will require additional transfer probes and wiring.

Solder-Side Components—it is preferable to have no components on the solder side, other than small SMDs. Test fixtures usually have a vacuum plate to hold the PCB assembly from the bottom. It may be necessary to mill the vacuum plate for accommodating components on the bottom side. As milling is an expensive process, it is necessary to restrict the milling for bottom components to only a few millimeters.

Locating Holes—add locating or tooling holes to the PCB (not in the panel), to allow the test jig to locate the PCB in the fixture. Preferably use non-plated holes of 3 to 4 mm diameter. Locating two tooling holes in diagonally opposite corners will allow the test jig to accommodate the PCB unambiguously. Keeping a free space of 5 mm around each hole will ensure the tooling pins of the fixture will not cause shorting of components or tracks during the test.

Pull-Up Resistors—use pull-up or pull-down resistors on all floating pins, rather than connecting them directly to the power rails. For pins that hold other devices to a reset state or high impedance state, the presence of these resistors allows the test jig to control the pins. Tying the pins through pull-up or pull-down resistors also helps in product functioning, as the circuit can reject spurious signals. These resistors also help the test jig in isolating individual components when locating a fault.

Space for Pusher Rods—these are necessary to push down on the PCB when testing. ICT jigs usually have fixtures with 2 mm diameter pusher rods and necessary space should be available between components on the top-side of the PCB under test. Spacing them evenly around the PCB helps the jig manufacturer locate individual positions for the pusher rods.

Programming Devices—although capable of programming devices such as EEPROMs during testing by ICTs, the cycle time per board may go up. RUSH PCB UK Ltd recommends pre-programming such devices before assembly, and allowing the ICT to control them during testing.

Batteries—preferably, fit batteries only after the testing is over. As an alternative, use a removable link to connect/disconnect them during the testing.

Review—reviewing the design to ensure proper functioning is important before committing to a fixture. Moving test pads or components on a PCB can mean a new fixture, leading to time and cost overruns, as an ICT jig can be expensive and take some time to prepare.

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We at the Electronic Manufacturing Services (EMS) from RUSH PCB UK Ltd offer advice to our clients on the above points and help them to design their PCB properly for compatibility with ICT. However, once the design meets DFA requirements, ICT provides fast and accurate testing, making the returns worth the investment.


Material and Methods for High Quality Rigid-Flex 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.


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.

Stretchable PCB

Where to use Stretchable PCB Technology

Traditional Printed Circuit Boards (PCBs) are rigid, meaning they are not meant to be bent during use. A different type of circuit board is available for use in applications that need the board to flex or bend repeatedly—flexible circuit boards. Both these are not very useful if the application demands the circuit board be stretched. For this, RUSH PCB UK LTD recommends using stretchable PCB technology. [1]

Stretchable PCB Construction

Although stretchable PCB technology uses classical processes for production and assembly of such PCBs, the laminate is either Polyurethane or Polyimide. This has the advantage of realizing stretchable PCBs with relatively low investments. For ease of assembly of components on the substrate, manufacturers use one of two methods as follows.

Manufacturers reinforce the laminate locally using an interposer or a special coating. The alternate method is to use Stretch-Rigid technology. Rather than connect two rigid boards with a flexible PCB as in Rigid-Flex construction, Stretch-Rigid technology connects multiple rigid boards using stretchable substrates with embedded copper interconnection traces. The electronic components are soldered on the rigid parts. [2]

Properties of Stretchable PCBs

PCBs with stretchable substrates are useful for applications that require the PCB to stretch, twist, bend, or any combination thereof. The stretchable substrate is ductile enough to decouple mechanical resonances, which reduces the effort necessary for compensating mechanical tolerances.

Stretchable PCBs come in single or double layers, with Polyurethane being the usual stretchable substrate. Typical base material thickness varies between 90 and 100 µm or 3.5 and 3.9 mil, while the copper weight is usually 0.5 Oz or 17.5 µm.

As the substrate must stretch, manufacturers take special care to give the copper a high peel strength of about 5 N/mm or 456 Oz/in, and a tensile strength of 6 MPa or 870 psi at 50% strain.

The above features of the substrate allow the stretchable PCB a maximum stretchability of 30% of its original length and 10% stretchability for repeated elongations. This however, depends on the structure of the copper pattern on the stretchable substrate. As the maximum allowed temperature for soldering on the substrate is about 150°C, the assembly process uses SnBi solder and FR4 interposers.

This allows a usable operational temperature range of 0 to 100°C for stretchable PCBs. Where the application requires a stretchable substrate of short length and low volume, manufacturers prefer to use Polyurethane as the substrate material. If the application demands a long and high-volume substrate link between the rigid parts, Polyimide is preferable. [2]

Advantages of Stretchable PCBs

Stretchable PCBs are very useful in the industry where two parts of a machine move relative to each other and must be interconnected electrically. For instance, a sensor executing complex movements on a stationary machine is best interconnected using a stretchable PCB as it allows the sensor to move in multiple degrees of freedom, including linear and rotational. Apart from being able to twist and bend, the stretchable interconnect can also allow the sensor to move linearly away from the machine (stretch) when needed, with a maximum elongation of 30% of its original length.

Therefore, two or more rigid boards connected by stretchable substrates can change their individual positions very easily, can change their positional angles relative to each other, and move apart or come close to each other, while remaining electrically tethered to each other all the time. However, for repeated stretching and contractions, RUSH PCB recommends limiting elongation of stretchable PCBs to 10% of the original length. [3]

Mechanism of Stretchable PCBs

Although the thermoplastic Polyurethane that manufacturers use as substrate for stretchable PCBs can stretch inherently, copper traces in straight lines on the substrate prevent it from doing so, as copper is not ductile enough for the purpose. Manufacturers use special press and confidential lay-up techniques for bonding the standard ED or RA copper foil on the Polyurethane substrate. Once this is done, they use regular subtractive wet-etching PCB processing steps such as drilling, metallizing, imaging, plating, and etching for fabricating stretchable circuits.

As adding multiple layers of adhesive and Polyurethane substrates reduces the stretchability of the product, stretchable PCBs are mostly double-sided and four layers at the most. To maintain a homogeneous elastomeric construction, manufacturers apply a Polyurethane solder-mask or coverlay on the finished PCB. [4]

Assembly of a stretchable PCB uses the standard off-the-shelf surface mounting components soldered on its copper tracks. As these components are rigid, the areas where the components are positioned cannot stretch. Therefore, the concept of the stretchable circuit is basically small islands of a rigid nature holding a few SMD components interconnected with conductive copper foil on stretchable substrates. For a mechanically reliable PCB, the manufacturing technique follows a gradual transition from the rigid area to the flexible area and ultimately to the stretchable region.

To allow the copper traces on the substrate to flex without damage, the designer gives the traces a horseshoe shape rather than allowing them to travel in straight lines. The designer then places the horseshoe shapes alternately facing 180°, allowing them to meander along the path the straight trace would have normally taken. When stretched, the horseshoe tracks will uncurl without much stress. Other shapes such as triangular and sinusoidal interconnect traces can also stretch, but exhibit higher stresses, leading to lower reliability. This has led manufacturers to standardize on the horseshoe shape. [6]

Designers must note that stretching copper traces leads to a change in their resistance. For instance, tests conducted on copper traces with thickness of 15 µm, width 1 mm, and length of 80 mm showed an original resistance of 7.4 Ω, which increased to 13.5 Ω when the trace was stretched by 10%, to 23.8 Ω when stretched by 20%, and to 37.6 Ω when stretched by 30%. However, lab tests have verified that the trace maintained its conductivity even after a 300% stretching. [5]

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

Uses of Stretchable PCB

Applications that demand the PCB be placed on a non-flat surface are the major users of stretchable PCBs. A conventional rigid PCB cannot be comfortably integrated on a non-flat surface such as that in wearable and implantable devices. Devices such as used in smart textiles, safety, sports and leisure, and biomedical applications often follow irregular shapes, and the printed circuit must follow the shape for proper integration.

Although it is possible to form a flexible circuit in the shape of a cone or a cylinder, only a stretchable circuit can be deformed onto any type of surface, as it has stretchable interconnects. [6]

For instance, stretchable PCB placed in the sole of a shoe can measure pressure with embedded sensors, collecting data with free movement of the user. Placed inside bandages, the pressure sensors on a stretchable circuit can measure the tightness of the applied bandage. [2]


A completely new range of electronic devices can make use of stretchable PCBs providing comfortability as their unique characteristic. Apart from the few uses listed above, stretchable PCBs are already being used in applications involving artificial skins, randomly shaped biomedical implants, and conformable light sources.


Printed Circuit Board (PCB)

Embedding Components within the PCB

The rise of the mobile industry on one hand and the increasing demand for wearables on the other, combined with the increasing use of IoT in the industry, has led to the complexity and density of electronic designs to increase substantially in the last two decades. Simultaneously, these demands have also increased the challenges for designers of printed circuit boards (PCBs) tremendously. One of the ways PCB designers are coping with the issue is by embedding electronic components within the PCB substrates. This is fast becoming a feasible step for eminent board manufacturers such as RUSH PCB UK LTD.

Advantages of Embedding

Before starting the design, it is imperative to understand the advantages that embedding components brings, while at the same time considering the drawbacks of adding the fabrication steps leading to the embedding. In fact, there are potential effects on cost and production yield that the design team must factor in when considering embedding components within PCBs. Some of these advantages are:

  • Reduction in size and cost
  • Minimizing electrical path lengths
  • Decreasing parasitic capacitance and inductance
  • Reducing EMI effects
  • Improving thermal management

For RUSH PCB UK LTD, innovation in PCB technology comes basically from reduction in size and cost. Embedding components within the PCB substrates help to reduce the size of the board assembly. For complex products, a PCB embedded with components can potentially reduce the manufacturing costs.

High-frequency circuits are highly susceptible to the parasitic effects of long electric path lengths during PCB design. Embedding components within the PCB helps in minimizing electrical path lengths, thereby reducing the parasitic effects to a large extent.

Such reduction in path lengths when connecting embedded passive components to the pins of an IC can decrease the parasitic capacitance and inductance, thereby reducing load fluctuations and noise within the system. For instance, it is possible to place embedded passive components directly underneath the pins of an IC. This not only reduces the via inductance, but also minimizes potential negative parasitic effects, and improves device performance. In fact, embedding components within the substrates of a board allows reduction of path lengths over surface mounting.

It is possible to integrate an electromagnetic Interference shield around an embedded component. For instance, simply adding PTH all around the component can reduce noise coupling from outside. In certain applications, this may even eliminate the need for any additional surface-mounted shield.

It is also possible to add heat-conducting structures to an embedded component for improving thermal management. For instance, embedding thermal micro-vias to be directly in contact with the embedded component can help it to dissipate the heat to a thermal plane on an external layer. Adding thermal micro-vias also reduces thermal resistance, as the amount of heat traveling through the PCB substrate reduces.

One of the major concerns when embedding components within a PCB is the long-term reliability of the design. Solder joints on embedded components formed and placed within the laminates of a PCB can be affected when the PCB undergoes soldering processes such as reflow during assembly of surface mount devices. Embedded components can be an additional problem after manufacturing, as they cannot be easily tested or replaced once they have failed.

What Components can be Embedded?

RUSH PCB UK LTD considers two main categories of components fit for embedding into PCB laminates—passive and active. They are used in different ways and for different applications. As a large majority of embedded components are of the passive category, embedded resistors and capacitors are the most popular.

However, an embedded passive component does not mean that a discrete resistor or capacitor is placed inside a cavity within the substrate of a board. Rather, it is the selection of a specific layer material to form the resistive or capacitive structure of an embedded passive.

Benefits such as listed above make embedded components an alternative to discrete surface-mount passive components. Applications such as series termination resistors benefit from this technology tremendously, a huge number of transmission lines terminate at dense memory devices and ball-grid array (BGA) ICs.

Embedding Chips

RUSH PCB UK LTD can embed a chip within a PCB, but steps for other manufacturers may vary. Typically, the fabricator has to create space for the body of the IC, and this takes the form of a cavity. Approaches to chip embedding technology may take the following approaches:

CIP or Chip in Polymer: this involves embedding thin chips when building up dielectric layers of the PCB, rather than integrating them within the core layers. The fabricator can use standard laminated substrate materials.

ECBU or Embedded Chip Buildup: this involves mounting chips on polyimide films and building up interconnect structures thereon.

EWLP or Embedded wafer-level package: this involves performing all technology steps at the wafer level. IO area available is limited to the footprint size of the chip, as this technology essentially requires fan-in.

IMB or Integrated module board: this involves aligning the components and placing them within a cavity and using controlled-depth routing to place the cavity within a core laminate. Filling the cavity with molding polymer ensures chemical, electrical, and mechanical compatibility to the substrate. Impregnation of isotropic solder in the polymer helps to form reliable solder joints while laminating the embedded part into the stack.

Component Design Considerations for Embedding

RUSH PCB UK LTD considers layout of components and their physical orientation as important factors when designing for embedded purposes. It is also necessary to select proper substrate materials and compatible components, as this reduces the chances of failure during PCB fabrication.

Selecting specific materials is the key to determine the electrical properties of embedded passives. For instance, an embedded resistor is simply a sheet of resistive film, its dimensions defining the value of the resistance. The resistance of such material is dependent on the resistivity of the material, its length, and its cross-sectional area. Resistive film materials vary in their resistivity, and this directly influences the final resistance value. Therefore, selection of the material is critical to the design and the manufacturing process.

Manufacturers make embedded capacitors by arranging properly dimensioned copper cladding to act as plates, and placing suitable dielectric material in between. Designers calculate capacitance based on the dielectric constant of the material, the permittivity of free space, distance between the plates, and the area of the plates. The final capacitance value increases with an increase in the dielectric constant of the chosen material, an increase in the area of the plane, and decreases with an increase in the plane-to-plane distance in the board layers. Manufacturers use special material for maintaining dielectric strength, with a thin but dimensionally stable dielectric layer for creating embedded capacitors for power supply decoupling.

For making other active components such as ICs, manufacturers and designers select materials that provide substrate durability and long-term reliability of components within cavities. CTE or coefficient of thermal expansion defines the manner in which the material will change during high-temperature events such as reflow soldering of surface-mount components. It is highly imperative for the designer to select substrate material and polymer with matched CTE for filling cavities to maintain the integrity of the board structure.

RUSH PCB UK LTD has two ways of aligning and placing embedded components in cavities—face-up and face-down, with face-down being the preferred process. For a face-down alignment, the cavity depth needs to match the package height, and therefore, the manufacturer can embed chips of different thicknesses on the same layer. This allows for good thickness control for the dielectric material, and accurate component placement during assembly.

Manufacturing Processes for Embedding Components

Individual manufacturers will vary their fabricating processes for embedding depending on the type of PCB and the available equipment at their disposal. Broadly, manufacturing process at RUSH PCB UK LTD for embedding components follow two methods—one, aligning component and placing them within cavities, and the other, molding components into the substrates, building up additional structures thereon.

Manufacturers use different manufacturing and configuration techniques to make cavities in PCBs. Advancement in technology has led to better and more efficient methods of developing cavities for embedding active components. The new methods offer additional benefits such as higher production yields and improved reliability.

Drilling cavities with lasers offers the highest positional accuracy and precision of all methods, as it is possible to control a laser beam precisely for achieving uniform depth and wear as it removes dielectric material. Using a longer wavelength prevents the laser from penetrating copper layers, thereby forming a distinct stop layer. After forming the cavity, the fabricator adds an anisotropic conductive adhesive before placing a component inside the cavity. Application of heat and certain amount of pressure helps to melt the solder particles in the adhesive material, thereby forming reliable solder bonds.

More conventional methods use milling for creating cavities, as milling is more cost-effective than lasers are. Although improved technology allows making miniature milling tools, there is a practical limit to using milling and routing for cavity creation. Even so, milling is more popular as compared to lasers.

Some manufacturers prefer using thin wafer packages, integrating them directly into dielectric layers during the buildup, rather than drilling or routing cavities into the core material. The fabricator begins by die-bonding the thin chip to the substrate, following it up with a layer of liquid epoxy or an application of a laminated RCC or resin-coated copper film as a dielectric. He/she then applies a heated press lamination process, optimizing it to embed the chip without void formation.

Documentation Requirements

Any design with embedded components will require proper documentation for reducing manufacturing time and cost. As the process of embedding components combines component assembly, packaging, and PCB manufacturing into a single manufacturing process, necessary documentation requires layer stack diagrams, NC drill files, fabrication notes, pick-n-place files, and assembly notes for effective PCB fabrication.


Market demand is pushing for high-density, low-profile electronic devices. Manufacturers are complying to this demand with the technology for embedding passive and active components within the board substrate. RUSH PCB UK LTD has successfully broken through potential barriers of reliability concerns and risks to production yields and cost.


PCB Vias and Everything You Need to Know About Them

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.

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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.


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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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.


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%.


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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Productive Approaches to Creating Prototype PCBs

So far, for printed circuit board or PCB prototype, designers limited their choice of PCB design tools to high-end, enterprise-level solutions, as these tools were expensive. The early design tools also added the cost of an extended setup time and learning curve, were limited in their capability, and most often, error-prone. Thankfully, modern tools are now affordable and come packed with all the features designers need for complex designs. Moreover, they are easy to use and focus more on ease of adoption.

Typically, a designer takes about two to three iterations for developing a custom PCB prototype for a working product with a high-speed computer-based design tool. However, with decreasing product life cycles, the time-to-market is steadily gaining in importance. Depending on overheads, board iterations can be expensive, because delaying the product’s market launch and the missed opportunity could cost the company several thousand dollars, or even the total loss of market share.

The above is prompting designers to employ simulation in the design cycle before they order PCB prototypes, as this dramatically reduces the cost of development. As the cost of change increases with development time, design changes occurring early in the design process cost substantially lower compared to those introduced during full-scale PCB fabrication. Using virtual prototyping has the advantage of identifying issues early on in the design process, and rectifying them is cheaper and simpler before they become a major problem.

Virtual Prototyping as a Productive Approach

Although entry-level tools did allow quick designs and prototype building, most designers relied on reference designs provided by chip vendors. Increasingly, designers are finding they cannot rely on reference designs to make their products work in operating environments—they need design for reliability and manufacturability for the real world.

Implementation of each new technology introduces multiple fast rise-time signals propagating at increasingly faster speeds. That does not allow the luxury of building prototypes, testing, reviewing, and revising the design approach with each build.

With virtual prototyping, designers can do with fewer PCB prototypes and improve their design efficiency. Virtual prototyping includes simulation of signal and power integrity, design for manufacturability, thermal analysis, and 3-D interference validation.

Earlier, skills of PCB designers and engineers were necessary for the entry-level tools to detect possible issues as they came up during the design process. High efficiency, complex designs, on the other hand, require a more constraint-driven approach, with correct-by-construction methodology. Once the engineers establish the rules, designers with downstream tools will follow them and use various design rule checks (DRCs) to conform their validation.

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Designing for Profitability

Modern PCB tools are able to handle several tasks other than simply laying traces. It is possible to use these design tools at every phase of the project from initial concept to the final assembly documentation. The addition of 3-D rendering engines in most of these design tools and the complete integration of 3-D component bodies in the footprint libraries makes this possible.

This capability allows designers to give shape to concepts very quickly. They can use virtual prototyping along with vendor-supplied 3-D step models to make a preliminary PCB layout. This allows a quick look at the finished product, such as the position of the I/O connectors, without the detailed board design.

This method helps designers handle request for change early in the design, as the visualization tools allows an upfront view of the design process, and everyone involved has a good idea of the direction the design is headed, and they can spot the misconception or conceptual errors early.

Virtual prototyping has an added side benefit. Every step of the process can confirm the mechanical fit of the product. Moreover, designers can now put together the fab drawing first and get quotes on any proposed design, material, and so on. They can pass the mocked-up fabrication drawing to the assembler for a review to uncover any issues. This not only saves time, but also the unnecessary expense of multiple prototypes. Silk-screeners, pick-and-place machines, and reflow ovens require tabs, holes, and other modifications that are not part of the original circuit design, but part of manufacturability. Assemblers reviewing the preliminary fab drawing often make recommendations based on their experience with materials and others.

Keeping a Dynamic Supply Chain Visibility

A major reason for design iterations is the supply chain information not being available to the designers in real time. Many a time designers have to manufacture printed circuit board prototypes only to have to change their design because of the non-availability or a certain component or components, which they have to replace with a suitable alternative and now requires a change in the PCB design. The cost of having to change the design at the prototype stage is much lower compared to that required once the product is in full production. Therefore, it is necessary to have a real-time view of the supply chain from the product management or procurement team.

Additive Manufacturing Rather than Subtractive

Regular PCB prototype making is a subtractive process. The fabricator starts with more material than needed, and removes the unnecessary parts. However, in an additive process, the fabricator starts with a thin substrate and adds the required copper traces with conductive ink.

Although still in its infancy, 3-D printing and additive manufacturing for electronics is a great way to generate less waste than traditional subtractive methods for PCB fabrication, especially for custom PCB prototypes. With additive manufacturing machinery deployed in-house, engineers can make necessary changes in a design without the traditional penalties in cost and time. The designer does not have to wait for the fab house to send the order PCB prototypes back, as he can create the prototype on his or her desktop.

Designers can select from 3-D printing and 2-D printing processes for additive manufacturing. The 3-D printing prints PCBs from scratch, using a variety of gels, inks, and substrates, layer by layer, manufacturing them at the nanoparticle level. This is a very new process and involves material complexity and extrusion requirements.

The 2-D inkjet-style printing matches more readily with the Gerber files designers generate to communicate designs to manufacturing. These machines only have print the conductive traces on a flat horizontal substrate. This is ideal for printing PCB prototypes for quick prototyping.


There are several ways to reduce the iterations involved with designing, PCB prototyping, and cutting down on the time to market factor. While virtual prototyping can actually save on the time required for ordering, testing, and reviewing prototypes, additive manufacturing can cut down the actual time for fabricating prototypes. Additionally, keeping a clear dynamic supply chain visibility precludes the necessity for redesigning the board at a later stage.


How to Detect Circuit Board Faults?

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.


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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Best PCB Layout Practices for 2018

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.


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.


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.


Benefits of HDI Flex Circuits

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.


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.



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.