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.


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.


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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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.


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.


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.


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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PCB Design

Up and Coming PCB Designs

We know the importance of PCBs in our tech-savvy world. The continual advancements in modern technology creates a need for continual advancements in printed board circuits. There is a demand for smaller, more elegant, and dependable PCB designs. These new designs are used from PC manufacturing to new medical technology. The constant development of new printed board circuit designs allows the technology industry to keep inventing new ways to make our lives a little easier and in some cases healthier. Today, we are going to explore some of the advancements that we are already benefitting from and what is in store for the future of the PCB Fabrication Industry.
Board Cameras are now being used in medicine to make diagnostic testing much more comfortable for patients. Instead of a large, uncomfortable scope or camera being inserted into the area of the body that is being examined, the patient can now swallow a camera that will collect the necessary information that a physician needs to make a diagnosis. One example of this is called Capsule Endoscopy, the patient will swallow the disposable “pill” which will take up to fifty thousand pictures of your digestive tract. The pill travels out of the body through a bowel movement and can be flushed away.
Vertical Conductive Structures (VeCS) is the invention of Joan Tourné will offers a less expensive alternative for challenging fan-out projects to fine-pitch grid array components. Although still in the testing phase of development, Tourné states “Not only can we achieve higher interconnection density by packing more vertical connections in a smaller space, at the same time we can increase conductor router channel density under grid array components.” He continues to explain that PCB Fabricators will not experience additional cost with this method since the technology required is already being used by high-end shops after the appropriate training and licensing. We will have to wait and see if this new method will prove beneficial to the PCB Fabrication industry.
GaNonCMOS project consortium is currently working on a project that will use energy efficiency using GaN power switches and CMOS drivers. Collaboration on this project began in January 2017, the goal is to work with optimized embedded printed circuit boards creating integrated power components for less expensive, better-functioning systems. That sounds wonderful, let’s hope that it works!
Newer, better, stronger. PCB designs are the backbone of any electronic device. They provide us the ease of access to information around the world, they allow us to stay in constant contact with our loved ones and are beginning to play a vital role in medical technology. Smaller, elegant, smarter. PCB Fabricators are constantly challenged to create innovative boards to further our thirst for the technological and make our world an easier place to live.

Starkey, Pete. 2017. Vertical Conductive Structures–a New Dimension in High-Density Printed Circuit Interconnect Accessed March 9, 2017
Prophet, Graham. 2017 Consortia to develop GaN processes and PCB panel-level packaging Accessed March 9, 2017


Understanding the Basic Aspects of Electronic Components

If you are not sure what an electrical component is rest assured, you are not alone.  For those of us who love our devices, but have no idea how they work, sit back, relax and get ready to learn!  Electronic components are the meat and potatoes of the electronic devices we use every day and can’t live without.  They are not flashy and are usually quite easy to overlook.  However, without them, we would literally be back in the dark ages. Today we are going to discuss some of the most basic electronic components.  Some examples include parts like resistors, capacitors, LEDs, transistors, and integrated circuits.  So, let’s get started with the basics.


Resistors have been named for their function, resisting the current.  The resistor is responsible for managing the volts and current in nearly any device that requires electricity.  It is the resistor that allows your device to continually operate without overheating or worse.  By controlling the voltage, it allows just the right amount of electrical current that is needed to operate the device, if it did not do this, then the device would receive too much electrical current and then overheat or in technical terms fry.


Capacitors are used to store an electronic charge for a small period which is released when the charge is needed.  The capacitor will release the stored-up charge when there is a disruption in the circuit of the device resulting in the need for additional power to keep it running.  Kind of like a backup battery, or you can think of it in terms of a generator.  When there is a blackout, facilities or homes that are equipped with a backup generator will not be left in the dark, the generator will save the day by providing electricity to the building.

LEDs (Limiting Emitting Diode)

Although this sounds like something that Luke Skywalker would use, LEDs are routinely used.  LEDs are used to provide light.  That little light that comes on when your cell phone is charging or the power light on your personal computer is the LEDs in action!


Transistors are a little more difficult to wrap your mind around.  They are made of three terminals;

  • Base: Voltage goes through first, it makes the collector “turn on”
  • Collector: Receive voltage from base has a positive charge
  • Emitter: Receives voltage from collector has a negative charge

They work together as a switch to turn the circuit “on”.

Integrated Circuits

An integrated circuit is a tiny component that may contain some of the components you have just learned about.  They are the cornerstone of the devices we know and love ranging from cell phones to our home and work personal computers.  Without them, we would not be in the age of technology that we currently enjoy!

So, there you have it!  Your first lesson in the basic aspects of electronic components is now complete!  We hope you have found this information valuable and informative.