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Rigid-Flex PCB Design Guidelines and Applications(Best Tutorials in 2021)

Rigid-flex PCB 1

Rigid-flex printed circuit boards are extensively useful in designing circuits for flexible products. They have enabled the development of many technological advancements, especially in wearable gadgets and sensors. But with their extraordinary abilities come complications in designing. Rigid-flex PCBs design is a challenging process that demands both skills and knowledge. That is why before designing a rigid-flex PCB, one should learn the factors that may affect PCB quality.

Before getting to the factors that impact PCB quality, let’s first understand how a Rigid-flex PCB is different from conventional PCBs.

All You Need to Know About Rigid-Flex PCB

Rigid-flex PCB is a hybrid board, developed with a combination of rigid and flexible parts. The combination can be either individual rigid and flexible parts or both rigid and flexible parts merged to form a single layer.

In the case of individual rigid and flexible parts, some portions of the board are solid like conventional PCBs and the rest are flexible like typical flexible PCBs. The components are soldered on the rigid part and the flexible part enables signal transmission through places where rigid PCBs cannot fit in or survive for long. Whereas in the case of a merged layer of rigid and flexible parts, the board gains flexibility up to some extent.

Substrate and Coverlay Films

High quality rigid-flex PCBs are developed with fiberglass with epoxy resin coating on both sides. The fiberglass provides flexibility whereas the epoxy resin makes it rigid. This combination gives the board ability to flex upon stress (but not as much as typical flexible PCBs).

A more suitable substrate is polyimide which is comparatively more flexible. Despite being flexible and thin it is strong enough to bear stress and does not tear off easily. Moreover, polyimide is heat-resistant which enables the development of high-temperature circuits. Apparently, polyimide enables the board to bend and bear stress more effectively.

Similarly, polyester (PET) is also used in the development of rigid-flex circuits and it also has flexible properties. However, polyester cannot withstand high temperatures for a long duration which is why polyester is used in only low-temperature applications.

For high-frequency circuits, polytetrafluoroethylene (PTFE) is a more suitable substitute. It is also useful in circuits that have low dielectric constant requirements.

To protect the insulation layer, a coverlay is applied on the surface. It prolongs the life of the insulation layer.

Conductor

Alike other complications in rigid-flex PCB board, the conductor selection also affects its efficiency. Typically, the conductors used in rigid-flex PCBs are copper foil, carbon film, silver-ink, and rolled annealed (RA) foils.

For small-scale, high power, and simple circuit designs, copper foils are most suitable. Whereas for sensitive and highly-stretchable applications, RA is a more suitable choice. However, is costly as compared to the copper foil.

Adhesives

As the name suggests, the purpose of adhesives is to form bondage between the copper layer and the polyimide layer. The bondage is not possible with just heat or pressure that is why and the adhesive is used to form a bond.

The commonly used adhesives are acrylic and epoxy-based but other adhesives such as silicones and hot-melt glues can also be used. Typically adhesive layer thickness is between 0.5-1 mil.

Rigid-flex PCB 2

                                                                                                               Rigid-flex Printed Circuit Board

Layer Stack-up Types in Rigid-Flex PCBs

The layer stack in rigid-flex PCBs is designed in the vertical direction; each layer is connected with the other layer to form a single unit. Proper layer stacking enables proper EMI blockage through a suitable arrangement of traces and ground planes. This is essential especially in sensitive and high-frequency applications. Proper layer stacking also reinforces PCB’s temperature-resistance.

  • Apart from the benefits of EMI blockage and temperature-resistance, it is essential in some cases for technical reasons like below.
  • It forms proper isolation between the signal plane and the ground plane.
  • It helps deal with thermal limitations and characteristics.
  • It enables the formation of circuits that consist of both analog and digital signals. It helps form isolation between both signals and reduces noise coupling.

Single-layer Stack up

Single-layer rigid-flex PCBs have usually one conductive layer with one or two polyimide layers. Like on conventional single-layer PCBs, electronic parts, pins and connectors can be connected on a single-layer rigid-flex PCB. But it is more useful for designs that are prone to static and dynamic flex.

Double-layer Stack up

Like in conventional PCBs, some rigid-flex PCB applications require double conductive layers; the connection has to be formed on two layers. For such applications, double-layer rigid-flex PCBs are more suitable.

Typically, in double-layer stacking, the insulation layer is sandwiched between the conductive layers. However, the conductive layers can be either coated for protection or more insulation layers can be added on the outer sides. Double-layer rigid-flex PCBs are most suitable for applications that have to deal with stationary or active flex.

Multi-layer Stack up

Just like double-layer PCBs, multi-layer PCBs have more than one conductive layer. However, in double PCBs, the conductive layers are maximum of two whereas, in multi-layer PCBs, the number of conductive layers is always three or more. One insulation layer is introduced between every two consecutive conductive layers. The conductive layers are vertically connected through vias.

The outer side of the conductive layer can be either covered with a coating or additional insulation layers can be added for protection. There is not much difference between double-layer rigid-flex PCBs and multi-layer PCBs except for the number of layers.

Rigid-flex PCB 3

                                                                                                                Rigid-flex Printed Circuit Board

Rigid-flex PCB Applications

Rigid-flex printed circuit boards are used in almost every electronic application that requires a robust circuit base with flexibility. Unlike conventional PCBs, rigid-flex PCBs give bending ability to the circuit; enhancing its reliability and ability to withstand mechanical stress.

The compact modeling of rigid-flex PCBs enables the development of certain circuit designs that cannot be achieved with simple PCBs. These features make it suitable for a variety of applications that are prone to mechanical stress. That is why rigid-flex PCBs are widely used in the development of both commercial and domestic applications.

What Makes Rigid-flex PCBs Suitable for Industrial Applications?

As the rigid-flex PCBs can be developed with busses and leads for connecting multiple circuits, the need for connecting parts declines; leading to a lower manufacturing cost. But with a reduced number of parts, rigid-flex PCBs also offer better reliability since the number of mechanical connections is also reduced.

Furthermore, the lesser the number of parts, the lesser the space PCBs consume.  As a result, the rigid-flex PCBs are now widely used in many applications

These characteristics make rigid-flex PCBs feasible for the development of various types of products. Some of the industrial applications of rigid-flex PCBs are as follows.

Medical Applications

  • Hand-held equipment
  • Cochlear monitor
  • Wearable sensors
  • Pacemaker
  • Imaging equipment
  • Robotic surgery equipment

Military Applications

  • Communication equipment
  • Global positioning system
  • Surveillance equipment
  • Military gadgets
  • Weapons guidance equipment
  • Air-defense systems

Aerospace Industry

  • Global positioning system
  • Control tower equipment
  • Sensing systems
  • Radar and navigation system
  • Environment monitoring system

Telecommunication Industry

  • Satellite communication system
  • Signal routing and expansion system
  • Handheld and wireless communication system
  • Signal transmission and processing system

Automotive Industry

  • Traction control system
  • Air conditioning system
  • Car navigation system
  • Music and entertainment system
  • LCD and camera
  • Other electronic systems

Manufacturing Industry

  • Automation systems
  • Switchgears
  • Air conditioning system
  • Close circuit Camera television system
  • Equipment testing system

Home Appliances

  • Washing machine
  • Ultraviolet light-based water purification system
  • Microwave oven
  • Television remote
  • Electronic iron machine
  • Solar power generation system

Rigid-flex PCB 4

                                                                                                                Rigid-flex Printed Circuit Board

How to Design a Rigid-flex PCB?

Rigid-flex PCB designing is a delicate process that involves technical detailing; even a small negligence can affect the overall durability and reliability of the PCB. Moreover, it can cause malfunctioning and circuit damage. Therefore, it is recommended to follow a standard operating procedure for the designing of rigid-flex PCBs. It is as follows.

  • Start with material preparation; choose a suitable material type (for the adhesive, conductor, etc) and the number of layers as per the application’s requirements. Plan manufacturing techniques; whether it is going to be pressure manufacturing or heat manufacturing.
  • Place the copper traces at the right angle to the rigid and flexible turns on the board.
  • Account for the potential electrochemical factors that may affect the flexible and rigid parts; for that, consider the ratio of bend radius to thickness. As per standard, the ratio between the bend radius and the flex material’s thickness should be 1:10.
  • Maintain a 0.5mm distance between the copper and adjacent vias; any less distance can cause contact between both upon frequent bending. Moreover, the vias on PCB must be stationary.
  • The overall PCB must be designed as per the temperature limitations of the circuit; the PCB must be capable to withstand atmospheric temperature. Test the PCB for maximum temperature, humidity, cooling, and flammability range. Make sure, the PCB can withstand bends and vibrations as per the application’s requirements.

Rigid-flex PCB Designing Software

Rigid-flex PCBs can be designed on a variety of software but for industrial-grade designing, the following software is used.

Altium Designer

Altium designers is an extensively technical software that enables proper detailing in rigid-flex designing. It supports PCB development up to 32 signals and 16 plane layers. Altium is one of the most commonly used rigid-flex PCB designing tools on an industrial scale.

Apart from the mainstream PCB designing tools it also offers 3D rendering tools that enable the 3D designing of PCBs. Moreover, the built-in PCB testing diagnostic tools enable reliable designing and design improvements.

Integrated ECAD/MCAD tools

Alike Altium, ECAD/MCAD tools enable designing from both electrical and mechanical perspectives. The 3D features of these tools allow visual inspection that helps improve board design.

Precautions to Consider When Designing Rigid-flex PCBs

As already stated, rigid-flex PCB designing is a delicate process that requires extensive detailing, even minor negligence can cause failure. Moreover, the designing flaws can cause wastage of parts and development costs too. However, with the proper operating procedure, such flaws can be avoided. Following are the steps that must be considered while designing a rigid-flex PCB.

  • Do not bend the corners unless until the corners are curved and are not sharp.
  • Make sure no vias are developed around the edges of the bend line as it affects the PCB’s strength.
  • For carrying power signal or ground signal, consider hatched polygons so that the medium remains flexible.
  • Do not introduce sudden trace width changes, slightly increase the width if needed. Sudden change in width affects the reliability of PCB.
  • Prefer using at least 2 flexible layers for proper flexing and reduced cost.
  • Highly thick PCB materials affect the overall flexibility and compactness of design. Moreover, they also complicate the development and assembly process. Prefer using a printed circuit board that has low thickness.
  • The arrangement of the wires shows the folding resistance requirements to enhance the shock resistance.

Rigid-flex PCB 5

                                                                                                          Rigid-flex Printed Circuit Board

Common Problems in Rigid-flex PCB Designing

  • The bending radius of a rigid-flex PCB can affect its reliability. If a PCB is frequently exposed to bends and vibrations, some parts of the circuit are more exposed to stressed than the rest of the parts in the circuit. Which means these parts are more likely to damage. If the radius is less rounded, it may lead to more mechanical stress.
  • Problems related to mechanical limitations commonly appear in rigid-flex PCB designing and development. Especially, problems such as printed circuit board thickness, drill-hole size to PCB thickness, and bending rate are common.
  • If the proper space between the copper trace and pads is not maintained, it can lead to malfunctioning especially in terms of unwanted short circuits. Make sure proper space is maintained.
  • If traces are stacked on a specific portion of the PCB on both sides, it can lead to fractures. This is because the dielectric experiences concentrated stress on that specific portion.

Apparently, Rigid-flex printed circuit board designing is a delicate process that demands extensive care and knowledge. But a designer can always design a flawless rigid-flex PCB if proper procedure is followed.

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2 thoughts on “Rigid-Flex PCB Design Guidelines and Applications(Best Tutorials in 2021)”

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