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Jun 15, 2020

Flexible and Rigid-Flex PCBs and Their Applications

Electronic gadgets are increasingly getting thinner and smaller. Wearable electronics must conform to body contours. Rather than use conventional rigid boards, OEMs find flexible and rigid-flex printed circuit boards (PCBs) to be more suitable to the requirements of modern electronic systems. The increasing popularity of flexible and rigid-flex PCBs for devices in the electronics industry stems from several advantages in their use such as reduced assembly costs, significantly higher reliability, and lower space requirements.

All types of PCBs provide the same basic functionality—acting as a substrate for electronic components. However, the key distinguishing features of PCBs are their design and the materials of their construction. Manufacturers can custom-tailor circuit boards depending on specific applications.

Differences Between Conventional and Flexible PCBs

As their names suggest, conventional boards are rigid, as they are stiff and do not bend, whereas flexible PCBs are pliable and users can bend or shape them to fit within the contours of designated systems. Manufacturers can design flexible circuits that can withstand repeated bending for hundreds of thousands of cycles without failure. Although this increase in versatility of flexible boards comes at a higher cost, their ability to function within limited space makes them very useful for several industrial applications including automotive, space, medical, and consumer electronics.

Fig 1: Flexible Vs. Conventional Rigid PCB

Their low cost had made conventional rigid boards very popular. Earlier, consumer electronic devices usually had greater space available within them, and conventional rigid boards allowed OEMs to save significantly on costs. However, with drastic reductions in the shape and size of consumer electronic equipment, of late, flexible PCBs are slowly siphoning away market share from rigid circuit boards, because flexible PCBs provide:

Flexibility: It is possible to fold, bend, and even crease flexible PCBs to fit the end application. Therefore, the designer can easily fit the circuit within the device, rather than building the end device around the electronics and circuit boards.

Durability: Although rigid PCBs have higher thickness and strength, flex PCBs can absorb vibrations and shocks much more effectively than rigid PCBs can. This makes flex PCBs more reliable, while increasing the functionality and life of the product.

Stability: While heat, radiation, or chemicals can easily damage or warp regular rigid PCBs, flex PCBs are more resistant to such detrimental environments.

Reduced Weight: With their lower weight, flexible PCBs result in a lighter product. Today’s electronics markets, including both designers and consumers, demand small and lightweight devices. Lightweight flexible PCBs are ideally suitable for drones and other unmanned vehicles.

Rigid-Flex PCBs

As a hybrid made from both rigid and flexible PCBs, a rigid-flex circuitry offers the best qualities of both while eliminating several of their individual limitations. Manufacturers incorporate flexible materials along with rigid materials within a rigid-flex PCB by layering the flexible substrates within the material making up the rigid circuit board. This method ultimately combines the versatility of flexible circuits and the strength, stability, and circuit density of the rigid PCB. The combination of a rigid and flexible PCB brings forth newer possibilities for mechanically challenging and designs of higher complexity.

Rigid-flex PCBs provide additional functionality as they help to streamline the electronic design. By eliminating flexible wire cables and connectors, rigid-flex PCBs offer better electrical performance compared to conventional PCBs.

Fig 2: Rigid-Flex PCB

The major advantage of rigid-flex PCBs is they contain all the electrical and mechanical connections as an integral part of their overall construction. This allows the rigid-flex PCB to provide improved electrical performance and service reliability.

Construction of Flexible PCBs

Manufacturers make standard PCBs with a base of fiberglass or metal, whereas the core of a flexible PCB is typically flexible polymer and the substrate is Polyimide film. The advantage is using Polyimide is it stays flexible after thermosetting, and does not soften when heated, unlike many other thermosetting resins do. Designers and manufacturers prefer upgraded Polyimide films more than the standard Polyimide films, as the former has good resistance to tearing and humidity.

Fig 3: Structure of Flexible PCB

For bonding its different layers, a flexible PCB also requires a special base material or an adhesive. While earlier manufacturers used adhesives for bonding, it reduced the reliability of the PCBs. Therefore, manufacturers developed a special Polyimide that could bond with copper without requiring an adhesive. This adhesive-less Polyimide also allows building thinner designs that have a lower risk of via damage.

For covering and protecting the copper surface, manufacturers use a coverlay film, also made of Polyimide film. This coverlay film works much the same way as a solder mask does for conventional PCBs.

If certain areas of a flexible PCB need to be rigid, manufacturers laminate a stiffener to that portion. The stiffener material, being an insulator, does not allow signal transfer.

Construction of Rigid-Flex PCBs

In a rigid-flex PCB, rigid sections connect flexible sections of the PCB. As rigid PCB materials make up the rigid sections, the board is unable to bend at these sections, and bends only at the flexible sections. This makes the board flexible, yet strong where necessary.

Electrical signals can easily transfer from the rigid section of the PCB to the flexible parts, as the entire design is one single circuit. Mechanical reinforcements make the rigid sections non-flexible. Often, manufacturers reinforce the rigid sections with fiberglass substrates just as they do for standard PCBs. For multi-layer rigid-flex PCBs, manufacturers often use prepreg fiberglass as a substrate for the middle layers.

Popular Stackups

Stackups for single- and double-layer flexible boards are similar in stackup construction, featuring a flexible polymer as base and Polyimide film as substrates. Flexible circuits may also include stiffeners made of FR-4 fiberglass or pressure sensitive adhesives (PSA).

Fig 4: Structure of Rigid-Flex PCB

Construction of rigid-flex PCBs can vary. One common approach is to use four rigid layers along with two flexible ones. This may include a core made of adhesive-less Polyimide to reduce risks of breakage. The section above this core and its copper film form the flex sections, with two layers of prepreg connecting them to the coverlay layers and its adhesives. Manufacturers fortify the rigid sections with fiberglass, additional copper layers, and solder mask.

Applications of Flexible and Rigid-Flex PCBs

With the myriad applications of flexible and rigid-flex PCBs, it is more comprehensive to list their usage industry-wise:

Industry Application
Consumer Appliances Electric irons, UV water purifiers, solar power systems, lighting systems, washing machines, TV remote controls, Microwave ovens, mobile phones, multimedia players, wearable electronics, and more.
Industrial/Manufacturing CCTV surveillance systems, control panels, industrial air conditioners, industrial automation systems, test equipment, electrical switches, and more.
Telecommunication Routers and servers, online signal expansion systems, transmission media, wireless communication systems, transmission systems, signal processing systems, communication satellites, base stations, hand-held units, and more.
Aerospace Climatic and environmental test chambers, motion sensors, noise and vibration testing systems, various sensors, control tower systems, radar equipment, radio communication systems, global positioning systems, avionics systems, and more.
Automotive Navigation systems, music systems, entertainment systems, traction control systems, liquid crystal displays, comfort control units, transmission control systems, electronic control modules, and more.
Military Tracking/Surveillance systems, launch detectors, aircraft missile launching systems, global positioning systems, communication systems, weapons guidance systems, and more.
Medical Wireless control systems, drug delivery systems, imaging equipment, handheld monitoring units, cochlear implants, pacemakers, and more.

Conclusion

Compared with traditional rigid boards, the advantages offered by flexible and rigid-flex circuits far outweigh their increased cost, as seen by their improved interface connection, reduced assembly costs, and reduced packaging weight. Moreover, flexible and rigid-flex circuits can resist sudden temperature excursions not only during manufacturing and assembly processes, but also during harsh conditions of application areas. Their flexibility allows the use of flex PCBs in areas where space is a constraint. All the above features make flexible and rigid-flex PCBs the most popular and suitable for use across domestic, commercial, and industrial sectors.