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FPCB Would a flexible printed circuit board be right for your next product design?

Von Nigel Charig

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Modern electronic devices are typically both compact and rugged–and this is due in part to their internal use of flexible (sometimes called flex) printed circuit boards (FPCBs). Originally designed during World War ll as a wire harness replacement, FPCBs have grown into an industry that is expected to reach US$27 billion by 2022.

A Flexible Printed Circuit Board (FPCB).
A Flexible Printed Circuit Board (FPCB).
(Source: Adobe Stock)

This article looks at the types of flexible printed circuit boards currently available, and the advantages, challenges, and costs associated with the technology.

Generally, flexible PCBs are made of materials such as plastic that can flex and move. This allows FPCBs to turn or shift during normal use without permanently damaging any onboard components or connectors. Additionally, FPCBs allow circuitry to be designed to fit an electronic device, rather than building the device to accommodate the PCB. FPCBs are pliable enough to configure around device edges and folds, and can survive 200,000 bending cycles.

Accordingly, FPCBs are found in applications like LCD displays, mobile phones, computer keyboards, antenna, and rechargeable battery circuits. Markets include consumer electronics, automotive electronics, and Internet of Things (IoT) devices as well as military, industrial, and aerospace.

FPCB technology has evolved so that many types are now available. This means that product designers have first to choose between rigid and flexible PCB solutions, and then, where flexible is the choice, decide on the best FPCB type. A product may well contain both rigid and flexible PCBs, while rigid-flex PCBs also exist. To better understand these options, we can look at the various FPCB types available, and then at the advantages and disadvantages associated with the technology.

FPCB types

Rigiflex Technology, Inc. has defined five FPCB types:

Single sided flex: These comprise a conductive copper layer on one side of the PCB. They suit equipment requiring high flexibility. Extremely cost-effective and easy to assemble, they make ideal wire harness replacements. They only require one tooling type, so multiple copies can be reproduced easily.

Double sided flex: This comprises a conductive copper layer on both PCB sides. These layers are generally connected using Plate Through Holes (PTH) or vias. Easy to manufacture, lightweight and readily reproducible, these are one of the most popular FPCB designs.

Multi-layer flex: These contain up to 10 conductive layers, connected by PTH holes or vias. This design is ideal where high connector density is required, with conductors routed through a small area. PTH mounting creates more reliable solder joints.

Rigid Flex: circuit boards combine rigid and flexible circuits. Flexible and rigid layers are integrated, and the PCB is assembled using PTH technology. This combination of rigid and flexible layers creates small interconnect areas, which reduces the chances of PCB failure.

High Density Interconnects: Also known as HDI, these boards provide more hi-tech solutions in terms of design, layout and construction. Each HDI includes extremely dense flex circuitry with precise features and microvias. These facilitate small-sized yet powerful PCBs with increased functionality. HDIs provide exceptional electrical performance, improved usage of advanced Integrated Circuits (ICs), and greater PCB reliability.

FPCB advantages, challenges and cost considerations

Space saving is a primary FPCB advantage; it consumes considerably less space and weight than a rigid circuit, allowing high-density, miniaturised high-reliability electronic products. With fewer interconnects, contact crimps, connectors and solder joints, potential sources of failure are reduced, and reliability is enhanced.

FPCBs are compatible with virtually any connector or component type, and work well with options such as ZIP connectors. They also perform very well in extreme temperatures and offer superior resistance to radiation and chemicals.

FPCBs have some disadvantages; changes and repairs are difficult, for example. Design changes must be made by modifying the basemap or programmed lithography. Modifications are relatively difficult, as a protective film must be removed from the board before work starts, and then replaced afterwards.

Length and width dimensions are limited by restrictions in production equipment size. Also, FPCBs can easily be damaged by careless handling—and trained personnel are then required to affect the necessary soldering and rework.

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Cost is always a critical consideration, however FPCBs’ cost-effectiveness compared with rigid PCBs depends very much on the application. FPCBs are designed and manufactured specifically for each application, so initial circuit design, layout and photographic plate costs are high for small quantities.

For larger production volumes, however, the reduced number of cables, connectors, wire harnesses and overall parts required for assembly can make FPCBs more cost-effective in the long run. This is especially true when the upstream and downstream benefits, such as the lowered supply chain risk and reduced maintenance requests that fewer parts offer are factored in.

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