Dimensional Gymnastics – Flexible Printed Circuit Assemblies
By David Pheteplace, Bishop & Associates Inc.

In the past, printed circuit boards (PCBs) were confined to one- or two-sided solid boards formed from woven glass and epoxy or treated cotton paper as the insulating layer. With the miniaturization of electronics over the last 20 years, these two-dimensional boards became a confining factor in the design of many products. The answer to the design engineer’s conundrum was the development of flexible printed circuit boards (FPCs). These FPCs are made by layering flexible insulating material (polyimide film) with copper foil that is chemically or mechanically etched to form the circuits and conductors. The resulting flexible circuit can be bent into a myriad of shapes to fit the application, or act as a point-to-point cable.

Hirose is one of the leading suppliers of FPC and flexible flat cable (FFC) connectors. Chris Tubbs, Hirose FPC and FFC assistant product manager, has seen a number of developments in FPC connectors and assemblies over the last 10 years. The most significant trend has been the miniaturization of the connectors to reduce the real estate they occupy, combined with a smaller pitch. The pitch, or spacing of the conductors, has gone from 1.25mm to 0.5mm during this time. Hirose’s FH28 connector has a 0.5mm pitch with a larger connector body, which is used for industrial applications where size is not an issue, but the ability for easy handling is.

Other innovations involve designing RoHS compliant products and eliminating halogen to address environmental concerns. While some of these connectors were low insertion force, most are now zero insertion force designs. Therefore, the FPC or FFC is held in place by a locking mechanism that retains the electrical interconnection. Hirose has also developed a passive feature to hold the FPC/FFC in place during assembly.

Tubbs identified devices with LCD screens as the largest application for FPCs and FFCs. Cell phones and PDAs are at the top of this list, particularly for flip phones, where the flexibility of the FFC was needed. The technology is also used in hand-held bar code readers and medical devices, digital cameras, hand-held games, and displays for in-flight audio/visual displays.

The fun with flexible printed circuit assemblies begins when designers take advantage of their three-dimensional capabilities. FPC assemblies come in two varieties: flexible and rigid flexible. The flexible circuit is comprised of an all-flexible insulating material. Portions of the insulating material are rigid in a rigid flexible assembly, such as an FR4 board (epoxy-impregnated woven glass). The advantage of the totally flexible assembly is that it can be bent and/or folded into complex shapes to accommodate the application. The advantage of the rigid flexible circuit is that it can provide a solid internal structure, if that is needed by the design, and the rigid structure more easily accommodates dense interconnect packages and tight lead spacing, such as ball-grid arrays. Both types of assemblies can reduce component count, assembly cost, improve interconnect reliability, and allow for dense package design with unusual shapes.

The manufacturing processes for these boards are similar to standard printed circuit boards. They can accept both through-hole and surface mount components. The boards do require fixtures, however, to go through the assembly and solder reflow processes.

The following are some of the typical (and unusually shaped) applications for this technology (some as seen on the Printed Circuits Inc. website).

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Flexible circuits and connectors will continue to grow due to the increased use of smaller and easily transportable devices in all areas of our lives.

David Pheteplace
Bishop & Associates Inc., Market Segment Director - Cable Assemblies
David Pheteplace joined Bishop & Associates in 2008 as its market segment director for cable assemblies. Pheteplace, a management consultant for the electronic and interconnect industry, specializes in operational and strategic analysis, problem solving, and solution implementation. He has more than 20 years of experience in the connector industry, including managing divisions for Amphenol, Cinch, and Robinson Nugent. Pheteplace can be reached at www.pheteplace.com.