CircuiTree - January 2009 - (Page 17) Flexible Thinking By Joe Fjelstad Back to Basics Part 22 – Connectors for Flexible Circuits n a case where the flexible circuit assembly is not a complete product unto itself, there is the inevitable need to connect a flexible circuit to other elements of an electronic system. Thus, understanding what options are available for making a connection, either separable or permanent, to those other elements is important to system design, manufacture, and assembly. There are a number of different ways of interconnecting flexible circuits, but the method of first choice commonly involves the use of some sort of connector and, as a result, virtually all electrical and electronic connector manufacturers have connectors that were either specially built for, or which are readily adaptable to, flex circuit designs. Basic connectors for use with flexible circuits can be relatively simple devices. Examples of these include insulation displacement and crimp- or clincher-type connectors. Connectors such as these have proven popular in applications where cost is important; however, they are not generally considered suitable for high-density, high-performance, or high-reliability applications. Fabrication of male/female pin in sockettype connectors generally involves swaged or brazed pins attached directly to the flex circuits. A variation is the multilevel etched circuit technology (aka sculptured® flex) that allows for the integration of the connector pins, and even socket of sorts, directly into the flex circuit itself. The method is suitable for a number of different electronic applications due to the fact that no discrete joining of pins to the flexible circuit is required. This approach allows for pin-like edge contacts, extending, unsupported, beyond the edge of the flex circuit. It is then possible to simply post form the leads as required to create a viable male pin connector for mating with a compatible socket of conventional construction. Edge card contacts, analogous to those found on rigid PCBs, can also be created by simply folding the contact area of a flexible circuit around a stiffener. The method pro- I vides a relatively inexpensive way to interconnect a flex circuit assembly to other system elements and is subject to the same limitations as its rigid brethren. One advantage is that there are numerous different types of mating female connecter solutions available for this type of connection. In addition, Figure 1 Making Flex Circuit Connection Directly Between Chip Packages Can Provide Significant Performance Improvements and Power Reduction While Greatly Simplifying PCB Design (Courtesy SiliconPipe) because the flex circuit itself is thin, it is possible to accommodate a wide range of different connector products by simply altering the thickness of the stiffener. One caveat is that the adhesive and unreinforced materials may creep over time so the connector should have some over drive capability to allow the contacts to follow. Surface-mounted connectors are an increasingly important and more common connector choice for use with flexible circuits for obvious reasons. With size reduction a common objective of flexible circuit technologies, it comes without surprise that low-profile connectors are another product well suited to the needs and abilities of flexible circuits. A number of low-profile connectors are presently in the market. These miniature connectors are very nicely suited to many space-constrained flexible circuit applications. Low-profile connectors offering low insertion force and zero insertion force connectors have been produced by commercial manufacturers that can handle contact pitches down to 0.30 mm (0.012 in). The profile height for such connectors can be as low as 0.60 mm (0.24 in). Another option for low-profile interconnection of flexible circuits is the creation of various lapped interconnections. For example, various materials have been used in this respect including solder, conductive polymers, and z-axis (anisotropic conductive) adhesives. Lapped interconnections using anisotropic conductive adhesives are a common method for interconnecting driver chips and circuits for flat panel displays. The method may be accompanied by the use of bumps on the flex circuit contact terminals, which can be produced by plating or mechanical deformation of the copper circuits. These methods have been suggested for service in prospective future designs where interconnection of high-speed, controlled impedance circuits is made directly between IC packages using a flexible circuit. This will ease the burden of having to design and manufacture complex printed circuits that often require several design and manufacturing cycles to get it right. An example of such a prospective construction is shown in Figure 1. In summary, making a connection between a flexible circuit assembly and a next level or same level interconnection device or system can be accomplished using any of a number of different options, both permanent and separable. The choice of which one to use should be predicated on the cost and performance requirements of the end product. The examples provided are not exhaustive in terms of options and it is recommended that the reader communicate with connector suppliers for their recommendations based on design needs. ■ Joseph Fjelstad, founder and president of Verdant Electronics and co-founder of SiliconPipe, is an author and innovator in electronic interconnection and packaging technologies. Download his flex circuit book free at www.flexiblecircuittechnology.com. E-mail: jfjelstad@siliconpipe.com circuitree.com • January 2009 17 http://www.flexiblecircuittechnology.com http://www.circuitree.com
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