CircuiTree - September 2008 - (Page 24)

[ Flexible Thinking ] By Joe Fjelstad Back to Basics Part 20 – Design Features That Provide Tear Resistance F lexibility is the obvious primary beneﬁt of using ﬂexible circuits. This important feature allows for elements of an electronic system to be moved relative to each other either dynamically, such as one might ﬁnd in a disc drive readwrite head or a laptop computer hinge or statically folded into a shape that allows the assembly to be packaged in a smaller volume, which is the most common type of ﬂex circuit application. Without regard to operation and application, all ﬂex circuit designs should be made tear resistant. The reason is that the thin, non-reinforced materials used for ﬂex are vulnerable to tear initiation, which is normally followed quickly by tear propagation and prospective or ultimate failure. However, in spite of the limitations of the material, tear resistance can be improved by employing certain well-established design features in the design. There are actually several possible methods, which are described as follows and illustrated in Figure 1. process. Though not ﬂexible, this method has been shown to provide a very robust corner construction and has been favored in the past by military product designers. It is an expensive solution, however, because of the type of preparation required and should be used only after careful consideration of the alternatives. the entire outline of the flex circuit, is a unique method to stop tearing of flex circuits. Such thin polymer fibers have very high strengths and are very pliable, minimally affecting flexibility. However, this is a labor-intensive method and should only be specified with the knowledge of a cost impact. The choice of which method used to prevent tearing of the ﬂex circuit is not of overriding importance. What is important is that the designer makes certain that some protection against tearing is used. In summary, all internal corners on a ﬂex circuit design should be provided with some radii and square or sharp internal corners should be rigorously avoided. Even if the area of the corner is to be permanently bonded to a rigid base, it is nevertheless recommended that a radius be used so that it becomes an automatic practice. ■ Use Fluoropolymer Coverlayer The use of fluoroplastics such as Teflon as coverlayers helps to improve tear resistance by virtue of the high tear resistance of the polymer itself. This is due to the fact that fluoropolymers tend to stretch rather than tear adding toughness to the substrate. An additional benefit of using fluoropolymer coverlayers for those involved in high frequency design is the fact that dielectric constant of the coverlayer is much lower. ® Use of Radiused Slots The use of slots having ends with a radius to access relieved circuit features also can serve to provide tear resistance. Normally such features can easily be provided for during the punching operation or other circuit fabrication process such as routing. Radius All Internal Corners The ﬁrst line of defense against tearing is to make certain all internal corners are provided with as generous a radius as possible. This is a fundamental practice in ﬂex circuit design and is the most important and simplest of all methods used to prevent tearing of the ﬂex circuit material. Drilled Holes at Corners or Ends of Slits For ﬂexible circuit designs with closely spaced tentacles, drilled or punched holes can be provided at the end of access slits. Such solutions have been used with success when ﬂexible appendages must be spaced very close to one another. This method allows the greatest use of material but the hole size chosen will impact tear resistance. If the hole is very small the overall robustness as a tear resistance feature will be reduced so care is advised. Figure 1 A Number of Methods for Tear Restraint Work Well for Flex: 1) Large Radius in Corner; 2) Embedded Glass Cloth; 3) Recessed Slot; 4) Hole in Slit; 5) Drilled Hole at Corner; 6) Embedded Aramid Fiber; and 7) Extra Copper in Corner Leave Tear Stop Metal in Corners If possible, the circuit designer should leave small areas of copper at internal corners to serve as tear stops at the inside of corner radii. This serves to prevent further or immanent propagation of a tear through the polymer after initiation should a tear in the material start. 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 Laminate Glass Fabric in Corners Glass cloths can be and have been laminated into corners during the fabrication 24 September 2008 • circuitree.com Provision of Fibers Inside Cut Line As an alternative to glass cloth, the use of strong material of fibers of nylon or aramid, routed through corners or along http://www.flexiblecircuittechnology.com http://circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - September 2008

CircuiTree - September 2008 Contents My Line Industry Review Tech Talk Flexible Thinking New Halogen-Free Materials: Their Time Has Finally Arrived Asian Section IPC Issues PCB and Package Convergence Ask the Flexperts Market Outlook IPCA Showcase Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - September 2008

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