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JANUARY 2019 I FLEX007 MAGAZINE 25 imation—is to create a paper model of the assembly design to see what traps might lie in the road ahead. This will be instructive in determining roughly how much extra length might be required to provide a service loop, which not only facilitates assembly but also creates some intrinsic strain relief in the circuit. Later, when the layer counts that might be required and the construction are defined, it can be repeated using materials of similar thickness and stiffness to the anticipated construction. This will expose any previously unforeseen potential for buckling or kinking of the circuit during assembly. 4. Use no right or acute angles in the flex- circuit outline: Stress risers are anathema in nearly every mechanical design, and with flexible circuits having a foot in an electrical, electronic, and mechanical world, stress risers are no less important. The first and arguably most important thing one must do is to make sure that right or acute angles be avoided everywhere possible in the circuit outline. The interface between the rigid and flex circuit is one spot where it is not really possible, and this should ideally be dealt with by providing some strain relief at the interface. Commonly, this is a bead of some resin or elastomer along the edge. I have seen failures occur at this interface, and I suspect other veterans of flexible circuits have as well over the years. This was not meant to be an exhaustive treatise on the topic. As I am certain every other flex circuit engineer has, I have a number of anecdotal experiences that involve nuanced elements of the design and assembly process that have resulted in failures, but these are some important ones in my opinion. I would add that when you experience failure—and it is likely you will at some point—embrace the moment. These are teaching moments and lessons that will not be forgotten. FLEX007 References 1. Flatland: A Romance of Many Dimensions by Edwin Abbott Abbott, 1884, available for free download at Project Gutenberg. Further Reading For more detailed discussions on this and other flex circuit topics, download your free copy of Flexible Circuit Technology, 4 th Edition at www.flexiblecircuittechnology.com. Joe Fjelstad is founder and CEO of Ver- dant Electronics and an international authority and innovator in the field of electronic interconnection and packag- ing technologies with more than 150 pat- ents issued or pending. To read past columns or contact Fjelstad, click here. Figure 2: Flexible circuits are plentiful inside the iPhone and vital, enabling elements of the phone's construction. (Source: Tech Insights)

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