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JANUARY 2019 I FLEX007 MAGAZINE 21 Lots of Prototyping and Testing If you have read some of my earlier PCB Norsemen columns, you probably understood that my heart pounds for PCB standards. At Elmatica, we value transparency and predictability, and IPC standards for PCB contributes to this, aligning the industry by following the same set of guidelines. When I started with FPCs, the focus was not on standards but tests and failures. It was all about finding the solution and having the knowledge and experience to know what was working or not. The customers relied on the suppliers to find their way around the challenges offered to them and the right solution for that exact flex circuit. The result was often a large number of prototypes and endless rounds of testing. I remember how satisfying it was to find a solution for a customer who had been elsewhere and was ready to throw the entire project in the bin. Now, the situation is different. Being a part of the development of several IPC standards, I often get that "throwback Thursday" feeling. It's quite funny to spot how many of our early recommendations are implemented into IPC- 2223D. Impedance: A Lesser Challenge With Improved Base Material Stability Today, we see more and more flex circuits with controlled impedance. This was a big challenge in the '80s. With the adhesiveless materials we have now—with improved base material stability and thickness tolerances— signal characteristics are much more predictable than with the adhesive-based materials we had before. In fact, some of the most advanced circuits with the smallest tracks and gaps are made on flexible materials. In my work with IPC developing a standard for medical applications, we see a trend in applications such as pacemakers and defibrillators where flexible polyimide materials are widely used. Whereas flexible design in my early flexible days (no, I was not a yoga master back then either) followed more or less the same design rules, the situation in 2019 is quite different. We see a big difference between those simple designs typically manufactured in volumes with general tolerances compared to those extreme high-end designs where dedicated state-of-the- art manufacturing equipment is required, and IPC Class 3 is barely a starting point. Thinking in Three Dimensions Is Still Vital In both ends of the design scale, a 3D understanding of the application is vital to provide sufficient DFM support, not to mention the specification. A complete specification (preferably digital) together with a 3D drawing is crucial to give a complete picture of the FPC and secure a product in compliance with the customer's expectations. It was like that in the '80s, and it still is today. Happy flexing! FLEX007 Jan Pedersen is a senior technical advisor at Elmatica. To read past columns or contact Pedersen, click here. Figure 2: Choosing a flexible circuit offers multiple advantages; thermal management, and increased design possibilities are just two.