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24 FLEX007 MAGAZINE I OCTOBER 2018 Feature by Dave Lackey and Anaya Vardya AMERICAN STANDARD CIRCUITS Designing Flex Circuits for First-Pass Success: Part 3 Editor's Note: This is the third in a series of excerpts from the book. The first two issues of Flex007 Magazine carried Part 1 and Part 2. Other Specific Issues of General Concern This series is not intended to cover all of the different issues related to flexible circuits, but instead provide a short list of key issues that often "fall through the cracks" based on our experience. The following examples are offered to help readers avoid certain traps that have ensnared flex circuit designers in the past. 1. High layer-count designs: There are a variety of challenges related to high layer- count flex designs (e.g., a 20-layer circuit with two rigid layers and 18 flex circuit layers). High layer-count designs are difficult for rigid circuit manufacturers and they are even more diffi- cult for flexible circuit manufacturers, owing in part to the inherent dimensional instability of flexible base materials. Because of this, it is highly recommended that the designer engages early on with the fabricator to understand these issues and to work through any concerns with the supplier. Every design is unique, but experience can be a great teacher and the manufacturing engineer can be a great source of help in steering clear of the major pitfalls. 2. Bookbinder constructions: The term "book- binder" comes from the days when book pages were bound in sections by sewing them down a center line. In the final construction, the center pages were more prominent and the outer pages withdrawn on the edges of the sections. Possibly it was noticed that all the pages could be made flush if they were cut to different widths before stitching, though it was generally much easier to cut the pages after stitching and folding. In flex circuits, the term

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