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Design007-Mar2020

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74 DESIGN007 MAGAZINE I MARCH 2020 Nobody designs a PCB, rigid or flex, without regard to cost. There is a continuing series of choices and trade-offs, some with greater cost impact than others. That topic alone could eas- ily fill an entire issue of this magazine. For this specific column, let's focus on flex and rigid- flex and discuss a few things that can easily be overlooked when putting together the puzzle pieces of a design and that may have an unin- tended impact on the overall cost. Size The cost of a flex or rigid-flex board is a de- rivative of the overall panel cost. Fabricators use varying panel sizes, with the most common panel sizes being 12" x 18" and 18 x 24". Piece price is the panel price divided by the number of parts manufactured on that panel. Manufactur- ing processes require a 1" border on the outside of the panel, leaving 10" x 16" and 16" x 24" of usable space. This is fairly straightforward in rigid designs. Most are squares or rectangles. Flex designs, on the other hand, often have unusual shapes, and how those shapes are "nested" on the production panel can have a big impact on cost. Most often, if you are re- questing a flex as a single piece, your fabrica- tor will place the parts on the panel in the most cost-effective manner. Arrays can be much trickier and present an opportunity for unin- tentionally increasing the cost. For example, a flex shaped like a "T" or and "L" are prime can- didates to "reverse nest" on the panel and flip parts around to better utilize the space. With an array, it may not be ideal to "reverse nest" for assembly reasons, and a fabricator is less likely to employ this strategy. It is highly rec- ommended that you engage with your fabrica- tor early in the process to discuss how to best utilize the production panel and drive out un- necessary costs. Another example is the overall length. Many flexible circuits are designed with a service loop. Depending on size, even 0.250" can make Are You Unintentionally Adding Cost? Flex Talk by Tara Dunn, OMNI PCB

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