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66 The PCB Magazine • August 2017 Here are a couple of examples. First, you are working with two different designs. One is a sin- gle-sided flex with 0.010" line/space. The second is a complex, 16-layer rigid-flex with stacked mi- crovias. Your approved supplier list consists of three fabricators who offer flex: Company A manufactures primarily single- and double-sided designs, Company B manufactures both flex and rigid flex, but typically works with designs that are 10 layers or less and Company C specializ- es in complex rigid-flex. It can get a little tricky. It is very likely that the company that will have the best lead-time and pricing for a complex rig- id-flex will not have the best pricing for the sim- ple flex. If cost isn't a factor, it can be easier to order both from the same fabricator , but if cost is a factor, then finding the best fit for each tech- nology level is going to be most cost-effective. The second example has to do with under- standing the capabilities matrix for each suppli- er. It is important to understand for each sup- plier that you work with, what is considered standard, advanced and emerging technology. Using drilled hole size as an example, certain manufacturers consider a 0.10" drill to be stan- dard and increased costs are incurred at 0.008". With others there is no increase in cost until you reach .006" drill. This in no way reflects on the quality of the product at each manufac- turer, but more reflects their comfort level and their specific cost drivers at a certain level of technology. Once you understand where those thresholds are, you can thoughtfully weigh the cost vs. benefit of moving beyond the standard technology. Select common materials that are in stock There are many different types of materi- al available for flexible circuits, and that num- ber grows exponentially when you consider rig- id-flex construction. To simplify, using the stan- dard copper/polyimide laminates as an example, the laminate is available in two types: adhesive- based and adhesiveless material. For both types, there are a vast number of combinations of ma - terials. Copper is typically available in ¼ oz. to 2 oz. copper and polyimide thicknesses typically range from 0.5 mil to 6 mil. Sounds great, right? Absolutely! But while these options are avail - able, it does not mean that they are all common- ly stocked at a fabricator or that they are low cost. The best advice I can give when designing Figure 1: Example of a rigid-flex stack-up. Image source: I-007eBooks, The Printed Circuit Designer's Guide to...Flex and Rigid-Flex Fundamentals. RIGID SECTION FLEX SECTION RIGID LAMINATE CORE 10 MILS COVERLAY 1 MIL POLYIMIDE ADHESIVE 1 MIL ACRYLIC 1 OUNCE COPPER Adhesiveless Polyimide Flex 1 mil RIGID SECTION LAYER 1 LAYER 2 LAYER 3 LAYER 4 RIGID LAMINATE CORE 10 MILS RIGID LAMINATE CORE 10 MILS RIGID LAMINATE CORE 10 MILS NO-FLOW PREPREG 6 MILS NO-FLOW PREPREG 6 MILS NO-FLOW PREPREG 6 MILS NO-FLOW PREPREG 6 MILS ADHESIVELESS POLYIMIDE FLEX 1 MIL 1/2 OZ. COPPER 1/2 OZ. COPPER 1/2 OZ. COPPER 1/2 OZ. COPPER COVERLAY 1 MIL POLYIMIDE MASK MASK MASK MASK ADHESIVE 1 MIL ACRYLIC 1 OUNCE COPPER KNOWLEDGE IS POWER