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70 The PCB Magazine • April 2015 know to keep copper geometries a certain dis- tance away from the routed edges. We know about hole size and outline tolerances, copper and board thicknesses and myriad other speci- fications that we don't even think about any more because it has become an inherent part of our thought process. Flexible circuit design rules are very simi- lar! We must pay attention to all of the same issues, minimum hole sizes, minimum trace/ space specifications and distance to edge and tolerances. First, let's discuss the fabrication process. Traditional PCBs and flex are fab- ricated in much the same way for the first several steps. The flex material, typically cop- per clad polyimide, is allo- cated, drilled, plated, imaged, developed and etched just as printed circuit boards are. The next step, however, is where the changes occur. The pan- els must be baked to remove moisture from the wet pro- cesses, then however where a PCB would go to a solder mask station, flex circuits go to a cover layer station. The insulating layer of a flexible circuit is made of poly- imide the majority of the time. This is not a screen process as it is in PCB fabrication, it is a lamina- tion process. Therefore the rules for oversize of openings are a little different. The cover layer material is made of 1-mil thick polyimide with a 1-mil thick adhesive attached. The material is drilled to create the openings that expose component pads. There- fore tolerances of drill location and size apply. Also during the lamination process the ad- hesive is heated to a temperature that allows it to flow easily. It must fill in all of the gaps between traces and pads so that there is no air trapped between layers. What this means to a flexible circuit designer is that the cover lay- er openings for a typical 1 oz. copper design needs to be oversized by .010" (10 mils). This is significantly larger than the typical PCB solder mask which oversize's 2–3 mils. The reason is first the tolerance of drill size and location, but also to account for the adhesive which squeezes out into the openings. We want to design such that the adhesive flows out to the pad, but not on top of the pad. That would affect the size of the annular ring. We call this phenomenon squeeze out and we want it to dam up on the thickness of the copper pad and not flow over the dam. Another area we must worry about that isn't common with PCBs is the "flex area." Some rea- sons for using flexible circuits are simply size and weight, but many of the ap- plications take advantage of the flexibility and use the circuit in this manner. There are two typical ways this is done. • Flex to fit: The circuit is flexed once only to fit into the assembly • Dynamic flex: This circuit will not only flex to fit into the assembly, but will be dynamic during operation Either scenario requires a bit of thought be put into the trace routing and pad place- ment in that area. First it's best not to have any components (solder pads) in this area. Whether the flex is formed once for fit or dynamic, the solder joint still will be the weakest part of the circuit. Solder, RoHS or leaded, is rigid and not intended to bend, flex or twist. Therefore if those joints are in the flex area you will likely see a fractured solder joint at some point. It's best to keep all solder points at least .100" (100 mils) away from flex areas. Further is better if real estate allows. Traces should route through the flex areas perpendicularly. This allows us to take advan- tage of the malleability of the copper. Rolled annealed copper has grain and if run horizon- tally in the flex area, may split or fracture leav- ing the engineers an intermittent issue to try to find which is very frustrating and difficult to the insulating layer of a flexible circuit is made of polyimide the majority of the time. this is not a screen process as it is in pCb fabrication, it is a lamination process. therefore the rules for oversize of openings are a little different. " " Flex mAtters DESIGN CONSIDERATIONS: FLExIBLE CIRCUIT VS. TRADITIONAL PCB continues

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