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PCB-Sept2016

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32 The PCB Magazine • September 2016 plains why the final assembly failed the burn test. Lesson Learned: Always clearly communi- cate UL requirements and include the require- ment in both the PCB fabrication notes and the assembly notes. Coverlay There were many stories along this line, but this one is classic; we have both seen this more than once. A particular application on a tiny flex circuit requires a very tight pad pattern. Standard, adhesive-based coverlay, was called out in the stack up. As the flex manufacturer was setting up the tooling, they asked if that area could be "gang opened" because the tight features would cause fabrication issues when aligning the drilled coverlay. That is a very com- mon question that I have seen asked and ap- proved hundreds of times. The designer agreed that this would be fine and that pad location was left free of coverlay. But once the parts ar- rived at the assembler and they went to screen print the paste, the area shorted out. The prob- lem was ultimately solved by using photoim- ageable coverlay to accommodate the tight fea- ture pattern. Lesson Learned: Review even the standard requests with a critical eye for the next processing steps the flex will see after fabrica- tion. The size of this particular flex combined with the tight features was the perfect combina- tion to cause an issue with something that is routinely done. Bend Radius By definition, flexible circuits are designed to bend, fold, and flex during installation and/ or use. That doesn't mean that the copper will not crack or break when it is overly stressed. There are two very important things to be aware of. First, let's look at rolled annealed (RA) copper versus electrodeposited (ED) copper. There re- ally is a significant difference in ductility. With a tight bend radius, or for a dynamically flexing application, specify RA copper. Second, involve your fabricator. The flex manufacturer is only going to see the design in a two-dimensional view. They will not know exactly how this is going to be used in your final assembly. If you are concerned about bend radius or otherwise stressing the copper, ask for their advice. There are many different tricks of the trade that a flex fabricator can recommend to ease the stress on the copper and improve performance. Use their knowledge! Array Configuration for Assembly It is common knowledge that assembling flex can create challenges. A lot of trial and er- ror is done to find the best way to handle it. Flex circuit size, array configuration, component placement and stiffener requirements all play into the decision, which just may be equal parts art and science. The first decision is whether the assembly will be done by hand or machine. If the assembly is not done by hand, whether to use a stiffened array or machined pallet needs to be determined. Here are a few examples: 1. For a small flex, with a few components on just one side and no stiffener requirements, consider creating an FR-4 stiffener pallet with adhesive on the outside perimeter only (Figure 1). After assembly, the flex can easily be peeled away from the stiffener pallet. Caution: a stiff- ener pallet with adhesive in selected areas only can easily be misunderstood during fabrication. Make the objective very clear in the fabrication files. 2. For a long flex with stiffeners, we suggest cross-hatching the copper, or adding in a cop- per pattern to maintain as much of the copper TROUBLESHOOTING FLEX CIRCUIT APPLICATIONS FOR MIL/AERO PROJECTS Figure 1: Array option with adhesive frame bonding the flex array to FR-4 for assembly.

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