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52 SMT007 MAGAZINE I AUGUST 2019 Feature by Dave Lackey AMERICAN STANDARD CIRCUITS Introduction What is a flex coverlayer? What's its pur- pose? Are they new or have they been around a while? Why do they matter to my design? Coverlayers are polymer materials used to cover and protect the copper traces of the flex circuit product. As implied, there are a num- ber of different options available for protect- ing the circuits, and they serve different design requirements in terms of cost, performance, and flexural endurance optimization. When specifying the choice, it is critical to call out not just the type of coverlayer material but also the thickness requirement. This can be very important in certain types of constructions, especially when a flex circuit will experience dynamic flexing during use. It is also important to know and understand that there are different types of materials avail- able for use as coverlayer materials and that there is no single, ideal solution. The appro- priate material choice will be based on a num- ber of factors, such as application, cost, pro- jected life, etc. Coverlayer selection requires a thorough analysis that balances both cost and performance to ensure the proper choice. Key considerations include how much will the flex circuits be bent in the field (install or dynamic), can a hybrid approach be taken (solder mask in SMT locations and coverlayer everywhere else), and is the performance improvement of laser machined coverlayers worth the sig- nificant cost increase (and potentially lead- time increase). In any case, due diligence is required to make the proper coverlayer tech- nology choice. Machining Options for the Coverlayer Both mechanical and laser machining are common fabricating processes used in today's printed circuit industry. Each method employs its own distinct equipment set and has its own advantages and disadvantages. Preference for coverlayers among the two typically depends on the application, volume/lead time, and cost. Mechanical Machining Certainly, mechanical machine is the tried- and-true historical method of fabricating cov- erlayers. The primary benefit of mechanical machining is that the fabricator can use their

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