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24 SMT Magazine • March 2017 circuits, because this final mechanical assembly provides a rigid structure to ship the flex board on and eliminates additional handling of pack- aging and unpackaging the PCBA. Addition- ally, doing it all in one place minimizes han- dling and over-processing, because if the prod- uct is shipped to a second facility, it must be packaged, retested and often repalletized for the next assembly step. Las Marias: Are there new technologies out there that will significantly impact or benefit the flex cir- cuit assembly process? Fraser: While not directly related only to flex, more automotive lighting applications are us- ing LEDs and often these are mounted on flex or rigid flex because of packaging needs. Reflow ovens with vacuum process are beneficial in terms of pulling air pockets out of solder joint to eliminate voids. This level of defect of miti- gation is good for both flex circuits and tradi- tional PCBAs. Las Marias: Thank you very much, Steve. Fraser: Thank you. SMT STRATEGIES FOR ADDRESSING FLEX CIRCUIT ASSEMBLY PAIN POINTS by Stephen Las Marias I-CONNECT007 Flexible circuits are increasing- ly being used in most end-markets amid the growing miniaturization trend and functionality needs of electronics products. "Flex cir- cuits facilitate product miniatur- ization through 3D and that is a growing trend in most markets," says Yousef Heidari, vice president of engineering at EMS firm Sig- maTron International. When it comes to assembly, the challenge varies based on the details of the all-flex or rigid-flex- rigid mechanical design, as well as the components that need to be assembled. The solder paste printing step is also one of the key challenging operations for de- signs that have multiple areas with fine-pitch com- ponents. Heidari notes that there is no specific new equipment needed to address the challenges for typical flex circuit assemblies once the design de- tails of handling the flex circuit during assembly have been worked out. The design must ensure that the different areas of the flex circuit get regis- tered and are well supported. Since the assemblies are moisture sensitive, one needs to ensure that the flex cir- cuits and components are dry pri- or to going through the correct reflow process. He adds that they must be handled appropriately and baked prior to use. Handling is a concern throughout the as- sembly as inappropriate handling can cause delamination. Do customers call out a spe- cific brand name of material to use when dealing with flexible printed circuits? Heidari says they deal with a lot of mission criti- cal products, and typically, those customers do have preferences in all aspects of defining the stack up material and details. Overall, the design for the flex circuit assembly as well as the subsequent han- dling of the assembly before final product integra- tion have the greatest impact on the quality of flex- ible circuit assemblies, according to Heidari, while the flex circuit panel design is the biggest contrib- utor to yields. For successful flex circuit assemblies, OEMs need to work closely with the design house, the fabricator, and the contract manufacturer, espe- cially on the design for manufacturability, says Heidari. He stresses that having a good flex fabri- cator is also critical. Achieving Successful Flex Circuit Assemblies Yousef Heidari