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PCBD-June2014

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30 The PCB Design Magazine • June 2014 Component Assembly Component assembly on flex circuitry is common and similar to methods used for rigid PCBs. More than 50% of flex circuits are sold with value-added component attachment. This can be done with hand soldering techniques or hand placement of SMT components for low volume. Both traditional tin/lead solders and lead-free solders (used to meet RoHS require - ments) are common as polyimide construc- tions are very robust at high temperatures. With higher-volume parts there is custom fix- turing used which can get complicated as it is adopted to accommodate the handling nu- ance characteristic of thin, flexible materials. Both high-volume wave soldering and surface mount soldering are common assembly tech- niques. Selective stiffeners are attached with either thermoset or high temperature pressure sensitive adhesives on regions of the circuits supporting components. The stiffeners are of- ten glass epoxy materials and prevent bending the circuit at a solder joint. Fracturing traces can occur absent adequate mechanical support from a stiffener. Additionally there is some process precondi- tioning recommended when soldering flex cir- cuits, especially if they are stored in a modestly humid environment. Polyimide is considered quite hydroscopic as it absorbs up to 2% of its weight in moisture. This absorption can cause circuitry to delaminate or blister when exposed to solder temperatures. Prebaking flex circuitry at 105°C for an hour eliminates this issue and is a good general recommendation prior to solder temperature exposure. As volumes increase beyond prototype quantities, fixturing becomes a requirement for both wave soldering and SMT soldering. This fixturing will be custom and design depends heavily on the individual parts and assembly required. With wave soldering, FR-4 glass epoxy is often used to mechanically support a soldered connector. This glass epoxy can also be used as feature FLExIBLE CIRCUITRY...A 3D PACkAgINg TooL continues Figure 4: Circuits with stiffener carrier.

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