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46 The PCB Magazine • June 2014 on regions of the circuits supporting components. The stiffeners are often glass epoxy materials and prevent bending the circuit at a solder joint. Fracturing traces can oc - cur absent adequate me- chanical 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 ep - oxy is often used to mechanically support a soldered connector. This glass epoxy can also be used as a carrier fixture with individual parts broken from the panel after component sol- dering. Carrier plates are also used with cavity openings allowing access to reflowed solder in a wave soldering machine. Fixturing for SMT assembly is often done with vacuum assist or by bonding the flex pan- el to a low tack carrier plate. Both methods al- low the panel to be held in place through pro- cesses including solder stencil, SMT placement and reflow. Pre-cutting or routing the individ- ual parts prior to assembly allows them to be easily removed from the panel post assembly. The myriad of individual parts and variety of assembly requirements leads to a wide variety of fixture options. There are companies special- FLExIBLE CIRCUITRy...A 3D PACKAgINg TooL continues izing in this type of tooling that can also assist with ideas and custom fixtures. Summary Flexible PCBs are used extensively through- out the electronic market for a wide variety of reasons, including their ability to move in an application, their light weight, and extreme thinness. They can also be used in high current- carrying applications and are often populated with components with assembly processes typi- cal of rigid printed circuits. There are certain important design, processing, and handling considerations that separate flex circuits from their rigid cousins. Many suppliers offer printed design guides and early involvement of a flex supplier is recommended as a design unfolds. This early involvement will help insure proper tolerances, costs, and constructions are consid- ered for a robust and reliable adaptation of the technology. PCB dave Becker is the V.P. of sales and marketing at all Flex Flexible circuits and Heaters, which specializes in low to medium volume flex- ible circuits and flex circuit assembly. Figure 4: circuits with stiffener carrier.