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46 FLEX007 MAGAZINE I APRIL 2019 bly because the processes are planar as well. Thus, it is wise to avoid these constructions if at all possible to improve both manufacturing and assembly yields. The second one is where the flexible layers are placed on the external layers of the board. These designs are rarer, but they do come in from time to time. Rigid-flex boards with flexi- ble layers as the external layers share the same risk of warp as any asymmetrical board con- struction due to the CTE mismatch between flexible materials and glass-reinforced materi- als. They also struggle in photoimaging and plating. Depending on the thickness of the base flexible laminate(s), the distance between the rigid boards, and the width of the circuits being imaged, these designs will have signifi- cant yield loss—typically, over 50%. Also, depending on your fabricator, these designs might be unmanufacturable. The reason is that the flexible areas of the board deform during dry-film lamination, which is crucial to faithfully reproduce the cir- cuits on your design. A material layup with the flexible layers on the outer layers is shown in Figure 2; layers four and five are flexible laminate. During dry-film lamination, the lam- inator applies pressure to the dry film, so it adheres to the base laminate. The rigid layers remain stable, but the flexible layers conform to the pressure, creating pinch points at the rigid-to-flex interface. The strain deforms the dry-film photoresist and can even deform the flexible layers at the same time. The copper on the flex layers is very soft—rolled annealed copper foil—and is prone to stretching easily when stressed. Rigid-flex Designs With Flex Arms A very common technique in rigid-flex design is to have one or more of the flex arms terminate in a flexible board rather than a rigid board. These are often used to accommodate ZIF connectors, hot bar solder joints, conven- tional connectors, through-hole assembly, etc. An example of a conventional rigid-flex board and a rigid-flex where one arm is a flex- ible board are shown in Figure 3. The board on the right is a conventional rigid-flex board where each arm terminates in a rigid board. The board on the left is very similar, but the center arm terminates in a flexible board designed to mate with a ZIF connector. This flex arm was designed to accommodate in- circuit test of the final assembly and was then removed and discarded. Rigid-flex designs similar to the board on the right are generally very high yielding and run through the manufacturing sequencing with few or no issues, which results in lower costs Figure 2: Pressure points at the rigid-to-flex interface.