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14 The PCB Magazine • February 2014 CLOSING THE GAP BETwEEN DESIGN AND FAB continues license to drive with little or no actual driving experience. It counts a lot to actually go to a PCB fab and take a tour, and fortunately most fabs with oil in their lamps are willing to oblige if you give them enough notice. I have done this my- self, recently spending a fair amount of time with a few fabs near me in California. And tak- ing tours of real PCB fabs and interrogating them has confirmed my suspicions. I can illustrate the sorts of problems that can occur between design and manufacturing with a tangible real-world example. On a visit to Sierra Circuits in Sunnyvale (also known as ProtoExpress.com) I got to walk through the entire process of preparing and laminating a rigid-flex panel "book." It was an eye-opening experience—not only for getting to see how a rigid-flex board is made, but just as much to be made aware of the fabrication process and its limitations. This was a particularly interesting example, because the PCB in question was very small and had to have thin flexible sections— about 3mm in width—between the rigid sec - tions, each about the size of a U.S. quarter. The final board prototype is shown in Figure 1. The big deal with this board was that with such narrow flex-circuit sections, it was not possible to use "bikini" coverlay for the flex. This meant that polyimide coverlay film had to extend through the entire lamination of the board, which in turn meant that acrylic adhe - sive layers had to be used to adhere the poly- imide coverlay layers to the rigid cores. This may not seem like a problem, except that the PCB is four layers in the rigid areas, and with such densely populated components on top and bottom layers, the vias were on the risky side of smallness. Why? Because the adhesive layers are known to expand in the Z-axis dur - ing solder reflow. In other words, the fab already knew that this board was going to have lowered yields in assembly caused by cracking vias, in turn caused Figure 1: difficult-to-make rigid-flex board.