Issue link: https://iconnect007.uberflip.com/i/1073397
62 FLEX007 MAGAZINE I JANUARY 2019 completely and will show up as low-resistance shorts (due to moisture-containing air) at elec- trical testing. Second, if your design has high- and low- pressure areas adjacent to one another, it can be very difficult to get enough pressure on the lower pressure areas to get them to yield. An example would be a board where one side of the design is eight layers of copper with many of them being ground planes, and immediately adjacent to that area is four layers of all sig- nal. It can be difficult to get enough pressure on the area of the board with four signal lay- ers. Another example would be pad stacking of non-functional pads on high layer-count boards. The pads form a very high-pressure area, and immediately adjacent to that is a very low-pressure area where air pockets will form. Third, because we use lamination materials that are designed to conform to the surface of the board, the surface of a rigid-flex board will have far more topography than a typical rigid board. Rigid boards often come out of lamina- tion with a very smooth surface as the outer layers of the board are right up against the lamination separator plate. Rigid-flex boards will more typically have a surface that can vary by ±0.005", which will affect subsequent processing, particularly filling through-holes for via-in-pad applications. Outer Layer Drilling After lamination, your boards go to drilling to add what will become the plated-through- holes. The drilling operation is identical to drilling conventional PCBs but with much more conservative parameters. The internal layers of a rigid-flex construc- tion have conventional HTE copper foils, glass- reinforced laminate, the polyimide base mate- rial of the flex laminate, and rolled-annealed copper foil on the flex. Each material has an optimum feed and speed for the drill, hit counts for the drills, and stack heights for the machine, but they each are also different from one another. The rolled-annealed copper foil is very soft. If we are too aggressive on any parameter, it will tear rather than drill the copper foil, resulting in nail-heading—a drilling defect. Similarly, any tearing or gouging along the flex-to-rigid interface will result in a plating condition called "folds" where there is a local - ized area of plating weakness that will fail during thermal excursions or when faced with other stressors. Laser Drilling of Microvias Earlier, we mentioned that the surface of your boards will have some topography that can be as great as ±0.005" from peak to val- ley. The same is true for any blind via pads that you are trying to reach successfully from the outer layers of the board with a laser via. They can vary across the panel in their vertical position from the laser beam. Lasers are simi- lar to cameras in that they have an ideal focal length. If the beam is too close, the via will receive too much energy and mushroom. If the focal length is too far and the via is incomplete or has some remaining resin on the surface of the via pad, that will result in an open circuit. Care is required to find the correct laser recipe to drill all of the vias with the correct energy accurately. Fabricating Filled Through Vias for Via-in-Pad Fabricating filled through-vias in rigid boards is a very common technique for providing designers with a via-in-pad solution, providing greater densities in their design, and is espe- cially helpful in high-density areas like BGAs. The surface topography that is typical to rigid-flex constructions, along with the cavi- ties created in the panel by the flexible areas, make fabricating filled through-holes on these boards very difficult and low yielding. We do build them, but every effort should be made to rout your boards in such a way as to not need filled vias for via-in-pad. To understand why, we will review how via filling and planarizing are accomplished on a rigid-flex board. After plating copper into the via-fill holes, the via-fill materials are epoxy based and are conventionally squeegeed out onto the panel under pressure to force the via-fill material