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PCB-Nov2017

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30 The PCB Magazine • November 2017 Via Formation When lasers are used to create vias, the dif- ference in ablation rates between the glass fiber and the surrounding resin can cause poor hole quality. Also, the fiberglass cloth is not uniform due to having areas with no glass, areas with one strand, and the intersections of strands (also known as knuckles), making it difficult to set up drilling parameters for all these vary- ing materials. Usually the drilling is set up for the hardest to drill region which is the knuck- le area. Figure 1 shows some examples of poor hole quality due to laser ablation of glass fiber- reinforced dielectrics. It seems many fabricators opt to not employ a glass etch in these types of conditions. A properly controlled glass etch will aid in removing the protruding glass fiber bun- dles and enhance overall plating uniformity. When migrating HDI manufacturing over to laser-drillable prepregs, improvement in via quality is achievable. These laser-drillable pre- pregs are made using spread yarns in both the warp and fill directions. Thus the reinforcement is more uniform helping to minimize the areas with no glass fiber, as well as the knuckle area [1] . Mechanical drilling is a proven technology that covers a large range of via diameters with high aspect ratios (depth to width). It is most economical for through vias and blind vias larg- er than 200 μm (8 mils) in diameter. To bridge the gap to microvias, special techniques are used to achieve depth-controlled drilling of small via sizes. High-speed spindles are combined with different kinds of depth sensors. Thanks to a technique called electric field sensing (EFS), a very high-precision blind via can formed. EFS is based on a simple antenna theory where the pressure foot is flooded with a low power mi- crowave field. The drill bit is used as an antenna to sense this field and monitor the output sig- nal. The drop of the signal indicates the drill bit touching a metal surface such as a copper sur- face of the board. From this "ZERO" position, the Z-axis drills with an accuracy of I5 μm (0.2 mil) into the board without the use of mechan- ical parts which are prone to wear and tear or optical elements which are prone to debris [3] . With improvements in software and drill bit quality, it is possible to manufacture blind vias mechanically. Generally, one must recog- nize that there are limitations as to the depth and minimum via diameter that are attainable. Yet, the capital cost for a mechanical drilling tool maybe already be fully depreciated, mak- ing the move to HDI fairly easy from a capi- tal equipment standpoint. This may allow for a cost competitive solution to via formation. The use of existing mechanical drilling equipment for microvias offers a great opportunity to start manufacturing microvias. Some of the depth control systems are even available as upgrades for existing machines. With very limited capi- tal expense, the cost model for mechanical drill- ing is very simple and is reduced to the cost of operation. An example of mechanically formed blind vias are shown in Figure 2. MOVING INTO MICROVIAS, PART 2 Figure 1: Via formation with glass fibers protruding. Figure 2: Mechanically formed via—note lack of taper.

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