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Design007-Nov2020

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60 DESIGN007 MAGAZINE I NOVEMBER 2020 This month's column will address what vias are and what they are used for, as well as how they are used in PCB design. I'll also cover some criteria on pad size vs. via size for fab- rication. Introduction to Vias But first, how did vias come about? That's easy. The first PCBs were all through-hole components, meaning a plated hole from one side to the other, from top to bottom. When board densities became more of an issue based on available real estate, components went from a 4-pin to an 8-pin or a 12-pin connec- tor, etc., using all through-holes to a surface- mount-type connector in an effort to use less board space. Thus, the first use of mechanical through-hole vias. The process for this is quite simple. Based on the tolerance expressed on the drill draw- ing, the vias (and component through-holes) are drilled larger (typically, 0.004–0.005") than the necessary finished hole size. If the part is a simple two-layer board, the process is as follows. The dielectric material is pulled then taken to the drilling department. Before this operation, the CAM department will specify the proper drill size to use, and a drill program- mer will set up the start and stop codes using those drill sizes for the N/C drill machine. Once drilled, the part would go through a series of cleaners and conditioners and then into a catalyst before the electroless copper stage, where the electroless copper deposition is done. The electroless stage is not an electro- winning process like electroplate and only lays down about 0.4 mils of electroless copper in the barrel of the hole and on the panel surface. This acts as a "tooth" or a bit of metal for the electroplated copper to adhere to. The Fundamentals of Via Design The Bare (Board) Truth Feature Column by Mark Thompson, CID+, MONSOON SOLUTIONS

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