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42 The PCB Design Magazine • September 2014 by Amit Bahl SIerrA CIrCuITS DESIGN FOR MANUFACTURING IPC-2581B Eases Stackup Development feature column PCB manufacturers often find that impor- tant details are unclear or missing from the designs that customers submit, and such proj- ects can't move forward until the designers are reached for clarification. In many of those cas- es, it is the way design data are organized and conveyed that is ultimately to blame. Whatever the EDA platforms that were used to create them, most PCB designs are output and sent to manufacturers as a collection of Gerber files that graphically define the layers, a drill file, a netlist, a board drawing including the stackup, and a readme text of notes and instruc- tions. Turnkey orders for combined fabrication and assembly also include a BOM file and coor- dinate data for pick-and-place operations. There is no common data format among all the files: They're simply bundled together and forwarded to the manufacturer to download and interpret. And that disparity among formats can lead to omissions on the design side and miscommuni- cation with manufacturers. Now consider what occurs (or should occur) long before any complex design is completed and sent for fabrication. The initial design step leading to a successful PCB layout for any com- plex, high-speed circuit is close consultation with your manufacturer to determine the op- timum stackup. The process is iterative, a back- and-forth collaboration to select the right mate- rials; determine line widths, spacings, and layer thicknesses to meet impedance values; and minimize the number of layers and nail down the via set within budget. Here again, there's no common protocol for how that's accom- plished. The process involves telephone calls, drawings, and emails until consensus. On one hand, there's stackup information the designer will use as the foundation for the layout. On the other hand, there's a detailed stackup from a manufacturing perspective, which describes how each layer will be constructed, and if the design will involve sequential laminations, how the layers will be grouped for fabrication.

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