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46 The PCB Design Magazine • February 2014 3. Etch and drill compensations not taken into account at the design and lay- out stage This can mean the difference between a quote and a "no bid" from a fabricator. Do not make the pads only .005" over the finished hole size or you will get a phone call from your fabricator, as this does not allow for drill compensation plus a minimum of .002" per side annular ring pad/hole. The same goes for slots or other edge-plated features. Additionally, make sure the design allows for etch compensations based on the copper weight you are specifying. Again the rule of thumb is half mil for every half-ounce of start- ing copper. Specific information about etch and drill compensations can be gleaned from the fabricator as different shops require different process minimums. Add to that, if your design needs to meet IPC, these things should have al- ready been factored into the design. Drawings and Data Don't Match Sometimes this happens due to rounding er- rors from metric to imperial or vice versa, but sometimes it happens because the part has been revised and the note is no longer applicable. Re- member to review the drawings to make sure all information contained is valid and/or still required. These are but a few examples of things a CAM department will run into. I hope this has helped you understand a small part of the CAM process as it relates to board fabrication. Feel free to send me your feedback! PCBDESIGN UNDERSTANDING THE TyPICAL CAM PRoCESS continues CliCk To View Video Interview Mentor graphics' Bruce Caryl discusses the company's move to incorporate 3D capabilities into high-speed simulation. is 3D the future of eDA? 3D for High-Speed Design by Real Time with... PCBDesign007 Mark Thompson is in engineer- ing support at Prototron Circuits. To read past columns, or to contact Thompson, click here, or phone 425-823-7000, ext. 239. the bare (board) truth

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