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50 DESIGN007 MAGAZINE I DECEMBER 2019 When is it appropriate to add full fabrica- tion notes instead of just a simple README file, and when should you just send the Gerber file for fabrication? Each of those approaches to documentation—or lack thereof—may be appropriate, depending on which stage you're at in your design process. Test Boards and Others For test boards, bench tools, or other boards that will never be used by anyone but you, you'll probably use a prototype or run-as-sent service. For these boards, and in these scenari- os, you likely won't have notes for UL marking, electrical testing, destructive testing, cross-sec- tion analysis, and so on. The extra documen- tation you create at this stage may be minimal or none; in fact, you might send in the Gerber file alone or attach a README file that says, for example, "Nonfunctional circuit for me- chanical fit only." This will clue the fabrica- tion house that the DFM checks they normally run—checks this board will fail—are not nec- essary. Occasionally, we also see a fully docu- mented fabrication drawing with a large note telling us to "ignore fabrication notes in con- flict with manufacturer's standard processes." Early Product Development When you're developing a new product, you can expect to go through several iterations of your boards to prove out your subsystems be- fore arriving at something that starts to resem- ble a finished product. At this stage of develop- ment, when you're ordering boards, you might want more control over elements like thick- ness or layer stack-up than you took with your test boards. Because you're still in early devel- opment, keeping development moving along may be your priority. You might add an elec- trical test to make sure the board works as de- signed, but you're not adding in a lot of process controls or special handling instructions that might slow development or add costs. Docu- mentation becomes a little more robust here than what you needed for your test boards, but just enough to get what you need to continue development. Fabrication Notes and the Design Process Connect the Dots by Bob Tise and Matt Stevenson, SUNSTONE CIRCUITS

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