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Design007-Nov2018

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76 DESIGN007 MAGAZINE I NOVEMBER 2018 "Will my parts fit on the board?" That seems like an easy enough question to answer as you transition from the design stage to manufac- turing, but issues with parts fitting are one of the most frequent causes of delays and cost overruns. We see it every day. Designs are sub- mitted that can't or shouldn't be manufactured because the parts will not fit properly. This creates issues with performance and durabil- ity—effectively reducing the overall quality of the board. The six following tips will help you avoid common parts-related manufacturability pitfalls. 1. Avoid Pinhole Pitfalls It's essential to check component physical dimensions, consider dimension tolerances, and account for varia - tions that could impact fit. Pins could be the wrong size or have the wrong spacing, and com- ponents could be much larger than their footprint or land pattern might indi- cate. Further, alternative or replacement components could be on the wrong end of the tolerance range. After all, it just takes a few mils before things don't fit and assembly starts going wrong. Finding a good alternative part can be a chal- lenge, which is why datasheets can be your best friends. In addition to watching part sizes, pay close attention to the minimum, nomi- nal, and maximum material conditions for the original part. 2. What to Do When the Land Pattern and Pin Size Differ One of the most frustrating mismatches with alternative through-hole parts occurs when the land pattern matches, but the pin size is off. If hole sizes are too tight, pins may not fit through the holes, or if they do go into the holes, they may not solder well. Solder will need to flow through the gap between the pin and the hole bar- rel. If there is not enough space to allow enough solder mass to flow through the hole, the circuit board will absorb heat from the molten solder and cause the solder to solidify partway up the hole. This is called a cold solder joint and can result in a premature failure of your circuit. To avoid this issue, make sure that when you design your land pattern, you know the pin size and tolerance range for your com- ponents. With that information, you can plan Six Tips to Ensure Parts Fit on Your Board Connect the Dots by Bob Tise and Dave Baker, SUNSTONE CIRCUITS

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