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Have you been using common pads in your designs? Please provide your feedback

Mark Laing

Mark Laing

Posted Mar 19, 2013
2 Comments

Hi there

I have heard of a couple of cases of PCB designs where a common pad is shared between two separate components. At it’s simplest case, two 0603 components lie end to end but instead of four pads, two for each, there are only three in total. The two pads closest to each other overlap meaning only one is visible on the board. In other words they share a common pad.

This scenario is typically done when there will be build variants on a single PCB layout but I have heard that in some cases instead of just one component of the two being fitted, both components would be fitted to a variant. I would think this would cause problems with solderability and placement accuracy when both are fitted hence why I would like your feedback on this subject to hear your views on the different scenarios.

Thanks

Mark

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It all depends. Pad sharing isn't a good idea, but it could work in some cases. The example you mentioned could certainly cause parts to tombstone. The component ends where there is a single part on the pad will reach the melting point of solder first and the wetting force on that end may cause the part to stand up. On an 0603 that may or may not happen. Try it with an 0201 and the will stand up and salute every time. Pad size, thermal mass of surrounding parts, solder paste and soldering temperature are other variables. Two pads with a normal trace between them will avoid the problem.

Jim Compton
10:01 PM Apr 28, 2013

Hi Jim Thanks for taking the time to respond to my blog post and sharing your thoughts on this topic. Sharing pads is certainly not something that appears to be commonplace but it came up from two different users of our software within a few weeks of each recently. Appreciate your input. I would like to ask for anyone else to provide their input on this subject. Mark

Mark Laing
2:30 PM May 8, 2013

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