Screaming Circuits: A Few More Via In Pad No-no's


A Few More Via In Pad No-no's

Move via to the left This first footprint will result in a minimal connection at best on the pad on the left. The via takes up nearly the whole thing so the pad barely has any metal and the open via will suck the solder down in it. There was plenty of spacing below, above and to the left on this board, so the easy solution would simply be to move the via in one of those directions far enough so that it's not in the pad and there is a web of solder mask between it and the pad.

Move via and equalize pads The second image has two problems. It has a similar via in pad issue and it also has unequal pad sizes. Having a small pad on one side and a larger pad on the other side encourages tombstoning.

Unequal pad sizes will likely exhibit different amounts of surface tension in the solder, which is a cause of tombstoning. The solder paste may melt at different times on the different sized pads due to different thermal mass, again a common cause of tombstoning in small parts. Also avoid putting different width connecting traces on the two different pads. Watch for anything that may act as a heat sink on one pad and not the other - large traces, inner layers, big components nearby. Things like that.

Duane Benson
Boot Hill is six blocks that way and to the right sixty paces.


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Hi Duane,

Thanks for all your blogs. I've learned a lot from your various tips.

In regards to this blog, I had a question about unequal pad sizes.

In my designs, I have ground pours that directly connect to pads. So as an example, a cap that goes from vcc to ground would end up with one pad attached to the ground pour. Because of the solder mask expansion, the ground side pad will be noticeable larger. What would you recommend?

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