Screaming Circuits: Unequal Pad Sizes, Take 2


Unequal Pad Sizes, Take 2

I wrote a post not long ago about via-in-pad and unequal pad sizes as a cause of tombstoning and I received a question from Aaron 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."

This is a pretty common practice. I do it myself sometimes. The risk of tombstoning or other solderability issues relates to a couple of things. First, the smaller the part, the more critical all of this is and the more likely it will be for problems to crawl up. I'm assuming we're talking about SMT parts. Thru-hole parts can have some issues with copper pours, but not near as many as can SMT.

Pour-with thermal Pour-no thermalThe first issue is that the copper pour on one side will act as a big heat sink and may lead to tombstoning, or at least a poor solder joint on the pour side. If high current isn't needed, then use thermal pads. That will help. I would guess that with a cap like you're describing, high current isn't a requirement.

The other issue Aaron mentioned was the soldermask expanding on the copper pour and making the aperture size smaller then on the other pad. You can try to make soldermask defined pads on both sides. Then, in theory, the mask expansion should be close to the same on both sides. You can also make a larger mask opening on the side that goes on the pour so that after expansion, it will approximate the other pad size. The problem with this approach is that if you change board houses, you may not have the same amount of expansion. The best option might be a call to your PCB fab house for a chat about control of the mask expansion.

Also, if you do use thermal pads, as in the illustration, that might just solve the expansion problem right there. Most CAD packages have either a global setting to make all pads in pours thermal pads or a properties setting for each part that would make thermal pads as illustrated on the right above.

Duane Benson
Pour poor pitiful me into a picture of a pitcher

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