QFN Solder Mask Issues

Here's another QFN oopsie. Presumably, in this case, the flag pad on the QFN is needed as a heat sink and those vias are designed to conduct heat to the other side of the board. That's all good. But, with the solder mask covering everything but the vias, it won't work. I'm guessing this was an accident.

QFN-reverse-mask The stencil looks okay. It's segmented to reduce the amount of paste. We here at Screaming Circuits recommend 50 - 75% paste stencil coverage, as do most QFN component manufacturers. But, again, on this board, it will all go to the wrong place. It will wick down the vias to the other side and may not connect the pad to the part at all.

The best option would be to have the vias filled with a thermally conductive material and plated over. Most fab houses can do this as a matter of course these days. The second option, would be to use slightly smaller via holes and reverse the mask so solder mask is capping the via holes and the rest of the pad is exposed for soldering.

Duane Benson
And keep moles out of your via holes too

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Comments

The vias in my photo look to be 12 or maybe even 15 mils. If you can't cap or fill them, then try to get the vias as small as your board house will let you - 6 mils is a good target.

What size drill holes would you recommend for the vias in the centre pad?

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