I haven’t written about via in pad in a while, but the issue hasn’t quite gone away yet. This particular QFN, below, has the vias tented, which is good. However, it could be better. If you look close, you’ll see that they’re tented on the bottom of the board with solder mask.
Tenting on the bottom will usually prevent solder spillage out on the back side of the PCB, but with vias this large, the solder will probably flow down into the space, leaving quite a bit of voiding under the part. Sometimes outgassing will pop open the little tents too causing the spillage. And with immersion silver boards, outgassing can cause corrosion in the vias if you have the bottom tented and the top also sealed — like by the part.
If it’s a low speed, low temperature QFN that just needs a little ground connection to the center pad, that voiding might not matter. But, in most cases with QFNs, you need minimal voiding for thermal or noise reasons.
The best option for manufacturability is always to have the vias filled and plated over at the board house, but that can be expensive. If you are going to tent with solder mask, this next image illustrates the three ways to do it.
A is the best: a cap on the component side about 100 to 125 microns bigger than the via. B, a larger cap on the component side, or C, a cap on the bottom, will also work but both come with a greater risk of excessive voiding.
Do solder mask tents need a rain fly?
In Oregon – probably yes.