I received a couple of good questions on my prior post about vias in QFN or QFP pads:
"I have a few questions about the second photo. The thermal vias in the center are masked over, doesn't this make it difficult to get uniform solder reflow on this pad? Also, what about the height differences due to the solder mask? Finally, what would the paste mask look like for this part?"
The part I used for the illustration is a QFP, but the same would go for QFNs also. All of the issues in question are somewhat more critical with QFN parts.
First, having the thermal vias masked over does make solder uniformity more difficult. The best option is to have the vias filled and plated over at the PCB fab house. That can be more expensive than is practical in many case though, so mask is still frequently used. A smaller mask size, 100 - 125 microns bigger than the via, is preferred to the larger mask are here, but this technique is used when registration is a concern. Again, it can be a cost issue. With a properly segmented mask, as illustrated on the right, reasonable solder deposition can be achieved.
Height isn't much of an issue with QFP parts, but can be with QFNs. Again, when the mask is properly segmented, height issues will be effectively mitigated.
Stanley Yelnats doesn't like vias.