Another problem with the QFN center pad is the thermal vias in the  pad, and 
their effect on solder distribution.  We don't get a lot of feedback from 
manufacturing, but we do know we were having soldering issues, even after 
segmenting the stencil.  They are using a 4 pad pattern, with a step down, 
but were still complaining.  I moved the thermal vias into the web of the 
stencil (non-pasted areas), thinking maybe the vias were either drawing solder 
away unevenly or outgassing and lifting the part.  I don't know the effect it 
had, but the complaints stopped.  

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