I am a PCB design engineer, and we have been getting conflicting
requests from our assembly houses regarding thermal pad styles. I am
hoping some technetter assembly folks can give some guidance as to
good practices for reliable mounting.

This part is a good example of the type of part in question:
http://www.linear.com/designtools/packaging/qfn/UHF38.pdf

In general, for most thermal pads we create our footprint matching the
recommended solder pad layout, and the stencil house windows the paste
appropriately for correct mounting. (the vias connecting the pad to
planes are outside of the thermal pad boundary of the part)

Our PCB assembly company we use for production in Japan has not had
any problems with this, and our QFN-style parts have mounted fine on
many different boards.

We have been using a local (US) assembly company to do our prototypes,
and we have been getting some conflicting requests from them. They
have requested that we add vias to the thermal pad, not for thermal
sinking reasons, but to "wick" away extra solder to make the parts
settle properly. In the spectrum of modifications to a thermal pad,
this sort of sounds like the worst case. Some web evidence seems to
agree with that:
http://blog.screamingcircuits.com/2009/01/speaking-of-common-qfn-issues.html

Our understanding has been that in the best case for vias in the
thermal pad they should be plated shut or otherwise plugged (coplanar
with the pad). Another option would be to window the paste as to not
overlap the vias (we have seen various app notes from IC vendors
suggesting this strategy) but that is still risky.

What is the correct answer? We have been going on the standard of
trusting the assembly company, and doing what they have qualified as
good practice, but we are at a point where the recommendations
conflict.

Thanks for any suggestions,
Tom

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