Hi Mark,

1.  You should read the QFN manufacturer application notes / guide rules
2.  Via holes in thermal pads, should be opened both sides
3.  The via hols should be layout, symmetrical from the center line of the
thermal pad.
4. Avoid via holes in pads termination or between termination pads to
thermal pads.

For full layout instruction, you can mail me offline.

Reuven

On Wed, Aug 31, 2011 at 10:52 PM, Mark Larson <[log in to unmask]> wrote:

> I am curious how others are handling these parts for soldering. Do you
> leave the thermal vias in the slug covered or uncovered with solder mask? Do
> you fill with anything? What size hole on a 62 mil board? Tie to all GND
> planes or just the back side? Thermal relief the via or bury it in the
> plane?
>
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