We fill the thermal vias with non-conductive epoxy and cap plate over them.   In a previous life, we plated them shut.  The solder voiding in the center slug was typically more trouble than anything else until you get the amount of solder paste printed.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mark Larson
Sent: Wednesday, August 31, 2011 12:52 PM
To: [log in to unmask]
Subject: [TN] QFN with solder slug

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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