Hi Mark,

I should have said that I do normally tent the vias (with soldermask on the
far side from slug).
This would help reduce thieving, which as Dave notes should be avoided.  I
remember following a mfr. white paper on thermal conduction of pth, and was
surprised at how few were needed.  Tenting the vias spoils some of their
thermal use, but it's a trade-off, and I've never been presented with solder
or thermal problems caused by the vias used under the part with the scheme I
described.

Richard

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, August 31, 2011 2:12 PM
To: [log in to unmask]
Subject: Re: [TN] QFN with solder slug

Hi Mark - if you leave the thermal vias open then the solder will thieve 
down them leaving the center pad "solder starved". I recommend you use 
some fabrication format which permits the vias not allowing solder to 
thieve - aka either capping them or filling them. The IPC-7093 standard on 
Bottom Termination Components also may have some helpful information.

Dave Hillman
Rockwell Collins
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Mark Larson <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
08/31/2011 02:52 PM
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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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