Wow, I hate to disagree with guys like Dave and Dewey, but "it depends" (Diet Dew is on the way, Doug) on many factors.
Final finish chemistries can make tenting a bad thing, plugging can be expensive and too large diameter will cause solder starvation.
We do lots of these and make sure all holes are small enough to minimize wicking with RoHS solders.
We have also found a little solder starvation (as long as there is still good wetting) doesn't cause to many reliability concerns.
We use small (smaller does not transfer heat as well but reduces wicking) vias under the part and as many large vias as can be fit around the part.
Thermal relief sorta cancels part of the value so everything gets flooded.
Also, tie to every layer possible, inner layers do not transfer heat real well but every little bit helps.
It is doable without extra fab/assy costs but may require some experimentation with your suppliers to find the right combination.
Good luck,
FNK
Frank N Kimmey CID+
Manager - PCB Design
Powerwave Technologies Inc.
Mobile - 916-670-0645
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey (EHCOE)
Sent: Wednesday, August 31, 2011 2:20 PM
To: [log in to unmask]
Subject: Re: [TN] QFN with solder slug
Vias are like puns (Dewey"isms"); they should all be in"tent"ed (capped, filled, plated-over). Thanks for agreeing.
Dewey
-----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
[log in to unmask]
Mark Larson <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
08/31/2011 02:52 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to Mark Larson <[log in to unmask]>
To
<[log in to unmask]>
cc
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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