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September 2011

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Subject:
From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Thu, 1 Sep 2011 10:42:45 -0500
Content-Type:
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Hi Karen - I was wondering when someone was going to ask that question. 
The IPC-7093 has some good information on the topic (section 7.6.1). There 
is no current requirement in JSTD-001 that I am aware of. The IPC JSTD-002 
requires that the ground pad have a minimum of 80% wettability. Most folks 
in the industry I have talked to are use a 50% minimum solder coverage 
unless the QFN has a specific thermal or grounding need for better 
coverage.

Dave



Karen Tellefsen <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
09/01/2011 10:30 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Karen Tellefsen <[log in to unmask]>


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<[log in to unmask]>
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Subject
Re: [TN] QFN with solder slug






What about voiding under these little monsters.  A lot of our customers
complain about voiding under the QFN ground slug.

Karen Tellefsen - Electrical Testing
[log in to unmask]
908-791-3069



  
  
  
   Re: [TN] QFN with solder slug  
  
  
   Wenger, George M.  
                     to:  
                        TechNet  
                                                        08/31/2011 05:57 
PM 
  
  
  
  
   Sent by:  
          TechNet <[log in to unmask]>  
   Please respond to TechNet E-Mail Forum, "Wenger, George M."  
  
  
  






I agree with Frank about disagreeing with Dave and Dewey and I agree with
him about final finish, and cost associated with tenting and plugging.
However, I also disagree with Frank about using small vias. A small via is
a much better capillary for wicking than a larger via.  The "it depends on
many factors" is absolutely correct.  Our concept is to design (i.e., size
and location) thermal vias under parts so that the copper barrels provide
the enough thermal path so we don't have to fill the vias and we size the
vias and via pattern and stencil print pattern to minimize any thieving.

Regards,
George
George M. Wenger
Senior Principal Reliability / FMA Engineer
Andrew Corporation - Wireless Network Solutions
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 Office (732) 309-8964 Mobile
E-mail: [log in to unmask]

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Frank Kimmey
Sent: Wednesday, August 31, 2011 5:33 PM
To: [log in to unmask]
Subject: Re: [TN] QFN with solder slug

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
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TechNet E-Mail Forum <[log in to unmask]>; Please respond to Mark Larson
<[log in to unmask]>


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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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