Hi folks - here is a late answer due to some " extra efforts" getting back 
from APEX. I agree with Roger Mack's response as we have seen the same 
situation before on a couple of designs. Components that have "base slugs" 
cause major process issues with solder wetting and buoyancy balance. 
Having thru vias in the slug pad is a poor design choice as they will 
thieve solder and impact your ability to balance solder volume/buoyancy. 
Roger's suggestion of filling and capping the vias removes them from the 
equation. Once you have the via issue resolved, then you have to balance 
the solder volume of the slug versus the solder on the leads such that you 
don't "float" either solder surface. The solder stencil suggestions 
provided by TechNet can and will resolve the solder buoyancy issue. 
Components with base slugs require additional effort to keep process 
yields high.

As for the sparrow question, I'll leave that to Doug - probably a half 
fractional element in the right response.

And Greg - there have been QFNs in avionics designs, both commercial and 
military, for at least 5 years so you have been flying around with them 
for some time now. Lots of good data showing their Class 3 product 
reliability.

Dave



From:   Gregory Munie <[log in to unmask]>
To:     <[log in to unmask]>
Date:   02/23/2013 01:41 PM
Subject:        Re: [TN] 48-pin QFN via-in-pad solder slug problem.
Sent by:        TechNet <[log in to unmask]>



I have been following this QFN discussion as about a year ago I worked a 
reliability problem for a QFN user.

One thing I noted is that the standoff is (of course) low. For anybody who 
is using QFNs: Have you done a Werner Engelmaier style mechanical 
reliability analysis on the parts?

Just asking. I saw them being used for avionics and was a little concerned 
about whether any planes I was to be flying on used QFNs. 

Greg Munie PhD
IPC Technical Conference Director
630-209-1683
[log in to unmask]
 

 
 
http://www.ipcapexexpo.org/
http://www.ipc.org


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Schaefer
Sent: Saturday, February 23, 2013 11:33 AM
To: [log in to unmask]
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.

I am currently working on a design with 2 similar QFNs requiring via in 
slug for thermal and electrical reasons.
IPC-4761 gives a good summary of methods for handling these components; 
IPC-4761 Type VII (filled and conductively capped) appears to be the only 
sure bet solution.

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