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February 2013

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Mon, 25 Feb 2013 13:57:17 +0000
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Yes. The QFN takes the LCCC and adds a belly pad. Ditto on master designer.

-----Original Message-----
From: Joyce Koo [mailto:[log in to unmask]] 
Sent: Monday, February 25, 2013 7:26 AM
To: 'TechNet E-Mail Forum'; Stadem, Richard D.
Subject: RE: [TN] 48-pin QFN via-in-pad solder slug problem.

Richard,
Before QFN, there is something called leadless ceramic package castellation. If I am not mistaken, many of them still out there after 15 years of service (being said that, Pb-Sn solder type helps too).  The CTE mismatch is even more if you just consider parts to the board materials only.  You just have to know how to design and manager the delta CTE (not everybody's cup of tea for sure).  A master of designer is golden (I admire them a lot).  My 2 cents.   

Joyce Koo
Materials Researcher - Materials Interconnect Lab Research In Motion Limited
Office: (519) 888-7465 79945
Mobile: (226) 220-4760

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, February 25, 2013 8:08 AM
To: [log in to unmask]
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.

Greg is correct: QFNs are not recommended for high-reliability applications.

With no compliant leads, and a fairly large component body, the delta CTE between the component and the PWB is usually too large to withstand very many service cycles before failure due to fatigue-induced cracking of the solder joints. Smaller ones are often used in commercial designs, etc, but they are too problematic for high-rel applications. There are also issues with large solder voids, solder balls under the component, flux entrapment, flexural stress cracking, etc. 
We do not use them in our designs, except for in cases where we perform a patented compliant lead attachment process (of which we do a LOT) so those are technically not really QFNs. 

Werner did write a few papers on the issues with these components, and because the solder height "h" is a prominent part of the fatigue formulas, QFNs simply do not do well. For some low-power devices they may perform adequately, but I would never use them if some other package was available no matter what the product is. Remember, even if high-reliability is not required, do you want to deal with the potential fallout of these parts from a high-volume failure standpoint?
I understand many have developed various processes for using them without any issues, but they nearly always require some special attention, including 100% X-ray process control. 
Designers need to think about the cost of that when they calculate the cost savings of the QFN versus other compliant-leaded or BGA package styles.

They are seldom cheaper.
 
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gregory Munie
Sent: Saturday, February 23, 2013 1:40 PM
To: [log in to unmask]
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.

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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