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February 2013

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From:
Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Joyce Koo <[log in to unmask]>
Date:
Mon, 25 Feb 2013 14:29:52 +0000
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Welcome to the engineer world.  No cut and dry.  Utilize the part to your environment requirements, - good enough with some safety factor is just about right ;-). 

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 Gregory Munie
Sent: Monday, February 25, 2013 9:14 AM
To: [log in to unmask]
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.

OK. Now I know the answer about QFN reliability: It's Yes. And No.

Starting off Monday more confused than finish of last Friday . . . 

:-)

Greg Munie PhD
IPC Technical Conference Director
630-209-1683
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http://www.ipc.org

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Monday, February 25, 2013 7:46 AM
To: [log in to unmask]
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.

QFNs are here to stay, and like Chinese semiconductors, are now or soon will be used in all of our military systems.  Years from now, while picking through the rubble of our civilization, anthropologists will speculate on what it was which made us all so stupid.  Not Pb in the water, like the Romans.  Maybe proof will arise that television really does suck out your brain!

Anyway, I had a good back & forth with Werner on QFNs (before he passed, of course!).  They are very interesting parts, and as noted below and elsewhere, they don't fall into standard analysis techniques because the solder joint is too thin.  There's no real solder in the joint--all intermetallics.  Intermetallics are very strong and can be brittle.  I don't think they stress relieve/creep like solder, which can be both good and bad for reliability.

On the other hand, the joint height on chip capacitors is also negligible.  We tell ourselves that the strength comes from the fillet. 

Werner claimed his formulae were only useful for solder joints greater than about 30 microns (my recollection--I have to dig that up if it is really of interest).

Regarding the actual subject at hand:
From the provided XRAY, I agree with Steve that the part has been reworked, or the solder deposition has poor control.  The little blobs of solder not wet to larger blobs (for example, 2nd pad down on right) or evenly distributed around the wetted areas indicates a poor reflow profile or exhausted/inadequate flux.  I presume that hazy blob on the lower right of the slug is solder that went through the via onto the pad for the via on the other side (IF you insist on using unfilled vias under a QFN slug, then at least encroach the mask on the opposite side via pad!).  The voids on the slug are irrelevant (not enough to make any significant thermal or electrical difference).

The uniformity of the solder deposition as seen both in the XRAY and in the pried off version suggest that the solder was in full contact with the device pad.

Hence, either the reflow profile/flux system failed, or the slug on the component just won't wet.  You have not responded to my earlier suggestion that you attempt to wet the part you pried off by placing it "slug up" on a hot plate and trying to get paste to flow onto it.

Wayne Thayer

-----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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