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February 2013

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From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Mon, 25 Feb 2013 12:19:37 -0600
Content-Type:
text/plain
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Hi Wayne - take a look at the SMTAI Conference proceedings - several very 
good QFN papers. Also the NASA DoD soldering study - high performance 
environment testing of the QFN package style.

Dave



From:   Wayne Thayer <[log in to unmask]>
To:     <[log in to unmask]>
Date:   02/25/2013 08:54 AM
Subject:        [TN] QFN reliability
Sent by:        TechNet <[log in to unmask]>



Getting joint height without spacers on QFN is a challenge.

Worst case:  There's a serial flash in a QFN package:  Has 8 small pads 
with no base pad at all.  Impossible to get joint height without spacers. 
Ditto for most chip caps, where the end fillets help suck the part down to 
no height.

Please provide paper references on spacers for QFN.

Thanks,

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, February 25, 2013 9:36 AM
To: [log in to unmask]
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.

Sorry for the confusion.
The statement "there's no real solder in the joint--all intermetallics" is 
not quite true. It all depends on the aperture size, type of component, 
etc, etc. Any amount of solder deposit is going to provide much more 
solder than the IMF alone, which is only a few microns thick at best. 
However, the solder in between the IMF on the belly pad of the part and 
the IMF on the pad is important only in its modulus of elasticity. The 
more you have, the better. But one can only print about a 45-50% reduction 
of the pad dimension, as otherwise you will have solder balls, etc.
Oftentimes a spacer is added. It has been shown to greatly increase the 
reliability. Many papers on that subject out there.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gregory Munie
Sent: Monday, February 25, 2013 8:14 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
[log in to unmask]
 

 
 
http://www.ipcapexexpo.org/
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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