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April 2012

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From:
"Post, Scott E" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Post, Scott E
Date:
Tue, 3 Apr 2012 11:41:43 +0000
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It's interesting that you're seeing failures of such a small QFN in such a benign test.  I've never tested an 8 pin QFN, but I've thermal cycled 32, 48, and 68 pin QFNs with both SnPb and SAC305 on 1.6 mm substrates with a variety of surface finishes from -40 to +125C.  The Weibull plots show an unintuitive trend - the reliability gets worse as the part size decreases.  In fact, differences of thousands of cycles.  1% fail for the 32 pin was right at 1000 cycles regardless of paste.  Based on my data it doesn't surprise me that a smaller QFN fails earlier, but I wouldn't have predicted it would fail that early in 0/100C testing.

The two things I've seen increase QFN reliability is a thicker paste stencil (more standoff) and the existence of a toe fillet.  You might try extending the pads enough to allow a toe fillet and use an aggressive flux in an attempt to create the fillet.  Of course, a toe fillet is not guaranteed since you're trying to solder to unprotected copper.

Scott Post
P.O.U. 0000-001E-0CTC
2151 E. Lincoln Road
Kokomo, Indiana    46904-9005
765-451-2983 (Phone)
765-451-0287 (FAX)

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of jonathan noquil
Sent: Monday, April 02, 2012 5:16 PM
To: [log in to unmask]
Subject: Re: [TN] how to pass solder joint reliability of a 3.2 mm board (16 layers) for QFN packae

Thanks Dennis:

I will check your report. Yes we have been passing with other components
with no issue on Cycling but not this QFN (8 leads), QFN seems easy to
crack on its joint when mounting on a thick PC board.

Iwill check and learn from the report
thanks

On Mon, Apr 2, 2012 at 2:13 PM, Dennis Fritz <[log in to unmask]> wrote:

> Johnathan,
>
> There is a great body of knowledge about solder joint reliability at both
> mild thermal cycle and harsh thermal cycle at the JCAA-JGPP report location
> under NASA TEERM:
> http://teerm.nasa.gov/LeadFreeSolderTestingForHighReliability_Proj1.htm
>
> The project 2 results report is at
> http://teerm.nasa.gov/nasa_dodleadfreeelectronics_proj2.htm
>
> I am puzzled why you think you cannot pass 1000 cycles at a delta of 100
> degrees C.  In both studies, but lead based and Pb-free solders passed far
> more cycles than that.   In fact, the Pb-free boards from test 2 are
> probably still being cycled at Boeing in Seattle.  In harsh cycle,
> lead-based solders perform best, in mild (100C) cycles, Pb-free solders
> perform best.
>
> Denny Fritz
>
>
>
>
>  -----Original Message-----
> From: jonathan noquil <[log in to unmask]>
> To: TechNet <[log in to unmask]>
> Sent: Mon, Apr 2, 2012 12:55 pm
> Subject: [TN] how to pass solder joint reliability of a 3.2 mm board (16
> layers) for QFN packae
>
> Hi All
>
> Need some suggestions on how to handle solderjoint issue (How to tweak to
> pass TMCL1000 cycles- 0 to 100C) on QFN packages ( 5 x 6 x 1.0 mm with 8
> pins).
> Finish is Pure Sn
> Board is 16 layers 3.2 mm, HASL finish.
>
> Does someone had an experience to share?
>
> thanks
>
>
> ______________________________________________________________________
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