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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:
Thu, 7 Aug 2014 19:05:42 +0000
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Yes. The fact that it is underfilled explained a lot of questions I had about the pictures. Dave is correct about the underfill epoxy. The CTE of the selected underfill should be very close to that of the solder alloy being used. So your statement that it is a wildcard is very true.
We use a small SRAM that I think is probably very similar to the one you are discussing here. We had trouble with the underfill until we discussed it with Werner Engelmaier a few years ago, and he put us on the right track.
Sure do miss that guy.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Hillman
Sent: Thursday, August 07, 2014 1:48 PM
To: [log in to unmask]
Subject: Re: [TN] SJ fracture signature

Hi Chris - great information. One comment, some underfill materials can actually make things worst so find out if they did any qualification testing with the underfill material.

Dave


On Thu, Aug 7, 2014 at 1:45 PM, Chris Mahanna <[log in to unmask]> wrote:

> My hypothesis goes something like this
>
> As Dave says "microstructure coarsening is more subdued".  I'd add 
> -with very little banding compared to the damage accumulated at the Sn 
> grain interfaces.  That is what I meant about this one being odd.  But 
> then again... we don't see SnPb failures much these days.
> My suspicion was that (relatively speaking) the sj was born that way - 
> as Joyce says: "way off eutectic, yielding high % of high melting 
> phase", rather than aged/evolved that way.
>
> I also suspected a relatively static mechanical load caused by 
> residual stress in the joints, as the board may not be too stable/flat 
> during reflow.  Average soldering cool-down rate was probably over 
> 6C/s at the time of solidus.  These fractures were found in a perimeter column.
>
> This BGA is a small 8M-bit SRAM.  I would have thought it to be pretty 
> flexible.  It is underfilled, which is a wildcard at the moment.  We 
> are asking for the material part# now.
>
> So the suspicion was on the reballing, but you can see why the 
> confidence level is low.
>
> This is actual dirty laundry, so can't say the use environment, but 
> there is relatively benign thermal cycling and enough vibe that it 
> should be taken into consideration.
>
> Chris
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of David Hillman
> Sent: Thursday, August 7, 2014 1:10 PM
> To: [log in to unmask]
> Subject: Re: [TN] SJ fracture signature
>
> Yes, Chris, come as clean as you can or we'll send Clump/Kloumpios to 
> visit you.
>
> Dave
>
>
> On Thu, Aug 7, 2014 at 11:44 AM, Stadem, Richard D. < 
> [log in to unmask]> wrote:
>
> > Okay, Chris. We've done our WAG speculating. Out with your 
> > suspicions, we must hear them.
> > dean
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
> > Sent: Wednesday, August 06, 2014 1:49 PM
> > To: [log in to unmask]
> > Subject: [TN] SJ fracture signature
> >
> > We see hundreds of cracked solder joints every year, but I can't say 
> > that I've ever seen one quite like this.  I have some suspicions, 
> > but I'll keep them to myself, as to not prejudice the crowd.
> > EDS maps are forthcoming.
> >
> > This is a caught in-the-wild field failure of a 48-pin BGA Reballed 
> > to SnPb ENIG on board side Ni barrier on component side
> >
> > http://stevezeva.homestead.com/45268-1.JPG
> >
> > http://stevezeva.homestead.com/45268-2.JPG
> >
> > http://stevezeva.homestead.com/45268-8.JPG
> >
> > Thoughts?  And Thanks to Steve!
> >
> >
> > Chris Mahanna
> > President, Technical Manager
> > Robisan Laboratory Inc.
> >
> >
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