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Subject:
From:
Yuan-chia Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Yuan-chia Joyce Koo <[log in to unmask]>
Date:
Thu, 7 Aug 2014 16:48:41 -0400
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text/plain
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text/plain (129 lines)
chris, are all the fracture at corner of the sj? or across the outer  
row? corner sj - thermal stress, outer row - you possibly deal with  
mechanical flex of either board or module in relation to the other...  
my 2 cents.  SRAM are low power device, j-heating possibly is not  
apply in this case (unless you got some hot components on the other  
side...).
            jk
On Aug 7, 2014, at 2:45 PM, Chris Mahanna 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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