Martin,

These are on field return units that are ~ 7 yrs old.  The application is consumer
electronics (shelf).  The quality of the original soldering looks fine (good volume and
wetting characteristics).  I have seen what appear to be interfacial failures on thermal
cycled PWAs.  However, on close examination the failure is through the solder just above
the intermetallic layer.  In the present case, the failure is between the copper and the
intermetallic layers.

Best regards,
Ed

Martin Christie wrote:

> Ed,
>
> Where do these failures tend to manifest themselves...post reflow, final
> inspection?
>
> If you were to cross section through the failed joints what type of solder
> fillet would you see?
>
> I've seen similar failures on solder mask defined pads where excessive
> solder has caused an effective wetting angle greater than  90Deg. This then
> acts as a stress raiser and failures can then occur during mechanical
> handling / thermal stress.
>
> Other than that, the only other time I've seen this failure is during
> thermal cycling.
>
> Martin
>
> ===== Original Message from [log in to unmask] (TechNet E-Mail Forum.) at
> 12/17/99 12:22 am
> >Hi everyone ... I have been looking at solder joint failures for quite a
> >while and have stumbled on something new to me.  I hope someone has seen
> >this and can give me some insights.  Here it is ...
> >
> >*  the failures occur on a number of different component types including
> >SOICs, QFPs, SMT caps (not chip caps)
> >*  the failure is complete interfacial failure of the mounting pad at
> >the copper/IMC interface (i.e the solder, Cu6Sn5 and Cu3Sn layers stay
> >with the component lead)
> >*  the board finish was HASL and the IMC layer appears normal (i.e. ~
> >1-2 microns total thickness)
> >*  the copper is heavily oxidized.  However, I believe this is because
> >the crack existed for a long period of time (perhaps?)
> >*  I see no convincing evidence of contamination at the failed interface
> >
> >*  the failure does not appear to be related to an electroplated copper
> >interface
> >
> >Based on past experience, this type of thing seems to be caused by gross
> >thermal shock!?!  Have any of you seen the type of failure I have
> >described?  If so, what was the conclusion as to cause?
> >
> >Best regards,
> >Ed Hare
> >VP SEM Lab, Inc.
> >
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--

               SEM Lab, Inc.
Scanning Electron Microscopy and Failure Analysis
               Snohomish, WA
               (425)335-4400
           http://www.sem-lab.com

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