Russ and all,
I have been following this thread with interest. Your question regarding how
you could see a nickel surface after soldering reminded me of the situation
we experienced with what some at IBM called "double reflow". My
understanding was that as the leads of say a QFP on the first build side of
the board were being reflowed during the second side build, the leads would
pass through liquidous at slightly different sides. What you could end up
with was only a few leads still being attached (at almost liquidous and
therefore with little strength), the rest were already molten. These few
attached leads now had all the stress from the package/board and since there
was little strength left, they would fracture right at the nickel layer of
the pad on the board. It seems to me that removing the BGA would cause
similar slight ball temperature differentials as they went through
liquidous. If there were stresses at this time due to warpage, could these
parts be fracturing at the package/ball interface?

Thanks,
             Robert Furrow
             SMT Process Engineer
             978-960-3224
             [log in to unmask]

> ----------
> From:         Russ Winslow[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Russ Winslow
> Sent:         Thursday, February 18, 1999 1:43 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] Defective BGAs?
>
> Hi Werner,
> My problem is that I agree with you whole heartedly.  Why come up with a
> complex explanation to a simple problem?
>
> We receive BGAs which have been pulled off of boards from all over.  We
> see
> the same phenomenon on most components we receive.  All from different
> sources and different manufacturers.
>
> At first I attributed the problem to the conversion of semiconductor
> assemblers to no-clean fluxes. Then I began looking into the possibility
> that they are just escapees which slip through the cracks.  Now I am
> wondering if they made it out of assembly with good ball attach and then
> were degraded by test sockets which can stress the solder balls while they
> are at elevated temperatures.  Regardless of what the root cause of the
> problem I guess the real question is:
>
> If you pull a BGA from a board (with a proper hot air reflow system) and
> find a bare nickel pad on the component who's fault is it?  As long as it
> is
> not a function of the removal process then I believe the responsibility is
> that of  the component manufacturer.  I believe the failure analysis (at
> the
> board level) is complete.  The component failed due to an open which was
> caused by a poorly attached solder ball (at the component).  My experience
> tells me that the nickel should never become exposed just by heating the
> solder balls to the molten state and then lifting the component from the
> board.
>
> Do you see any reason why the board removal process would expose a bare
> nickel pad of a good BGA?
>
> -----Original Message-----
> From: [log in to unmask] [mailto:[log in to unmask]]
> Sent: Wednesday, February 17, 1999 6:43 PM
> To: [log in to unmask]; Russ
> Subject: Re: [TN] Defective BGAs?
>
>
> Hi Russ,
> Well, your problem comes down to two possible causes:
> 1) no solderability; your tests seem to exclude that.
> 2) inadequate soldering process; it certainly is possible that the solder
> balls were never properly attached to the component. Your problem is not
> likely related to shear strength of (or crack propagation in) nickel-tin
> intermetalics, but the absenceof any IMCs which are the by-product of a
> good
> intermetallic bond resulting from good wetting. Solder attachment strength
> to
> nickel is about half that to copper.
>
> Werner Engelmaier
> Engelmaier Associates, L.C.
> Electronic Packaging, Interconnection and Reliability Consulting
> 7 Jasmine Run
> Ormond Beach, FL  32174  USA
> Phone: 904-437-8747, Fax: 904-437-8737
> E-mail: [log in to unmask]
>
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