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October 2007

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Subject:
From:
"Igoshev, Vladimir" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Igoshev, Vladimir
Date:
Mon, 1 Oct 2007 10:28:40 -0400
Content-Type:
text/plain
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text/plain (72 lines)
Hi Wayne,

Have you cross-sectioned one of the failed balls to look at the fracture
path? If the failure doesn't look like BP, then I'd assume it didn't
fail at the Ni-P/intermetallic interface. In such a case, a
cross-sectional view would tell you where (which interface(s)) failure
actually happened at.

Regards,

Vladimir

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: Monday, October 01, 2007 9:39 AM
To: [log in to unmask]
Subject: [TN] BGA Intermetallic failure

0.5 mm pitch BGA, board has via-in-pad.

On 5 of 7 via-in-pad locations, the balls separated in the intermetallic
zone at the substrate interface.  All other balls look ugly, but not out
of the usual ugliness when the BGA is SAC305 and we are attaching with
Sn/Pb, and they seem well attached.  The ugliness of the bulk of the
balls is very uniform, indicating thorough mixing of the alloys.  The
separated intermetallic zone is lumpy gray, not like cases of black pad
I've seen.

Boards had ENIG, then had SnPb SIPAD done on them (requires 2 reflow
cycles).  After that, the BGA's were attached and then the other side
was stenciled, populated, and reflowed.  So the ENIG interface had been
reflowed a total of 4X.

The surface of the via-in-pad locations is fairly lumpy, like is common
with CB100 via fill.  Perhaps this lumpiness was responsible for too
much gold in the joint?  I can't think of any other reason why these
locations are where the problems are.

Additional insight would be very much appreciated.

Thanks,

Wayne Thayer


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