Werner,

Is the epoxy placed before reflow? Before placement? What I'm trying to
figure out is how you keep from disturbing the BGA placement during
epoxy application.

Blair

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Tuesday, April 17, 2007 11:32 AM
To: [log in to unmask]
Subject: Re: [TN] Tacking corners of BGAs

Hi Richard,
The improved reliability should not have come as a surprise. Consider
what is
happening: 
(1) the solder balls are at their full diameter when BGA is placed,
(2) the epoxy fixes the distance between PCB and BGA at that diameter
value,
(3) during reflow, the epoxy expands opening the solder gap further,
(4) on cooling, that solder balls are prevented by the epoxy from
collapsing,
(5) you have the equivalent reliability of using non-melting 10Sn90 Pb
solder balls, and (6) this results in the reliability improvement
observed.
I have been pushing this for   ceramic chipp components for some time.



Werner



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