Werner is (naturally) correct. This phenomenon was first observed I believe
during the '80's when some companies used a "belt and braces" approach to
chip placement and screen printed paste, followed by center glue dot and
reflow (the components generally at that time were 1206).

As Werner points out the glue expands and sets spacing the component from
the board leaving the component connected by a diablo (waisted)  shaped
solder column.

John

 
 
John Burke
 
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Tuesday, April 17, 2007 8:32 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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