Hi Werner,

We tried globtops on the corners many years ago, but we found that the
registering became disturbed during wave soldering (superBGAs). Since
then we have shipped 2 million boards with BGAs with no corner fixation.
When/why do some need these extras?

Inge

-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Werner Engelmaier
Skickat: den 17 april 2007 17:32
Till: [log in to unmask]
Ämne: 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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