Is this soak at heat up of cool down?  For heat up the flux functionality is
likely to be destroyed.  If the flux is not destroyed, one possible
benefit(?) is the likely reduction of voids in the solder balls, but even
this is not clear because some evidence shows improved SMT T/C results if
fine voids are present near the solder ball to substrate interface.  But
this benefit(?) can be realized with much less reduction in effective heat
up rate.  Five to 10 minutes is very extreme.

For cool down you are likely to significantly coarsen the solder
microstructure which is likely to reduce surface mount temp cycle
reliability performance.

This soaking is not a good idea.

Regards,
Leo

------------------------------------
Leo M. Higgins III, Ph.D.
Director of Applications Engineering / ASAT Inc.
[log in to unmask]
3755 Capital of Texas Hwy-So
Suite 100
Austin, TX     USA     78726
tel: 512-383-4593
fax: 512-383-1590
mobile: 512-423-2002
------------------------------------


-----Original Message-----
From: Rick Thompson [mailto:[log in to unmask]]
Sent: Thursday, July 01, 2004 10:54 PM
To: [log in to unmask]
Subject: [TN] Long Reflow Soak Times for BGAs?


I'm having a philisophical discussion with my boss regarding long reflow
soak times to improve soldering under BGA devices.  Is there anything to be
gained by extending soak times at the 150C level from 60-90 seconds to 5-10
minutes? I've never seen times this extended and am wondering if they make
sense in some situations?  The boards in question are not terribly
difficult, .063" thick, 14 layer, 12x16 inches with maybe a couple thousand
components on top.  Any comments on this practice?

Thanks in advance,

Rick Thompson

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