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August 1997

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Subject:
From:
"Gary D. Peterson" <[log in to unmask]>
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Date:
Wed, 6 Aug 1997 09:36:13 -0600
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[log in to unmask] wrote:
>
>      Hello,
>
>      Many articles recommend rapid cooling rate during rework for reliable
>      solder joints. What cooling rate can be readily achieved with most
>      rework stations?  One article recommended 100 C in 5 seconds, is it
>      reasonable? I thought it would be hard to achieve such a rapid
>      cooling.
>
>      Thanks,
>
>      Yuan

Yuan,

I have recently completed characterization of our Air Vac DRS22 machine
for replacement of Amkor's 352-ball and 432-ball Super BGA packages.
I was able to force cool from 212 C (measured against the underside of
the BGA with a 5-mil diameter coaxial thermocouple) to 185 C in the
first 1/2 minute; 185 C to 176 C in the next 1/2 minute; 176 C to 141 C
in the next 1/2 minute and 141 C to 119 C in the next 1/2 minute.  I am
restricted by company guidelines to 30 PSIG air pressure on the machine
which impacts maximum air flow rate available.  The nozzle for the
super-BGA package is quite large, which supports maximum air flow, thus
a "fast" cooling rate for this machine.  A quick calculation shows a
cooling rate of 36 C in the first minute (it took 2-minutes to cool 93 C
which is about 24 times as long as your target of 100 C in 5 seconds).
My experience...our DRS22 rework machine can't get close to that cooling
rate.  My opinion... none of the others can either.

Maybe some of those metallurgists out there will tell us just what the
tradeoffs are between cooling rates of 100 C per 120 sec. vs 100 C per 5
sec.

Gary P.
--

                                  Gary D. Peterson
    _/_/_/   _/    _/  _/        SANDIA NATIONAL LABORATORIES     _/_/_/
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