Dave
I recall some studies coming across my desk at the Lake that promoted
decreased temperature (70 - 80C) of bake out to below vapor temp with
extended time for reduced intermetalic and solderability impact. Of course
it is all dependent a great deal upon the release rate of substrate
materials, mask, and number of layers. Poly is pretty slow compared to
glass, etc.
Mel Parrish
201 Cedar Road
Mullica Hill, NJ 08062-2503
856 223 5828
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of David Hillman
Sent: Thursday, June 01, 2000 3:18 PM
To: [log in to unmask]
Subject: Re: [TN] Baking of boards
Hi Ryan! Get a copy of the following article: "Removing Moisture from
Electronic Components and Assemblies", authored by Charles S. Leech, JR. ,
Circuits Assembly May 1994, it covers some of the basic physics of water
loss in detail - especially the issue of vacuum moisture removal. In the
article the vaporization of water via vacuum is discussed as not being the
most effective moisture removal method in all instances - the change of
physical states (liquid to vapor) can result in the formation of ice (the
article describes a neat demonstration of this reaction) which impedes the
removal of water from a board. I participated in a DOE in which we showed
that a standard baking procedure in a convection oven was more effective
than a vacuum bake process. However, you don't get anything for free - the
convection bake process degraded the board solderability much more the
vacuum bake process . Good Luck.
Dave Hillman
Rockwell Collins
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O.K. Doug and Ron, I'm going to put you an the spot.
Two weeks ago, I set up an experiment to see how long I would have
to vacuum to get some PBGA's dry. The vacuum drying was run in parallel
with 125?C bake and just air drying (20% RH here in the high desert). The
vacuum drying didn't do much better than air drying.
The part were supersaturated with water, and the experiment only
ran
for 24 hours. But projecting the desorption of the moisture, it would have
taken a week to dry them, compared to 24 hours with the bake.
The vacuum was done with our plasma etcher at 0.5 torr (no plasma),
so I fear that I may have frozen the moisture in the component. Any ideas
as to why the moisture didn't come out? In theory, vacuum should work!
Thanks
Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]
>
> I agree with that, Doug. For many years we have vacuum baked all of our
> boards. It helps even more if you back-fill with dry Nitrogen as we do.
> Ron Dieselberg
> BAE SYSTEMS
> Cincinnati Electronics Corp.
>
>
>
> If you are really concerned about the effects of heat on the polymer
> qualities or the oxidation of the surfaces, consider using a vacuum or
> vacuum-make operation. Gets the water out with less heat.
>
> Doug Pauls
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