Ryan -

An additional note - Charles S. (Stu) Leech can be reached at
[log in to unmask] - they have done a considerable amount of additional
work since the publication that Dave refers to.


Regards - Kelly

-----Original Message-----
From: David Hillman <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Friday, June 02, 2000 8:58 AM
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
>[log in to unmask]
>
>
>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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