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March 1999

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Subject:
From:
"Hogue, Pat (AZ76)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 25 Mar 1999 10:53:21 -0700
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Hi Phil:

    When I used to perform outgassing tests (per NASA SP-R-0022 or ASTM E595
at 125C at <10E-6 torr) at Ball Bro. in Boulder, CO I used a rule-of-thumb
(developed by those kind folks) that for a 24 hour test the diffusion of
volatile material, including water, was limited to 1/32 inch below the
surface.  This means that a typical organic material can be depleted of low
molecular weight volatile material in a 24 hour period at 125C and 10E-6
torr if it is no more than 1/16 inch thick, assuming diffusion is from top
and bottom surfaces.

     Ultrahigh vacuum systems can take days or weeks to reach vacuums like
10E-10 torr, depending upon size.  Water is very difficult to desorb,
especially as the number of atomic layers decreases (i.e., changes from
physically adsorbed to chemically adsorbed).

     Some recent work at NASA (that I have not followed up on) seems to
indicate that residual hydrogen and react with natural surface oxide to form
water.

Pat Hogue
Materials and Processes Engineering
Honeywell Satellite Systems Operation
Glendale, AZ 85308
(602) 561-3748


        ----------
        From:  Phil Hersey [SMTP:[log in to unmask]]
        Sent:  Wednesday, March 24, 1999 9:00 AM
        To:  [log in to unmask]
        Subject:  [TN] How to get water out?

        I am chronically failing RGA (residual gas analysis) despite several
hours
        of vacuum bake.  I vaguely remember from my sputtering days that
vacuum
        alone is not the best way to remove moisture - vacuum dep. machines
use a
        "nitrogen trap".

        Does anyone know of better ways than vac. bake to remove H2O from
PCB
        assemblies which will later be hermetically sealed and tested for
RGA?

        Resp.
        Phil Hersey, Carson City Nevada USA

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