Hi Phil,

        I looked at the Vacuum bake in some detail in years gone by. The
particular application was with laminate and prepregs. Since the pre-pregs
could not undergo baking (that advances the cure) to help remove water you must
draw a vacuum that is lower than 24 millitorr before the water will begin to
elute. With the cured laminates you can raise the temperature to aid the water
elution time. I'm not sure about the complexities of a populated board, but I
would expect that a Greater Vacuum  is necessary to elute the water, due to the
many paths necessary for the water vapor to flow. I also expect that one must
stay at a fairly low temperature so as not to damage silicon devices,
laminates, etc. This will probably nessitate long times at High vacuum with a
"Mean free path' in the vacuum system large enough to handle the amount of
water vapor eluted.
        We used liquid nitrogen traps, perhaps in a similar application that
you mentioned in a gas chromatograph system to drop out volitles from the gas
stream while doing some gas analyses several years ago. I don't quite know how
you might implement this unless the trap was placed between your Vac. Chamber
and the pump to eliminate backstreaming.

For What it's worth, I hope this helps,

Regards,
Les
>  From: Phil Hersey <[log in to unmask]>, on 3/24/99 7:59 AM:
>  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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