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Subject:
From:
David Hillman <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, David Hillman <[log in to unmask]>
Date:
Mon, 16 Apr 2018 07:10:41 -0500
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Hi folks - we did some vacuum baking experimentation a few years back and
yes, vacuum baking can be effective but it is not as straight forward as
you would imagine. You need to carefully select the vacuum bake parameters
or you will actually impede water diffusion. You can cause ice formation
rather than water evaporation depending on your parameters (put a shallow
pan of water in a vacuum chamber and try it, its kinda fun). Wayne's
approach is much simpler and far more practical/effective in terms of
moisture removal.

Dave Hillman
Rockwell Collins
[log in to unmask]


On Sun, Apr 15, 2018 at 11:43 AM, Wayne Showers <
[log in to unmask]> wrote:

> The problem with vacuum drying is not so much the boiling point of water
> as in an unimpeded environment, those values are: At 25" Hg : 56C, 27" Hg :
> 45C, 28" Hg :40C, and so on.  The problem is degree of impediment caused by
> the actual plastic.  To build off what Yuan-chia Joyce Koo has stated but
> with a different vector in mind, the plastic material of moisture bags for
> instance is highly resistant to diffusion.  Different polymers will follow
> accordingly.
>
> For information, many plastics must be dried immediately prior to molding
> otherwise they suck up moisture like a sponge. Common hygroscopic polymers
> are Nylon, ABS (Acetyl Butyl Styrene), PC (Polycarbonate), and PMMA (Poly
> (methyl methacrylate)).  Nylon and PMMA are very common materials in
> component shells.  ABS and PC index* at temperatures not much higher than
> boiling water and are not typically used.  PP (Polypropylene) and PE
> (Polyethylene) are not so bad, hence their use in capacitors, but these
> index in the 200C range making them a bear for SMT placement.
> My general rule is the higher the MSL number, i.e., the more likely a part
> is to absorb moisture, the easier it is to dry.
>
> For normal bake (NO vacuum), here is the formula I derived several years
> ago:
> Thickness (mm) X Thickness (mm) (Divided by) Bake Temp Factor
> D125oC= 0.4427 // D90oC= 0.1270 // D40oC= 0.0131, Result is time in hours.
>
> I have never experimented with Vacuum Acceleration, but I will query the
> Penn State Plastics Engineering group with what we are trying to resolve
> and post those results to the J-STD-033 committee as well as here.
> * Index is the temperature at which a plastic flows well enough to be
> molded.  It is neither solid nor liquid, but soliquid.
>
> This all sounds like a job for Clumpy and Kloumpios when they are next
> available.
>

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