Hi,

 If you have a stack of PCBs, lets say 25 or 50 boards which is a stack 2 or 3 inches high, can water vapor diffuse out the space between the boards? Does laying a thick tissue between the boards help or is it best to place each board in edge supported carriers?  I worry about warp if a board is edge supported while at elevated temps. 

Does anyone has knowledge here or even an educated guess?

Thanks,
Bob K.

-----Original Message-----
From: TechNet <[log in to unmask]> On Behalf Of David Hillman
Sent: Monday, April 16, 2018 8:11 AM
To: [log in to unmask]
Subject: Re: [TN] Vacuum Baking

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
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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.
>