Bob Neves wrote:
> 
> Doug,
> 
> There are several factors other than PWB substrate material which will also
> influence moisture release rates: Percentage Copper (planes & circuits);
> Thickness; Storage Environment; Soldermask; and probably a few others that
> don't come to mind right now.  The point is that it is going to be difficult
> to throw a specific "one time fits all" into moisture prebakes.
> 
> The longer your PWB's bake, the more likely solderability problems will
> creep in due to oxide formation and intermetallic growth acceleration.
> Obviously an inert environment will retard oxidation, but will do little for
> heat accelerated intermetallic growth.
> 
> The easiest way to calculate the best prebake times for your set of PWB
> moisture circumstances is to perform a simple weight loss study.  Take a
> couple of your ready to assemble PWB's and cut them into 2" x 2" squares.
> Very accurately weigh and number each sample with a laboratory scale.  Place
> all of the samples into your prebake oven, removing samples at 15 minute
> intervals.  Immediately re-weigh the removed samples, and calculate the
> percentage weight loss due to moisture.  It will become very clear where the
> weight loss percentage levels flatten out, allowing you to calculate your
> point of diminishing "prebake time" returns.
> 
> At 08:26 AM 9/26/96 -0700, you wrote:
> >We are looking for reports or other references for moisture release rates
> >for different PWB substrate materials.  Also recommended prebake parameters
> >for time and temperature in air and for other atmospheres. Any leads will be
> >appreciated
> >
> >Mel Parrish


Bob, Doug, Mel

While you are doing your weight loss tests, continue taking measurements during the 
cool down.  I remember some in-house tests (from another life) which showed that over 
50% of the moisture is reabsorbed in Epoxy systems after only 30 minutes.  Make sure 
you find out how long the board stays "dry enough".  By the way, if you can determine 
what "dry enough" is, write a paper.  I don't think anyone knows.  

Another interesting baking effect:  The more often the epoxy is baked, the quicker 
the water gets out, and the quicker it gets back in again.  Almost like micro 
erosion.  (We hated the results of the study.)

Saerching for Tabasco in Virginia,
George Franck
Raytheon E-Systems
Falls Church Va.

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