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1996

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Subject:
From:
Bob Neves <[log in to unmask]>
Date:
Thu, 26 Sep 1996 11:46:51 -0700
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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
>We can be reached at:            e-mail: [log in to unmask]
>Manufacturing Technology Training Center (MTTC), Inc
>543 Graaf Street                     Phone 619 446-5571 
>Ridgecrest CA 93555              Fax  619 446-4337
>Home Page: http://www1.ridgecrest.ca.us/~mttc/mttc.htm
>Bulletin Board: 619 446-4087 
>
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Bob Neves
Director of Technical Services
MICROTEK LABORATORIES
(800) 878-6601
[log in to unmask]
http://www.TheTestLab.com


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