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July 2003

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Subject:
From:
"FOX, Ian (York Rd)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 14 Jul 2003 08:46:22 +0100
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text/plain (73 lines)
Kaye, the addition of material to the exterior surfaces of PCB's does little
to prevent moisture absorption. If you carry out periodic weight gain
measurements under RTCA DO160 humidity testing conditions (we always use
category B, the 10 day test) on a resist coated PCB,  you find that better
coating materials reduce the absorption rate by about 20%, most by 10%, but
after 4 days of humidity exposure the PCB's are just about saturated (that
equates to about 0.7% for a basic FR4 and 2% plus for some aramids),
irrespective of what you have on the outside. Hope this is of some help.

Regards
Ian Fox
Goodrich Engine Controls

-----Original Message-----
From: Kaye Clayo [mailto:[log in to unmask]]
Sent: 11 July 2003 16:38
To: [log in to unmask]
Subject: [TN] Low moisture absorbing soldermask


Dear Technet:

I have just been asked if there is a soldermask that has a low moisture
absorption property. We have a pcboard that uses Taconics TLC material,
which is absorbing moisture, and the engineer would like to "encase" the
board with a low moisture absorbing soldermask to solve this problem.

Does anyone know of anything that will work, or another way around this
problem?

The engineer also asked me if anyone knew how to relate IPC-TM-650 2.6.2.1
"Water Absorption, Metal Clad Plastic Laminates" to real life. What he means
is, how does this test method, which dunks a pcboard into distilled water
for 24 hours, relate to 70% humidity at 23C.

Thank you for your help.

Kaye Clayo
Sr. P. C. Board Designer
Keithley Instruments, Inc.


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