Anders,
In "high" humidity the absorbed(condensed) water can form an electrolytic
solution of high enough conductivity to carry current with subsequence
anodic dissolution and cathodic dendtries to form and may cause electrical
breakdown. The easiest thing to do is put a section of soldermask in the
oven and measure the conductivity.
Hopes this helps.
Richard Haynes

-----Original Message-----
From: Pedersen, Anders <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, November 05, 1998 11:10 AM
Subject: [TN] Soldermask Breakdown in Humidity Chamber


>We have recently seen a failure of an assembly in a humidity chamber.
There
>were two things observed.  First, on an external layer, power traces (+25V)
were
>very close to ground traces (~20 mils), and an apparent electrical bridge
caused
>a short and a burned the area.   Secondly, other areas of the board
appeared to
>have white residue and broken down soldermask bridging between connector
pins.
>Some of these areas took up a number of pins (thumb print size).
>
>I vaguely remember and issue where assemblies that were not thoroughly
cleaned
>could leave residuals (chlorides?) and that when subjected to a humid
>environment could break down soldermask.  I also believe that humid
chlorides
>can crystallize and can be influenced by electrical potentials between
pins -
>causing shorts.  However, that's where my memory breaks down.
>
>Can anybody provide some clarity on potential reasons for soldermask to
>breakdown in a humidity chamber?
>
>Thank you,
>
>                Anders P. Pedersen                         * 407-729-2143
(ph)
>        Harris Aerospace                                   * 407-729-2097
(fax)
>        P.O. Box 94000, MS22-4842                  * [log in to unmask]
>        Melbourne, FL 32902                        http://www.harris.com/
>
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