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June 2004

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Subject:
From:
"Brooks,Bill" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 8 Jun 2004 12:21:46 -0700
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Scott,

Moisture in and of itself is non-conductive. It's the ionic contamination or
under etching or surface finish dendrite growth that typically causes
shorts, or voltage break over failures.

All things being equal, I would expect that the shop that did the boards
skipped baking them before applying the solder mask... This may have allowed
dendrites to form between conductors through the moisture which would have
performed as an electrolyte.

I would suggest making a section of the bad area and take microscopic
pictures of the affected area and you might see the failure mode for the
boards.

Best regards,

Bill Brooks
PCB Design Engineer , C.I.D., C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
http://pcbwizards.com

-----Original Message-----
From: Scott Lefebvre [mailto:[log in to unmask]]
Sent: Tuesday, June 08, 2004 11:32 AM
To: [log in to unmask]
Subject: [TN] Soldermask Dielectric DC properties

We have a PCB that two traces have shorted and blown a set of traces on the
PCBs.  The trace width and spacing is correct for 150 V and is not a new
design.  We recently changed PCB shops.  We found some residual moisture
about 1/8" square in close proximity of two traces after assembly and after
test.  The traces were entirely covered with LPI soldermask.

I understand that the moisture should be removed prior to test.  My question
is shouldn't the soldermask have insulated the two traces from the moisture?

Scott

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