David Fulmer,
In my experience at AMP with fine pitch tin plated connector contacts and in
some cases, pcbs for same, I'd go with covering your guard rings with a good
moisture resistent solder mask. You see, moisture and closely spaced tin
plated surfaces with a voltage potential between them, starts dendritic growth
of tin "whiskers". If allowed to go far enough, you could end up with a
profound low impedance short between the entities involved. And the term
"closely spaced" is somewhat relative to the amount of moisture around.
Depending on the "input pin" functions in question, even a higher impedance
level of feedback leakage or grounding could definitely cause output drift
in an op amp circuit, particularly if the gains are high.
Another culprit may be the post soldering cleaning process (or lack thereof).
Certainly the general effects of this issue are well documented some where
else and are usually appreciated, if not totally understood by most assembly/
fab houses. Check with your assembler if you feel this may be part of
the problem. When the lab I work for at AMP prepares board mounted product
for testing, it is SCRUPULOUSLY and THOROUGHLY cleaned AND DRIED, using
processes appropriate to the fluxing system used to solder the parts.
Hope the above helps.
Leonard F. Bendiksen
Test Engineer and
PCB/Fixture Designer
AMP Incorporated
Harrisburg,PA
USA
[log in to unmask]
(717)780-6493
----- Begin Included Message -----
[log in to unmask]: Wed, 21 Aug 1996 14:06:29 PST
Subject: Analog guard rings
X-Mailer: Juno 1.00
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From: [log in to unmask] (David M Fulmer)
Topic: Surface moisture affecting operational amplifier output
drift on a FR-4 multi-layer PCB.
(board layup) - 1). component layer, 2). ground plane, 3).
power plane, 4). solder side layer
Question: When shielding the input pins of an operational
amplifier with guard rings, is it recommended to
expose the guard traces (solder over tin reflow), rather than
covering them with solder mask ?
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