I've checked the technet archive and have not found a similar problem, so
here goes.

I have a problem with a small mobile product that is failing in field
trials.  One area we have considered is a combination of RH and the level
of ionic contamination affecting high impedance areas of the PWA to cause
the problem.  (The problem exhibits itself as a RAM corruption and the S/W
engineers are busily making sure it isn't their problem.)

An external lab checked a sample of the PWA's using the Omega method and
found the average level to be about 18 ug/sq.in.  (I'm aware that 10
ug/sg.in. is the recommended limit.)  The PWA uses 0402 sized resistors and
is the finest pitched assembly that the manufacturer in China who builds
this and other boards for us uses.  A no clean process is used.  The other
products built for us by this manufacturer are OK in terms of ionic
contamination and field performance.

Most of the failures in field trials have occurred not too far from the bay
in one of the suburbs of Sydney where the field trials are in progress.
The temperature is not high (15 - 20 degrees C), but we are now in the
rainy season.  One day, when it rained in the suburb nearer the bay, and
not in the other field trial areas, 6 of the field trial units failed in
the one area.

My questions are:
1.  Does anyone have experience that indicates that the 10ug/sq.in. level
for ionic contamination is OK for PWAs that use 0402 sized components?  Or
does experience indicate that a lower level is more appropriate and if so,
what level?  (It don't know the exact pitch of this board, but am waiting
for the answer.)
2.  Is it possible to determine an impedance above which one should be
concerned with a level of ionic contamination for a given voltage?
3.  If it turns out that for this design, RH and the level of ionic
contamination are causing the problem, can the situation be improved by
conformally coating the board or areas of the board?  (If so, can someone
please point me to the applicable standards for conformal coating?  Thanks.)

Best Regards,

Chuck Mays

Invetech Operation Pty Ltd
[log in to unmask]

Tel 613 9211 7700
Fax 613 9211 7702

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