I don't know if you have the ability to accept or ship the PWBs on decisions
made by your own internal MRB. You can furnish test data to help the
customer in the final decision on acceptability.
As for a test to establish a warm fuzzy( I'll pass up the request for
royalties for the use of that term in a technical nature), I would test the
PWBs for 48 hours using the 85/85 humidity test. Wait one hour and perform
Hi-pot test at a minimum of 500 vdc or a 1000 vdc in your dielectric spacing
is adequate. If they pass and no excessive leakage is noted then you have
some grounds(or the lack there-of)to accept.
Assemblies are tougher. You need to replicate the thermal/humidity exposures
without exceeding the ratings of the components as a whole and perform an
electrical stress test, while still maintaining functional test level
capabilities.
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of - Bogert
Sent: Wednesday, September 15, 2004 3:36 PM
To: [log in to unmask]
Subject: Re: [TN] Photos of Conductive Contamination Under Solder Mask
...
September 15, 2004
The process used is to strip off the tin/lead resist to give bare copper
traces. then the scotch brite abrasion is done prior to solder mask.
Although most of the slivers are silver in color denoting a tin/lead/copper
sliver (as confirmed via SEM on a sample PWB), we have seen a few pure
copper slivers. We believe the tin/lead/copper slivers are being abraded
off the PTH, PADs and SMT land Areas.
We intend to prohibit future mechanical scrubbing as one of the corrective
actions, along with determining what needs to be done to prevent excess
copper build-up that can ultimately flake off.
However, we have not determined what to do with previously manufactured
product. About 2,600 suspect PWBs have been manufactured over the last
couple years. About half of these have been populated with components and
delivered to customers as completed modules. To date, there have been no
functional failures on any of the PWB's or modules pertaining to electrical
shorts or other failures that could be attributed to the presence of
conductive slivers.
The PWB's passed the shorts test at 50 VDC, and for those that were
populated a final functional test. The ionic contamination test just prior
to solder mask was performed (sample basis) and passed for each LOT of
boards. Each board is coated with LPI solder mask and ultimately Type UR
conformal coating.
For delivered equipment, it is not acceptable (for various reasons) to scrap
existing product and start over. Also, since the delivered product is used
in military equipment, it is not readily accessible for visual inspection
and repair/rework.
Based on the above criteria, we are leaning at accepting current product as
is and just fixing on go forward basis. Since the ionic contamination test
was passed, we don't believe there is any trapped ionic contamination (just
trapped conductive particles) under the solder mask. Since the boards are
solder mask and conformal coated, and no ionic contamination was found, we
feel there is a low risk for creating future dendrite growth under the
solder mask.
I would like your thoughts on this, and advice as to whether there is any
additional testing on either an unpopulated PWB or a completed assembly that
could be done to give us a warmer fuzzy feeling that accepting existing
product is technically acceptable. If you recommend additional testing can
you recommend who could do it, and a ball-park cost.
I cannot provide you with the specific environment our product is subjected
to. Just assume it is normal Class 3 military product subject to a humid
and temperature environment (< 85C).----- Original Message -----
From: Susan Mansilla<mailto:[log in to unmask]>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Wednesday, September 15, 2004 8:56 AM
Subject: [TN] Photos of Conductive Contamination Under Solder Mask ...
Thanks for posting the photos, Steve.
The photos look like peeling off of overhanging material like Tin - they
don't look like scrubbing debris.
The areas showing the conductors show pronounced grooves across the
conductors.
This remains a puzzle, but my previous indictment of the scrubbing process
coupled with inadequate removal of the tin/lead etchresist are the best
choices
for sources until you can tell us more about the samples.
Susan Mansilla
Robisan Lab
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