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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