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September 2004

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Subject:
From:
Jeffrey Bush <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Jeffrey Bush <[log in to unmask]>
Date:
Thu, 16 Sep 2004 07:42:51 -0400
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I would have the fab house begin to plate at lower ASF and look at
thicker photo resist (2 or 2.5 mil) to prevent the plate-over that they
are experiencing.  In a manual plating line this become more critical as
the plating control is more difficult due to the nature of manual rack
placement in the tank. 

I would also suggest retesting any raw boards at 250VDC/100 MegOHM
isolation to ensure that spacing is maintained.  A HYPOT test on a
sample of boards is also a good idea to ensure utility isolation is
robust - 500 VDC for 30 sec.  This should provide some feedback to
functionality of the boards that have the condition.  

As a note, if these PCB's are military they may violate the
undercut/overhang spec as well as the plating adhesion requirement for
no slivers or loose metal.



Jeffrey Bush
Director, Quality Assurance and Technical Support

                          76 Technology Drive - POB 1890
                             Brattleboro, Vermont 05302-1890
                                Tel. 802.257.4571.21 Fax. 802.257.0011
                                    [log in to unmask]
                              http://www.vtcircuits.com	



-----Original Message-----
From: - Bogert [mailto:[log in to unmask]] 
Sent: Wednesday, September 15, 2004 6:36 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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