Craig: As pointed out by others peelable ammoniated latex temporary masks
will cause problems with exposed copper, however you are saying this product
is water soluble. First of all are you sure this is not a peelable type of
mask, that it really is water soluble (WS)?
If it is WS most are made up of acrylic polymers, water, surfactant and
alcohol. They can also have rheology modifiers depending on how the material
is applied to the board. These typically don't but "could" produce an
interaction with the copper.
It's possible that when the WS mask is applied wet to the board it could be
skinning over and remaining wet underneath causing a reaction with the
copper over time. This reaction with the copper surface results in a wicking
back at the copper/LPI interface causing delamination of the soldermask in
the application area.

As a test apply some of the WS mask directly to the copper on the board away
from any permanent soldermask. Do this on two different boards. Let one sit
for a couple of days to air dry, bake the other one immediately after
application at 250-270F for 5-7 minutes. Wash both and look at the copper.
Does the air dry look worse than the baked board? If it does than there
could by a compatibility problem with the WS mask and copper over time. If
they both look bad then there is definitely a compatibility problem, try a
different WS mask. If they both look good then the board may have improper
surface prep and/or under cured mask, review your prep process and increase
the bake time/temp and try a UV bump. Your tape adhesion testing is not a
guarantee of properly cured mask.

In any case it wouldn't be a bad idea to try a couple of different WS masks
in conjunction with a few different permanent soldermasks over copper boards
to see if there are chemical compatibility issues.



Regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett WA

425-446-6076
-----Original Message-----
From: Craig Ropp [mailto:[log in to unmask]]
Sent: Friday, March 09, 2001 10:56 AM
To: [log in to unmask]
Subject: [TN] Solder Stop affecting LPISM adhesion of pad openings on
Copper Groundplanes


Has anyone had any experience with Solder Stop (water soluble mask)
affecting LPI adhesion around pad openings on Groundplanes. Typically
adhesion issues almost always relate back to inadequate surface prep during
the board manufacturing process. I had to "see it to believe it". We have
reviewed every step of the process and cannot find any disconnects. We have
only one Customer experiencing this problem. They have used the same Solder
Stop for approx 5-6 yrs. At 1st it was believed that the peeling didn't
occur until after Wave Solder. However, they had coated some bds and
allowed them to set over the weekend and after washing them off found the
peeling mask. We tape test all boards at our facility per IPC-TM-650
procedure. There has been no evidence of peeling at all. After visiting
with the Customer it seems that the time the Solder Stop is on the bds
(i.e. longer than 2 days) determines whether or not peeling will occur. The
peeling is always on pad openings which are on Copper groundplanes. We have
no evidence of any peeling on pads which are surrounded by FR4. The peeling
is usually limited to an area concentric to the mask opening and extends 5-
30 mils into the surrounding mask. We have run tests at one of our other
facilities consisting of running numerous bds across the wave solder at
different preheat and solder pot temps (we even matched the temperatures
the Customer with the issue is using) with no evidence of peel. The
customer is adding approx 1 cup of water to 1 Gal of Solder Stop to aid in
flow. It was requested not to refer to the Material's manufacturer until we
have additional data. Any suggestions or help you can provide will be
greatly appreciated.
Thanks,
Craig

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