Ron Rawlyk asked:

>We are a medium sized company that designs and manufactures CCAs for commercial
>and military applications.  We currently have four different manufacturers of
>PWBs.  We are experiencing a problem with one type of soldermask that is
>provided by our vendor of choice.  The soldermask is liquid photo imagable
>and is IPC-SM-840 class 3 certified.  The name of the soldermask is PSR 4000
>manufactured by Taiyo America.

During a PWB vendor qualification in 1993, I was given a proprietary report
on independent qual testing of masks by a major board fab vendor. Taiyo
PSR-4000 was tested against 5 other well known LPI masks. It was rated
superior for hydrolytic stablility and thermal shock. I know this is a
popular product in Japan, and IMO the product has good industry acceptance.

>Our vendor feels that the soldermask is deteriorating after our semi automated
>saponified aqueous cleaning process.  Over copper substrates the soldermask
>develops small white spots that range in size approximately .010 to .020 inches
>in diameter.  Soldermask covering copper traces.010 to .015 inches wide has
>turned white over 50 to 80% of the trace length.  This occurs both over the
>solder side and component side of the PWB.  The deteriorated soldermask on the
>traces then looses adhesion to the copper traces leaving bare copper exposed.
>
>Our assembly process is as follows:

(snip)

>The washer recipe is as follows:  ( CCAs are washed in a Unit Design model
>ABC 2001 machine.)
>1. Wash cycle. Duration 4 min., temperature 165 F, 4% saponifier solution
>added.
>2. Quantity 3 rinse cycles.  Duration 1 min., temperature 165 F, 0% saponifier
>solution added.
>3. Dry cycle.  Circulating fan. 15 minutes at 150 F.
>
>The pH value for the washing solution is 10.8.

Your wash cycle is pretty long, uses very hot water, and adds caustic
agents. Polymers will nearly all absorb water. Could you use an in-line
cleaner with a much shorter cycle? Do you really need the saponifier? Is
your dry cycle effectively a high temp 100% RH environmental chamber?

I suggest you might just shorten the "dry" cycle, open up that dishwasher,
and let the boards dry several hours in open room air with beneficial
results.

Many people disagree with me, but I strongly favor the organic acid water
soluble paste and flux with a straight water wash. Soldering with pine tree
sap was developed by the Egyptians several thousand years ago. IMO, a more
advanced chemistry is available with OA flux, and residue removal is
comparatively easy. Of course, you may be faced with a customer requirement
that prohibits your use of this process.

The military (especially the China Lake site) was resistant to any change
from rosin fluxes for years. If the environmental movement had not
influenced use of CFC's and trichloroethane (which readily dissolve rosin
residues before taking out that ozone), I bet the military standards
requiring their use might still be in effect.

>This problem was first noticed about 9 months ago and only on one vendors PWBs.
>At that point discussions with the PWB vendor and soldermask manufacturer
>resulted in adjustments to our curing parameters and wash recipes.  The problem
>did not totally disappear but dissipated to a level that was acceptable.  The
>problem has resurfaced this week with a vengeance.

It may be due to variations in curing and/or thickness of the mask.

>I am interested in any feedback on occurrences of this problem and solutions.
>Specifically these questions.
>
>1. Are we the only people in this industry that are experiencing this problem?

Don't know, but I bet not.

>2. Is our cleaning process indeed causing the damage, or is it a process
>problem with our vendor?

Perhaps this is NOT an either/or situation. I _do_ think your long, hot,
chemically fortified "cleaning" cycle will be tough on _any_ thin polymer
coatings. Remember that rosin is essentially an unreacted natural polymer,
and you are probably getting every last bit of that stuff off. If the wash
solution diffuses into the mask, perhaps it is affecting the underlying
copper...

Try shortening the cycle, reducing the additives, and reducing the
temperature with split lots and look for changes in the effect.

>3. Is our saponified aqueous cleaning harmful for IPC-SM-840 class 3 LPI's
>in general?

I think so, but that does not mean it would be impossible to buy a mask
which _could_ stand up to it; irrespective of the IPC qualification
results. If that's what you have to do, move that direction with full
force.


****Your mileage may vary. No offense meant to any Egyptians, pine tree
fans, or China Lake residents.****


Jerry Cupples
Interphase Corporation
Dallas, TX
http://www.iphase.com