Ooh! This is a dodgy area...

First - Rick

Your question is the classic: "Is your conformal coating compatible with my
no-clean process?"

The answer is MAYBE!

Remember that a conformal coating - especially those that are qualified to
various standards and specifications - seals in as well as out. They are not
impervious, but allow air that might contain water to migrate in and out
through the film. If there any water soluble residues, especially remaining
salts from preceding processes, will very probably lead to electro-chemical
reactions that could be cause premature product failure.

It is frankly dangerous, or at the very least extremely misleading, for any
flux vendor or coating supplier to claim total compatibility with no-clean
processing.

There is a huge amount of new research data that PROVES this point, and is
why there shall soon be available a new "Process Characterisation" spec..

As to the issue of adhesion - this is a confusing area:

Does the coating stick to the component legs - the bare track - the
component bodies - yet it does not stick to the solder resist for example?

To my experience and knowledge, I have not yet come across an intelligent
conformal coating - so how the hell does it know the difference?

When carrying out some adhesion test, what are you trying to prove?

If the coating is a silicone, I virtually guarantee that the adhesion test
as suggested, will prove positive....but it shouldn't, because nothing
sticks to silicones, especially the adhesive tape used in the test!

If your end-product PASSES all your environmental tests, yet upon subsequent
examination the coating is seen to be peeling from the surface, do you have
a problem?

Now, Mike:
> Rick:
> We ran a fairly in depth study with Kester, looking at the compatibility of
> their no-clean fluxes and our non-cleaned PCA's with conformal coating(CC).
> The test used Kester's 256 no-clean solderpaste and the 245 no-clean core
> solder. We looked at them individually and also combined on the same board.
> This was done for both acrylic and urethane CC. We did not test Humiseal's
> 1B31 LOC acrylic.
>
This should not make much difference as the only change to this product is
the type of solvent used in the formulation to meet EPA VOC limits in the
USA.

> The test consisted of SIR and adhesion.

Urghh! Adhesion tests.
>
> SIR was done on B25 coupons, 65C/85%RH, 168 hours, readings were taken at
> 1,4,7 days under test conditions and recovery after removal from the
> chamber. We did not see any significant difference between controls,
> fluxed/coated and fluxed uncoated boards. They were all within one order of
> magnitude under test conditions.
> Visual inspection after SIR testing showed some signs of
> blistering/vesication on the boards with 256/245 with urethane coating but
> not on the acrylic. However there was no drop off in SIR on the urethane
> boards.

Ooohhh! This is even more dodgy.

Research has shown many major drawbacks with prevailing SIR test protocols,
which is what I am working hard to address and revise.

I shall not dwell too long here on this issue, but rather invite those who
want more data to contact me off-line. Briefly however:

B25 coupons might not be representative of your end-product.

65/85 testing may yield misleading data because at above 40DegC, no-clean
residues volatilise from the surface

Dendrites can form and collapse (unless preserved via current limiting
resistors in the test equipment) within 20 minutes or less. Thus making 4
measurements over 7 days might miss huge amounts of electro-chemical
activity that shows failure conditions.

The test voltage used, might not be representative of your end product.

The variation that you observed Michael, between the acrylic and urethane,
could be representative of the cured condition of the coating.
>
> The adhesion testing was done per ASTM Method D 3359-92a. They were all
> acceptable however the combined 256 and 245 residues showed the lowest level
> of adhesion. The 245 alone was better. The acrylic had better adhesion than
> the urethane.
>
> Conclusion:
> If you are applying the acrylic CC over just the 245 no-clean you "maybe"
> OK. When you start mixing different flux residues with other conformal
> coating chemistries you can create a chemical stew that's not compatible.
> Obviously you also need to consider incoming board cleanliness,
> contamination added by your handling and processing, adhesive properties of
> the soldermask, electrical requirements of the board, service life and
> conditions of the product, etc etc. So you will need to do some testing
> yourself to qualify the materials and performance.
>
>
>
> Regards
>
> Michael Barmuta
>
> Staff Engineer
>
> Fluke Corp.
>
> Everett WA
>
> 425-446-6076
>
>
Amen to all that. I should only add that liquid fluxes create undoubtedly
more serious process issues, because they are not as controlled in
application as compared to a paste flux.

Too often we have found customers who, because of poor solder joints, simply
turn up the flux amount and or pre-heat temperature, and thus grossly change
the amount of residues that may be present.

In short - do more testing before you commit. You might want to find out if
your various vendors can help you....

--
Regards, Graham Naisbitt
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Mobile: +44 (0)79 6858 2121

Concoat Limited - Engineering Electronics Reliability

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