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March 2000

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 8 Mar 2000 17:27:01 +0200
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Steve

Kyzen XJN is quite alkaline (pH c. 9,5) and, at the very least, may be expected to
swell a film which is developed in an alkali, possibly even with a slightly lower pH.
Furthermore, it contains alcohols which will lower the surface tension and therefore
aid absorption. If the curing of the mask is not 100% complete, my guess is that you
would have problems, including such as you describe, but possibly even worse ones that
you are not aware of. For example, to have a high pH, there are almost certainly some
ionic components in the cleaner. When the mask swells, the liquid will, to some extent
be absorbed. The first rinse, presumably with DI water with a pH in the 5-7 range. This
will have the immediate effect of shrinking the surface of the mask again, inhibiting
the egress of the imprisoned cleaner, which will leach out very slowly and surely not
within the time range of your total rinsing operations. This meanes that if your board
becomes damp in service, such as in a humid atmosphere, it is not impossible for some
ionic components to leach out. This is well-known and the developer itself will exhibit
a similar effect.

As mentioned before, 100% curing is essential. This means that the photoinitiation must
provide sufficient free radicals for the polymerisation (i.e. cross-linking of the
prepolymer) to be completed. I'm not sure of this, but I believe that cross-linking may
be inhibited if the material is not fresh or, at least, the curing cycle must be
modified. Check with the technical support of the mask maker.

My general attitude is that aqueous developed masks, especially dry film types, which
must, by definition, be applied in a thermoplastic form, exhibit a strong risk of
problems with alkaline cleaning and utmost caution must be exercised to ensure overall
process compatibility, possibly with a very narrow operating window between acceptable
and not acceptable. Plus, my experience is that some aqueous-developed masks, under any
conditions, may leach ionisable material under humid conditions.

Of course, surface preparation before laminating the film is, as you suggest, of first
importance, as well. Poor adhesion is usually symptomatic of at least one of the
following
- inadequate surface preparation
- low laminating temperature
- old material
- stockage of the material in the wrong conditions.

FWIW

Brian



"Stephen R. Gregory" wrote:

> Hi All!
>
> Maybe some of you remember quite a while back, I had a problem with some
> boards with the soldermask flaking off all the copper after 3-cleanings in
> our MCS-1000. It's basically a fancy-smancy batch cleaner. We use Kyzen
> Aquanox XJN at between a
> 25-30% concentration (recommended by Kyzen). We also run our wash temps at
> 140º F., and our rinse at 120º F.
>
> Back when it happened last time we narrowed things down to the soldermask not
> being able to take the Aquanox, it was a dry-film (Dynachem Dynamask). We had
> other boards that had dry-film on them that we built and showed no
> ill-effects from the cleaning. I looked into things and learned that these
> boards were masked with a Dupont 8100 series dryfilm...so made it a point to
> specify Dupont dry-film on the boards that require dry-film.
>
> Well guess what? I'm starting to see the problem again. Our fab vendor swears
> up and down they used a Dupont dry-film. What I see, is that after the second
> wash the mask will start to "bubble" and lift on the vias (and sometimes the
> traces too), the film hasn't broken yet (it's still intact, it appears to
> just be lifting from the copper), but after you wash the board the third
> time, the mask starts flaking off all the copper... vias, traces, everywhere.
>
> So I guess I'm starting to wonder now if it's really a mask compatibility
> issue with the Aquanox, or is it possibly something about  the quality of the
> surface prep prior to solder masking?
>
> Have any of you seen this before?
>
> Thanks!
>
> -Steve Gregory-
>
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