Peter,
I will try to help with rough finish issue.
More than likely it is caused by "pin holes" or micro bubbles.
This can occur if the coating is drying to quickly (warm surface, wrong viscosity, etc.).
High humidity (above 60% RH) can also cause a rough surface appearance.
This condition is not good because it compromises the coating integrity.
Hope this helps.
David A. Douthit
Manager
LoCan LLC
[log in to unmask] wrote:
> Hi, Folks,
>
> I have a theory about one phenomenon out of two that I have just witnessed,
> but seek your sage thoughts on the matter.
>
> Our fabrication shop was spray coating an FR4 / HASL-finished board with
> Humiseal 1B31, which was masked with some very expensive Kapton tape. The
> tape is meant to be a no-static, no-residue type. We noticed that, under
> the UV inspection lamp in the spray booth, the coating appears to recede
> from the edge of the Kapton tape - at least there is a "line" 1 or 2 mm
> wide at the edge of the masking, that indicates (to me) a different
> thickness of coating in that zone compared with the rest of the board. We
> tried over-spraying the affected areas again once the initial coat had set
> a bit, but the line reappeared. It only happens where there is Kapton tape.
>
> At the same time, the coating on the solder side of the board is a nice
> shiny finish, while the component side has quite large areas that have a
> more matte, slightly "rough" appearance. I doubt there is a connection
> between the two phenomena, but you never know.
>
> My theory about coating recession from the tape is that there is a static
> charge on the Kapton tape, in spite of the "no-static" presumption, and
> that the coating particles in the spray are likewise charged from their
> passage through the spray gun. The two like charges are then repelling one
> another. I noticed that neither board nor spray gun are grounded, and think
> that they should be (comments, anyone?).
>
> My secondary theory is that the coating is reacting with the tape adhesive
> at the edge of the tape, creating a line of contamination that the coating
> doesn't stick to properly. Kapton tape is a commonly used masking material,
> though, and I haven't heard of this problem occurring with it before.
>
> I haven't come up with an explanation for the matte areas of coating on the
> component side of the board. The board had just come from the customary
> cleaning and drying that I give all boards just prior to coating.
>
> Can you come up with possible explanations and cures for what is happening
> here?
>
> MTIA
>
> Peter
>
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