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Reply To: | TechNet E-Mail Forum. |
Date: | Thu, 12 Mar 1998 14:36:36 -0500 |
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I have come across this problem at an account in Montreal -but never found
out what caused it.
we looked into the cure cycles on our LPI -different hold times
did a UV bump to fully cross link/cure
used different cleaning methods/solutions
different finishes -gloss-semi matte
matte was not an option
tried different mask
looked into the method or gun used to apply the
conformal coating
i felt the wave solder was removing whatever caused the
dewetting and that may explain why the solder
side was ok
We also found there was no dewetting when we sprayed a few sample boards
at another location. you could have your supplier do the same
afri
On Wed, 11 Mar 1998 [log in to unmask] wrote:
> Many of you provided useful information to us last year when we were
> preparing to change from dry film solder mask to liquid photoimagible (LPI)
> mask. I could use some of your expertise again.
>
> We have now converted to LPI. But recently we had a run of boards on which
> there was significant dewetting randomly scattered over the surface--on the
> silk screened side only. We have now experimented with a number of test
> boards and we get the same phenomenon--dewetting on the silk screened side
> only. Cleaning the boards in alcohol before coating helps some. Aqueous
> cleaning helps much more (DI water with detergent added per manufacturer's
> specifications--our normal board cleaning process). But excessive
> dewetting remains.
>
> In all the tests dry film boards were included as controls and no dewetting
> occurred on any of them. (Our supplier can find no change in his process.)
>
> In addition to the possibility of a contaminant being added in the silk
> screening process we question whether there is adequate curing of the mask.
> However, there is no dewetting on the non-silk screened side. Moreover,
> the silk screen ink is UV cured so that it seems the mask on that side
> would be more thoroughly cured if its degree of cure differed at all. (We
> use a polyurethane conformal coating).
>
> Any suggestions would be appreciated. Thanks.
>
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