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March 1998

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Subject:
From:
Afri Singh <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 12 Mar 1998 14:36:36 -0500
Content-Type:
TEXT/PLAIN
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TEXT/PLAIN (69 lines)
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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