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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