There are a few reasons why you would be seeing variation in the amount of UV
engery required to achieve a full exposure in  LPI processing and as well as
not always being able to achieve complete exposure at 300 mj/cm2.

Just a couple. . . 

1) The thickness of s/m deposit has a large effect on how much light is
required to achieve the recommended stouffer step reading.  I have seen LPI
applied generally from 0.7 mils - 2.0 mils dry thickness, and in terms of
exposure, this will give you very different results with the same amount of
exposure energy.

2) The tack dry cycle prior to exposure can effect the results of a known
amount of exposure with all other factors being equal.  The longer or harder
an LPI is tack dried, the higher the final stouffer step will read.  This has
to do with the solvents left in under-tack dried LPI and how they inhibit UV
cross-linking.

3) The amount of time a solder mask has been mixed for will eventually also
effect the stouffer step reading as well as other working parameters for the
material.

4) Many manufacturers recommend 300 - 600 mj/cm2 for exposure.  Please be
aware that there are many types of radiometers on the market, and for the
most part read differently.  I use an International Light IL390B light bug
and when measuring against one of the old blue Dynachem hockey pucks for
example, the pucks will generally measure 25% less.

Best Regards,

Van Miller
Electra Polymers
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