[log in to unmask] wrote:
> 
> 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
> [log in to unmask]
> 
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If you are trying to use an IL light meter (what ever series) you should
discuss it with IL and also read the very informative manual they
provide with the units.  In their words. "The only way to test exposure
of photo resist or mask is with a Stouffer (or Eqiv.) step scale".  Due
to the extreme wide variation of the light output freq. per exposure
unit the meter is only a referance to go back to a known machine and
compare readings from one time frame to another for the same machine.
hockey puck as we affectionetly call it.  I loved discovering the same
          verbage in my manual.

          I was shocked after spending $1400.00 only to find out it
          really has little value compared to the Stouffer when you
          move from Machine to Machine, Shop to Shop.

I do 100% agree with all your other comments about thickness, mixing
time and cure.  Trust your Stouffer but run various known thickness and
cure cycles on various panels to determine the right exposure for your
unit. 

You will deal with variations in thickness that is very broad on a
single setup, mesh, viscosity, screen technique and panel configuration. 

Hopfully Fisher from Dexter will also answer.  He is a master on this
process.

Kerry Grimes

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