In our experience, reducing the pigmentation in LPISM's can improve 
undercut considerably.  This can be particularly important in the case of 
nickel gold finishes or when fine pitch solder dams (around 3 mil) are 
required.  (Nickel gold has a tendency to 'get into' LPISM undercuts at 
pad edges reducing adhesion).  The down side used to be strikethrough on 
thin laminates.  Dark coloured LPISM's tend to stop UV passing through 
the laminate and affecting the 'reverse side' image during exposure. The 
advent of substrates containing UV blockers has largely overcome this 
problem.  AS a result most LPISM suppliers are now looking to produce 
masks with lower pigmentation.  Unfortunately such masks don't hide flaws 
in the circuit underneath..... but that's another story!
-- 
Bob Courtney
Coates Circuit Products
Tel (+44) 1761 414471
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