Randy, though I can't comment on your particular laser system, it is
well known that microvias can have a profound effect on density.  A
chapter in Coombs' Printed Circuits Handbook (4th ed.) details the
effect.  The chapter suggests a concentration on reducing via dia. is
more significant than reducing line width, at least at this stage of
PWB development.  ITRI (co. I work for is a member) has lots of info.
on microvias, including their reliability.  A personal observation
and caution:  Look carefully at the pad arounnd the via.  The hole
dia. is (almost) not relevant because in-plane routing has to go
around the pads (annular ring), not just the hole.  So a 4 mil dia.
with a 10 mil dia. pad has 7X as much area in the annular ring
portion as for the via (pi-r-squared).  (We are working on
demonstrating padless microvias to eliminate this problem, for outer
layers only).  There are a number of ways of making microvias,
perhaps 6-8+ ways, and the debate over which will "win" rages on (we
have our own bias).
The "Initial Three" are plasma, laser and PID <-photoimageable
dielectric>.  My own co. - here's the comercial! has an emerging
method callled IMPRINT Patterning.  There will be a number of papers
re microvias and sequential build up at IPC Long Beach conference the
week of 4/26/98 (I'm speaking 4/28 AM).  Good luck.
George Gregoire, Dimensional Circuits Corp.
DCC is the inventor of Imprint Patterning, a new method of patterning
that does not depend on photosenstivie materials at all.  It is a
form of "precision hot stamping", similar to the way phono records
and compact disks (submicron features) are "imprinted" in volume.
For further info. fax me at (619) 549-4292

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