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November 2001

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Subject:
From:
Ian Hanna <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 22 Nov 2001 08:49:28 -0500
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just a quick note on lasing...i found it, once dialed in, to be a very good
process.  the biggest deal besides ablating the holes is plating
them...sounds like an old song now.  it took me less than a week to work up
a very wide process window for 1080 fr-4 with an nd/yag UV laser.
everything looked great on the microsections but we found we had about .5%
fall out due to plating voids in the finished boards...a difficult number to
really work with, too slim for easy failure analysis yet one or two opens
per board didn't work...used really low amps and a couple of 'cheats' at
electroless (metalization) to get the job out...and ended up going to
complex waveform reverse pulse plating...which made things more or less a
cake-walk...bottom line is that if i was to bring laser drilling in house
again i'd sub-contract the drilling (that is hold off on the rather
substantial capitol investment in a laser via machine) until i had the rest
of my processes dialed in...registration on n-1 layers...coordinating the
registration between the mechanical hole grid and the micro-via
placement...and of course reliable process....one of the more 'holistic'
processes i've worked with...everything needs to work together, you can't
just ship out panels for drilling and process them the same old way...

might as well throw in my $.02 worth on your grinding idea....i'm a little
concerned about the integrity of the 'knees' of the plated barrels...it's
always nice to see an even coating of rich, thick, well leveled copper going
from the surface all the way into the barrel of the hole...interrupting
this, then re-plating over top -might- not give you as solid a 'knee' and
with board expansion from z-axis CTE mismatches so common in fr-4
construction, you might see a higher than normal drop out well after the
fabrication fact...i'd do some fairly aggressive solder floats, at least 5x
10 seconds in a pot of 550 degrees and look for any signs of failure...to
ensure that there are no long-term reliability issuesthat being said,
i've plated, filled and ground lots of blind vias without too many
reliability issues in avionics applications...so it's do-able...just so
you're aware of potential pit-falls really...


good luck...sounds like an exciting project



ian

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