Laser trimming for resistors on circuit boards is "available" from both ESI
and GSI Lumonics.  Both have presented papers on this subject at IPC meetings
in the past year.  Also, panel discussions on embedded components have been
held at IPC meetings.  There will be a feature area on embedded components,
including laser trimming, at next years IPC Expo in Long Beach in late March.

I understand all the lasers can easily trim the thin metallics used for some
of the resistors.  PTF formulations are more similar to the epoxy resin used
in the board, so it is a bit harder to stop the laser when the PTF is cut
thru, but the I understand the laser pulse power is set to do minimal damage
to the underlying epoxy.  The ceramic material DuPont is using for
fired-on-copper-foil resistors presents the most problem for laser trim.
However, I understand DuPont has incorporated a TiO2 containing "covercoat"
for their material, which turns out to be below the resistor in normal trim
operation.  This TiO2 containing material absorbs and scatters the laser
energy after it cuts the resistor and before it can damage the epoxy and
glass in the stack-up.

Embedded capacitors are presenting a bigger challenge in trim, as the
measuring equipment will take more development than the straight forward
resistor trim equipment.

Hope this helps all better understand the embedded passives capability.

Denny Fritz
MacDermid
Member of NIST Embedded Passives Consortium

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