Photimagable Laminates for Thin Applications

Photimagable Laminates for Thin Applications

      A process is described here for making thin cores containing
interconnects without the use of drilling.  The process uses a
photoimagable dielectric material and conventional fabrication
processes.

      The fabrication of a thin 2S structure is best accomplished
with a photoimagable dielectric in a dry film form.  A suitable
material is one which can be applied as a film, have rheological
characteristics making the material tack-free but not brittle, able
to be thermally cured, and developable with acceptable contrast in a
suitable developer solvent.  While a photoimagable material is not
absolutely necessary, these properties can be met using a dry film
soldermask such as Vacrel* 8230 or ASM (*).

      The first step is to laminate or otherwise apply the dielectric
to the treated side of copper sheets.  Next, two sheets are laminated
together to give a sandwich structure of two layers of dielectric
between two sheets of copper.  Alternatively, a sheet of copper could
be laminated to give a sandwich containing one layer of dielectric.
The copper is etched using a photoresist and process to protect the
copper except for the via openings.  Next the resist is stripped, and
the dielectric developed out of the holes.  Curing the dielectric is
then done using a thermal bake cycle sufficient to fully cross-link
the dielectric polymer.  The sandwich core is now ready for plating
in the vias, after which it is circuitized using typical subtractive
technology.

      In one example, ASM dry film soldermask was used for the
dielectric.  ASM was applied to 1 oz.  copper foil using a hot roll
laminator at 110C.  Two layers of ASM on copper were laminated
together using the same technique.  The via openings were etched as
described above.  Since the ASM is a solvent developable material,
the dielectric was not affected by the subtractive aqueous resist
processing.  Next, the sandwich cores were developed in
gamma-butyrolactone for 60s.  In this step the ASM is still
completely unpolymerized, but the copper acts as an etch mask to
ensure the dielectric is developed only out of the holes and is not
significantly undercut by the developer.  After develop, the cores
are cured by baking at 185 degrees C for...