New developments in the adhesion promotion of electroless Ni or Cu films to polyimide substrates - Polyimides and Other High Temperature Polymers - page 309

fers to PI surfaces a specific chemical reactivity allowing a direct attachment
of palladium-based catalysts. Two routes were used for this purpose. The
former involves the chemisorption of the Pd +2 ions present in a dilute acidic
PdCl 2 solution onto the grafted nitrogenated functionalities, and consequently
the formation of strong covalent C-N-Pd bonds. The latter uses the deposit of
a metal-organic thin film (here a palladium acetate thin layer) from a solution
containing the precursor.
2. Grafting of oxygenated functionalities (through the use of plasma or VUV
treatments in an O 2 atmosphere at reduced pressure) also confers to PI sur-
faces a specific chemical reactivity allowing, in this case, a direct attachment
of tin species. Under these conditions, the creation of Pd +2 sites requires the
use of the conventional sensitization / activation method (involving both tin
and palladium species). This route suggests the formation of strong C-O-Sn
(Pd) covalent bonds.
3. Regardless of the process used to attach the palladium-based catalyst (PdCl 2
or Pdac route) on surfaces grafted with nitrogenated groups, the catalytic spe-
cies are chemisorbed in the Pd +2 state while the triggering of the electroless
metallization occurs only when these bonded Pd +2 species are, at least, partly
reduced to the Pd 0 state. In this study, both laboratory-made and industrial Ni
and Cu plating baths were studied. In the case of the laboratory-made Ni plat-
ing solution, the Pd +2 reduction occurs spontaneously in the solution due to
the presence of the hypophosphite reducing agent. In all the other cases (in-
dustrial Ni bath and laboratory-made or industrial Cu bath) the Pd +2 reduction
does not take place spontaneously in the solutions. This is due to the presence,
in the industrial baths, of stabilizers which probably block the Pd +2 catalytic
sites. Furthermore, in the case of Cu baths, the Pd +2 reduction by formalde-
hyde is very slow and does not take place in a reasonable period of time. Un-
der these conditions, it is necessary to carry out the Pd +2 reduction either in a
hypophosphite solution or under VUV-irradiation under vacuum before sub-
strate immersion in the previously cited plating baths.
4. Electroless Ni or Cu films deposited onto PI substrates through the simplified
process involving surface functionalization in a nitrogenated atmosphere, and
a subsequent “activation” through the PdCl 2 or Pdac route, and qualified in
this work as bright and adherent, exhibit good adhesion characteristics when
the relevant metal / polymer systems are subjected to a cross-cut peel test.
However, the effects of different surface treatments of the polymer surface
cannot be distinguished using this test. On the other hand, an original ap-
proach based on measurements of the change in the electrical resistance of the
metallic film during a fragmentation test on the coated specimens appears to
provide interesting prospects. Experiments carried out in this work dealt only
with electroless Ni deposits. Nevertheless, they show that the tin-free proc-
esses described in the present study can be advantageously compared in terms
of practical adhesion of Ni deposits to those involving the conventional sensi-