New developments in the adhesion promotion of electroless Ni or Cu films to polyimide substrates - Polyimides and Other High Temperature Polymers - page 287

New developments in the adhesion promotion of
electroless Ni or Cu films to polyimide substrates
M. CHARBONNIER, Y. GOEPFERT and M. ROMAND ∗
Laboratoire de Sciences et Ingénierie des Surfaces, 43 Boulevard du 11 Novembre 1918,
Université Claude Bernard - LYON 1, 69622 Villeurbanne Cedex, France
Abstract—Electroless metallization (or electroless plating) of polymers and composite materials is
a multi-step chemical process requiring: (i) the pre-conditioning of the sample surface to render it
chemically reactive, (ii) the activation of the so-modified surface through the adsorption of a palla-
dium-based catalyst, and (iii) the metallization operation itself. In the present work, electroless plat-
ing was carried out on polyimide substrates and step (i) was performed via plasma or VUV irradia-
tion-assisted treatments operating especially in nitrogenated atmospheres (NH 3 or N 2 ). In addition,
step (ii) was accomplished using two different routes. The one is based on the chemisorption of Pd +2
species (from a PdCl 2 solution) on surfaces which are previously grafted with nitrogenated function-
alities. The other involves deposition (e.g. by spin-coating) of a palladium acetate (Pdac) thin film
on the same substrates functionalized in the same way. In both cases, the so-grafted Pd +2 species
have to be subjected to a reduction to Pd 0 before becoming catalytically active towards the metalli-
zation initiation. In laboratory-made Ni plating baths in which there are no stabilizers this occurs
through the reducing agent (sodium hypophosphite NaH 2 PO 2 ). On the other hand, in laboratory-
made Cu plating baths, such a reduction by formaldehyde (HCHO), although thermodynamically
possible, does not take place because of a too slow kinetics. Note that a similar behavior is observed
with both commercial Ni or Cu plating baths due to the presence of stabilizers which probably mask
the potential catalytic sites when the Pd +2 -grafted surfaces are dipped in these baths, and therefore
prevent Pd +2 species from being reduced. Under these conditions, the reduction of the Pd +2 species
has to be performed prior to the surface immersion in the Ni or Cu plating baths. For this, one sim-
ple method consists in dipping the Pd +2 -grafted surfaces in a hypophosphite solution. Another route
consists in subjecting the same samples to a VUV irradiation under vacuum. However, the experi-
ments show that the kinetics of the Pd +2 reduction to Pd 0 is clearly more favorable in the first case
and under the experimental conditions used. In short, the work described in this paper has been suc-
cessful in leading to a better understanding of the chemisorption mechanisms of the palladium-based
catalysts on functionalized surfaces and of their role in initiating the electroless deposition, as well
as in allowing the development of new approaches for electrolessly metallizing polymer-based ma-
terials.
Finally, this work has shown that electrical measurements carried out in conjunction with a
fragmentation test appear to be able to distinguish the effects of different surface treatments of
polymer substrates on the practical adhesion of the metal film on the polymer surface.