Adhesion Enhancement of Electroplated Cu/Electroless Ni Composite Layer to (PMDA-ODA)-Type Polyimide Films

Abstract

In order to improve the adhesion strength of electroplated Cu/electrolessly plated Ni (EN)/PI laminates, four kinds of potassium hydroxide (KOH)-based solutions with different amounts of ethylenediamine were prepared and the surfaces of pyromellitic dianhydride-oxydianiline (PMDA-ODA)-type polyimide (PI) films were modified with these solutions. The surface of PI films was characterized by contact-angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The surface energy of PI films was calculated by the Owens–Wendt equation using the contact angle data of de-ionized water and diiodomethane (purity 99%). Adhesion strength of modified Cu/Ni/PI laminates was measured in terms of peel strength using the T-peel test. The results showed that the surface energy of PI films increased from 38.9 (unmodified) to 65.9 mJ/m2 (modified with solution No.4, i.e., solution containing 5 mol/l KOH and 90 mol/l ethylenediamine, for 180 s). XPS spectra showed that the surface of modified PI films incorporated a potassium-based product which is due to the formation of potassium polyamate in the modified layer through imide ring-cleavage reactions. AFM results revealed that the surface modification decreased the surface roughness of PI films, which is ascribed to the removal of the surface protrusions by etching during the modification process, and also revealed that it is desirable to modify PI films with a KOH-based solution containing a suitable amount of organic additive (ethylenediamine) to obtain a smooth EN layer. The T-peel test results showed that the peel strength of Cu/Ni/PI laminate reached a maximum when the PI film was modified with solution No. 4. The correlation between the peel strength and the surface energy of PI films revealed that the enhanced peel strength was mainly attributable to the increased surface energy of PI films by the modification process.