Interface adhesion between copper lead frame and epoxy moulding compound: Effects of surface finish, oxidation and dimples

A comprehensive study is made of the effects of dimple and finishing material of copper alloy lead frame on interface adhesion with epoxy molding compound. The surface plating materials studied include copper alloy, microetched copper, Ag, black oxide coating, Ni, Pd/Ni and Au/Ni plating. Special emphasis was placed on the effects of various stages in manufacturing processes, including post mold cure, thermal cycling, IR reflow and temperature and humidity storage, on the changes in interface adhesion. The interface bond strength was measured using the lead pull test. The lead frame surface was characterized using various techniques, such as atomic force microscopy, X-ray photoelectron spectroscopy, ellipsometry, Raman spectroscopy and contact angle measurements, to correlate the measured interface bond strength with the surface characteristics of various natures. It was confirmed that dimples etched on the lead frame surface offered several mechanical anchoring mechanisms and much improved interface bond with moulding compound. The interface bond strength and the number of dimples exhibited roughly a linear relationship. The black oxide and Pd coated surfaces were shown to provide the strongest interface adhesion, with the highest wettability amongst various surface finishes studied. The measurement of contact angle is proven to be an efficient way of evaluating the metal surface quality for interface adhesion with polymer resin.