Copper-oxide coating applied onto the copper substrate has emerged as an alternative to metallic coatings to improve adhesion with polymeric adhesives and molding compounds. The interfacial-bond strengths between the black oxide-coated Cu substrate and epoxy-based, glob-top resin were measured in button-shear tests, and the failure mechanisms were identified from the fracture-surface examination. The emphasis was to establish the correlation between the coating thickness, the surface roughness, and the interfacial adhesion with respect to treatment time. It was found that at the initial stage of treatment a thin layer of flat, cuprous oxide developed, above which fibrillar-cupric oxide was formed with further treatment until saturation with densified fibrils at about 150 sec. The interfacial-bond strength between the oxide-coated copper substrate and glob-top resin increased gradually with increasing treatment time, until the bond strength reached a plateau constant after a treatment for about 150 sec. There was a functional similarity between the oxide thickness, the surface roughness, and the interface-bond strength with respect to treatment time. A treatment time of 150 sec is considered an optimal condition that can impart the highest interface adhesion.

Key words: Interfacial adhesion, copper substrate, black oxide, coating thickness, surface roughness

INTRODUCTION

Delaminations at various interfaces are one of the most critical reliability issues in plastic packages,1 so significant research has been performed to improve the interface adhesion between copper-lead frame/substrate/foil and many different types of organic resins/adhesives/prepregs. Strong adhesion of metal to polymer has been achieved by modifying the chemistry and topography of the metal surface. Surface chemistry has been modified through plating with metals, such as Au, Ag, Ni, and Pd;2 applying a primer with an organic inhibitor, such as benzotriazole;3 vacuum deposition;4 ion implantation; and ultraviolet cleaning. Mechanical roughening has also been successfully employed to modify the topography of the metal surface.

In addition to the aforementioned metal plating, cleaning, and mechanical roughening techniques, copper oxide is another useful chemical-conversion coating for improved adhesion of copper with various polymers and has been widely employed since the early days of printed-circuit technology.5 The black-oxide-treatment process was originally developed to produce an antique black finish on copper and brass parts for decorative applications. To form the oxide coatings, sodium chlorite and sodium hydroxide are used at high concentrations and temperatures near boiling points. The black oxide treatment is commonly used to increase the bond strength between the copper and organic prepreg layers in multilayer printed-circuit boards (PCBs).6,7 The effects of coppe-roxide coating in PCB applications are known to be two-fold: one is to increase the available surface area for bonding by growing copper-oxide crystals; and the other is to passivate the copper surface to prevent contamination during manufacturing processes at elevated temperatures. The recent rapid growth in multilayer PCB production and the extension of copper-oxide coating technology to integrated circuit (IC) packages, especially for copper-lead frame and tape ball-grid array (TBGA) packages,8 have necessitated a more detailed optimization of coating materials and processes for improved adhesion characteristics and manufacturability in assembly. In a typical TBGA package, a glob-top resin encapsulates the die and wire-bonding assembly on a copper heat sink/stiffener, whose surface needs to be treated to achieve strong adhesion with the glob-top resin and tape adhesives.

Three different types of copper oxide with distinct colors, namely, black oxide, brown oxide, and white oxide, have been developed with different performance and applications because of their different crystal structures. The classic black oxide is most common and widely used, as it can be processed at low temperatures. The outer surface of the black oxide layer is primarily cupric oxide (CuO), and there is a gradient through the thickness that becomes progressively richer in cuprous oxide (Cu^sub 2^O).9,10 The long, acicular-cupric crystals pose significant problems of easy removal from the base copper when subjected to scrubbing and contact during subsequent assembly processes. To avoid unwanted removal of the loosely bonded oxide layer, several surface-hardening techniques have been applied based on a shot-peening process with tiny, hard particles, such as glass beads or alumina. To retain the bond strength in high-temperature PCB lamination processes, new oxide processes were developed, producing more densely packed, shorter needle crystals with brown color across the surface.11 White oxide is a primer system, consisting of multiple layers of tin/tin oxide and a cross-linking silane agent on the copper surface. Passing through a tin-sulphate spray chamber forms tin/tin oxide layers, while applying a solution of organosilane forms the cross-linking layer on top of the tin oxide layer. The tin-silane multilayers can have very strong covalent and polar bonds through double-bonding mechanisms between the silane and polymeric resins.12