Journal of ELECTRONIC MATERIALS, Vol. 32, No. 6, 2003 Regular Issue Paper 558 Optimization of Black Oxide Coating Thickness as an Adhesion Promoter for Copper Substrate in Plastic Integrated-Circuit Packages MOHAMED LEBBAI, 1,2 JANG-KYO KIM, 1,3 W.K. SZETO, 1 MATTHEW M.F. YUEN, 1 and PIN TONG 1 1.—Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. 2.—QPL Ltd., QPL Industrial Bldg. Tsuen Wan, NT, Hong Kong. 3.—E-mail: mejkkim@ust.hk Copper-oxide coating applied onto the copper substrate has emerged as an al- ternative 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 frac- ture-surface examination. The emphasis was to establish the correlation be- tween the coating thickness, the surface roughness, and the interfacial adhe- sion 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 densi- fied 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 thick- ness, 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 im- prove the interface adhesion between copper-lead frame/substrate/foil and many different types of or- ganic 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 plat- ing with metals, such as Au, Ag, Ni, and Pd; 2 apply- ing a primer with an organic inhibitor, such as ben- zotriazole; 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 com- monly used to increase the bond strength between the copper and organic prepreg layers in multilayer printed-circuit boards (PCBs). 6,7 The effects of copper- oxide 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 pre- vent 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 neces- sitated a more detailed optimization of coating ma- terials and processes for improved adhesion charac- (Received May 21, 2002; accepted October 11, 2002)