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Research on Discoloration Problem of Electrolytic Ni/Au on High Density Package Substrate

As an important component of integrated circuit, package substrate is an essential intermediate product for electrical interconnection between chip and printed circuit board. The functions of package substrate are protect circuit board, dissipate heat, and provide electrical interconnection. Electrolytic Ni/Au is generally selected as the surface plating layer in the bonding pad of the package substrate. Gold has oxidation resistance and low resistance, and nickel is used as the barrier layer to prevent copper element from diffusing to the surface of the gold layer. The excellent gold-plated layer should have a compact surface and a golden yellow surface, but there will be discoloration problem of gold surface in production. During wire bonding, the discolored substrates will have problems such as non-stick bonding and insufficient tension, which will have adverse effects on the yield of the factory and the service life of components. In this paper, the discolored substrate and normal substrate are the main experimental objects. Under the guidance of electroplating theory, the surface properties and section states of different package substrates are studied. From macroscopic to microcosmic, a series of detection methods are adopted, such as optical microscope (OM), micro spectrometer, scanning electron microscope (SEM), atomic force microscope (AFM), X-ray energy spectrum analysis (EDS) and argon ion cross-section polishing instrument (CP). These methods are used to analyze the surface morphology, composition, roughness, cross-section thickness and element distribution of different pad areas. Combined with the technological process of electrolytic Ni/Au and related knowledge of electro-crystallization, the reasons for the discoloration problem of gold surface and poor wire bonding ability of the pad are found out.