Materials interfaces in flip chip interconnects for optical components; performance and degradation mechanisms

Flip chip solder joint reliability is dependant on final microstructure. The interface between the solder and the underlying metallization is of primary concern since the majority of joint failures occur at or near the interface. We report the results of our investigation on the structure-property relationships of the as deposited metallization and electroplated Pb/Sn solder, as they affect failure mechanisms. Samples investigated were prepared by electron beam evaporation of a Cr/Cu/Au metallization. Electroplated solders were prepared using a high tin solder bath and current density of 3.2mA/cm(2). It is shown that interdiffusion and intermetallic formation between gold and copper occurs during evaporation deposition. It is also shown that a significant amount of interdiffusion and intermetallic formation between the tin in the solder and the seed layer metals (Cu and Au) occurs during the electroplating operation. It is believed that this is due largely to resistive heating of the samples. The degree of interdiffusion in the as deposited state determines the ultimate reliability of the flip chip solder bumps in an optical configuration. (C) 1998 Elsevier Science Ltd. All rights reserved.