Abstract
Lead-tin solder has played an important role in microelectronic packaging for the past few decades. The current density and the operation temperature in the packaged devices rise substantially with the increase in device density and functionality for integrated circuits, which may result in severe reliability problems of solder joints. In this study, the soldering reaction between eutectic lead-tin solders and copper thin films and the evolution of solder microstructure and wetting precursors under electric current stressing were investigated. The patterned copper thin film samples were prepared by conventional photolithography techniques. The sample was placed on a preheated hotplate with a solder ball sitting on the center of the Cu strip. The solder melted and reacted with the Cu metallization after applying an electric current through the Cu strip. The results showed that there was an asymmetric distribution of Pb precipitates in the anode side and the cathode side of the solder. It was found that Pb precipitates aggregated and formed in columnar shape at the anode side, and the length of the Pb columnar precipitates increased proportionally with both the current density applied and the reaction time. The direction of the Pb columnar precipitates appeared to depend on the direction of the current applied. Besides, the activation energy for the growth of Pb columnar precipitates was measured to be 165.1 KJ/mole. The effect of electric current on solder wetting behavior and solder precursor was also investigated. An asymmetric distribution of Pb precipitate and intermetallic compounds was observed in the precursor bands of both sides of the solder. It was also found that the width of the solder precursor band was mainly affected by the temperature and the flux applied on the samples during soldering reaction instead of the stressing electric current.