Evaluation of the Effects of Au Content on Intermetallic Compound Layer Growth in Pb-Sn and Sn-Ag-Cu Solder Joints

摘要

Gold (Au) is often used in the electronics industry as a protective finish to preserve the solderability of underlying finishes and/or substrate materials. However, the presence of Au in the molten solder increases the risk of embrittlement by: 1) Au saturation above the solubility limit of the solder, resulting in formation of Au-Sn intermetallic compound (IMC) particles and 2) long-term solid-state diffusion of Au toward the base material interface, resulting in the formation of a Au-rich IMC layer. These effects are being studied by liquid-state and solid-state aging studies. Ring-and-plug test samples were constructed with Pb-Sn and Sn-Ag-Cu solders having additions of 0, 2, 4, and 8 wt.% Au. Solid-state aging was performed at 25 to 170°C for up to 200 days to simulate long-term storage and to determine IMC layer growth kinetics. For solid-state aged samples, electron probe microanalysis (EPMA) showed an increase in the amount of Au in the Cu_6Sn_5-type interfacial IMC layer as Au is increased in the solder. In contrast, the Cu_3Sn-type IMC layer shows essentially no solubility for Au. Analysis of IMC growth kinetics showed enhanced growth of the (Cu,Au)_6Sn_5 layer as Au was added to the IMC composition. Development of a second, higher Au, IMC phase in the (Cu,Au)_6Sn_5 layer contributed to the enhanced layer growth.