Dissolution and precipitation of Ag3Sn plates in ultra fine solder joints using synchrotron radiation real-time imaging technology

With the continuous miniaturization of solder joints, the formation of large Ag₃Sn plates in Ag containing solders has a detrimental effect on the reliability of solder joints. In the present work, synchrotron radiation real-time in situ imaging technology was used to study the dissolution and precipitation behavior of Ag₃Sn plates, as well as the undercooling of Ag₃Sn plates and the solder balls, in Sn-4.0Ag-0.5Cu/Ni-P solder joints with a diameter of 45 μm. The precipitation temperature of Ag₃Sn plates was in the range of 160-02°C and the undercooling was in the range of 15-57°C. The undercooling of small solder balls was in the range of 29-63°C, which was slightly larger than that of Ag₃Sn plates. Once nucleated, Ag₃Sn plates rapidly precipitated at the interface within 2 s, and the precipitation rate was estimated to be at least 5 μm/s. The dissolution rate of Ag₃Sn plate was significantly slower than the precipitation rate. It took about 40 s for Ag₃Sn plates to dissolve into the molten solder, and the dissolution rate was estimated to be 1 μm/s. Synchrotron radiation real-time in situ imaging technology provides a direct way to observe the microstructural evolution and measure the undercooling of Ag₃Sn plates and solder bumps.