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 mu m. The precipitation temperature of Ag3Sn plates was in the range of 160-202 degrees C and the undercooling was in the range of 15-57 degrees C. The undercooling of small solder balls was in the range of 29-63 degrees 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 mu m/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 mu m/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.