Size Effect on the Intermetallic Compound Coalescence in Sn-Ag-Cu Solder and Sn-Ag-Cu/Cu Solder Joints

For micro-scale solder interconnects, the microstructure of the solder joint and the soldering defects are directly related to the solidification behavior of the undercooled solder. In this study, the undercooling and solidification behavior of Sn3.0Ag0.5Cu solder balls with different diameters (0.76, 0.50 and 0.30 mm) and the joints of soldering these balls on Cu pads of two different diameters (i.e., 0.32 and 0.48 mm) was investigated using differential scanning calorimetry (DSC) incorporated into the reflow soldering process. Results show that the decrease in diameter of both solder balls and solder joints brings about an obvious increase of the undercooling. Coarse primary solidification phases form in smaller solder balls which have a larger value of the undercooling. EDS analysis results indicated that only bulk primary Ag3Sn phase existed in the solder balls, while only large primary Cu6Sn5 phase formed in the solder joints. Contrast with the solder ball samples, the interfacial reaction and dissolution of Cu atoms into the solder matrix led to a decrease in the degree of the undercooling of the solder joints during solidification process, and the primary Ag3Sn phase was suppressed due to the increase of Cu content in the solder joints.