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Synchrotron radiation X-ray real-time imaging technology was used to study the interfacial reactions of Sn and Sn3.5Ag solders with Cu and Ni substrates during the heat preservation stage. This experiment allows us to in-situ observe the interfacial intermetallic compounds (IMCs) growth, provides a stage to distinctly separate their growth at isothermal reflow and cooling durations, and thus avoids the measurement errors for size and morphology of interfacial IMCs. The results show that the CuSn morphology is scalloped at Sn-xAg/Cu interfaces, whereas (Cu,Ni)Sn is wide-stripe shaped at Sn-xAg/Ni interface during the heat preservation stage. It is interesting to note that the grain boundary molten liquid gaps is observed at both of the Sn3.5Ag/Cu and Sn3.5Ag/Ni interfaces. Compared with pure Sn solder, Sn3.5Ag solders are observed with interfacial IMC of greater thickness, smaller base width, lower aspect ratio and more substrate consumption. In addition, it is confirmed that AgSn grains appear in Sn3.5Ag solder, but not on the surface of existing interfacial CuSn grains at Sn3.5Ag/Cu interface during the heat preservation stage. All of the growth kinetic index (n) for Sn-xAg/Cu is close to 1/3 indicating that grain boundary diffusion determines the interfacial reaction at the heat preservation stage. The mechanism by which Ag addition affects interfacial reaction and changes the mirostructure of solder matrix through creation of grain boundary gaps and decrease of the growth activation energy of IMC is also clarified. The knowledge for morphology and thickness control of interfacial IMCs by alloy elements addition as outlined by this study can be utilized to improve the reliability of solder joints in electronic packaging and solar photovoltaic (PV) cells array manufacturing sectors. [ABSTRACT FROM AUTHOR]

The morphology evolution mechanism and dynamics of CuSn intermetallic compound (IMC) in cooling stage were studied by using pure Sn solder ball with a diameter of 1 mm to react with polycrystalline Cu substrate and form CuSn. A nearly uniform height of the scallop-like IMC grains is attained by using high pressurized air to remove excess liquid solder (that is, for acquisition of the IMC morphology identical to heat preservation stage). But the morphology evolution of IMC in the solder joint was greatly affected by the cooling phase of interfacial reaction, and IMC morphology partition phenomenon was found in the spherical joints. The IMC at the edge portion characterized scallop morphology whereas the central portion possessed prismatic shape morphology. Finite element simulation for the temperature field distribution in the solder ball-substrate domain showed temperature gradient in solder's internal core was significantly greater than that at the edge. Using this simulation results with kinetic equations, it could be understood that the central core region is conducive to the formation of small plane structure morphology. These small planes would eventually form the prismatic morphology IMC in the central region. As the soldering temperature increases, the area ratio of IMC with scallop-like morphology to the prismatic shaped IMC reduced from 0.2178 down to 0.1680. Moreover, for an elevated temperature of 300 °C, it is observed that the central region IMC (with prismatic structure) grows upto an average thickness of 7.7245 μm, whereas the thickness value for scalloped IMC at the edge portion is around 4.556 μm. [ABSTRACT FROM AUTHOR]