Abstract：Two kinds of rapid and slow cooling solder joints of Sn-35Bi-1Ag/Ni-P/Cu were prepared by reflow soldering and then cooling in water and oven, respectively. The kinetics of two kinds of layers were investigated using isothermal aging. Scanning electron microscope and energy dispersive spectrum were employed to characterize the microstructure and composition of the interfacial layers. Experimental results show that the structure of rapid-cooling interface is composed of (Ni,Cu)3Sn4+Ni3P, while the slow-cooling interface consisting of (Ni,Cu)3Sn4+Ni3P+(Cu,Ni)6Sn5.During the isothermal aging process, the growth of (Ni,Cu)3Sn4 layer slows down for the thin Ni-P gradually depleting, but the (Cu,Ni)6Sn5 layer follows the dynamics of the square root of aging time. The growth of interface IMCs is overall controlled by the diffusion mechanism, and these two kinds of interfacial layers eventually exhibit the composite lamellar structure of (Cu,Ni)6Sn5+(Ni,Cu)3Sn4+(Cu,Ni)6Sn5.The IMCs layer thickness in slow-cooling solder joints is greater than that in rapid-cooling solder joints under the same condition during soldering and aging. The growth rates of total IMCs are 4.670×10-18m2/s for slow-cooling joints, 3.816×10-18m2/s for rapid-cooling joints, respectively. It indicates that the cooling rate affects the aging behavior of solder joints during soldering and service.
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