Abstract: In order to optimize the welding process of high reliability solder SAC-SBN alloy for automotive electronics, the influence of different cooling rates on microstructure of the SAC-SBN solder alloy and its solder joints was investigated. The test results indicate that the microstructure of the SAC-SBN alloy gradually coarsens with the decrease of cooling rate. Under the condition of a cooling rate of 48 ℃/s, the refinement degree of the alloy structure is optimal, while at a cooling rate of 0.13 ℃/s, the intermetallic compound (IMC) coarsens in the alloy and the Bi element exhibits grain boundary segregation. Compared with traditional SAC305 alloy, the SAC-SBN solder joints have a larger number and smaller size of IMC grains at the solder joint interface, which is attributed to the precipitation of Bi element at the interface grain boundary, which promotes the heterogeneous nucleation growth of (Cu, Ni)₆Sn₅. When the cooling rate is 48 ℃/s, the IMC layer at the SAC-SBN solder joint interface is the thinnest, as 1.96 μm. However, due to the difference in thermal expansion coefficients, the stress at the interface is relatively high, and voids appear in the IMC layer at the interface. When the cooling rate is 0.13 ℃/s, the IMC layer of the solder joint is the thickest, as 3.18 μm. And due to the interface reaction and maturation mechanism, there is stress between the Sn alloy matrix and IMC, resulting in voids in both the IMC layer at the interface and the (Cu,Ni)₆Sn₅ that has matured and grown inside the solder. When the cooling rate is 1.33 ℃/s, the solder joint structure is fine and there are no obvious defects.

Key words: SAC-SBN alloy, cooling rate, intermetallic compound, solder joint, interface morphology