| SnPb solder was widely used in electronic packaging industry because of its excellent performance. However, Pb is harmful to the environment and human health. As a result, all countries banned the use of SnPb solder. Among the alternatives of SnPb solder, due to its low melting point, good physical properties and excellent weldability, SnAgCu lead-free solder is the main alternatives. But compared with the traditional SnPb solder, it still exists many deficiencies. Researchers added mixed rare earth to solder to improve the performance of the solder. At the same time, in order to study the relationship between aging temperature and the intermetallic compound layer (IML), samples were aged at high temperatures for different time.In this paper, the solder alloy studied was Sn-3.8Ag-0.7Cu. Various amounts of La (0,0.05,0.1,0.2,0.5,1.0,1.5wt.%) were added to the solder to study its effect on the microstructure and properties. At the same time, samples were aged at different temperature to study the effects of aging temperature on the microstructures of solders and joints. In addition, the Sn-3.8Ag-0.7Cu-1.5La solder was aged at middle-temperature for 30 min, compressive stress was loaded to solder by four-point loaded specimen, then growth of tin whisker was observed, the effect of aging temperature and compressive stress on the growth of tin whisker was studied. The main content and some conclusions are summarized as follows:(1) While the La adding amount was 0.05wt.%, compared with the Sn-3.8Ag-0.7Cu solder, melting temperature range reduced about 48%, spreading area increased about 34.74%, the wettability was improved, solder microstructure was very small and uniform, shear strength increased about 13.28%. However, when the La adding amount was more than 0.5wt.%, compared with Sn-3.8Ag-0.7Cu solder, there didn’t exist any noticeable change of solidus and liquidus temperature, wettability was decreased, the shear strength of joints fell sharply, decreased about 25%. The fracture mode of Sn-3.8Ag-0.7Cu-0.5La/Cu joint contained ductile fracture and brittle fracture, meanwhile, there existed small amount of cracks at the interface of the joint.(2) Small amount of La addition produced none noticeable change on the morphology of the IML, but it influenced the thickness of the IML. When the adding amount of La was 0.5wt.%, compared with Sn-3.8Ag-0.7Cu solder, the thickness of Cu6Sns layer fell about 16.23%, the thickness of Cu3Sn layer fell about 25.98%. Further increase in addition of La resulted in increasing of the IML thickness.(3) Both the solder samples and joints were aged (aging temperature T was 75℃, 125 ℃,160℃). It was found that as long as the aging temperature increased, solder microstructure coarsened, intermetallic compounds was related, bamboo shape IML gradually became flat, its thickness also increased. The relationship between aging temperature and the IML thickness is roughly linearly, small amount of La decreased the proportion coefficient. When aging temperature reached 160℃, the proportion of Cu3Sn layers in the IML rose sharply. Cracks were found in the Sn-3.8Ag-0.7Cu-0.5La/Cu joint after aging at 75℃ for 24h. Solder samples were respectively aged at 25℃,50 ℃,75 ℃ for 30 min, and the tin whiskers were observed by SEM. While the aging temperature was 25 ℃, the tin whisker didn’t grow. There existed several tin whiskers while the aging temperature was 50℃ and 75℃, and the whisker length was longer when the aging temperature was 50 ℃. Loading 5 MPa compressive stress to a sample, at the same time, select a sample which was not loaded, both samples were aged at 60℃ for 24h. It turned out that no whiskers grew on the sample which was not loaded, while the other one grew some. Compressive stress and middle-temperature aging can promote the growth of tin whisker. |
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