Shear Properties Of Sn-Cu Based Bulk Solders And Joints

Sn-0.7Cu eutectic alloy is one of the most promising candidates in the soldering applications due to its excellent properties. Previous studies showed that the growth ofβ-Sn dendrite was inhibited by adding a small amount of Zn into the Sn-Cu solder, while the microstructure was refined effectively. New phase, with the grain size of 2-3μ, was detected at the phase boundary betweenβ-Sn and eutectic region. However, there are few reports on mechanical properties of Sn-Cu-Zn solder joints, and there is no report about the effects of the temperature, reflow time and aging time on the mechanical properties.In this work, the effect of 1.0wt.% Zn addition on the microstructure of Sn-0.7Cu was investigated, and the effect of reflowing time and aging time on the IMC morphologies of the solder joints were also researched. In addition, the shear strengths of the solders with solder joints were compared under different temperatures, strain rates and aging times. As a result, a more profound understanding of Sn-based alloy deformation and the factors on the joint strength and fracture can be obtained, which will provide reliability data for lead-free solder development and application. The results show that:1. The addition of 1.0wt.% Zn refined the microstructure of Sn-0.7Cu solder and improved the fraction of eutectic area. A small amount of Cu-Zn was detected at the phase boundary between P-Sn and eutectic region. The shear strengths of the both solders decreased as the shear temperature increased. The shear strength of the Sn-0.7Cu-1.0Zn solder was higher than that of Sn-0.7Cu solder when sheared at 25℃, but they were the same when sheared at-80℃. However, the shear strength of Sn-0.7C-1.0Zn solder was much higher than that of Sn-0.7Cu when shear at-40℃, which is less significant than the difference when sheared at 25℃. The reason of the above is that the addition of Zn refined microstructure and the dislocation movement was prevented, resulting in the improvement of the strengths of two solders. With the temperature increasing, the effect of grain refinement mechanism was weakened, consequently, the internal strength became dominant and Sn-0.7Cu-1.0Zn solder exhibited the same strength as that of Sn-0.7Cu solder.2. The IMC of Sn-0.7Cu/Cu is composed of Cu6Sn5. The IMC thickness of the Sn-0.7Cu/Cu and the size of the Cu6Sn5 particles became larger with the reflow time increasing. For Sn-0.7Cu-1.0Zn, the IMC thickness was not affected by reflow time. It is interesting that the Cu5Zn8 particles were found adhered to the Cu6Sn5 particles. The Cu3Sn layer existed between the Cu6Sn5 layer and the Cu substrate in the case of Sn-0.7Cu/Cu after aging but was not found in the case of Sn-0.7Cu-1.0Zn/Cu. The Cu6Sn5 particles of Sn-0.7Cu/Cu became larger as the aging time became longer. However, for Sn-0.7Cu-1.0Zn/Cu, the size of the particles seemed to be the same even if the aging time became longer.3. The shear strength of Sn-0.7Cu solder joints increased with shear temperature decreasing and strain rate increasing. When sheared at 25℃, the shear strengths of Sn-0.7Cu/Cu was higher than that of Sn-0.7Cu-1.0Zn after reflowing for 1 min., but they were identical after reflowing soldering for 10 min. The shear strengths of Sn-0.7Cu/Cu reflowed for 10 min was the same as that of reflowed for 1 min. For the case of Sn-0.7Cu-1.0Zn/Cu, the shear strengths increased with the reflowing time increasing. When sheared at 80℃and -40℃, the shear strengths of all the samples showed no difference. All fractures occurred in the solder of the joints and exhibited ductile fracture although brittle fracture occurred only in a few samples when shear at-40℃.4. The shear strength of the joints decreased after aging and decreased a little with the aging time extending. What's more, differences of the shear strengths were not significant as a result of reflow time and the addition of Zn. Fracture occurred in the solder of the joints and exhibited ductile fracture.