| As for the danger of Pb on health and environmental safety, lead-free solders are being applied more and more widely in the manufacture of modern electronic products. Sn-Ag-Cu solder has been proven to be the most promising alternative lead-free solder to replace the conventional Sn-Pb solder. However, due to the global economic downturn in the electronics industry, higher requirements are put forward for lead-free electronic assembly, the eutectic or near-eutectic Sn-3.0~3.8Ag-Cu solders are unable to meet the “low-cost, high-quality” requirements of the electronics industry. In this paper, the low-Ag Sn-0.5Ag-0.7Cu lead-free solder was studied, alloying element Ga was selected to optimize the properties of the solder, and the impact law of Ga on the microstructure and properties was explored.The results indicated that the addition of rare element Ga could significantly refine the Sn-0.5Ag-0.7Cu solder matrix structure and the IMC particles in the matrix became small and uniformly distributed. When the content of Ga reached 0.5 wt.%, the microstructure was the most uniform and the grain reached the highest degree of refinement as well. However, black gallium-riched phase would precipitate with excess addition. The melting point of the solders were lowered slightly by Ga addition, which would not cause too much impact on the existing soldering equipments.Wetting balance method and high-temperature oxidation weight gain method were adopted to study the wetting and the oxidation resistance properties of the Sn-0.5Ag-0.7Cu solder respectively. The results showed that: small amounts of Ga could effectively improve the wetting and oxidation resistance properties. As a surface-active element, Ga could accumulate on the solder’s surface, greatly reducing the surface tension of the liquid solder, and the flowability of the solder was enhanced, hence the wettability and the oxidation resistance properties were improved. However, when the Ga content exceeded 0.5%, the solder wettability was adversely affected due to the uneven distribution of the gallium-riched phase. Since Ga could not be easily oxidized, the oxidation resistance property was increased significantly, the oxidation weight gain of Sn-0.5Ag-0.7Cu-0.5Ga solder was half of that of Sn-0.5Ag-0.7Cu solder at the condition of 245℃ and 60 h.From the microstructure analysis of the Sn-0.5Ag-0.7Cu-x Ga/Cu solder joints, it was found that when the addition of Ga was≤0.5%, the influence of Ga was mainly performed in the refinement of IMC particles. The grain growth was inhibited and the Cu6Sn5 compound layer became thinner and smoother. The shear strength of the solder joints was greatly improved and increased by 17.9% at the content of 0.5%.For the isothermal aging at 150℃, it could be concluded that during the aging process, the thickness of the interface layer compound of Sn-0.5Ag-0.7Cu and Sn-0.5Ag-0.7Cu-0.5Ga solders was increased almost linearly, while the growth of the solder containing Ga was slower. Ga played an evidently suppression effect in the growing of IMC layer and the reliability was improved. The micro-mechanical properties of the solder joints were decreased with the aging time prolonging, however the shear strength of the Sn-0.5Ag-0.7Cu-0.5Ga solder joint after aging for 720 h was still higher than that of the Sn-0.5Ag-0.7Cu solder joint before aging. Adding Ga could apparently reduce the growth rate of the IMC layer during the failure process of the solder joints, which was beneficial for the maintaining of mechanical properties, thus greatly improving the reliability of the Sn-0.5Ag-0.7Cu-0.5Ga solder joint. |
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