Effects Of Ag, Ga, Al And Ce On The Properties Of Sn-9Zn Lead-free Solder

Due to the toxicity of Pb and legislative pressures, the research on the lead-free solder alloys has been widely concerned around the world. Among lead-free solders, Sn-Zn solder is widely recommended because of its low melting point (198℃, near that of the Sn-Pb alloy -183℃). However, due to its poor oxidation resistance and poor wetting properties, this solder is not able to be directly applied to electronic products. In this paper, according to the development of lead-free solder and the characteristics of the production process, the effects of Ag, Ga, Al and Ce on the properties of Sn-9Zn lead-free solder were studied.Experimental results show that, the optimum additive amount of Ag in Sn-9Zn solder is about 0.3wt.%. By adding 0.3wt.% Ag, the oxidation resistance of solder is enhanced, so the solderability is almost the best. The dimples of the Sn-9Zn-0.3Ag soldered joints are much finer than that of Sn-9Zn, and the pull force is enhanced by 27.2%. However, when the content of Ag is upto 1wt.%, some Cu-Zn and Ag-Zn intermetallic compounds appear in the bottom of dimples, and the pull force of soldered joint decreases. Sn-9Zn eutectic alloy consists of two phases, namely, theβ-Sn and Zn-rich phases with less than 1% Sn. With the addition of Ag, the number of rod like Zn-rich phases decreases. Some Ag-Zn phases appear in the solder when the content of Ag is 0.5wt.%. A planar layer of Cu5Zn8 intermetallic compound formed at the Sn-9Zn/Cu interface. As for the Sn-9Zn-0.3Ag/Cu interface, some additional nodules, AgZn3 IMCs, are found beside the aforementioned planar layer. And the additional nodules beside the planar layer grow up to a scallop-shaped layer gradually as the content of Ag increases.The optimum additive amount of Ga in Sn-9Zn solder is about 0.5wt.%. As an active element, Ga tends to accumulate at the interface of molten solder, and the solderability of solder increases. After adding 0.5wt.% Ga, there is no obvious change in mechanical property, and the pull force is enhanced by 2.8% and the fracture micrograph shows that the joint failed in a ductile manner. When the content of Ga is upto 3wt.%, some Ga-rich phases are found on the grain boundaries and the fracture micrograph of soldered joint implies an intergranular fracture mode. Correspondingly, the pull force of Sn-9Zn-3Ga soldered joint is reduced obviously. When the content of Ga is 0.5wt.%, the grains of microstructure are small and a fine microstructure is obtained. When the content of Ga is 3wt.%, some black phases appear on the grain boundaries. The Cu5Zn8 IMCs become thicker with the addition of Ga. The optimum additive amount of Al in Sn-9Zn solder is about 0.005 wt.%. After adding trace amount of Al, a dense alumina membrane appeares at the surface of the molten solder which reduces the oxidation of Zn, thus the solderability is improved. After a small amount of the addition of Al element, there is no obvious change in mechanical property or fracture morphology. With a small amount of the addition of Al element, the size of rod like Zn-rich phases decreases. When the addition of Al is 0.1wt.%, the Al element appeares on the grain boundaries. Because the addition of Al is rather small, there is little change in the interface morphology.The optimum additive amount of Ce in Sn-9Zn solder is about 0.08wt.%. The solderability is improved because Ce tends to accumulate at the interface of molten solder. The pull force is enhanced by 27.6% when the content of Ce is 0.08wt.%, and the dimples are small and uniform. When the content of Ce is upto 1wt.%, the pull force of soldered joint decreases and some Cu-Zn intermetallic compounds appear in the bottom of dimples. Adding 0.08wt.% Ce makes solder alloy form finer microstructure, and some particulate-shaped Sn-Ce compounds appear in the solder when the content of Ce is 0.1wt.%. The addition of 0.1wt.% Ce makes the Cu5Zn8 IMCs at the Sn-9Zn/Cu interface become thicker.The result of this paper will be theoretically helpful for the development and application of new Sn-Zn lead-free solder.