Research On New Kind Of Low Temperature Sn-9Zn Based Lead Free Solder

Because of low melting point, good mechanical property, low cost, andenvironment friendliness, Sn-9Zn eutectic solder is potential to be new kind of leadfree solder. However, bad oxidation resistance and wettability make it difficult to beused broadly. So Sn-9Zn eutectic solder attracts lots of attention, and the research onperformance improvement has become one of the most important issues.In this study, Cr, Al, Ti, La were selected as alloying elements for improvingperformance of Sn-9Zn solder. The aim was improving oxidation resistance, and otherperformance, such as wettability, melting point and mechanical property. And it isfound that Sn-9Zn-Cr alloy has good oxidation resistance and mechanical property.Furthermore, the preparation method, the effect of Cr content on performance, andinterfacial reaction behavior and mechanism were studied.The result shows that Cr, Al, Ti can improve oxidation resistance of Sn-9Zn alloyexcept La, and they have different mechanism. Because of preferential oxidization, Alor Ti addition forms compact and stable oxide film on the surface of Sn-9Zn solder,which inhibits further oxidation. Cr addition forms a barrier layer, which is so call“inner barrier layer”, in the sub-surface of Sn-9Zn solder to protect the solder fromfurther oxidation.Then influence of alloying element, Cr, Al, Ti, and La, on other properties ofSn-9Zn alloy was studied. In general, microstructure and melting point are notchanged by adding these alloying elements, neither are solidus/liquidus temperature,melting range. Although Al and Ti addition apparently decrease wettability of Sn-9Znsolder, Cr, and La improve it slightly. According to our research, Cr addition canimprove oxidation resistance while maintaining wettability and melting point. Sn-9Zn-Cr alloy is new kind of low temperature lead free solder with broadapplication prospect.For high quality of sample, exploration of preparation of Sn-9Zn-Cr solder wascarried out. Because of finer microstructure, the Sn-9Zn-Cr solder prepared by SnCrmaster alloy method with medium frequency induction furnace is the best choice. Andthe optimal content of Cr addition is0.1wt.%.Further research on the effect of Cr content on performance of Sn-9Zn solder isperformed. It seems that the best oxidation resistance and wettability were providedby the solder which has0.1wt.%Cr. And Sn-9Zn-0.1Cr alloy shows the bestelongation value, which is approximately66%more than that of Sn-9Zn alloy.Finally, morphology, thickness, and growth dynamics of intermetallic compoundlayer at Sn-9Zn-Cr/Cu interface were studied. The results show that:（1） Sn and Cr donot take part in the interfacial reaction at Sn-9Zn（-xCr）/Cu interface, and thecomposition of intermetallic compound layer is Cu₅Zn8. The thickness of theinterfacial IMC layers increases as the aging time is extended.（2） Interfacial reactionof Sn-9Zn solder with Cu plate is diffusion controlled. When Cr content is0.1wt.%,the diffusion coefficient shows the lowest value, while the activation energy is themaximum value. The reason of this phenomenon is that Cr addition reacts with Zn toform intermetallic compounds, such as～Zn₁₇Cr and/or～Zn₁₃Cr, which candecrease diffusion of Zn atom in the solder and make them more stable. So adding Crinto Sn-9Zn solder leads to slow down the growth of the interfacial IMC layer. Thestabilization effect of Cr addition on Zn atom is maximum when Cr content is0.1wt.%.