Effect Of The Addition Of Cu And Zn On SnBi Lead-free Solder

Sn42Bi58, Sn59.9Bi40Cu0.1and Sn57.9Bi40Zn2Cu0.1solder alloys were selectedin this study for research. The effects of Cu and Zn elements on melting characteristic,microstructure, thermal conductivity, electrical conductivity, wettability and mechanicalproperty of SnBi solder alloys were systematically studied by differential scanningcalorimetry （DSC）, Scanning electron microscopy （SEM）, Energy-Dispersive Spectro-meter （EDS）, thermal conductivity test, wetting behavior test, microhardness test andtensile test. Meanwhile, the formation of interfacial intermetallic compounds （IMCs）and shear strength of SnBi/Cu solder joints were studied by SEM, EDS and forceanalyzer （Instron5564）. The fracture mechanism of SnBi/Cu solder joints wereinvestigated depend on the morphology of fracture. The results are summaried asfollows:The effects of Cu and Zn elmements on the melting characteristic, microstructure,thermal conductivity and electrical conductivity of SnBi solder alloys were studied. Theresults showed that the melting point of Sn59.9Bi40Cu0.1solder alloy is close to that ofSn42Bi58solder alloy, while the mushy temperature zone of Sn59.9Bi40Cu0.1is wider.Moreover, the microstructures of Bi-rich phase and β-Sn phase in Sn59.9Bi40Cu0.1andSn57.9Bi40Zn2Cu0.1solder alloys were refined because of the formation of Cu₆Sn₅IMC. The addition of2wt%Zn element decreased the melting point and liquidtemperature of Sn59.9Bi40Cu0.1solder alloy apparently, while it exerted little effect onthe mushy temperature zone. Even though the majority of Zn element inSn57.9Bi40Zn2Cu0.1sloder alloy was Zn-rich phase, other fraction of Zn reacted withCu to form Cu₅Zn₈IMC, which refined the Bi-rich and β-Sn phase inSn57.9Bi40Zn2Cu0.1sloder alloy. In addition, the thermal and electrical conductivityof Sn59.9Bi40Cu0.1solder alloy was superior to that of Sn42Bi58sloder alloy, whereasinferior to that of Sn57.9Bi40Zn2Cu0.1solder alloy. As a semimetallic element, thethermal and electrical conductivity of Bi are low, hence, the decrease of Bi element wasfavorable to improve the thermal and electrical conductivity of SnBi solder alloy.Furthermore, Zn-rich phase exhibited high thermal and electrical conductivity. Thus, thethermal conductivity and electrical conductivity of Sn57.9Bi40Zn2Cu0.1is the bestamong these alloys.The effects of Cu and Zn elements on the mechanical properties of SnBi solder alloy were investigated. The result showed that the tensile strength, microhardness andmalleability of Sn59.9Bi40Cu0.1were improved due to the grain refinement andprecipitation strengthening of Cu₆Sn₅IMC. The tensile strength and microhardness ofSn57.9Bi40Zn2Cu0.1solder alloy were greater than Sn59.9Bi40Cu0.1because bothCu₆Sn₅and Cu₅Zn₈IMCs played the roles of grain refinement and precipitationstrengthening. However, the malleability of Sn57.9Bi40Zn2Cu0.1solder alloydecreased apparently compared with that of Sn42Bi58and Sn59.9Bi40Cu0.1solderalloy. This is because the β-Sn matrix in Sn57.9Bi40Zn2Cu0.1solder alloy wassegmented by needle-like Zn-rich phase. The tensile curve and fracture morphologyindicated that the tensile ruptures of SnBi solder alloys were quasi-cleavage fracturewhich occurred in the Bi-rich phase.The effects Cu and Zn elements on the wettability of SnBi solder alloy werediscussed. From the point of wetting force, the wettability of Sn63Pb37solder alloy wasbetter than that of SnBi solder alloys because Pb reduced the surface energy of solderalloy effectively. However, the wetting time of Sn63Pb37solder alloy was longer thanthat of SnBi solder alloys. This is because the viscosity of Sn63Pb37solder alloy ishigher than that of SnBi solder alloys at250℃. When moderate water-white rosin wasused, the wettability of Sn57.9Bi40Zn2Cu0.1solder alloy was improved a lot, becausewater-white rosin reduced the surface tension by eliminating the oxide film of Zn andpreventing the re-oxidation of Zn in liquid Sn57.9Bi40Zn2Cu0.1solder alloy. Thespreading test showed that the wettability of Sn57.9Bi40Zn2Cu0.1solder alloy wasworse than that of other SnBi solder alloys. This is because the surface energy of Zn ishigher and it is easier to be oxided, which suppressed the spread ofSn57.9Bi40Zn2Cu0.1solder alloy on Cu substrates.The shear property of SnBi/Cu solder joints were studied. The result showed thatall the SnBi/Cu solder joints ruptured soon after the elasticity deformation and thefractures occurred at the interface of SnBi solder and Cu substrates. This is becausevoids and volume stress between solder alloys and substrates lead to the stressconcentration along solder joints. In addition, the shear strength of Sn57.9Bi40Zn2Cu-0.1/Cu solder joint was the lowest among these three SnBi/Cu solder joints, since morevoids in Sn57.9Bi40Zn2Cu0.1/Cu solder joint decreased its shear strength. According tothe fracture morphology of SnBi/Cu solder joints, the fracture mode of Sn42Bi58/Cuand Sn57.9Bi40Zn2Cu0.1/Cu solder joints were the mixture of quasi-cleavage andintergranular brittle, while it was quasi-cleavage in Sn59.9Bi40Cu0.1/Cu solder joint. The microstructure analysis results showed that all the IMCs of SnBi/Cu solder jointswere continuous Cu₆Sn₅phase. In addition, a large amount of fine Bi-rich particlesdistributed in solder matrix close to the Sn59.9Bi40Cu0.1/Cu and Sn57.9Bi40Zn2Cu-0.1/Cu solder joints, while only coarse Bi-rich particles distributed in the Sn42Bi58/Cusolder joint.