Effect Of Cr Addition On Solder Joint Interface Reaction And Properties Of Sn58Bi New Lead-free Solder

The new lead-free solder plays a role in electronic packaging regarding heat conduction,electricity conduction,electrical connection,etc.Compared with the traditional Sn Pb solder,lead-free solder does not contain toxic Pb and is friendly to the human body and the environment.Therefore,the research and development of new lead-free solders have become one of the hot spots in the electronic packaging industry.Sn58 Bi composite solder has been widely concerned by researchers because of its advantages of low melting point,high joint strength,high creep resistance,low thermal expansion coefficient and good wettability.However,the brittle Bi-phase in Sn58 Bi solder leads to low mechanical properties of solder joints.At the same time,solder joints are used in electronic devices.With the extension of aging time,the IMCs layer between solder and substrate will change correspondingly,affecting the reliability of solder joints.Therefore,to improve the microstructure of Sn58 Bi composite solder and improve the mechanical properties of solder joints,the Sn58 Bi composite solder was modified by the alloying method.In this study,Sn58Bi-XCr(X = 0,0.1,0.2,0.3 wt.%)solder was prepared by mechanical ball milling,cold pressing,and vacuum arc melting.According to the possible performance requirements of lead-free solder in production,application,and waste,the effects of Cr addition on the physical properties,interface reaction,solder joint mechanical properties,and corrosion resistance of Sn58 Bi solder were discussed and analyzed.The results show that:(1)In the study of physical properties: the addition of Cr significantly refines the microstructure of Sn58 Bi composite solder.Sn58 Bi composite solder has the best microstructure refinement effect when Cr content is added to 0.2wt.%.Under 100 °C isothermal agings,the average phase spacing of Sn58Bi-0.2Cr solder in each aging time is the smallest with the aging time extension.Sn58Bi-0.2Cr composite solder has higher hardness and lower density among the four alloys.The conductivity of Sn58 Bi composite solder increases with the addition of Cr,and the conductivity of Sn58Bi-0.2Cr solder is second only to that of Sn58Bi-0.1Cr solder.The melting point of Sn58 Bi composite solder increases gradually with the addition of Cr,but the melting range gradually widens.The wettability of Sn58 Bi composite solder on three Cu alloy substrates(Cu,Cu30 Zn and Cu7Sn)decreases with the addition of Cr,and the wettability of Sn58 Bi composite solder on Cu30 Zn substrate is better than the other two substrates.(2)In the study of solder joint interface reaction: under the condition of isothermal aging,after the Sn58Bi-XCr composite solder is wettled on three kinds of Cu alloy substrate,the thickness of the IMCs layer on the interface gradually becomes thinner with the addition of Cr,so the growth rate of IMCs layer also gradually decreases.However,the IMCs layer thickens with aging time.Among them,the interface IMCs layer of Sn58Bi-XCr on Cu30 Zn substrate is the thickest,but Bi segregation is significantly improved,and the existence of Kirkendall voids is almost not observed.(3)In the study of mechanical properties of solder joints: under the condition of isothermal aging,the tensile strength of Sn58Bi-XCr composite solder is better than the shear strength after wetting on three Cu alloy substrates.And the comprehensive mechanical properties of Sn58Bi-XCr/Cu30 Zn soldered joints are better.The comprehensive mechanical properties of Sn58Bi-0.2Cr composite solder joints are better than those of other alloys.(4)In the study of corrosion performance: with the addition of Cr content,the impedance value of Sn58 Bi composite solder increases gradually,and its corrosion potential moves along the negative direction,the corrosion current density also gradually decreases,and the corrosion resistance of the composite solder is improved.When the Sn58 Bi solder is leached in Na Cl and Na OH solutions,the leaching amount of Sn decreases gradually with the addition of Cr.The Sn58Bi-0.3Cr composite solder has the best corrosion resistance among the four alloys.