Effects Of Sn-Ag-x Layers On The Microstructure And Mechanical Properties Of Sn-58Bi Solder Joint

In recent years,portable electronics place in an explosive growth stage.Electronic packaging technology is also constantly pursuing development with the constant creation of related technologies in high-tech industries.To meet the developing needs of packaging technology,the properties and reliabil ity of solder joints need to be further improved.Among lead-free solders,Sn-58Bi(Sn Bi)solder has attracted increased attention because of its low melting temperature and low cost.Meanwhile,Sn Bi shows acceptable solderability and bonding strength.However,the defects of the Sn Bi solder alloy are coarse structure and the segregation of the Bi phase at the interface during service.The microstructure coarsening has a serious threat to the reliability of solder joints.Therefore,improving the performances and reliability of Sn Bi solder joints has great significance.Four Sn-Ag-x(Sn-3.0Ag-0.5Cu,Sn-0.3Ag-0.7Cu,Sn-0.3Ag-0.7Cu-0.5Bi-0.05Ni,and Sn-3.0Ag-3.0Bi-3.0In)leveling layers were coated on Cu pads to prepare Sn Bi/Sn-Ag-x/Cu solder joints.The microstructure,hardness,shear strength and fracture morphology of solder joints before and after aging,as well as the changes of microstructure and reliability of solder joints before and after thermal cycling were studied.Experimental results indicate that all the Sn-Ag-x leveling layers have positive effects on improving the wettability of Sn Bi solder.Due to the addition of the leveling layers,the grain size of the?-Sn phase in the Sn Bi/Sn-Ag-x/Cu solder joint is significantly larger than that in the Sn Bi/Cu eutectic solder joint.Meanwhile,the hardness of the solder bulk in the Sn Bi/Cu solder joint shows a decrease trend because of the addition of the leveling layers.The addition of Sn-Ag-x layers increased the thickness of the interfacial intermetall ic compound(IMC)but had limited effects on the shear force of the Sn Bi solder joint.Due to the addition of the interfacial layers,the brittleness of the Sn Bi/Cu solder joints during the shear test was slightly suppressed.The Sn Bi/Cu solder joint shows obvious strength drop and interfacial brittle fracture after aging.Through the addition of the Sn-Ag-x layers,the brittle failure caused by aging is effectively suppressed.In addition,the Sn-Ag-x leveling layers improve the shear strength of the Sn Bi/Cu solder joint after aging.Among them,the Sn Bi/SACBN/Cu solder joint shows the highest shear strength.The results show that the microstructure of solder joints becomes coarsened after the thermal cycling test.After 700 thermal cycles,the Sn Bi/Sn-Ag-x/Cu solder joints are more finely microstructure than those in the Sn Bi joints.The interfacial IMC of Sn Bi/Cu solder joints and Sn Bi/Sn-Ag-x/Cu solder joints become thicker with increasing thermal cycles.However,the IMC growth rate of the interfacial in the Sn Bi/Sn-Ag-x/Cu joint is significantly slower compared to the Sn Bi/Cu joint without the addition of the Sn-Ag-x leveling layers.And the Sn-Ag-x leveling layers effectively suppress the growth of the Cu ₃Sn layer in the interface during thermal cycling.In the shear test,the shear strength of each solder joint decreases after thermal cycling.However,the shear strength of Sn Bi/Sn-Ag-x/Cu solder joints is greater than that of Sn Bi/Cu joints under the condition of the same cycle.The brittle damage induced by thermal cycling is virtually suppressed with the addition of the Sn-Ag-x leveling layers.Thus,the Sn-Ag-x layers enhance the heat resistance of the Sn Bi solder.It can efficiently improve the mechanical properties of Sn Bi solder in the actual service.