Effect Of Alloying On Solderability And Reliability Of SnBi Based Low Temperature Solder Alloys

With the development of packaging technology,packaging tends to be integrated,miniaturized,multi-functional and high-density.The development of 3D packaging and Sip technology requires the use of low-temperature solder technology.Traditional Sn-Ag solder alloys cannot meet the requirements of packaging technology,which is urgent to develop new high-reliability low-temperature solder alloys.In this paper,the SnBi-based low-temperature solder alloy was used as the matrix to form five-component SnBi57AgCuCo and SnBi45AgCuNi solder alloys by adding trace amounts of Ag,Cu,Co and Ni.The solderability of solder alloys was studied by means of microstructures,thermal conductivity,melting properties,wettability and mechanical properties.The reliability of hybrid joints was investigated by analyzing the effects of thermal action on microstructure,failure mode and shear strength and the board-level drop test.Specific research contents and conclusions are as follows:The melting thermal conductivity and wettability of SnBi57AgCuCo and SnBi45AgCuNi solder alloys were investigated by adding trace Ag,Cu,Co and Ni elements.The results show that the addition of trace Ag,Cu,Co and Ni elements does not affect the melting onset temperature of SnBi57AgCuCo and SnBi45AgCuNi solder alloys.When Sn and Bi elements in solder alloys are near eutectic composition,the addition of trace elements reduced the liquidus temperature of SnBi57AgCuCo solder alloys and shortens the melting range.The decrease of Bi content led to the increase of liquidus temperature and melting range of SnBi45AgCuNi solder alloy.The additions of trace Ag and Cu elements made the thermal conductivity of SnBi57AgCuCo solder alloy slightly higher than that of Sn42Bi58 solder alloy.The thermal conductivity of SnBi45AgCuNi solder alloy significantly increased by decrease of Bi element.The addition of trace Ag,Cu,Co and Ni elements reduced the surface energy of solder alloys.The wettability of SnBi45AgCuNi solder alloy is better than that of SnBi57AgCuCo and Sn42Bi58 solder alloys.The effects of the addition of trace Ag,Cu,Co and Ni elements on the microstructure and mechanical properties of SnBi57AgCuCo and SnBi45AgCuNi solder alloys were investigated.The results show that the addition of trace Ag,Cu and Ni elements resulted in the formation of intermetallic phases Ag₃Sn and Cu₆Sn₅ as a nucleantion particle in the solder alloy,which increased the nucleation rate of the solder alloy and formed the?-Sn phase in which Bi-phase particles were dispersed,and the?-Sn phase was increased and the Bi-rich phase was coarsened due to the decrease of Bi element content of SnBi45AgCuNi solder alloy.The intermetallic phase refines the Bi phase particles in the?-Sn phase,which played a role in fine grain strengthening and dispersion strengthening on the mechanical properties of the solder alloy.A trace amount of Co element dissolved in the?-Sn phase acted as a solid solution strengthening,which made the tensile strength,elongation and hardness of SnBi57AgCuCo and SnBi45AgCuNi solder alloys better than those of Sn42Bi58eutectic solder alloy,and the dimple-like structure was observed in the tensile fracture morphology,which indicated that the plasticity of the solder alloy got enhanced,and the fracture was intergranular fracture along Bi phase.The reliability of SnBi-based solder alloy hybrid joints was investigated by observation of the microstructure evolution and failure analysis under different thermal effect,the shear strength under aging and the drop test.The results show that Bi phases in solder joints of SnBi57AgCuCo,SnBi45AgCuNi and Sn42Bi58 solder alloys were diffused and aggregated obviously under isothermal aging at 100?for 500 h,which formed coarse bulk Bi phases.Dye and pry test shows that the aggregation of Bi phases weakened the solder joint strength.The Bi phases in the three solder alloy joints gradually aggregated with the increase of cycle,and the strength of solder joints gradually decreased under the condition of thermal cycle.The shear strength of improved SnBi57AgCuCo and SnBi45AgCuNi solder alloys by adding trace Ag,Cu,Co and Ni elements was obviously higher than that of Sn42Bi58 solder alloys.The shear strength of solder joints increased under aging at 100?for 250 h and then decreased with further extension of aging time.According to the results of board-level drop test,the drop properties of SnBi57AgCuCo and SnBi45AgCuNi solder alloys were significantly prior to that of Sn42Bi58 solder alloys.The failure position of hybrid joints was located at the outermost corner of BGA chip.The voids in solder joints were the main path of microcrack initiation and propagation,and the failure cracks tended to propagate towards voids and IMC.