Study On Microstructure And Properties Evolution Of Sn-15Bi Solder With Trace Ag And Cu Addition

Electronic components are developing towards miniaturization,integration and lead-free.Sn-Bi solder joints have attracted extensive attention due to their good wettability and high strength.With the development of electronic packaging to high density,the chip heat production increases in the work.It is of great significance to study the thermal reliability of solder joints during service.In this paper,the effects of adding a small amount of Ag and Cu elements on the microstructure,wetting behavior and mechanical properties of Sn-15Bi solder were studied.The results show that the addition of alloying elements Ag and Cu makes the grain refinement of the solder.The ultimate tensile strength of Sn-Bi-Ag alloy increases with the increase of Ag content,and the ultimate tensile strength of Sn-Bi-Cu alloy decreases slightly with the increase of Cu content.The addition of Ag and Cu improves the wettability of Sn-15Bi solder.The wettability of the alloy on Cu substrate is better than that on Ni substrate.Secondly,this paper focuses on the microstructure and shear properties of Sn-15Bi-x Ag/Cu（Ni）and Sn-15Bi-x Cu/Cu（Ni）solder joints with BGA structures with different Ag and Cu contents before and after aging.The interface metallurgical reaction,growth kinetics of interfacial intermetallic compounds（IMC）and fracture mode of solder joints were discussed.The results show that the addition of Ag inhibits the growth of Sn-15Bi-x Ag/Cu intermetallic compound Cu₆Sn₅,but has little effect on the growth of Sn-15Bi-x Ag/Ni intermetallic compound Ni₃Sn₄.The addition of Cu promoted the growth of Sn-15Bi-x Cu/Cu intermetallic compound Cu₆Sn₅and Sn-15Bi-x Cu/Ni intermetallic compound（Cu,Ni）₆Sn₅.The addition of Ag increases the ductility and shear strength of solder joints,while the addition of Cu increases the brittleness and reduces the shear strength of solder joints.Finally,the microstructure and interfacial IMC growth kinetics of Cu（Ni）/Sn-15Bi-x Ag/Cu（Ni）and Cu（Ni）/Sn-15Bi-x Cu/Cu（Ni）structures before and after thermal cycling as well as the shear properties of BGA solder joints under thermal cycling were studied.Bi dissolved in Sn precipitates and aggregates during thermal cycling in solder joints.Cracks were found between solder and intermetallic compound layer at the interface of Ni/Sn-15Bi-x Ag/Ni solder joints after 1200 cycles of thermal cycling,while no cracks were found in other solder joints.In the thermal cycling process,the shear strength of Sn-15Bi-1Cu/Ni solder joints is the most stable.The shear strength of other solder joints decreases with the extension of thermal cycling time.The shear strength of solder joints with Ag addition decreases more slowly than that of solder joints with Cu addition.