Research On The Mechanical Properties And Solder Joints Reliability Of Sn-Bi Solder Alloys

In modern microelectronic packaging systems,due to the complexity of the electronic packaging level,the dense overall structure,and the wide range of materials involved,any reliability problem cause the entire packaging system to fail.Lead-free solder solder joints,as secondary packaging connection materials in microelectronic packaging systems,also have important roles such as mechanical structural support,electrical connections,and heat dissipation^[1],which are important factors that can affect the reliability of the entire packaging system.Based on Sn-Bi alloys,this paper analyzed the weldability and wettability characteristics of Sn57BiAgCuCo,Sn45BiAgCuNi,64Sn-35Bi-1Ag,and 69.5Sn-30Bi-0.5Cu alloys.The reliability of the four solder alloys at different temperatures in the tensile,creep mechanical behavior was researched,and microstructure evolution of BGA solder joints coupled with electromigration and rapid temperature cycling was analyzed.The specific research contents and conclusions are as follows.The melting characteristics and wettability of Sn57BiAgCuCo,Sn45BiAgCuNi,64Sn-35Bi-1Ag,69.5Sn-30Bi-0.5Cu were studied,and it was found that the solidus phase temperature does not change much,nor does it change the main phase transition reaction during solidification,but the liquidus temperature of the Sn57BiAgCuCo solder alloy is reduced.The addition of Cobalt has a significant inhibitory effect on wettability,and the addition of Ag element can enhance the wetting performance of the alloy to a certain extent.The mechanical behavior and temperature of the four alloys in static tensile under the temperature condition of 50?125?and the fracture m echanism w ere studied.T he results show that except for Sn57BiAgCuCo alloy,the strength and elastic modulus of the other three alloys generally decrease with increasing temperature when stretching at50?125?.T he increase is related to the reduction of the critical cutting stress required for slip.The slight increase of Sn57BiAgCuCo strength at 100?is related to the dynamic recrystallization to form more high-strength bulk Bi-rich grains.The tensile fracture analysis are as follows:as the temperature rises,the fracture modes of Sn57BiAgCuCo and Sn45BiAgCuNi changed from cleavage/quasi-cleavage to ductile fracture,64Sn-35Bi-1Ag and 69.5 Sn-30Bi-0.5Cu changed from a mixed fracture mode with ductile fracture to ductile fracture.The low stress creep behavior and creep fracture mode of four alloys under the temperature condition of 75?100?w ere studied,and the behavior of nanoindentation creep of Sn57BiAgCuCo and Sn45BiAgCuNi alloy under the condition of 50?100?was investigated.The tensile creep results show that the activation energy of Sn57BiAgCuCo,Sn45BiAgCuNi,64Sn-35Bi-1Ag,69.5Sn-30Bi-0.5Cu alloys is 43.964kJ/mol,152.695 kJ/mol,189.643 kJ/mol and 149.947 kJ/mol,respectively.Sn57BiAgCuCo is easier to creep.And the creep behavior is mainly controlled by the dislocation climbing mechanism under high-temperature.The strength of Sn57BiAgCuCo alloy is more sensitive to temperature,which is related to the stronger ability of Sn-Bi lamellar tissue to deform and coordinate after 75?.The activation energy of Sn57BiAgCuCo during nanoindentation under high stress conditions is 6.323kJ/mol,and the creep stress index is between 3.62 and 5.28.The creep activation energy of Sn45BiAgCuNi is 7.322 kJ/mol,and the creep stress index is between 3.00 and 4.93.The calculation results show that the dislocation climbing mechanism is the main mechanism controlling the creep.The microstructure evolution of BGA solder joints of four alloys after electromigration and temperature cycling was studied.It was found that when the current density was greater than 2×10²A/(8(8(8(8²,the solder joints can undergo directional movement of atoms inside the metal.It is calculated that the strain field of the four alloy solder joints is about-0.448%?0.325%during the temperature cycling.The SEM results of the solder joints show that under the influence of electronic wind and Joule heat,the Bi phase growth shows a certain direction.The directional movement of metal atoms in the solder joint during electromigration will cause the internal holes to gather and grow,which will adversely affect the reliability.The dispersion of Bi-rich phases in is related to the dynamic recrystallization under alternating strain fields to form new fine grains.The grain boundaries provide more diffusion channels for Bi atoms.In addition,the generation of new grain boundaries when Ag,Cu atoms and Sn diffused can also provide additional diffusion paths.