The Influence Of Ge And Bi On The Thermal Reliability Of SAC105 And Sn0.7Cu

Sn0.7Cu and SAC105 are used widely for their perfect wetting and spreading properties,as well as good mechanical properties.In electronic packaging,reliability research is important particularly.There have been a lot of researches on the performance improvement of Sn0.7Cu and SAC105 solders.However,the thermal reliability of BGA joints is still blank.The addition of Ge and Bi made SAC105 and Sn0.7Cu grains refined,and changed the melting point and melting range of the solders,furthermore,the alloy elements improved the solderability and reliability of the solders.The addition of Ge improved the toughness of SAC105 and Sn0.7Cu.The addition of Bi not only improved the mechanical properties of the joints,but also increased the brittleness of the solder alloy.It is found that the addition of alloying elements slowed down the rate of Sn atomics and Cu atomics diffusion,which inhibited the growth of the Cu₆Sn₅ IMCs.During constant temperature aging（CT）,the thickness of the interface compounds had a linear relationship with the square root of the aging time.The solder joints can maintain good connection performance during constant temperature aging at 125°C.Most of the shear fracture of joints occurred in the solder.The toughness of the solder alloy induced with the increase of aging time,while the shear strength decreased with the increase of aging time.In the later stage of aging,Ge can maintain the reliability of Sn0.7Cu and SAC105 joints.But,the solder at the SAC105-2Bi joints separated from the Cu₆Sn₅,for its significant brittleness and large size grains.During-40°C～125°C thermal cycling（TC）,the thickness of the interfacial compound increased slowly with the increase of the thermal cycles,and the form of the IMC growth is more complicated.The large temperature change and the mismatch of the material’s coefficient of thermal expansion（CTE）,made the solder joints stress concentration,as well as,the extreme low temperature made Sn phase changed,the cold brittleness of the metal material was prominent,caused the extrusion between the grains,and the toughness of the solder alloy decreased gradually.which led the Cu₆Sn₅ layer fractured,and the fracture interface recrystallized due to heat,which made the compounds stratified,so the cracks of SAC105,SAC105-2Bi and Sn0.7Cu-2Bi joints propagated between the Cu₆Sn₅ layers,and the joints failed.The addition of Ge made the thermal expansion coefficient of the solder tend to be toward the metal material such as the Cu substrate,so Ge can better maintain the reliability of the solder joint in the thermal cycling.With the introduction of the acceleration factor K,the calculation found that,the growth rate of the IMC in CT was faster than that in TC.During CT,the thermal equilibrium state was relatively stable,the internal stress of the solder joint was small,and the compound layer would not break or even dissolve.The growth of IMC relied on the diffusion of Cu and Sn atoms.During CT,atoms migrated at a stable rate,and the IMC grew stable;in the low temperature stage of thermal cycling,Cu and Sn atoms cannot reach the activation energy for a long time,and the diffusion cannot proceed.When the temperature rose to 125°C,the atoms need to absorb heat to make them move.This process consumed some time.Therefore,during thermal cycling state,the growth rate of IMC slowed down.