Study On Electromigration And Shear Behavior Of BGA Structure Micro-Solder Joints Under Electro-Thermo-Mechanical Multi-Field Coupling

To meet the demand for low-temperature reflow soldering in electronic packaging in recent years,Sn-58 Bi solder,with the advantages of low melting point and low cost,has attracted attention in low-temperature packaging and has gradually been applied in boardlevel packaging of ball grid array(BGA)components.However,due to the effect of Joule heating on the solder joint,the temperature of the solder joint may rise sharply when the electronic product is powered on,which may cause local or complete melting of the solder,leading to serious electromigration(EM)problems.In addition,the Joule heating generated by the current can cause thermal mismatch and generate thermal stress between different materials,resulting in the electro-thermo-mechanical coupling field during the service of the solder joint.Therefore,this paper comprehensively studies the solid-solid(S-S)and liquidsolid(L-S)electromigration of Cu/Sn-58Bi/Cu solder joints and Cu/SAC305/SnBi/Cu hybrid solder joints and conducts in-depth research on the shear fracture behavior of Cu/SAC305/SnBi/Cu solder joints under electro-thermo-mechanical coupling loads with different current directions and current densities.The research results on the S-S and L-S electromigration of BGA structure Cu/Sn-58Bi/Cu solder joints indicate that during the S-S electromigration process,the current stress has a reverse polarity effect on the growth of intermetallic compounds(IMCs)at the solder joint interface.It significantly promotes the development of cathode-side interface IMCs,while the opposite effect occurs on the anode side.In addition,with increasing temperature,the electromigration effect becomes more severe,and the Bi-rich layer on the anode side of the solder joint disappears.However,during the L-S electromigration process,the thickness of anode-side interface IMCs significantly increases.The current stress induces polar growth of interface IMCs,and increasing temperature and current density promote electromigration within the solder joint.Furthermore,no Bi-rich layer is formed on the anode side under different conditions.The research results on the S-S and L-S electromigration of BGA structure Cu/SAC305/SnBi/Cu hybrid solder joints indicate that,in S-S electromigration,there are significant differences in the migration behavior of Bi atoms in the solder joint with partially mixed SAC305 and SnBi solders under different current directions.However,the thicknesses of the interface IMCs on the anode side are greater than those on the cathode side,and the current stress determines the polarity of the interface IMCs growth.In the hybrid solder joint with thoroughly mixed SAC305 and SnBi solders,the interface IMCs grow with the same polarity,and the migration behavior of Bi atoms is not significant and independent of the current direction.However,the partially mixed hybrid solder joints show more severe electromigration effects than the fully mixed solder joints.However,during the L-S electromigration process,regardless of the electron flow direction,when the current density is low,the growth of the interface IMCs in the partially mixed hybrid solder joint shows a polarity effect.After increasing the current density,the growth behavior of the interface IMCs changes,offering a reverse polarity effect.Bi atoms did not undergo significant diffusion and aggregation during the L-S electromigration process.The results of the study on the shear fracture behavior of BGA structure Cu/SAC305/SnBi/Cu with different solder mixing degrees under the electro-thermomechanical coupling load show that under the electro-thermo-mechanical coupling load,as the current density increases,the shear strength of hybrid solder joints decreased with increasing current density due to significant electromigration and Joule effects within the solder joint.In addition,the fracture mode of the fully mixed hybrid solder joints changed from brittle fracture to brittle-ductile mixed fracture due to severe local Joule effects,independent of the current direction.However,for partially mixed hybrid solder joints consisting of two different solders,the fracture mode of the joints changed from ductile fracture to brittle-ductile mixed fracture with increasing current density when the electron flow was from the SnBi/Cu side to the Cu/SAC305 side.After changing the direction of current,the solder joint always maintains ductile fracture.