Creep And Thermomechanical Fatigue Behaviors Of Lead-free Solder Joint Under High Current Density

Electronic packaging technology is developing toward miniaturization and high density packaging to meet consumer's requirement of portability.As an important connection structure in secondary electronic packaging,lead-free solder joint plays roles in structure support,electrical connection and thermal dissipation.Therefore,the reliability of lead-free solder joint is one of the most important factors that affect the operation of electronic devices.Currently,most attention is focused on individual reliability problems such as creep,thermomechanical fatigue(TMF)and electromigration(EM),the researches on the combination of them are insufficient.In fact,as the size of solder joint scaling down,the current density through the solder joint is escalating,which becomes an outstanding problem.Since EM changes the microstructure and interfacial structure significantly,the influence of EM cannot be simply ignored when evaluating the performance of creep or TMF of a real device.Therefore,the research of creep or TMF behavior under high current density is a more practical reliability problem.Although some researches had already developed theories related to the coupling of creep and EM,experimental verification is still needed.Besides,the coupling of TMF and EM is seldom reported in literatures.Based on above consideration,this study investigated creep and TMF behaviors of anisotropic Sn0.3Ag0.7Cu and lamellar eutectic Sn58 Bi solder joint under high current density condition.Firstly,orientation evolution,growth and formation mechanisms of interfacial intermetallic compound(IMC)and matrix IMC were revealed to achieve a fundamental understanding of EM.Secondly,effects of electromigrated microstructure on creep and TMF behaviors were discussed.Finally,failure mechanisms of solder joint under the coupling of high current density and creep stress or thermal cycling were elaborated systematically.Moreover,to gain a better understanding of recrystallization and plasticity of tin crystal,a slip trace analysis method was introduced to correlate the slip trace(twinning)with slip system(twinning system).The results indicated that EM had no ability to change grain orientation of Sn0.3Ag0.7Cu solder joint,but could cause a rigid rotation of local grains within few degrees by grain sliding process.The migration of copper atoms in Sn0.3Ag0.7Cu solder joint during EM presented a selective feature,that is,copper atoms tended to migrate through the grain whose c axis oriented close to the direction of electron flow.During EM of Sn0.3Ag0.7Cu solder joint,interfacial IMC grew up with the size increasing and the morphology transiting from scallop-shape to planar,resulting in the decrease of shear strength.During EM of Sn58 Bi solder joint,tin-rich and bismuth-rich phases coarsened significantly by enrichment and segregation process.During thermal cycling of Sn0.3Ag0.7Cu solder joint,new orientations formed by continuous recrystallization process and the migrated IMC facilitated this process.Thermal cycling with high current density changed grain orientation significantly by discontinuous recrystallization process.By slip trace analysis method,we correlated the morphology of slip trace(twining)with slip system(twinning system).Slip in <110},<211} and <011} planes and twining in <301} and <101} in Sn0.3Ag0.7Cu solder joint were detected.During creep of Sn0.3Ag0.7Cu solder joint,fracture in solder matrix was the main failure mechanism.Both electromigrated microstructure or creep with high current density increased creep rate of Sn0.3Ag0.7Cu solder joint.Preliminary treatment of EM formed a two-planar layer structure by solid reaction,which provided more cracking sites and decreased the energy for crack propagation.Consequently,the fracture path transited from solder matrix to interface.Under high current density condition,crack tips caused current crowding and subsequently softened the solder at the local region.This process facilitated crack propagation along the interface.Under multifield coupling condition,the nucleation and spread of fatigue or creep cracks were found to be relieved by high current density in Sn58 Bi solder joint.However,once the fatigue or creep crack formed,the tip of a crack caused current crowding and subsequently accelerated EM process.Serious solid reaction during EM induced a final failure at the interface.