Research On Creep Behavior Of Lead-free Solder Joint Under Thermomigration

With the densification and miniaturization of electronic products, the current density of solder joints become lager and lager, which leads to a high temperature gradient in the solder joints, resulting in thermomigration(TM). The thermal expansion coefficients(CTE) of the solder and packaging materials are different. The electronic products will produce different thermal stress during the service process. This will make the solder joints have a creep behavior under TM, so a separate study on the creep behavior of lead-free solder joints under TM is particularly important. Creep experiment platform coupled TM and pure creep experimental platform are designed in this paper. The creep performance of Cu/Sn0.7Cu/Cu solder joints under different stress and different temperature are studied, which provides a theoretical basis for the reliability of solder joint interconnects.Firstly, the creep properties of the Cu/Sn0.7Cu/Cu solder joints under shear stress of 1.78 MPa, 2.53 MPa and 3.62 MPa and the temperature of 100℃ and 150℃ are studied. The results indicate that the temperature accelerates the creep rupture of solder joints. The creep life, fracture morphology, fracture morphology, fracture position and the interface IMC evolution of solder joints are observed, and the creep index n and the creep activation energy Q are solved. The values of n and Q show that the dislocation sliping is the main deformation mechanism.Secondly, the Cu/Sn0.7Cu/Cu solder joints are subjected to thermomigration with the 250 h, 500 h, 750 h and 1000 h under the temperature gradient of 1046℃/cm. The creep behavior of Cu/Sn0.7Cu/Cu solder joints under the shear stress of 2.53 MPa and 3.62 MPa with the creep temperature of 100℃ is investigated. The research showes that compared to the pure creep behavior of Cu/Sn0.7Cu/Cu solder joints under the temperature of 100℃,the creep life of coupled TM is reduced. With the increasing of TM’s time, creep life of solder joints increases firstly and then decreases. When the time of TM is 250 h, 500 h and 1000 h, the creep stress exponent n of Cu/Sn0.7Cu/Cu solder joints is 3-4, the main creep mechanism of the solder joints is the dislocation sliping. When the time of TM is 750 h, the creep stress exponent n of Cu/Sn0.7Cu/Cu solder joints is approximately equal to 2, the main creep mechanism of the solder joints is micro-void aggregation and grain boundary sliding.Thirdly, the Cu/Sn0.7Cu/Cu solder joints are subjected to thermomigration with the temperature gradient of 1046℃/cm and shear stress of 1.78 MPa, 2.53 MPa and 3.62 MPa. The creep life, fracture morphology, fracture morphology, fracture position and the interface IMC evolution of solder joints are observed and the creep index n and the creep activation energy Q are solved. The values n and Q show that the creep mechanism of solder joints changes from the dislocation sliping to micro-void aggregation and grain boundary sliding.Finally, isothermal aging at the temperature of 150℃ for 750 h is conducted on the Cu/Sn0.7Cu/Cu solder joints. Then the creep expeiments under the shear stress of 1.78 MPa, 2.53 MPa and 3.62 MPa with the temperature gradient of 1046℃/cm are investigated. They show that after isothermal aging, solder joints are more likely to fracture. This is due to the shear strength of the solder joints decreases after isothermal aging for 750 h. In the process of creep deformation, when the shear strength is 3.62 MPa and 2.53 MPa, TM has little influence on the changes of the IMC macrostructure owing to the shorter creep life. However, when the shear stress is 1.78 MPa, the fraction occurres in the IMC. The fracture mechanism changes from ductile fracture to ductile brittle mixed fracture.