Study On Interfacial Structure Evolution And Shear Fracture Mechanism Of Sn-based Solder/Kovar Joints

With the rapid development of electronic packaging industry,new requirements have been put forward for solder joints that combine metal,ceramic(hard glass)advantages.The thermal expansion coefficient of Kovar alloy in the fixed temperature range is very similar to that of ceramic(hard glass),which leads to its irreplaceable position in the field of electronic packaging.The wide application of Kovar alloy in vacuum electronic products will cause a series of problems in service.However,the study on interfacial reaction between Kovar alloy and Sn-based solder is extremely scarce.In-depth and detailed study on the solder joint interface reaction and reliability is of great significance to the application of Kovar alloy in the field of electronic packaging.In this paper,Sn37Pb solder paste with excellent comprehensive performance and Kovar alloy are selected as the solder and substrate.The interfacial reactions and microstructural evolution of Sn37Pb/Kovar with Au/Ni platings solder joints were investigated during isothermal aging at 120? 150? and 170? for different duration,respectively.The experimental results indicated that a large amount of IMCs(AuSn4,(Au,Ni)Sn4)is distributed in the bulk solder and a double-layer IMC,which consist of(Ni,Au)3Sn4 and AuSn4 is formed between the Sn37Pb solder and Ni layer after reflowing process.During the aging process,the compositions of interfacial IMCs of solder joint obviously changed,which transformed from AuSn4/(Ni,Au)3Sn4 to(Au,Ni)Sn4/Ni3Sn4.Furthermore,the total IMC thickness increased dramatically with increasing isothermal aging time,as well as aging temperature.The total thickness of IMCs layer at the joint interface is linearly correlated with the square root of aging time,which indicates that the growth process of IMC layer is mainly dominated by the volume diffusion mechanism during the solid-state aging process.The activation energy(Q)value of the Sn37Pb/Kovar couple was calculated to be approximate 95.96 kJ·mol-1.The effects of isothermal aging on the interfacial reactions and microstructural evolution of Sn37Pb/Cu/Kovar solder joints were investigated during isothermal aging at 150? for different duration.The shear test was performed to explore the shear strength and failure mechanism of solder joints with different aging time.The experimental results revealed that the introduction of Cu film on Kovar significantly improved the shear strength of Sn37Pb/Cu/Kovar solder joints.The shear strength of Sn37Pb/Cu/Kovar solder joints with same aging time increased first and then decreased with the increase of deposited Cu film thickness.This phenomenon can be explained by the thickness of interfacial IMCs layer in the solder joint had a significant effect on its shear strength.The shear strength of aged solder joints decreased with increasing aging time,which could be ascribed to the thicker IMC layer generated at the joint interface.The synergetic effects of strain rate and Cu film on the shear fracture behaviors of Sn37Pb/Cu/Kovar solder joints were investigated after reflowing at 250 0C for 5 min.The experimental results indicated that a single interfacial AuSn IMC layer(Kovar side)was observed between the Sn37Pb solder and Kovar substrate,which was covered by a discontinuous AuSn2 IMC layer.The interfacial IMC layer changed from the AuSn IMC to(Au,Cu)Sn IMC after electroplating Cu film on the Kovar substrate.The shear tests of Sn37Pb/Cu/Kovar joints were performed to study the shear fracture behavior and the experimental results showed that the shear strength of solder joint demonstrated increment at first and then decrement as the applied strain rate increased.As the applied strain rate was 6.67×10-1s-1,the shear strength of solder joints reached a maximum value.This phenomenon can be explained by considering the solder-controlled and IMC-controlled fracture mechanisms over a range of strain rates.Subsequently,the shear strength started to drop as the deposited thickness further increased.Moreover,the electroplated Cu film on Kovar substrate could effectively reduce the strain rate sensitivity.