Study On The Pressure Sintering Process And Reliability Of Nano-Silver For Packaging

With the advancement of science and technology,the third-generation of semiconductor power devices are continuously developing towards integration,precision and multi-functionalization.At the same time,the service conditions for these devices have become more severe and complex,often requiring them to withstand challenges such as high temperature,high frequency and large electric current.This places higher demands on the connection materials and connection reliability of the chips in power device packaging.With the characteristics of "low temperature interconnection,high temperature service",as well as the high reliability and excellent electrical thermal properties of the resulting sintered body,nano-silver particles have become an ideal connection material in the field of high-power device packaging.The application of nano-silver paste to copper substrate with mechanical interconnection is an important direction to advance the development of sintered silver technology.However,for the sintered silver joint on the copper substrate,its reliability will decrease after long-term service under different environmental tests.Therefore,it is of great importance to study the degradation mechanism of joint performance.In this paper,the effects of different sintering process parameters on the microstructure,shear strength and shear behavior of nano-silver joint were studied,the fracture mechanism of the joints was analyzed.Based on the optimum sintering process,the evolution of the microstructural and performance degradation mechanism of sintered nano-silver joint on the copper substrate during temperature cycling and high-temperature aging tests were investigated respectively.The results showed that the increase of sintering temperature,time and pressure would improve the densification degree of the microstructure and the shear strength of the joints.With the increase of sintering temperature,the fracture location of the joint moved from the interface to the inside of the sintered silver layer.The plastic deformation degree of the fracture surface structure intensified.At 260℃,the average shear strength of the joints was up to 90.85 MPa,and the cross-sectional structure showed a tendency of coarsening.Prolonging the sintering time could promote the fracture of the joint from the sintered silver layer to its interior,and make the amount of plastic deformation of the fracture structure increased.When the sintering time exceeded 5 min,the change of microstructure porosity was not significant and the shear strength tended to saturate.The effect of sintering pressure on the fracture behavior of the joint had a pattern similar to that of sintering time.The increased pressure made the interface connection more complete.To avoid damage to the chips caused by excessive pressure,the optimal process parameter for this study was determined to be sintering at 240℃ and 10 MPa for 5 min,at which the shear strength of the joint was 71.92 MPa.After temperature cycling and high-temperature aging tests,the shear strength of the sintered silver joints on copper substrates were both degraded,however,the reliability of long-term service at-40～125℃ and 250℃ could still be guaranteed.The fundamental reason for the weakening of the joints performance during temperature cycling was the different coefficients of thermal expansion between the sintered layer and the substrate.The severe thermal mismatch under the effect of temperature cycling that caused the joints to be subjected to alternating stress internally.As the number of cycles increased,defects such as copper oxide layer and cracks were produced in the structure,which deteriorated the overall stability of the sintered joints.The structure degradation during high-temperature aging was mainly attributed to the problem of pores gathering at the interface caused by the inter-diffusion of interface elements,the oxidation of the copper substrates,and the formation and growth of copper oxides within the silver layer.In addition,the severe coarsening of the sintered silver structure was also an important reason for the failure of the joints in high temperature.