| In recent years,power modules have a developing trend of higher power and higher current density.As an important part of the electrical conduction and heat dissipation,soldered joints will face more severe service conditions.As a new high-temperature material,nano-silver paste has attracted more and more attention for its excellent properties.In this paper,sintering process and reliability have been made a preliminary esploration.An ultrasonic-assisted pressureless sintering for nano-silver paste is developed and the effect of ultrasonic vibration on the microstructure and mechanical properties of pressureless sintered nano-silver joints is investigated.The results have shown that ultrasonic vibration can not only increase the density of sintered silver,but also transfer the fracture mode of joints to cohesive failure and increase shear strength to 18.9 MPa.There exists a insufficient sintered zone at the edge of joints.Ultrasonic vibration could reduce the transition zone,thus improving the reliability of joints.Pressure sintering process for silver paste is studied using self-designed constant pressure sintering fixture.A sintering pressure of 1.5 MPa at 120 °C can make the chip and silver paste good wetting,reduce voids and significantly improve shear strength.For sintered silver joints,the grains have gradually grown and coarsened during aging at 175 °C.There are micro-cracks and delamination in the interface chip and sintered silver in the late of aging.The shear strength has a significant decline at the beginning of aging,but can be stabilized to 20 MPa and better reliability compared with soldered joints.During aging,the compounds component,microstructure and shear strength have changed.IMC layers get thicker and flatter by element diffusion and grain growth.The shear strength has declined rapidly after 500 h aging,and the fracture surface is also transferred form the chip to the interface of Cu3 Sn and Cu6Sn5. |
