Processing And Mechanism Of Electric Current Assisted Sintering Nanosilver Paste

Nanosilver paste is a promising lead-free die-attach material suitable for high temperature and high power electronic packaging because of its good mechanical performance, high thermal and electrical conductivity. Nanosilver is now attracting many scientists' and engineers' attention in the electronic packaging industry. However, most of the studies on nanosilver paste were focused on hot press sintering process and on mechanical properties of sintered silver joints. Traditional hot press sintering method is relatively complicated, time-consuming and it is difficult to automate. Therefore, in this study a rapid sintering method namely Electric Current assisted sintering(ECAS) was put forward to realize rapid sintering of nanosilver paste within one second.Real-time temperature detection system was established based on infrared thermal image technology to record temperature change on nanosilver layer. The temperature change can be divided into five stages: rapid heating, slow heating, keep balance, rapid drop and slow drop. And it can be controlled by adjusting current magnitude and current-on time.The effects of copper surface finish, current and current-on time on sintering temperature, shear strength and microstructure of sintered joints were studied respectively. Results show that when peak temperature is higher than 325 oC and current-on time is longer than 400 ms, the sintered joints have high shear strength. And the shear strength increases with peak temperature; the shear strength increase as the current-on time increases. However, the rate of increase of shear strength decreases. And at current-on times longer than 1000 ms, the shear strength of the joint shows a small decrease.The rapid sintering behaviors including temperature profile, particle size, pore size and shape, and shear strength, of nanosilver joint at different stages of ECAS were studied to help understand the fundamental mechanism. Based on the results, ECAS can be divided into three stages. At the initial stage, the removal of most organics and the rearrangement of nanosilver particles cause rapid shrinkage of the joint. After initial stage the nanosilver particles come into direct contact with one another, and the shear strength of the joint increases quickly due to the diffusion of atoms through the melted region between the particles. At the final stage, further shrinkage of the joint proceeds by plastic deformation under high temperature(about 400oC) and loading. The elimination of crystal defects also contributes to the shrinkage at the final stage. The sintering of nanosilver finally increases the shear strength of the joint to 50 MPa.Finally the reliability of sintered joint by ECAS and hot press were evaluated by cyclic shear fatigue test. The result shows that the joints sintered by ECAS are more reliable than those of hot pressing.