Investigation On Current-assisted Sintering Nanosilver Paste For Die Attachment And Joint Reliability

As a novel lead-free interconnection material, nanosilver paste is quite suitablefor high temperature and high power electronic packaging because of its goodmechanical performance, high thermal and electrical conductivity. Nanosilver is nowattracting many scientists’ and engineers’ attention in the electronics industry. It hasthe potential to replace the traditional solder, and to be the first choice of the futureelectronic packaging interconnection material. However, at present the study of thenanosilver paste is still in the initial stage. Most investigations of nanosilver pasteespecially the large area device interconnection were focused on its hot press sinteringprocess and its mechanical property evaluation. Therefore, in this study a newsintering method namely Current-assisted sintering technology (CAST) was putforward to accomplish the sintering of nanosilver paste. This method can realize rapidsintering of nanosilver paste. The fatigue behavior of5×5mm2silver joint bycurrent-assisted sintering have been studied in this work. It will provide guiding andinspiration for applications of nanosilver paste in the field of electronic packaging.This approach has broadened the limitation of traditional hot press sintering methodand can significantly improve the efficiency of device connection. It also provides anew way of thinking for the future research on technique of power electronics deviceconnection.By applying the pulses of alternating electrical current through the joints, itrealized rapid sintering of nanosilver paste and achieved strong shear strength.Investigated processing variables on the joint strength were copper surface finish,current level, current-on time, nanosilver bondline thickness, pre-drying temperatureand time. Through studying the effect of different processing variables on the bondingstrength, the results showed that current level and current-on time were moreimportant. The bonding strength increased substantially with current level andcurrent-on time. Other variables have certain influence on the bonding strength, butnot very obverous.By changing alternate current (AC) to direct current (DC), the nanosilver jointscould also be sintered rapidly. From the shear results we can see that the current levelof DC could be lower than that of AC in order to obtain the almost same bonding strength. It proves that the energy efficiency of DC is much higher thanAC.In order to evaluate the reliability of the current-assisted sintered nanosilverjoints, cyclic shear fatigue test in the mode of stress control was used for testingcurrent-assisted sintered nanosilver joints and hot press sintered nanosilver joints. Anon-contact measurement system was build up to measure the deformation of sinteredsilver joints. The results showed that the joints sintered by CAST were more reliablethan those of hot pressing. Basquin formula was used to obtain the fatigue life modelof these two kinds of sintered joints. And the predicted results coincide well with theactual result.Finally the effects of mean stress and stress amplitude on the ratcheting behaviorof the CAST sintered joints were observed by stress controlled cyclic shearexperiments, respectively. The result showed that the ratcheting displacement rate ofthe sintered specimens increased with mean stress or stress amplitude and fatigue lifedecreased with mean stress or stress amplitude.