Investigation On Fatigue Behavior Of Sintered Nanosilver For High Density Electronics Packaging

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. Therefore, in this study a non-contactdisplacement detecting system (NDDS) by combining infrared heating and a chargecoupled device (CCD) camera with digital image correlation method (DICM) hadbeen established. The NDDS was introduced to perform real-time non-contactdisplacement measurement for uniaxial cyclic/ratcheting/fatigue tests of metal jointsat room and high temperatures. The force-displacement hysteresis loops could also beaccurately recorded through synchronizing the force and displacement. Furthermore,the system was used to study the reliability of nanosilver joint at room and hightemperatures.Uniaxial ratcheting behavior and the fatigue life of sintered nanosilver joint wereinvestigated at room temperature. Effects of stress rate, mean stress, stress amplitude,and stress ratio on the uniaxial ratcheting behavior of the sintered nanosilver jointwere discussed. Stress-life (S-N) curves of the sintered joints were also obtained.Smith-Watson-Topper (SWT) model, Gerber model and modified Goodman model,all of which took effect of mean stress into consideration, were compared forpredicting the fatigue life of the sintered joint. Both the ratcheting strain and its rateincreased with increasing the stress amplitude or the mean stress. The increase of thestress amplitude and the mean stress both reduced the fatigue life of the sintered joint,while the fatigue life prolonged with the increase of the stress rate and the stress ratio.The failure of the sintered joint in the case of large stress amplitude and small meanstress was dominated by ratcheting damage, but that in the case of small stressamplitude and large mean stress was dominated by low cycle fatigue damage. Themodified Goodman model predicted the fatigue lives of the sintered joints best amongthree life models. Ratcheting behavior of sintered nanosilver joints was examined by cyclicshearing tests at high temperatures. Effects of temperature, mean stress, and stressamplitude on the ratcheting behavior of the nanosilver joints were analyzed. Animproved temperature-dependent modified Goodman model was proposed and wasproved to be able to accurately predict the fatigue life of the sintered nanosilver jointsand the soldered SAC305joints by introducing the temperature-dependent fatiguestrength exponent and the fatigue strength coefficient. In order to illustrate thesuperior reliability of nanosilver paste at high temperature, ratcheting behavior andfatigue lives of sintered nanosilver joints were compared with those of solderedSAC305joints. Shear strength of the sintered nanosilver joints was larger than that ofthe soldered SAC305ones. The fatigue life of sintered nanosilver joint was muchlonger than that of soldered SAC305one under the same loading conditions.Therefore, nanosilver can be recommended as a promising die-attaching material forpower electronic packaging, especially for high-temperature applications because thesintered nanosilver shows significantly higher resistance to cyclic fatigue, e.g.,ratcheting displacement, than the solder SAC305.