| Nano-silver paste,a kind of novel lead-free interconnection material that can be used in wide-band-gap semiconductors working at high temperatures for its high thermal conductivity,electrical conductivity,and melting point,is gradually becoming an alternative to traditional solder alloys and conductive adhesives.During service,significantly thermo-mechanically stress will be induced in the sintered-silver bond-line due to the mismatch of thermal expansion coefficients between die,substrate and sintered-silver.Sintered silver may suffer fatigue,ratcheting,creep or dwell-fatigure damage under the co-existence of high temperature and thermal stress.Therefore,it is necessary to investigate the elevated-temperature mechanical reliability of sintered silver.Sintering processes of nano-silver paste for substrate level bonding were introduced and evaluated.Bonding plates with uniform porous structure,crack free,and average shear strength higher than 50MPa were achieved by employing double-layer printing method and optimizing the temperature to apply pressure.After thermal aging,the joints maintained reliable with high shear strength after aging at 125?and 225?.Aging at 275?and 350?weakened the shear strength significantly.Three type of failure modes involving cohesive failure were described and analyzed in detail by microstructral observation.Creep damage evolution of nano-silver sintered lap shear joint was investigated at the elevated temperature of 325oC.Through non-destructive X-ray three-dimensional?3D?visualization,the crack growth behavior of the joint was clearly observed,which could be divided into three stages.3D volume-rendered images and fractographic analysis showed that the growth of macroscopic initial cracks at the interfaces dominated the creep fracture process.Initial failure of the joints often occurred at the interfacial sintered-silver layers.Both the size and position of the initial interfacial cracks had significant effects on the final creep failure of the joints,and higher stresses led to higher porosity and earlier failure.Creep lives of nano-silver sintered lapp shear joint were predicted by four kind of constitute equations.Kachanov damage equation showed the highest predicted accuracy.A series of fully reversed loading tests were conducted at elevated temperature to describe the fatigue behavior of nano-silver sintered lap-shear joints.The evolution of hysteresis loops and shear strain range were analyzed in detail.The results showed that both hysteresis loops and shear strain range increased with increasing temperature and shear stress amplitude.As a result,the fatigue life decreased.Basquin model was used to assess the fatigue life of the joints at elevated temperature and the constants in the model were figured out,which yielded good prediction for experimental data.Dwell-fatigue tests at different temperatures were conducted on nano-silver sintered lap-shear joint.Effects of temperature,and loading conditions on dwell-fatigue behavior of nano-silver sintered lap shear joint were systematically studied.With higher temperature and longer dwell time,creep played a more important part in dwell-fatigue tests.Creep strain accumulated during maximum shear stress hold was found partly recovered by the subsequent cyclic unloading.The initial fracture of dwell-fatigue tests occurred at the interfacial sintered-silver layer.Plastic deformation,which occurred prior to intergranular fracture of the joints,decreased with increasing temperature.The minimum mean shear strain rate??minsynthesized the effects of various factors,such as temperature,shear stress amplitude,mean shear stress,and dwell time,by which the fatigue and dwell-fatigue life of nano-silver sintered lap shear joint could be well predicted within a factor of two. |
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