A Study On Pressureless Low Temperature Sintering Of Tin Doped Nanosilver Paste

Compared to silicon semiconductor material which is currently widely used,new electronic semiconductor materials,such as silicon carbide,gallium nitride,have a higher power density,higher thermal management and a wider operating temperature.These high-power devices have to withstand high operating temperature.And if the heat generated during the device operation cannot be diffused in time,it will cause mismatch in the packaged device due to thermal expansion,and finally result in instability.At present,the die attach materials commonly used in electronic packaging,such as solder alloy and conductive adhesive materials,cannot meet the power electronics with higher power density and increasing operating temperature because of their low operating temperature,producing lead contamination and metal compounds,higher elastic modulus,and mismatch between devices caused by the accumulation of heat and so on.However,nanosilver paste can be sintered at a low processing temperature.And after sintered completely,silver layer is a porous material with excellent thermal,electrical,mechanical performance and reliability.Moreover,it can be used in high temperature environment.So nanosilver paste becomes a potential die attach material for high-power device.But nanosilver paste is not suitable for industrial application because of its sintering temperature as high as 280?.In order to lower the sintering temperature,at first,we produced TDSP?tin doped silver paste?according to the theory of transient liquid phase sintering?TLPS?.We added a certain amount of micro sized tin particle to nanosilver paste.Then we dissolved them in an organic solvent,and well mixed by mechanical oscillation and ultrasonic vibration.At last we evaporated the mixture in a water bath at a constant temperature to a suitable viscosity.Finally we achieved resulting TDSP paste.We used scanning electron microscopy?SEM?to evaluate the uniformity performance of TDSP paste.The results showed TDSP was mixed uniformity.And agglomeration was not obvious among the particles.Secondly,the TDSP was analyzed by thermal gravimetric analyzer?TGA?aimed to investigate the mechanism of volatilizing of organic compounds in the sintering process and to determine a reasonable experimental parameters.Finally,we developed the appropriate sintering process of TDSP.In the sintering process,the highest sintering temperature was 235?,which was lower than that of nanosilver paste which was 280?.Then we carried on microstructure analysis of sintered TDSP by SEM.The results indicated that the sintered TDSP was Ag/Ag₃Sn/Ag-Sn solid solution composites.With the increasing of tin's content,the amount of Ag₃Sn increased.Ag₃Sn was existing in a spherical shape.Then we analyzed the sintering mechanism of TDSP by TLPS.TDSP's sintering was a process of solid-solid and solid-liquid diffusion.The appearance of the liquid phase accelerates the mass' s transportation.Substance was also more compact after sintering.Finally,we evaluate the bonding strength of sintered TDSP joint by shear tester.And the results showed that when tin's content added to nanosilver paste was 4wt.%,the joint owned the highest strength.With the increasing of tin's content,the joint's bonding strength dropped to a lower level.After testing,we found that with the increase of the tin's content,the XRD peak of Ag matrix shifted to the right.The intensities of the Ag₃Sn diffraction peaks continuously increased indicating more Ag₃Sn was formed.From above results,we suspected that the second phase strengthening and solid solution strengthening mechanism played an important role to improve joint's bonding strength.However,when the addition of tin's content exceeded 5wt.%,the joint's bonding strength steeply dropped due to crack caused by the increasing brittle phase of Ag₃Sn.