Fabriation And Study Of Ni@Sn Core Shell Structure Bimetallic Powder For High Temperature Applications

Global awareness of environment issues has been promoted.Generally,the gasoline car is being replaced by hybrid electric and electric vehicles,and the electrified vehicle is the development trend of vehicles.Electric devices are the key part of the power electronic system operating stably on electrified vehicles,and the operation reliability of them is essential to vehicles.SiC has been the new generation of semiconductor material after Si,and the devices based on SiC could withstand higher operation temperature.If the device based on SiC is used in power units,the total volume and weight of the system could be reduced largly,without being limited to the cooling.Nickel is a kind of stable metal element,and it could be used as the diffusing barrier layer for example UBM(under-bump metallization)in electronic packaging because the reaction rate is about two orders of magnitude slower than that of Cu.Three IMC namely Ni3 Sn,Ni3Sn2 and Ni3Sn4 exist in Ni-Sn binary phase system.The experiment of Ni/Sn diffusing couple shows that Ni3Sn4 appears firstly,and the next are Ni3 Sn and Ni3Sn2 in turn.But the formation rate of the latter is relatively slow.Ni3Sn4 is the mian phase as it is reflowed at relatively low temperature 250 ?.The melting point of Ni3Sn4 is 794.5 ?,which meets the requirement of operation at high temperature after reflowing at low temperature.The processing time is as high as 1-2 h at 250 ? with a mass of residual tin and inhomogeneous Ni-Sn IMCs in Ni-Sn TLP.If the processing temperature is promoted,the relatively large residual stress would have negative effects on the reliability.The paper presents that Ni@Sn core-shell structure is used to solve the problem brought by Ni-Sn TLP.Ni@Sn was made of Ni as core and Sn as shell,accelerating the reaction of between Ni and Sn to reduce the process time with the larger interface between them.Besides,Ni@Sn fabricated by electroless plating is the relatively fixed component structure when the thickness of shell is stable,which directly ensures the uniform distribution of Ni-Sn IMCs.Due to the fact that there are a lot of voids in solder paste made of Ni@Sn powder,reducing the shear strength at high temperature,adding the bad effect brought by the relatively poor wettability,the preform pressed by Ni@Sn powder was used in the study.In the reflow experiment,Ni and Ni3Sn4 are the main phases in the preform reflowed for about 40 min at 250 ? after XRD analysis firstly,which meets the predict.The effect of pressure,residual organics and over tin separating out on the bondline has been analyzed in optimizing reflow parameters using Ni-Ni bonding.The corresponding measures were put forward,to gain better bonding.Electrical resistivity,shear strength at high temperature,coefficient of thermal expansion and heat conductivity were measured.At 500?,the averge shear strength was gained,which proves the feasibility of the perform pressed by Ni@Sn core-shell powder at high temperature.Then,the excellent shear strength was analyzed in detail.