Preparation And Process Optimization Of High-performance Pressureless Sintering Nano-particle Copper Paste For Die Attachment Of Power Electronics

With the increase of working temperature of chips in power electronics,there are the greatly growing demands for die attachment materials capable of high temperature processing.As a promising bonding material and a potential substitute for the nano-particle Ag paste,the nano-particle copper paste has attracted plenty of attention for its excellent electrical and thermal conductivity as well as low cost.The past ten years have seen many attempts made by researchers and engineers in the electronic package field for development of commercially viable Cu pastes,but there are still many challenges,such as easy oxidation of copper nano-particles(Cu NPs),rigorous sintering processing conditions of the copper paste,low strength and poor reliability of sintered joints.This thesis study aims to develop a low-temperature and pressureless sinterable nano-particle Cu paste with high performance.Two kinds of Cu NPs with different size distributions were synthesized by a chemical reduction method;and the performance of copper pastes prepared by these two Cu NPs was compared.Further,the role of capping agent on the surface of Cu NPs in the sintering process was clarified.The Taguchi method was used to optimize the fabrication process of Cu paste joints so as to further increase the bonding strength of the joints.Finally,the Cu paste joints with excellent bonding performance and high reliability can be obtained after pressureless sintering process at low temperatures.The results show that both polydispersed Cu NPs(P-NPs)with size in the range of 4 nm to 110 nm and bimodal-sized Cu NPs(B-NPs)with large particles(160 nm)surrounded by small NPs(9 nm)are synthesized by changing the amount of capping agent and the dropping rate of the reaction solution.The bonding strength of B-NPs Cu paste performs better than the P-NPs one due to formation of bulk-size copper islands in sintered Cu paste joints.Moreover,the formation process of bulk-size copper islands indicates that the bimodal-sized feature and wrapped structure of B-NPs exhibit heritable characteristics during sintering process.In addition,the results also demonstrate that the lactic acid capping agent on the surface of prepared Cu NPs can react with copper oxides at low temperature and the reaction products can promote the subsequent sintering process.This helps solve simultaneously two major obstacles in low-temperature pressure-less sintering of the Cu paste,one is the decomposition of organic capping agent on Cu NP surface at low temperature and the other is the poor sintering performance caused by surface oxidation of Cu NPs.The sintered Cu paste joints show high shear strength up to 65.2 MPa after perssureless sintering at 280 ?C for 10 min in nitrogen atmosphere,which is mainly attributed to formation of bulk-size Cu islands in the joints and appearance of an Ag-Cu interdiffusion layer of 300 nm thick at the interface between the silver-coated copper substrate and the sintered Cu paste.Further,it is worth indicating that the surrounded structure of bimodal-sized Cu NPs and the coexistence of ethylene glycol and glycerol in organic carrier of paste are sufficient conditions to ensure the formation of bulk-size copper islands in sintered Cu paste joints.In storage tests,the shear strength of sintered Cu paste joints is higher than 60 MPa after storage of the Cu NPs at 8 ?C for 90 days and the nano-particle Cu paste at-40 ?C for 30 days.Besides,the strength of sintered Cu paste joints is above 60 MPa after conducting high-temperature storage test at 200 ?C for 600 h.Therefore,this nano-particle copper paste is a promising material for die attachment of power electronics.