A Study Of Connection Process And Properties Of Die Bonding On Bare Copper Substrate By Using A Silver Paste Hybrid With Micro And Nano Particles

The demand of high power density and high reliability for integrated power electronic module has put forward higher requirements for chip level interconnect technology.Nano silver paste has become the most popular chip interconnect material because of its low sintering temperature,high working temperature,and excellent conductive and thermal properties,gradually replacing the traditional solder alloys.However,the dispersants used to prevent the aggregation of silver nanoparticles could improve the sintering temperature,and the production cost of the nano silver particles is higher.All of these hinder the market promotion of nano silver paste.In addition,a potential application of nanosilver sintering is bare copper bonding,where replacing substrates with auxiliary silver or other plating that can damage bonding would be advantageous.However,die bonding on bare copper substrates remains difficult because copper is easily oxidized during sintering and there exists a lattice mismatch between silver and copper.In this study,a novel silver paste containing nano and microparticles was demonstrated for the bare-copper bonding of a high-power chip by pressure-free sintering in air.The effect of sintering atmosphere on the decomposition behavior of the paste was studied by TGA and Raman.The results show that oxygen must be present for successful low-temperature sintering.The effect of oxygen content on the sintering layer and interface bonding of bare copper joint was investigated by shear strength and transient thermal resistance.The results show that joints sintered in air at 265°C without applied pressure exhibited both excellent strength and thermal properties(shear strength: 53 MPa,transient thermal resistance: 0.132°C/W).The joints sintered in low vacuum at 265°C presented better thermal and mechanical performance(shear strength: 64 MPa and transient thermal resistance: 0.114°C/W).Microscopic analysis shows that the high surface energy of the nanoparticles in the paste and porous sintered layers may have contributed to the formation of chemical bonds between the activated silver and copper/copper oxides.In order to evaluate the reliability of the bare copper joints sintered in air and low vacuum using the paste and sintering process above,the temperature cycling tests were further carried out.The results show that the bare copper joints sintered and tested in low vacuum exhibited better resistance to temperature cycling than that in air,which is due to the absence of oxides,reducing the thermal stress caused by the mismatch of thermal expansion coefficients between the layers.According to the fracture mode of the joints,microstructure of sintered layer,and the diffusion behavior of the interface,the sintering mechanism of hybrid size silver paste and bonding mechanism of the interface under different oxygen content were proposed in this paper,and the aging failure behavior of the bare copper joints sintered in air and low vacuum were analyzed.