| With the development of highly dense,micro-scale and multi-functional electron components,as well as the development of wide bandgap semiconductors,the reliability of microelectronic packaging system is facing greater challenge.Conventional lead-free tin-based solder is not suitable for the next generation of “solders” to form joints in electronic devices,because the excessive growth of intermetallic compounds,lower resistance to electrical migration and lower melting point.Based on the surface effect of nanomaterials,copper nanoparticles can be sintered at a temperature that is much lower than the melting point of bulk copper.And bonding with sintering copper nanoparticles has attracted much attention of international and domestic academics due to its considerable resistance to electrical migration,thermal and electrical conductivity.However,it is difficlut to store copper nanoparticles and protect them from oxidization,and high temperature or pressure is necessary to achieve the copper nanoparticles solder joints with considerable bonding strength,thus limiting the application and development of copper nanoparticles as electronics interconnection in microelectronic packaging field.This research aims to promote the bonding strength of copper nano/micro-particles solder joints by designing the mass ratio between copper nano/micro-particles and selecting optimal surface roughness of copper substrate,and improve the oxidation resistance of copper nanoparticles for high temperature and long-term storage by phosphating treatment.Modeling,calculation of formula and investigation of fracture surface microstructure were also used to analyze the strengthening mechanism of these three approaches on the shear strengths of joints.The main conclusions are given as follows:(1)The copper nanoparticles can fill the interstitial space between the copper microparticles and tend to be around of them during sintering,thus producing the S-Structure which can increase the density of copper sintering structure.The mass ratio of 9/1(nano/micro)was optimal and the joint with it showed a highly dense structure and a highest bonding strength of about 20.5 MPa at a lower temperature of 250 °C and pressure of 4 MPa.Copper nano/micro-particles paste possesses higher oxidation resistance,bonding strenghth and lower cost than copper nanoparticles paste.(2)The sawtooth structure,formed between the grooves of copper substrates surface and the sintering structure of copper particles,can increase the bonding area and promote the mechanical interlocking between copper substrates and copper particles.The optimal surface roughness for maximum bonding area and strength was Ra = 189.9 ± 5.4 nm,and it depends on the material of substrate and the size of copper particle.(3)The optimal phosphating time for copper nanoparticles was 30 min,the nanoparticles after phosphating for 30 min can be effectively protected by the phosphating films from oxidation at temperature below 300 °C and they can be stored for months in air without any oxidation.The mean diameter of the nanoparticles increased to 200 nm from 50 nm after phosphating for 30 min due to the formation of phosphating reactants.The shear strength of the Cu-Cu joint by phosphating treated nanoparticles was higher than that of untreated nanoparticles due to the better oxidation resistance when the temperature was over 250 °C,while the performance of the joint by phosphating treated nanoparticles was also better than that of untreated nanoparticles in storing and aging tests. |
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