Study Of Microstructure Evolution And Mechanical Properties Of Low Silver Composite Solder Alloys And Solder Joints

With the continuous development of miniaturization and multi-function of microelectronic products,the size of solder joints in microelectronic packaging is getting smaller and smaller,and the proportion of intermetallic compounds(IMC)between the interfaces increases.Due to the brittleness of the interfacial IMCs and the thermal mismatch between components,the possibility of solder joint failure is increased to a certain extent.Meanwhile,the mechanical properties and microstructure evolution of solder joint are also important factors affecting the reliability of solder joints.Therefore,the research on the mechanical properties and microstructure evolution of solder joint has become one of the key scientific issues in microelectronic packaging reliability research.In this thesis,in order to further optimize the properties of low silver lead-free solder,the composite solder was prepared by low-silver content solder Sn-0.3Ag-0.7Cu doped with ceria(CeO₂)nanoparticles.The influence mechanism of CeO₂ nanoparticles doping on the growth of intermetallic compounds and interfacial microstructure during reflow soldering was explored.The shear properties,creep behavior and low cycle fatigue properties of Sn-0.3Ag-0.7Cu-x CeO₂composite solder alloy were systematically studied.The main contents and results are as follows:The influence of CeO₂ nanoparticles doping on the growth of intermetallic compounds and interfacial microstructure of the composite solder alloy during reflow soldering was studied.Results show that the microstructure of the composite solder alloy could be optimized by adding CeO₂ nanoparticles,and the IMC grains in the solder/Cu interface and Sn matrix are refined.When the concentration of CeO₂ is 0.5 wt.%,the best refining effect can be obtained.The thickness and grain size of interfacial IMC increase with the increase of reflow time.The addition of CeO₂ nanoparticles can significantly inhibit the growth of IMCs.In order to further study and analyze the IMC growth mechanism of composite solder alloys during reflow soldering,a diffusion-controlled IMC growth kinetics model is proposed based on the mass conservation law,Fick's first law,and quasi-steady-state approximation.The modeling results show that the diffusion rate of Cu atoms at the interface has a maximum value at the early stage of reflow soldering and a rapid downward trend appeared soon,and then tends to be stable,which is consistent with the formation and growth data of the interfacial IMCs.The calculated results based on the model are in good agreement with the theoretical analysis,which indicates that the diffusion model can be used to further understand the growth process of IMC and the diffusion mechanism of Cu atoms at the interface during the reflow soldering process.The influence of the addition of CeO₂nanoparticles on the diffusion behavior of Cu atoms during the interfacial reaction is consistent with the heterogeneous nucleation mechanism.The influence of the addition of CeO₂ nanoparticles on the shear properties of Sn-0.3Ag-0.7Cu-xCeO₂ composite solder alloys was investigated.Results show that the shear properties of the composite solder alloys were improved.The mechanism could be that the addition of CeO₂ nanoparticles can inhibit the growth of eutectic phase,and the pinning effect of smaller eutectic phase is more obvious,therefore the shear properties of the composite solder alloy are enhanced.When the concentration of CeO₂ is 0.5 wt.%,the best shear properties can be obtained.A modified model of the relationship between temperature,strain rate and max shear stress is established based on the power law and the Anochie-Boateng relationship.The calculated results based on the model are in good agreement with the experimental data.Therefore,the new model can be used to analyze the shear properties of composite solder alloys and to well understand the influence mechanism of temperature and strain rate on shear properties.After doping CeO₂ nanoparticles,the size of pores and dimples formed by shear action is getting smaller,so it is more difficult to link the holes to form cracks or larger holes,thus delaying the formation of larger holes and cracks,so as to improve the shear properties of the composite solder alloys.The creep behavior of Sn-0.3Ag-0.7Cu-0.5CeO₂ composite solder alloy under different stresses and temperatures was studied.The results show that the creep activation energy of the composite solder alloy is in the range of 40.87-48.61 kJ/mol,which is close to the level of high silver content solder of Sn-3.9Ag-0.6Cu and Sn-3.8Ag-0.7Cu.It is indicated that CeO₂nanoparticles can effectively improve the creep activation energy of low silver solder alloy,and make it have the creep performance similar to that of high silver solder alloy in the case of low Ag content.The Peierls-Nabarro stress and dislocation slip activation energy of Sn-0.3Ag-0.7Cu and Sn-0.3Ag-0.7Cu-0.5CeO₂ are small,which indicate that the obstacle resistance of dislocation slip in solder alloys is small and dislocation slip is very easy to occur.The Peierls-Nabarro stress and dislocation slip activation energy of the solder alloy containing CeO₂ nanoparticles are higher than those of the solder alloy without CeO₂ nanoparticles,indicating that the dislocation slip resistance of the composite solder alloys is higher,so the mechanical properties of the composite solder alloy are better than those of the solder alloy without CeO₂ nanoparticles.The threshold stress of the composite solder alloy at different temperatures is discussed based on Orowan stress theory,and a modified creep model is proposed.The results show that the creep behavior of composite solder alloy simulated by the modified creep model is better than that of the original model.The low cycle fatigue behavior of Sn-0.3Ag-0.7Cu-0.5CeO₂ composite solder alloy is studied.The results show that the low cycle fatigue life index and material ductility coefficient decrease with the increase of temperature,which means that the low cycle fatigue life and ductility of composite solder alloy decrease with the increase of temperature.In addition,fatigue index and ductility coefficient are strongly dependent on frequency.Therefore,the Coffin-Manson model is modified,and the relationship between frequency index,low cycle fatigue life index,material ductility coefficient and temperature is established.The modified model can eliminate the influence of different frequency and temperature on the low cycle fatigue life of composite solder alloy.