| As the direction of development of electronic devices is miniaturization and high density, the service condition for solder joint is more and more rigorous. Therefore, a new type of composite solder is demanded to satisfy the high reliability requirement of modern electronic products.Graphene nanosheets(GNSs) is a kind of nano-size reinforcement, wich has extraordinary performances. In this thesis, SAC composite solder reinforcement by GNSs was prepared by the powder metallurgy process, and the mass fraction of GNSs in the composite solder changes from 0% to 0.1%. During sintering process, through atoms diffusion between interface of solder powers, the solder powers become a solder whole under pressure and with the help of hot.Basic properties testing show that after adding GNSs the density and coefficient of thermal expansion will decrease, while the hardness will increase. Through solder microstructure observation found that adding GNSs can refine microstructure and improve interface energy per unit volume. When the mass fraction of GNSs belows 0.02%, GNSs disperse in solder matrix homogeneously with gap between GNSs, and in this “sea-island two-phase system” situation the thermal diffusivity will descent. However, the thermal diffusivity will ascent if the mass fraction of GNSs is more than 0.02%, because the GNSs form a thermal conductive network by touching each other. Measuring the contact angle of solder joint between solder and its composite with copper substrate illustrates that adding GNSs could decrease the contact angle and this phenomenon will be more obvious with more GNSs added.The growth thythm of interfacial IMC had been researched before and after adding GNSs. After soldering 5min, most of interfacial Cu6Sn5 is scallop-type shape and other is hexagonal prism-type shape. The research consequence shows that the GNSs added can reduce interfacial IMC growth rate. The statistic data of average IMC thickness is analysed after isothermal aging from 100℃ to 175℃. The activation energy for SAC and SAC+0.02%GNSs composite soder are 46.20KJ/mol and 59.72KJ/mol respectively calculated by empirical diffusion formula and Arrhenius equation. In addition, during the isothermal aging process the shape of Cu6Sn5 translate form scallop-type to triangle-type through crosssection of solder joint observation.The shear load of SAC and its composite solder BGA joints have bean tested. Test results show that adding GNSs can improve resistance of shear fracture of joints, and slso it can be foud that the improvement is more obvious at high additive content. After joint crosssection observation, the fracture modes are divided into three typical modes. Because of refine grain strengthening and dispersion strengthening, solder joints’ fracture mode is changed and their shear load is improved. Then SAC and its composite solder joints are placed in aging aven for 7d at 175℃. The shear load of those joints experienced isothermal aging descent significantly. This phenomenon can be attributed to two causes. On the one hand, the thickness of interfacial IMC increased significantly during isothermal aging process. On the other hand, the strength of solder matrix will decrease after IMC coarsening and micropore aggregating in solder matrix. |
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