| Sn-Pb alloy is widely used due to its excellent performance in traditional electronic industry. Considering the inherent toxicity of lead and lead containing alloys to environment and human healthy, lead-free solders are developed to comply with the trend of lead-free electronic packing. However, most researchers prefer to carry out the researches by changing alloy composition, optimum mixture ratio, adding rare earth elements and altering cooling rate. Few people pay attention to the effects and rules of melt structure and status on microstructure and properties of lead-free solders. In this paper, Sn-3.8Ag-0.7Cu-0.5Ce solder was chosen to study the effect of liquid-liquid structure transition and aging treatment on solidification structure, soldering interface and mechanical properties of the solder joint, and the growth behavior of Cu6Sn5 on soldering interface and Sn whiskers were also studied. The main studies are as follows:(1) Electrical resistivity of liquid Sn-3.8Ag-0.7Cu-0.5Ce solder was investigated by DC four probe method. The abnormal changes of resistivity of melts was found at certain temperature range, which indicated that temperature-induced liquid-liquid structure transition (L-LST) probably occurred in Sn-3.8Ag-0.7Cu-0.5Ce solder.(2) Microstructure was observed after solidification experiment and aging treatment. Results show that the microstructure was remarkably refined by liquid-liquid structure transition, and the spreadability was also enhanced. After aging treatment, the morphology of microstructure changed a lot.(3) Sn-3.8Ag-0.7Cu-0.5Ce/Cu interfacial IMC of different melt structure after aging treatment was investigated. Results show that IMC grows slowly after the melts experienced L-LST and the morphology is more constant. The thickness of IMC is increased with elevating aging temperature.(4) Shear test of Sn-3.8Ag-0.7Cu-0.5Ce/Cu lap joint which was after aging treatment was conducted. Results show that the shear strength was increased by L-LST and with elevating aging temperature the shear strength first increased and then decreased.(5) Growth behavior of Sn-3.8Ag-0.7Cu-0.5Ce/Cu interfacial Cu6Sn5 was studied with different immerging time. Results show that the morphology of Cu6Sn5 is more compact after the melts experienced L-LST, which results in a more stable joint.(6) Sn whisker on different melt structure solders which was after aging treatment was observed by SEM. Results show that the growth of Sn whisker was restrained by L-LST and aging temperature has a great influence on the growth of Sn whisker. |
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