Fundamental Research On The Melting Temperature Depression Of The Sn-based Lead-free Solder Alloy Via Size Effect Of Nanopaticles

Due to the toxicity of lead (Pb), Pb-containing solder alloys are being phased out from the electronic industry, which has promoted the development and implementation of lead-free solders. These lead-free solder alloys possess, however, some weaknesses, mainly root in their higher melting temperature compared to the Sn-Pb eutectic solders. To solve this issue, researchers have tried to decrease the melting temperatures of the lead-free solders. A feasible approach to decrease the melting temperatures of a solder alloys is to decrease the particle size of the solder alloys down to the nanometer range.In this dissertation, we focus on the research to decrease the melting temperature of the lead-free solder alloy by decreasing their particle size down to nanometer range. The Sn-based lead free solder alloy nanoparticles with different sizes and size distribution were synthesized by chemical reduction method. The size-dependent melting temperature of these synthesized nanoparticles was studied by differential scanning calorimetry (DSC) and theoretical calculation. Based on these researches, the approach to prepare smaller nanoparticles of Sn3.0Ag0.5Cu lead-free solder alloy was studied, and nanoparticles of Sn3.0Ag0.5Cu lead-free solder alloy with equivalent melting temperature to SnPb solder alloy was obtained. Moreover, the undercooling of the Sn-based lead solder alloy nanopaticles was studied by DSC at different cooling rates. The major contents of this dissertation are epitomized as follows:1. Synthesis of Sn-based lead free solder alloy nanoparicles with different particle sizes and size distribution. The effect of surfactant on the nanoparticles size and size distribution was studied. It was found that the larger ratio of the weight of the surfactant to the precursor resulted in smaller particle size. Due to the capping effect caused by the surfactant molecules coordinating with the nanoclusters, a larger amount of surfactant would restrict the growth of the nanoparticles. Meanwhile, the effect of the reduction adding rate on the particle size and size distribution of Sn3.5Ag nanoparticles was studied. The results showed that the particle size increased and then decreased as decreasing the reduction adding rate, which can be explained by the competitive growth of the primary particles and second particles. The effect of the precursor concentration on the particle size and size distribution of Sn3.0Ag0.5Cu nanoparticles was also studied. The results showed that the particle size of Sn3.0Ag0.5Cu nanoparticles increased as increasing the precursor concentration. Finally, the formation of Sn3.5Ag and Sn3.0Ag0.5Cu nanoparticles was correspoindingly analyzed.2. The size-dependent melting temperature of Sn-based lead free solder alloy. The melting temperatures of Sn nanoparticles showed strong size-dependent tendency. The size-dependent melting properties of tin nanoparticles were comparatively analyzed by employing different size-dependent theoretical melting models and the differences among these models were discussed. The Sn3.5Ag and Sn3.0Ag0.5Cu nanoparticles also showed size-dependent melting temperature tendency. The melting temperature of Sn3.5Ag nanoparticles with average particle size of 30nm was about 210℃, 11℃lower than that of the bulk alloy. The melting temperature of Sn3.0Ag0.5Cu nanoparticles with average particle size of 30nm was 201℃, much lower than that of the bulk alloy. Theoretical analysis showed that the melting temperature can be as low as that of eutectic Sn-Pb solder alloy when the particle size was decreased to 10nm.3. The research on the Sn3.0Ag0.5Cu nanoparticles with large melting temperature depression. The Sn3.0Ag0.5Cu nanoparticles with smaller particles size and narrower size distribution were prepared by the self-developed consumable electrode direct current arc (CDCA) technique. The prepared nanoparticles were characterized by various methods such as X-ray diffraction (XRD), field emission scanning electronic microscope (FE-SEM) and high-resolution transmission electron microscopy (HRTEM). The melting temperatures of the Sn3.0Ag0.5Cu nanoparticles were measured by DSC, and the results showed that the calorimetric melting onset temperature of the nanoparticles of SnAgCu solder alloy could be as low as 180℃, which was equivalent to that of the traditionally used SnPb eutectic alloy (183℃). The CDCA technique showed promising prospect in manufacturing large amount of nanoparticles with controlled shape, small size, narrow particle size distribution and nearly oxide-free composition. This undoubtedly puts forward a novel feasible approach to manufacture high quality lead-free solders for electronic packaging.4. The research on the undercooling of the Sn-based lead-free solder alloy. The solidification properties of the Sn3.5Ag and Sn3.0Ag0.5Cu nanoparticles were studied by DSC at different cooling rates. The undercooling of these nanoparticles showed strong cooling rate dependent tendency. The undercooling of the Sn3.5Ag and Sn3.0Ag0.5Cu nanoparticles was in the range of 85.0~91.0℃and 82.0~88.5℃, respectively. In addition, the undercooling of Sn3.0Ag0.5Cu nanoparticles was much larger than that of the Sn3.0Ag0.5Cu micro-sized particles, hopefully producing better mechanical properties of the solder joints.