The effect of cobalt nanoparticles on the microstructure and mechanical properties of hybrid solder joints

Abstract

The restriction on the use of lead-based solder has created an urgent need to develop alternative materials, that match or exceed the performance of traditional lead-containing solders. Since conventional alloying processes have not been sufficient to achieve the desired properties, new approaches are needed to enhance the performance of solder materials. One promising strategy for improving solder performance is the incorporation of reactive nanoparticles, by adding them to the solder or the flux. They interact with the material at microscopic level. This process enhances key characteristics such as mechanical strength, heat resistance and overall longterm stability. As a result, so called advanced "hybrid solder systems" have developed. This thesis investigates the impact of cobalt nanoparticles (Co NP) on mechanical properties and microstructure of hybrid solder joints, with a key focus on the growth of brittle intermetallic compounds (IMC), the shear strength and time and temperature dependent mechanical properties. Sn-3.5Ag solder and flux doped with up to 1.00 % cobalt nanoparticles was used to prepare the solder joints. Subsequently, some of the specimens were subjected to a variety of thermal treatments. Metallographic analysis was used to observe microstructural differences and measure IMC thickness to evaluate growth kinetics and the formation of brittle intermetallic compounds. Push-off shear tests provided insights into the mechanical performance. Stress-relaxation tests (SRT), were conducted to measure the relaxation behaviour and calculate the stress exponent to determine the creep resistance. The results show that the addition of 0.10 % cobalt nanoparticles significantly reduces the growth of the Cu6Sn5 IMC layer, with only a slight effect on the Cu3Sn layer, for as reflow and thermally treated specimen. This reduction in brittle intermetallic compounds growth leads to an improved long-term stability. However, this effect is most pronounced at low concentrations of Co NP additions. Push-off shear tests and the shear tests with the stress-relaxation samples both showed an significant increase in performance with 0.10 and 0.25 % Co NP additions, leading to an improvement of the mechanical properties. Increased concentrations showed no enhancement. The load drop measured in stress relaxation tests did not exhibit any significant change with the addition of cobalt nanoparticles. However, the stress exponent revealed a clear trend, demonstrating that the creep resistance could be considerable improved with the addition of 0.10 % cobalt nanoparticles. These findings highlight the potential of adding reactive nanoparticles to solve significant issues related to lead-free solder joints, by improving both their initial performance as well as their long-term stability. A key and recent discovery is that the increase in shear strength does not directly correlate with an improvement in creep behaviour of the cobalt nanoparticle doped solder joints. Future research could concentrate on optimizing the nanoparticle concentration and evaluating the effects of other metallic nanoparticles to further improve solder joint properties.