Lead-free Sn-based/MW-CNTs nanocomposite soldering: effects of reinforcing content, Ni-coating modification, and isothermal ageing treatment

Abstract

In this article, soldering of sheet-form copper substrates in a lap-joint design by using the newly developed lead-free nanocomposite solders based on the ternary eutectic system of Sn–3.5Ag–0.7Cu (in wt%) alloy reinforced with multi-walled carbon nano-tubes (MW-CNTs) was assessed. Different weight percentages of MW-CNTs (0.05, 0.1, 0.15, and 0.2 wt%) were incorporated within the SAC solder matrix by using the powder mixture system and employing of mechanical alloying (MA) processing route. The main object was controlling the formation morphology and growth kinetics of intermetallic compounds (IMCs) layer at the interface with the Cu substrate during soldering process. Also, to improve the compatibility of reinforcing MW-CNTs and solder alloy, decoration of nickel particles on the surface of nanotubes by using the electroless plating system was considered. The results showed that by increasing the amount of reinforcing nanotubes and implementation of Ni-coating on the surface of MW-CNTs, the thickness of IMC layer at the interface between Cu substrate and solder alloy is continuously refined. This important issue yielded to a continuous and significant improvement of tensile strength up to ~ 50%, as compared to the un-reinforced SAC solder alloy, despite of considerable ductility reduction. In following, the influence of post isothermal ageing treatment at a temperature of 150 °C with a holding time up to ~ 100 h on the microstructural characteristics and mechanical property of the soldered joints was elaborated. Employing of such isothermal ageing treatment revealed very effective in more elevating the mechanical strength of soldered joints (to attain the tensile strength of up to ~ 30 MPa). Furthermore, a simultaneous improvement of the elongation to failure more than ~ 22% was noted caused by increasing the thickness of the IMC layer at the interface. To this end, acceleration in the solid-state diffusion of elements played the main role.