Experimental investigation of mechanical and electro-magnetic behaviour of lead-free binary solders with trace addition of copper

Abstract

Lead-free solders are now-a-days of great practical importance, especially, for the electronic industries all over the world from both environmental and human health points of views. Several lead-free solutions for the solders are available in the industrial applications. Among these lead-free solutions, tin (Sn) based bismuth (Bi) and zinc (Zn) systems are the two main systems that are constantly being looked into for their superior performance. Aluminium (Al) is used as a minor alloying element on occasion. A new comparative analysis of the performances of three tin-based (Sn) potential lead-free binary systems, namely, Sn-Bi, Sn-Zn and Sn-Al with identical proportions (Sn80%x20%; x = Bi/Zn/Al) is conducted in the present thesis. More importantly, the effect of trace-addition (0~0.5% wt.) of copper (Cu) on the mechanical and electromagnetic characteristics of the binary solder systems is investigated along with their microstructural changes. For mechanical characterization, the tensile properties, microhardness and impact toughness are investigated. The bulk electrical conductivity, AC responses in terms of impedance, complex dielectric function and loss as well as complex permeability characteristics are studied for the purpose of electromagnetic characterization of the solder materials. The microstructure of the test alloys as well as their constituent elements is also investigated to explain the observed mechanical and electromagnetic behaviours. Optical micrographs and SEM images as well as EDS spectrum are included for this purpose. Addition of trace-amount of copper causes formation of various intermetallic phases/compounds (IMC) in the tin-based systems, which is verified to have significant effects on mechanical as well as electromagnetic properties. Owing to the formation of IMCs and refining of the eutectic microstructures, the mechanical strength and hardness of the alloys increase as the Cu percentage increases, which are found to be in contrast with those of elongation and impact toughness characteristics. The electrical conductivity is found to improve up to a certain Cu composition; further increase in Cu decreases the conductivity due to excessive formation of IMCs and increased grain boundary area. By acting as electron scattering points, the intermetallic phases, however, negatively impact on the dielectric function and loss characteristics. Finally, from the comparison of characteristic behaviours of the present lead-free solder-alloy systems with those of traditional lead-solder (Sn63Pb37) as well as currently used lead-free solder (SAC305), it has been verified that the Al and Zn-based ternary alloy systems containing 0.3% Cu are highly promising candidates for lead-free solders in terms of their mechanical as well as electromagnetic performances.