Morphological and mechanical characterisation of BI-AG alloy as high temperature lead free solder

In welding and metallurgical processes, solder is used to join two or more metallic surfaces. Today, the use of solder in modern microelectronic technology is ubiquitous. Lead solder has been widely used in the semiconductor industry for a long time. In soldering process, demands for elevated temperature application are increasing and they require large amount of lead in solder alloys like Pb-5Sn. Due to the lead toxicity, an impetus has been provided towards the development of leadfree solders by worldwide environmental legislation that prohibited the use of lead in solders. Despite numerous studies on lead free solders in recent years, only limited number of research are available on high-temperature lead-free alternative solders. Bismuth based alloys, zinc based alloys, and tin based alloys have been introduced as a tailored lead-free alternatives under consideration to substitute Pb- Sn solder alloys. Therefore, this research focused on several candidate alloys (Bi-1.5-2.5-3.5 wt.% Ag) as alternative solders in order to replace the high lead solders. To reach this end, candidates of Bi-Ag alloys with different mass compositions were put in oven at 160°C for 0, 100, 200, 350, and 500 hours . Morphological characterizations such as, grain growth behavior, Cu-rich particles size behavior in solder bulk and wetting angle formed between the solder alloys on copper plate during aging process, were performed by Optical Microscope (OM) and Scanning Electron Microscope (SEM). Area elemental analysis was performed using Energy Dispersive X-ray (EDX). Candidates of Bi-Ag alloy joint’s behavior and resistance were investigated using single lap-shear test method by 10 kN universal testing machine. Failed areas were investigated by optical microscope and SEM-EDX. It was observed that wt% of Bi and Ag, and aging time affected the overall results of shear test and isothermal aging process. By increasing percentages of Ag in solder matrix, the length of Cu-rich particle size and grain boundary grooving thickness increased. Also by increasing percentages of Ag in solder matrix and gaging time,wetting angle decreased. For shear test results, by adding of Ag content in solder matrix, the maximum shear stress and shear strain at maximum shear stress were also increased. The highest maximum shear stress and strain were observed at Bi- 3.5 wt.% Ag with the value of 13.355 MPa and 44.136, respectively. Furthermore,the lowest maximum shear stress and strain were observed at Bi-1.5 wt.% Ag with the value of 11.032 MPa and 24.955, respectively.

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