Study on the wetting properties, interfacial reactions and mechanical properties of Sn-Zn and Sn-Zn-Bi solders on copper metallization
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Date
2007
Authors
Ramani, Mayappan
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Abstract
Practically all microelectronic assemblies in use today utilize Sn-Pb eutectic solder for interconnection. Due to the increase in the use of electronic devices within the industry as well as personal use, the usage of solder connections has increased. Emerging environmental regulations worldwide, most notably in Europe (WEEE) and Japan, have targeted the elimination of Pb usage in electronic assemblies, due to the inherent toxicity of Pb. This has made the search for suitable Pb-free solders an important issue for microelectronics assembly. In this research, Sn-9Zn and Sn-8Zn-3Bi lead-free solders were investigated as potential replacements for the Sn-Pb solder. A systematic study was conducted on the solders characteristics, wetting behaviour, the interfacial reaction, mechanical properties and growth kinetics of solders on Cu substrate. The addition of 3wt% of Bi to the Sn-Zn system lowered the melting temperature by 3.5oC which is only 12oC higher than Sn-40Pb solder. It improved the wettability on Cu substrate by reducing the surface tension of the molten solder. The -Cu5Zn8 phase is the main interface intermetallic formed between Sn-Zn solders and the Cu substrate and this intermetallic thickness increased with time and temperature. The Sn-8Zn-3Bi/Cu solder joint had higher joint strength than Sn-9Zn/Cu and Sn-40Pb/Cu joints but solder joint degradation occurred at 150oC aging temperature. The activation energy for the growth of -Cu5Zn8 phase in Sn-9Zn/Cu and Sn-8Zn-3Bi/Cu systems are 44.05 and 55.36 kJ/mol, respectively. The addition of Bi to the Sn-Zn system had retarded the growth of -Cu5Zn8 phase by increasing the activation energy for the intermetallic growth. The direct interface between Cu and Sn-8Zn-3Bi solder should have a temperature limit to be used below 100oC, which is enough for most of the commercial electronics applications.
Description
Ph.D
Keywords
Materials and Mineral Resources Engineering , Copper metallization