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SMT Perspectives and Prospects
By Dr. Jennie Hwang
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The Role of Bismuth (Bi) in Electronics, Part 1
Following the prelude for this series (SMT Magazine, August 2017), we will now sequentially address the topics as outlined therein.
Bismuth (atomic number 83 and atomic weight 208) is classified as a metal. It normally appears as grayish white with reddish tinge and can be grown into colorful iridescent crystals. It is soft, but brittle. It has rhombohedral crystal structure in contrast to a cubic structure of lead and tetrahedral of tin. It melts at 271°C (520°F) and boils at 1,560°C (2,840°F). In comparison with tin and lead, its density (9.80g/cm3) is higher than Sn (7.31g/cm3) and lower than Pb (11.34g/cm3).
With respect to conductivity, the electrical conductivity of bismuth is measured at 0.8 (104 Ohm-1 cm-1) versus tin at 9.1 and lead at 4.8, and the thermal conductivity is around 8 (w/m-k, 300°K) compared to tin’s 66 and lead’s 35. Among metals, bismuth’s electrical and thermal conductivity are the lowest. The coefficient of thermal expansion (CTE) is also lower than that of tin or lead, at 13.4 x 10-6/C (Sn=22.0, Pb=28.9). The lower CTE can be leveraged as a useful property to design a proper CTE of solder materials.
Its lower surface tension (378mN/m, 270°C) than tin (574mN/m, 232°C) and lead (465mN/m, 327°C) is also a useful property, contributing to the improvement in wetting ability. This improvement is expected to be observed in bismuth-containing solders with other conditions being equal. This is considered a significant advantage over other elements when the specific performance requirement is needed.
Two other unique properties of Bi are that it has the greatest Hall effect of any metals (i.e., its resistance increases in a magnetic field), and that Bi expands upon solidification by 3.32%.
Overall, bismuth’s versatile properties make it an intriguing element to design alloys.
Table 1 summarizes the properties of bismuth.
The level of natural resources of bismuth is about the same as Ag. Elemental bismuth may occur naturally, although its sulfide and oxide form important commercial ores.
It is found most in Australia, Bolivia, Canada, China, Japan, Korea, Peru, Mexico, and USA. However, Bolivia, Australia and China are the only lands where native Bi is available. Due to its lack of wide presence in the native state, bismuth is usually associated with copper, lead, tin, tungsten, silver and gold ores. Yet in China, a major proportion is associated with tungsten. Bismuth has always been produced mainly as a by-product, and its price historically reflected the cost of recovery and the balance between production and demand.
The world mine production in 2016 stands at approximately 10,000–11,000 metric tons, with major contributions from China, Vietnam and Mexico, and the world reserves at 370,000 metric tons (Source: USGS). The United States ceased production of primary refined bismuth in 1997 and is highly import-dependent for its supply.
Bi Safety Data
Bismuth is widely recognized by the scientific community as one of the safest elements available. In practical terms, it has been regarded as non-toxic. Its only regulated uses are in pharmaceutical and cosmetic applications. It was approved by US EPA/NSF for substitution of Pb in free-cutting brasses for drinking water fittings in the U.S. A number of literatures cover the studies of Bi and Bi-compound safety.
Table 2 lists the relative rank of toxicity per U.S. OSHA – PEL.
Bi-compounds have been used for burn bandage dressings, antiseptic powders, and the treatment of venereal diseases. As reported, other pharmaceutical areas engaging Bi-compounds include its use as a pre-treatment to reduce the lethal toxicity of several forms of cancer therapy and the use as an additive to special polymers for bone implants. In addition, Bi-Ge oxide crystals have their place in diagnostic devices by its virtue of neutralizing lethal gamma rays and improving overall imaging quality.
In the chemical world, Bi catalysts are widely used in industrial organic chemistry, and Bi-compounds are popular pigments for health and beauty care. Toys and industrial applications that require non-toxic yellow to red or green pigments also rely on Bi compounds. For example, bismuth oxychloride pigment’s brilliance and luster is an effective ingredient to generate the pearlescent effect in lipsticks, nail polishes and make-up powders. Another compound, Bi citrate, is found to contribute to improved hair dyes. Additionally, bismuth is used in metallic paints and glass coloration products, as well as in heat/energy absorption coatings, such as solar panels.
For the metal industry, Bi has been successfully used as a lubricant to steel and aluminum, improving the machinability of free-cutting steels and aluminum. In galvanizing, Bi in place of Pb has been used to increase fluidity of the bath and wettability of steel. This Bi function makes the lead-free galvanizing possible. Then there are “green bullets.” U.S. Army engineers have been trying to develop lead-free, combat-ready cartridges since the early 2000s. Reportedly, more than 1,000 indoor military shooting ranges have closed because of the Pb-contamination problem and the resulting high airborne Pb-level. For this purpose, Bi is found to be a successful replacement for Pb.
For electronic applications, Bi-compounds are found to effectively improve and alter the properties of ceramic materials, such as lowering the processing temperature and improving the properties of varistors (zinc oxide). Bismuth telluride, a semiconductor, is found to be an excellent thermoelectric material. Additionally, bismuth has been used in electronic solders, which will be the subsequent topics of this series.
More Columns from SMT Perspectives and ProspectsSMT Perspectives and Prospects: Critical Materials—A Compelling Case, Part 3
SMT Prospects and Perspectives: AI—A Prelude to Opportunities, Challenges and Possibilities
SMT Perspectives and Prospects: Pearls of Wisdom
SMT Perspectives and Prospects: The Role of Bismuth (Bi) in Electronics, Part 7: A Case Study in Fillet-Lifting
SMT Perspectives & Prospects: Creating a Better World Through Engineering
SMT Perspectives and Prospects: Cybersecurity Requires an Active Approach
SMT Perspectives and Prospects: In Search of Wisdom
SMT Perspectives and Prospects: Critical Materials—A Compelling Case, Part 2