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SMT-Oct2017

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12 SMT Magazine • October 2017 Following the prelude for this series (SMT Magazine, August 2017), we now sequentially address the topics as outlined therein. Elemental Properties Bismuth (atomic number 83 and atomic weight 208) is classified as a metal. It normal- ly appears as grayish white with reddish tinge and can be grown into colorful iridescent crys- tals. 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/cm 3 ) is high- er than Sn (7.31g/ cm 3 ) and lower than Pb (11.34g/cm 3 ). With respect to conductivity, the electrical conductivi- ty of bismuth is mea- sured at 0.8 (10 4 Ohm –1 cm -1 ) versus tin at 9.1 and lead at 4.8, and the thermal con- ductivity is around 8 (w/m-k, 300°K) com- pared to tin's 66 and lead's 35. Among metals, bismuth's electrical and ther- mal 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 proper- ty, contributing to the improve- ment in wet- ting ability. This improvement is expected to be obser ved in bis- muth-containing solders with oth- er conditions being equal. This is con- sidered a significant advantage over oth- er elements when the specific performance requirement is need- ed. 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 solid- by Dr. Jennie S. Hwang H-TECHNOLOGIES GROUP The Role of Bismuth (Bi) in Electronics, Part 1 SMT PROSPECTS & PERSPECTIVES

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