SMT007 Magazine

SMT-Dec2017

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14 SMT Magazine • December 2017 addition, tests were performed on alloys with Bi replacing an equal amount of both Sn and Pb. This resulted in six solder alloy compositions. The solder alloy compositions along with their melting temperature (T m ), yield strength (s y ), tensile strength (s TS ), Young's modulus (E), plas- tic strain (e p ) at fracture, and fatigue life (N f ) at a total strain of 0.2% are summarized in Table 1. All compositions are expressed in weight per- cent unless otherwise specified. Also included is the reference solder alloy of 63Sn37Pb. As exhibited in Table 1, the addition of 2 wt.% Bi depressed the original melting tem- perature of 63Sn37Pb by 2–3°C. There was al- most no distinction in the melting temperature change when 2 wt.% Bi replaced Sn or Pb or both Sn and Pb in an equal amount. At 5 wt.% Bi, both the alloy liquidus temperature and sol- idus temperature were lowered. The melting temperature for the solder alloys with 5 wt.% Bi in place of Sn (Alloy 5) was about 2–3°C lower than that with 5 wt.% Bi in place of Pb (Alloy 6). This indicates that Bi at 5 wt.% lowers the melting temperatures of Pb-rich phase more ef- fectively than Sn-rich phase. Comparing the strength of the solder alloys containing 2 wt.% Bi with that of 63Sn37Pb, the Bi addition largely increased the alloy strength and plasticity. There were no measurable differ - ences in the tensile behavior among the solder compositions containing 2 wt.% Bi in place of Sn (Alloy 2) or Pb (Alloy 3) or equally both Sn and Pb (Alloy 1). When the content of Bi in 63Sn37Pb increased to 5 wt.%, the strength maintained, but the alloy plasticity reduced. The differences in tensile behavior among the solder compositions containing 5 wt.% Bi in place of Sn (Alloy 5) or Pb (Alloy 6) or equally both Sn and Pb (Alloy 4) were within the data-scattering range. The content of Bi up to 2 wt.% was the most effective amount to increase both the alloy strength and plasticity. Any further increase in the Bi content from 2 wt.% to 5 wt.% exhibited little effect on the alloy strength, however sig- nificantly reduced the alloy plasticity. The fatigue life (N f ) increased with the Bi content up to 5 wt.%, a contrast to the reduc- tion in plasticity at 5 wt.% Bi. This is attributed THE ROLE OF BISMUTH (BI) IN ELECTRONICS, PART 2 Table 1: Melting temperature range and mechanical properties of 63Sn37Pb containing 2 – 5 wt.% Bi.

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