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The Role of Bismuth (Bi) in Electronics, Part 2

Click here to see Part 1 of this column series.

Part 2 of the series outlines the Bi effects on 63Sn37Pb solder material, which have been substantiated by years of field performance prior to lead-free implementation. This should serve as the sound baseline for further discussion on the subject.

The incorporation of Bi in Sn-containing solders is expected to affect both physical properties and mechanical properties of the resulting solder materials. This includes melting temperature, wetting ability, strength, plastic strain and fatigue behavior. The direct addition of a sufficient amount of Bi to a eutectic alloy (e.g., 63Sn-37Pb) also alters its eutectic behavior or deviates from eutecticity. The DSC thermogram below of 63Sn37Pb plus 1 wt.% Bi indicates that 63Sn37Pb essentially maintains its eutectic property. However, at 2 wt.% Bi, the range of melting starts to appear, departing from the eutectic point.

Figure 1: DSC Thermogram of 63Sn37Pb added with 1 wt.% Bi.

An extensive study was carried out on the effects of a minor addition of Bi to SnPb eutectic solder. The following table summarizes the effects of addition of Bi to 63Sn37Pb up to 5 wt.% on the basic mechanical properties and the melting temperature. The dosages of 2 wt.% and 5 wt.% Bi were added to 63Sn37Pb, respectively, by separately replacing Sn or Pb. In 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 (Tm), yield strength (sy), tensile strength (sTS), Young’s modulus (E), plastic strain (ep) at fracture, and fatigue life (Nf) at a total strain of 0.2% are summarized in Table 1. All compositions are expressed in weight percent unless otherwise specified. Also included is the reference solder alloy of 63Sn37Pb.

As exhibited in Table 1, the addition of 2wt.% Bi depressed the original melting temperature of 63Sn37Pb by 2–3°C. There was almost 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 solidus 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 effectively than Sn-rich phase.

Table 1: Melting temperature range and mechanical properties of 63Sn37Pb containing 2 – 5 wt.% Bi.

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 differences 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 significantly reduced the alloy plasticity.

The fatigue life (Nf) increased with the Bi content up to 5 wt.%, a contrast to the reduction in plasticity at 5 wt.% Bi. This is attributed to the amplitude of fatigue strain range used (De = 0.2%), which is well below the plastic strain at fracture (ep = ~12%). Under the relatively low fatigue strain range, the high strength is a leading factor contributing to the high fatigue resistance.

In a Bi-Pb system, a solid solubility of Bi in Pb is 23.5 wt.%, and a Sn-Bi system indicates a solid solubility of Bi in Sn is 21 wt.%. When Sn-Pb-Bi constructs a ternary system, the underlying basic physical interactions are not expected to be grossly changed. Overall, the Sn-Pb-Bi ternary solder alloys containing 2 wt.% Bi demonstrated a much higher strength, a higher fatigue life, as well as a higher plasticity than 63Sn37Pb eutectic solder. The alloy melting temperatures slightly decreased. When Bi increased to 5 wt.%, the strength and fatigue life of the Sn-Pb-Bi still remained higher than 63Sn37Pb, but their plasticity decreased significantly.