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SEPTEMBER 2024 I SMT007 MAGAZINE 75 phase adjacent to the smaller Bi-rich particles is higher that adjacent to the larger Bi-rich par- ticles, where it is estimated to be 2.9 at%. is Bi concentration gradient in the Sn-rich phase drives the Bi atoms to diffuse from the smaller Bi-rich particles to the larger Bi-rich particles. In addition, since the Bi content of the Sn-rich phase adjacent to the larger Bi-rich particles is lower, the microstructural coarsening will result in a lowering of the equilibrium Bi con- tent of the Sn-rich phase. So, the two phenom- ena, one being the Sn-Bi alloy composition moving towards thermodynamic equilibrium from point c to point b on the phase diagram and the other being the microstructural coars- ening, both result in the lowering of the Bi con- tent of the Sn-rich phase and therefore, as per the lever rule, the overall increase in the vol- ume fraction of the Bi-rich phase in the alloy. Room temperature aging thus increases the volume fraction of the Bi-rich phase. A recent paper reported a decrease in the volume frac- tion of the Bi-rich phase when a Sn-Bi alloy was aged at room temperature 4 . e explanation of this error can be found in the next section. Temporal Decrease of the Area Fraction of the Bi-rich Phase at Free Surfaces e Bi-rich phase area fraction at free sur- faces has been experimentally observed to decrease with time even at room temperature. is decrease is not due to the Bi-rich phase particles dissolving into the Sn-rich matrix but due to the Bi-rich phase particles mov- ing away from the free surfaces into the bulk of the alloy. is can only happen if the sur- face tension between the Bi-rich phase and air is higher than that between the Sn-rich phase and air. Figure 3a shows the surface of a planar solder specimen of eutectic Sn-Bi alloy flanked by copper on both sides having been subjected to high electric currents at high temperatures. Figure 3: a) Scanning electron micrograph of a planar solder specimen that was subjected to 3 Amp current at 100, 90, 80, 70 and 60°C over a total of 52 days; b) Surface 1 µm below the surface shown in (a); c) Surface 5 µm below the surface shown in (a); d) Surface 10 µm below the surface shown in (a).