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March 2014 • SMT Magazine 35 curs during solidification of Pb-free solders and the relatively low temperature of the binary Sn- Bi eutectic, the location of Bi particles is inter- preted as evidence that the boundary between primary tin dendrites and eutectic regions is the last part of the melt to solidify. After aging, Bi precipitates are found predominantly found at grain boundaries. The solubility of Bi in Sn at the 423 K aging temperature exceeds the Bi con- tent of the alloy, so the preferential nucleation and growth of Bi at heterogeneous nucleation sites due to aging suggests either that the Bi dif- fused rapidly at room temperature after aging or was retained at grain boundaries even during extended aging. The trends observed in SAC-3.4Bi are also true for SnAg-4.8Bi, but the distribution of Bi through the alloy is different due to differing Bi content. In the as-cast SnAg-Bi the Bi particles are found in the interior of the Sn dendrites (Figure 2), not at the dendrite-eutectic interface as in SAC-Bi. The logic for the SAC-Bi leads to the conclusion that the solidification of the Sn phase proceeds from the eutectic region to the interior of the dendrites. After aging the Bi dis- tribution consists of 1–5 µm equiaxed particles along grain boundaries and a fine dispersion of sub-micrometer particles throughout grain in- teriors. The room-temperature solubility of Bi in Sn is approximately 1.8 wt %, so the SnAg-Bi al- loy contains approximately twice the Bi beyond the solubility limit as the SAC-Bi alloy. This dif- ference accounts for the different microstruc- tures in the aged condition of the two alloys. The anisotropy of the tetragonal β-Sn unit cell has been noted for its influence on both mechanical behavior of Pb-free solder 11 and sol- der joint reliability 12 . The crystallographic ori- entation of various samples was not assessed in the current work but there is no indication that results were unduly influenced by preferred orientation or very large grain sizes. The influ- ence of preferred orientation of the Sn unit cell with respect to applied loads in the various tests should thus be considered part of the system- atic error in the reported results. Tensile Properties Tensile properties (yield strength and ulti- mate tensile strength) for tensile tests performed at three temperatures are summarized in Table 1. The Castin™ (AIM Solder) alloy was also test- ed and is included in the table. The change in yield or ultimate strength due to aging is shown as a percentage. There are two primary trends in Table 1. The first is the large difference in strength imparted to the SAC-Bi and SnAg-Bi alloys by the addition of Bi, and the second is the differences among the alloys in response of strength to aging. In SAC305, the considerable drop off in strength after aging is attributed to the loss of intact eu- Figure 2: SeM bSe micrographs (1000x) of as-cast microstructure of SaC-bi (left) and Snag-bi (right). feaTure mEchaNIcaL BEhavIOr OF BISmUTh PB-FrEE SOLDErS continues