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36 SMT Magazine • March 2014 feaTure tectic regions which constrain the deformation of the relatively softer primary β-Sn. The aged microstructure contains relatively less abundant and much larger intermetallic particles that are primarily found at grain boundaries. In uniaxial tension it is expected that these grain bound- ary particles do not strengthen SAC305 and the drop in strength due to aging is dramatic, a re- sult consistent with other investigations 13, 14 . The influence of Bi on Pb-free solder me- chanical properties have been attributed to a combination of solid solution and particle strengthening for both SAC-Bi and SnAg-Bi 9 . The results in Table 1 show that the while the strengths of SAC-Bi and SnAg-Bi respectively are not too different from each other in the as-cast condition, the alloys respond to aging differ- ently. The strength of SAC-Bi decreases after ag- ing, although less in absolute and much less in relative terms than was observed for SAC305. In contrast, the strength of SnAg-Bi remains nearly constant. The microstructural changes that oc- cur in aging of SAC-Bi are similar to those ob- served in SAC305, with the additional feature of precipitation of grain boundary Bi particles. Therefore, the strength is diminished in a simi- lar way by the loss of particle-reinforced eutec- tic regions, an effect which is countered by the presence of Bi in solid solution. Aging of SnAg- Bi leads to a ripening of Ag3Sn particles and ero- sion of the eutectic region but it is accompanied by a redistribution of Bi particles that results in a fine dispersion of Bi throughout grain interiors, not only at grain boundaries. The small changes in tensile properties for this alloy are somewhat surprising in light of the dramatic differences between the as-cast and aged microstructure. This too is understood as the dual contribution to strengthening by Bi as particles and in solid solution, although the relative contributions of the two types of strengthening cannot be deter- mined based on this data. Creep Creep data for Pb-free solder alloys and multiple sample scales (e.g., bulk, lap shear and solder-joint scale) have been analyzed us- ing both hyperbolic sine models and power law models 15, 16 : Hyperbolic Sine Model: Power Law Model: Creep data were analyzed in detail using both models in a previous publication 10 , but it Table 1: Tensile properties of Solders (all values in Mpa; average of three or four specimens per condi tion). mEchaNIcaL BEhavIOr OF BISmUTh PB-FrEE SOLDErS continues