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34 SMT Magazine • March 2014 attained so a minimum creep rate was substi- tuted for steady-state creep rate where neces- sary. Specimens for dynamic mechanical analysis (DMA) were prepared from rectangular graph- ite molds that yielded bars measuring approxi- mately 60 x 6.4 x 3.2 mm. These rectangular bars were reduced to a thickness of approxi- mately 1.7 mm by manually grinding and lap- ping roughly equal amounts of material from each face. DMA was performed using a TA In- struments Q800 DMA in dual cantilever mode, in which the rectangular beam sample was fixed at its ends and the middle. Tests were run in both isothermal and temperature sweeping modes. For the former, measurements at fre- quencies from 0.1 to 200 Hz were collected at 0 and 24°C and displacements of 5, 10 and 20 µm. A complete set of alloys and aging condi- tions were run in duplicate only at 5 µm, which for a nominal beam thickness of 1.7 mm was equivalent to a strain of approximately 7.5∙10 -5 . Measurements above 50 Hz were discarded due to instrumental factors. Comparison of alloys in isothermal mode is made here most exten- sively for tests run at 0°C because at the lower temperature there was less dependence of mea- sured tan δ on strain. Additional tests were run as temperature sweeps using a displacement of 5 µm at fixed frequencies from temperatures of -100 to +100°C. results Microstructures Details of as-cast and aged microstructures, including micrographs, of SAC-Bi, SnAg-Bi and SAC305 for the tests discussed herein have been previously published 9, 10 and are only summa- rized here. As reference, previously unpublished metallographic images of SAC305 at 400X mag- nification are provided in Figure 1, and SAC-Bi and SnAg-Bi in Figure 2. All three alloys share similarities in their microstructures and re- sponse to aging. In the as-cast condition the microstructure is dominated by the dendritic structure exemplified in Figure 1. Individual in- termetallic particles that formed upon solidifi- cation (SnAg3 and Sn5Cu6) were too small to be resolved even at 10,000x magnification in a scanning electron microscope. Aging at 150°C for 336 hours leads to ripening of these particles and loss of dendritic structure. Intermetallic particles are typically found stabilizing irregular grain boundaries. The story for the two Bi-containing alloys is similar with respect to grain boundaries and Sn-Ag and Sn-Cu compounds. Bi does not form compounds with Sn, Cu or Ag, so it is present ei- ther in solid solution or as a separate elemental phase. In as-cast SAC-Bi, Bi particles solidify at the edges of individual Sn dendrites. Given the undercooling of the primary β-Sn phase that oc- mEchaNIcaL BEhavIOr OF BISmUTh PB-FrEE SOLDErS continues feaTure Figure 1: optical micrographs of as-cast (left) and aged (right) SaC305.