Mechanical properties and tensile deformation behavior have been investigated on Sn-Ag-Bi system lead-free solders. Tensile deformation behavior was evaluated by strain rate changing method to estimate the thermal fatigue properties of the alloys. Tensile tests were conducted on cast solders shaped into rod type tensile specimens at room temperature. Strain rate was changed repeatedly during the test to obtain strain rate sensitivity index (m). The addition of bismuth up to 5% to Sn-3.5Ag eutectic remarkably increases the tensile strength, however, it drastically lowers elongation, especially the elongation becomes lower than 10% after aging at 393 K for 605 ks. Further addition of bismuth up to 15% gave only a little effect on tensile strength and elongation. The solution hardening of bismuth in tin is the main reason of the high strength of bismuth added alloys, because the increment of the lattice constant of tin well coincided with the change in tensile strength. The plots of m vs. strain where m was measured showed linear relations, therefore, the m value at strain zero (m0) and the gradient of the line (k) were obtained by extrapolating the line to strain zero. The lower values of m0 and positive low values of k have been believed to have excellent thermal fatigue resistance. The m value was decreased by addition of bismuth, however, it increased k. After aging the alloys more than 5%Bi, the gradient k showed negative values, indicating the alloys became susceptible to cracking, therefore, these alloys are believed to have inferior resistance to thermal fatigue. The alloys with 2% Bi showed lower m0 and lower k than Sn-3.5Ag, therefore, the alloy is estimated to have superior or similar thermal fatigue properties to Sn-3.5Ag, however, the melting temperature range is rather high.
