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March 2014 • SMT Magazine 37 should be noted that in that publication the use of hyperbolic sine model was used to compare the data for SAC-Bi and SnAg-Bi alloys to pub- lished data for dilute Sn-Bi alloys 17 and to com- pare as-cast and aged SAC305 to published data for as-cast 18 and aged 19 bulk SAC396. The hyper- bolic sine model may have merit as a modeling tool for solder joint deformation calculations, but when confronted with a change in creep behavior as a function of applied stress, it can obscure the actual operative creep mechanisms by rolling all data into a single expression with one stress exponent and activation energy. For example, comparing creep data for SAC305 at 75°C reveals key differences between the as-cast and aged condition, as shown in Figure 3. The as-cast SAC305 can be fit using a single power-law model, while the aged alloy could be fit using either a single hyperbolic sine function or a combination of a low-stress and high-stress power law function. In the case of aged SAC305, the change from a lower stress exponent at low- er stress (n = 2.5) to a higher stress exponent (n = 7.8) at applied shear stress above roughly 10 MPa at 75°C may mean that a different de- formation mechanism needs to be considered at different loads, and that the transition be- tween the two regimes is not likely to occur at the same shear stress in aged SAC305 as it does in as-cast SAC305. In general for all three alloys the transition between low and high stress oc- curs at a temperature-compensated shear stress τ/G (where G is shear modulus) of approximate- ly 4 x 10-4. While the previous publication provides de- tails of the creep model parameters n and Q for each equation, a rough summary of the obser- vations for all three alloys can be made using a single figure showing the measured steady-state or minimum shear strain rate as a function of temperature under a constant stress of 8.7 MPa, as shown in Figure 4. While higher applied loads were necessary during testing to complete tests in a reasonable amount of time, lower stresses such as 8.7 MPa (~1250 psi) are more representative of the types of thermomechanical loads that might be ex- pected in solder joints in actual applications. In this case, the data show that SAC305 is most creep resistant in the as-cast condition, but in the aged condition it deforms more rapidly than any other alloy. The 8.7 MPa stress in Figure 4 Figure 3: Creep data for as-cast and aged SaC305 tested at 75°C. feaTure mEchaNIcaL BEhavIOr OF BISmUTh PB-FrEE SOLDErS continues