Issue link: https://iconnect007.uberflip.com/i/270052
38 SMT Magazine • March 2014 figure 4: comparison of shear strain rates of alloys in aged and as-cast condition. Previously unpub- lished results for Castin (SaC-Sb) are also shown. is a larger fraction of the yield strength in as- cast SAC305 than for SAC-Bi or SnAg-Bi, so the change in creep strain rates in SAC305 with ag- ing is due to the shift to the high stress regime with higher stress exponent n for the aged alloy even though at the same applied load. At the other extreme, the measured creep strain rates for SnAg-Bi do not change due to aging. While the details of creep analysis are im- portant, the quantitative results of creep testing can be summarized as follows 10 : • Aging decreases the apparent activation energy of creep for SAC305 and SAC-Bi; • Aging has minimal effect on the activation energy of SnAg-Bi; and • The activation energy of creep in SAC305 is lower than SAC-Bi and SnAg-Bi in either as-cast or aged conditions. In practical terms, the implications of the first two of these three statements are captured to a great extent in Figure 4: The creep rates of SAC305 and SAC-Bi will change with the mi- crostructural changes that occur during aging, but the creep rate of SnAg-Bi will remain nearly constant. It has been found that extended aging of SnPb at 125°C had minimal effect on room- temperature creep rates of Sn-37Pb 13 , so the behavior of SnAg-Bi is similar to SnPb in that respect. Damping capacity Damping capacity is reported as loss tangent (tanδ), where the loss angle δ represents the phase angle between stress and strain. The loss angle is related to the engineering quantity spe- cific damping capacity Ψ by the relationship: Ψ =2πtanδ The specific damping capacity refers to the ratio of the energy dissipated to maximum elastic energy stored 20 ; in the DMA testing the source of the elastic energy is the cantilever bending imposed by the test frame. Presumably higher damping capacity could lead to better dynamic performance, with less elastic energy from impacts available to cause failures in inter- metallics or pad cratering. For SAC solders the energy stored during the input of elastic energy may provide a similar driving force for micro- mEchaNIcaL BEhavIOr OF BISmUTh PB-FrEE SOLDErS continues feaTure