Issue link: https://iconnect007.uberflip.com/i/270052
40 SMT Magazine • March 2014 structural evolution as observed during isother- mal aging, although by a different route than particle coarsening. For example, decrease in hardness was measured as a function of shear fatigue cycles even though the typical interme- tallic particle size did not change as a result of the applied elastic/plastic loads 21 . The testing temperature is suffi- ciently high relative to the melt- ing points of the alloys that at 0°C, the damping behavior is attributed to thermally activat- ed dislocation movement that is sensitive to microstructural factors such as grain size and dislocation density 22 . Literature data for high purity tin 23 are seen to be consistent with the present data, including the small absorption peak that occurs in several alloys but not Sn-37Pb. The general behavior of all of the Pb-free al- loys appears to be consistent with the response of the β-Sn matrix to the elastic input, while the differences between them are attributed to indi- vidual characteristics of the microstructures. While the log-log nature of the tanδ vs. fre- quency plots obscures the magnitude of the change between as-cast and aged Sn-37Pb, there is a roughly 0.011 decrease in tanδ due to aging, which is the largest measured change of all the al- loys. The coarsening of lamellae in Sn-37Pb leads to reduction in the number of phase boundaries which is presumably responsible for the decrease in damping. In the Pb-free alloys, the behavior of SAC305 with respect to aging is the opposite of SAC-Bi and SnAg-Bi, as aging SAC305 leads to an increase in damping capacity while it is de - creased in the Bi-containing alloys. For SAC305, the positive change is attributed to the forma- tion of a continuous Sn matrix. For SAC-Bi and SnAg-Bi the decrease is consistent with the gen- eral notion that a coarsening of the microstruc- ture and annealing reduces damping capacity. The plot of tanδ vs. temperature at a single frequency (in this case 10 Hz) in the presentation charts shows that considering the data at only 0°C does not provide a complete picture of the behavior of these alloys. As the test temperature reaches room temperature and above, tanδ of the Bi-containing alloys increases dramatically rela - tive to SAC305, so that above 50°C damping ca- pacity of SnAg-Bi actually exceeds SAC305 while SAC-Bi exceeds SAC305 in the as-cast condition. The trends in damping capacity as a function of temperature can be compared to results for com- mercially pure Sn and Sn-37Pb. This comparison shows that SAC305 behaves like Sn, exhibiting a gradual and relatively small change over the 200-degree change from –100 to + 100°C. SAC-Bi and SnAg-Bi are more like SnPb, in which a relative - ly large increase in tanδ occurs as temperature increases past room temperature. Given the similarities between the bi- nary phase diagrams of Sn-Pb and Sn-Bi, the results for damp- ing capacity indicate that the Sn- Bi interfaces are more important in determining the behavior of SAC-Bi and SnAg-Bi than the intermetallic particle-Sn matrix inter- faces in SAC305. conclusions The objective of this research was to charac- terize the mechanical metallurgy of alternative solder alloys for high-reliability applications. Key differences in use of high-reliability electronic systems compared to mass-produced consumer electronic devices include harsh deployment en- vironments and length of service life. Electronic systems installed in a satellite, for example, may be built years before the spacecraft is launched and then must survive many years in orbit with no opportunity for repair or replacement. The system life cycle, sustainability requirements and usage environments for space vehicles dif - fer from airborne systems and there is no reason to expect that the optimal solder alloy for space is the same for aircraft. Characterizing behavior and performance of solder joints over long time scales may not be a high priority for consumer electronics manufacturers, although the docu- mented changes in SAC alloy microstructures due to isothermal aging and their impact on reli- ability have been documented 24, 25 . The selection of Bi-containing alloys was The coarsening of lamellae in Sn-37Pb leads to reduction in the number of phase boundaries which is presumably responsible for the decrease in damping. " " mEchaNIcaL BEhavIOr OF BISmUTh PB-FrEE SOLDErS continues feaTure