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MAY 2020 I SMT007 MAGAZINE 27 strong indication of HRL1 improved mechanical and thermal strength. Creep tests of the bulk alloys were performed using a constant load (150 N) at 80°C. This type of test offers an opportunity for gaug- ing the thermo-mechanical properties of sol- der joints before any assembly takes place. The total time until rupture, also called creep strength, of HRL1 is 30% higher than in the eutectic 42Sn58Bi, providing additional evi- dence of HRL1 improved resistance to mechan- ical and thermal stresses. Mechanical and Thermal Reliability Portable and handheld devices have quickly become part of our daily lives and, conse- quently, resistance to drop and shock turned into a must-have property of solders used in such devices. Since the testing of actual elec- tronic devices is quite cumbersome and expen- sive, proxy tests (such as the JESD22-B111 standard) can be used instead. JEDEC's service condition B (1500 Gs, 0.5 ms duration, and half-sine pulse) is probably the most common drop shock test at the board level and serves as a reference for the results presented later. Reducing the Bi content below 58 wt.% can effectively improve the alloy ductility while maintaining its strength and results in improved drop shock performance, as shown in Figure 2 [10] . However, Sn-Bi alloys with 40 wt.% Bi or lower have liquidus temperatures above 178°C and would require reflow tem- Additionally, the DSC curve of HRL1 shows a 79.7% conversion into a liquid at 139°C and 99% at 144°C [10]. The densities of 42Sn58Bi and HRL1 alloys are higher than SAC305, as Bi density is considerably higher than Sn. The lin- ear coefficient of thermal expansion (CTE) of HRL1 alloys is somewhere in between 42Sn58Bi and SAC305. At room temperature, both Sn-Bi alloys have considerably higher ultimate tensile strength (UTS) than SAC305. How- ever, HRL1 alloy yield strength and elongation are similar to those of SAC305. Comparatively, the eutec- tic 42Sn58Bi higher yield strength exemplifies its brittle nature. Tensile data of 42Sn58Bi at 75°C is not available, as the tensile samples started deforming and slipping from the test- ing grips. However, at 75°C, HRL1 tensile and yield strength still match SAC305 performance, which is a Figure 2: Weibull distribution of drop shock failures of various LTS alloys using CTBGA84. Table 1: Key bulk alloy properties. (Notes: 1) CTE between 25 and 80°C, 2) CTE between 25 and °C, 3) Sample deformed at °C.