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March 2014 • SMT Magazine 33 er the results for this alloy in these test articles are due to the low-melting Sn-Pb-Bi eutectic or other factors. Subsequent analysis of the test re- sults 7 showed that test alloys were a less impor- tant factor in reliability testing than component type for both thermal and vibration testing. Sample Preparation and Testing Solder alloys were purchased as bars in pre- alloyed form from commercial vendors. In ad- dition to the two Bi-containing alloys, SAC305 was included as a reference alloy, as were SnPb and commercially pure Sn for selected test- ing. Specimens for various tests were prepared in bulk form in graphite molds by heating to 270°C and holding for 20 minutes, followed by rapid quenching to room temperature or 0°C, resulting in as-solidified microstructures similar to that observed in actual solder joints. Samples were tested in as-cast condition or aged condition, which consisted of 336 hours in an inert environment at 150°C. After casting or ag- ing, specimens were stored in a freezer at -10°C except dur- ing machining or testing. Tensile specimens were machined from cylindrical blanks as round dogbones with threaded grips to con- form to requirements in ASTM E8 standard for tensile testing of metallic materials. The cylindrical castings had a diameter of 9.5 mm, and the tensile specimens were ma- chined with a gauge diameter of 4.1 mm and a gauge length of 16.3 mm. Tensile tests were performed on an Instron (In- stron Corp., Norwood, MA) 8800 series test frame at a strain rate of 8.3∙10 -4 sec -1 at temperatures of 24, 75 and 125°C. Three or four specimens were run at each temperature for both as-cast and aged con- ditions of each alloy. Testing conditions were ambient air and samples were allowed to equili- brate for 15–20 minutes prior to beginning the test. Strain measurements were made using a video extensometer. Creep specimens were of a double shear ge- ometry inspired by earlier work on the creep of pure Sn and pure Pb 8 . The specimen fea- tures three grip sections of identical diameter and two reduced gauge sections. The specimen is held at each end and pulled in the middle by a fixed load, and approximates a constant stress condition to high strains. Specimens were machined from cylindrical castings with a diameter of 12.5 mm to dimensions of ap - proximately 23 mm in length and an overall di- ameter of 11.5 mm. The reduced gauge sections were each 2 mm long and 5.7 mm diameter. The specimen geometry is intended to keep the specimen at constant shear stress during the test, although pure shear is not strictly attained. Tests were performed at 42, 75, 100 and 125°C by submersing the sample and fixturing into a heating silicone oil bath. Room temperature tests were performed under ambient laboratory conditions. Tempera - ture fluctuations in both cases were less than ±1 K over the duration of the tests. Creep tests were performed at rang- es in applied stress equivalent to approximately 20–90% of the yield strength in tension, which worked out to a range of applied shear stress from 3.5 MPa to 60 MPa, depend- ing on the alloy and condi- tion. The higher-strength Bi-containing alloys required higher creep test stresses and accounted for the higher test loads. Displacement of the sam- ples was measured with an accuracy of 5 x 10 -4 mm, or a strain of approximately 9 x 10 - 5 . Tests were concluded no sooner than having met one of two conditions: a) rupture or clear evidence of the onset of ter- tiary creep, or b) extended period of apparent steady-state creep through a minimum test duration of 2.5 x 10 5 seconds (approximately three days). Several tests were run in excess of 5 x 10 5 seconds. The nature of the alloys was that true steady-state secondary creep was not feaTure mEchaNIcaL BEhavIOr OF BISmUTh PB-FrEE SOLDErS continues Solder alloys were purchased as bars in pre-alloyed form from commercial vendors. In addition to the two Bi-containing alloys, SAC305 was included as a reference alloy, as were SnPb and commercially pure Sn for selected testing. " "