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32 SMT Magazine • March 2014 Bi) and Sn-3.4Ag-1.0Cu-3.3Bi (SAC-Bi) and compared to SAC305. Tensile properties, creep behavior and damping capacity of these alloys were evaluated. The results show significant dif- ferences in these properties between the as-cast and aged conditions. In particular, the properties of SAC305 respond to aging differently from the Bi-containing alloys. For ex- ample, while aging at 150°C for 336 hours (two weeks) de- creases the yield and tensile strength of SAC305 by nearly a factor of two, the same ag- ing treatment leads to a much smaller relative change in the strength of SAC-Bi. The rela- tionship between aging, mi- crostructure and mechanical properties do not necessarily explain performance in board- level thermal cycling tests but the metallurgical differences observed between the alloys due to Bi addition could con- tribute to fundamental basis for improved reliability. Experimental Alloy Selection The alloys that were selected for this study had shown good performance in circuit board reliability testing but had not been extensively characterized and had not been adopted for use by the consumer electronics industry for Pb-free assembly. The alloy selection strategy was based on three suppositions. First, the selection crite- ria for Pb-free solders in consumer electronics assembly would emphasize cost considerations and regulatory compliance deadlines in product design and manufacturing. Second, the reliabil- ity and product lifetime expectations for Pb-free consumer products are entirely different from high-reliability systems, especially space flight hardware where repair and replacement on or- bit are not possible. Consequently, the perfor- mance and reliability requirements for indus- tries whose products were initially exempt from RoHS would not be considered in alloy selec- tion and development of best assembly practic- es. The third supposition in alloy selection was that the consumer electronics industry would extensively characterize the Pb-free solder al- loys for their systems. Therefore, alloys chosen for this study were those which had shown early promise in circuit- board reliability testing but were subsequently abandoned by the electronics assembly business. The main sources used for the selection process were the two published by the National NCMS 1, 2, 5 and the U.S. Department of Defense- sponsored JGPP-JCAA 3 . Two Bi-containing alloys were selected based on review of these sources: 1. Sn-3.4Ag-4.8Bi (wt. %): This alloy performed well in studies reported by NCMS in 1997 1 , and was the best per- former in the initial screen- ing in the 2001 NCMS report 2 . It was dropped from consideration at this point because of its high Bi content. Bi forms a low melting ternary eutectic with Sn and Pb (melting point 96°C), so this alloy was potentially problematic in a mixed-alloy envi- ronment. The actual composition of the alloy reflects the largest amount of Bi that could be added to the Sn-Ag eutectic without showing evidence of Sn-Bi eutectic (melting point 138°C) during thermal analysis 6 . 2. Sn-3.3Ag-1.0Cu-3.4Bi: This alloy was the highly rated in the NCMS 2001 study and was one of the alloys selected for extensive charac- terization in the JCAA-JGPP study 3 . The alloy outperformed SAC and SnPb in manufactured circuit boards (high Tg board materials) in the latter effort, but did not do as well in reworked boards, in which hand soldering of eutectic SnPb was used to simulate repair of low-temperature circuit boards that had been originally soldered with Pb-free alloys. This specific parameter was intended to duplicate actual practices for aircraft circuit board repair, and it is not certain wheth - mEchaNIcaL BEhavIOr OF BISmUTh PB-FrEE SOLDErS continues feaTure The alloys that were selected for this study had shown good perfor- mance in circuit board reliability testing but had not been extensively characterized and had not been adopted for use by the consumer electronics industry for Pb-free assembly. " "