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14 SMT Magazine • October 2014 introduction In response to government legislation and regulation around the world regarding lead con- tent in consumer electronics 1 , electronics pro- ducers have quickly converted to lead-free tin/ silver/copper (SAC) solder. In 1997, a compre- hensive $11 million National Center for Manu- facturing Science (NCMS) project was conduct- ed by 11 private and public institutions to de- termine a suitable replacement for tin/lead sol- der (SnPb) 2 . After studying 70 candidate alloys, it was concluded that alloys near the eutectic SAC composition would be best suited for their applications. Additional studies and consortia (National Electronics Manufacturing Initiative, Center for Advanced Vehicle Electronics, IN- EMI, HDPUG, IDEALS, Pb-Free Electronics Risk Mitigation) perpetuate near eutectic SAC alloy as the most popular lead-free replacement out of the now more than 300 alloys in use. Its reli- ability has proven acceptable to the consumer electronics industry where short product life cycles and relatively benign operating environ- ments are common. The primary difficulty of the SnAgCu system is its high melting point at 217°C, requiring processing temperature in ex- cess of 250°C to ensure complete melting of the high melting phases Ag3Sn and Cu6Sn5 that could be present (Figure 1). Such high temperatures can damage the polymeric materials used for components, stak- ing, and boards, unless more expensive compo- nents and materials are used and limit the num- ber of rework cycles 2 . Additionally, it brings new and reemerging failure modes in electronics, including tin whisker growth 3 . For defense and space applications, reemergence of this issue with SAC raises concerns regarding increased infant mortality, latent failures, and the need for complete requalification. Establishing new qualification procedures is made more onerous as the behavior of SAC alloys are still not fully characterized or understood. Additionally, the most common alloy system exhibits a num- ber of known drawbacks, making it unreliable for long-term use in harsh environments with shock, vibration, heat or thermal cycling 4–8 . When alternatives to SnPb solder are consid- nanOcOPPer-baseD sOLDer-Free eLectrOnic assembLY materiaL continues FEATurE Figure 1: phase diagram of the Sn-Ag-Cu system. graph generated from niST data.