Effect of Rare-Earth (La, Ce, and Y) Additions on the Microstructure and Mechanical Behavior of Sn-3.9Ag-0.7Cu Solder Al
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EMERGING environmental regulations are dictating the elimination of Pb in microelectronics, due to the intrinsic toxicity of Pb. This had led to the development of Pb-free solder alternatives such as Sn-Ag-Cu alloys. Although these alloys have been used as a replacement for Pb-Sn solders, they have a higher melting point and often exhibit poorer damage tolerance than Pb-Sn.[1–6] The Pb-free solders containing small amounts of rareearth (RE) elements have been shown to exhibit improvements in both physical and mechanical properties, in particular, an enhancement in ductility. In addition to decreasing the alloy melting temperature[7–9] and improving the wetting behavior,[10–13] RE elements have been shown to refine solder microstructures[7,12,14–18] and improve mechanical performance.[7,8,11–13,15,17] In particular, we have shown that the addition of RE elements to Sn-Ag-Cu alloys can refine the microstructure by decreasing the Sn grain size,[12] refining the size of Ag3Sn and Cu6Sn5 intermetallics,[12,19–21] and decreasing the thickness of the Cu6Sn5 interfacial layer formed between Sn and the Cu metallization.[19,22] Improvements in M.A. DUDEK, Graduate Research Associate, and N. CHAWLA, Professor of Materials Science and Engineering, Professor of Mechanical Engineering, and Director of Mechanical Behavior of Materials Facility, are with the School of Materials, Fulton School of Engineering, Arizona State University, Tempe, AZ 85287-8706. Contact e-mail: [email protected] Manuscript submitted March 25, 2009. Article published online January 30, 2010 610—VOLUME 41A, MARCH 2010
strength,[8,12,13,17,20,21,23] ductility,[12,23] and creep resistance[11,13] have also been reported. In previous work,[15,16] we have shown that small additions of La (0.1 and 0.5 wt pct) to Sn-Ag-Cu refine the Sn dendrite microstructure and significantly increase the ductility of solder joints compared to Sn-Ag-Cu. In addition, a new intermetallic phase is present, namely LaSn3, which is homogenously distributed in the solder. We have shown that these dendritic LaSn3 particles are directly responsible for the higher ductility observed in these materials by allowing microscopic voids to nucleate throughout the solder volume (instead of the localized strain at the solder-intermetallic interface) and homogenizing the strain in the solder joint. Although promising results have been observed with La-containing Sn-Ag-Cu solders, the reactive nature of most RE elements with oxygen causes some of these solder systems to severely oxidize even under ambient conditions. Indeed, we[24] have found that La-containing solders can oxidize an appreciable amount, especially when exposed to higher temperatures (373 K (~100 C)). The oxidation can also result in whiskering, due to the phase separation that occurs during oxidation (formation of La2O3 and pure Sn), causing compressive stresses on Sn.[25] It should be noted that Ce-based alloys seem to be less prone to oxidation and Sn whiskering. This oxidation could affect the viability of Sn-Ag-Cu-La solders
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