Rapid Solidification of Sn-Cu-Al Alloys for High-Reliability, Lead-Free Solder: Part I. Microstructural Characterization
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ALUMINUM micro-alloying additions have been shown to affect the solidification and microstructure of Pb-free solder alloys, particularly in Sn-Ag-Cu and Sn-Cu solder alloys, by reducing solder undercooling, eliminating or reducing the Ag3Sn intermetallic compound (IMC) plate morphology, and forming CuxAly IMCs.[1–6] To form solder joints, solder alloys are commonly converted into powder particles, incorporated into a solder paste, and melted and solidified in final assembly to connect components to circuit boards. In the context of designing Sn-Cu-Al solder joint alloys, it is important to establish what the optimal CuxAly microstructures should be in both the as-fabricated powder and after soldering of the KATHLENE N. REEVE, Ph.D. Candidate, and CAROL A. HANDWERKER, Reinhardt Schuhmann Jr. Professor, are with Purdue University, 701 West Stadium Ave., West Lafayette, IN 47907. Contact e-mail: [email protected] STEPHANIE M. CHOQUETTE, Graduate Student, and IVER E. ANDERSON, Senior Metallurgist, are with the Ames Laboratory (USDOE), Iowa State University, Ames, IA 50011. This manuscript has been authored, in whole or in part, under Contract No. DE-AC02-07CH11358 with the U.S. Department of Energy. The U.S. government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. government purposes. Manuscript submitted March 17, 2016. Article published online October 6, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
components is completed. Design factors that must be considered include: 1. 2. 3.
The role of solidification cooling rate and alloy composition on the formation, morphologies, and size distributions of CuxAly particles The ability of CuxAly to serve as a heterogeneous nucleation catalyst for the Cu6Sn5 IMC phase, and the resulting morphologies and sizes of the Cu6Sn5 The possible presence of the ternary-b phase during Sn-Cu-Al alloy solidification and its subsequent decomposition at low temperatures, as predicted by thermodynamics calculations, and its possible effect on Cu6Sn5 nucleation.[2,5,6]
Reeve et al. confirmed the initial formation of the CuxAly phases in the temperature ranges of 723 K to 823 K (450 C to 550 C) for seven different Sn-Ag-Cu-Al alloys and two different Sn-Cu-Al alloys.[2] The observation of this high solidification temperature provides the possibility of CuxAly IMC particles serving as persistent heterogeneous nucleation catalysts for the Cu6Sn5 phase, where ‘‘persistence’’ is defined as having minimal coarsening after multiple solder reflow cycles to a maximum temperature of 523 K (250 C), as typically required for circuit board assembly. Xian et al. observed Cu6Sn5 formation on CuxAly particles within Sn-4.0Cu-xAl and Sn-0.7Cu-0.05Al wt pct alloys, and they identified the orientation relationship between the two IMC phases:[5,6]
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