Rapid Solidification of Sn-Cu-Al Alloys for High-Reliability, Lead-Free Solder: Part I. Microstructural Characterization of Rapidly Solidified Solders
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Particles of Cu x Al y in Sn-Cu-Al solders have previously been shown to nucleate the Cu6Sn5 phase during solidification. In this study, the number and size of Cu6Sn5 nucleation sites were controlled through the particle size refinement of Cu x Al y via rapid solidification processing and controlled cooling in a differential scanning calorimeter. Cooling rates spanning eight orders of magnitude were used to refine the average Cu x Al y and Cu6Sn5 particle sizes down to submicron ranges. The average particle sizes, particle size distributions, and morphologies in the microstructures were analyzed as a function of alloy composition and cooling rate. Deep etching of the samples revealed the three-dimensional microstructures and illuminated the epitaxial and morphological relationships between the Cu x Al y and Cu6Sn5 phases. Transitions in the Cu6Sn5 particle morphologies from faceted rods to nonfaceted, equiaxed particles were observed as a function of both cooling rate and composition. Initial solidification cooling rates within the range of 103 to 104 °C/s were found to be optimal for realizing particle size refinement and maintaining the Cu x Al y /Cu6Sn5 nucleant relationship. In addition, little evidence of the formation or decomposition of the ternary-β phase in the solidified alloys was noted. Solidification pathways omitting the formation of the ternary-β phase agreed well with observed room temperature microstructures.
KeywordsDifferential Scanning Calorimetric Melt Spin Ribbon Cu6Sn5 Phase Water Quenching Sample Cu6Sn5 Particle
This work was supported by Ames Laboratory, Purdue University, and Nihon Superior through Ames Lab Contract No.DE-AC02-07CH11358. Additionally, this research was conducted with government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. The research group is grateful for the support and helpful communications provided by Nihon Superior and for the assistance of Fukuda Co. in producing the drip atomized solder sample presented in this article. The group would also like to thank Kevin Dennis (firstname.lastname@example.org) of Ames Laboratory for help in producing the DSC cooled and melt spun ribbon alloys, Warren Straszheim (email@example.com) of Iowa State University’s MARL microscopy facility for assistance and expertise in the use of the field emission SEM, and John Holaday (firstname.lastname@example.org) of Purdue University for calculating the Thermo-Calc solidification paths presented.
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