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Kinetics of Pb-rich Phase Particle Coarsening in Sn–Pb Solder Under Isothermal Annealing–cooling Rate Dependence

Published online by Cambridge University Press:  01 June 2005

Paul Vianco ,
Jerome Rejent ,
Gary Zender  and
Alice Kilgo
Paul Vianco
Affiliation:
Sandia National Laboratories, Albuquerque, New Mexico 87185
Jerome Rejent
Affiliation:
Sandia National Laboratories, Albuquerque, New Mexico 87185
Gary Zender
Affiliation:
Sandia National Laboratories, Albuquerque, New Mexico 87185
Alice Kilgo
Affiliation:
Sandia National Laboratories, Albuquerque, New Mexico 87185
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Abstract

The coarsening behavior of the Pb-rich phase particles in 63Sn–37Pb (wt%) solder was investigated following isothermal annealing treatments. Samples were exposed to cooling rates of 0.1, 1.0, 10, and 100 °C/min. Annealing temperatures were 25, 55, 70, 85, and 100 °C, and times were 2–100 days. The mean particle diameter decreased from 1.8 × 10−3 to 0.8 × 10−3 mm with increased cooling rate, indicating two solidification regimes: one for cooling rates ≤1 °C/min and the other for cooling rates of ≥10 °C/min. The Pb-rich phase particles coarsened more quickly in samples made at the two fastest cooling rates. There was little Pb-rich phase particle coarsening at 25 and 55 °C for all annealing times. Coarsening rate kinetics were examined specifically for the 10 and 100 °C/min data using the expression Atnexp[−ΔH/RT]. The values of n were 0.23 ± 0.11 and 0.36 ± 0.13, respectively; n was not sensitive to annealing temperature. The corresponding 1/n values indicated that the coarsening mechanism changed from a fast diffusion to a bulk diffusion controlled process with a faster cooling rate. The apparent activation energy ΔH ranged from 16 ± 8 to 41 ± 8 kJ/mol; the values increased with cooling rate from 10 to 100 °C/min. The ΔH value was sensitive to annealing temperature only for the faster cooling rate of 100 °C/min. Together, the n and ΔH values indicated that an accelerated, fast diffusion mechanism with low activation barriers characterized the Pb-rich phase coarsening in samples exposed to a slower cooling rate, greater annealing, or a combination of the two conditions. That mechanism likely originated from the in situ development of recover/recrystallization microstructures in the Sn-rich phase. At faster cooling rates, those microstructures were not as well developed, so coarsening was controlled more by the higher activation barriers of bulk diffusion processes.

Keywords

AgingElectronic materialMicrostructure
Type
Articles
Information
Journal of Materials Research , Volume 20 , Issue 6 , June 2005 , pp. 1563 - 1573
Copyright
Copyright © Materials Research Society 2005

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