Diffusion Parameters and Growth Mechanism of Phases in the Cu-Sn System
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THE reaction between Sn-based solders and Cu-based metallization remains an important topic. Increased miniaturization and conversion to Pb-free solders place increasing demands on the electromechanical integrity of solder joints, whether they be chip to chip, chip to substrate, or package level. Because of interdiffusion, two intermetallic compounds, Cu3Sn and Cu6Sn5, usually form during soldering and or pretinning of Cu substrates and can continue to grow during storage or service. Because these intermetallics affect the integrity of joints, many articles have been published on this topic (e.g., References 1 and 2). The diffusion literature in Sn-Cu alloys was reviewed in 1981 by Butryrmowicz.[3] In Cu solid solutions, Sn is the fast diffuser.[4,5] On the other hand, Cu is known to be an extremely fast interstitial diffuser in the c direction in Sn.[6] However, in the present research, we focus on diffusion in the intermetallic compounds. This subject was studied using diffusion couples by Onishi and Fujibuchi.[7] They obtained parabolic growth constants for the layer thicknesses, measured the range of stoichiometry of the phases, and determined values for the interdiffusion coefficients for the Cu3Sn and Cu6Sn5 phases between 463 K and 493 K (190 C and 220 C), as follows: ~Cu6 Sn5 ¼ 1:55 108 exp 64:8 kJ mol1 =RT m2 =s D ~Cu3 Sn ¼ 1:43 108 exp 70:7 kJ mol1 =RT m2 =s D
A. PAUL, Associate Professor, and C. GHOSH, PhD Student, are with the Department of Materials Engineering, Indian Institute of Science, Bangalore, India. W.J. BOETTINGER, NIST Fellow, is with the Metallurgy Division, NIST, Gaithersburg, MD 20899. Contact e-mail: [email protected] Manuscript submitted April 22, 2010. Article published online January 7, 2011 952—VOLUME 42A, APRIL 2011
Further, in their Cu/Sn couples, Kirkendall markers were observed only in the Cu6Sn5 phase. Thus, they observed marker motion toward the Sn-rich side of the Matano interface and concluded that Sn was the faster diffuser in Cu6Sn5. A detailed analysis of the mobilities of species was not possible in their work, because the marker movement was less than their diameter (5 lm). In the present study, 1-lm markers were employed. We also employ integrated diffusion coefficients, as introduced by Wagner,[8] due to the uncertainty of the exact composition range of an intermetallic compound as measured from a diffusion couple with a microprobe. Tu and Thompson[9] interestingly found that only Cu6Sn5 grew, and not Cu3Sn, at room temperature in thin film Cu/Sn bilayers. Cu3Sn could grow only above 423 K (150 C). Their W markers after room temperature diffusion were found in the Cu6Sn5 phase much closer to the Cu/Cu6Sn5 interface than to the Cu6Sn5/Sn interface. Thus, both Sn and Cu diffuse in Cu6Sn5, and the authors concluded that Cu was the faster diffuser in Cu6Sn5. Paul et al.[10] prepared incremental diffusion couples of Cu/Cu6Sn5 and Cu3Sn/Sn with Kirkendall markers to measure separately the diffusion properties in the intermetallics Cu3Sn and Cu6Sn5, respectively
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