Phase formation at the Sn/Cu interface during room temperature aging: Microstructural evolution, whiskering, and interfa
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Carmen Krüger Institute for Materials Science, University of Stuttgart, D-70569 Stuttgart, Germany
Udo Welzela) and Jiang-Yang Wangb) Max Planck Institute for Metals Research, D-70569 Stuttgart, Germany
Eric Jan Mittemeijer Max Planck Institute for Metals Research, D-70569 Stuttgart, Germany; and Institute for Materials Science, University of Stuttgart, D-70569 Stuttgart, Germany
Werner Hügel Robert Bosch GmbH, D-72770 Reutlingen, Germany (Received 2 October 2010; accepted 4 May 2011)
The time-resolved evolution of intermetallic phase formation in the system pure Sn (polycrystalline coating with a thickness of several microns) on pure Cu (polycrystalline bulk substrate) was investigated in detail by means of focused ion beam and transmission electron microscopy and x-ray diffraction during aging at room temperature for a period of about 1 year. The availability of this coherent data base allowed interpretation of the evolution of intermetallic compound (IMC) formation in terms of interface thermodynamics and interdiffusion kinetics. On this basis spontaneous Sn whiskering on the surface of the Sn coating as a consequence of intermetallic phase (Cu6Sn5) formation along, specifically, Sn grain boundaries intersecting the Sn/Cu interfaces could be discussed. Moreover, a treatment to mitigate spontaneous Sn whiskering on the basis of thermodynamic control of the IMC morphology was proposed.
Address all correspondence to this author. e-mail: [email protected] b) Present address: Department of Physics, Shantou University, Shantou 515063 Guang Dong, People’s Republic of China DOI: 10.1557/jmr.2011.162
during aging at room temperature4–7 through continuous addition of material to their base.8 Even though the growth mechanism of a Sn whisker5,9–18 still remains to be revealed, the “driving force” for Sn whisker growth could be related to the build-up of residual mechanical stresses in the Sn coating5,6,15,16,18–25 and thus whisker growth serves as a stress relief mechanism.5,13,15–20 After Sn deposition on a Cu substrate and subsequent aging at room temperature, Cu diffuses rapidly by an interstitial mechanism into the Sn coating6,26,27 and the formation of the intermetallic phase Cu6Sn5 takes place on the Sn side of the Sn/Cu interface, preferentially along Sn grain boundaries.5,6,19,20,28,29 Continued growth of Cu6Sn5 on the Sn side occurs along the Cu6Sn5/Sn interface by a continuous supply of Cu atoms, which takes place via Cu diffusion through Cu6Sn5 grain boundaries and interstitially in Sn.28 Very recently it was shown experimentally that the growth of the intermetallic phase Cu6Sn5 along Sn grain boundaries intersecting the Sn/Cu interface induces in-plane residual compressive stresses, in combination with residual stress gradients (in out-of-plane22 and in-plane23 directions) in the Sn coating, which act as driving forces for the transport of Sn atoms to the whisker nucleation site and thus control
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Ó Materials Research Society 2011
I. INTRODUCTION
Pure Sn and Sn-based alloys deposited on electronic c
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