Tin Whisker Formation - A Stress Relieve Phenomenon
- Infineon Technologies AG, Wernerwerkstr. 2, 93049 Regensburg (Germany)
- Philips Semiconductors, Gerstweg 2, 6534 AE Nijmegen, The Netherlands (Netherlands)
- STMicroelectronics, Via C. Olivetti2, 20041 Agrate Brianza (Italy)
- Freescale Semiconductor, 7700 West Palmer Lane, Austin TX 78729-8084 (United States)
With the move towards lead-free electronics also the solderable finish of electronic components' terminations are converted. While the typical finish was containing 5 % to 20 % lead (Pb) and thus was almost whisker free, lead (Pb)-free finishes such as pure tin or high tin alloys are rather prone to grow whisker. These whiskers are spontaneous protrusions that grow to a significant length of up to millimeters with a typical diameter in the range of few microns and are suspect to cause shorts in electronic assemblies. The latest details of the mechanisms are not yet understood. However it appears to be well established that the driving force for tin whisker growth is a compressive stress in the tin layer and that this stress is released by whisker formation. Besides the mechanism for whisker growth therefore the mechanism of the stress induction is of interest. The origin of that stress may have multiple sources. Among others the most important one is the volume increase within the tin layer due the formation of intermetallics at the interface to the base material. This applies to all copper based material. For base materials with a coefficient of thermal expansion (cte) significantly different from the tin finish another mechanism plays the dominant role. This is the induction of stress during thermal cycling due to the different expansion of the materials with every temperature change. Another mechanism for stress induction may be the oxidation of the finish, which also leads to a local volume increase. Based on the knowledge of stress induction various mitigation strategies can be deducted. Most common is the introduction of a diffusion barrier (e.g. Ni) in order to prevent the growth of the Cu-Sn intermetallics, the controlled growth of Cu-Sn intermetallics in order to prevent their irregularity or the introduction of a mechanical buffer material targeting at the minimisation of the cte mismatch between base and finish material. With respect to the stress release various theories are actually under discussion, which deals with the influence of grain orientation, which may have influence due to the strong anisotropic properties of tin, the diffusivity of atoms, which is necessary to 'feed' the whisker and other effects such as the role that oxide or other protective layers may play.
- OSTI ID:
- 20798194
- Journal Information:
- AIP Conference Proceedings, Vol. 817, Issue 1; Conference: 8. international workshop on stress-induced phenomena in metallization, Dresden (Germany), 12-14 Sep 2005; Other Information: DOI: 10.1063/1.2173568; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-243X
- Country of Publication:
- United States
- Language:
- English
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