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Tin whiskers spontaneously grow from electrodeposited Sn-containing films, but the growth mechanism is not well understood. The current tin whisker testing outlined in JEDEC22A121 fails to provide information on film properties necessary to predict the propensity of a film to whisker. Evaluating local grain structures, grain boundary mobility and grain orientations around whiskers and correlating these film properties to whisker growth provide valuable information regarding the propensity of a film to whisker.Grain Structures
Defects-whiskers and hillocks-originate from surface grains at the top of the films.Cross sections reveal largely columnar structures. All defects appear to originate from "surface grains." Surface grains may arise from recrystallization of the film or be present in the as electrodeposited films. The physics-based mechanisms by which these happen are not known. Defect growth occurs in effort to relax stress within the film  .
Cu6Sn5 intermetallic forms at the interface between Sn film and Cu substrate. Addition of Cu to Sn film results in additional Cu6Sn5 intermetallic formations at the grain boundaries . Addition of Pb to Sn film results in finer grains.Figure 1: Schematics of whisker and hillock formation on Sn and Sn alloys electrodeposited films on Cu substrate.Figure 2: FIB cross sections revealing microstructures of the Sn, Sn-Cu, and Sn-Cu-Pb films near defects.
Grain Boundary Mobility
Limited grain boundary mobility due to pinning leads initially to vertical growth and promotes whisker formation.Varying film composition alters the film microstructure, grain boundary pinning and stress. The properties affect the film defect density, type and morphology.Figure 3: Decrease in grain boundary mobility due to increased grain boundary pinning from Cu addition to Sn films.Figure 4: Defect phase diagram showing defect density, type and morphology as a function of Cu and Pb addition to the Sn films.
Cu6Sn5 intermetallics promotes whisker growth:
- Higher film stress  ;
- Increased defect density;
- Limited grain boundary mobility   (pinning from formation of Cu6Sn5); and
- Vertical growth morphology.
Pb mitigates whisker growth :
- Lower film stress ;
- Extremely low defect density;
- High grain boundary mobility; and
- Complex lateral growth morphology.
In the vicinity of defect grains, there are always other grains that are highly misoriented with respect to their neighbors, leading to local stress variations within the film, inducing defect growth .
Examined in-plane and out-of-plane orientations to determine the highly misoriented grains. Stress distribution is nonuniform. Stress variation in different grains can be as large as a factor of 3  for a given film strain due to elastic anisotropy. Stress gradient causes diffusion of Sn atoms leading to defect growth .Figure 5: Schematic of grain orientation in IPF colors.Figure 6: Grain orientations and nonuniform stress distribution near whisker root on Sn-Cu films.Figure 7: Grain orientations and nonuniform stress distribution near hillock root on Sn-Cu films.ConclusionsCorrelating local film properties to whisker growth provides valuable information regarding the propensity of a film to whisker. Conditions for whisker formation in the electrodeposited Sn films on Cu substrates include, but are not limited to, presence of surface grains as defect origins, limited grain boundary mobility due to pinning, and presence of highly misoriented grains with respect to their neighbors leading to local stress gradient and subsequent whisker growth.AcknowledgementsThis work is supported by Cisco Systems, Inc. and Naval Surface Warfare Center, Crane Division. The authors would like to thank Denny Fritz, SAIC and MacDermid for valuable discussions.References: 1. Fisher, R. et al. "Accelerated Growth of Tin Whiskers," Acta Metallurgica, Vol. 3, No. 2 (1955), pp. 200-201.2. Williams, M. et. al. "Hillock and Whisker Growth on Sn and SnCu Electrodeposits on a Substrate Not Forming Interfacial Intermetallic Compounds," Journ. Elec. Mat., Vol. 36, No.3 (2007), pp. 214-219.3. Pedigo, A. et al. "Whiskers, Hillocks, and Film Stress Evolution in Electroplated Sn and Sn-Cu Films," Proc. of the IEEE Elec. Comp. Conf., (2008), pp. 1498-1504.4. Boettinger, W. et al. "Whisker and Hillock Formation on Sn, Sn-Cu, and Sn-Pb Electrodeposits," Acta Materialia, Vol. 53 (2005), pp. 5033-5050.5. Sarobol, P. et. al. "Changes in Defect Morphology and Texture in Sn-Cu and Sn-Cu-Pb Electroplated Films," in press IEEE Transactions on Electronics Packaging Manufacturing, 2010.6. Arnold, S. "The Growth of Metal Whiskers on Electrical Components," Proc. of the IEEE Elec. Comp. Conf., (1959), pp. 75-82.7. Sarobol, P. et. al. "A Synchrotron Micro-Diffraction Investigation of Crystallographic Texture of High-Sn Alloy Films and its Effects on Whisker Growth," in press ECTC Conf., 2010.8. Rayne, J. and Chandrasekhar, B., "Elastic Constants of β Tin from 4.2oK to 300oK," Phys. Rev. Vol.120, No.5 (1960), pp.1658-1663.9. Pedigo, A. et. al. "Crystallographic Texture of Sn-based Electroplated Films," in press, IMAPS International Microelectronics and Packaging Society, 2009.