Sn whiskers are a serious reliability problem in Pb-free electronics manufacturing. Whiskers grow out of pure Sn coatings and have been responsible for numerous system failures, such as the Galaxy IV satellite.
To increase our understanding of whisker growth, we have performed systematic measurements to quantify the evolution ofthe Sn-Cu intermetallic compound (IMC), stress in the Sn and whisker nucleation rate. Comprehensive measurements of these quantities simultaneously on the same set of samples elucidates the correlations among them and their dependence on important parameters (layer thickness, grain size, alloy content, etc.).
Results in fig. 1 indicate that, although the IMC grows continuously, the stress saturates at different values depending on the film thickness (and hence grain size). In each case, whiskers start to form only after the stress reaches its compressive saturation value.
In situ SEM observations (fig. 2) have allowed us to observe how the whisker grows in real time. We find that there are no pre-existing defects where the whiskers first start to appear. In addition, in situ removal of the surface oxide is not sufficient to make the whisker grow, indicating that the underlying grain structure is critical to whisker nucleation. Simultaneous measurements of the IMC, stress and whisker density
We are using these measurements to develop models of the stress and whisker growth rate (fig. 3). This allows us to both predict whisker formation for different microstructures and develop scientifically-based mitigation strategies.
Real-time SEM/FIB studies of Whisker Growth and Surface Modification, Nitin Jadhav, Eric Buchovecky, Eric Chason, Allan Bower. JOM, July 2010.
Understanding the correlation between intermetallic growth, stress evolution and Sn whisker nucleation, N. Jadhav, E. Buchovecky, L. Reinbold, K.S. Kumar, A. Bower and E. Chason, IEEE Trans on Pack. Manuf., (2010), Inprint.
Finite element modeling of stress evolution in Sn films due to growth of Cu6Sn5 intermetallic compound, Eric J. Buchovecky, Nitin Jadhav, Allan F. Bower, and Eric Chason, Journal of Electronic Materials, 38, 2676 (2009) (Selected as editor’s choice).
Stress behavior of electroplated Sn films during thermal cycling, Jae Wook Shin, Eric Chason, J. Mater. Res. 24 (2009).
Compressive stress generation in Sn thin films and the role of grain boundary diffusion, Jae Wook Shin, Eric Chason, Phys. Rev. Lett. 103, 056102 (2009).
Relation of Sn whisker formation to intermetallic growth: Results from a novel Sn-Cu "bimetal ledge specimen", L. Reinbold, N. Jadhav, E. Chason and K.S. Kumar, L. Reinbold, N. Jadhav, E. Chason and K.S. Kumar, J of Materials Research 24, 3583 (2009).
Whisker formation in Sn and Pb-Sn coatings: role of intermetallic growth, stress evolution and plastic deformation processes, E. Chason, N. Jadhav, W.L. Chan, L. Reinbold and K.S. Kumar, Appl. Phys. Lett. 92, 171901 (2008).
Plastic Deformation Processes in Cu/Sn Bimetallic Films, K.S. Kumar, L.Reinbold, E. Chason, J. Mater. Res. 28, 2916 (2008).
Whisker Formation in Sn Coatings on Cu, Eric Chason, Lucine Reinbold and Sharvan Kumar, MRS Symp Proc. 851, 239 (2005).