Bilsland Dissertation Fellowship
Reinhardt Schuhmann Jr. Professor of Materials Engineering
Professor of Materials Engineering
Degree in Progress
PhD Materials Engineering
- MSE 230 - Structure and Properties of Materials (Fall 2010 & Spring 2012)
Thin Film Characterization and Stress Analysis on the Formation of Whisker in Pb-Free Sn-alloy Solder and Electroplated Films
Tin whiskers grow spontaneously from Sn-based films, posing a reliability risk in Pb-free electronics. The propensity to form whiskers in electroplated beta-Sn films is correlated to crystallographic textures because beta-Sn has highly anisotropic elasticity and thermal expansion. A strategy for identifying potential whisker orientations and whisker-resistant beta-Sn films based on crystallographic textures and storage conditions was developed using the elastic strain energy density (ESED) as the driving force for stress relaxation. Using Object-Oriented Finite element analysis (OOF), ESED distributions in two dimensional beta-Sn films with a range of textures were simulated for elastic stresses induced due to intermetallic compound formation at room temperature and for thermoelastic stresses induced by the coefficient of thermal expansion mismatch during thermal cycling. These simulations demonstrate that the properties of beta-Sn films and response to external stresses are highly dependent on the textures and the stress-generating mechanisms, suggesting that whisker growth risk can be reduced by engineering specific textures for specific applications and temperature conditions, but local grain orientations relative to the film texture also matters.
In addition, surface defect (whiskers and hillocks) nucleation and grain boundary sliding were observed along grain boundaries in Sn-3.0Ag-0.5Cu solder films after thermal cycling from -40 degree C to 85 degree C for 1000 cycles. Surface morphology and grain orientation distribution were characterized using SEM with electron backscattering diffraction (EBSD). Evolution of local grain orientation, relative dislocation density, and ESED before and after surface defect formation was determined using x-ray synchrotron micro-diffraction. The results demonstrate that recrystallization occurs, forming shallow grains and relaxing the thermoelastic stress induced by thermal cycling as they grow. In addition, the geometries of the newly-formed grains and surface uplift due to grain boundary sliding of the parent grains were examined using FIB. The observed surface grain geometries will be related to whisker growth models, including our recent model of grain boundary faceting, sliding-limited Coble creep, and conditions to mitigate whisker formation will be studied.
P. Sarobol, J.P. Koppes, W.H. Chen, P. Su, J.E. Blendell and C.A. Handwerker, "Recrystallization as a Nucleation Mechanism for Whiskers and Hillocks on Thermally Cycled Sn-Alloy Solder Films," Material Letters, Vol. 99, pp 76-80, May 2013.
P. Sarobol, W.H. Chen, A.E. Pedigo, P. Su, J.E. Blendell and C.A. Handwerker, "Effects of Local Grain Misorientation and beta-Sn Elastic Anisotropy on Whisker and Hillock Formation," Journal of Materials Research, Vol. 28, Issue 05, pp 747-756, Jan. 2013.
W.H. Chen, M.Y. Lai, K.T. Tsai, C.Y. Liu, Y.L. Wang, "Spontaneous Formation of Ordered Nanobubbles in Anodic Tungsten Oxide during Anodization," Journal of Physical Chemistry C, Vol. 115, Issue 38, pp 18406-18411, Sep. 2011.