(323b) Atomistic Modeling of Grain Boundary Diffusion in Sn-Ag-Cu Solder

With clock speeds of microchips increasing and their size decreasing, the relative current density (electric flux) through the solder ball is trending towards 10⁶ Amp/cm². Experimental results for SnAgCu solder show that current densities of this magnitude cause cracking and subsequent failure of the solder joint within a few hundred hours. The process by which these interconnects fail is called electromigration and a collaborative effort with UB's Electronic Packaging Laboratory is in place to study this phenomenon. Basaran, et al.[1] have formulated a thermodynamic model of this damage evolution by describing the movement and generation of voids in solder. A first step in refining this model is the inclusion of accurate atomistic transport information.

With this as our goal, we have conducted simulations of solder-like materials using molecular dynamics and the Dimer method [2] for both surface diffusion and grain boundary diffusion models. Investigating surface diffusion, we simulated adatom movement on an Sn surface. Simulations of Sn grain boundaries were also performed. These are particularly important, as grain boundaries present in solder create a ?short-circuit? path for atoms to travel?increasing the effect of electromigration. We present our findings on the transport properties of these two models, including some quantitative values. Our diffusion results, such as activation energies and diffusion constants, will fit into a multi-scale model using simulations performed by finite element methods.

[1] Basaran, C., M. Lin, and H. Ye. A thermodynamic model for electrical current induced damage. International Journal of Solids and Structures, December 2003.40(26):p.7315-7327.

[2] Henkelman, G. and Jonsson H. A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives. J. Chem. Phys, October 1999. 111(15):p.7010-7022.