Electro- and thermomigration in micro bump interconnects for 3D integration

Following the “More-than-Moore´s” law, mobile electronic devices have to offer several functions without taking up too much room or consuming too much power. To address these requirements Chip-on-Chip (CoC) structures have been developed as a high performance solution for three-dimensional packaging. One basic part of such CoC structures are micro bump arrays, they become necessary to connect the I/O (in/out) contacts of the ICs. Connecting thousand of I/O contacts on an area of a few square centimeters, the diameter of a single micro bump is smaller than 25μm. The combination of low melting materials and high current densities, caused by small contact surfaces, leads to a strong influence of electro- and thermomigration on the aging process of the micro bump array. Simulations were performed to determine the migration induced material transport and the resulting mass flux divergences in FEM models of 10μm micro bumps. The simulation results for several solder materials including SnAgCu (SAC) solder, nickel, silver and gold were compared to each other. Based on the results the geometry of the contacts has been varied to reduce the effect of current crowding with the effect of smaller mass flux divergences in consequence of electromigration. Furthermore the pitch between the bumps was varied. The expected lifetime of the micro bumps was compared with conventional SAC bumps for PSvfBGAs (Package Stackable Very thin Fine Pitch Ball Grid Arrays). For the conventional BGA bump a diameter of 280μm was chosen.