Influence of trace geometry on the current crowding effect in ultra-fine pitch MicroBump

On one hand, with the rapid increase of integrated circuits (ICs) I/O number, the traditional package technique does not meet the need of pioneer development. On the other hand, three-dimensional integrated circuits (3-D ICs) become more and more popular to satisfy the multi-function chip development. One of the best solutions of these needs is the ultra-fine pitch micro-bumps. The micro-bumps can conduct the power and signals between chips in the stack and significantly shorten the distance between chips. However, the line-to-bump structure of micro-bump results in the serious current crowding near the entrance point of current flow. The current crowding effect, which makes the "REAL" resistance of micro-bump much higher than the theoretical one, is a phenomenon of non-uniform current density distribution on the interface between Al pad and solder joint. In this study, a finite elements analysis (FEA) method and a parametric method are applied to analyze the electrical behavior, current crowding, of ultra-fine pitch micro-bump. A series of models with different thick Al traces are built to observe the influence of trace geometry in the ultra-fine pitch micro-bump structure. The result of FEA shows that there is a significant correlation between the thickness of Al trace and the resistance of micro-bumps. The thicker the Al trace, the lower the bump resistance. Moreover, the crowding ratio (CR), a factor defined to indicate the level of current crowding, also decreases dramatically with the thicker Al trace. When the thickness of Al trace increase from 0.4 μM to 6.0 μM, the resistance of a single micro-bump decreases from 161.2 MΩ to 27.3 mil, and the CR decreases to a fourth of its initial value, too. Besides the FEA, a simplified numerical which is constructed by a micro-resistance network is proposed in this study to illustrate the relationship between the geometry of Al trace, the resistance of micro-bump, and CR. This numerical model sho- - ws that the bump resistance with a non-uniform current density distribution can be presented as (2R_taceR_joint)^0.5. Furthermore, the CR equals to R_bump / R_joint and can also be presented as (2R_tace / R_joint)^0.5. These results indicate the influence of trace geometry on bump resistance and CR, and it fit the FEA results well. With the Kelvin bump structure, experimental measurement of bump resistance is also performed to examine the credibility of the FEA results.