Shape Effect of Passivation Opening on the Electric Behavior in Flip-chip SnAg Solder Joints under Electromigration

Abstract

Flip-chip solder joints have become the most important technology for high-density packaging in the microelectronic industry. As the size of the joints progressively shrinks, the carried current density increases. The octagonal passivation opening is now used in packaging industry. The shape effect of passivation opening on its electric behavior during electromigration is clear. This study investigates current density and temperature distribution in eutectic SnAg solder joints with various dimensions on different shapes of passivation opening, including circles, semicircle, squares, octagons, and D-shapes. Three-dimensional electric simulation was carried out to simulate the current and distribution of different passivation opening. According to the results, the maximum current density in the solder joints with square, D-shaped, semicircular passivation opening are lower than that with circular passivation opening under the same contact area. Because the electric current is dispersed along one edge near to the entrance of the Al trace, the current crowding effect is relieved to some extent. The linear measure of the edge near to the entrance of the Al trace is defined as contact length. The contact length dominates the dispersion of current. The semicircular passivation opening has the longest contact length which makes current spread uniformly before entering the solder joint, so the crowding ratio is lowest. However, the D-shaped and square passivation opening also possess higher capability to reduce current crowding effect. Furthermore, the D-shaped passivation opening has two advantages which include longer contact length and larger contact area. The contact area make current travel further before entering the solder joints and offers more space for the depletion of voids. It also enhance the life time. The sequence of cutting circular from one side near the current entrance to the center of the circular passivation opening is also studied by electric simulation. Even though the contact area decreases, the smaller distance from the center of circular passivation opening to the cur edge has better ability to reduce the crowding effect due to the longer contact length. Finally, the current density that is obtained from the simulation can be used to estimate mean-time-to-failure (MTTF) of the joints. Through changing the shape of passivation opening from circle to semicircle, the MTTF can be enhanced to 3.84 times. This study provides the guideline of an optimal design rule for the shape of the contact opening flip-chip solder joints.