Effects of Voiding on Thermomechanical Reliability of Copper-Filled Microvias: Modeling and Simulation

The increase in the I/O density and the decrease in the size of electronic packages have driven the need for high-density interconnect (HDI) circuit boards that use microvias as interconnects. One challenge for HDI circuit board development is to fabricate microvias without generating voids in deposited copper structures. A parametric study was conducted to investigate the effects of voids on the thermomechanical reliability of copper-filled stacked microvias using 3-D finite-element analysis and strain-based fatigue life estimation. It was found that microvia fatigue life was affected by geometrical void characteristics, such as shape, size, and location; microvia aspect ratio; and dielectric material properties. While voids are defects in copper-filled microvias, the existence of voids is not always detrimental to microvia reliability. For example, a microvia with a spherical void of 8% volume ratio had a 10% longer fatigue life than the same microvia without voiding. On the other hand, large voids decreased the durability of microvias-a 16% conical void resulted in a microvia fatigue life that was only 1.4% of that of a nonvoided microvia. Moreover, microvia aspect ratio is a critical parameter for reliability. The fatigue life of a voided microvia of 0.25 aspect ratio was more than two orders of magnitude longer than the fatigue life of a voided microvia of 0.75 aspect ratio with the same void volume ratio. As an outgrowth of this study, a microvia virtual qualification method is proposed. Using a combination of finite-element analysis and fatigue life prediction, the required amount of HDI board reliability testing is reduced, cutting overall development time and cost.