Electroless nickel/gold bump in the flip-chip-type CMOS package for mobile-phone camera modules

Wafer bumping technology using electroless nickel bump with a thin gold layer was studied for the flip-chip-type CMOS image-sensor (CIS) package for mobile-phone camera modules. The precise control of process parameters in the electroless nickel solution was undertaken by synthetic consideration of the effects of solution temperature, pH, stabilizer concentration, and aluminum pad size on the electroless nickel/gold bump formation. The flip-chip bonding process fitting the electroless nickel/gold bump is conducted by optimization of bonding temperature, pressure, and time. Reliability tests are performed to ensure the robustness of the image-sensor module. The impurity particle density is estimated to be one per 300,000 pixels by 3D laser analysis on the image-sensor surface. The phenomenon of the drop in chip yield during the wet process is attributed to the organic particles produced during the wafer backside coating. From all the experimental results, we first suggest that electroless nickel/gold can be applied to the flip-chip-type CIS package for the mobile-phone camera module without any significant impurity contamination of the image-sensor surface.