Cause and Prevention of Large Void Formation in Flip Chip Solder Connections

X-ray inspection of large die bonded into ceramic packages revealed a high incidence of voids, whose size approached 20 percent of the solder volume. All voids appeared to form on the chip bond pads. Die attachment was performed on a high precision bonder with in-situ reflow under a formic acid atmosphere. The die were bumped with 98.2 tin / 1.8 silver weight percent solder on nickel coated copper pads. The cofired ceramic packages were metallized with electroless gold over nickel. Our investigation determined that the voids were initiated by formation of a gold-nickel-tin intermetallic compound that released hydrogen gas at the solder interface. This compound formed preferentially on the chip pads because after completing reflow, the bonder cooled the assembly by blowing nitrogen on it, which caused the chip to cool faster than the package because of its much lower thermal mass. The propensity for void formation was all but eliminated by reducing the peak reflow temperature, time above reflow, and making modifications to the bonder cooling system to reduce the temperature difference between the chip and its package. We believe that after the voids nucleated at the solder-intermetallic interface, they continued to grow by release of hydrogen from the molten solder. Our calculations indicate that a concentration of 0.0048% hydrogen atoms in the solder is sufficient to grow voids of the size that we observed. Our hypothesis that release of dissolved hydrogen drives void growth was supported by a hydrogen degassing experiment that we conducted. As-received solder bumped chips were baked under high vacuum for several days, and then bonded to silicon chips with gold/ nickel metallized bond pads. X-ray inspection of these assemblies revealed no voids in contrast to the multiple voids observed in similar samples that were bonded without the hydrogen degassing step.