Micro-bump challenges and reliability improvements in stack-die packages

The fact that Sn-Cu reaction rate is much faster than Sn-Ni reaction and releasing much more gibbs free energy causing Sn prefers to have reaction with Cu instead of Ni. Moreover, at the temperature below 170°C, Sn side-wall reaction rate is larger than IMC formation rate at the temperature below 200 °C because the long distance Sn side-wall migration is faster than Cu traveling through to center Sn. This makes Sn in the micro-bump favors to flow to the Cu side-wall to have side-wall reaction during the high temperature (maximum 170 °C) storage process (HTS). This will result in large voids formed at the center of the micro-bump after HTS, causing malfunction of the product. Therefore, detail DOEs needs to be done to evaluate the reaction rate mentioned above in order to design a highly reliable microbump that can last over 3000 hours under HTS test.