Investigation of electromigration behaviour in lead-free flip-chip solders connections

Packaging technology has to continuously evolve in order to keep pace with the demand for smaller and lighter products. One manifestation of this is the need to drastically reduce the size of flip-chip bumps and their pitch. To make things more challenging, the changes have to be mastered with new materials in response to lead-free legislation. At the same time the reliability of the electronic devices should not be sacrificed. In this respect, a relatively new challenge is also posed by the observation, that solder connections are vulnerable to electromigration. The aim of this research was to evaluate the long-term electromigration behaviour of lead-free (SAC305) flipchip solder connections with a nominal diameter of 60 μm or 50 μm, which have been assembled in flip-chip organic packages having electroless Nickel under bump metallization with a pitch of 100 μm. Test vehicles were subjected to electromigration tests for over 25,000 hours at constant current densities of 8 kA/cm² or 5 kA/cm² respectively, and nominal temperatures of 125 °C, 100 °C, or 28 °C until failure. The failure data has been evaluated employing Weibull statistics as well as lognormal distribution and the mean time to failure (MTTF) has been calculated. Only three out of twelve samples have failed after 25,000 h for 50 μm solder bumps tested at a current density of 5 kA/cm² and a temperature of 100 °C; no failures at all have been observed at an ambient temperature of 28 °C. The comparison of the MTTFs for the different bump diameters leads to the result that, under the same testing conditions of current density and temperature, the life time of smaller bumps is considerably longer. This — on first sight — surprising finding can be explained by lower heat generation due to current flow, and thus by a lower temperature of these bumps. The results were subsequently used to estimate the parameters of Black's equation. The evaluation yielded an activation energy of E
a
= 1.13±0.18 eV and a current density exponent in the range of n = 4.9–2.2. The relatively high value of E
a
points to a good robustness of the bump metallization investigated in our study; the variation of n indicates a change in failure mechanism.