Reliability of lead-free SnAg solder bumps: influence of electromigration and temperature

Temperature and current are the main factors that determine intrinsic degradation of flip chip solder bumps. The intrinsic reliability of bumps is independent of other package features, like geometry and materials. Therefore, bump lifetime can be generically assessed for a particular bump system independently of product and package specifics. The investigated bump type must always be seen as part of a system which includes chip pad, under bump metallization (UBM), solder bump and substrate pad with or without barrier layer. Using high temperature storage (HTS) and high current stress experiments the degradation of SnAg bumps due to metal diffusion, formation of intermetallic compounds (IMC´s), void formation and electromigration is investigated at accelerated conditions. We found a common activation energy of ∼ 1.0 eV, which rules the temperature dependence of bump degradation both with and without current and for both current directions. Current dependence in the low current range follows a current density exponent of = 1.7, which is close to published data on eutectic SnPb bumps. The test results are extrapolated to estimate operating lifetimes as a function of temperature and current. Conversely, the same data yields the maximum current specification of SnAg bumps as a function of temperature and required lifetime.