Electromigration failures of UBM/bump systems of flip-chip packages

In this research, a flip-chip package under high temperature operation life test is conducted to evaluate the structural integrity of under bump metallurgy (UBM) and solder bumps. To understand the impact of current polarity on solder bumps, a reverse engineering methodology is employed to calculate bump resistance histories having different current directions and metal trace resistance between two adjacent bumps as well. It is concluded that the observed high resistance increase is from bumps with electrical current flowing upward into UBM/bump interface (cathode), while bumps having opposite current polarity cause only minor resistance change. The directions of electrons flow affect the reaction rates of UBM and flip-chip solder, therefore result in different failure modes and, degradation rates. Besides current polarity, aspects of current density and operation temperature are also probed. It is observed that operation temperature has more direct and significant influence then current density does to bump failures owing to low melting point characteristic of eutectic solder. Failed test vehicles are subjected to cross section analysis via SEM, the identified failure sites are from aforementioned high resistance bumps with structural damages at the region of UBM and UBM/bump interface.