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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 UBMbump interface.

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This paper aims to investigate the electromigration phenomenon of under-bump-metallization (UBM) and solder bumps of a flip-chip package under high temperature operation life test (HTOL). UBM is a thin film AIMi(V)/Cu metal stack of 1.5 pm; while bump material consists of Sn/37Pb, Sn/90Pb, and Sd95Pb solder. Current densities of 2,500 and 5,000 Ncm2 and ambient temperatures of 150 to 160 "C are applied to study their impact on electromigration. It is observed that hump temperature has more significant influence than current density does to bump failures. Owing to its higher melting point characteristics and less content of Sn phase, high-lead bumps are observed to have 8-12-fold improvement in Mean-Time-To-Failure (MTTF) than that of eutectic Sd37Pb. Individual bump resistance history is calculated to evaluate UBMibump degradation. The measured resistance increase is from bumps with electrical current flowing upward into UBMibump interface (cathode), while bumps having opposite current polarity cause only minor resistance change. The identified failure sites and modes from aforementioned high resistance bumps reveal structural damages at the region of UBM and UBMhump interface in forms of solder cracking or delamination. Effects of current polarity and crowding are key factors to observed electromigration behavior of flip-chip packages. I CellNo. I Current I CurrentDensitv 1 Temp. I 0-7803-7649-8/03/$17.00 02003 IEEE