The progress of three-dimensional integrated circuit (3D IC) micro-bump joining technology has led to an increased volume fraction of intermetallics (IMC) in the post reflow joints, to an extent that a solder micro-bump may consist almost entirely of IMCs. Therefore, the current carrying capability and electromigration (EM) life time of the purely IMC micro-joint needs to be understood as functions of stressing conditions and degradation mechanisms. Superior EM performance and robustness of IMC joints is demonstrated with no resistance fluctuation under ultra-high stressing condition for over 9000 hrs while solder micro-bumps led to an open failure within 500 hrs. At least an order of magnitude greater current carrying capability of IMC micro-joint compared with solder micro-joint is observed experimentally. The observed degradation mechanism is void formation within Al trace rather than damage inside IMC joint. IMC joint is not the EM reliability bottle neck of the test circuit.