A first metallic diffusion barrier layer is formed on a last level metal plate exposed in an opening of a passivation layer. Optionally, a metallic adhesion promotion layer is formed on the first metallic diffusion barrier layer. An elemental metal conductive layer is formed on the metallic adhesion promotion layer, which provides a highly conductive structure that distributes current uniformly due to the higher electrical conductivity of the material than the layers above or below. A stack of the second metallic diffusion barrier layer and a wetting promotion layer is formed, on which a C4 ball is bonded. The elemental metal conductive layer distributes the current uniformly within the underbump metallurgy structure, which induces a more uniform current distribution in the C4 ball and enhanced electromigration resistance of the C4 ball.