The invention provides an embodiment of a bump structure which includes a contact element formed on a substrate; a passivation layer overlying the substrate, the passivation layer having a passivation opening exposing the contact element; a polyimide layer overlying the passivation layer and having a polyimide opening exposing the contact element; an under bump metallurgy (UMB) feature electrically coupled to the contact element and having a UBM width; and a copper pillar on the under bump metallurgy feature, a distal end of the copper pillar having a pillar width, the UBM width greater than the pillar width. The invention also provides a method of forming a bump structure.