Effect of the Cu thickness on the stability of a Ni/Cu bilayer UBM of lead free microbumps during liquid and solid state aging

The present study focuses on liquid and solid phase reactions between an electrodeposited bilayer UBM and Sn, which is chosen as worst case solder material for lead free bumping. Small sized bumps were realized by electrodeposition using one lithographic mask for electrodepositing the UBM and the solder. The bilayer UBM consists of an electrodeposited Ni and Cu layer (Ni_ed, Cu_ed) on a sputtered Cu seedlayer (Cu_sp) and a sputtered TiWN diffusion barrier. The Cu_ed layer is situated between solder and the Ni_ed layer and is suitable to act as a sacrificial layer forming a diffusion barrier layer between Ni and the solder. This barrier layer avoids or decreases the degradation of the Ni_ed layer during solid or liquid phase reaction with the solder. Rules for a proper design of the Cu layer thickness are deduced taking into account the solubility of Cu in the solder material as well as the formation and growth of intermetallic phases between solder and UBM. Electrodeposition of the UBM is suitable to realize small sized bump sizes up to 50 μm diameter.