Critical Factors Impacting bonding strength of UBM in Bumps and Methodologies for Optimization

Bumping is an indispensable part of Flip Chip packaging. Bumping technologies require Under Bump Metallization (UBM) to meet package reliability requirements. UBM provides the critical interface between the metal pads of the IC chip and solder bumps. To prevent direct interaction of die's metal pad with Cu or solder, a diffusion barrier layer of Ti or TiW is sputtered on top of metal pad of IC chip. The solder wetting layer can be a single layer or double layer like combination of copper and Nickel. One of the potential weak links in UBM stack up is the bonding strength between the UBM layers, namely between Ni & Cu. If the adhesive strength is below requirement, the layer can give way and fail under the applied voltage combined with moisture and operating temperature. Galvanic reaction is the predominant failure mechanisms with applied voltage and moisture. This paper discusses the critical factors affecting the bonding strength between Cu & Ni layers, the inputs required for maintain the critical factors and the results.