underbump

Underbump metallization (UBM) is a thin metallization stack deposited on the solder pad of a silicon die or package substrate to serve as the interface for solder bumps in flip-chip and wafer-level packaging. The UBM provides adhesion to the substrate, acts as a diffusion barrier to prevent solder elements from migrating into the substrate, and promotes reliable solder wetting and bonding.

A typical UBM stack comprises multiple sublayers. An adhesion layer, such as titanium-tungsten (TiW) or titanium nitride (TiN), helps the stack attach to the underlying surface. A diffusion barrier, commonly nickel-containing alloys (e.g., Ni, NiV), further prevents intermixing between the solder and the substrate. A seed layer, often copper, enables electroplating of the solder bumps. Some processes may use variations of these materials or additional layers depending on the application and solder chemistry. Bumps are subsequently formed by electroplating onto the defined UBM areas and then reflowed to form the final joint.

Fabrication typically involves thin-film deposition by sputtering or evaporation to create the UBM stack, followed by patterning through photolithography and etching or lift-off. A seed layer may be deposited beforehand to facilitate copper electroplating of the bumps. Surface finishing, planarization, and cleaning are important to ensure consistent bump formation.

Applications are prevalent in flip-chip and wafer-level packaging, including microbumps with SnAgCu or pure Sn solder. The UBM design and materials influence reliability, affecting diffusion, intermetallic formation, and joint integrity under thermal cycling. Related topics include under bump oxide (UBO), a surface oxide sometimes observed on UBMs that can impact solder wetting and requires proper surface preparation.