solder bump

Solder bumps are small spherical solder connections formed on die bond pads that enable flip-chip bonding by providing both electrical connection and mechanical attachment to the substrate. Bumps are typically 50-150μm diameter on 100-250μm pitch, though advanced packages use finer pitches. Bump formation processes include evaporation (sputtering under-bump metallization and solder), electroplating (plating solder on patterned seed layer), and solder ball placement (attaching pre-formed balls). Common solder compositions include lead-tin (now restricted), lead-free SAC (tin-silver-copper), and high-lead for die attach. Under-bump metallization (UBM) provides adhesion, diffusion barrier, and wettable surface—typical stacks are Ti/Cu/Ni or Al/Ni(V)/Cu. Bump height is 50-100μm after reflow. Solder bumps must provide reliable electrical connection, mechanical strength, and accommodate thermal expansion mismatch between die and substrate. Bump inspection uses X-ray to verify voiding and wetting. Electromigration in bumps is a reliability concern for high-current applications. Advanced packages use copper pillar bumps with thin solder caps for improved electromigration resistance and finer pitch capability.