solder bump

A solder bump is a tiny ball of solder deposited on a chip's I/O pad that becomes the electrical and mechanical joint in flip-chip assembly. Picture a sheet of bubble-wrap shrunk to microscopic scale: a regular grid of little metal beads, each one a future connection. When the die is pressed face-down onto the substrate and heated, every bump melts and fuses to its landing pad at once, forming the whole connection array in a single reflow step.

Bump size sets how densely you can connect. Conventional flip-chip C4 bumps are around 100 micrometres on a ~150 µm pitch; 'microbumps' used to stack dies in 2.5D/3D shrink to ~10–25 µm; and the newest hybrid bonding eliminates the solder ball entirely, fusing copper-to-copper at single-digit-micrometre pitch. Finer pitch means more connections per millimetre — the headline metric driving chiplet and 3D-IC integration.