micro-bump

**Micro-Bump Copper Pillar Assembly** is **fine-pitch interconnects via copper pillars with solder caps bonding chiplets to substrate** — enables chiplet assembly at high density. **Pillar Structure** copper (~2-5 μm diameter, 5-15 μm height) on bond pad; solder cap (lead-free SAC, SnPb). **Pitch** 10-20 μm spacing (advanced), 20-50 μm (conventional). **Fabrication** copper electroplating; height controlled by current, time. **Solder Cap** melts during reflow, wets pillar. **Reflow** controlled thermal cycle melts solder, bonds chiplets. **Shear Strength** solder joint mechanical integrity tested via shear. **Thermal Cycling** repeated −40 to +125°C cycles stress joint. Solder fatigue life important. **Under-Bump Metallurgy** Ni-Pd-Au or Cr-Ni prevents diffusion, enables wetting. **Micro-Void** solder voiding reduces joint strength. Flux chemistry, vacuum bonding mitigate. **X-Ray Inspection** detects voiding, positioning; non-destructive. **Bridging/Opens** defect detection; yield critical. **Assembly Yield** micro-bump precision challenging; yields ~99%. **Rework** thermal rework enables chiplet replacement. **Underfill** optional potting protects bumps; distributes stress. **Electromigration** high-current vias require design margin. **Micro-bump assembly enables chiplet bonding** at required density and reliability.