copper pillar process plating

**Copper Pillar Bumping** is **electroplated copper column technology with solder cap enabling sub-100 µm pitch flip-chip interconnect and superior electromigration reliability**. **Copper Pillar Geometry:** - Height: 20-80 µm (pitch-dependent, taller = coarser pitch) - Diameter: 20-50 µm (aspect ratio 1-4:1) - Pitch capability: 40-100 µm (vs C4 traditional 200 µm) - Stand-off: copper height ensures solder gap for underfill flow **Nickel Barrier Cap:** - Ni thickness: 5-10 µm plated on top of copper - Purpose: prevent solder wetting during initial placement/storage - Sacrificial layer: Ni dissolves into solder during reflow - Composition: pure Ni or Ni-plated alloy **Solder Tip:** - SnAg solder: plated on Ni cap (2-5 µm) - Melt point: 217°C SAC, enables reflow bonding - Thickness: thin layer prevents excessive solder volume **Electroplating Process Flow:** - Photoresist pattern: lithography defines pillar locations (pitch-dependent) - Cu seed layer: PVD evaporated Ti/Cu foundation (300-500 nm) - Cu electroplating: high-speed ECD (electrochemical deposition) fills resist windows - ECD chemistry: CuSO₄ bath with accelerators/suppressors for uniform plating - Ni plating: separate plating cell with Ni(II) sulfamate bath - SnAg plating: final solder cap - Resist strip: photoresist removal, Cu seed etched in trenches (optional) **Electromigration (EM) Advantage:** - Cu higher melting point (>1000°C) vs solder (217°C SAC) - EM resistance: copper pillar lifetime >10x SnPb bump at same current density - Current carrying capacity: higher reliability for power bumps - Black-pad risk: reduced vs Ni-plated C4 (nitriding) **Fine-Pitch Implementation:** - Pitch scaling: 50 µm and below challenging (photoresist window definition) - Aspect ratio control: taller pillars for coarser pitch, shorter for finer pitch - Photoresist: thick resist (30-50 µm) required for tall pillars - Plating uniformity: current distribution across pillar ensures consistent filling **Thermal Compression Bonding (TCB):** - Heated tool: applies force + temperature during bonding - Reflow alternative: TCB enables micro-bump bonding (sub-3 µm pitch research) - Tool precision: must ensure simultaneous contact across all bumps - Coplanarity requirement: ±1-2 µm variation critical **Reliability and Manufacturing:** - Process variability: plating bath control (pH, temperature, additives) - Defect modes: protrusion (pillar too tall), short (pillar-to-pillar contact), voids - Cost vs C4: higher process cost but superior EM performance justifies premium - Yield: mature process achieving >99% yield for standard pitches Copper pillar technology represents industry mainstream for flip-chip bumping—enabling fine-pitch ASIC packaging and superior long-term reliability versus solder-only alternatives.