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.