Wafer Bumping and Flip-Chip technologies are solder/copper interconnection processes enabling face-down die bonding with fine-pitch capability (scaling from 200 µm C4 to <10 µm hybrid bonding).

C4 (Controlled Collapse Chip Connection):

• Solder bump: electroplated SnPb or SnAg on top of UBM (under-bump metallurgy)
• Collapse mechanism: reflow melts solder, surface tension pulls die down
• Pitch: traditional 200 µm, fine-pitch versions 150-100 µm
• Process: pattern resist, evaporate/plate UBM (Au/Ni/Cu), plate solder, reflow
• Advantages: mature, proven reliability, multiple suppliers

Copper Pillar Technology:

• Copper electroplating: plated Cu column (height 20-80 µm, diameter 20-50 µm)
• Solder cap: thin SnAg solder on top of Cu pillar
• Pitch advantage: sub-100 µm pitch enabling finer interconnect than C4
• EM reliability: Cu higher melting point vs solder-only bump
• Cost: plating complexity vs C4 simplicity

Bump Electrolytic Plating Process:

• Seed layer: evaporated Ti/Cu provides initial conductivity
• Resist patterning: photoresist defines bump locations (pitch-dependent)
• Plating: Cu or Ni electrochemically deposited
• Over-plating: metal grows column shape
• Resist strip: photoresist removal, optional barrier removal (Ti/Cu etched)
• Solder plate: SnAg electroplated on top (if needed)

Microbump for 3D/Advanced Packaging:

• Pitch scaling: <50 µm pitch possible (<10 µm hybrid bonding research)
• Aspect ratio: height-to-width ratio critical (>1 preferred for coplanarity)
• Coplanarity requirement: all bumps same height ±2 µm (affects yield)
• Bumping tool precision: placement accuracy determines assembly yield

Underfill Process:

• Capillary underfill: low-viscosity capillary flow into bump gap (no vacuum needed)
• Molded underfill: underfill overmold dies (more expensive, better reliability)
• Purpose: mechanical reinforcement (thermal cycling stress), moisture barrier
• Cure: UV initiation or thermal cure depending on resin chemistry

Flip-Chip Assembly Integration:

• Bump thermal compression bonding (TCB): heated tool applies force during reflow
• Interconnect uniformity: ensures simultaneous bump contact
• Rework challenge: unlike wire-bond, flip-chip difficult to repair (expensive)
• Yield improvement: automated optical inspection (AOI) for bump placement QA

Reliability Testing (JEDEC):

• Thermal cycling: -40°C to +125°C cycles (1000+ cycles expected)
• Drop test: mechanical shock 2 meters (consumer electronics standard)
• HTOL (high-temperature operating life): functional test at 85°C/85% RH
• FTAB (fluid thermal aging bucket): moisture absorption stress

Wafer bumping technology directly enables chiplet integration and 3D stacking—progress in pitch scaling and reliability remains bottleneck for sub-100 µm pitch mainstream adoption.