Flip Chip Bump

Flip Chip Bumping is the process of forming solder or copper pillars on chip I/O pads that enable direct electrical and mechanical connection to a substrate without wire bonding — the standard interconnect method for high-performance ICs requiring high I/O count and short interconnect length.

How Flip Chip Works

1. Bump Formation: Deposit solder or Cu pillars on chip bond pads (UBM first).
2. Flip: Invert chip so bumps face down toward substrate.
3. Align: Optical/IR alignment of bumps to substrate pads.
4. Reflow: Heat to melt solder → bonds form between chip bumps and substrate.
5. Underfill: Dispense and cure epoxy between chip and substrate for mechanical strength.

C4 Bump (Controlled Collapse Chip Connection)

• IBM's original flip chip technology (1960s, still widely used).
• Eutectic SnPb or lead-free SnAgCu solder balls, 100–250 μm pitch.
• Self-centering: Liquid solder surface tension aligns chip during reflow.
• Typical bump height: 80–120 μm.

Copper Pillar Bumps

• Electroplated Cu column + thin solder cap (SnAg or SnAgCu).
• Fine pitch: 40–100 μm (vs. C4's 100–250 μm).
• Lower solder volume → reduced bridging risk at fine pitch.
• Better electromigration resistance than pure solder.
• Standard for <28nm devices: Apple A-series, Qualcomm, AMD CPU/GPU.

Under Bump Metallization (UBM)

• Adhesion layer (Ti or TiW) + barrier layer (Ni) + wettable layer (Au or Cu).
• Prevents Al pad corrosion, promotes solder adhesion, blocks Cu/Al interdiffusion.

Microbump (2.5D/3D IC)

• For die-to-die bonding: 10–40 μm pitch.
• Used in HBM (High Bandwidth Memory), TSMC CoWoS packages.

Flip chip bumping is the enabling technology for high-density chip-to-package interconnects — essential for every modern high-performance processor, GPU, and networking chip.