Wire Bonding
Wire Bonding and Die Attach are the fundamental semiconductor packaging assembly processes that mount the die onto a substrate and create electrical connections between die pads and package leads — collectively responsible for ensuring electrical, thermal, and mechanical integrity of every packaged chip, from $0.10 microcontrollers to $50,000 server processors.
Die Attach
Purpose: Mechanically and thermally bond the silicon die to the package substrate or leadframe.
Methods:
- Epoxy Die Attach: Silver-filled epoxy adhesive — most common for standard packages.
- Thermal conductivity: 2-25 W/m·K depending on silver loading.
- Low cost, easy rework.
- Solder Die Attach: AuSn or SAC solder — for high-power devices requiring low thermal resistance.
- Thermal conductivity: 50-60 W/m·K.
- Used in power amplifiers, high-brightness LEDs, automotive.
- Sintered Silver: Nano-silver paste sintered at 200-300°C — emerging for SiC/GaN power.
- Thermal conductivity: > 200 W/m·K.
- Handles junction temperatures > 200°C.
Wire Bonding
Purpose: Connect die bond pads to package substrate pads using thin metal wire.
Types:
| Type | Wire Material | Diameter | Process |
|---|---|---|---|
| Ball Bonding | Gold (Au) | 18-50 μm | Thermosonic (heat + ultrasonics + force) |
| Ball Bonding | Copper (Cu) | 18-50 μm | Thermosonic with forming gas (N2/H2) |
| Wedge Bonding | Aluminum (Al) | 25-500 μm | Ultrasonic only |
- Ball Bond: Spark melts wire tip → forms ball → pressed onto die pad → loops → wedge bond on substrate.
- Cu wire replaced Au wire ($50/oz Cu vs. $2000/oz Au at 2024 prices) for >80% of consumer packages.
- Speed: Modern wire bonders: 30-60 bonds per second per unit.
Wire Bond vs. Flip Chip
| Aspect | Wire Bond | Flip Chip |
|---|---|---|
| I/O count | < 1000 | > 10,000 |
| Inductance | Higher (wire loop) | Lower (direct bump) |
| Cost | Lower | Higher |
| Thermal | Die face up (heat through substrate) | Die face down (heat through bumps + underfill) |
| Package types | QFP, BGA, QFN | BGA, CSP, CoWoS |
Advanced Wire Bonding Applications
- Stacked Die: Wire bonding connects multiple dies stacked vertically — memory packages (LPDDR).
- Reverse Wire Bonding: Ball-on-substrate, wedge-on-die — enables thinner profiles for stacked packages.
- Heavy Wire Bonding: 100-500 μm Al wire for power modules (IGBT, SiC) carrying 10-100+ amps.
Wire bonding and die attach are the packaging workhorses of the semiconductor industry — while advanced packaging (flip chip, hybrid bonding) captures headlines, wire bonding still accounts for over 75% of all semiconductor interconnections produced globally, processing billions of bonds per day.