heel crack
**Heel Crack** is a wire bond failure mode where fractures develop at the transition point (heel) between the wire and the second (stitch) bond.
## What Is a Heel Crack?
- **Location**: Junction of wire loop and stitch bond
- **Cause**: Excessive ultrasonic energy, improper tool geometry, thermal fatigue
- **Failure Mode**: Crack propagates until complete wire separation
- **Detection**: Pull test shows low force with neck break location
## Why Heel Cracks Matter
The heel is the weakest point in a wire bond due to work-hardening during bonding. Cracks here cause reliability failures after thermal cycling.
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**Heel Crack Prevention**:
| Parameter | Optimum | Effect if Wrong |
|-----------|---------|-----------------|
| US power | Medium | High = cracks, Low = weak bond |
| Bond force | Balanced | High = thin heel, Low = poor bond |
| Loop height | Adequate | Low = stress concentration |
| Tool angle | Correct | Wrong = asymmetric heel |