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.

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 |