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.

<svg viewBox="0 0 376 283" xmlns="http://www.w3.org/2000/svg" style="max-width:100%;height:auto" role="img"><rect x="0" y="0" width="376" height="283" rx="12" fill="#0d1117"/><g font-family="ui-monospace,SFMono-Regular,Menlo,Consolas,&quot;Liberation Mono&quot;,monospace" font-size="14"><text xml:space="preserve" x="20" y="31.7"><tspan fill="#c9d1d9">Wire Bond Geometry - Heel Location:</tspan></text><text xml:space="preserve" x="20" y="50.7"><tspan fill="#c9d1d9">         Wire loop</tspan></text><text xml:space="preserve" x="20" y="69.7"><tspan fill="#c9d1d9">      ╭</tspan><tspan fill="#6e7681">────────────</tspan><tspan fill="#c9d1d9">╮</tspan></text><text xml:space="preserve" x="20" y="88.7"><tspan fill="#c9d1d9">     ○              ╲</tspan><tspan fill="#6e7681">═════</tspan><tspan fill="#c9d1d9"> </tspan><tspan fill="#6e7681">←</tspan><tspan fill="#c9d1d9"> Stitch bond</tspan></text><text xml:space="preserve" x="20" y="107.7"><tspan fill="#c9d1d9">  Ball bond          </tspan><tspan fill="#6e7681">↑</tspan></text><text xml:space="preserve" x="20" y="126.7"><tspan fill="#c9d1d9">                   HEEL</tspan></text><text xml:space="preserve" x="20" y="145.7"><tspan fill="#c9d1d9">                 (crack site)</tspan></text><text xml:space="preserve" x="20" y="164.7"><tspan fill="#c9d1d9">                 </tspan></text><text xml:space="preserve" x="20" y="183.7"><tspan fill="#c9d1d9">Heel Crack Cross-Section:</tspan></text><text xml:space="preserve" x="20" y="202.7"><tspan fill="#c9d1d9">     Wire        </tspan><tspan fill="#6e7681">┌─────</tspan></text><text xml:space="preserve" x="20" y="221.7"><tspan fill="#c9d1d9">         ╲      ╱</tspan></text><text xml:space="preserve" x="20" y="240.7"><tspan fill="#c9d1d9">          ╲____╱  </tspan><tspan fill="#6e7681">←</tspan><tspan fill="#c9d1d9"> Crack initiation</tspan></text><text xml:space="preserve" x="20" y="259.7"><tspan fill="#c9d1d9">         Heel area (work-hardened)</tspan></text></g></svg>

Heel Crack Prevention: