Wirebonding Failure - Wedge Bond Heel Breaks

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Wedge Bond Heel Breaks

Heel Breaking is the severing of the wire from its wedge or crescent bond due to a fracture in the heel. The heel is the portion of the wire where the wire tapers off into the wedge or crescent bond. It is equivalent to the neck of a ball bond.

Heel break is commonly due to poor wirebonder set-up. Poor set-up includes improper bonding parameter settings, bond head movement settings, and worn-out or contaminated tools. Incorrect bonding parameters can deform the bond excessively, resulting in a very thin and weak heel which can easily fracture. Improper bond head movements and low loop settings may subject the wires to excessive stresses that tend to pull them backward and away from the bonds, resulting in gross heel cracks which may propagate into total fracture. Worn-out and contaminated tools can produce mechanical damage or defects in the wires which can act as starting points for crack propagation.

Figure 1. A Heel Break

Heel breaks have also been traced to mechanical stresses applied to the package. In hermetic packages, for instance, the high-frequency vibrations produced by ultrasonic cleaning or sandblasting tend to subject the wires to violent vibratory stresses that can easily propagate heel cracks to fracture. Thermo-mechanically induced delaminations in the lead finger areas of plastic packages also result in second bond heel cracks.

Corrosion, which can significantly reduce the diameter of a bond wire, can also lead to heel breaks. It is often due to the presence of corrosive contaminants such as Cl and S in the wires and bonds.

Heel breaking may be accelerated by SHRT, Temp Cycle, and Thermal Shock.

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