Wirebonding Failure - Ball Bond Neck Breaks

Ball Bond Neck Breaks

Neck Breaking is the severing of the wire from its ball bond due to a fracture in the neck. The neck is the portion of the wire where the wire meets the ball bond. The geometrical transition of the wire at the neck makes this portion vulnerable to the thermo-mechanical stresses that the wires are subjected to.

Neck Breaking 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 thin, weak, or cracked neck which can easily fracture. Improper bond head movements and low loop settings may subject the wires to excessive stresses that can result in gross neck cracks which may propagate into total fracture (see Figure 1). Worn-out and contaminated tools can produce mechanical damage or defects in the wires which can act as starting points for crack propagation.

Die overcoat material reaching the neck regions of the bonds can likewise cause neck breaks. Silicone gel coat exerts tremendous forces on the wires if the unit is subjected to excessive thermal stresses, pulling the necks away from their ball bonds. This mechanism can be easily prevented though by limiting the die coat thickness, i.e., not letting the die coat level reach the neck portion of the wires.

Figure 1. Neck Cracking that can lead to a Neck Break

Neck breaks as a result of wire sweeping during molding can also occur. Excessive wire sweeping produces both a linear and angular displacement of the wire, which tend to pull on and twist the neck of the ball bond. The neck is severed if its fracture strength is exceeded by the resultant stress. Taut wires and low loop heights aggravate the effects of wire sweeping. Die-to-plastic delaminations, such as those induced by an improper DTFS process, can likewise result in neck breaks, since any movement of the delaminated plastic with respect to the die would tend to pull away the wires from the die.

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

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

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