Interface Degradation
Interconnects between a semiconductor die and its package lead frame suffer from mechanical weakening over repeated stress events. Wire bond fatigue describes the progressive damage to the gold or aluminum wires caused by thermomechanical movement. This condition eventually leads to an electrical open circuit or a high resistance contact.
Thermal Cycling
Power cycles generate heat within the silicon, causing the wire to expand and flex at the heel of the bond. Since the die and the lead frame expand at different rates, the wire loop acts as a spring that absorbs the displacement. Repeated bending work hardens the metal, making it more brittle over time and increasing the likelihood of failure at the neck.
Crack Propagation
Small fissures begin to form at the point of maximum curvature or at the bond interface itself. These cracks grow during each subsequent temperature fluctuation until the cross sectional area of the wire is reduced. Eventually, the wire snaps or lifts away from the bond pad.
Reliability Test
Accelerated life testing uses high temperature cycling to evaluate the durability of the interconnect design. Pull tests and shear tests provide a measure of the initial bond strength. Monitoring the resistance of the bonds during these cycles helps identify the onset of the failure.
