Dynamic Four Wire Testing Essentials for Microvia Reliability Assessment
Dynamic four-wire testing isolates ambient and transient microvia resistance shifts during thermal stress to catch latent target pad defects before shipment.
Copper Grain Boundary Fatigue
Structural Degradation
Mechanical failure in metallic traces occurs when repeated thermal or vibrational stress causes micro-cracks at the junctions of individual metal crystals. This copper grain boundary fatigue leads to an increase in electrical resistance or a complete loss of continuity over time. It typically appears in high-strain environments where the mismatch in thermal expansion coefficients pulls at the internal structure of the metal.
Nucleation Process
Dislocations within the crystal lattice accumulate at the borders of the grains during cyclic loading. As copper grain boundary fatigue progresses, these dislocations form voids that eventually coalesce into a visible fracture. High temperatures accelerate the rate at which these atoms migrate and weaken the interface.
The presence of impurities or large grain sizes often makes the material more susceptible to this type of structural failure. Fatigue life decreases notably when the copper is deposited with high internal stress or low ductility.
Detection Method
Monitoring resistance during thermal cycling provides a way to catch the early stages of the crack. Since copper grain boundary fatigue starts as a microscopic separation, it might not cause a failure during initial electrical testing. Resistance spikes during the heating phase signal that the boundaries are separating.