Microvia Fatigue Defect
Thermal cycling in electronic assemblies generates mechanical stress that can fracture the copper plating at the transition point between a microvia wall and the target capture pad. The occurrence of microvia knee cracking represents a major reliability concern because it introduces intermittent open circuits that escape initial electrical testing. This defect typically occurs due to the mismatch in thermal expansion coefficients between the copper and the resin substrate.
Stress Concentration Point
During assembly reflow or operational heating, the resin of the printed circuit board expands vertically much faster than the copper plating. This expansion puts a high tensile load on the microvia structure, concentrating the stress at the sharp bend or knee where the via cylinder meets the surface pad. If the copper plating is too thin or has uneven grain structure, this stress leads to a microscopic crack around the circumference of the knee, which can separate fully when the board warms up and rejoin when it cools.
Testing Verification Protocol
Reliability labs use highly accelerated thermal shock testing to force this crack to manifest before the assemblies ship to the field. Measuring the resistance of daisy-chained microvia test patterns during thermal transitions catches these failures, which are otherwise invisible at room temperature.
