Quantifying Microvia Plating Fatigue and Target Pad Separation Rates under Rapid Thermal Cycling
Continuous high-speed Kelvin monitoring during rapid thermal cycling isolates latent target pad separations that re-nest and pass static ambient tests.
Stacked Microvia Fatigue
Mechanical Degradation
Mechanical degradation occurs in vertical columns of laser-drilled connections that are subjected to repeated thermal expansion and contraction within the dielectric layers. Stacked microvia fatigue is the progressive weakening of the bond between the microvia base and the underlying target pad due to repeated heating and cooling. This condition eventually leads to an open circuit or an intermittent connection that is sensitive to temperature.
Placing Method
Placing microvias in a stack creates a continuous column of copper that expands at a different rate than the surrounding resin and glass. Because of stacked microvia fatigue, the interface between the plating of the upper via and the copper of the lower via is subjected to significant shear and tensile forces. These forces are concentrated at the corners and the center of the pad, where small separations can begin to form after only a few reflow cycles.
Thermal Cycle
Thermal cycle testing and micro-sectioning are the standard tools used to evaluate the integrity of these complex structures. When stacked microvia fatigue is detected, it often points to an issue with the cleanliness of the target pad before plating or the ductilities of the copper itself. Modern standards often recommend staggering the microvias instead of stacking them to distribute the stress more evenly throughout the substrate.
This design change significantly improves the lifespan of the board in applications that require multiple soldering steps or operate in extreme temperatures.