Structural Fatigue
Cumulative fatigue within an electronic assembly due to alternating temperature changes causes electrical open circuits. This damage, categorized as thermal cycling failure, arises from the mismatch in coefficients of thermal expansion among the board materials. Over repeated temperature fluctuations, the different expansion rates generate cyclic stresses that exceed the strength of the materials.
Protecting against this fatigue requires careful selection of laminates and plating thickness.
Stress Distribution
Cracks in plated through-holes and corner microvia fractures are common ways this structural breakdown shows itself. The resin expands along the z-axis much faster than the copper barrel, putting the plated cylinder under continuous tensile and compressive stresses. Eventually, this mechanical flexing leads to copper fatigue, resulting in cracks that typically form around the center of the barrel or at the corner of the microvia pad.
In addition, internal solder joints can experience fatigue failure, where the copper traces detach from the solder pad, causing intermittent electrical connections.
Environmental Chamber
Reliability assessment involves exposing test boards to continuous temperature swings in specialized environmental chambers. The testing standard, such as IPC-9701, defines the temperature extremes and dwell times for each cycle to simulate years of field service. Engineers monitor the resistance of daisy-chained test circuits continuously during the cycles to detect the exact moment a crack occurs.
This continuous tracking captures intermittent failures that occur only at high temperatures when the board is fully expanded, providing the data needed to calculate the mean time to failure. The resulting fatigue logs help engineers evaluate new solder formulations or copper foil types before approving them for high-volume product manufacturing.
