Thermomechanical Failure
Repetitive expansion and contraction of plated through-hole structures during thermal cycling constitutes a leading cause of open circuits in double-sided printed circuit boards. The specific structural degradation known as barrel fatigue results in circumferential cracking of the copper plating along the central region of the drilled hole. This vulnerability arises because the organic substrate expands in the vertical direction at a rate several times greater than the copper plating during temperature excursions.
Stress Concentration
High local stress accumulates where the copper barrel meets the internal layer foils or near the center of the vertical column. Under thermal load, barrel fatigue develops as these forces exceed the tensile strength of the electrodeposited copper. This defect is particularly common in high-aspect-ratio holes that suffer from uneven copper plating thickness or poor ductility in the electroplating bath.
Reliability Assessment
Detecting this defect before system integration involves thermal shock testing and continuous resistance monitoring during environmental stress screening. If the resistance of a daisy-chained test vehicle increases abruptly during a temperature transition, it indicates that barrel fatigue has broken the copper cylinder. Ensuring proper copper ductility and maintaining a minimum plating thickness of twenty-five microns in the plated through-holes are standard mitigation strategies used to prevent this failure.
This testing must continue for hundreds of cycles to guarantee that the assembly can withstand the thermal stresses of repeated solder reflow and subsequent field operations.
