The effect of copper foil properties on low cycle fatigue of a multi-layer flex cable

A multi-layer high performance flex cable was used as a test vehicle for studying the effect of copper foil properties on low cycle fatigue properties of fine line conductors. We fabricated the cable using copper foils for the three conductor layers, and used advanced organic composite dielectric materials for the internal dielectric layers. We fabricated the conductors using subtractive etching as opposed to using an additive plating process. The cable had to survive multiple cycles of fatigue bending around a sharp radius and we found that selection of copper foil was critical in determining fatigue life. We identified three distinct failure mechanisms in the cables that depended strongly on the surface roughness of the foil. We then studied the mechanical properties of free-standing copper foils of varying roughness and thickness to explain the behavior of the cables. We found that the surface roughness of the foils, and the ratio of roughness to thickness, strongly effected their fatigue performance