Definition

Z-axis CTE (coefficient of thermal expansion) is the expansion rate through the PCB thickness. Unlike X-Y axes (constrained by fiberglass), Z-axis expansion is resin-dominated and much higher. Below Tg, typical FR-4 has Z-axis CTE of 50-70 ppm/°C; above Tg it can exceed 250 ppm/°C. High Z-axis CTE stresses plated through-holes during thermal cycling, potentially causing barrel cracks. Thicker boards, higher aspect ratio vias, and more thermal cycles increase failure risk. Low-CTE materials are used for high-reliability applications.