Definition

Coefficient of Thermal Expansion (CTE) measures how much a material expands per degree of temperature change, typically expressed in parts per million per degree Celsius (ppm/°C). In PCBs, mismatched CTE between materials causes stress during thermal cycling. FR-4 has a Z-axis CTE of 50-70 ppm/°C below Tg and much higher above Tg. Matching CTE between the PCB and components (especially ceramic BGAs at ~7 ppm/°C) is critical for reliability.