The next four data-sheet parameters cover the thermal properties of circuit-board materials. The coefficient of thermal expansion (CTE), for example, indicates how a circuit material expands and contracts with temperature. It is typically of greatest concern in the z direction or thickness of a circuit material because of the use of plated through holes (PTHs) for electrical connections from one side of a PCB to the other and in multilayer constructions. Excessive z-axis CTE during thermal cycling can damage the PTHs. As a general rule of thumb, a z-axis CTE of less than 70 ppm/°C is considered acceptable. The optimum CTE value is closely matched to the CTE of copper, at 17 ppm/°C.
The CTE of a circuit material in the x and y directions can also be a concern, especially when placing surface-mount devices on the PCB material. If the x-y plane CTE of the material is significantly mismatched from the CTE of a surface-mount device, the expansion and contraction of the circuit material will differ than that of the surface-mount device. With thermal cycling over time, the differences in CTE can cause a work-hardening to occur at the solder joints between the PCB and the surface mount device, potentially harming long-term reliability.
A circuit material’s glass transition temperature, Tg, is the temperature at which the material exhibits a modulus change or transition in its physical characteristics. Most high-frequency PCB materials have different CTE below the Tg temperature than above it. This is sometimes referenced as alpha1 CTE for the CTE below Tg and alpha2 for the CTE above Tg. For most circuit materials, the CTE value is much greater above Tg than below it, and this