The main ones that we would consider first include:
A measure of how much the material expands when heated. Critical in Z-axis – typically above Tg and the expansion is greater. If CTE insufficient for then failures can occur during assembly as the material expands rapidly above Tg.
Materials can have same Tg yet different CTE’s – lower CTE is better. Equally some materials can have higher Tg values, yet also have a higher (worse) CTE post Tg.
Tg / GLASS TRANSITION TEMPERATURE
The Tg value is the temperature at which the material changes from a reasonably stiff glass-like material to a more elastic and bendable plastic-like material. Important as above Tg, the materials properties will change.
Td / DECOMPOSITION TEMPERATURE
This is a measure of the degredation of the material. The analysis-method measures when 5 % of the material is lost by weight – the point as which reliability is compromised and delamination may occur.
Higher reliability PCB will require Td ≥ 340℃
T260 / T288 / TIME TO DELAMINATION
This is the method to determine the time when the thickness of the PCB is irreversibly changed at a predefined temperature (260 or 288 in this case) – i.e. when the material expands to such a degree that it delaminates.