The glassy substance reversibly transforms between the glassy state and the high elastic state. That is to say, if the temperature of the epoxy resin used as the adhesive of the FR-4 substrate is lower than Tg, the material is in a hard “glass state” at this time. When the temperature is higher than the Tg value, the material will appear as soft as rubber.
When the resin material is below Tg, and its state is a hard solid, it is in a glass state. Under the action of external force, there is a certain deformation, but the deformation is reversible; that is after the external force disappears, the deformation also disappears.
High elastic state
When the heating temperature of the resin exceeds Tg, the molecular chains in the amorphous state begin to move, and the resin enters a highly elastic state. The resin in this state is similar to the elastomer in the rubber state but still has reversible deformation properties.
After the temperature exceeds the Tg value, the material gradually becomes soft. As long as the resin does not decompose, it can still return to the same rigid state as before when the temperature is cooled below the Tg value.