The glass transition temperature, or Tg, of a printed circuit board (PCB) is the point at which the material of the PCB changes from a rigid state to a more flexible and pliable one. This temperature is specific to the type of material used in the PCB and can vary depending on the laminate selected. For instance, the Tg of a PCB made from FR4 material is typically TG 130 (130°C), meaning that the PCB will become more flexible when it reaches temperatures above 130°C. It's important to note that the Tg is not the same as the maximum operating temperature of a PCB, as the Tg refers to the point at which the material properties of the PCB change, while the maximum operating temperature is the highest temperature at which the PCB can function properly.