The substrate of High Tg circuit board will change from “glass state” to “rubber state” when the temperature rises to a certain area, and this temperature is called the glass transition temperature (Tg) of the board. In other words, Tg is the maximum temperature (℃) at which the substrate remains rigid. That is to say, ordinary PCB substrate materials at high temperature not only produce softening, deformation, melting and other phenomena, but also performance in the mechanical, electrical characteristics of a sharp decline (I don’t think we want to see their products appear this situation).

Generally speaking, the board of Tg is more than 130 degrees, high Tg is more than 170 degrees, and medium Tg is more than 150 degrees.
Usually Tg≥170℃ PCB, called high Tg PCB.
When the Tg of the substrate increases, the circuit board’s heat resistance, moisture resistance, chemical resistance, stability resistance and other characteristics will be improved. The higher the TG value, the better the temperature resistance of the plate. And the high TG is often applied in the lead-free process,
High Tg refers to high heat resistance. With the rapid development of the electronic industry and the electronic products represented by computer, the higher heat resistance of PCB substrate materials have become an important guarantee  when the development towards into a high function and high multilayer. What’s more, the appearance and development of high-density installation technology represented by SMT and CMT make PCB more and more inseparable from the support of high heat resistance of substrate in small aperture, fine circuit and thin form.
Therefore, the difference between the general fr-4 and the high Tg fr-4 is that in the thermal state, especially in the case of heat after moisture absorption. And there are many differences in the mechanical strength, dimensional stability, adhesion, water absorption, thermal decomposition, thermal expansion and other conditions of the materials. The products with high Tg are obviously better than ordinary PCB substrate materials. In recent years, the number of customers requiring the production of high Tg circuit board increased year by year.