Immersion Gold FR4 High TG PCB With HASL Leadfree Surface Finishing

What is high Tg PCB?

What are the advantages of using high PCB?

Glass Transition Temperature (Tg)

The thermal properties of the resin system are characterized by the glass transition temperature (Tg), which always is expressed in °C. The most commonly used property is the thermal expansion. When measuring the expansion versus the temperature, we can get a curve as shown in following picture. The Tg is determined by the intersection of the tangents of the flat and steep parts of the expansion curve. Below the glass transition temperature, the epoxy resin is rigid and glassy. When the glass transition temperature is exceeded, it changes to a soft and rubbery state.

For the most commonly used types of epoxy resin (FR-4 grade), the glass transition temperature is in the range 115-130°C, so when the board is soldered, the glass transition temperature is easily exceeded. The board expands in the Z-axis direction and stresses the copper of the hole wall. The expansion of epoxy resin is about 15 to 20 times greater than that of copper when exceeding Tg. This implies a certain risk of wall cracking in plated-through holes, and the more resin around the hole wall, the greater risk. Below the glass transition temperature, the expansion ratio between epoxy and copper is only three times, so here the risk of cracking is negligible.

As above stated, when the temperature of a high Tg PCB rises to a certain area, the substrate will be turned into "rubbery state" from “glassy state”, the temperature at this time is called the glass transition temperature (Tg) of the PCB board. That is, Tg is the maximum temperature (° C) at which the substrate remains rigid. In other words, the substrate materials of ordinary PCB not only be soften, deform, melt and so on at high temperatures, but also have a sharp decline in the mechanical and electrical properties (I do not think you are willing to see this happen to your products).

The Tg of general board is above 130 celsius degrees, high Tg is generally greater than 170 celsius degrees, medium Tg is about greater than 150 celsius degrees. PCB boards with Tg ≥ 170 ° C are usually called high Tg PCBs.

If the Tg of a substrate is increased, then the characteristics of heat resistance, moisture resistance, chemical resistance, and stability resistance etc., of the PCB will be improved and enhanced. The higher the Tg value, the better the temperature resistance of the board, especially in the lead-free process. High Tg is more widely used nowadays.

High Tg refers to high heat resistance. With the rapid development of the electronics industry, especially the development of the electronic products represented by computers to the trend of high-functionality and high multilayered, higher heat resistance of the PCB substrate material is required as an important guarantee. The emergence and development of high-density package technology represented by SMT, CMT, makes the PCB more and more inseparable from the high heat resistance support in aspects of small via, fine circuits and thinning tendency.

Therefore, the differences between normal FR-4 and FR-4 with high Tg are that the mechanical strength, dimensional stability, adhesiveness, water absorption, thermal decomposition, thermal expansion and other conditions are different in thermal state, especially being heated after moisture absorption. High Tg product is obviously better than ordinary PCB substrate materials. In recent years, the customers who ask for high Tg PCB have been increasing year by year.