FR4 material and Tg: what the numbers mean
A short guide to the standard PCB laminate, to the glass transition temperature and to the question of which Tg your board needs.
What FR4 is
FR4 is the most widely used base material for printed circuit boards: glass-fibre cloth impregnated with epoxy resin and made flame retardant ("FR"). It is mechanically strong, electrically insulating, absorbs almost no water and costs little, which is why it is used for most boards.
Typical properties
| Property | Typical value |
|---|---|
| Dielectric constant | about 4.2 to 4.8, depending on frequency and resin content |
| Thermal conductivity | about 0.3 W/m·K, practically an insulator |
| Water absorption | very low |
| Flame retardancy | self-extinguishing (the "FR" in the name) |
The data sheet of the laminate that is actually used is decisive; the values above are guides.
What Tg means
The glass transition temperature (Tg) is the temperature at which the resin starts to soften. Below Tg the board expands little; above it the expansion — especially across the thickness — grows several times. This puts stress on the copper in the plated holes. The Tg is not a maximum operating temperature: keep the working temperature well below it.
Tg options at PCB-ELECTRA
| Tg | Use |
|---|---|
| 135 °C (standard) | Simple 1- and 2-layer boards, normal assembly |
| 150 °C | More thermal stress: several soldering passes, thicker boards, higher operating temperature |
| 170 °C | Multilayer boards, repeated reflow, demanding thermal conditions |
Higher Tg options are priced separately; the calculator shows the effect immediately. As a guide, from 4 layers upwards a higher Tg is the safer default, because the thicker stack goes through more thermal stress during lamination and soldering.
Why lead-free soldering raises the question
Lead-free solder is processed at higher temperatures than leaded solder. A board that goes through lead-free reflow once or twice, or is reworked, benefits from a higher Tg, while a simple board that is wave-soldered once is fine with the standard value.
How to choose
- 1- or 2-layer, normal assembly, moderate operating temperature: Tg 135 °C.
- Several soldering passes, thick board or heat near the board: Tg 150 °C.
- 4 or 6 layers, repeated reflow or high temperatures: Tg 170 °C.
- If heat is the main issue and not the laminate, consider an aluminium board: see FR4 or aluminium.
Choose the Tg in the price calculator; all options are listed under capabilities. More on the layer structure in the stack-up guide.
Frequently asked questions
What does the Tg value mean?
The glass transition temperature, the temperature at which the laminate starts to soften. Above it, the board loses stiffness and expands more along its thickness.
Which Tg values do you offer?
Tg 135, 150 and 170 °C.
When do I need a higher Tg?
For lead-free soldering with several reflow cycles, for boards that run warm in use, and for multilayer boards with many holes.
Browse the questions and answers →
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