Everything you should know about the Tg of PCB
Temperature changes can have a critical impact on the operation, reliability and quality of PCBAs. Temperature rises results in materials expanding, however, the principal materials that PCB are made of have disparate thermal expansion coefficients. This causes mechanical stress that can create micro-cracks that may be undetected during electrical tests (Open / short tests) carried out at the end of production. Best-case scenario, these micro-cracks cause failures detected after brazing. Worst case, it may cause random failures of the finished product!
The introduction of RoHS directives in 2002 required the use of lead-free alloys for soldering. Removing lead results in the rise of melting temperature, PCBs are therefore subject to higher temperatures during soldering (reflow and wave). Depending on the chosen reflow process (single, double…), it is necessary to use a PCB with appropriate mechanical characteristics, especially one with suitable glass transition temperature (Tg).
What is Tg?
Tg is a mechanical property that designates glass transition temperature, that is, the temperature at which the base material (polymer or glass) shifts from a glassy, solid, rigid state to a rubbery state. When the Tg is exceeded, materials don’t melt but undergo structural change and become rubbery.
Precise measurement of this temperature is hard to achieve because many factors come into consideration, including the material’s molecular structure. Different materials therefore have a different glass transition temperature. Two materials may have the same Tg even though they have different technical characteristics (eg Material A and C in the graph below). Materials’ viscosity increases when heated. After cooling, they are more prone to cracking or breaking.
High Tg materials have the following properties:
- Resistance to high temperatures,
- Long delamination durability (aging of materials to consider for safety reasons),
- Low thermal expansion.
Tg for PCBs
For PCBs, the Tg corresponds to the temperature at which fiberglass becomes amorphous during lamination at high temperatures and under pressure of the different material layers. It is not the PCB maximum operating temperature, but rather that which the PCBA can endure for a short time before it deteriorates.
Copper-coated laminate (FR4) treated by internal layer imaging is the most frequently used material for PCB production but it requires PREPREG layers application, that are only rigid after lamination. The heat required for the PREPREG rigidity must be applied without exceeding the FR4 Tg to preserve the PCB stability. The standard FR4 Tg is between 130 – 140°C, the median Tg is 150 °C and the high Tg is greater than 170°C. In the hot state, high Tg FR4 will have better mechanical and chemical resistance to heat and moisture than standard FR4.
Below the Tg of different PCB base materials:
|Base material||PREPREG||FR2||CEM1||CEM3||FR4||FR4 High Tg||Teflon||Polyimide|
If the PCB’s operating temperature exceeds its Tg for an extended period of time, the PCB will change from glassy state to rubbery state and its performance will be affected. Tg guarantees the mechanical stability and proper functioning of the PCB during its life time.
Tg is one of the key features to consider when specifying your PCBs. It is very important to determine, at a very early stage of designing, the temperature at which the PCBs will be exposed to select the appropriate material, especially for PCBs exposed to high operating temperatures.