For printed circuit boards exposed to high thermal loads, the necessary long-term operating temperature must be determined early on, in order to select an appropriate material.
The Tg value of the base material can be used for this purpose as a reference. Below we have summarized the details for you.
Tg Value
The glass transition temperature (Tg) is an important normative dimension for the base material that determines the temperature at which the resin matrix converts from a glassy, brittle condition into a soft, elastic one.
The Tg value of the base material sets here an upper boundary, at which the resin matrix decomposes and a subsequent delamination occurs. The Tg is thus not the value of the maximum operational temperature, but rather that which the material can endure for only a very short time.
A guideline for a continuous thermal load is an operating temperature approximately 25°C below the Tg.
When the glass transition temperature (Tg) is over 170°C, it is referred to as a high Tg material.
High Tg materials have the following properties:
- High glass flow temperature value (Tg)
- High temperature durability
- Long delamination durability
- Low Z axis expansion (CTE)
| Material for High-Tg boards | Tg | CTE-z | εr, Dk- Permittivity | Electric strength | Surface resistivity | CTI | Thermal conductivity | Td value | Peel strength |
|---|---|---|---|---|---|---|---|---|---|
| °C | ppm/°C | @1GHz | KV/mm | MO | PLC | W/m*K | °C | N/mm | |
| ISOLA IS410 FR4 HTg, CAF-Enhanced | 180° | 55 | 4,0 | 44 | 8,0 x 10^6 | 3 | 0,5 | 350° | 1,2 |
| ISOLA IS420 FR4 HTg, CAF-Enhanced | 170° | 45 | 4,0 | 54 | 3,0 x 10^6 | 3 | 0,4 | 350° | 1,3 |
| ITEQ IT-180A FR4 HTg | 175° | 45 | 4,4 | 45 | 3,0 x 10^10 | - | - | 345° | 1,4 |
| Shengyi S1000-2 FR4 HTg | 180° | 45 | 4,8* | 63 | 7,9 x 10^7 | 3 | - | 345° | 1,4 |
| ARLON 85N Polyimid HTg | 250° | 55 | 4,20* | 57 | 1,6 x 10^9 | - | 0,2 | 387° | 1,2 |
CAF - Conductive Anodic Filament: an undesirable conducting filament in the substrate of a circuit board
CTE-z
The CTE value shows the thermal expansion of the base material. CTE-z represents the z-axis and is e.g. due to the stability of the vias, of high importance. A higher Tg value favors a low CTE-z value which represents the absolute expansion in the z-axis. Errors like pad lifting, corner cracks and cracks within the via can be prevented through a low CTE-z value.
T260 - T288 value, Td
The decomposition temperature Td of a resin system depends on the binding energies within the polymers, and not on the glass transition temperature Tg. A good indicator for this characteristic is the T260 or T288 value, which specifies the time until delamination at 260°C or 288°C, respectively.
A very important indicator of the heat resistance is the time-to-delamination at a certain temperature. This test is preferably performed at 260 °C or 288 °C. The T260- or T288-value is the time to delamination of the tested material at 260 °C or 288 °C, repectively.
Td: Temperature-of-decomposition indicates the temperature at which the base material has lost 5% by weight and is an important parameter for the thermal stability of a base material. Through exceeding this temperature an irreversible degradation and damage to the material by the decomposition occurs.