As the demand for high-performance electronics continues to rise, the role of PCBs has become more critical than ever. For many applications, traditional FR-4 PCBs may not perform well because of it’s inconsistent performance in high temperatures. This is because of a parameter of PCB laminates called the Glass Transition temperature, or more popularly known as Tg. This article will give you a brief idea of the importance of Tg in PCBs, the characteristics of high Tg PCBs, and applications of high Tg.
What is Tg PCB?
High Tg PCB refers to a circuit board made with a high glass transition temperature material. Tg, formally known as Glass transition temperature, is a parameter of PCB laminates that denotes the temperature at which the laminate starts to shift mechanical form. It changes from a hard glassy state to a soft rubbery state until the temperature remains above the Tg threshold temperature. After the temperature of the laminate reaches below the Tg threshold temperature, it starts returning to its hard glassy state. However, even after cooling, there might be a chance that the surface of the PCB gets deformed from the initial shape before it reaches the Tg temperature, which in extreme cases might cause an open or a short circuit, resulting in PCB failures.
What are High Tg PCBs, and why is High Tg important?
While choosing FR-4 for a PCB, you might have encountered a query, “Please specify the temperature gradient of FR-4”, and you get three options to choose from –
- Standard Tg- 130 °C to 140°C
- Mid Tg- 150°C
- High Tg- Greater than or equal to 170°C
Although not limited to FR-4, high Tg PCBs are PCBs that are fabricated by laminates that have a Tg of 170°C or higher. Usually, most of the materials other than FR-4 PCBs have a Tg of more than 170°C and hence are considered high Tg PCBs. Ideally, a higher Tg value is deemed as better than a lower Tg value. A higher Tg value crudely has the following benefits-
- Reduced risk of delamination and warpage in fabrication.
- Improved thermal performance even for prolonged durations.
- Reduced chances of shorting and open circuit in the event of excessively high temperatures.
Popular High Tg Materials
The following are a few high Tg PCB laminates popularly used in various applications –
| Material | Tg Value |
| Rogers RO4350B | 280°C |
| Isola FR408HR | 180°C |
| Isola 370HR | 180°C |
| Arlon 85N | 250°C |
| Megtron 6 | 185°C |
| Rogers RO4003C | 280°C |
Properties of High Tg PCBs
The key properties of high Tg PCBs that are common to all high TG materials are –
- Good Electrical Performance – Although, higher Tg value has nothing to do with electrical performance directly, as a general trend with many exceptions, a higher Tg constitutes better electrical performance. Additionally, most of the high-performance materials have a Tg of more than 200°C.
- High Thermal Stability – High Tg PCB materials can maintain dimensional and structural integrity even at prolonged exposure to high temperatures. Thermal stability ensures a smooth surface is maintained at high temperatures, which is essential for high-speed PCBs.
- High PTH Reliability – PTH, also known as plated through hole reliability, refers to the durability and mechanical integrity of PTH. A high Tg PCB material does not soften at higher temperatures during soldering cycles, especially in the case of a via in pad, which reduces the stress on the copper in the PTH barrel.
- Support of HDI Designs – Although high Tg does not directly affect the HDI capabilities, HDI designs are mostly supported by high Tg materials.
- High Cost – High Tg PCB materials tend to cost more than their lower Tg counterpart. Most of the materials follow the same trend, even when compared to different materials. This trend can be best observed with FR-4, where higher temperature grade FR-4 costs more than the lower Tg FR-4.
When Should You Consider High Tg PCB Materials?
Usually, other than the case of FR-4, you won’t get to decide if the Tg should be higher or lower for a similar material. For a higher Tg, you would have to choose a different material altogether. Nevertheless, there are cases that might force you to choose a higher Tg material. Some of the cases that might require a high Tg PCB material are –
- LED PCBs – If you are not using a metal core PCB for LED application, then Tg plays an important role in the performance of the PCBs. For applications like outdoor signage or LED displays, the Displays are required to work in all weather. In Southeast Asia and Middle Eastern countries, environmental temperatures are already high, and in broad daylight, the displays are required to operate in full brightness, which produces a lot of heat. To ensure the reliability of these displays, the PCBs of these displays (The LED PCB and the driver PCB) are generally made using high Tg PCB materials (usually FR-4).
- PCBs with Heavy Copper Pours – Some PCBs host components that require heavy copper pours as a part of their application requirement or safety requirement. Components with EPADs, chassis ground, etc., are a few examples of components requiring heavy pours. Now, the problem is that with such heavy copper pour pads of these components do not heat up easily since the copper in them dissipates the heat, which may cause difficulty in soldering them manually. However, a bigger problem is posed by removing such components from the PCB. A good example is the SFP cage that requires a heavy chassis ground pour. SFP cages are notoriously difficult to remove from the solder pads. So, hot air guns (which blow hot air of 300°C to 400°C) are used to remove the cages, but the process also heats up the PCB laminate around it. The best chance for the PCB not to deform in the process of desoldering is when the laminate has a high Tg value.
- PCBs are Required to Operate at very High Temperatures – Some electronics are required to operate in extremely high temperatures. Applications may require military or aerospace grade components that are required to operate reliably at -55°C to 125°C and 150°C in some cases. To host such components, special materials are used that have superior electrical and mechanical properties along with a high Tg.
Applications of High Tg PCBs
Due to its outstanding properties, High Tg PCB can be applied to many areas. Some of the applications of high Tg PCBs are –
- Outdoor Signages
- Military and aerospace-grade electronics
- Automotive Grade electronics
- High-performance computers.
- Networking Equipment
- Power inverters
Conclusion
Although many general applications (which cover around 70% of all the electronics) may not require consideration of high Tg PCBs. But they are an integral part of some key technologies that fully utilise the benefits of high Tg laminates. It is recommended to use high Tg PCB materials that are required to have high reliability and longevity. Although it costs more, the benefits of using a high Tg laminate outweigh its limitations.
