In the world of printed circuit boards (PCBs), understanding material properties is crucial. One such property is Tg, or glass transition temperature. For FR4 PCBs, knowing what Tg means can significantly impact design and performance. In this blog, we’ll explore what Tg is, how it’s measured, its importance, and other related questions.
What Does Tg Mean in PCB Manufacturing?
Tg stands for glass transition temperature. It refers to the temperature at which a polymer material, such as the resin in a PCB substrate, transitions from a hard, glassy state to a softer, rubbery state.
For the circuit board, the Tg value reflects the highest temperature it can withstand. When the temperature exceeds Tg values, the molecular structure of the circuit board material will be changed, lead to changes in its physical properties. Such as mechanical strength, size stability, adhesive, water imbibition, thermal expansion, etc. Therefore, the level of TG value of the PCB directly affects the stability and high temperature resistance of the circuit board.
In the high temperature environment, if the TG value of the circuit board is too low, it is easy to cause PCB deformation, fracture, cracking and other problems, affecting the reliability and life of the printed circuit board. If the TG value of the circuit board is too high, the PCB is easy to become hard and brittle, affecting the processability and flexibility of the circuit board.
How is Tg Measured in FR4 Materials?
Accurate measurement of Tg is crucial for ensuring the performance and reliability of FR4 PCBs. Knowing the exact Tg helps in selecting the appropriate material for specific applications, particularly those involving high temperatures and thermal cycling. Using materials with a Tg well above the operating temperature ensures the PCB will maintain its mechanical and electrical properties under expected conditions.
The primary method for measuring Tg in FR4 materials is by differential scanning calorimetry (DSC). It involves heating a small sample of the material at a controlled rate, typically between 10°C to 20°C/min. As the sample heats, the DSC measures the heat flow into the material and records it as a thermogram. The Tg is identified as the temperature where there is a noticeable change in heat capacity, appearing as a step or inflection point on the thermogram. DSC is widely used because it provides precise and reliable Tg measurements.
Additionally, Dynamic Mechanical Analysis (DMA) and Thermomechanical Analysis (TMA) also can be used to measure the Tg value.
Why is Glass Transition Temperature Critical in PCBs?
Glass transition temperature is critical because it affects the thermal and mechanical properties of the PCB. If a PCB operates near or above its Tg, it can experience issues like delamination, warping, and electrical failure. Here are some benefits of Tg.
- Thermal stability
One of the main reasons is its role in determining thermal stability. During operation, PCBs are subjected to varying temperatures due to power dissipation from electronic components and environmental conditions. Make sure the Tg of the PCB material is well above the maximum operating temperature, it helps maintain mechanical stability and reliable performance under thermal stress.
- Mechanical Strength
The Tg value also affects the mechanical strength and durability of the PCB. Below the Tg, the material is rigid and can withstand mechanical stress without significant deformation. This rigidity is essential for maintaining the structural integrity of the PCB, especially during assembly processes like soldering, where the board is subjected to mechanical and thermal stresses.
- Thermal Management
Thermal management is very important to electronics, especially for those miniaturized electronic devices. If the PCB itself has good thermal management, then designers no need to install heatsink or coolers. Materials with higher Tg values exhibit lower coefficients of thermal expansion (CTE), meaning less expansion and contraction with temperature changes.
- Application-specific requirements
Different applications have varying thermal and mechanical requirements. High-performance computing, automotive, aerospace, and industrial electronics often operate in environments with high temperatures and thermal cycling. For these applications, using PCB materials with appropriate Tg values is crucial.
What is the Typical Tg Value for Standard FR4 PCBs?
Standard FR4 materials generally have Tg values in the range of 130°C to 140°C. For high-performance applications, materials with higher Tg values are necessary. The table below lists some common high Tg materials used in PCB manufacturing, along with their Tg values and characteristics.
| Material | Tg Value (°C) | Characteristics | Applications |
| ISOLA IS410 | 180 | Excellent thermal and mechanical properties | High-reliability applications |
| Nanya NP-175 | 175 | High thermal stability, good mechanical strength | Automotive, industrial electronics |
| Shengyi S1000H | 170 | Cost-effective, widely used | Consumer electronics, industrial applications |
| Ventec VT-47 | 170 | High Tg and low CTE | High-density interconnect (HDI) PCBs |
| Rogers 4350B | 280 | Extremely high Tg, excellent thermal stability | RF and microwave applications |
Can You Explain the Difference Between Tg and Td in PCBs?
Tg and Td (decomposition temperature) are two different thermal properties. Unlike the reversible physical change associated with Tg, decomposition is an irreversible chemical process. At Td, the polymer chains in the material break down, leading to a loss of structural integrity and release of volatile compounds. Key points about Td include:
1. Chemical Stability
Td represents the upper thermal limit of a material’s chemical stability. Exceeding Td results in the breakdown of the polymer structure, rendering the material unsuitable for further use.
2. Material Selection
Knowing the Td helps in selecting materials for high-temperature applications. A material’s Td should be well above the maximum operating temperature to prevent degradation.
3. Safety and Reliability
Operating a PCB above its Td can result in catastrophic failure, including charring, delamination, and loss of electrical performance. The operating temperature remains below Td is critical for safety and reliability.
How Do You Choose the Right Tg Value for Your PCB Design?
Choosing the right Tg value depends on the operating environment and thermal demands of your application. Consider factors such as the maximum operating temperature, the thermal cycling the PCB will undergo, and the mechanical stresses it will face. Consulting with material suppliers and using simulation tools can help make an informed decision. According to our 18 years experience in PCB, we summarized a recommend Tg value for each application. Hope this is useful for you guys.
- Consumer electronics
If your PCB will be used in standard consumer electronics, just like smartphones and tablets, a Tg value of around 130°C to 140°C is typically sufficient. These devices generally do not encounter extreme temperatures or thermal cycling.
- Automotive applications
Automotive PCBs are always exposed to high temperatures and significant thermal cycling. A Tg value of 170°C or higher is recommended to ensure reliability and performance under the hood. So, Rogers material is good for this kind of PCB.
- Aerospace and defense
Applications in aerospace and defense require materials that can withstand extreme temperatures and harsh environments. High Tg materials with values of 180°C or more are often used to meet these stringent requirements.
- Industrial Electronics
Industrial environments can vary widely, but generally, high Tg materials are preferred due to the exposure to high temperatures and mechanical stress. A Tg value of at least 150°C to 170°C is advisable.
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