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Middle Tg PCB vs High Tg PCB

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Middle Tg PCB vs High Tg PCB

As the demand for high-performance circuit boards continues to grow, the choice of substrate material becomes crucial. Two commonly used options are PCB Middle Tg and High Tg PCB materials. While both offer enhanced thermal properties and performance, they differ in certain aspects. This article aims to provide a comparative analysis of PCB Middle Tg and High Tg PCB, highlighting their unique characteristics and applications.

Understanding Glass Transition Temperature (Tg)

The glass transition temperature (Tg) is the temperature at which an amorphous material, such as a polymer, transitions from a rigid, glassy state to a more flexible and rubbery state. It is a critical characteristic of the material as it indicates the temperature at which it undergoes a significant change in its physical properties.

At temperatures below Tg, the material is brittle and exhibits a glass-like behavior, while above Tg, the material becomes more elastic and exhibits a rubber-like behavior. Tg is usually measured in degrees Celsius (°C) and varies depending on the specific material.

For example, in standard FR-4 materials commonly used in PCB manufacturing, the Tg is typically around 130-150°C. This means that below this temperature range, the material remains rigid and can withstand mechanical stress without significant deformation. However, exceeding this temperature can cause the material to soften and lose its structural integrity, leading to potential failures or performance issues in electronic devices.

It’s important to consider the Tg of a PCB material when designing electronic devices, especially for applications with elevated temperatures, as selecting a material with a higher Tg can ensure better reliability and performance in challenging thermal conditions.

What Does PCB Tg Mean?

The glass transition temperature (Tg) of a polymer system in a Printed Circuit Board (PCB) refers to the temperature at which the material undergoes a transition from a rigid and stiff state to a softened or pliable state. This thermodynamic change can be reversed as long as the polymer system is not degraded. When the material is heated above its Tg and then cooled back to the Tg, it will return to its stiffer state and exhibit its initial properties.

In PCBs, the base materials typically contain organic components, such as hydrocarbon binders, which have lower temperature stability compared to the inorganic constituents of the composite materials. Chemists have made significant efforts to improve the thermal properties of polymers used as binders. However, these properties still fall short of the thermal stabilities exhibited by metal foils, reinforcing glass fibers, and ceramic fillers commonly found in PCBs.

Despite this, these polymers are suitable for use in various lead-free technologies. While a slight increase in soldering temperature initially posed a challenge, it was found that the stability of temperature of the PCB binders’ polymers was significantly higher than anticipated.

Therefore, the glass transition temperature of a polymer system in a PCB signifies the temperature at which the material transitions from a rigid to a pliable state, and this transition can be reversed without degradation. Although the thermal stability of PCB binders’ polymers is lower than that of inorganic components, they are still effective in lead-free technologies.

Key Characteristics of PCB Tg Materials

The major properties of a PCB material with respect to its Tg (glass transition temperature) include:

1.Thermal Stability:

The Tg value indicates the maximum temperature at which the material can maintain its structural integrity and physical properties without significant degradation or softening. A higher Tg material offers better thermal stability and can withstand higher operating temperatures.

2.Mechanical Strength:

The Tg temperature affects the mechanical strength of the material. Below the Tg, the material is rigid and stiff, while above the Tg, it becomes softer and more pliable. The Tg value determines the temperature range within which the material retains its mechanical strength and dimensional stability.

3.Electrical Performance:

The Tg temperature can impact the electrical properties of the PCB material. Certain electrical characteristics, such as dielectric constant and dissipation factor, may vary with temperature, particularly around the Tg range. It is important to select a material with a suitable Tg to ensure consistent electrical performance.

4.Chemical Resistance:

The Tg temperature can influence the material’s resistance to chemicals and solvents. Generally, a higher Tg material tends to exhibit better chemical resistance, making it more suitable for applications where exposure to harsh chemicals is expected.

5.Moisture Absorption:

The Tg temperature can affect the moisture absorption properties of the material. Materials with a higher Tg typically have lower moisture absorption rates, which is desirable for applications where moisture resistance is important.

6.Processing Temperature:

The Tg temperature also impacts the processing and manufacturing of PCBs. During fabrication processes such as soldering, the temperature should not exceed the Tg of the material to avoid softening and potential damage to the PCB.

Overall, the Tg value of a PCB material is a critical parameter that influences its thermal, mechanical, electrical, chemical, and processing properties. Selecting a material with the appropriate Tg is crucial to ensure the reliability and performance of the PCB in its intended application.

PCB Tg: Categorizing Temperature Grades

It is commonly believed that higher Tg (glass transition temperature) values in PCBs are generally superior. However, this notion does not always hold true. While it is accurate that higher Tg values can delay the onset of significant thermal expansion in a specific polymer system, the overall expansion characteristics can still vary among different materials. Materials with lower glass transition temperatures may exhibit lower net expansion compared to those with higher glass transition temperatures.

PCB high Tg is achieved when the temperature reaches a specific range, causing the substrate to transition from a glassy and rigid state to a more rubbery state. This temperature is known as the glass transition temperature of the circuit board.

To elaborate further, it represents the maximum temperature in ℃ at which the substrate will maintain its rigidity. Without reaching such high temperatures, standard PCB materials may experience melting, deformation, softening, and a significant decline in their electrical and mechanical properties.

Typically, the Tg of ordinary PCB substrates is higher than 130℃, while high Tg FR4 PCBs usually have a Tg value greater than 170℃. Medium Tg materials, the Tg values higher than FR4 Tg150 PCBs.

Increasing the Tg of the substrate enhances moisture resistance, heat resistance, chemical resistance, and overall stability of the PCB. Higher Tg values also indicate greater temperature resistance, particularly in lead-free processes where PCB high Tg applications are prevalent.

Characteristics of High Tg PCBs

With the rapid growth of the electronics industry, high Tg (glass transition temperature) materials for PCBs have become widely utilized in various applications such as communication equipment, computers, precise instruments, and other electronic apparatus. As the demand for higher functionality and increased multilayer development of PCBs rises, the need for substrate materials with higher heat resistance becomes a prerequisite.

The utilization of high Tg materials in PCB substrates is crucial, particularly when dealing with small apertures, thinning, and fine wiring due to the emergence and development of high-density mounting technologies like Chip Mount Technology (CMT) and Surface Mount Technology (SMT). The difference between high Tg materials, such as Tg150 FR4, and general FR-4 lies in their adhesion, mechanical strength, water absorption, thermal decomposition, and dimensional stability in hot states. High Tg PCBs exhibit superior performance in these areas, especially after water absorption, and offer better thermal expansion characteristics compared to regular PCB substrate materials. Despite their higher cost, there has been a significant demand for high Tg PCBs recently.

Furthermore, high Tg materials have gained recognition in the LED lighting industry. This is due to the higher heat dissipation requirements of LEDs compared to conventional electronic components. However, the FR-4 board structure using high Tg materials is more cost-effective than metal core PCBs.

The properties of high Tg materials can be summarized as follows:

1. Low thermal expansion.
2. High resistance to elevated temperatures.
3. Excellent mechanical properties.
4. Excellent reliability of plated-through holes (PTH).
5. High resistance to thermal stress.

Characteristics of Middle Tg PCBs

PCB Middle Tg material offers a range of features that greatly contribute to the overall performance of circuit boards.

One notable feature is its resistance to Conductive Anodic Filament (CAF). CAF is a concern for PCB manufacturers as it occurs between adjacent PCB vias. The use of PCB Middle Tg material helps mitigate the risk of CAF, making it a suitable choice for circuit board fabrication.

Another advantage of PCB Middle Tg material is its environmental friendliness. It does not contain phosphorus or halogen, making it a safe and eco-friendly option for PCB manufacturing.

Furthermore, PCB Middle Tg material meets the UL-94 V-0 flammability standard, which is widely recognized in the PCB industry. This standard determines the flammability characteristics of PCB materials and ensures compliance with safety regulations.

The thermal performance of PCB Middle Tg material is also noteworthy. It exhibits excellent thermal properties, including low Z-axis coefficient of thermal expansion (CTE) and a middle glass transition temperature. These characteristics contribute to superior thermal performance, making this material well-suited for the fabrication of rigid circuit boards and high-performance boards.

PCB Middle Tg vs High Tg PCB: How Do They Differ?

PCB (Printed Circuit Board) is an essential component in electronic devices that provides mechanical support and electrical connection between various electronic components. Two common types of PCBs are Middle Tg and High Tg PCBs, which differ based on their glass transition temperature (Tg).

1. Glass Transition Temperature (Tg):

●Tg is the temperature at which the PCB material changes from a rigid, glassy state to a softer, rubbery state.

●Middle Tg PCBs have a Tg range of around 150°C, while High Tg PCBs have a Tg range of above 170°C.

●The higher Tg of High Tg PCBs makes them more resistant to heat and ensures better mechanical stability at elevated temperatures.

2. Thermal Performance:

●High Tg PCBs have better thermal performance compared to Middle Tg PCBs.

●High Tg PCBs can withstand higher operating temperatures without undergoing structural changes, ensuring reliable performance under challenging thermal conditions.

3. Reliability:

●Due to their higher Tg, High Tg PCBs offer improved reliability and longevity in high-temperature environments.

●Middle Tg PCBs may experience degradation or material breakdown at elevated temperatures, leading to reduced reliability.

4. Material Selection:

●High Tg PCBs are typically made using high-performance materials with a higher glass transition temperature.

●Middle Tg PCBs can be made from standard FR-4 materials, which have a lower Tg compared to high-performance materials.

5. Cost:

●High Tg PCBs generally cost more than Middle Tg PCBs due to the specialized materials required for their construction.

6. Applications:

●Middle Tg PCBs are suitable for most general electronic applications that do not involve extreme temperatures.

●High Tg PCBs are preferred for applications where high reliability and resistance to thermal stress are crucial, such as automotive, aerospace, and industrial equipment.

Overall, the main difference between Middle Tg and High Tg PCBs lies in their glass transition temperature, with High Tg PCBs offering higher thermal resistance, better reliability, and enhanced mechanical stability at elevated temperatures.

When Do I Need Middle Tg PCBs?

Middle Tg (glass transition temperature) PCBs, also known as Tg150 PCBs, are typically used when the operating temperature requirements fall between those of standard PCBs and high Tg PCBs. Here are some situations when middle Tg PCBs may be needed:

1.Moderate Temperature Environments:

If your electronic device or application operates in moderately elevated temperatures, but not as extreme as those requiring high Tg PCBs, middle Tg PCBs can provide the necessary thermal stability. These PCBs can withstand temperatures higher than standard PCBs (Tg > 130℃) but may not be required to handle the highest temperature ranges.

2.Lead-Free Assembly:

Middle Tg PCBs are often preferred in lead-free assembly processes. Lead-free soldering typically involves higher temperatures during the manufacturing process. Middle Tg PCBs can offer improved resistance to thermal stress and maintain their structural integrity during the soldering process.

3.Power Electronics:

Middle Tg PCBs are commonly used in power electronics applications where components generate significant heat. These PCBs can handle the heat dissipation requirements of power electronics devices such as power supplies, inverters, motor drives, and other high-power applications.

4.Consumer Electronics:

In certain consumer electronics devices, especially those with higher power consumption or compact form factors, middle Tg PCBs can provide better thermal management. This helps to prevent overheating and ensures the reliability and longevity of the electronic device.

5.Industrial Control Systems:

Industrial control systems often require PCBs that can withstand higher operating temperatures and temperature fluctuations. Middle Tg PCBs can be suitable for such applications, providing enhanced thermal stability and reliability in industrial environments.

6.Automotive Electronics:

In automotive applications that do not require the extreme temperature resistance of high Tg PCBs, middle Tg PCBs can still offer improved thermal performance and reliability. They can be used in various automotive electronics components, such as control modules, sensors, and displays.

When Do I Need High Tg PCBs?

High Tg (glass transition temperature) PCBs are needed in several situations where standard PCBs may not provide sufficient thermal stability and reliability. Here are some instances when high Tg PCBs are required:

1.High-Temperature Environments:

If your electronic device or application operates in elevated temperatures or experiences frequent temperature fluctuations, high Tg PCBs are essential. These PCBs can handle higher operating temperatures without losing their mechanical and electrical properties, ensuring the reliability and performance of the device.

2.Power Electronics:

Power electronics applications, such as power supplies, inverters, and motor drives, generate significant heat due to high power levels. High Tg PCBs are necessary to dissipate heat effectively and maintain stable operation under these demanding thermal conditions.

3.Automotive Electronics:

Automotive electronics often face harsh operating conditions, including high temperatures under the hood. High Tg PCBs are crucial in automotive applications to ensure the reliability and longevity of electronic components, such as engine control units (ECUs), sensors, and infotainment systems.

4.Aerospace and Defense:

In aerospace and defense applications, high Tg PCBs are necessary due to extreme temperature variations and demanding environments. These PCBs are used in avionics systems, radar equipment, communication systems, and other critical electronic components where reliability is paramount.

5.High-Speed Communication Systems:

High-speed communication systems, such as data centers and telecommunications infrastructure, generate heat as a result of high-frequency data transmission. High Tg PCBs are used to manage the heat dissipation and maintain signal integrity in these applications.

6.LED Lighting:

High-power LED lighting applications produce significant heat, which can affect the performance and lifespan of the LEDs. High Tg PCBs with excellent thermal management properties are employed to dissipate heat efficiently and ensure the reliability and longevity of LED lighting systems.

7.Harsh Industrial Environments:

Industrial applications often involve high temperatures, vibrations, and exposure to chemicals or moisture. High Tg PCBs are required in industrial control systems, automation devices, and other industrial equipment to withstand these harsh operating conditions.


The choice between PCB Middle Tg and High Tg PCB materials depends on specific requirements and applications. PCB Middle Tg offers a balance between cost and performance, making it suitable for a wide range of applications. On the other hand, High Tg PCB provides superior heat resistance and dimensional stability, making it ideal for high-density mounting and advanced technologies. By understanding the differences between these two options, electronics manufacturers can make informed decisions and select the most appropriate material for their specific needs.

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