How to Select Tg of PCB ？

What is the Tg of PCB?

In PCB manufacturing, "Tg" stands for Glass Transition Temperature. It is the temperature at which the PCB substrate material transitions from a rigid, glassy state to a soft, rubbery state. PCBs are flame-retardant (UL94 V-0) and do not burn easily; instead, they soften above Tg.

The Critical Correlation Between Tg and Z-Axis CTE (Coefficient of Thermal Expansion)

When the temperature exceeds the Tg point, the PCB substrate material (such as standard FR-4) undergoes a physical transition from a brittle, hard glass state to a soft, rubbery state. This transition inherently leads to significant dimensional changes and structural distortion of the PCB, ultimately degrading both its mechanical stability and electrical properties.

As illustrated in the thermal expansion curves above, this degradation occurs because the Coefficient of Thermal Expansion (CTE) remains relatively low and stable only when the temperature is below Tg. Once the threshold is crossed, the material's molecular links loosen, causing the Z-axis CTE to increase sharply (often by 3 to 5 times).

This rapid, uneven dimensional change exerts enormous thermomechanical stress on the plated through-holes (PTH). During automated reflow or wave soldering, such intense stress frequently induces inner-layer copper cracking, pad delamination, or severe board warpage. Therefore, selecting a higher Tg material is fundamentally about restricting this post-Tg Z-axis thermal expansion to safeguard the physical and electrical integrity of the vias.

PCB Tg Class

In PCB manufacturing, engineers select the appropriate FR-4 substrate based on the application's requirements. Standard base material is an epoxy resin system, and Tg value is the most common indicator used to classify FR-4 base material grades.

According to temperature, it is generally divided into three levels, namely the general Tg, medium Tg, and high Tg.

General Tg:

The Glass transition temperature (Tg) of low FR4 sheets is typically around 130-140℃. However, due to its excellent physical properties, well-established production technology, and affordability, it remains the preferred material for the majority of circuit board manufacturers. For example, when it comes to general 3C digital consumer electronics products, the temperature requirements are not high, and it is sufficient to choose a standard Tg value. That's it.

Generally, the processing technology of Tg plate is mature and simple, the production cost is low, and it has good economic efficiency.

Medium Tg

The medium Tg value is around 150°C. Compared to ordinary FR4 sheets, Tg150 sheets offer improved heat resistance, moisture resistance, and chemical resistance. They can operate reliably for extended periods in high-temperature, high-humidity, and corrosive environments. Therefore, Tg150 boards are suitable for use in fields with higher requirements, such as high-speed digital circuits, microwave frequency circuits, and high-frequency circuits. However, processing Tg150 plates is more challenging than processing ordinary Tg plates, and the cost of these plates is high.

High Tg

If the glass transition (Tg) value of the substrate is above 170°C it indicates that the PCB will have high heat resistance, moisture resistance, chemical resistance, constant resistance and other important characteristics. This is crucial in the lead-free tin spray process, where a high Tg value is essential.

If there are time constraints during PCB processing, multiple PCB layers, high welding temperature (≥230℃), high working temperature (above 100℃), or significant welding thermal stress (such as wave soldering), it is advisable to select a high Tg plate.

High Tg sheets are primarily used in high multi-layer printed circuit boards (10 layers), automotive industry, packaging materials, embedded substrates, precision instruments for industrial control, routers, and other industries. fields.However, it should be noted that the higher the Tg value, the higher the cost of the PCB circuit board.In general, a higher Tg value indicates better temperature resistance for the board, resulting in less board deformation and improved resistance to warpage.

However, due to the high glass transition (Tg) point, it indicates that the temperature requirements of the board are also high during processing and pressing. As a result, the particle board will be relatively hard and brittle, which will affect the quality of subsequent mechanical drilling and the electrical characteristics of it’s use to some extent. The cost should be based on your needs, taking into account the same time.

Electrical, mechanical, chemical, and thermal properties need to be considered when choosing the appropriate laminate. In general scenarios, medium and low Tg FR-4 can completely satisfy.

For multi-layer PCBs, JLCPCB currently provides high TG FR-4 options. For example,  " High Precision PCB " service can manufacture PCBs up to 20 layers, which is very useful for projects that require complex layouts and large-scale integration. This high TG FR-4 material can maintain stability in high temperature environments and provide good performance.

JLCPCB FR-4 Substrate Selection Matrix and Engineering Guidelines

To help electronics engineers make precise design choices that align with manufacturing realities, the following matrix defines the thermal capabilities and processing impacts based on JLCPCB's standard production specifications:

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How to Select TG of PCB ?

Choosing the appropriate Glass Transition Temperature (Tg) of a PCB is a critical design decision that determines the reliability, structural integrity, and electrical performance of the circuit board under both thermal assembly stress and operational environments.

It is important to remember that Tg is not the maximum continuous operating temperature of the PCB. Instead, Tg is the temperature at which the mechanical properties of the resin matrix transit from a rigid, glassy state to a flexible, rubbery state. To ensure long-term product reliability, the actual safe continuous operating temperature of a PCB should typically be maintained at least 20°C to 30°C below its specified Tg.

Here are the professional guidelines for selecting the correct PCB Tg based on JLCPCB's engineering standards:

1. Evaluate the Continuous Operating Environment: First, determine the maximum ambient temperature and internal heat generated by the device during continuous operation. If your product's internal operating temperature remains below 90°C–100°C, a standard TG 135–140 board is highly cost-effective and sufficient. However, if the continuous operational temperature climbs to 110°C–130°C, upgrading to JLCPCB’s FR-4 TG155 or TG170 is mandatory to prevent material degradation and pad lifting.

2. Analyze Component Thermal Dissipation Profiles: Check the datasheets of the high-power components you plan to use (such as MOSFETs, CPUs, or power modules). High-power components dissipate concentrated thermal energy into the localized PCB copper layers. The substrate's Tg must be high enough to handle these localized hot spots without causing resin softening, ensuring that the continuous operating temperature around these components remains well below the board's Tg.

3.Match the Substrate Class to Mechanical and Electrical Requirements: Selecting a higher Tg material directly improves the PCB's thermal stability and reduces its Z-axis Coefficient of Thermal Expansion (CTE). For multi-layer boards or designs requiring low signal distortion, choosing a high Tg material like JLCPCB's FR-4 TG170 ensures excellent dimensional stability, minimizes impedance drift under temperature fluctuations, and drastically enhances resistance to board warpage during long-term operation.

4.Account for Manufacturing Processes and Assembly Thermal Stress: Different assembly methods exert different levels of thermal shock on the PCB substrate. Lead-free assembly processes (such as Lead-Free HASL or ENIG reflow) typically peak at temperatures ≥ 230 ℃. While this peak temperature exceeds the Tg of all FR-4 materials, a high Tg board (TG170) possesses significantly higher thermal decomposition endurance, preventing severe multi-layer delamination and plated-through-hole (PTH) cracking during the intense heat of reflow and wave soldering.

FAQ about Tg of PCB

Conclusion

In short, selecting the correct Tg for a PCB requires a comprehensive evaluation of the continuous operating environment, localized component heat dissipation, multi-layer design complexity, and the thermal stress of lead-free assembly line processes. Engineers must ensure the selected PCB has a high enough Tg safety margin to guarantee long-term performance without over-engineering unnecessary material costs.