What Is High TG Flex PCB?

A high TG flex PCB is a flexible printed circuit built with materials that have a high glass transition temperature (TG)—typically above 170°C. The TG value defines the temperature at which a PCB material transitions from a hard, glassy state to a soft, rubber-like condition.

In flexible circuits, the TG value determines the substrate’s ability to resist deformation during heat exposure. If the TG is too low, the base material softens, leading to delamination, warping, or electrical failure during soldering and reflow.

PCB Type	TG Range (°C)	Typical Applications	Material Example
Standard TG	<130°C	Consumer electronics	PI + adhesive film
Mid-TG	130–170°C	Automotive control	Polyimide film，Shengyi SF305
High TG	>170°C	Aerospace, industrial	DuPont Pyralux HT, Panasonic R-F775

High TG flexible circuit boards maintain dimensional accuracy and strong layer adhesion during repeated reflow or operation at elevated temperatures.

Why TG Is Critical in Flex PCB Design?

The glass transition temperature defines the threshold at which the polymer’s molecular structure becomes mobile. When a flex substrate exceeds this point, mechanical stability and insulation strength drop significantly.

If TG is insufficient, designers may face:

1. Misalignment or expansion of copper traces

2. Cracks or microvoids in plated vias

3. Loss of adhesion between copper and the dielectric

4. Impedance drift under temperature cycling

According to IPC-4101 and ASTM D3850, high TG laminates (>170°C) are recommended for long-term exposure to high thermal stress.

6 Major Challenges in Designing High-Temperature Flex PCBs

Designing for high temperature requires balancing flexibility and thermal durability. Common challenges include:

• Thermal stress from CTE mismatch

• Copper adhesion weakening at elevated temperatures

• Micro via reliability issues under thermal cycling

• Signal impedance variation due to expansion

• Solder joint fatigue and oxidation

• Reduced flexibility in ultra-high TG materials

(Reference: IPC-2223 & NASA EEE-INST-002 guidelines)

How to Choose the Right High-Temperature Flex PCB Materials?

Selecting suitable material depends on TG value, flexibility, and cost-performance ratio.

Material	TG (°C)	Flexibility	Cost Level	Common Use
Kapton HN	360	Excellent	Very High	Aerospace & defense
DuPont Pyralux HT	260	Excellent	High	Industrial, sensors
Panasonic R-F775	210	Good	Mid-High	Automotive
AP Polyimide Film	240	Good	Mid	Automotive ECUs

When Should You Use a High TG Flex PCB?

Use high TG materials when:

Operating temperatures exceed 150°C

Products require multiple reflow or rework cycles

Devices are used in industrial temperature sensing, engine control, or power modules

Among cost-efficient options, Shengyi SF305 (TG150°C) provides good balance between flexibility, adhesion, and affordability, making it a preferred choice for industrial temperature sensors.

Advanced Testing and Reliability Methods for High TG Flex PCB

High TG flexible circuits undergo stringent validation to ensure stable performance under heat stress.

Test Type	Objective	Standard Reference
Thermal Cycling	Verify reliability after heat/cool cycles	IPC-TM-650 2.6.7
TGA (Thermogravimetric Analysis)	Measure weight loss vs. temperature	ASTM E1131
DMA (Dynamic Mechanical Analysis)	Evaluate stiffness across TG range	ASTM D4065
Peel Strength	Test adhesion between layers	IPC-TM-650 2.4.8
Flammability	Check material safety	UL94 V-0/VTM-0

Key Characteristics and Advantages of High TG Flex PCB

1. Stable dimensions under continuous heat

2. Improved soldering endurance

3. Enhanced adhesion between copper and dielectric

4. Resistance to humidity and chemicals

5. Longer lifespan during repeated heating cycles

These properties make high TG flexible PCBs essential in temperature sensors, automotive power systems, and industrial controllers.

Techniques to Improve Heat Dissipation in Flex PCB Design

For circuits facing sustained heat exposure, the following 16 techniques can help:

1. Increase copper thickness

2. Add thermal vias

3. Use thermally conductive stiffeners

4. Integrate flexible graphite sheets

5. Optimize component layout

6. Use air gaps to isolate heat zones

7. Apply high TG adhesive layers

8. Employ thermal interface materials (TIM)

9. Choose polyimide coverlays with high heat rating

10. Use ENIG or OSP finish for oxidation resistance

11. Control trace width for even heat spreading

12. Apply thermal relief patterns

13. Add stiffeners to spread localized heat

14. Design hybrid rigid-flex sections

15. Use aluminum stiffeners for power modules

16. Maintain balanced copper distribution

(Based on IPC-2221B & JEDEC JESD51 thermal design guidelines)

Applications of High TG Flex PCBs

High TG flexible PCBs are widely used in:

• Industrial temperature sensors

• Automotive ECUs and LED modules

• Aerospace systems and avionics

• Medical imaging and control equipment

• Power management and high-frequency devices

According to Grand View Research (2024), high-temperature electronic substrates are projected to grow at a CAGR of 8.6% through 2030, with strong demand from industrial automation and EV markets.

Summary

High TG flexible PCBs offer the thermal stability and durability required in industrial, automotive, and aerospace applications.While ultra-high TG materials like Pyralux HT provide exceptional resistance, cost-effective options such as Shengyi SF305 (TG150°C) deliver reliable performance for industrial temperature sensors and control systems where both heat endurance and flexibility are important.The key lies in selecting the right TG range, material structure, and lamination parameters to ensure consistent reliability throughout the product’s lifecycle.

FAQ

Q1: What does “TG” mean in a flexible PCB?

TG (Glass Transition Temperature) is the point at which a material changes from a hard state to a soft, rubber-like state. Higher TG means better resistance to deformation and delamination under heat.

Q2: Is higher TG always better?

Not necessarily. Extremely high TG materials (above 250°C) can become less flexible and more expensive. Choosing a material around 150–210°C TG often offers the best balance of cost and performance for industrial uses.

Q3: What TG value should I choose for my flex PCB?

Below 130°C: Consumer electronics

130–170°C: Automotive

170°C or above: Aerospace or continuous high-temperature operation

For industrial temperature sensors, TG150°C materials like Shengyi SF305 are often ideal.

Q4: Can high TG flex PCBs handle multiple soldering cycles?

Yes. High TG laminates are designed to resist reflow and rework cycles without losing adhesion or dimensional accuracy.

Q5: Which tests confirm high TG performance?

Common evaluations include thermal cycling, TGA, DMA, and peel strength testing according to IPC and ASTM standards.