What does the Tg value mean

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The Tg value (glass transition temperature) (Tg) is a decisive parameter for electronics engineers and PCB designers when choosing base materials. Below we discuss the important considerations for the downstream operation of the electronic assembly process and provide a rule of thumb for the appropriate Tg value.

The role of the Tg value in electronics design

The Tg value (temperature) of FR-4 is a crucial selection criterion for engineers and PCB designers. At this temperature, the resin transitions from a hard, rigid state to a soft, elastic one, causing a significant increase in Z-axis expansion (CTEz). This expansion can lead to cracks in the vias and tearing of internal conductive tracks from the via tubes. Additionally, delamination may occur, resulting in the partial separation of the press-moulded laminate, which ultimately leads to the failure of the electronic assembly or device.

Typical Tg values for FR-4 materials are:

Standard Tg 130°C
Medium Tg 150°C
High Tg 170°C

Why is there a focus on the Tg value?

The Tg value of the FR4 material used must be carefully selected to ensure that the operating temperature, e.g. the ambient temperature or the temperature in the neighbourhood of ICs, always remains below this threshold with a certain tolerance.
Typically, the Tg value should be about 20 to 30°C above the highest operating temperature of the device.

For example: The maximum temperature of a module is set at 85°C. Plus the additionally 30°C tolerance, means the Tg value of the base material should be at least 115°C. For an assembly with a maximum operating temperature of up to 125°C, plus the 30°C tolerance, means that the Tg value must be ≥ 155°C. In this case, a high Tg material of 170°C is required.

Costs vs. performance

The soldering process when assembling the electronic components places a high thermal load on the PCBs.Here, the Tg value is only a limited selection criterion. Higher Tg values do not necessarily mean higher temperature resistance during processing. It means that the probability of the PCB being damaged during the soldering process is reduced. However, higher Tg values mean higher material costs and more complex manufacturing processes.

And there is something else that developers need to bear in mind: Measures used to increase the Tg value often go hand in hand with a reduction in copper adhesion strength which means more brittle materials; hence the more complex production process at the PCB manufacturer.

In order to minimise the risk of thermo-mechanical defects, such as delamination in PCB assembly process, other parameters must be taken into account, above all the delamination temperature Td.

To provide a clear understanding of how soldering components on PCBs can impact the base material, we published a detailed article on this topic in 2013, which remains relevant today.

> How Often Can You Raise a Eurocircuits PCB to Lead-free Soldering Temperatures?

The properties of FR-4

In a separate blog we explain the properties, processing methods and interrelationships of the key parameters of FR-4 for PCB design. Understanding the interrelationships helps in the design of reliable hardware.

> What designers should know about FR-4

Eurocircuits has tested the performance of FR-4 materials specifically for lead-free soldering processes. In our pooling production we use base material with an average Tg value of 145°C, Td 350°C and CTEz 3%: Isola IS400 and NanYa NP-155-F.

> PCB from Eurocircuits standard pool PCB & Assembly Service – STANDARD pool

The most important facts summarised:

The Tg value is a decisive parameter in the selection of FR4 base materials.
The guideline is the continuous operating temperature of the final device. The Tg value should be 20 to 30°C above the highest operating temperature.
Higher Tg values offer advantages in terms of thermal stability and reliability, but make the PCB more expensive.