What is Tg? Tg is one typical term that emerges when discussing the mechanical properties of glass, plastics, or polymers.
Abbreviated as Tg, glass transition temperature is critical, especially when picking circuit board materials. So why does it matter? Read on to learn more!
What Is Tg?
Tg is a mechanical property that defines a glass or polymer’s transition temperature when the material changes state.
This state change is from solid and glassy to a viscous/rubbery condition.
The transition is reversible and occurs via a vitrification process. Vitrification involves supercooling the viscous, rubbery material back into the glass state.
Units for Glass Transition Temperature
Since glass transition temperature involves heating, the units for Tg are:
- K (Kelvin)
- °C (Degrees Celsius)
- °F (Degrees Fahrenheit)
The transition temperature usually varies between 170K (-103°C) and 500K (227°C) for most synthetic polymers. This variation is due to polymer chain mobility.
Factors Affecting Glass Transition Temperature
The primary factors that affect glass transition temperatures include the following.
Chemical Structure
This factor encompasses the following.
- Molecular Structure: Injecting bulky, inflexible side groups decreases the material’s mobility, which raises the transition temperature.
- Molecular Weight: Increasing the molecular weight of straight-chain polymers lowers the chain-end concentration. This action reduces the free volume at the end group region, which increases the Tg.
PET granules and their chemical formula
- Polar Groups: These groups increase intermolecular forces, cohesion, and interchain attraction, which raises the Tg due to the decreased free volume.
- Chemical Cross-Linking: Increasing the cross-linking lowers the polymer’s mobility. The result is an increase in Tg due to the reduced free volume.
Moisture Content
Water creates hydrogen bonds between the polymer chains, which increases the distance between chain structures.
So moisture increases the free volume, which lowers the Tg.
Plasticizers
Like water, plasticizers space polymers further apart, increasing the material’s free volume.
The spacing makes the polymer chains slide past each other more easily, becoming rubbery at lower temperatures. So adding plasticizers lowers the Tg.
Entropy Effects
Entropy is usually high in amorphous materials but low in crystalline polymers. If entropy is high, the transition temperature is also high.
Free Volume and Pressure
A pressure increase around the material compresses it, reducing the free volume and raising the Tg.
Other factors include the following.
- Alkyl chain length
- Polymer chain flexibility
- Branching
- Bond interaction
- Film thickness
Which Polymers Undergo Tg?
As stated earlier, Tg depends on the polymer’s chemical structure.
Polymers consist of long molecule chains, and the crystallinity of these polymers defines their chemical structure.
This chemical structure can be either of the following.
Amorphous Polymers
Amorphous materials have random molecular structures that lower the glass transition temperature.
Their random structure means the polymeric chains inside entangle physically and have gaps between them or at the ends.
Also known as the free volume, this space enables the polymers to move at lower temperatures, hence the low glass transition temperature.
And the more the free space, the lower the Tg.
Pure amorphous polymers don’t have a melting temperature. They only have a glass transition temperature.
So as the temperature increases, these materials soften and become more rubbery.
Typical examples include PMMA, PVC, and PC. The materials are more sensitive to stress failure because they contain hydrocarbons.
A comparison between amorphous, crystalline, and semi-crystalline polymers
Crystalline Polymers
Crystalline materials are the opposite of amorphous structures because they contain highly ordered molecular structures.
So they do not soften and become rubbery as the temperature increases. Instead, they melt at a narrow, defined melting point.
Therefore, these materials have no glass transition temperature but have a melting point.
This point is usually higher than the Tg for amorphous materials.
Typical examples include POM, PET, and PEEK.
Semi-Crystalline Polymers
Semi-crystalline materials combine amorphous and crystalline (random and ordered) structures.
So the polymers have a glass transition temperature and melting point. And the Tg is lower than the melting temperature.
The ordered crystals limit the movement of the polymer chains, reducing the free volume inside. So the materials have a higher glass transition temperature than amorphous polymers.
Low Tg vs. High Tg Polymers
Remember, only amorphous and semi-crystalline have a glass transition temperature when considering the chemical structure.
But putting all the factors above into consideration, you get categorizations of polymers with low and high Tgs. They include the following.
Low Tg Polymers
These polymers are naturally flexible and soft because their Tg is below room temperature. They include rubber elastomers like polyisobutylene and polyisoprene.
High Tg Polymers
High Tg polymers naturally exist in a glassy, brittle state because their transition temperature is higher than the room temperature.
They include polystyrene and poly(methyl methacrylate).
The chemical formula for polystyrene
Why Glass Transition Temperature Matters
Tg is a critical property to check because it helps with research, quality control, and development by determining these factors.
Flexibility Considerations
If your application needs a rubbery/flexible coating or adhesive, use a polymer with a Tg below the service temperature.
Service/Operating Temperature Considerations
High operating temperature applications require a high Tg polymer. But do not go too high.
Polymers can be brittle and crack or lose adhesion if the service temperature is many degrees below the Tg.
There needs to be a little flexibility to make the material last longer.
Cohesive and Shear Strength Properties
Cohesive and shear strength properties are better if the Tg is significantly higher than the service temperature. In this scenario, the polymer will remain glassy throughout the operation.
Adhesive Properties
Applications requiring polymers with adhesive properties need a glass transition temperature close to the service temperature.
This property enhances the wetting ability and adhesive bonding of the material.
Thermal and Mechanical Shock Considerations
Hard, glassy materials are not good at absorbing mechanical and thermal shock.
So applications requiring these properties need low Tg polymers to make them soft and rubbery when in service.
Methods of Determining the Glass Transition Temperature
There are three primary ways of determining a material’s Tg.
Differential Scanning Calorimetry (DCS)
This thermo-analytical technique uses a differential scanning calorimeter to check the heat flow difference in the sample material.
The calorimeter references this difference against temperature or time while programming the temperature change in a specified atmosphere.
A scientist analyzing a sample to get its glass transition temperature using the DCS method in a lab
Some test standards that determine Tg using this process include the following.
Differential Thermal Analysis (DTA)
This technique exerts a sample material to several heating and cooling cycles.
After that, it determines the temperature difference between the sample and reference materials.
And it maintains similar temperatures during the thermal cycles for both the sample and inert reference materials to ensure uniformity.
An example of a test standard that uses this method is the ASTM E794-06(2018).
Dynamic Mechanical Analysis (DMA)
DMA relies on a dynamic mechanical analyzer to measure a material’s stiffness and plot it against either of the following.
- Humidity
- Temperature
- Frequency
- Dissolution media
The analyzer operates by applying mechanical stress to the sample material and measuring the strain.
ASTM uses this method to determine the Tg with the ASTM E1640-13 standard.
A DMA graph showing the characteristics of polycarbonate when heated
Other methods include:
- Thermo-mechanical analysis
- Specific heat requirements
- Isothermal compressibility
- Thermal expansion measurement
- Heat capacity determination
- Micro-heat-transfer measurement
When Do You Need a High Tg PCB?
Multilayer, high-density, and high-frequency circuit boards generate a lot of heat.
And even if they have adequate heat dissipation mechanisms, the materials will get hot.
So you should use high Tg materials to build the substrate and prepreg layers in these circuit boards.
Ideally, a PCB should be able to handle a thermal load of at most 25°C below the glass transition temperature.
If higher than that, build the board using a high Tg material. And if the circuit operates at 130°C or more, switch to a high Tg PCB material.
The operating environment also matters when considering the glass transition environment.
If the board does not generate heat but operates in a hot area, it should have high Tg materials.
An overheating CPU mounted on a PCB (motherboard)
Lastly, you should consider the soldering process.
Most assemblers use Lead-free solders (ROHS), which have higher melting points than non-ROHS (Leaded solder).
So modern circuit boards need substrates with a high glass transition temperature to handle the high heat.
Wrap Up
In summary, glass transition temperature ties directly to a polymer’s mechanical properties, such as impact resistance, tensile strength, and elasticity.
When plotted on a graph against temperature, Tg marks a region of dramatic mechanical and physical changes to a polymer.
So it is a critical factor to consider when selecting PCB materials.