The reliability of printed circuit boards plays a crucial role in electronics production. Particularly in applications that are exposed to high temperatures, it is essential that the materials from which the PCBs are made can withstand these conditions. An important parameter for assessing the thermomechanical resilience of PCBs is the glass transition temperature, also known as the Tg value. In this article, we take a closer look at the importance of the Tg value.
What is the glass transition temperature (Tg)?
The glass transition temperature is a critical parameter for evaluating the temperature resistance of polymers. It marks the transition from a brittle, glassy state to a plastic, soft state. In the context of printed circuit boards, the glass transition temperature corresponds to the point at which the resin material from which the printed circuit board is made begins to soften and change its mechanical properties.
- Tg = kink or inflection point in the curve
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Tg depends on the degree of cross-linking of the resin matrix
(number of epoxide groups per molecule) - The higher the degree of cross-linking, the higher the Tg
- The Tg value only provides information about the continuous operating temperature of the PCB
- The values are usually between 130° C and 260° C
Why is the Tg value of the base material important?
The thermomechanical load capacity of a PCB depends on the base material used. Base materials are made up of a resin matrix, fillers and embedded glass fabrics. The resin matrix of the base material acquires its final properties during the pressing process of the PCBs (curing of the prepregs). The resin matrix is subjected to an optimum temperature-pressure-pressing regime and a three-dimensionally cross-linked polymer structure is created, which has the desired properties in terms of temperature resistance and strength.
Tg value base material and ∆Tg value printed circuit board
Even if we as PCB manufacturers already have knowledge of the Tg value of the base material before the pressing process, it is still important to check the ∆Tg value (Delta Tg) of the finished PCB. The ∆Tg value is a measure of the degree of hardening of the prepregs after the pressing process.
There are several reasons why this review is necessary:
Quality control
Checking the ∆Tg value of the finished PCB is an important step in quality control. It ensures that the manufactured PCBs have the necessary thermomechanical properties to meet the requirements of the specific application environment. PCBs are often used in environments that are exposed to high temperatures, whether in industrial applications, automotive or aerospace. Knowing the Tg values allows us to ensure that the PCBs can withstand the required temperatures without losing their structural integrity. A PCB whose operating temperature is approx. 20 Kelvin higher than the glass transition temperature of the resin matrix becomes soft and deformable. This can lead to mechanical failure phenomena such as delamination, cracks or component displacement, which impairs the reliability of the entire electronic assembly. By precisely determining the Tg values of the PCBs, potential weak points can be identified and avoided.
Process variation
During the manufacturing process, various factors can lead to variations that can affect the ∆Tg value of the finished PCB. These include fluctuations in the pressing time, the pressing pressure, the curing temperature and the material composition. For this reason, the three important parameters of pressing time, pressing pressure and temperature are automatically monitored and documented using envelope curves.
Material composition
The Tg value of the base material is a very important material parameter. It is determined by the materials used, such as resin systems, fillers and glass fabrics, which are combined in the manufacturing process of the base material. It is therefore important to determine the Tg value specifically for the material composition used.
Customer requirements
Customers often define specific requirements for the thermomechanical load capacity of their PCBs. By selecting the correct Tg value of the base material and checking the ∆Tg value of the printed circuit board, we can ensure that our products meet these requirements and guarantee customer satisfaction.
Methods for determining Tg
There are various methods for determining the glass transition temperature of printed circuit boards:
DSC - Differential Scanning Calorimetry
In differential scanning calorimetry (DSC), a PCB sample is sealed in a small aluminum crucible and subjected to a controlled heating regime. The temperature change of the sample is scanned, i.e. detected at any time with a sensor on the sample. With the help of a reference measurement, the temperature deviations can be converted into the amount of heat absorbed or released by the PCB sample. A resin matrix made of epoxy resin, which is most commonly used for printed circuit boards, shows a typical range of heat absorption (endothermic behavior) above 120° C. This range corresponds to the softening range of the PCB. This area corresponds to the softening area of the resin matrix, also known as the glass transition area. The center of the range is defined as the glass transition temperature (i.e. Tg value). If the heating process is repeated with the same PCB sample and a shift in the Tg value is detected, this is referred to as post-curing.
This means that the PCB sample tested was not yet optimally cured. In this way, pressing programs in PCB production can be adapted and optimized for specific materials.
TMA - Thermal Mechanical Analysis
In thermomechanical analysis (TMA), the physical property changes of the PCB are measured as a function of temperature under a constant load. To determine the Tg value, a sample is clamped in the TMA apparatus and heated in a controlled manner. During the heating process, the dimension or coefficient of expansion of the sample is measured continuously. Typically, the expansion rate of the material changes significantly when the glass transition temperature is reached: below the Tg the material is hard and brittle, above it is softer and more flexible. This change leads to a characteristic kink or flattening in the thermomechanical curve, which marks the Tg value. This offers the advantage that direct dimensional changes in the material are measured in response to a change in temperature. Therefore, this method can even be used with very small samples. However, with particularly weak transitions, the sensitivity of the measurement can be lower than with DMA and DSC.
DMA - Dynamic Mechanical Analysis
In dynamic mechanical analysis (DMA), a sample of the PCB base material is subjected to an oscillating mechanical stress while it is heated in a controlled manner. The DMA records the changes in a storage modulus (measure of stiffness) and in a loss modulus (measure of damping). A marked drop in the storage modulus together with a peak in the loss modulus indicates the Tg, where the material transitions from a glassy to a rubbery state. DMA therefore provides comprehensive information about the mechanical behavior of the PCB material (stiffness, damping) over a wide temperature range and is very sensitive in detecting transitions, even if they are very weak. DMA can provide module data that is useful for mechanical specification. A disadvantage is the comparatively more complex preparation and execution of the test. The analysis and interpretation of the results can be more complex than with TMA or DSC.
- Tg (DMA) > Tg (DSC) > Tg (TMA)
- Guide value: Tg (TMA) = Tg (DSC) - 10° C = Tg (DMA) - 20° C
- The basis for comparison in data sheets and publications is usually Tg (DSC)
Glass transition temperature vs. continuous use temperature
- Rule of thumb: Working temperature = Tg (DSC) - 30 Kelvin
- The higher the Tg, the longer the expansion coefficient remains constant over a larger temperature range
- Short-term overruns are permissible (production, soldering)
Conclusion
The choice of measurement method depends heavily on the specific application and the desired information. The determination of glass transition temperature and ∆Tg value is an essential step in evaluating the thermomechanical load capacity of printed circuit boards. It enables us to select materials and optimize processes to ensure that our products meet the requirements of demanding applications. By precisely controlling these parameters, we at KSG can produce highly reliable PCBs that guarantee long-term performance under a wide range of conditions.
Any questions?
Do you have further questions about PCB design or need support with the layout and development process?
Then simply contact the experienced experts in our Technical Support team:
Phone: +49 3721 266-555
E-Mail: ts@ksg-pcb.com
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