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High-quality PCBs thanks to Tg value determination

Basismaterial Leiterplatte

The reliability of printed circuit boards plays a decisive role in electronics production. Especially for applications that are exposed to high temperatures, it is essential that the materials from which the printed circuit boards are made can withstand these conditions. An important parameter for assessing the thermomechanical load capacity of printed circuit boards is the glass transition temperature, also known as the Tg value. In this article, we take a closer look at the significance 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, glass-like 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.

Graph TG-Wert bei FR4 Material

Why is the Tg value of the base material important?

The thermomechanical load capacity of a printed circuit board depends on the base material used. Base materials consist of a resin matrix, fillers and embedded glass fabrics. The resin matrix of the base material acquires its final consistency during the pressing process of the printed circuit boards (curing of the prepregs). The resin matrix is subjected to an optimum temperature-pressure-pressure 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 printed circuit boards produced have the necessary thermomechanical properties to meet the requirements of the specific application environment. Printed circuit boards are often used in environments that are exposed to high temperatures, whether in industrial applications, the automotive sector or the aerospace industry. Knowing the Tg values allows us to ensure that the PCBs can withstand the required temperatures without losing their structural integrity. A printed circuit board whose operating temperature is approx. 20 Kelvin higher than the glass transition temperature of the resin matrix becomes soft and malleable. 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. This is why 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. This 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 PCB, 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 heat absorption range (endothermic behavior) above 120° C. This range corresponds to the softening range of the resin matrix, also known as the glass transition range. 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.

Ofen mit Aluminiumtiegel
DSC: Furnace with PCB sample and reference each in an aluminum crucible

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 expansion coefficient 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 of measuring direct dimensional changes in the material in response to a change in temperature. This method can therefore 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 oscillating mechanical stress while being 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 characterizes the Tg, where the material changes 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 these are very weak. DMA can provide module data that is useful for mechanical specification. One disadvantage is the comparatively more time-consuming preparation and implementation of the test. The analysis and interpretation of the results can be more complex than with TMA or DSC.

Diagramm Tg-Wert Messkurven
Measurement curves with evaluations for determining the glass transition temperature

Glass transition temperature vs. continuous use temperature

Graph der Dauerbetriebstemperatur

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 the evaluation of the thermomechanical load capacity of printed circuit boards. By selecting materials and optimizing processes, it enables us to ensure that our products meet the requirements of demanding applications. By precisely controlling these parameters, we at KSG can produce highly reliable printed circuit boards that guarantee long-term performance under a wide range of conditions.

Any questions?

Do you have further questions on the subject of PCB design or do you need support with the layout and development process?
Then simply contact the experienced experts at our Technical Support:

Phone: +49 3721 266-555
E-mail: ts@ksg-pcb.com

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