PCB catching fire due to high temperature.

High Temperature PCB Materials for Class 3 Applications

In electronics, PCBs are manufactured in three different categories of IPC: Class 1, Class 2, and Class 3. While Class 1 refers to general electronic products, Class 2 and Class 3 refer to dedicated service and highly reliable electronic products, respectively. IPC Class 3 boards are designed for harsh environments and are commonly used in military, aerospace, and medical applications. Harsh environments include temperature or humidity fluctuations, electromagnetic interferences, or power surges (lightning or man-made). Despite these odds, Class 3 boards are expected to perform vital functions without any downtime. Thus, designers must consider a myriad of factors during fabrication, including the effect of external temperatures on PCB materials.

Considerations for High Temperature PCB Materials

For a new design engineer, opting for materials with high temperature-bearing capacity is the obvious choice. But it’s not so easy. You have to consider board design, the application type, and the environment as well. It is also imperative to know the temperature at which the material starts to expand or chemically decompose. In this regard, here are some factors to consider when selecting a board material for high-temperature applications.

Thermal conductivity (K)

Thermal conductivity refers to the ability of a material to conduct heat through its body. Better conductivity indicates that the material can conduct more heat outside and thus can handle higher temperatures. For instance, boards used for lighting applications are composed of aluminum at the core so that they can conduct more electricity emanating from LEDs and heat regulators.

Glass Transition Temperature (Tg)

Glass transition temperature indicates the stiffness of the material. In datasheets, you’ll typically find Tg mentioned as a temperature range indicating the value at which the board starts to heat up and the polymer substrates start to soften. This value is extremely useful when there are vias on the board that may fracture during thermal expansion. For high-temperature applications, the Tg value should be above 170 degrees Celsius. It’s common practice to select a PCB material with a higher Tg value than your application requirement (a minimum difference of 20 degrees Celsius, at least).

Dissipation Factor (Df)

Also known as the loss tangent, this value helps project the expected loss level in signal value as it passes through the board. Designers use this value to acknowledge the limit of cooper tracing required on the board.

Coefficient of Thermal Expansion (CTE)

CTE indicates the rate of a material’s expansion as the temperature increases. It is used to predict the linear increase in height of a material in response to a known temperature condition. Materials with a high Tg value tend to have low CTE. By opting for high temperature PCB materials, you tend to avoid potential errors occurring due to z-axis expansion such as pad lifts, via cracks, and corner cracks.

Decomposition Temperature (Td)

As the name suggests, decomposition temperature indicates the extreme triggering temperature at which a board may start to chemically decompose. Once the board starts to decompose, the loss is irreversible. Thus, your PCB material temperature (T) should be lower than the stated decomposition temperature (ideally, Td < T < Tg).

Now, let’s discuss some of the commonly used high temperature PCB materials.

Ideal High Temperature PCB Materials

In addition to the above, choosing the right high temperature PCB material also depends on the application environment. Boards for high-heat application environments may require different materials than those deployed in toxic chemical environments. Here are some of the most popular high temperature PCB board materials listed in order of high to low thermal conductivity measured in Watts per meter-Kelvin (W/mK).

Ceramic (25 W/mK)

Ceramic PCBs can safely operate in temperatures up to 350°C. With high thermal conductivity and low CTE, ceramics are reliable for high-power circuits, transmission/receiving modules, and memory modules. They are also naturally resistant to hard chemicals and moisture.

High-temperature ceramic products:

  • RO4500 (Tg = 250°C) from Rogers

Aluminum (2 W/mK)

PCBs with the regular FR4 and aluminum cores are your best bet for high-temperature environments. The presence of a metal substrate enables rapid heat dissipation, unlike CM3 or FR4. Thus, aluminum PCBs are ideal for the thermal contraction and expansion of components as well. Even when under exposure to wide temperature fluctuations ranging from 20 °C to 170 °C, aluminum’s dimensional change remains between 2.5 ~ 3.0%.

High-temperature aluminum product:

  • VT-4A1 (Tg = 250°C) from Ventec

BT-Epoxy (0.35 W/mK)

BT-epoxy is a popular high temperature PCB material for multilayer board applications. The combination of Bismaleimide/Triazine (BT) and epoxy resin makes for excellent thermal, mechanical, and electrical qualities. BT-epoxy is primarily used for designing PCBs for high-voltage applications and high-layer count printed wiring boards.

High-temperature BT-epoxy products:

  • G200 (Tg = 180°C) from Isola

PCBA Manufacturing for Extreme Environments - Part 1

Download Now

FR4 (0.25 W/mK)

In China, FR4 is the most widely used PCB base material next to FR1 and FR2. But FR1 and FR2 are typically used for 1-layer PCBs as they are not ideal for passing through holes. FR3 is not recommended for building multi-layer PCBs; FR4 is the best option. FR4’s popularity can be attributed to its flexibility. With it, PCB companies can create various types of single-layer and multilayer PCBs—leaving management and quality control much easier, and eventually reducing costs.

FR4 material is otherwise known as glass-reinforced epoxy laminate. As opposed to a general FR material, high-Tg FR is one of the most frequently used high temperature PCB materials. “FR” stands for Flame Retardant, which indicates the ability of the material to resist ignition during prolonged exposure to heat. FR4 PCBs maintain their quality in dry and humid environments. Moreover, FR4 has a very low coefficient of thermal expansion, impressive dielectric properties, and doesn’t absorb moisture like other materials.

High-temperature FR4 products:

  • TU-752 (Tg = 180°C) from Taiwan Union
  • IT-180A (Tg = 175°C) from ITEQ
  • N4000-6 (Tg = 175°C) from Nelco
  • KB-6168 (Tg = 180°C) from Kingboard
Tempo's Custom Avionics for PCB Manufacturing Service
  • AS9100D and IPC J-STD-001E with Space Addendum certified manufacturing processes.
  • ISO-9001, IPC-600 and IPC-610 commitment to quality certifications.
  • Execute your full development cycle from proto to validation, NPI, and low volume production.
  • Accurate quote in less than a day.
  • DFX support, including DFM, DFA, and DFT from Day 1 of design.
  • Entire turnkey PCB manufacturing in as fast as 4 days.
  • Extreme space environment targeted manufacturing.
  • Use reputable components suppliers to ensure quality, security and traceability.
  • Performs multiple automated inspections during PCB assembly to ensure quality for prototyping.

Selecting high temperature PCB materials can be overwhelming for new designers. In this regard, partnering with a knowledgeable CM can help you select the right materials and customize the fabrication process as per your application requirements. Tempo Automation has years of experience manufacturing high Tg PCB for different application parameters and specifications.

Tempo employs a white-box turnkey PCBA manufacturing process that promotes collaboration and transparency between engineers and CMs. This allows us to quickly deliver high-quality boards for both standard and non-standard designs that meet rigorous defense and aerospace industry criteria for prototyping and on-demand production. We also provide downloadable DRC files in Altium Designer, Cadence Allegro, Mentor Pads, other CAD formats, and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the role of electronics in aerospace systems, contact us.

The latest PCB news delivered to your inbox.

Search Sign In