Fr4 Maximum Temperature and Thermal Dissipation

January 17, 2020 Cadence PCB Solutions

Circuit board with flames coming from it

 

Pacifying my upset child isn’t a pleasant experience, particularly when I’ve suggested leaving his cherished-balloon in the car on a hot sunny day. It took me a while explaining how the heat trapped in the car causes the balloon to expand, and eventually burst, without much avail.

Eventually, I found myself getting a new balloon and a cone of his favorite ice-cream to atone for my oversight. Call that over-pampering but that’s better than dealing with an unhappy child, particularly when I’m at fault.

In PCB design, the same oversight in the temperature requirement of the PCB will cost more than balloons and ice creams. You’ll need to know how much heat the PCB could take before it starts suffering structural damage, particularly if it’s being deployed in high-temperature environments.

What Is The Maximum Temperature of FR4 PCB?

As you’ve realized, FR4 denotes the grade of the fire-retardant F4 attribute of a PCB. FR4 PCB is made of multiple layers of glass fiber epoxy laminate material. FR4 PCB is preferable amongst manufacturers, due to its consistency of physical properties.

FR4 PCB is quite robust when exposed to heat, but at a certain temperature, its physical property changes. The heat-resistance characteristic of FR4 is denoted by Tg or glass transition temperature where it changes from solid to a soft and rubbery state. Typically, FR4 PCB is rated to have a Tg of 130°C.

In other words, if a PCB rated at 130°C is heated beyond its glass transition temperature, it loses its solid form. Not only you’ll have an unstable mechanical structure, but the electrical properties of the PCB will also deteriorate beyond the rated Tg. This is why it is important to consider the fr4 maximum temperature when you’re designing PCBs for applications like oil & gas and automotive, where the temperature goes beyond the typical Tg value.

Medium & High TG-PCB: Beyond Typical FR4 Maximum Temperature

If an application demands PCB with higher Tg value, you’ll need to opt for a medium or high Tg PCBs. Medium Tg PCBs generally have a maximum temperature of above 150°C while high Tg PCBs are rated above 170°C. PCBs with higher Tg value also demonstrate better moisture and chemical resistance, as well as sturdier physical structure in heat. 

Unless specified, manufacturers will use low Tg PCB for fabrication. Medium & high Tg PCBs are generally more expensive. Materials like S1141 and S1002-M are usually used to produce high Tg PCB. Due to the higher glass transition temperature, the lamination of high Tg PCB involves a higher amount of heat. Price-wise, high TG PCBs are more expensive. 

 

Working on the circuitry of the engine of a car

Some applications require high Tg PCBs.

 

Important Considerations When Using FR4 At Its Maximum Temperature

A common mistake is to base the operating temperature of the PCB with its tG value. You should always allocate a minimum of 20°C of allowance when choosing the right FR4 PCB. For example, a low Tg FR4 of 130°C should have an operating temperature limit of 110°C. 

As PCB designers, you’ll need to be aware of thermal regulation techniques in the design. Power regulation modules emit heat and proper dissipation techniques should be applied. Employing heatsinks or thermal dissipation vias help in preventing hot spot on that stretches the temperature of the PCB beyond its limit. 

The Tg rating and the operational temperature of the PCB aren’t the only factors that determine its functionality in a high-temperature environment. The operating temperature limits of individual components must also be considered in your design.

 

Heat sinks for use on electronic circuits

Heatsink helps to regulate thermal dissipation.

 

For example, military-grade, automoative and extended temperature components have a wider temperature tolerance than commercial or consumer grades part. While components with higher temperature tolerance are more expensive, they are crucial in ensuring the reliability of the PCB circuit in the field.

As for microcontrollers, it helps to be aware that lowering its system clock usually increases its limit of maximum operating temperature. 

To optimize your design and staying clear of the FR4 maximum temperature, it helps to use reliable and flexible design and analysis software with thermal analysis capability like Cadence’s. Especially if you’re looking for a layout package with any customizable ability for vias, net management, and proper analysis, look no further than OrCAD.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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