Before you measure PCB temperature, it’s essential to determine the primary heat source in the PCB — typically the microcontroller or microprocessor — as well as to locate the temperature sensors. Once you have these pieces of information, temperature monitoring can begin. PCB temperature measurement typically involves three different steps:
Place a ground plane between the temperature sensors and the heat source.
Connect the GND pins of each temperature sensor to the heat source’s ground plane.
Make sure the temperature sensors and the heat source remain close to each other on the PCB.
Following these steps will allow you to measure the temperatures of the primary heat source — and thus of the PCB as a whole — with accuracy and precision.
How much heat can a PCB stand?
How much heat a PCB can stand depends on the materials it is composed of. Materials with optimal thermal properties provide reliable resistance against the effects of high heat, whereas some materials stand up less reliably to high temperatures. The metric known as glass transition temperature (TG) helps indicate this resistance. For instance, FR-4 has a TG of about 135 degrees Celcius.
PCBs are typically defined as high-temperature PCBs if they can withstand temperatures of 150 degrees Celsius. Some high-temperature PCBs may be able to withstand even higher heats, but boards manufactured from materials with less heat resistance will be able to operate safely only at much lower temperatures. High-temperature PCBs are becoming more and more common in applications such as automotive and industrial applications where extreme temperatures are part of the working environment.
PCB temperature is important
PCB temperature measurement is also critical because the problems that result from high temperatures do not remain localized. They can quickly spread to other components of the PCB and cause a cascade of malfunctions and damage.
Excessive heat in a PCB can cause the following types of damage:
Loss of structural integrity: Excessive heat can damage the integrity of a PCB. The layers of a PCB are highly sensitive to fluctuations in temperature, and when they get too hot or cold, they expand and contract. Excessive heat can lead to warping in the lengths, widths and thicknesses of different PCB layers.
Disruption of circuit lines: Excessive heat can result in circuit damage as well. Circuit lines expand and change shape when they overheat. Once this happens, the circuits become susceptible to frequency shifts, distortions and straight losses. Their conductor impedance can also shift from its standard value of 50 ohms. Millimeter-wave circuits and microwave circuits, in particular, have tiny, delicate components that can easily sustain damage if they expand and deform at high temperatures.