Checking Emissivity Before Trusting a PCB Thermal Image

A thermal image is a map of detected infrared radiation interpreted through measurement settings. On a circuit board, plastic packages, solder, exposed copper, and metal shields can produce very different apparent temperatures even when their actual temperatures are similar. A bright or dark region needs interpretation before it becomes evidence for a thermal redesign.

Identify the surface being measured

FLIR explains in its emissivity guidance that reflected radiation and surface emissivity affect temperature readings. A suitable high-emissivity reference surface can help assess a reflective target, but its temperature must represent the target. Do not apply one assumed emissivity value to every material on a populated board.

Mark the component and surface of interest on a visible-light photograph. Record whether the camera observes a molded package, a metal lid, coating, label, or temporary reference material. The case surface is also distinct from the semiconductor junction; a surface reading does not directly establish junction temperature.

Control the image conditions

• Use appropriate focus, working distance, and spatial resolution for the target size.
• Record emissivity and reflected-temperature settings with the image.
• Check for reflections that move when the viewing angle changes.
• Keep airflow, enclosure state, orientation, and load conditions consistent.
• Allow a defined stabilization period or capture a documented thermal transient.

A small hot component may occupy too little of the measurement area for its reported peak to be representative. Increasing display magnification does not create additional optical resolution. Check the camera’s measurement requirements rather than relying on the number of colored pixels on screen.

Cross-check surprising results

If a shiny shield appears cooler than neighboring plastic, compare it using a suitable reference method before concluding that the shield is effective. Any added tape, paint, or sensor can change the local thermal condition, so keep the modification small and appropriate to the test. Record it explicitly.

Compare board revisions with the same color scale when visual differences are being discussed. Automatic scaling can make a modest temperature range look dramatic or make a serious change appear familiar. Preserve numerical regions of interest and their measurement assumptions alongside the image.

The final thermal report should separate observed surface temperature, inferred behavior, and any calculated junction estimate. Include the instrument setup and uncertainty sources that could change the engineering decision. A thermal camera is an excellent way to locate patterns and investigate heat flow, provided its attractive image is supported by a repeatable measurement of the surfaces that matter.