Reliability Thermal Test
Thermal reliability ensures electronic products or equipment operate under a wide range of temperature conditions without affecting user experience.
Thermal Throttling happens when CPU temperature governor starts to downscale CPU frequency or wireless power amplification to limit over all system power output.
Proper thermal design has NO thermal throttling under any practical system load conditions.
Product plastic or metal skin temperature Must be under maximum allowed temperature under worst thermal load conditions
Note: There is a safety regulatory limit to ensure product safety; however product designers should have more stringent limits to improve product competitiveness.
Define power budget for each high power output components such as processor and and wireless chips and amplifiers
Use heat sinks on large ICs such as processor.
Use thermal interference materials (TIM) to pull heat from IC to the surface effectively
Common TIM: thermal pad, thermal glues, graphite sheet, and thermal pipe.
Connect component ground pads to the main ground planes to increase thermal relief areas.
Practical Test Setup
Have sense resistor for each main IC component to log the total power per each IC
Log the individual power and total input power from all these sense resistors.
Glue thermal couple on top of each IC package to sense case temperature
Have thermal couple for bottom and top enclosure of the product
Put the device under test inside a temperature chamber that is set to the maximum ambient temperature per requirements of the product (40C is generally used for consumer electronics)
Log CPU frequencies, CPU junction temperature, all IC case temperature and product enclosure temperature under thermal stress (i.e fully load the CPU) for 30 to 90 mins.
The cpu frequencies can be read from cpuinfo in the ARM core via linux shell commands such as this:
Acceptance Criteria: CPU does not scale down, and all temperatures does not exceed IC maximum case temperature and enclosure skin temperature.
Summary & Conclusion
Thermal reliability ensures product operate under a wide range of temperature and humidity under any thermal load conditions.
Electronic product surface temperature Must under a safety limit
Electronic product performance Must not thermal throttle to affect user experience.
Develop a power budget is essential to understanding and limit thermal hot areas within a PCB board
Use thermal interference materials (TIMS) to pull heat out of product to ambient air more effectly
Thermal design is critical to product long term operability and user experience .