Introduction

Thermal reliability ensures electronic products or equipment operate under a wide range of temperature conditions without affecting user experience.

Overview

Product performance

• • 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 Safety

• • 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.

Mitigation

• 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: 

    • more /proc/cpuinfo 

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 .