Description:
We are qualifying a potted device which uses the nRF52840 and is intended to run in ambients up to 50 °C external air temperature.
The nRF52840 uses its PWM outputs to switch high-current loads, which generates heat in nearby PCB shunt resistors. Because the unit is fully encapsulated in a thermally conductive potting compound, we would like to confirm that our thermal test methodology and acceptance criteria are aligned with Nordic’s specifications.
Parameters (per nRF52840 Product Specification – Recommended Operating Conditions):
- Max Operating Ambient Temperature (TA): 85 °C
- The maximum permissible temperature surrounding the chip.
- Max Junction Temperature (Tj): 90 °C
- Measured via the internal temperature sensor inside the nRF52840.
Test Setup:
- Thermocouple placement (before potting):
- T1: Directly on top of the nRF52840 package (primary reference).
- T2.x: On top of the shunt resistors (reference).
- T3: On the enclosure surface (reference).
- Chamber conditions:
- Device operated in a thermal chamber at 50 °C external ambient.
- Worst-case load applied:
- Maximum sustained current through shunts.
- nRF52840 operating at high RF duty cycle & TX power.
Acceptance Criteria:
- Measurements taken during steady-state after at least 45 minutes of thermal soak.
- T1 (above the MCU) remains less than 85 °C, demonstrating that the local effective ambient around the chip is within the 85 °C spec.
- The internal temperature sensor reports less than 90 °C, ensuring the junction temperature is within specification.
Request:
Could you please confirm that:
- Using 90 °C as the maximum allowed junction temperature is correct for continuous operation of the nRF52840.
- This test method is a valid approach to verify compliance with device specifications.
Thanks!
