This paper presents an investigation of the impact of board level package underfilling on the temperature drift performance of MEMS-based inertial measurement units (IMUs). Previous research identified significant temperature-induced drifts in MEMS sensors within IMUs, leading to reduced accuracy in various applications. The MEMS sensor package used in the examined IMU is a land grid array (LGA) package. To mitigate these effects, a comprehensive temperature testing protocol was developed, covering a range from -40 °C to 120 °C, aimed at evaluating the performance of IMUs before and after the application of underfill encapsulation. The study involved both accelerometers and gyroscopes embedded in the IMUs, comparing the untreated devices with those subjected to underfilling. Results from the temperature tests indicate a remarkable improvement in performance, with significant reductions in data variability and temperature drift across the tested temperature range. Moreover, warpage measurements were performed covering the addressed temperature range to assess the mechanical stability of the devices. The application of underfill encapsulation resulted in a noticeable reduction in warpage, further supporting an improvement of the IMU's thermo-mechanical robustness. These findings suggest that thermally induced package warpage is a major cause of sensor drift that can be mitigated by underfilling. Hence, temperature-driven drift can be compensated as well as thermo-mechanical robustness can be improved leading to an enhancement of the functional and structural reliability of MEMS-based IMUs in environments subject to wide temperature fluctuations.