Avionics Unit Thermal Study

Overview

This case study presents a steady-state thermal analysis of a fictional avionics enclosure housing two PCB assemblies with multiple power-dissipating semiconductor devices. The objective is to predict junction temperatures (Tj) for all critical components and verify that they remain within manufacturer-specified maximum ratings after applying appropriate derating per MIL-HDBK-217 and internal derating guidelines.

The enclosure is a sealed, conduction-cooled chassis with no forced airflow. Heat generated by internal electronics transfers through the PCB substrate and card guide contact interfaces to the chassis sidewalls, and then to the external environment via natural convection and radiation. The analysis characterizes each stage of this thermal resistance path to identify bottlenecks and recommend design improvements where margins are insufficient.

Thermal Model & Boundary Conditions

The ANSYS Mechanical thermal model includes the aluminum chassis (Al 6061-T6), two PCB card assemblies modeled as orthotropic composite plates, all major semiconductor packages represented as heat sources, card guide contact conductance at the PCB edges, and thermal interface material (TIM) between the PCB and chassis rail.

The external ambient temperature is set at 71°C per MIL-E-5400 Class II operating requirements. A natural convection film coefficient of 8 W/m²·K is applied to all external chassis surfaces. Internal radiation between PCB assemblies is included using surface-to-surface radiation with emissivity values of 0.85 for PCB laminates and 0.05 for bare aluminum surfaces.

Thermal Resistance Path

The total thermal resistance from junction to ambient (Rja) is the sum of each resistance in the heat flow path: junction to case (Rjc, per manufacturer datasheet), case to PCB via solder joints or thermal pad (Rcs), PCB conduction to card edge (Rpcb), card guide contact resistance (Rcg), chassis wall conduction (Rwall), and chassis external convection (Rconv).

Thermal interface material (TIM) with conductivity of 3.0 W/m·K is applied between the component thermal pad and PCB copper pour, and between the PCB edge and the aluminum card guide rail. The card guide contact conductance is modeled as 500 W/m²·K based on published values for aluminum-to-aluminum interfaces with light clamping pressure.

Junction Temperature Results

Predicted junction temperatures are computed for each power-dissipating device using the total thermal resistance path and the device power dissipation. Results are compared against the derated maximum junction temperature, which is set at 110°C (25°C below the absolute maximum rating of 135°C) per standard derating practice for avionics applications.

Conclusions & Recommendations

Six of seven critical devices meet the derated junction temperature limit of 110°C with positive margin. The FPGA (U1) exceeds the derated limit by 2°C, and the power regulator (U5) has only 2°C of margin — both are flagged as design action items before qualification.

Recommended corrective actions are: (1) replace the TIM under U1 with a higher-conductivity material (target ≥ 6.0 W/m·K) to reduce Rcs by approximately 0.4°C/W, and (2) add a copper heat spreader layer in the PCB stackup under U1 and U5 to improve lateral heat spreading and reduce Rpcb. These two modifications are predicted to bring U1 Tj to approximately 107°C and U5 Tj to approximately 104°C, providing acceptable margin in both cases.

No chassis redesign is required. Card guide contact conductance is adequate, and chassis wall and external convection resistances are within expected ranges for a naturally cooled sealed enclosure.