Full Thermal Chain

Formula

Description

The full thermal chain models heat flow from the semiconductor junction through each thermal interface to the ambient air, analogous to series resistors in an electrical circuit. Each thermal resistance represents a barrier to heat flow: Rθjc is junction-to-case (determined by the package and die attach), Rθcs is case-to-sink (determined by thermal interface material like thermal paste or pads), and Rθsa is sink-to-ambient (determined by the heatsink design and airflow). The total thermal resistance is the sum, and the temperature rise is proportional to power dissipated.

Variables

• Tj — Junction temperature (°C)
• Ta — Ambient temperature (°C)
• P — Power dissipation (W)
• Rθjc — Junction-to-case thermal resistance (°C/W)
• Rθcs — Case-to-sink thermal resistance (°C/W)
• Rθsa — Sink-to-ambient thermal resistance (°C/W)

Practical Notes

Typical Rθcs values: thermal grease 0.1-0.5°C/W, silicone pad 0.5-2°C/W, mica insulator 1-3°C/W, bare contact 1-3°C/W. Forced air convection can reduce Rθsa by 3-10x compared to natural convection. Always derate for the maximum expected ambient temperature in the end application.