MPS (Monolithic Power Systems) defines thermal resistance as a chain of $R_\theta$ values that must be summed from junction to ambient. Each resistor represents a physical interface or material.

• $R_{\theta JC}$: Junction-to-Case — from die to package surface. Datasheet value, cannot be reduced.
• $R_{\theta CS}$: Case-to-Sink — thermal interface material (TIM/grease). Minimize with good TIM and torque.
• $R_{\theta SA}$: Sink-to-Ambient — heatsink performance. Depends on fin count, airflow (LFM).
• $R_{\theta JA}$ (JEDEC): Total junction-to-ambient. Used for single-layer PCB with no heatsink reference.

Parallel paths: When heat can flow via multiple routes (heatsink + PCB copper pour), combine as parallel resistances: $\frac{1}{R_{eff}} = \frac{1}{R_1} + \frac{1}{R_2}$.

Derating: MPS recommends keeping $T_j \leq T_{j,max} - 20°C$ for full reliability. Design margin is $P_{derate} = \frac{T_{j,max} - T_a}{R_{\theta JA}}$.