Influence of Solder Joint Thermal Resistance on Power MOSFET Heat Dissipation

The study focuses on computing the solder joint thermal resistance ($\mathbf{R}_{\text {thSolder }}$) between the power transistor case and the Printed Circuit Board (PCB), as well as analyzing the temperature distribution in the tested sample. The simulation results were validated using measured waveforms of junction-to-ambient thermal resistance $\left(\mathbf{R}_{\text {th }(\mathrm{j}-\mathrm{a})}\right)$ for a power Metal-OxideSemiconductor Field-Effect Transistor (MOSFET). A comparison was made between the simulated and measured thermal resistances ($\mathbf{R}_{\mathbf{t h}}$) for two substrates - FiberglassReinforced Epoxy Laminate (FR4) and Insulated Metal Substrate (IMS). For the FR4 substrate with liquid cooling, the measured junction-to-ambient thermal resistance was 19.57 K/W, while the computed value was $15.96 \mathrm{~K} / \mathrm{W}$. The solder joint thermal resistance accounted for $2.63 \%$ of the computed junction-ambient thermal resistance, corresponding to 0.42 K/W. In the case of IMS, the solder joint thermal resistance had a more significant role in thermal management, as its thermal resistance constituted $13.1 \%$ of the junction-ambient thermal resistance also $0.42 \mathrm{~K} / \mathrm{W}$. The computed junction-ambient thermal resistance for IMS was $3.21 \mathrm{~K} / \mathrm{W}$, while the measured value was $3.68 \mathrm{~K} / \mathrm{W}$. This study highlights the critical impact of the solder joint thermal resistance between the transistor case and the substrate on the efficiency of heat dissipation.