The steady-state FEA thermal simulations were performed with a loss dissipation of 10 W applied on the power device and a constant temperature of 30 °C set on coldplate. It was assumed that the heat generated by device only transfers downward to the coldplate through conduction and there is no heat removal by convection or radiation mechanisms in this thermal model. The system temperature distributions of all investigated package models are illustrated in Figure 4-4. The temperature at different nodes shown in the R_θ circuit model
(Figure 3-1) can be obtained from these simulation results and used for thermal resistance calculation between different nodes. For example, the R_θJC/P was calculated using the Equation 3 shown below:

Figure 4-4 Simulation Results of Temperature Distribution for Different Packages: (a) QFN 8x8, (b) QFN 12x12 (50 mΩ), (c) QFN 12x12 (30 mΩ), (d) TOLL, and (e) D²PAK

Figure 4-5 Comparison of R_θ Simulation Results for Different Surface-Mount Packages

Based on the thermal simulation results, Figure 4-5 presents the calculated R_θJC/P values for different products packaged in various formats. It is clearly demonstrated that for the same 50-mΩ on-resistance device the R_θJC/P of QFN 12x12 package (2.64 °C/W) decreases more than 40% compared to that of QFN 8x8 package (4.77 °C/W). The 30-mΩ device in the same QFN 12x12 package performs slightly better than the 50-mΩ one mainly because the die size is larger for the lower on-resistance device. From the R_θ breakdown analysis also shown in Figure 4-5, it can be observed that the reduction of R_θJC/P for QFN 12x12 package compared to its QFN 8x8 predecessor is mostly attributed to the decrease of R_θPCB and R_θTIM. Such thermal performance improvement is realized by taking advantage of its larger thermal pad and package size which enables the use of a thermally more effective Cu mounting pad and more thermal vias in PCB design. Comparing with other bottom-side cooled, surface-mount packages (i.e., TOLL and D²PAK) used for discrete power devices, QFN 12x12 package has a lower R_θJC/P by about 16%, showing its thermal advantage.