Perform thermal analysis of PCB mounted SOP(3)

Perform thermal analysis of PCB mounted SOP

Table 2 shows the results for maximum power (P_max) allowed in the slug-down configuration in still air with and without a heatsink for the two die-attach materials. Based on these results, we focused our next study with the more conductive die-attach material (Cookson).

We wanted to find the shortest heatsink sufficient to dissipate 10 W of heat at the die. We used the parametric study capability in the CFD software to quickly set up and solve for different scenarios.^[3] The variable parameter in this case was the heatsink fin height. The results are shown in figure 5; junction temperature (T_j) is represented by circles and case temperature (T_c) by squares. We found that a heatsink with fin height of 10.36 mm is sufficient to dissipate 10 W.

Table 2: Thermal resistance for different die-attach materials.

Figure 5: Junction temperature (Tj) and case temperature (Tc) for different heatsink fin heights.

These results prompted us to investigate further to find Pmax that could be dissipated if there were tighter constraints on the size of board and heatsink, and hence we reduced the size of both to 30 x 30 mm. We also studied the effect of different fin heights on junction-to-ambient thermal resistance, θ_ja (table 3).

Table 3: Thermal resistance vs fin height in still-air environment.

With forced airflow, the junction-to-ambient thermal resistance could be further reduced, allowing higher powers to be dissipated and T_j to be kept under 150°C. Figure 6 shows the package simulation in a forced-air environment. Table 4 shows the results for heatsink optimisation in forced air. It is interesting to see that, with forced airflow of 2 m/s, the package could dissipate over 20 W of heat for a fin height of 21 mm and 17 W with fins just 10-mm high.

Figure 6: Package with heatsink in a forced-air environment.

Table 4: Thermal resistance versus fin height in forced air. θja: junction-to-ambient thermal resistance, Pmax: maximum power.

We did a similar parametric study for the smaller heatsink with a base of 30 x 30 mm for different fin heights in forced air (table 5). This smaller heatsink with 10-mm high fins (lower weight) offered the same performance as a larger heatsink with 5-mm fin height.