For this demonstration example, test vehicles were constructed by mounting a MOSFET on three different PWBs: 30x40mm PWB with 2 inner layers, 15x20mm PWB with 2 inner layers, and 15x20mm PWB with no inner layers. The other side of test coupons were then bonded to a heat sink. All impedances measured are referenced to the heat sink surface temperature. Previous tests of the MOSFET directly mounted to the heat sink revealed a junction-to-sink thermal resistance of 0.66°C/W. The following heating curve plot compares the transient response of the three test vehicles from initial heating to steady state.
The next figure shows the structure function plot associated with this heating curve data. In the structure function plot, starting from the left, all three cases show virtually identical structure function plots from 0 to 3.5°C/W. The common peak at about 0.2°C/W which is associated with thermal interface between the die and the copper heat spreader inside the MOSFET. Moving to the right, we next see a valley at about 0.6-0.7°C/W which is indicative heat flow transitioning out of the component's copper spreader. This is consistent with the previously measured thermal resistance of 0.66°C/W for the direct-mounting to the heat sink.

Moving further rightward, the peak at 1.0°C/W indicates the transient heat flow across the thermal interface between the transistor body and the PWB. The peak is associated with both the change in material properties as well as the significant change in the heat flow geometry. Continuing further to the right, a broad anti-peak occurs between 2-3.5°C/W indicating the transient two-dimensional spreading within the PWB. Note that all three cases show virtually identical structure function plots from 0 to 3.5°C/W despite that fact that from 1 to 3.5°C/W heat is substantially flowing within the three different PWBs.

Moving further rightward we begin to see a divergence in the plots for the different test vehicles. The two multilayer boards diverge at about 3.5°C/W and the two small boards diverge at about 4.3°C/W. Looking at the heating curve, we see a similar divergence at 8 seconds at an impedance of 3.5°C/W and 10.5 seconds at about 4-4.5°C/W for these cases respectively. Thus both the heating curve and structure function are in close agreement although the transitions are much more easily discernable on the structure function plot.
At impedances above 4.5°C/W, the three cases are diverging with each showing a similar peak but at different impedances. This peak indicates saturation of the transient lateral spreading within PWB and that heat is beginning to move out of the PWB in a transverse direction. Looking further to the right, the vertical asymptotes of 7.7°C/W, 9.5°C/W, and 10.5°C/W equal the final equilibrium values shown on the heating curve and would also equal the thermal resistances measured with a simple steady-state test method.

In summary, the combination of heating curve and structure function analysis when applied to a few variations of component design allow accurate interpretation of the thermal performance of various thermal designs implementations. Without multiple variations in component design, the transient response interpretations are much more speculative. Importantly, the structure function does not independently differentiate between changes in transient heat-flow geometry within a consistent material and consistent transient flow-geometry within a body composed of material with different thermal properties (conductivity and heat capacity). The peaks and anti-peaks of the structure function are indicative of both changes in material properties and flow-geometry. The final vertical asymptote of the structure function is simply the steady state thermal resistance of the component in the particular cooling configuration.