## Results
### Inspection of Adhesive Interfaces

Table 1: Average termination value of each group normalized to HS-M or LS-M of M1. The termination value does not represent R\({}_{\text{th}}\) since thermal equilibrium was not reached. Figure 9: (**a**) Image of the inspected power inverter module in a semi-fabricated state for adhesive investigation with five half bridges on five DBCs attached with nonconductive thermal adhesive to an aluminum body. The sixth DBC holds peripheric components. (**b**) Schematic description of adhesive manipulations on a single DBC/half bridge used for grouping. No difference between the manipulations was recognizable for X-ray inspection in (X1) to (X5) meaning that this method was inadequate to detect adhesive defects. The density of the adhesive was too small compared to the surrounding high-density metals of the aluminum body and DBC to have significant influence. With SAM in (S1) to (S5), a failure detection in the adhesive layer was partly possible. Especially distinct was the manipulation for M3, where the removed adhesive on the left side appeared as a white area. A difference between M4 and M5 with opposite tilting directions was also detectable with a brighter section for the lifted area of M5 without a homogenic adhesive distribution. However, SAM was only useful in areas of the DBC without components or wire and ribbon bonds, disturbing the measurement by incorrect reflection angles or absorption, appearing as black shadows in the images. Therefore, for M2, no difference was detectable since the manipulated area was directly below the HS-M.  
With TTA, all manipulations were detectable, including M2 having the strongest thermal degradation but not visible with SAM. All \(Z_{th}(t)\) curves overlapped in (Z) until approximately 100 ms. Afterward, the thermal adhesive defined the behavior and the curves following different courses depending on the type of manipulation. The tilt toward the HS-M of M4, thus, showed no degradation since there was no mechanical difference compared to the reference. A tilt to the opposite direction of M5 instead had a high influence on HS-M because the lift-off was directly below. Figure 10: Comparison of NDTs on adhesive interconnects. (**XI**) to (**X5**) show X-ray images of one high-side MOSFET (HS-M) of each module and (**S1**) to (**S5**) show the SAM image with a 30 MHz transducer from the same HS-M. Markings of the manipulation types are included. The \(Z_{th}(t)\) of these HS-Ms are plotted in (**T**) normalized to the reference M1 and the normalized logarithmic derivative in (**B**), with a normalization interval between 0.1 ms and 1 ms, and measured peak increases compared to M1 of 1.04 for M2, 0.68 for M3, \(-\)0.037 for M4, and 0.54 for M5 were measured. M3 showed only a slight increase affirmed by a squeezing of the thermal adhesive into the removed areas and, therefore, an increase in area and a reduction in height. A transformation of \(Z_{th}(t)\) using the normalized logarithmic derivative showed the same results with different peak heights in (B). Comparing the average termination values in Table 1 reveals that, with TTA, a distinction of manipulations was possible. By taking HS-M and LS-M into account, an identical trend was not observed for the two groups. It is notable that all tilting directions showed different behaviors, even for a tilt to the left or to the right. For tilting to the left (M6), the LS-M was more strongly depredated than by tilting to the right (M7). The reverse effect applied to the HS-M, where tilting to the left had a smaller influence. In conclusion, X-ray was not applicable for adhesive investigation. The high-density material surrounding the low-density adhesive prohibited detection. With SAM, evaluation was possible only for areas without disturbing components or wire and ribbon bonds. This was problematic since the adhesive degradations directly below the MOSFETs had the highest thermal impact. An inspection with TTA seemed the most promising, whereby All defects were detected and also distinguishable.