## Results  
### Inspection of Soldered Interface  
For a comparison of the NDTs on solder interconnects, two different ceramic-based white power LEDs were analyzed with the thermomechanical set-up described in Figure 1b. An Al IMS-PCB with 70 \(\upmu\)m copper, 50 \(\upmu\)m dielectric material with 4.2 W/mK thermal conductivity, and 1.6 mm aluminum core was used as the substrate, and SAC105 paste was applied via stencil-printing over a 75 \(\upmu\)m thick stencil. The package, concept, and electrical parameters of the two LED types were quite similar. Both used a ceramic carrier with three backside large-area solder pads for heat dissipation, two for electrical connection of anode and cathode, and the third as an additional insulated thermal pad. The LED dies with a surface area of approximately 1 mm\({}^{2}\) were attached to the top side copper structure of the ceramic carrier with a planar die attach for LED type A compared with gold bumps for LED type B. On top of the die was a phosphor platelet for light conversion, and the die and phosphor were laterally optically coated with titanium oxide to prevent light emission to the sides. Both LEDs were classified for 1 A with a forward voltage \(V_{f}\) of about 3 V.  
For SAM, the measurements were performed with a 100 MHz transducer from the backside of the substrate to investigate the solder layer. An inspection from top (LED first) was not possible, since different materials in the complex internal structures of the LED led to different propagation velocity of the acoustic wave and forbid an adequate gating to inspect specifically the solder layer. TTA measurements were run with t\({}_{\text{Heat}}\) = t\({}_{\text{Sense}}\) = 3 s to reach thermal equilibrium, I\({}_{\text{Heat}}\) = 1A, and I\({}_{\text{Sense}}\) = 20 mA with 10 repetitions to reduce noise. For the standard X-ray inspection, there were no noteworthy settings. Several LEDs were inspected with the NDTs. However, a detailed discussion of the results was done only for four exemplary devices with suspicious behavior in Figure 6 including X-ray images, SAM images, and \(Z_{th}(t)\) and B(z) plots. The figure can be vertically divided in the center with the left side belonging to LED type A and the right side to LED type B. For LEDs of Type A, a nonwetted area was observed for one sample (LED2), and its NDT results are depicted in Figure 6 (X2) for X-ray, (S2) for SAM, (T1) for \(Z_{th}(t)\), and (B1) for normalized logarithmic derivative. For benchmarking, the results of a good reference are additionally shown in Figure 6 (X1) and (S1) and the \(Z_{th}(t)\) and B(z) curves added to (T1) and (B1). In (X1) of the reference, the three solder joints (top, left bottom, and right bottom) show a uniform grayish color disturbed only by voids (brighter, irregular shapes) and internal LED structures, e.g., die, copper structures, and ceramic vias (darker, orderly rectangles and circles). In (X2), this was different. The top solder joint showed an irregular-shaped dark area being darker compared to the top solder joint of (X1) and the remaining area of the top pad was brighter compared to (X1). The solder was hoarded in the darker area while the brighter area held no solder and, therefore, was nonwetted, which also resulted in a tilting of the LED by 6.5\({}^{\circ}\). With SAM, the nonwetted area was also detectable in (S2) but not the hoarding, since the top pad here was not apparently different compared to the other two pads. According to the reduced soldering area, the thermal performance of the LEDs was also decreased shown in (Z1). The \(Z_{th}(t)\) curves separated at approximately 10 ms and terminated with a difference of 1.9 K/W, being an increase of over 20% in R\({}_{\text{th}}\). The same was visible while comparing the normalized logarithmic derivative in (B1) with a peak increase of 0.12. A possible reason for the nonwetted area could be a contamination of the solder pad of the LED or substrate. Figure 6: Comparison of nondestructive testing methods (NDTs) for soldered interconnects on four power LEDs.