## 3 Results and Discussion  
The values of thermal resistance \(R_{th}\), and the waveforms of transient thermal impedance \(Z_{th}(t)\) for different samples are presented in Figures 5 and 6. In Figure 5, the small squares represent the average; the horizontal lines indicate the median; the borders of the boxes indicate the \(\pm\sigma\) standard deviation; the crosses mark the min-max values. The differences in thermal resistances were observed. Figure 5: Thermal resistance (\(R_{th}\)) of NGB8207 BN transistors soldered onto FR4 substrate in different soldering ovens. Figure 6: Transient thermal impedance waveforms of NGB8207 BN transistors soldered onto FR4 substrate in different soldering ovens. Single-factor variance analysis of the results proved that there were statistically important differences between the samples at a significance level of 0.05. The letters A, B, and C in Figure 5 mean statistically important differences between the samples. According to [PERSON]'s test, the samples soldered in a batch oven had the lowest thermal resistance, which was surprising, and they differed significantly from all of the other samples. Thermal resistance in the case of the tunnel ovens was slightly but statistically higher. The highest values were obtained from the samples soldered in the vapour phase (VPS) oven and manually with hot air. As was proven earlier, vapour phase soldering can be a very effective soldering method, especially for large-component soldering, but it has some limitations. First of all, because of the intensive heating, solder joints performed with VPS can exhibit greater voiding and tombstone failures. A vacuum option seems to be necessary for this type of soldering. Without the application of underpressure in the last 5 s of soldering, the quality and the solder joint was much worse [12; 20]. The results of the transient thermal impedance (Figure 6) showed differences in the course of the curves after 10s for the different soldering ovens, which reached 15% between the samples. This is due to the fact that solder joints are responsible for the longest thermal time constant of the IGBTs under investigation. Before 10s, the smaller differences in the curves are the only measurement errors. Figure 7 shows a thermal image of a transistor which was soldered in the batch oven. The heat distribution was similar for all the samples. Heat was effectively removed from the transistor by the liquid flowing through the heat exchanger. A large temperature gradient was observed between the edges of the sample and the middle. The temperature of the FR4 substrate started to decrease considerably from 4 mm from the IGBT component and did not exceed 30 \({}^{\circ}\)C. The temperatures of the exchanger and the liquid flowing through it were close to the ambient temperature. The detected temperature maximum was 15 \({}^{\circ}\)C higher than the measured junction temperature. This was due to the difference between junction and case temperatures of the IGBT, which was particularly visible in the conditions where the device being tested was effectively cooled [21]. Moreover, such a high value of discrepancy may also be affected by averaging the temperature over an area larger than the structure of the transistor, which is the heat source used by the infrared camera [22]. This suggests that our measurement method (Figure 4) is more accurate and suitable for microelectronics applications than the frequently used thermo-vision camera. The voids ratio for the samples is presented in Figure 8 and Table 2. While the voids were being calculated, only the area of the thermal pad was taken into account; since most of the heat was dissipated here, it is marked by a rectangle in Figure 8. The voids percentage differed significantly. The highest voids percentage area was observed for the samples soldered in a 4-zones tunnel oven, reaching 30%. For the rest of the samples, this parameter did not exceed 12%. However, the standard deviation was relatively high.