# Influence of Quality of Mounting Process of RF Transistors on Their Thermal Parameters and Lifetime
## 5 Results of Measurements  
The tests were carried out for four modules described as PCB1, PCB2, PCB3, and PCB4. Each of them contained two pairs of the investigated transistors. First, indirect measurements were made with an electrical method and then with an optical method. The measurements of transient thermal impedance \(Z_{\text{th}}\)(t) and thermal resistance R\({}_{\text{th}}\) of the BLC9G22 LS-160 VT transistor mounted on a heat exchanger of the dimensions \(150\times 70\times 10\) mm in a forced cooling system [45] were performed using the indirect electrical method. This method was described, inter alia, in the paper [42] and the measuring system used in the paper [45]. The measurements were made with free cooling (the pump was turned off in the cooling system) and forced cooling (the pump was turned on in the cooling system). The investigated transistor mounted on the heat exchanger is shown in Figure 9, and the obtained results are shown in Figures 10 and 11. During the measurement, the tested transistor gave off the power equal to 21.5 (blue line and red line) and 41.5 W (black line). Figure 10 shows that the steady state Z\({}_{\text{th}}\)(t) value of about 5.2 K/W was obtained with free cooling and the steady state was obtained after about 6000 s. In turn, with forced cooling the steady state occurred after about 100 s and the steady state value of Z\({}_{\text{th}}\)(t) was approximately 2.6 K/W. With forced cooling, it can be seen that doubling the power dissipated in the transistor caused a decrease in the thermal resistance value by only 0.5%. It is also worth noting that the waveforms Z\({}_{\text{th}}\)(t) determined for both the cooling conditions in the time range t < 5 s are practically indistinguishable. Therefore, it can be concluded that this part of the waveform Z\({}_{\text{th}}\)(t) describes the heat transfer from the semiconductor die to the cold plate. Simultaneously with the measurements performed with the indirect electrical method, the case temperature of the tested transistor was measured with the use of a pyrometer. Based on the obtained measurement results, values of thermal resistance between the transistor die and the surroundings R\({}_{\text{thj-a}}\) and between the case and the surroundings  
Figure 9: View of the tested transistor mounted on the heat exchanger. Rthc-a were determined for both the considered sets of cooling conditions. These results are shown in Figure 11. The difference between the mentioned parameters for both the sets of cooling conditions is about 1.1 K/W and corresponds to the thermal resistance between the die of the transistor and its case Rthjc. It is more than twice the value declared by the manufacturer. Using the measurement method described in Section 4, the temperature of the transistor case was measured with the IR camera for selected power values. For example, the temperature distributions on the surface of the case of the investigated transistor are shown in Figures 12 and 13. In these figures, the image from the IR camera is presented in its upper part. The areas with a higher temperature are lighter than those showing a lower temperature. Additionally, the temperature control lines were introduced through the transistor's longitudinal section. At the bottom of the thermograph there is a graph of the temperature distribution: in area 1--blue line; and the transistor cross-section--green line. On the bottom line of the application, one can also read the reference temperature. Figure 11: Measured values of thermal resistance of the tested transistor mounted on the heat exchanger with free (1) and forced (2) cooling. Figure 10: Measured waveforms of transient thermal impedance of the tested transistor mounted on the heat exchanger.