# Influence of Quality of Mounting Process of RF Transistors on Their Thermal Parameters and Lifetime
## 6 Influence of Thermal Parameters on the Lifetime
These parameters were equal to the minimum value, the maximum value, and the average value of thermal resistance, respectively. The results for the BLC10G22 LS-240 PVT transistor are shown in Figure 18.  
It can be seen from Figure 18 that, regardless of the \(\rm R_{th}\) value, with an increase in the dissipated power \(\rm P_{D}\), the lifetime sharply shortens, and the quotient \(\rm\tau/\tau_{0}\) decreases as much as 10 times with an increase in the power from 1 to 10 W. At the power value of 30 W this time is reduced even 1000 times. For both the transistors it is clearly visible that for the highest values of \(\rm R_{th}=\rm R_{thmax}\) the changes in the value of \(\rm\tau/\tau_{0}\) are the biggest. Apart from the dependence \(\rm\tau/\tau_{0}(P_{D})\), the relationship \(\rm\tau/\tau_{0}(R_{th})\) was determined. These characteristics were calculated for several values of the dissipated power equal to 8.5, 22, and 44 W and they are shown in Figure 19. One should pay attention to the slope of the characteristics and thus changes in the value of the quotient \(\rm\tau/\tau_{0}\) especially at intervals, where there are actual \(\rm R_{th}\) values of both the transistors. For the BLC10G22 LS-240 PVT transistor, the measured value \(\rm R_{th}\) under the same conditions range from 2.1 to 3.1 K/W. For such a range of \(\rm R_{th}\) values with power \(\rm P_{D}=22\) W and \(\rm T_{j}\) about 80 \({}^{\circ}\)C, the changes of \(\rm\Delta\tau/\tau_{0}\) were 19.7 times. This proves that the lifetime in the discussed range of \(\rm R_{th}\) changes with the given parameters shortens from 2.6 to 19.7 times. As for the BLC9G22 LS-160 VT transistor, there appeared similar dependences. For the measured \(\rm R_{th}\) values of this transistor in the range from 1.9 to 3.2 K/W, the changes of \(\rm\tau/\tau_{0}\) were from 5.5 to even 50.7 times. Figure 17: Influence of temperature on the lifetime of a soldered joint depending on the size of the soldered component. Using the measurement results presented in Figure 14, the statistical distribution of the results of thermal resistance \(R_{\rm th}\) is also checked for the transistors TRA and TRB [24,37] with the dissipated power \(P_{\rm D}\leq 10\) W and \(P_{\rm D}\geq 20\) W. From the obtained distributions it can be observed that in terms of measuring thermal resistance \(R_{\rm th}\) under the described operating conditions both the transistors have similar properties, as shown in Figure 20.  
Figure 19: Relative change in the transistor lifetime as a function of thermal resistance. Figure 20: Distribution of \(R_{\rm th}\) measurement results for the TRA transistors (marked as A) type BLC10G22 LS-240 PVT and the TRB transistors (marked as B) type BLC9G22 LS-160 VT in the tested PCB1–PCB4 modules. The results of the measurements of thermal resistance \(R_{\rm th}\) presented in Figure 20 show that it does not statistically depend on the tested transistor types. Its value depends on the power dissipated by these transistors. For the transistor TRA with \(P_{\rm D}\leq 10\) W, the median of the measured \(R_{\rm th}\) values is 2.93 K/W, and for the transistor TRB type it is 2.99 K/W. For \(P_{\rm D}>20\) W the values of \(R_{\rm th}\) are even 20% lower than for \(P_{\rm D}<10\) W. The values of the median of the measured \(R_{\rm th}\) are presented in Table 2.  
From the results of the measurements presented in Table 2 it can be seen that in the tested cases, for both the types of transistors, a two-fold increase in the dissipated power from 10 to 20 W caused a decrease in the \(R_{\rm th}\) value by 15%. In order to determine how the gaps in the solder affect the parameters of the tested RF power transistors [24,37], including the value of thermal resistance and the internal temperature of the component, the 6 Sigma methodology and the Quality House analysis [39] were used. The Minitab tool includes a \"Transfer Function Identification\" function that allowed us to create a matrix graph showing the relationship between selected parameters.