# Influence of Quality of Mounting Process of RF Transistors on Their Thermal Parameters and Lifetime
## 6 Influence of Thermal Parameters on the Lifetime  
Temperature changes affect the lifetime of electronic circuits. This lifetime is related both to the lifetime of elements used to build electronic modules and to the degradation time of solder joints. In order to check the strength of solder joints and their degradation during the life of the devices, tests of electronic modules are performed in the properly established thermal cycles. As it is presented in [47], brazed joints are a subject to stresses resulting from thermal cycles and constant elevated temperatures (from 40 to 70 \({}^{\circ}\)C) during the normal operation of electronic modules, which leads to thermomechanical fatigue and the formation of intermetallic compounds. From the tests carried out by the authors of the cited paper on PV modules, the strength of solder joints at the operating temperatures of 85, 95, and 105 \({}^{\circ}\)C was tested in 15 min cycles [47]. The series resistance values of the PV cell obtained from the three temperature zones made it possible to calculate the activation energy for the degradation of the solder joint. The beneficial effect of Ag in the solder material in the mitigation of the solder degradation is also illustrated. The analyses show that the temperature at the ends of electronic components has a significant impact on the service life of the solder joint [48]. The strength of solder joints is also related to the size of the component. The graph of the risk of a solder joint failure due to the component size and the temperature value is shown in Figure 17.  
As it is visible, the risk of damaging the soldering joint decreases with a decrease in temperature and with an increase in the component size. This relation is a result of an increase in thermal capacitance of the component with an increase in its size. In the case of the considered RF power transistors [35; 36], such a temperature, which has a significant impact on the life of the joint, is achieved with the continuous operation of the transistors and the power dissipated, depending on the type of a transistor, from 20 to 25 W without forced cooling and thermocouples. Setting the transistor's operating point at \(\mathrm{P_{D}}=22\) W illustrates the real nature of heating of the power devices in the environment in which they must operate in a typical application. The junction temperature of the transistor, according to the Formula (1), varies linearly with the increasing power. The slope of line \(\mathrm{T_{j}}\) (\(\mathrm{P_{D}}\)) is equal to the value of \(\mathrm{R_{th}}\). Thus, it is the highest for the highest \(\mathrm{R_{th}}\) values. Figure 16: Dependences of \(\mathrm{R_{th}}\) on the case temperature for the tested transistors at \(\mathrm{P_{D}}=8.5\) W (**a**) and \(\mathrm{P_{D}}=21.5\) W (**b**). The changes in \(\rm T_{j}\) caused by power \(\rm P_{D}\) dissipation at a fixed \(\rm R_{th}\) value have a significant impact on the transistor lifetime [49; 50]. According to the Arrhenius formula [51], the lifetime \(\rm\tau\) to failure of a transistor is described by the formula [52]  
\[\rm\tau=\tau_{0}\cdot exp\left(\frac{E_{a}}{k}\cdot\left(\frac{1}{T_{j}}-\frac{ 1}{T_{0}}\right)\right) \tag{3}\]  
where \(\rm E_{a}\) is the activation energy of the damage process, \(\rm k\) is the Boltzmann constant, and \(\rm\tau_{0}\) is the lifetime of the transistor operating at temperature \(\rm T_{0}\). In order to determine the value of \(\rm\tau\), aging tests should be performed to determine the value of parameter \(\rm\tau_{0}\). In this study, the lifetime reduction factor is determined by determining the value of the quotient \(\rm\tau/\tau_{0}\). In the calculations, the value of the activation energy of the damage process \(\rm E_{a}\) equal to 1 eV was assumed. Since the value of \(\rm T_{j}\) depends on thermal resistance \(\rm R_{th}\) and power \(\rm P_{D}\), the relationship \(\rm\tau/\tau_{0}(P_{D})\) was determined for parameters \(\rm R_{thmin}\), \(\rm R_{thmax}\), and \(\rm R_{th,average}\) for both the tested transistors.