# Influence of Quality of Mounting Process of RF Transistors on Their Thermal Parameters and Lifetime
## 1 Introduction
The device junction temperature \(T_{j}\) increases proportionally to the power dissipated. The dependence of the junction temperature on the dissipated power \(P_{D}\) is described by the equation [1,16,17]  
\[T_{j}=T_{a}+R_{th}\cdot P_{D} \tag{1}\]  
where \(T_{a}\) is the ambient temperature and \(R_{th}\) is thermal resistance of the semiconductor device. The above dependence is valid at the steady state, when power losses do not change with time or change very slowly. In transient states, thermal properties of semiconductor devices are characterized by transient thermal impedance \(Z_{th}(t)\)[17, 18, 19]. The value of thermal resistance of semiconductor devices depends, among others, on the quality and the method of their assembly [20, 21, 22]. In particular, the quality of the solder connection of the thermal pad to the PCB (Printed Circuit Board) is important [23]. In the soldering process, the maximum of 30% void [24] not covered with the solder is allowed under power semiconductor devices. Additionally, according to the IPC-610 [24], other criteria related to voids should be agreed on with the manufacturer. Typically, the maximum individual void sizes are set at 8%. The paper [25] presents the results of the experimental research on the use of dies for MOSFET RF transistors in a case made of plastic materials. Unfortunately, the cited paper does not provide the results of thermal parameters measurements. Yet, it was shown that the use of plastic materials has led to a deterioration of the RF performance of these transistors. The paper [26] presents the design and the results of experimental tests of the integrated RF amplifier modulated with the WCDMA signal, operating in mobile phones. Unfortunately, in the cited paper there is no data concerning the assembly of amplifiers and external RF power stages. The paper [27] presents the results of measurements of thermal parameters of VDMOS transistors mounted on glass substrates. Attention was paid to the measurements of thermal resistance and thermal analyses carried out. In the paper [28], a junction temperature rise in GaN-HEMT RF transistors was analyzed. The dimensions of voids and their number in the brazed joint were also estimated. This parameter was taken into account in the thermal analyses carried out in the ANSYS software. In turn, the paper [29] contains the results of the research on thermal phenomena in high-power GaN RF transistors operating in the switching mode. The paper [30] presents a new non-linear electrothermal model of RF transistors of the LDMOS type. Algorithms based on neural networks are used for modeling this device. The paper [31] describes the results of the study of the correlation between a change in electrical parameters of the N-LDMOS power RF transistor and the damage that occurred after the impulse RF lifetime tests (1500 h). The tests were first carried out in real time with the use of a test stand designed for experiments with radar systems. The subject of the research presented in [32] is the influence of temperature on properties of N-channel LDMOS transistors operating in the radio frequency range, subjected to reliability tests in the temperature range from 10 to 150 \({}^{\circ}\)C.  
The article [33] presents the measuring method and exemplary results of the measurements of transient thermal impedance of microwave transistors. Particular attention was paid to the measurements in very short periods of time after a power step change. The paper [34] presents a thermal model of RF transistors taking into account voids in solder joints. The aim of this article is to investigate the influence of imperfections in the soldering process on thermal parameters of power transistors used in RF circuits. In particular, the influence of the dissipated power on the transistor case temperature is considered for different sizes of the unsoldered spaces (voids) under the tested power transistors.