# Influence of Quality of Mounting Process of RF Transistors on Their Thermal Parameters and Lifetime
## 4 Measurement Method and Set-Up  
In order to check if there is an influence of void spaces on thermal parameters of power transistors, values of their thermal resistance R\({}_{\text{th}}\) were measured. According to the classical theory [40], thermal resistance between the semiconductor die and the surroundings R\({}_{\text{th}}\) is the sum of thermal resistance R\({}_{\text{th-c}}\) between the semiconductor die and the case and thermal resistance R\({}_{\text{th-a}}\) between the case and the surroundings. The first of these components depends on the quality of the assembly process of the semiconductor die in the case and the case construction [41]. The other component depends on the external cooling system and the mounting method of the RF power transistor [20; 22]. The indirect electrical method described in [42] was used to measure thermal resistance R\({}_{\text{th}}\). This method requires the inclusion of the tested transistor in a special measuring system and periodic switching of the transistor's drain current. This measurement can be performed only for the transistor, which is not connected to a special application circuit. In order to determine the value of thermal resistance R\({}_{\text{thly-c}}\), it is necessary to measure the junction temperature of transistor T\({}_{\text{j}}\) at a predetermined value of temperature of the case T\({}_{\text{C}}\). The stability of this temperature is ensured by placing the tested transistor on a cold plate, which is a part of the forced liquid cooling system of this transistor. For the considered transistors mounted on the tested PCBs, values of thermal resistance R\({}_{\text{th-a}}\) were measured using the measuring set-ups shown in Figure 7. These systems allow separating the power of the fixed value in the tested transistors and measuring the case temperature T\({}_{\text{C}}\) in the steady state using an IR camera. Both the set-ups enable power supply of the tested transistors in such a way that it is possible to generate enough power in them to significantly increase their junction temperature. The transistor's operating point coordinates are measured with voltmeters V\({}_{1}\), V\({}_{2}\) and an ammeter A\({}_{1}\). At the set voltage V\({}_{\text{GS}}\) and voltage V\({}_{\text{DS}}\) with flowing current I\({}_{\text{D}}\) in the tested transistor, power P\({}_{\text{D}}\) is dissipated. As a result of self-heating, the junction temperature of the tested transistors T\({}_{\text{j}}\) and the case temperature T\({}_{\text{C}}\) increase. The temperature T\({}_{\text{C}}\) is measured with an IR camera. On the basis of the measured temperature T\({}_{\text{C}}\) and the information about the reference temperature T\({}_{\text{a}}\), also indicated by the IR camera, thermal resistance R\({}_{\text{th-a}}\) is determined using the formula:  
\[\text{R}_{\text{thc-a}}=\frac{\text{T}_{\text{C}}-\text{T}_{\text{a}}}{\text {P}_{\text{D}}} \tag{2}\]  
The unit of thermal resistance is K/W. \begin{table}
\begin{tabular}{c c c c} \hline
**PCB** & **Transistor** & **Total Void [\%]** & **Maximum Individual Void [\%]** \\ \hline
4 & TR1\_A & 12 & 2.4 \\
4 & TR3\_A & 16.9 & 8.2 \\
1 & TR2\_B & 18.9 & 11.7 \\
1 & TR4\_B & 16.8 & 7.2 \\
2 & TR2\_B & 14.4 & 2.9 \\
2 & TR4\_B & 16.6 & 9.2 \\
3 & TR2\_B & 19.9 & 8.3 \\
3 & TR4\_B & 9.9 & 5.6 \\
4 & TR2\_B & 15 & 8 \\
4 & TR4\_B & 21.1 & 8.7 \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._The circuit shown in Figure 7a is prepared to measure Rthc-a of the BLC9G22 LS-160 VT transistor, and in Figure 7b of the BLC10G22 LS-240 PVT transistor. The view of the test stand is shown in Figure 8.  
The following measuring instruments were used for the measurements: the Agilent E3645A series power supply, the Agilent E3633A series power supply and the Optris IR camera and the Optris PI connect software [43]. The measuring systems together with the IR camera were placed in a specially prepared chamber, isolating it from the external factors, such as blasts of air or sunlight.