# Influence of Quality of Mounting Process of RF Transistors on Their Thermal Parameters and Lifetime
## 2 Investigated Transistors  
The subjects of the investigations are power transistors LDMOS and HVLDMOS manufactured by Ampleon, type: BLC9G22 LS-160 VT [35] and BLC10G22 LS-240 PVT [36] placed in the RF circuit containing the Doherty amplifier. Both these transistors are characterized by high efficiency and excellent thermal stability [35; 36]. Both the transistors are capable of operating with a load mismatch corresponding to a standing wave ratio VSWR = 10:1 (perfect match is 1:1). The manufacturer's declared value of thermal resistance between the junction and the case Rthj< of these transistors is 0.47 K/W [35] and 0.35 K/W [36], respectively. At typical application of the considered transistors are RF power amplifiers for base stations and multi carrier applications in the 2110-2200 MHz frequency range. Some data given by the manufacturer on RF characteristics of the considered transistors was obtained for the test signal characterized by: 2-carrier W-CDMA; 3 GPP test model 1 with 64 DPCH; PAR = 8.4 dB at 0.01 % probability on the CCDF; f1 = 2112.5 MHz; f2 = 2117.5 MHz; f3 = 2162.5 MHz; f4 = 2167.5 MHz. RF performance is obtained at VDS = 28 V; DQq = 1600 mA; T\({}_{\rm C}\) = 25 \({}^{\circ}\)C; in a water-cooled class-AB test circuit. In such conditions, the following typical values of parameters are obtained [36]:* Power gain \(\rm G_{P}=18.5\) dB at the dissipated power \(\rm P_{L}=60\) W;
* Drain efficiency \(\rm\eta_{D}=27\%\);
* Input return loss \(\rm RL_{in}=-14\) dB;
* Adjacent channel power ratio (5 MHz) \(\rm ACPR_{SM}=-30\) dBc. The cases of the investigated transistors are shown in Figure 1.  
Both the transistors have gold-plated terminals, which, according to the experience of one of the companies producing electronic equipment, has a significant impact on the quality of SMT (Surface Mount Technology) [37]. The area of the thermal pad (transistor sources) is 201.88 mm\({}^{2}\) for both the transistors. The tested transistors are mounted on the PCB shown in Figure 2.  
This PCB has the dimensions of 245 \(\times\) 280 mm. The PCB laminate has 12 conductive layers of the thickness 75 \(\rm\upmu\)m (internal layers) or 100 \(\rm\upmu\)m (outer layers). The insulation layers are 75 \(\rm\upmu\)m thick. On the PCB there are two PA circuits containing 2 pairs of LDMOS and HVLDMOS transistors, which are marked on the main page (top) of the PCB. The BLC9G22 LS-160 VT and BLC10G22 LS-240 PVT transistors are part of the transceiver TRX, where they are placed in the PA. They play the role of an amplifier in the Doherty amplifier. They are assembled in the SMT process. There are pockets in the printed circuit board in the place where each transistor is mounted. The pocket dimensions in the PCB laminate are: x = 21 mm (max 21.6 mm), y = 10.15 mm (max 10.4 mm), and depth = 1.63 mm. The area of the pocket is only 11.12 mm\({}^{2}\) larger than the area of the transistor source soldered to the pocket. As mentioned in the Introduction, one of the applications of power transistors in RF circuits is the Doherty amplifier. In the tested modules, the considered transistors work in such amplifiers. The main amplifier, called the carrier signal amplifier (Main or Carrier), is connected to the peak amplifier (Peak). Connecting these amplifiers requires that the amplitudes of the signals from both the outputs are in the appropriate phase with regard  
Figure 1: Cases of the investigated RF power transistors and their dimensions. Figure 2: Selected parts of the PCB on which the investigated RF power transistors are mounted. to each other, which is ensured by the use of additional phase shifters on the inputs and outputs [4]. The phase change introduced by the impedance inverter is compensated for at the input of the peak amplifier. The load is connected to the main amplifier via an impedance inverter, which is typically a quarter-wave transmission line. In this way, only the Main amplifier works at a low signal level, up to 13 dBm--amplifier classes A and AB [3].