# Influence of Quality of Mounting Process of RF Transistors on Their Thermal Parameters and Lifetime
## 7 Conclusions  
This article analyzes selected problems related to the impact of the quality of a soldering process of RF power transistors on multilayer printed circuit boards on the efficiency of the dissipation of heat generated by these transistors. The considerations were carried out for power LDMOSFETs operating in the Doherty amplifiers. Various solutions of the cases of the transistors and a different manner of their mounting on the PCB were considered. Thermal parameters of these transistors were measured for many transistors mounted on identical multilayer PCBs. The performed measurements included the determination of thermal resistance between the case and the surroundings, and the non-uniformity of temperature distribution on the surface of these transistor cases. The obtained measurement results were compared with the results of the measurements made by the indirect electrical method for one of the considered transistors operating in the forced liquid cooling system. It was shown that for all the tested transistors, the tested PCB ensures more effective cooling than a heat-sink in the form of an aluminum block of the dimensions 150 \(\times\) 70 \(\times\) 10 mm. Thermal parameters were measured for each of the tested transistors and additionally, X-rays of the soldered joints were taken. Based on the obtained images, the area of the total and individual voids under each transistor was determined. The obtained values change in a wide range. The results of the measurements of thermal resistance R\({}_{\text{th}}\) were compared with the determined void values and it was found out that there was no correlation between these values. This is inconsistent with the common opinion given in the literature that increasing the surface area of the void causes worsening in the heat removal efficiency of the transistors. The 6 Sigma method was used to statistically check whether there was indeed a correlation between the above-mentioned quantities. This analysis showed no correlation between the considered parameters. It was also shown that an influence in the dissipated power causes a decrease in thermal resistance. For the transistors of the same type mounted in the same PCB in the same mounting process, very big differences in the measured values of thermal resistance were obtained. They were equal even to 20%. It was also shown that the quality of the assembly, characterized by the R\({}_{\text{th}}\) value, has a decisive influence on the lifetime of the components under consideration. Changing this parameter by only 10% causes even a several time change in the lifetime of these elements. The risk associated with the SMT process and the manufactured product was estimated in terms of the soldering quality requirements, product specifications, and the assumptions related to its use. The compliance of the parameters of power transistors with the specification and the main functional assumptions of the manufactured product was verified. The activities related to the described validation of the influence of voids on the lifetime of the module may also allow defining the acceptable limits of unsoldered areas for subsequent, newly introduced products. It was shown that the information obtained using X-ray imaging cannot give the proper information concerning the estimated lifetime of the mounted devices. A better solution is to measure the device thermal resistance. The further work by the authors will be focused on investigating the correlation between the parameters characterizing the soldering process and the reliability of RF transistors. Conceptualization, K.G. ; methodology, K.G. and P.P. ; validation, K.G. and W.K. ; investigation, W.K. and K.G. ; writing--original draft preparation, W.K. and K.G. ; writing--review and editing, K.G., W.K. and P.P. ; visualization, W.K. and K.G. ; supervision, K.G. All authors have read and agreed to the published version of the manuscript.