The RF power amplifier demonstrators containing each one GaN-on-SiC, HEMT, CHZ015A-QEG, from UMSin SMD quad-flat no-leads package (QFN) were subjected to thermal cycles (TC) and power cycles (PC) andevaluated electrically, thermally and structurally. Two types of solders, Sn63Pb36Ag2 and lead-free SnAgCu(SAC305), and two types of TIM materials, NanoTIM and TgonTM 805, for PCB attachment to the liquid cold platewere tested for thermo-mechanical reliability. Changes in the electrical performance of the devices, namely thereduction of the current saturation value, threshold voltage shift, increase of the leakage current and degradation ofthe HF performance were observed as a result of an accumulated current stress during PC. No significant changes inthe investigated solder or TIM materials were observed.