The objects under investigation in this study are RF-transistors, solder joint materials and thermal interface materials (TIM). In total 50 RF power amplifier demonstrators consisting each of a PCB board containing one GaN-on-SiC, HEMT, CHZ015A-QEG, from UMS in SMD quad-flat no-leads package (QFN) were assembled for the study. Two types of lead-free solders (Sn63Pb36Ag2 and SnAgCu (SAC305)) and two types of TIM materials (NanoTIM and TgonTM 805) for PCB attachment to liquid cold plate were tested for thermo-mechanical reliability. The demonstrators were subjected to thermal cycles (TC) and power cycles (PC) and evaluated electrically, thermally and structurally. A set of 24 demonstrators were subjected to thermal stress by 2300 thermal cycles (TC) between -20°C and 80°C and remaining 26 demonstrators were subjected to the electrical stress by power cycling (PC) with a drain current of 100 mA at a drain voltage of 45 V and a cycle time of 2 min. High-frequency (HF) characterization of all the boards was done before exposure to the thermal and electrical stresses. Static electrical characterization of all the devices was performed by measuring threshold voltage and blocking voltage and output characteristics. The static electrical and HF characterization was repeated after 2300 thermal cycles and after 1100, 4700 and 14500 power cycles. In addition, the packages were inspected by optical microscopy and by 2D-Xray microscopy, in order to reveal possible failures in solder joints. The main reason for the thermal cycling tests was to test the solder joint reliability. The suggested tests were based on standard IPC 785, treating HEMT devices and solder. One thermal cycle took 113 min. One month of the accelerated test is equal to about one year use in field conditions. The rampup slope was 2-3°C/min and the dwell time was 15 min. The temperature in the chamber and the temperature on at least one board were recorded during the whole test.

The main reason for the power cycling tests was to investigate degradation of the transistor package and TIM, including the die during cyclic heating and cooling. The temperature distribution on the HEMT package and PCB was measured, several times during the test by thermal imaging and by thermocouples. Power cycling was performed at room temperature with demonstrator boards mounted on a liquid cold plate. The power cycling implies DC-test with on/off sections where the demonstrators reach steady state in-between. The cycle time was set to 2 min. The boards were ID labeled and underwent a screening procedure under which some were visually inspected, powered up and investigated with X-ray. After finishing TC and PC tests selected boards were visually inspected in white light, X-rayed and a couple of boards were subjected to cross-section investigations for failure analysis.