Purpose To improve productivity and reach better quality in assembling, measurements and proper process controlling are a necessary factor. This study aims to focus on the monitoring heat-level-based vapour phase reflow soldering (VPS), where – as it was found – different thermocouple constructions can affect the set parameters of the oven and resulting soldering profiles significantly. Design/methodology/approach The study experiments showed significant alteration of the heating profiles during the process of the reflowing using different construction of k-type thermocouples. In a heat-level-based VPS oven, polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA) gas and water-resistant, fibreglass, thin PFA and ultrathin PFA-covered thermocouples were tested with ±1 °C precision. The oven parameters were swept according to the heating power; the length of the introduced thermocouple cables was also taken into account. An FR4-based sample PCB was used for monitoring the temperature. Findings According to the results, due to the mass and volume of the thermocouples’ wires, different transients were observed on the resulting soldering profiles on the same sample PCB. The thermocouples with lower thermal mass result in faster profiles and significantly different heating factor values compared to the thermocouples that have larger thermal mass. Consequently, the length of the thermocouple wires put in the oven has also considerable effect on the heat transfer of the PCB inside the oven as well. Originality/value The paper shows that the thermocouple construction must be taken into account when setting up a required soldering profile, while the thermal mass of the wires might cause a significant difference in the prediction of the actual and expected soldering temperatures.