## 3 Results and Discussion  
### General Validation of the Process: Observation of the Pressure Changes during Heat-Level VPS  
Initially, the validation of the process was performed with the analysis of the sensor data over a repeated setting of the oven. The phases were also followed by personal observation, which is--in general practice--not necessary, due to the automated control.  
It was found that the identification of the heat-level VPS process is possible in sufficient detail with one gauge-sensor based pressure measurement inside the workspace. During the soldering process, the output of the pressure sensor reveals the phases (PH1-4) of the process in fine details. The four phases are distinguishable by analysing the pressure signal (Figure 5). The different phases are as follows: the immersion of the PCB, heating, solder-break, and cooling (see Figure 2 for a graphical representation).  
Phase 1 (PH1) represents the immersion of the carrier lift to the depth of the workspace after the closure of the cover (initial state). The pressure decrease is due to the downward movement of the colder carrier, which causes vapour condensation and perturbs the initial state around the heater at the bottom. It should be noted that this phase is approximately similar for every setting, even if the heating power or other process parameters are changed.  
During Phase 2 (PH2), the heating phase is presented, which is the main part of the soldering temperature profile. The first part of the phase is suggested to heat the TCs with unsaturated vapour--heating with saturated vapours would result in a step-response feedback on the TC. Heating is more intense in the second part of the phase--concurrently, saturated vapour height increases. The peak of the pressure is much narrower than the temperature peak (shown at around 380 s in the case of Figure 4.) This indicates that the vapour column generation and reaching its maximum height can be easily monitored by pressure sensors (similar result was already validated on standard VPS processes but with flow sensors [11]). Usually, the control of the oven is solely based on a temperature basis. According to our findings, it is recommended to expand the general concept of  
Figure 5: The phases of the heating profile during the soldering process.  
solder profiling with the measurement of pressure peak in fusion with the signals from the temperature sensors. The pressure peaks are later observable in Figures 6 and 7 too. Generally, we can assume, that temperature-feedback based control takes a longer time at the detection of the exponential saturation--the time can be reduced, and productivity increased when the temperature is combined with pressure peak detection.  
In Phase 3 (PH3), the workpiece carrier moves up after the heating is done, to prepare the VPS for the cooling stage. This step was noted in the introduction section as the soldering \"break\". Just as in the first phase, the movement of the support can easily perturb the homogeneity and overall state of the vapour. Therefore, the pressure suddenly shows a decreasing and increasing nature in a short time.  
At the start of Phase 4 (PH4), the oven gives a signal that the PCB is ready to be taken out. Additional condensation is caused by the coolers; a significant amount of the vapour transforms back again to the liquid state, which results in a negative swing in the pressure curve. In this stage, the temperature and the pressure inside the oven return to the initial state.  
Figure 6: The height derived from pressure results and temperature measurement during the heating phase inside the workspace of a heat-level VPS oven with 80% heating power (HP).  
Figure 7: Pressure and temperature measurement with the variation of the heating power (*HP = heating power).