## 3 Results and Discussion
### Gauge Pressure Sensor Response to the Heating Control  
The heating power was set to specific values: 60%, 80%, and 100% to investigate the effect of the heating on the pressure and temperature profiles. This setting ultimately affects the actual product--the soldered PCBA and, more specifically, the joint quality. The actual wattage is masked behind the controls of the system, but it is expected that the given maximum reported power drawing of the oven is close to the actual 100% setting. Each set heating value was obtained from experience on the shop floor, from actual manufacturing. This coarse adjustment makes considerable changes in real-life production; finer tuning of the oven was therefore not considered. Figure 6 presents a selected saturated vapour height plot calculated from the pressure in Figure 5, according to equation (8). The figure also shows that the pressure curve could well define the most critical part of the heating process. The approximate peak of the pressure reaches around 100 mm height (\(\sim\)20 Pa); the peak occurs in accordance with the saturation of the temperature at the 230 \({}^{\circ}\)C. The minor overshoot on 230 \({}^{\circ}\)C suggests an insignificant boiling point drift in the applied heat transfer medium. With the detection of the pressure peak, we are able to enable a faster medium type recognition process in the working cycles of the oven. The recorded temperature is representing the boiling point of the medium when the pressure peak appears. The signal of the pressure sensor shows a decreasing trend after reaching the peak, which represents the full distribution of the vapour inside the designated workspace. The sensor plot shows a slight relaxation, which eventually settles to a flat plateau (it can be seen after 400 s in the pressure curve in Figure 5). After the initial peak of the hydrostatic pressure, the height of the column, also the available saturated vapour settles to a steady height. This means that at this point, the oven is practically generating as much vapour, as much is condensed on the PCBA. When the vapour is able to extend above the level of the PCBA, the heated TC breaks the soldering process. It is important to note that heating comes from an unsaturated mixture of vapour and air, which heats up the integrated TC to a pre-programmed level of 150 \({}^{\circ}\)C, which also shows that non-saturated vapours are used to break the process. Figure 7 shows the curves for various heating settings; the maximum of the temperature is not affected in a significant manner; only the time is prolonged. Heating power affects the resulting measured hydrostatic pressure; consequently, the height of the saturated vapour, which is an important finding--the pressure signal shows that the saturated vapour column reaches different levels when the power of heating is modified (see Tables 1 and 2). The power is presented in percentage as noted before and is suggested to be close to the 3.3 kW indicated in 2.1. It is clear that the time of the saturation depends on the heating power. In case the heating goes lower than 60%, the pressure sensor loses stable and repeatable performance, with flat response curves, meaning that the oven is working on the boundary of producing saturated vapour at the measurement height. For delicate heating, this can be a good approach in reflow soldering; however, the process becomes long and non-practical for actual mass-production. In addition, the importance of the pressure sensor decreases in such cases. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & & **Saturation Time, s** \\ \hline Heating Power, (oven & & \\ setting, \%) & 60\% & 80\% & 100\% \\ \hline Gauge-type pressure & & & \\ sensor & 477 & 282 & 186 \\ Thermocouple & 498 & 284 & 186 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of two different sensors by saturation time [s].