## 4 Conclusions  
This paper presents the versatile and novel application of gauge-type pressure sensors in heat-level--vapour phase soldering (HL-VPS) ovens. It was shown in a commercially available VPS oven that the gauge-type sensors could reveal the time point of vapour saturation at the PCB level and the state of the saturated vapour column precisely, improving the characterization of the VPS reflow process.  
The current findings show that the usual measurement method with temperature sensors has limitations validating the process of HL-VPS. We showed that hydrostatic pressure measurement at the suggested vertical point of the PCB assembly could distinctively reveal the four phases of the heat-level VPS method. The pressure as a function of time is representing the state of the oven; the exact hydrostatic pressure value provides information about the build-up and exact height of the saturated vapour column. The following four phases are distinguishable in the pressure curves: immersion of PCB, heating, solder-break, and cooling. The pressure sensor readings can be used to validate the heating delay settings of the oven as well. The novel measurement opens new possibilities in smart HL-VPS oven control and more precise time usage of the oven. It was shown that a saturated vapour column for practical use is presently above 50-60% of the heating power, which is a practical lower limit for soldering profiles in the acceptable time window. Our suggestion is to combine the sensor types to enable better profile and peak control, and to reduce idle times of the oven.  
It was shown that the high sensitivity gauge-type pressure sensor is applicable for leakage detection as well. The leakage on a VPS oven can cause slow but significant loss of the expensive heat transfer medium. In addition, the leakage can lead to harmful gas contamination of the surrounding working area around the oven. The presented results can not only improve current oven design principles, but they can be retrofitted for OSH (occupation safety and health) improvements in the wake of Industry 4.0 directives on the manufacturing shop floor. For future work, the application of the sensor in VPS ovens based on different heating principles (e.g., Soft-VPS [7] or Vacuum VPS [18]) is planned. Analysis of large PCBAs with increased height (e.g., power electronics) is also planned for future work. The current results can be improved with the upcoming generations of gauge sensors with narrower pressure ranges and improved sensitivity.  
Figure 11: The profiles of the pressure and temperature in a hermetically and non-hermetically sealed oven.  
**Author Contributions:** M.A.A. contributed with concept, measurements, data processing and visualization, manuscript writing; B.I. contributed with concept, manuscript writing and revisions; B.D. contributed with thermal measurement evaluation, manuscript writing and revision and A.G. contributed with concept, data processing, manuscript writing and revisions. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The research reported in this paper and carried out at the BME has been partially supported by the NRDI Fund based on the charter of bolster issued by the NRDI Office under the auspices of the Ministry for Innovation and Technology. Also this research was partially funded by NKFIH, grant number, FK 132186.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the ongoing state of the research.  
**Acknowledgments:** The research was supported by the National Research, Development and Innovation Office--NKFIH, FK 132186.  
**Conflicts of Interest:** The authors declare no conflict of interest.