## 3 Experimental Method
Test program descriptions are as follows:  
* Reference soldering program: Soldering program used in the previous experiment; first chamber cleaning in the HCOOH atmosphere for 60 s; temperature during HCOOH cleaning about 215 \({}^{\circ}\)C.
* Test V1: Increased temperature in the first chamber during HCOOH cleaning, equal to the increase in activation energy of a reaction between formic acid and metal oxides; temperature measured on modules should be below the SnSb\({}_{5}\) solid's temperature, which is 235 \({}^{\circ}\)C; dwell time after HCOOH insertion was 120 s; the second chamber process was without changes. * Test V2: First chamber: dwell time after HCOOH insertion was 120 s; second chamber: after process start, there is a mixture of HCOOH from the first chamber and N\({}_{2}\) from the second chamber; the first step is to create soft evacuation, flooding with HCOOH up to 30 L/min; after reaching final pressure 90 s dwell time for oxide reduction; temperature measured on modules should be below 220 \({}^{\circ}\)C; then evacuate to 5 mbar, flood with N\({}_{2}\) to 100 mbar, and then perform the standard steps of the base program. * Test V3: First chamber dwell time after HCOOH insertion was 120 s; second chamber: after process start, in the chamber, there is a mixture of HCOOH from the first chamber and N\({}_{2}\) from the second chamber; first step is the soft evacuation (longer flooding and more fresh HCOOH compared to version 2), flooding with HCOOH up to 30 L/min; after reaching pressure, 90 s is the dwell time for oxide reduction; temperature measured on modules should be below 220 \({}^{\circ}\)C as well; evacuate to 5 mbar, flood with N\({}_{2}\) to 100 mbar, and then perform the standard steps of the base program. * Test V4: First chamber: dwell time after HCOOH insertion was 240 s; second chamber: without changes. * Test V5: First chamber: HCOOH lower flooding with 60 s dwell time; the idea of using a lower flow rate is the intention to possibly increase HCOOH concentration in the carrier gas; the longer flooding time is also the reaction time; second chamber: evacuate to 5 mbar, HCOOH flood with the slow flow rate up to 15 L/min, evacuate to 5 mbar, flood with N\({}_{2}\) to 100 mbar, then perform the standard steps of the base program. Overall, the results clearly indicate that increasing the duration of formic acid presence significantly enhances void reduction in soldered units (Figure 13). This improvement is likely due to more effective chemical reactions between formic acid and metal oxides, lead ing to the better cleaning and preparation of surfaces for soldering. This data underscores the importance of optimizing formic acid exposure time to achieve high-quality solder joints with minimal void presence, and also highlights the substantial benefits of prolonged exposure in improving soldering outcomes. Highly flexible diffractometer enabled us to perform various types of X-ray diffraction analysis and reflectivity measurements of thin films with thicknesses ranging from micrometers down to a few nanometers. A diffractometer, Bruker D8 DISCOVER, equipped with a rotating Cu anode operating at 12 kW, was used (standard \(\theta/20\) and Grazing incidence X-ray diffraction) for soldering preform analysis to identify the impurities on the surface that were removed by formic acid in the soldering process. For analysis, we chose a solder preform with yellowish surface discoloration that showed poor wetting and a silver preform surface that showed satisfactory wetting (Figure 5). Results from the X-ray diffraction analysis are shown on Figure 14. On the preform with a yellowish surface, we observed antimonoxyde Sb\({}_{4}\)O\({}_{6}\). The thickness of the oxide was not measured. Figure 13: X-ray diffraction analysis of soldering preforms with yellowish surface discoloration.