## 3 Experimental Method
To better quantify the impact of cleaning time on processing time and average void area, cleaning tests from 200 s up to 600 s were performed as well (Figure 5) to find out the minimal stable process requirements (Figures 6 and 7). Higher formic acid HCOOH presence has a direct impact on cleaning the materials during the soldering process. As for the selected reference group and process settings, there is a visible improvement visible when decreasing the average void area. In comparison to the reference group, an extended time of 281 s decreased average void ratios to 1.21%, and, with a cleaning time of 401 s, to 0.8%. Higher cleaning times than 420 s (420 s, 450 s, and 608 s) have, for average void areas, similar results (0.53%, 0.47%, and 0.55%), which represents a virtual boundary for cleaning process stabilization. Moreover, the increase in temperature during this process plays a crucial role in accelerating the chemical reaction between formic acid and metal oxides. Higher temperatures enhance the reactivity of formic acid, facilitating a more efficient breakdown of oxides and other impurities on the metal surfaces. This accelerated reaction not only aids in reducing voids, but also improves the overall quality and reliability of the solder joints. By optimizing both the duration and temperature of formic acid exposure, the soldering process can achieve superior results with minimal void presence, ensuring robust and durable connections between components. This approach underscores the importance of precise control over environmental conditions to enhance the effectiveness of soldering operations. Prior to the oxide removal process, the metal surfaces were prepared using a controlled etching methodology to ensure uniform exposure and enhance the effectiveness of subsequent treatments. This preparation step involved the application of a mild acid solution to remove surface contaminants and native oxides, creating a clean and reactive surface for formic acid treatment. The experimental results demonstrate that the oxide removal process using formic acid is time-dependent. As the exposure time increases, the removal of thick metal oxides becomes significantly more effective, indicating a strong correlation between treatment duration and oxide reduction efficiency. We can assume that no contamination in the volume of solder is present because formic acid vapor can sufficiently remove the oxide layer only from the surface. Different surface structures of solder preforms were observed with the optical Microscopy LEICA DM4M. In general, surfaces with many pinholes and valleys showed worse wetting behavior and void ratios after soldering compared to surfaces with lower roughness. Solder preforms of the desired thickness are manufactured using the multiple rolling processes, which could have an impact of the surface's roughness. Rough surfaces (Figure 8, solder batch #B) tend  
Figure 4: Boxplot of C-SAM total void area [in%] for various solder batches from “Supplier A” with a comparison between two extended cleaning times, 300 s and 860 s.  
Figure 5: Individual plot for average void area [in%] for different test programs. Figure 8: Optical microscopy of the solder preform surface with 100\(\times\) magnification. Scratches and “lines” are visible on the solder surface from the rolling process of the casted alloy. Figure 9: C-SAM scan of the modules assembled with a DCB with and without soldering voids and de-wetting in the soldering joint (soldering voids and de-wetting is shown as a white area); comparison of the quality of two different preform surfaces (preform dimension 48.8 mm \(\times\) 47 mm). The soldering experiment results provide insightful data on the effectiveness of varying durations of formic acid's presence in reducing void areas within soldered units (Tables 2 and 3) and Figure 10.  
The reference group, with a formic acid exposure of 212 s, serves as the baseline, showing an average void area of 2.18%, a minimum void area of 0.74%, and a maximum void area of 4.28%.