Formic acid atmosphere vacuum Reflow Soldering imposes far more stringent requirements on Solder Paste than conventional SMT, because formic acid gas accelerates the reduction reaction of the copper substrate oxide layer at high temperatures; however, if rosin or organic acid residues in the solder paste are excessive, violent vaporization during the vacuum stage will instead cause uncontrollable microvoids inside solder joints. Therefore, during selection, we must confirm that the weight loss rate of the TGA curve of this batch of vacuum solder paste between 200℃ and 300℃ is lower than 1.8%, otherwise solder paste collapse or spattering will occur before the vacuum chamber is pumped down to 50Pa.

The stencil aperture design in the printing process cannot simply apply lead-free solder paste parameters. The metal content of formic acid Reflow Solder Paste generally reaches 90.5%±0.3%, and the viscosity fluctuation range is as narrow as 450±30Pa·s. This means that if the squeegee pressure is higher than 0.35MPa and the angle is less than 55°, solder paste will accumulate at the aperture edge and overflow outside the pad after thermal expansion during the vacuum stage. We once experienced consecutive solder bridging issues in three batches on an IGBT module production line, and finally traced the root cause to the surface roughness Ra increasing from 0.21μm to 0.37μm after stencil laser polishing, which directly deteriorated the solder paste release performance.

The formic acid concentration inside the vacuum reflow oven must be stably maintained at 1.2%±0.15% (v/v). This value has a strong coupling relationship with the halogen activator content in the solder paste. When the Cl⁻ content of formic acid reflow solder paste exceeds 380ppm, even if the formic acid concentration in the oven meets the standard, CuCl₂ residues will still be generated at the solder joint interface and induce later electrochemical migration. Therefore, each incoming batch must be retested by IC ion chromatography, and cannot rely solely on the supplier's COA report.

The reflow peak temperature setting must synchronously consider changes in formic acid partial pressure. Actual measurements show that under the conditions of 50Pa vacuum degree and 1.2% formic acid, the actual liquidus temperature of the Sn96.5Ag3.0Cu0.5 alloy will decrease by 2.3℃. If the peak temperature is still set at 235℃ under atmospheric pressure, it will easily cause insufficient solder joint wetting and uneven IMC layer thickness. We recommend tightening the peak temperature window to 232.5℃～234.5℃ and extending the holding time to 45 seconds to ensure sufficient nucleation of the Cu₆Sn₅ phase.

When operators wearing nitrile gloves replace solder paste, they must avoid the area directly below the formic acid gas outlet, because formic acid vapor will instantly generate white flocculent substances upon contact with the amine corrosion inhibitor in the solder paste. Deposition of this substance on nozzles or tracks will cause subsequent placement offset. Last week, the AOI false alarm rate on a certain production line suddenly increased to 12.7%, and the root cause was that the operator opened the vacuum chamber door without closing the formic acid injection valve.