The Working Principle of Vacuum Reflow Oven

1. Vacuum Environment and Atmosphere Control

• Vacuum Pumping and Gas Replacement: Before soldering, the furnace chamber undergoes multiple vacuum pumping cycles (to a pressure of 1-100Pa, for example) and is filled with reducing gases like nitrogen or formic acid. This replaces the internal atmosphere, reduces residual oxygen, and prevents solder oxidation.
• Advantages of Low Pressure: The vacuum environment eliminates thermal conduction interference from gaseous media, enabling more accurate temperature control. At the same time, it promotes the escape of bubbles from the molten solder, reducing the solder joint void rate (which can be controlled below 5%).

2. Infrared Radiation Heating and Temperature Profile

• Heating Method: Infrared quartz lamps are used to directly radiate a silicon carbide heating plate, or a hot air circulation system is adopted. This ensures an extremely small temperature difference (within ±1℃) on the PCB surface, achieving uniform heating.
• Multi-stage Temperature Control:
  • Preheating Stage (100-180℃): Evaporates flux volatiles to avoid thermal shock.
  • Soldering Stage (e.g., 245-260℃): After the solder melts, it remains in a molten state under vacuum conditions. Negative pressure (e.g., 0.1Pa) is applied to expel bubbles.
  • Cooling Stage: Controls the cooling rate to optimize the microstructure of solder joints.

3. Process Optimization and High-Reliability Soldering

• Reducing Gas Assistance: A stream of formic acid is introduced at 160-180℃ to activate the metal surface and enhance solder wettability.
• Positive-Negative Pressure Process: Some new-type equipment integrates alternating positive and negative pressures (e.g., vacuum pumping after applying a negative pressure of 50,000-100,000Pa). This further reduces voids and is suitable for high-density packaging (such as BGA and CSP).

4. Application Scenarios and Advantages

• Applicable Fields: It is used in fields with extremely high requirements for solder joint reliability, such as military electronics and aerospace. It can solder high-melting-point solders like gold-tin.
• Core Advantages:
  • Low Void Rate: Through vacuum degassing, the void rate is significantly lower than that of traditional reflow soldering.
  • Oxidation-Free Soldering: The vacuum or reducing atmosphere prevents solder oxidation.
  • Compatibility with Complex Materials: It is compatible with gold-tin solders for components such as ceramic substrates and metal-packaged devices.

Summary