How to improve the vacuum degree of vacuum brazing furnace

Understanding the Principle of Vacuum Brazing Furnaces: Why Can’t the Vacuum Level Be Increased?
The vacuum level within a vacuum brazing furnace reflects the gas pressure inside. The fundamental reason for a low vacuum level is that the “intake volume” exceeds the “exhaust volume.”
1. Excessive intake volume (leakage, outgassing):
Real leakage: Outside air enters the vacuum chamber through tiny gaps in seals, welds, valves, electrodes, and other locations.
Virtual leakage: Water vapor and oil vapor adsorbed on the furnace walls, workpieces, fixtures, and other surfaces, as well as gases contained within the material, are released during the heating process.
2. Insufficient exhaust capacity:
Degraded vacuum pump performance: Oil contamination, wear, deterioration or insufficient pump oil, and other factors can reduce the pump’s pumping speed and ultimate vacuum level.
Clogged or blocked pipes: Dust and condensation increase flow resistance in filters, cold traps, valves, and pipes.
Improper pump configuration: For example, improper startup timing of a high vacuum pump (such as a diffusion pump or molecular pump) or insufficient pre-evacuation of the foreline pump.

Conventional Methods and Maintenance Measures for Improving the Vacuum Level of a Vacuum Brazing Furnace
This is an essential part of daily work and regular maintenance.
1. Ensure Good Sealing
Sealing Rings:
Regular Replacement: Regularly replace all O-rings for static seals (such as the furnace door and observation window) and dynamic seals (such as the electrode drive) in accordance with the equipment manual or based on frequency of use.
Cleaning and Maintenance: Before each furnace start-up, carefully clean the sealing grooves and sealing ring surfaces to ensure they are free of dust, metal shavings, and other foreign matter. Apply an appropriate amount of vacuum silicone grease evenly, avoiding excessive amounts.
Damage Inspection: Inspect the sealing rings for scratches, indentations, and cracks due to aging.
Welds and Flanges: Regularly perform helium mass spectrometry leak testing to locate and repair minor leaks on the vacuum chamber housing.
Valves and Connections: Check the sealing surfaces of all vacuum valves for integrity and ensure they are properly closed. Check the vacuum seals at the water, electricity, and gas inlets for reliability.

2. Reduce Internal Outgassing (Virtual Leaks)
Regularly clean the vacuum chamber interior, heat shields, heating elements, and other surfaces with anhydrous ethanol or acetone (safety precautions apply) to remove oil and dust. For stubborn stains, lightly sand with fine sandpaper before cleaning.
Pretreatment of Workpieces and Fixtures:
Strict Cleaning: Workpieces must be thoroughly degreased, deoiled, ultrasonically cleaned, and dried to ensure they are free of contaminants.
Tooling and Fixture Selection: Prefer materials with low outgassing rates and that have undergone high-temperature degassing treatment, such as stainless steel, molybdenum, and graphite. Avoid using ordinary steel, copper (containing volatile zinc), and parts with coatings or paint.
Tooling Degassing: New or long-unused fixtures should be pre-baked and degassed at a vacuum temperature above the normal process temperature.
Using a Cold Trap (Cryogenic Trap):
Installing a liquid nitrogen cold trap between the diffusion pump and the high vacuum chamber effectively captures water vapor, oil vapor, and other vapors, significantly improving the vacuum level and protecting the high vacuum pump.

3. Ensure effective vacuum pumping of the vacuum pump group
(1) Mechanical pump (foreline pump):
Regular oil change: Regularly change the vacuum pump oil according to the frequency of use and the manufacturer’s recommendations. If the oil becomes darker, emulsified, or foam appears, it should be replaced immediately.
Check the oil level and oil quality: Always keep the oil level at the center line of the window.
Maintain the inlet filter: Prevent dust from entering the pump body.
(2) Roots pump:
Check whether the pump body temperature and operating sound are normal.
Ensure that the interlock protection function with the foreline pump is normal to avoid starting when the inlet pressure is too high.
(3) Diffusion pump/molecular pump (high vacuum pump):
Diffusion pump: Regularly change the oil to prevent oil oxidation and deterioration. Ensure that the cooling water is unobstructed and the heating power is normal.
Molecular pump: Regularly check the bearings and listen for any abnormal operating sounds. Avoid working in a vibrating and dusty environment.
(4) Piping system:
Check whether all vacuum valves can be fully opened to ensure that the air flow channel is unobstructed. Clean condensation from the cold trap and pipes regularly.

Measures to Improve Vacuum Brazing Vacuum Levels

Adding a Pre-Bake System: For brazing processes requiring extremely high vacuum levels (e.g., above 10⁴Pa), consider pre-baking the tooling and components before entering the furnace. This can significantly accelerate moisture desorption from the tooling and components.
Upgrading a Vacuum Gauge: Ensure your vacuum gauge (especially the high-vacuum gauge) is calibrated and accurate to avoid misjudgments.
Upgrading Equipment: If, despite all efforts, the ultimate vacuum still does not meet the new process requirements, you may need to consider upgrading your vacuum pump system, such as replacing an older diffusion pump with a faster molecular pump.
Improving the vacuum level of a vacuum brazing furnace requires patience and meticulous care. Establishing and strictly implementing a regular preventive maintenance plan and developing a habit of keeping detailed equipment operation records will help you quickly identify the cause of any problems, improve vacuum brazing vacuum levels, and increase your first-pass weld pass rate.