Vacuum Brazing Technology of Water-cooled Plates for Large Computer Electronic Components

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A large number of ultra-high-power electronic components are installed on large computers, and their heat dissipation problems must be solved. Conventional high-power electronic components use air-cooled heat dissipation, but it is powerless to use air-cooled heat dissipation for ultra-high-power electronic components, so a water-cooled heat sink with higher cooling efficiency must be used for cooling. The water-cooling plate requires a light and corrosion-resistant aluminum alloy as the base, on which a tortuous water flow channel is machined by a CNC machining center, and then the water flow channel is closed by brazing. The welds are required to be uniform and dense, resistant to the impact of high-pressure water flow, without the slightest leakage, and the deformation should be less than one thousandth. In this paper, an experimental study on the vacuum brazing process of the water-cooled plate of high-power electronic components is carried out.

The material used for the product is LF21 (AL-1.5Mn) standard aluminum alloy and composite plate, and the brazing material is Al-11.5Si-1.5Mg alloy. The Al-Si-Mg ternary eutectic temperature is 551℃, the Al-Si binary eutectic temperature is 577℃, the solidus temperature of the Al-Si alloy containing 9.79%～10.16%Si is 577℃, and its liquidus temperature is 577℃. The temperature was 591°C.

The main technical indicators of vacuum aluminum brazing furnace are as follows.

①Furnace type: single room, horizontal, double door

②Maximum temperature: 800℃

③Working temperature: 620℃

④Effective hot zone temperature difference: ≤±3℃ (empty furnace, 600℃, temperature measurement at nine points)

⑤Ultimate vacuum degree, ≤6.7×10-4Pa

⑥ Working vacuum degree: ≤6.7×10-4Pa

⑦Pressure rise rate: ≤0.2Pa/h

⑧ Heating rate; the empty furnace rises from room temperature to 600℃ in less than 30min

⑨ Cooling rate: the empty furnace is cooled from 600 ℃ to 200 ℃ less than 20min

⑩Maximum loading volume: 400kg (including fixture)

⑪Temperature control, FP21 programmable temperature controller automatically controls temperature

⑫ Process control, PLC programmable controller automatic control

Compared with other metals, aluminum and aluminum alloys are more difficult to braze. The main reason is that aluminum has a great affinity for oxygen, and it is easy to form a dense, chemically stable, and high melting point (2050°C) aluminum oxide film on the surface. When the thickness is about 50 × 10-10m; at the brazing temperature of 500 ℃ ~ 600 ℃, the film thickness greatly increases to 2000 × 10-10m, which hinders the wetting and bonding of the solder and the base metal. When vacuum brazing without flux, it is difficult to remove this oxide film only by vacuum. Therefore, in order to remove the oxide film and prevent the oxidation of aluminum, it is necessary to resort to the action of some metal activators at the same time. Practice shows that magnesium is the best activator. Because of the high vapor pressure of magnesium, it is easy to volatilize in a vacuum, which is beneficial to remove the oxide film. Magnesium is added to the Al-Si alloy as an activator, part of it is dissolved in aluminum, and part of it exists in the form of MgSi2 compound. The melting temperature of magnesium is 649°C, the evaporation point of magnesium under normal pressure is 621°C, and the evaporation point of magnesium is 250°C under the vacuum condition of 10-3Pa. In the range of 500°C to 580°C, a large amount of magnesium evaporates. In the vacuum chamber, the magnesium vapor reacts with the oxidizing atmosphere (O2, H2O) to purify the medium; the magnesium vapor contacts the oxide film and undergoes a reduction reaction, which deteriorates the dense and smooth Al2O3 film and generates loose and porous new. Oxide layer MgAl2O4.

Aluminum and aluminum alloy vacuum brazing can only use aluminum-based brazing filler metals, and their melting points are close to the melting point of the base metal, so the heating temperature must be strictly controlled. should.

Since any contamination source remaining on the surface of the workpiece will make the brazing fail, the lubricant in the material manufacturing process and the contamination source contaminated in the storage process must be completely removed, and the surface oxide film should also be minimized. method.

①Clean with gasoline or acetone to remove lubricating oil and dirt on the surface.

②Immersed in 5% NaOH (weight concentration) aqueous solution for 1min, temperature 60℃.

③ Rinse with water

④Immersion in 5% HNO2 (volume concentration) aqueous solution for 30s, temperature 30℃.

⑤ Rinse with water

⑥ Drying at 80℃

In order to control the deformation of the workpiece in the hot state, a grid-shaped fixture made of superalloy material is used to assemble and position the workpiece.

Put the water-cooled plate assembly clamped in the jig into the vacuum furnace and make the whole workpiece level. The thermocouple for monitoring the temperature of the workpiece is introduced into the center of the workpiece through the process hole on the side, so as to monitor the actual temperature of the workpiece and prevent the workpiece from overheating or underheating.

Brazing process:

After the workpiece is put into the furnace, start the mechanical vacuum pump and preheat the diffusion pump. When the vacuum degree in the furnace reaches 10°Pa, start the Roots pump to make the vacuum degree in the furnace reach 103Pa. After the diffusion pump starts to work, open the high vacuum valve and put The vacuum degree in the furnace is pumped to 6.7×10-3Pa, and the heating is started. Before 400°C, the temperature can be increased at a faster rate, generally 20°C/min; at 400°C, the temperature should be kept for 30-60min, on the one hand, the temperature of the workpiece is close to the furnace temperature, and on the other hand, the vacuum degree can be increased; at 400°C Between ～570℃, the heating rate should not be too fast, generally controlled at 10～15℃/min; at 570℃, the temperature should be kept for 20～40min, so that the temperature difference between the surface and the core of the workpiece is reduced; between 570℃～620℃ , the heating rate should be accelerated, generally controlled at 20 ℃/min, so that the workpiece can quickly rise to the brazing temperature; keep the temperature at 620 ℃ for 10-20 minutes, when the monitoring thermocouple on the workpiece reaches 595 ℃, turn off the heating power and start vacuum cooling ;When the monitoring thermocouple on the workpiece drops to 540°C, fill the furnace with 99.999% high-purity nitrogen, and start the forced cooling fan for rapid cooling; when the monitoring thermocouple on the workpiece drops to 150°C, turn off the strong Cold system, work done.

The water-cooled plate of electronic components that has been vacuum brazed maintains the metallic luster of aluminum on the surface without oxidation; the welding fillet is well formed, and the welding seam is uniform and dense; product design requirements.

Equipment selection: The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.