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Vacuum Brazing Furnace

Ceramic package shell vacuum brazing

A customer uses a hydrogen furnace to braze ceramic packaging shells. In dry hydrogen, the entire brazing process requires precise control of the dew point. At the same time, hydrogen will embrittle metals such as copper, titanium, copper, copper, and aluminum. , affecting the welding quality of the workpiece. In addition, when the hydrogen content in the air is higher than 4%, it will become an explosive gas, which poses certain potential safety hazards when using a hydrogen furnace for welding. The most important reason is that the porosity rate of the ceramic package shell soldered in a hydrogen furnace is as high as 30%-40%, and the national standard requires that the porosity rate should not be higher than 25%. The existence of large-area holes makes the base and Insufficient chip connections lead to a decrease in conductive properties and affect other physical properties. For this reason, we use a vacuum brazing furnace instead of a hydrogen furnace to weld the ceramic package shell.

1 Vacuum brazing furnace process exploration

Vacuum brazing refers to a method of connecting metal parts by heating the solder to reach the melting temperature under vacuum conditions, and then utilizing the solidification of the liquid solder and its diffusion with the base metal to connect metal parts. . It has the following advantages:

1) Vacuum brazing does not use flux, and will not cause defects such as pores and slag inclusions. It can significantly improve the corrosion resistance of the product, avoid environmental pollution, and create necessary conditions for safe production;

(2) Vacuum brazing can save a lot of expensive metal flux and does not require complicated flux cleaning processes, reducing production costs;

(3) The vacuum brazing filler metal has good wettability and fluidity, and is easy to weld workpieces with complex shapes or narrow channels;

(4) It is suitable for brazing stainless steel and high-temperature alloys, especially heat-resistant alloys with high aluminum and titanium content. It can obtain high-quality welding joints. It is also suitable for brazing various difficult-to-braze stainless steel, titanium, iron, molybdenum, tin and buttons. Joining of molten alloys or dissimilar metals:

(5) Since the parts to be welded are heated under vacuum conditions, there will be no problems such as oxidative carburization, decarburization, contamination or deterioration. The parts to be welded are in an overall heating state in the furnace, so the thermal stress can minimize the amount of deformation. limit;

(6) The base metal and solder are in a low-pressure state, which makes it easy to eliminate volatile impurities and gases released at the brazing temperature, which can improve the performance of the base metal itself;

(7) The heat treatment process of the welded parts and the vacuum brazing process can be carried out together.

The brazing process can also be arranged as the final process;

(8) The surface of the workpiece after vacuum brazing is not only clean and bright, but also easy to inspect the quality of the weld.

The materials for this brazing are shown in the left picture in Figure 1. From top to bottom, they are the sealing ring, silver copper solder and ceramic packaging shell. The purpose of brazing is to use silver copper solder to weld the sealing ring to the welding surface of the ceramic package shell. The welding finished product is shown in the figure on the right. The sealing ring material is Kovar alloy (4J29), the solder is silver-copper alloy (Ag-Cu28), the black part of the ceramic package shell is ceramic, the gray part is the soldering surface, and the material is nickel.

The vacuum brazing production process includes surface preparation before brazing, assembly, placement of solder, brazing and post-welding treatment. Each process will affect the final quality of the product. Among them, the brazing process is the decisive process for forming a good welded joint.

1.1 Workpiece surface preparation

Before vacuum brazing, oxides, grease, paint and other impurities on the surface of the workpiece must be carefully removed. Because the melted solder cannot infiltrate the unclean material surface, nor can it fill the joint gap. Therefore, the workpiece needs to be cleaned: generally first clean it with alcohol, then clean it with water, then clean it with carbon tetrachloride, and finally clean it with water, and place it in an oven to dry at 50°C. At this time, the grease on the surface of the part will Removed cleanly.

For parts with large shapes and complex quantities, ultrasonic cleaning can be used. Ultrasonic oil removal is highly efficient. Cleaned workpieces must not be stored in the air for more than 8 hours, otherwise they need to be cleaned again. After cleaning, use clean tweezers to operate, and do not touch it directly with your hands to avoid secondary contamination.

If the workpiece is not cleaned, tiny bubbles or holes will remain between the solder and the weldment.

1.2 Assembly and fixation

Brazed parts should be assembled and positioned to ensure their relative position. For parts with small size and simple structure, simple methods can be used to fix them, such as relying on self-weight closing, rivets, etc. For parts with complex structures, special fixtures are generally used to position them. The requirements for brazing fixtures are that the fixture material should have good high temperature resistance and oxidation resistance; the fixture and the part materials should have similar thermal expansion coefficients; the fixture should have sufficient stiffness, but the structure should be as simple as possible and the size should be as small as possible. The fixture not only works reliably, but also ensures high production efficiency. If the workpiece is not assembled properly, the sealing ring and the ceramic packaging shell will easily become misaligned, resulting in scrap.

2 Vacuum brazing process parameters

The main parameters of the vacuum brazing process are brazing temperature and holding time. The brazing temperature is usually 25°C to 60°C higher than the solder liquidus to ensure that the solder fills the gap. For some solders with wide crystallization temperature intervals, there is a certain amount of fluidity due to the existence of a considerable amount of liquid phase below the liquidus temperature. At this time, the brazing temperature can be equal to or slightly lower than the solder liquidus temperature.

The length of brazing holding time is also a very important process parameter. If the holding time is too short, the interface reaction will be insufficient, and the joint strength is not high; if the holding time is too long, the long-term reaction will produce too many brittle phases at the interface, which will also cause the joint strength to decrease and may even cause cracking. Therefore, in order to ensure the quality of brazing, the holding time must be strictly controlled at a certain brazing temperature.

The brazing temperature and holding time depend on the size of the workpiece and the intensity of the interaction between the filler metal and the base metal. Large items should be kept warm longer to ensure even heating. The interaction between the filler metal and the base metal is strong, so the heat preservation time should be short. Generally speaking, a certain heat preservation time is necessary to promote the mutual diffusion of the solder and the base metal to form a strong bond, but too long a heat preservation time will lead to the occurrence of defects such as corrosion. Based on past experience, the holding time is about 30 minutes.

3 Analysis of causes affecting the welding quality of workpieces

After a series of experiments, the factors affecting the welding quality of the workpiece were analyzed and summarized as follows:

(1) The filler is defective and some gaps are not filled.

The joint design is unreasonable, the assembly gap is too large or too small, and the parts are tilted during assembly; the brazing temperature is too low or unevenly distributed; the solder material is improperly selected, such as the wetting effect of the solder material is poor and the amount of solder material is insufficient; the placement of the solder material Improper; poor preparation before brazing, such as unclean cleaning, etc.

(2) brazing holes

The joint gap is improperly selected; the parts are not clean before brazing; the protective gas deoxidation effect is poor; the brazing material precipitates gas during brazing or the brazing material is overheated.

(3) Brazing seam welding slag

Uneven heating; improper selection of joint gap; solder filler from both sides of the joint; melting temperatures of solder and flux do not match.

(4) Solder seam cracking

Due to the different thermal expansion coefficients of dissimilar base materials, the internal stress formed during the cooling process is too large; the same type of brazing material is heated unevenly, resulting in inconsistent shrinkage during the cooling process; when the solder solidifies, the parts move mutually; the solder crystallization temperature interval is too large Large; the brazing seam is too brittle.

(5)Cracking of base material

The base metal is overburned or overheated; the brazing filler metal penetrates into the base metal to form a brittle phase; uneven heating or excessive internal stress due to rigid clamping of the tool; stress caused by the internal stress of the workpiece itself; dissimilar base materials The difference in thermal expansion coefficient is too large and its ductility is low; solder loss.

(6) Solder loss

The brazing temperature is too high and the holding time is too long; the interaction between the base metal and the brazing filler metal is too severe; the amount of brazing filler metal is too large.

4 Conclusion

Through the above experiments, it is fully proved that the vacuum brazing furnace can replace the hydrogen furnace to weld the ceramic packaging shell. The vacuum brazing process can fully meet the customer’s needs. The vacuum brazing equipment can be used to replace the hydrogen equipment to braze the ceramic packaging shell.