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Vacuum Heat Treatment Furnace

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PCB micro-drill vacuum brazing

1 PCB

Multi-hole drilling of multi-layer printed circuit boards (PCBs) by CNC machine tools has high processing efficiency, accurate hole position accuracy and high hole quality, which makes this method widely used. In actual production, in order to save the cost of cemented carbide materials and processing man-hours, in recent years, a micro-drilling tool structure with butt welding of cemented carbide tip and stainless steel body has appeared. The figure below shows the schematic diagram of the PCB microdrill after vacuum brazing. However, there are also tools after vacuum brazing of the cemented carbide cutter head and the stainless steel cutter body. The vacuum brazing of the cutter head is not firm and falls off the cutter body. Therefore, how to improve the vacuum brazing strength of PCB micro-drill tools and seek the best vacuum brazing conditions is of great significance for improving tool life and promoting the application of PCB micro-drills with butt welding structure.

2 The following factors need to be considered in actual vacuum brazing

(1) The parts to be welded are inevitably covered with oxides, grease and dust during the processing and storage process before vacuum brazing. Since these surface coatings can hinder the wetting and spreading of the liquid solder on the base metal, they must be completely removed before vacuum brazing.

(2) The process of removing the oxide film on the surface of the base metal before vacuum brazing cannot meet the requirements. This is because during the heating process of vacuum brazing, the surface of the base metal and the molten brazing material will be rapidly oxidized to form an oxide film again. Therefore, vacuum brazing flux is widely used in actual vacuum brazing. It has the functions of removing film, protecting and activating to promote the smooth formation of vacuum brazed joints.

(3) The uncleaned surface layer of cemented carbide after vacuum sintering often contains more carbon in free state, which hinders the wetting of the brazing filler metal, resulting in poor vacuum brazing performance of cemented carbide, and the linear expansion coefficient of cemented carbide. Relatively low, and the linear expansion coefficient of stainless steel is quite different. During vacuum brazing, a large thermal stress will be generated at the joint, which is prone to cracks. The silver-based solder with a relatively low vacuum brazing temperature was selected in the experiment, because the low vacuum brazing temperature can effectively prevent the occurrence of cracks, and at the same time, it is necessary to control the heating speed (controlled by high frequency), the cooling speed and the heat preservation treatment after welding. , This can reduce and remove the residual stress of vacuum brazing joints and avoid vacuum brazing cracks.

(4) Since the size of the vacuum brazing gap has a significant impact on the joint, and for different forms of joints and different types of loads, as well as different combinations of base metals and brazing materials, there are different requirements for the gap. Factors, this experiment selects the weld gap of 0.02-0.04mm. Production has proved that the secondary pressure on the weld in a short period of time after welding plays a great role in strengthening the welded joint and eliminating residual stress. And the secondary pressing delay time (the time interval between the end of welding and the secondary pressing) is used as a factor affecting the strength of vacuum brazing.

3 Bending strength testing method

The main failure mode of the tool is fracture when the carbide micro-drill is processing multilayer circuit boards. When the PCB is processed on the CNC machining equipment with a high degree of automation, the breakage of the tool will seriously affect the processing quality and efficiency of the PCB. Therefore, it is very important to ensure the strength of carbide micro-drills.

As shown in the figure, the fracture damage of the tool is mainly due to the bending load generated by the radial force Fy, and the torsional load generated by the torque T.

In the actual flexural test, the fracture position generally occurs at the weld, and for the fractured position in the flexural test process, the maximum normal stress during transverse bending can be obtained.

4 PCB microdrill vacuum brazing experiment

The low quality of PCB micro-drilled welded joints is mainly due to the combined effect of various factors such as wetting, insufficient filling, excessive welding residual stress and η that generate brittleness. In actual production, the vacuum brazing temperature holding time, cooling time, high frequency power, secondary pressure and time of the tool holder and delay time of secondary pressure, vacuum flux flow rate and vacuum flux inlet air pressure within the length of the welding piece The flexural strength and holding time of pressing down, as well as EMISSIVITY (reflectivity of dissimilar materials, the experimental value can be set by the machine) will affect the strength of vacuum brazing to a certain extent. Because there are many factors that affect the strength of vacuum brazing, each factor has different effects on it, and there is also an interaction between the factors. The combination of different factors will inevitably affect the quality and output of the product. Therefore, the significant effect of each factor on production can be found out by orthogonal experiment, and the influence of the level combination of factors on the quality and yield of the product can be determined at the same time. Orthogonal experimental design method is a scientific method to study and deal with multi-factor experiments. It uses a normalized form, an orthogonal table, to select experimental conditions, arrange experimental plans and conduct experiments, and pass a smaller number of experiments. Find out the better production conditions, that is, the optimal or better experimental program. It is mainly used for new product development, product or process improvement, and installation services. Through a small number of tests, it can find a combination of factors of high quality, high yield and low consumption to achieve the purpose of improvement.

5 Conclusion

In this paper, the vacuum brazing process of the micro-tool used in PCB is discussed from the vacuum brazing method, the strength testing method and the application of experimental design method, and the advantages of the orthogonal experiment are fully utilized. The factors that have a greater influence on the welding strength are the secondary pressure of the tool holder, high frequency power, vacuum brazing temperature, secondary pressure delay time, anti-folding pressure holding time, in-line anti-folding force, etc. The actual production proves that these optimized process parameters It has a significant effect on improving welding strength and reducing scrap rate.