Contact with low-melting-point brazing filler metal layer and preparation method thereof

Abstract

The invention discloses a contact with a low-melting-point brazing filler metal layer and a preparation method of the contact. The preparation method comprises the following steps: performing surface grinding, polishing and cleaning treatment on the contact of which the brazing filler metal layer needs to be prefabricated, preparing a soldering lug matched with the boundary dimension of the welding surface of the contact through a stamping process, jointly placing the contact and the soldering lug on a graphite plate for assembling, heating the assembled part in an atmosphere protection furnace to be above the liquid phase line of the welding flux, and performing welding. And the molten welding flux wets the contact and is spread on the welding surface, and finally cooling is conducted. The method is simple in process, stable in quality, environmentally friendly and widely suitable for preparing various prefabricated layers with the brazing filler metal melting point lower than the contact sintering forming temperature, and the problem that a reliable low-melting-point brazing filler metal layer cannot be prefabricated on the surface of a contact in the prior art is solved.

Description

Technical Field

[0001] The invention relates to the technical field of electric contact manufacturing, and in particular to a contact with a low-melting-point solder layer and a preparation method thereof. Background Art

[0002] Contacts are core components in electrical contact systems, playing the role of connecting, carrying and disconnecting currents, and are widely used in various types of high and low voltage electrical equipment. Sheet contacts are a type of contact structure often used in circuit breakers and contactors, mostly in the form of round or square sheets, with relatively simple dimensions. Depending on the application scenario and load conditions, sheet contact materials include silver graphite (AgC), silver tungsten carbide (AgWC), silver tungsten (AgW), silver nickel graphite (AgNiC) and silver-based metal oxides (AgSnO2In2O3), etc. The preparation methods of these materials are mainly through powder metallurgy near-net forming or powder metallurgy combined with secondary pressure processing. Sheet contacts are generally connected to a support body composed of copper and copper alloys by welding, and then assembled into circuit breakers or contactors as contact assemblies for use, but due to the presence of tungsten, graphite and ceramic oxides, the connection quality of direct welding is very poor. In order to meet the high reliability requirements of electrical equipment, solder with specific components must be added to improve the welding quality. At present, electrical appliance manufacturers widely use automated welding and assembly production lines. Adding solder during the welding process is obviously not conducive to automation or will reduce production efficiency. Therefore, manufacturers generally require contact material manufacturers to pre-prepare a solder layer on the contact welding surface.

[0003] Patent CN1008151B disclosed a method for manufacturing an electrical contact with a prefabricated solder layer in the 1980s. The method first prepares a contact intermediate with a zigzag groove (the groove surface is the welding surface) by pressing and sintering, then lays a layer of solder powder on the groove surface, and finally performs re-pressing to obtain a finished product. However, the prefabricated solder layer is only mechanically pressed with the contact, which is easy to cause partial detachment when subjected to vibration or collision, and causes pollution to the working surface of the contact. Patent CN1645532A once disclosed a method for coating a solder layer on the surface of a copper-based contact, which is to place a solder coated with a solder flux on the contact welding surface, and then make the solder adhere or melt and spread on the contact surface by bonding, pressure processing or heating. However, in the report, the form of the pre-placed solder is not disclosed (such as powder, paste or foil, etc.), and the method of compounding the solder to the contact surface is also very broad. None of the corresponding specific process parameters are disclosed in all the embodiments, and the guiding role for practical application is limited. In addition, coating the solder surface with brazing flux also increases the complexity of the operation and affects production efficiency. Patent CN106548880B also disclosed a method for preparing a brazing composite material layer on the surface of a silver-based electrical contact, which mixes brazing alloy powder, flux, adhesive and curing agent into a paste fluid according to a ratio, and then prints the solder paste on the welding surface of the electrical contact through a dispensing machine, and then sintering and curing it under the protection of an inert gas. This method has very high requirements on the precision of the dispensing machine, otherwise it is easy to have inconsistent dispensing amounts; at the same time, due to the inconsistent specific gravity of each component in the solder paste, stratification is easy to occur during the placement process, exacerbating the quality fluctuation of the prefabricated solder layer of the contact. In addition to the above reports, there are also some materials that disclose a method for preparing a contact with a composite layer (CN115810494A), which is to simultaneously cold-press the composite layer powder and the contact material powder, and then sinter and re-press them to obtain it. However, the prerequisite of this method is that the melting point of the composite layer must not be lower than the sintering temperature. The melting point of the solder layer prefabricated on the surface of the silver-based or copper-based sheet contact is mostly lower than the temperature when the contact powder is sintered, so this process cannot be used. Summary of the invention

[0004] In order to meet the demand of prefabricating a low melting point solder layer on the surface of a sheet contact and overcome the shortcomings of the prior art, the present invention provides a method with simple process, wide applicability and good quality reliability. The method uses a solder sheet that matches the outer dimensions of the contact welding surface, then assembles it with the contact, and then melts the solder sheet and spreads it evenly through furnace brazing under vacuum or protective atmosphere, thereby obtaining a sheet contact with a prefabricated solder layer.

[0005] To achieve the above purpose, the main steps and contents of the technical solution of the present invention are as follows:

[0006] S1. Contact surface treatment

[0007] The contact points to be preformed with the brazing material layer are ground and polished by a high-speed centrifugal polisher. The grinding media are grinding balls and deionized water, preferably zirconia balls with a grinding ball diameter of 1-5 mm. The mass ratio of the contact points, grinding balls and deionized water is preferably 1:(2-6):(3-10), the polishing time is 10-60 minutes, and the polishing frequency is 10-50 Hz. The polished contacts are cleaned with deionized water or an organic solvent (preferably anhydrous ethanol or isopropanol, etc.), and dried by hot air.

[0008] S2.Welding piece punching

[0009] According to the thickness of the required prefabricated solder layer, select a solder strip of corresponding thickness, and punch it into a solder piece that matches the outer dimensions of the contact welding surface through a punching machine. The corresponding relationship between the length L and width W of the solder piece and the length and width dimensions (L0, W0) of the contact is: L = L0-(0.5-1) mm, W = W0-(0.5-1) mm. The melting point of the solder in the solder piece is lower than the lowest melting point of each component in the contact;

[0010] The S1 and S2 are not sequential and can be performed simultaneously;

[0011] S3. Assembly

[0012] The graphite plate made of high-purity isostatic graphite is used as the carrier. The ash content of the graphite plate is not higher than 500ppm and the density is not less than 1.8g / cm 3 , the plate thickness is not less than 5mm, the welding piece is placed above or below the contact surface of the solder layer to be coated, and the two are placed together on the graphite plate to complete the assembly;

[0013] S4. Heating the solder

[0014] The assembled contacts, solder sheets and graphite plates are placed in a heating furnace together by heating in a box furnace or mesh belt furnace with an inert gas or reducing gas as the protective atmosphere. The heating temperature is 10-50°C above the liquidus temperature of the solder, the insulation time is 1-10 minutes, and then cooled to below 60°C at a rate of 5-20°C / min, thereby obtaining contacts with a prefabricated solder layer.

[0015] In step S1, the contact surface state (such as surface composition, micromorphology, etc.) is significantly affected by the grinding ball material and grinding ball size, which leads to different results when heating the solder coating in step S4. The present invention finds that the use of zirconia balls with a diameter of 1-5 mm is the best.

[0016] In step S2, the ash content of the graphite plate is required to be no higher than 500ppm and the density is required to be no lower than 1.8g / cm 3, mainly because too many impurities or voids in the graphite will affect the effect of heating the solder coating in the furnace in the S4 step; in addition, S2 requires the plate thickness to be no less than 5mm, in order to ensure the strength of the carrier, which is sufficient to support the contacts.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] (1) Simple process and wide applicability

[0019] The present invention adopts a solder sheet that matches the outer dimensions of the contact welding surface, and melts and spreads the solder sheet by heating in a furnace to obtain a contact with a prefabricated solder layer. The method is applicable to all solders whose melting points are lower than the powder metallurgy forming temperature of the contact, and the solder sheet can be obtained by only selecting a solder strip of corresponding thickness and stamping it. The box furnace or mesh belt furnace is a heating method widely used in the industrial field, so the operation of the present invention is simple and has strong popularization and applicability.

[0020] (2) Green and environmentally friendly, low pollution

[0021] The surface state of the parent material has a great influence on the wettability of the solder. The present invention improves the cleanliness of the contact surface through physical and chemical methods (polishing, cleaning), so that the molten solder can be evenly spread when the solder is heated, eliminating the use of solder or flux. Since the organic matter in the solder will form volatile gas when heated, it is often harmful to the human body or the environment and needs to be specially collected and processed. In addition, the deionized water or anhydrous ethanol cleaning agent, zirconium oxide abrasive, etc. used in the present invention do not have biological or environmental toxicity, which is more in line with the green and environmentally friendly production concept.

[0022] (3) Stable quality and good consistency

[0023] The present invention ensures that the blanking die can easily control the shape and size of the solder piece, thereby first reducing the quality fluctuation of the finished product caused by the deviation of the amount of solder. In addition, the surface treatment of the contact before solder coating significantly improves the cleanliness and wettability of the welding surface. The ratio of the prefabricated solder layer area to the welding surface of the obtained contact product is close to 100%. At the same time, the soldering rate between the solder layer and the contact is high, the interface bonding is good, and the pores are small, which helps electrical appliance manufacturers to further improve the welding quality and ensure the reliability of electrical equipment.

[0024] In summary, the melting point of the solder layer prepared on the contact welding surface of the present invention is lower than the lowest melting point of each component of the contact. At present, most of the prefabricated contact solder layers in the field are pure silver, and its melting point is the same as the main component of the contact (usually silver). The corresponding preparation method is not suitable for the case where the melting point of the solder is lower than the contact forming temperature or the contact melting point. As for other methods that have been disclosed for prefabricating solder layers on the contact surface, although some technologies can be applied to the case where the melting point of the solder is low, there are obvious deficiencies in their preparation process or effect. The technical solution disclosed in the present invention can overcome the above problems. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The corresponding relationship diagram between the solder piece size (L and W) and the contact size (L0 and W0).

[0026] Figure 2 The Cu strip with a thickness of 0.2±0.05 mm prepared by the present invention in Example 1 80 Ag 15 Cross-sectional metallographic structure of AgC(5) contact of P5 solder.

[0027] Figure 3 The Cu strip with a thickness of 0.1±0.03 mm prepared by the present invention in Example 2 80 Ag 15 Cross-sectional metallographic structure of AgWC(40) contact of P5 solder. DETAILED DESCRIPTION

[0028] Embodiments of the present invention are described in detail below, and the examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals represent the same or similar elements or elements with the same or similar functions from beginning to end. The embodiments described below by reference to the accompanying drawings are exemplary, are intended to be used to explain the present invention, and are not to be construed as limitations of the present invention. Those who do not indicate specific techniques or conditions in the embodiments are carried out according to the techniques or conditions described in the documents in this area or according to the product specification. Those who do not indicate the manufacturer of reagents or instruments used are all conventional products that can be obtained commercially.

[0029] Example 1 with Cu 80 Ag 15 Preparation of AgC(5) Contacts with P5 Solder Layer

[0030] S1. Contact surface treatment: The AgC (5) contacts were ground and polished using a high-speed centrifugal polisher. The grinding and polishing media were Φ2 mm zirconium oxide balls (grinding balls) and deionized water. The mass ratio of the contacts, grinding balls, and deionized water was 1:3:6. The polishing frequency was 20 Hz and the time was 30 min. The polished contacts were cleaned with anhydrous ethanol and then dried with hot air at a temperature of 80 ± 15 °C.

[0031] The AgC (5) contact is made by powder metallurgy combined with secondary pressure processing, wherein the mass fraction of graphite is 5%, referred to as AgC (5), and the length×width×thickness of the contact is 6×6×2 mm respectively.

[0032] S2. Welding sheet punching: Select Cu with a thickness of 0.2 mm 80 Ag 15 The P5 solder strip is punched out by a punching machine to obtain solder sheets with a size of 5.5×5.5×0.2 mm.

[0033] The S1 and S2 are not sequential and can be performed simultaneously;

[0034] S3. Assembly: Place the welding piece and the contact on the graphite plate so that the welding piece is below the welding surface of the contact;

[0035] S4. Heating the solder coating: The three are placed together in a box-type furnace, and the protective atmosphere in the furnace is a nitrogen-hydrogen mixed gas, which is kept at 680°C for 5 minutes. After solidification and cooling, the prefabricated Cu 80 Ag 15 P5 AgC(5) sheet contact with solder layer thickness of 0.18mm.

[0036] The metallographic structure of the contact cross section is as follows Figure 2 As shown, it can be seen that the interface is well bonded and there are basically no pores.

[0037] Example 2 with Cu 80 Ag 15 Preparation of AgWC(40) contacts with P5 solder layer

[0038] S1. Contact surface treatment: AgWC (40) contacts were ground and polished using a high-speed centrifugal polisher. The grinding and polishing media were zirconium oxide grinding balls and deionized water. The grinding ball size was Φ1.8 mm. The mass ratio of the contacts, grinding balls, and deionized water was 1:4:5. The polishing frequency was 30 Hz for 20 min. The polished contacts were cleaned with deionized water and then dried with hot air at 80 ± 15 °C.

[0039] The AgWC (40) contact is made by a powder metallurgy near-net-shape forming process, wherein the mass fraction of tungsten carbide is 40%, referred to as AgWC (40), and the length×width×thickness of the contact is 6×4×2 mm respectively.

[0040] S2. Welding sheet punching: Select Cu with a thickness of 0.1 mm 80 Ag 15 The P5 solder strip is punched out by a punching machine to obtain solder sheets with a size of 5.7×4.5×0.1 mm.

[0041] The S1 and S2 are not sequential and can be performed simultaneously;

[0042] S3. Assembly: Place the soldering sheet and the AgWC (40) contact on the graphite plate so that the soldering sheet is above the contact welding surface;

[0043] S4. Heating the solder coating: The three are placed together in a mesh belt furnace, and the protective atmosphere in the furnace is nitrogen with a purity of 99.95%. The furnace is kept at 660°C for 3 minutes, and the prefabricated Cu is obtained after solidification and cooling. 80 Ag15 P5 AgWC(40) sheet contact with solder layer thickness of 0.09mm.

[0044] The metallographic structure of the contact cross section is as follows Figure 3 shown.

[0045] Example 3 with Ag 56 Cu 22 Zn 17 Preparation of AgWC(40) Contacts with Sn5 Solder Layer

[0046] S1. Contact surface treatment: The AgWC (40) contacts were ground and polished using a high-speed centrifugal polisher. The grinding and polishing media were zirconium oxide grinding balls and deionized water. The grinding ball size was Φ1 mm. The mass ratio of the contacts, grinding balls, and deionized water was 1:5:8. The polishing frequency was 30 Hz and the time was 30 min. The polished contacts were cleaned with isopropyl alcohol and then dried with hot air at 80±15°C.

[0047] The AgWC (40) contact is made by a powder metallurgy near-net forming process, wherein the mass fraction of tungsten carbide is 40%, referred to as AgWC (40), and the length×width×thickness of the contact is 4×3×1.5 mm respectively.

[0048] S2. Solder sheet punching: Select Ag with a thickness of 0.1 mm 56 Cu 22 Zn 17 The Sn5 solder strip is punched out by a punching machine to obtain solder sheets with a size of 3.5×2.5×0.1 mm.

[0049] The S1 and S2 are not sequential and can be performed simultaneously;

[0050] S3. Assembly: Place the soldering sheet and the AgWC (40) contact on the graphite plate so that the soldering sheet is below the contact welding surface;

[0051] S4. Heating the solder coating: The three are placed together in a mesh belt furnace, and the protective atmosphere in the furnace is a nitrogen-hydrogen mixed gas, which is kept at 670°C for 3 minutes. After solidification and cooling, the prefabricated Ag is obtained. 56 Cu 22 Zn 17 AgWC(40) sheet contacts with Sn5 solder layer thickness of 0.08mm.

[0052] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and intent of the present invention.

Claims

1. A method for preparing a contact with a low melting point solder layer, characterized in that: The following steps are included: S1. Contact surface treatment: The contacts to be preformed with the solder layer are ground and polished by a high-speed centrifugal polisher, and grinding balls and deionized water are used as grinding media; preferably, the grinding balls are zirconium oxide balls with a diameter of 1-5 mm; the polished contacts are cleaned with deionized water or an organic solvent, and dried by hot air; preferably, the organic solvent is anhydrous ethanol or isopropanol; S2.Welding piece punching: According to the required thickness of the prefabricated solder layer, a solder strip of corresponding thickness is selected, and a solder sheet matching the outer dimensions of the contact welding surface is punched out by a punching machine. The corresponding relationship between the length L and width W of the solder sheet and the length and width dimensions (L0, W0) of the contact is: L = L0-(0.5-1) mm, W = W0-(0.5-1) mm; the melting point of the solder in the solder sheet is lower than the lowest melting point of each component in the contact; The S1 and S2 are not sequential and can be performed simultaneously; S3. Assembly: The graphite plate made of high-purity isostatic graphite is used as the carrier. The ash content of the graphite plate is not higher than 500ppm and the density is not less than 1.8g / cm 3 , thickness not less than 5mm; place the solder sheet above or below the contact surface of the solder layer to be coated, and place both together on the graphite plate to complete the assembly; S4. Heating the solder: The assembled contacts, solder sheets and graphite plates are placed in a heating furnace together with an inert gas or reducing gas as the protective atmosphere, heated to above the solder liquidus temperature and kept warm, and then cooled to obtain contacts with a low melting point solder layer.

2. The method for preparing a contact with a low melting point solder layer according to claim 1, characterized in that: In the step S1, the mass ratio of the contact, the grinding ball and the deionized water is 1:(2-6):(3-10); the polishing time is 10-60 min; and the polishing frequency is 10-50 Hz.

3. The method for preparing a contact with a low melting point solder layer according to claim 1, characterized in that: In the step S4, the heating furnace is a box-type furnace or a mesh belt furnace.

4. The method for preparing a contact with a low melting point solder layer according to claim 1, characterized in that: In the step S4, the heating temperature is 10-50°C above the liquidus temperature of the solder and not higher than the forming temperature of the contact material or the lowest melting point of each component in the contact, and the holding time is 1-10 minutes.

5. The method for preparing a contact with a low melting point solder layer according to claim 1, characterized in that: In the step S4, the cooling rate is 5-20°C / min, and the furnace exit temperature is below 60°C.

6. A contact with a low melting point solder layer prepared by the method of any one of claims 1 to 5.

Images