A vacuum brazing method for chip resistor

Abstract

The present invention discloses a vacuum brazing method for chip resistors, which belongs to the technical field of resistor processing; the method includes five steps: welding environment, pre-welding treatment, solder and weldment mounting, welding specifications, and post-welding inspection and testing. The welding environment requires operation in a fully enclosed clean workshop, and the air purification degree and temperature and humidity are controlled to meet the welding requirements; the pre-welding treatment requires degreasing and surface cleanliness inspection of the solder and weldment; the solder in the solder and weldment mounting can be selected as welding wire or solder paste, and sent to the vacuum brazing furnace after the plate is placed; the welding specifications require that the vacuum brazing furnace should meet the temperature control requirements during welding; the post-welding inspection and testing requirements include full inspection items and random inspection items. The present invention can improve welding efficiency and welding quality, and the welding pass rate can reach more than 95%.

Description

Technical Field

[0001] The invention belongs to the technical field of resistor processing, relates to a chip resistor, and specifically relates to a vacuum brazing method for a chip resistor. Background Art

[0002] Chip resistors are widely used in aviation, aerospace, navigation, weapons, national defense engineering, communications, automation control, instrumentation and other electronic products due to their outstanding advantages such as high resistance accuracy, small temperature coefficient and high reliability. Most of their structures are designed in the form of a middle resistance band plus electrodes at both ends ( Figure 1 As shown in the figure, the electrodes and the resistor bands are usually connected by welding. In order to achieve stable and reliable performance, general industrial-grade products require that the joints have no defects such as cold solder joints and desoldering. However, the requirements for the joints of aerospace-grade and military-grade products are that the appearance of the joints should not have defects such as melting, solder accumulation, outflow, and incomplete flow; there should be no defects such as holes, thin solder, and no solder inside; the resistor bands should not have defects such as cracks, embrittlement, lattice changes, and resistance mutations; in addition, the joints must have high shear strength. The existing welding methods for chip resistors include: fusion welding (gas welding, arc welding, laser welding, plasma welding, etc.), pressure welding (diffusion welding, contact welding, ultrasonic welding, etc.), brazing (electron beam welding, flame welding, high-frequency welding), etc. The existing technology has the following defects: (1) The high thermal conductivity of the parent material makes it extremely difficult to melt, and the joints are prone to cold solder joints and desoldering. (2) The parent material has a large expansion coefficient, which is easy to generate welding stress, and the joints are prone to thermal cracks. (3) Hydrogen, oxygen and water vapor cause pores in the joints. (4) Uneven welding temperature causes the base material to melt and the solder to spread unevenly. (5) The base material is not clean and the solder has poor wettability, which causes holes inside the joint and discontinuous welds. (6) The welding time is difficult to control and the shear strength of the joint is low. The welding qualification rate of aerospace-grade products in the existing technology is less than 10%, and welding requires complex fixtures and high welding costs; too many welded parts cannot be mounted at one time, and the production efficiency is extremely low. Summary of the invention

[0003] The object of the present invention is to provide a vacuum brazing method for chip resistors to solve the defects pointed out in the background technology.

[0004] In order to solve the above technical problems, the technical solution provided by the present invention is: a vacuum brazing method for chip resistors, including five steps: welding environment requirements, surface treatment of solder and weldment, solder and weldment mounting, welding specifications, and post-welding inspection and testing; The specific steps include: (1) Welding environment: Operate in a fully enclosed cleanroom, control the air purity, temperature and humidity to meet welding requirements, and remove dirt and oil from tools and equipment. Wear gloves when installing solder and weldment. (2) Pre-welding treatment: Degreasing and surface cleanliness inspection of solder and weldment until the surface cleanliness meets welding requirements; (3) Solder and weldment mounting: Solder can be selected from welding wire or solder paste, control the position of the resistor band and the total length of the resistor to meet the requirements of the drawing, and then place the plate and send it into the vacuum brazing furnace; (4) Welding specifications: The following specifications should be met when welding in a vacuum brazing furnace: ① When the vacuum degree in the furnace is not less than 1×10 -4 P a Start heating when the vacuum degree in the furnace should not be less than 1×10 -3 P a ;② Heating specification: 10min from room temperature to 200℃, 15min from 200℃ to 400℃, 15min from 400℃ to 600℃, 20min from 600℃ to 800℃, 15min from 800℃ to 860℃~920℃;③ Insulation specification: Insulation for 20min;④ Cooling specification: You can choose natural cooling or cooling by filling the furnace with nitrogen; (5) Post-weld inspection and testing: including full inspection items and random inspection items. Full inspection items include visual inspection, dimensional inspection, tool microscope inspection, X-ray inspection and bridge test; random inspection items include destructive (DPA) inspection and shear stress test.

[0005] Furthermore, in step (1), the air purification degree is not less than 100,000, the humidity is not greater than 45%, and the temperature is 20-25°C. The surfaces of tools and instruments are decontaminated and degreased with alcohol or acetone before use. When solder and welded parts are mounted on the rack, fine sand or medical PVC gloves are worn.

[0006] Furthermore, in the step (2), when degreasing, the solder and weldment are placed in an acetone solution, ultrasonically cleaned for 10 minutes to remove surface oil stains, taken out and placed on a gauze pad, and dried with a hot air blower; the solder and weldment are tested with a cleanliness detector, requiring the surface cleanliness RFU value to be ≤5.0, otherwise they are cleaned again.

[0007] Furthermore, in step (3), when the solder is selected as welding wire: A. Solder rack: Shape the welding wire into a straight strip, then bend and tie it to both ends of the resistor band, and cut off the excess solder at the interface; B. Insert the resistor band with the tied welding wire into the electrode and place it on the table, and fix the position of the resistor band in the electrode; C. Measure the total length of the resistor. If the total length does not meet the requirements of the drawing, adjust it. D. Push the welding wire to the weld seam and make it as close to the weld seam as possible; E. Place the resistors loaded with solder neatly on the tray, with the spacing preferably so that they do not touch each other; F. Place the tray with the welded parts in the heating zone of the furnace, check to make sure that the solder is at the weld seam and the welded parts are not in contact with each other; G. Close the furnace door and evacuate; When solder paste is selected for solder: a. Insert the resistor strip into the electrode and place it on the table, fix the position of the resistor strip in the electrode; b. Measure the total length of the resistor, if it does not meet the requirements of the drawing, adjust it; c. Apply solder paste on the weld, clean up the excess solder paste outside the weld, and blow dry the solder paste with a hot air blower; d. Place the resistors filled with solder paste neatly on the tray, with the spacing preferably not touching each other; e. Place the tray with the welded parts in the heating zone of the furnace, check to make sure that the solder is at the weld and the welded parts are not touching each other; f. Close the furnace door and evacuate.

[0008] Furthermore, in step (4), the relationship between the insulation temperature of welding and the number of welds is: 870°C for less than 200 pieces, 890°C for 201 to 600 pieces, and 910°C for 601 to 1000 pieces.

[0009] Furthermore, in the step (5), the post-welding inspection and test are as follows: (I) Full inspection items: ① Visual inspection: The weldment has no defects such as desoldering, embrittlement, melting, cracking, solder nodules, excessive dispersion, and incomplete welds; ② Dimension inspection: The total length of the resistor measured by a vernier caliper and the position of the resistor band are in accordance with the drawings; ③ Tool microscope inspection: The weld has no leaks, cold solder joints, and the resistor band has no cracks; ④ X-ray inspection: The weld line is clear, uniform, without ghosting, and without interruption; ⑤ Bridge test: The resistance tested at 25°C meets the requirements of the drawings; (II) Random inspection items: Destructive (DPA) inspection: ① Metallographic microscope inspection: There are no defects such as holes, no solder, thin solder, and discontinuous solder inside the weld; ② Shear stress test: The shear strength of the joint is ≥230MPa.

[0010] The beneficial effects of the present invention are: A vacuum brazing method for chip resistors has the following advantages: The present invention provides a vacuum brazing method for chip resistors; it not only solves the welding defects of the prior art such as solder nodules, overflow, discontinuity; holes in the seams, lack of material, no material; cracking of resistor bands, sudden change in resistance, etc., but also does not require any complex tooling and fixtures during the welding process, thereby reducing the welding cost; and can weld hundreds or thousands of pieces at a time, greatly improving the production efficiency; using the chip resistor welding technology of the present invention, the welding qualification rate of aerospace-grade and military-grade products can reach more than 95%. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 A schematic diagram of a chip resistor of the present invention; Figure 2 Metallographic microscope photo of the vacuum brazing joint of the chip resistor of the present invention; Figure 3 X-ray inspection photograph of the vacuum brazed joint of the chip resistor of the present invention; In the attached figure: 1-electrode; 2-resistance band. DETAILED DESCRIPTION

[0012] The technical solution of the present invention is further described in detail below, but the protection scope of the present invention is not limited to the following.

[0013] (1) Welding environment: Welding environment requirements: 1. The entire operation process is carried out in a fully enclosed clean room. 2. The air purification level is not less than 100,000, the humidity is not more than 45%, and the temperature is 20-25°C. 3. The surface of tools and equipment should be decontaminated and degreased with alcohol or acetone before use. 4. Wear fine sand or medical PVC gloves when installing solder and weldment.

[0014] (2) Pre-welding treatment: Pre-welding treatment includes degreasing and surface cleanliness inspection: 1. Put the solder and weldment into acetone solution, ultrasonically clean for 10 minutes to remove surface oil, take out and place on gauze pad, and dry with hot air blower. 2. Use cleanliness tester to test the surface cleanliness of solder and weldment RFU value ≤5.0, otherwise re-clean.

[0015] Embodiment 1: Vacuum brazing of 200 chip resistors: 1. Parts to be welded: 200 resistors (400 copper electrodes, 200 manganese copper resistor strips). 2. Solder: AgCu28 welding wire, 0.5 mm in diameter.

[0016] (3) Solder and weldment mounting: A. Solder mounting: Use flat-nose pliers to shape the AgCu28 welding wire with a diameter of 0.5 into a straight strip, then bend and tie it to both ends of the resistor strip, and use diagonal pliers to cut off the excess solder at the interface. B. Insert the resistor strip with the tied welding wire into the electrode and place it on the table. Use a depth gauge to determine the position of the resistor strip in the electrode according to the drawing size. C. Use a vernier caliper to measure the total length of the resistor. If it is larger than the drawing size, use manual flat-nose pliers to squeeze the total length of the resistor to meet the drawing requirements. D. Use tweezers to push the welding wire to the weld seam so that it is as close to the weld seam as possible. E. Place the resistors with solder neatly on the tray, with the spacing being such that they do not touch each other. F. Place the tray with the welded parts in the heating zone of the furnace, and check to make sure that the solder is at the weld seam and the welded parts are not in contact with each other. G. Close the furnace door and evacuate.

[0017] (4) Welding specifications: ① When the vacuum degree in the vacuum brazing furnace is not less than 1×10 -4 P a Start heating when the vacuum degree in the furnace should not be less than 1×10 -3 P a② Heating specification: 10min from room temperature to 200℃, 15min from 200℃ to 400℃, 15min from 400℃ to 600℃, 20min from 600℃ to 800℃, 15min from 800℃ to 870℃. ③ Insulation specification: 870℃ insulation for 20min. ④ Cooling specification: Turn off the heating power, cool naturally to 50℃, open the furnace door and take out the soldered resistor.

[0018] (5) Post-weld inspection and testing: (I) Full inspection items: ① Visual inspection: The weldment has no defects such as desoldering, embrittlement, melting, cracking, solder nodules, excessive dispersion, and incomplete welds. ② Dimension inspection: The total length of the resistor measured by a vernier caliper and the position of the resistor band are in accordance with the drawings. ③ Tool microscope inspection: The weld has no leaks, cold solder joints, and the resistor band has no cracks. ④ X-ray inspection: The weld line is clear, uniform, without ghosts or interruptions. ⑤ Bridge test: The resistance tested at 25℃ meets the requirements of the drawings. (II) Random inspection items: Destructive (DPA) inspection: ① Metallographic microscope inspection: There are no defects such as holes, no solder, thin solder, and discontinuous solder inside the weld. ② Shear stress test: The shear strength of the joint is ≥230MPa.

[0019] Embodiment 2: Vacuum brazing of 1000 chip resistors: 1. Parts to be welded: 1000 resistors (2000 copper electrodes, 1000 manganese copper resistor strips). 2. Solder: AgCu28 solder paste.

[0020] This embodiment is basically the same as the first embodiment, except that the solder in this embodiment is AgCu28 solder paste, the welding temperature is 910°C, and in order to shorten the cooling time, nitrogen is introduced into the furnace after cooling to 400°C.

[0021] The specific operations of steps (3) and (4) are as follows: (3) Solder and weldment installation: a. Insert the resistor band into the electrode and place it on the table. Use a depth gauge to determine the position of the resistor band in the electrode according to the drawing size. b. Use a vernier caliper to measure the total length of the resistor. If it is larger than the drawing size, use manual flat-nose pliers to squeeze the electrode and resistor band so that the total length of the resistor meets the drawing requirements. c. Use a needle tube filled with solder paste to apply solder paste to the weld seam, use a cotton swab to clean the excess solder paste outside the weld seam, and use a hot air blower to dry the solder paste. d. Place the resistors filled with solder paste neatly on the tray, with the spacing between them not touching each other. e. Place the tray with the weldment in the heating zone of the furnace, check to make sure that the solder is at the weld seam and the weldments are not touching each other. f. Close the furnace door and evacuate.

[0022] (4) Welding specifications: ① Start heating when the vacuum degree in the vacuum brazing furnace is not less than 1×10-4Pa. During the heating process, the vacuum degree in the furnace shall not be less than 1×10-3Pa. ② Heating specifications: 10min from room temperature to 200℃, 15min from 200℃ to 400℃, 15min from 400℃ to 600℃, 20min from 600℃ to 800℃, 15min from 800℃ to 910℃. ③ Insulation specifications: Insulation at 910℃ for 20min. ④ Cooling specifications: Turn off the heating power supply, fill the furnace with nitrogen when the temperature drops to 400℃, and open the furnace door to take out the soldered resistor after the temperature drops to 50℃.

[0023] The existing brazing method was used to inspect and test the resistors produced in batches with feed amounts of 300, 200, 100, and 50, respectively; the method of the present invention was used to inspect and test the resistors produced in batches with feed amounts of 100, 300, 500, and 600, respectively; the statistical data of the pass rates of the two welding methods are shown in Table 1: Table 1: Statistics of welding pass rates of existing brazing methods and the method of the present invention It can be seen from Table 1 that the welding qualification rate of the existing brazing method is less than 10%, while the welding qualification rate of the method of the present invention reaches more than 95%.

[0024] In the present invention, the solder and the weldment have high cleanliness, the solder has good wettability to the weldment, and there is no desoldering, cold soldering, thin solder, inability to flow, or incomplete flow defects; the weldment is heated as a whole, the welding temperature is uniform, and there is no weldment corrosion, solder nodules, unmelted solder, discontinuous welds, etc.; the welding stress is small, and there are no defects such as weld deformation, embrittlement, and fracture; the welding is completed in a vacuum to remove hydrogen, oxygen, and water vapor, and the weld is free of defects such as pores, impurities, oxidation, and decarburization; the welding temperature is controllable, and the resistor strip has no cracks or resistance value mutation defects; the joint has high shear strength (≥230MPa); no complex tooling and fixtures are required, the process is simple, and the welding cost is low; hundreds or thousands of pieces can be welded at one time, and the production efficiency is high.

Claims

1. A vacuum brazing method for a chip resistor, characterized in that: The steps include: (1) Welding environment: Operate in a fully enclosed cleanroom, control the air purity, temperature and humidity to meet welding requirements, and remove dirt and oil from tools and equipment. Wear gloves when installing solder and weldment. (2) Pre-welding treatment: Degreasing and surface cleanliness inspection of solder and weldment until the surface cleanliness meets welding requirements; (3) Solder and weldment mounting: Solder can be selected from welding wire or solder paste, control the position of the resistor band and the total length of the resistor to meet the requirements of the drawing, and then place the plate and send it into the vacuum brazing furnace; (4) Welding specifications: The following specifications should be met when welding in a vacuum brazing furnace: ① When the vacuum degree in the furnace is not less than 1×10 - 4 P a Start heating when the vacuum degree in the furnace should not be less than 1×10 -3 P a ;② Heating specification: 10min from room temperature to 200℃, 15min from 200℃ to 400℃, 15min from 400℃ to 600℃, 20min from 600℃ to 800℃, 15min from 800℃ to 860℃~920℃;③ Insulation specification: Insulation for 20min;④ Cooling specification: You can choose natural cooling or cooling by filling the furnace with nitrogen; (5) Post-weld inspection and testing: including full inspection items and random inspection items. Full inspection items include visual inspection, dimensional inspection, tool microscope inspection, X-ray inspection and bridge test; random inspection items include destructive (DPA) inspection and shear stress test.

2. A chip resistor vacuum brazing method as claimed in claim 1, characterized in that: In the step (1), the air purification degree is not less than 100,000, the humidity is not greater than 45%, and the temperature is 20-25°C. The surfaces of tools and instruments are decontaminated and degreased with alcohol or acetone before use. When solder and welded parts are mounted on the rack, fine sand or medical PVC gloves are worn.

3. A chip resistor vacuum brazing method as claimed in claim 1, characterized in that: In the step (2), when degreasing, the solder and weldment are placed in an acetone solution, ultrasonically cleaned for 10 minutes to remove surface oil stains, taken out and placed on a gauze pad, and dried with a hot air blower; the solder and weldment are tested with a cleanliness detector, requiring the surface cleanliness RFU value to be ≤5.0, otherwise they are cleaned again.

4. A chip resistor vacuum brazing method as claimed in claim 1, characterized in that: In the step (3), when the solder is selected as welding wire: A. Solder rack: shape the welding wire into a straight strip, then bend and tie it to the two ends of the resistance band, and cut off the excess solder at the interface; B. Insert the resistance band tied with welding wire into the electrode and place it on the table, and fix the position of the resistance band in the electrode; C. Measure the total length of the resistor. If the total length does not meet the requirements of the drawing, adjust it. D. Move the welding wire to the weld and make it as close to the weld as possible; E. Place the soldered resistors neatly on the tray, with spacing so that they do not touch each other; F. Place the tray with the welded parts in the heating zone of the furnace, check and make sure the solder is at the weld seam and the welded parts are not in contact with each other; G. Close the furnace door and evacuate the air; When solder paste is selected for solder: a. Insert the resistor strip into the electrode and place it on the table, fix the position of the resistor strip in the electrode; b. Measure the total length of the resistor, if it does not meet the requirements of the drawing, adjust it; c. Apply solder paste on the weld, clean up the excess solder paste outside the weld, and blow dry the solder paste with a hot air blower; d. Place the resistors filled with solder paste neatly on the tray, with the spacing preferably not touching each other; e. Place the tray with the welded parts in the heating zone of the furnace, check to make sure that the solder is at the weld and the welded parts are not touching each other; f. Close the furnace door and evacuate.

5. A chip resistor vacuum brazing method as claimed in claim 1, characterized in that: In the step (4), the relationship between the insulation temperature of welding and the number of welds is: 870°C for less than 200 pieces, 890°C for 201 to 600 pieces, and 910°C for 601 to 1000 pieces.

6. A chip resistor vacuum brazing method as claimed in claim 1, characterized in that: In the step (5), the post-welding inspection and test are as follows: (I) Full inspection items: ① Visual inspection: The weldment has no defects such as desoldering, embrittlement, corrosion, cracking, solder nodules, excessive flow, incomplete weld filler, etc.; ②Dimension inspection: Vernier caliper is used to measure the total length of the resistor and the position of the resistor belt is in accordance with the drawing; ③Tool microscope inspection: There are no leaks, cold welds, and cracks in the resistor belt; ④X-ray inspection: The weld line is clear, uniform, without ghosts or interruptions; ⑤Bridge test: The resistance tested at 25℃ meets the requirements of the drawing; (II) Inspection items: Destructive (DPA) inspection: ① Metallographic microscope inspection: no holes, no solder, thin solder, discontinuous solder and other defects inside the weld; ② Shear stress test: joint shear strength ≥ 230MPa.

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