Brazing structure and brazing method of lightweight flexible graphite thermal conductive wire

Abstract

The present invention discloses a brazing structure and brazing method of a lightweight flexible graphite thermal cable. The brazing structure comprises a graphite film stack composed of multiple layers of graphite films, and heat-conducting metal connecting ends located on both sides of the graphite film stack; the graphite film stack comprises a middle flexible section, two end connecting sections of the graphite film, and a gold-based solder layer is arranged between two adjacent layers of graphite films of the two end connecting sections of the graphite film; sandwich structure composite solder layers are respectively arranged between the two end connecting sections of the graphite film and the heat-conducting metal connecting ends, and the sandwich structure composite solder layers are composed of a gold-based solder foil, a lead-based solder sheet, and a gold-based solder foil arranged in sequence. The brazing method of the graphite thermal cable of the present invention adopts the brazing structure, and the prepared graphite thermal cable has a small thermal resistance at the connection interface, a large heat flux, a high bonding strength between the flexible graphite section and the end metal, resistance to mechanical vibration and temperature alternating shock, and high service safety and reliability.

Description

Technical Field

[0001] The invention relates to a brazing structure and a brazing method of a lightweight flexible graphite heat-conducting cable, belonging to the technical field of heterogeneous material connection. Background Art

[0002] Thermal cables, also known as thermally conductive flexible connections, are heat transfer channels between heating devices and heat sinks. The heat of the heating device is quickly transferred to the heat sink heat dissipation end through the flexible thermal conductive section. The flexible thermal conductive section of the metal thermal cable generally uses oxygen-free copper or aluminum alloy filament braids or foil laminates with high thermal conductivity, which are consolidated with the heating device connection end and the heat sink connection end to achieve flexible heat transfer and flexible installation functions. Since all parts of the metal thermal cable are metal, its consolidation process is relatively simple, and conventional pressure welding or fusion welding can achieve good connection and 100% interface metallurgical bonding.

[0003] With the advancement of electronic technology and aerospace technology, the electronic devices integrated on the single-machine payload of spacecraft are becoming more and more dense, the power consumption of the single machine is getting higher and higher, and the working heat makes it difficult to maintain a constant temperature of its structural parts and electronic devices. There is an urgent need to find high thermal conductivity materials with higher thermal conductivity to achieve efficient heat transfer, especially for thermal cables used in satellites and various spacecraft. In addition to high thermal conductivity, they are also expected to be light and flexible. Based on the above-mentioned application traction and the emergence of high thermal conductivity materials, the low-density, high thermal conductivity carbon material "flexible graphite film" has attracted the attention of designers in the field of thermal management. At present, the thermal conductivity of lightweight flexible polyimide flexible graphite film with a thickness of less than 30μm in the plane direction has exceeded 1000W / (m·K) and has good flexibility. It has become an ideal alternative material for the flexible thermal conductive section of metal thermal cables. The density of graphite film material is 1.8g / cm 3 , has great potential for weight reduction compared to metal.

[0004] A graphite film with a thickness of less than 30 μm is selected to prepare a graphite film thermal cable. Since the single layer thickness is relatively thin, in order to achieve sufficient heat flux, a multi-layer stacking method is required to form a graphite film stack. Figure 1 , 2As shown, the two ends of the multilayer graphite film need to be compositely consolidated to form a graphite film stack 3, and the graphite film stack 3 is consolidated and connected with the heating device connection end 1 and the heat sink connection end 2 at both ends. Public information from several professional thermal cable manufacturing companies in Europe and the United States shows that the assembly and consolidation of graphite film thermal cables adopts a crimping process, that is, the layers at both ends of the graphite film stack are bonded with organic adhesives, and after curing, they are inserted into copper or aluminum sleeves for crimping. Zhang Xuexun and others from the Shanghai Satellite Equipment Research Institute in China also adopted the method of applying thermal grease and then tightening with bolts in the assembly and connection design of flexible graphite film and heat sink metal. Although the process of gluing and mechanical crimping is simple and easy to implement, the low thermal conductivity of organic adhesives and the high thermal resistance of the crimping interface greatly weaken the effect of efficient heat transfer in the plane direction of the graphite film. At the same time, this connection method also reduces the structural strength and thermal fatigue life of the graphite film thermal cable to a certain extent.

[0005] The use of brazing process to prepare graphite film thermal cables can avoid the problems of high thermal resistance and low thermal fatigue life of conductive adhesives and mechanical crimping to a certain extent. The use of metal brazing filler metal (also known as filler metal) can achieve interface metallurgical bonding. Limited by the low starting temperature of graphite film oxidation (400°C), direct brazing with active brazing filler metal cannot be implemented, and only indirect brazing process scheme with graphite film surface metallization can be adopted. Almost all public documents and patents use tin-based solder, that is, eutectic tin-lead alloy solder. The use of tin-lead solder requires flux, so in the preparation of graphite film thermal cables, tin-based brazing paste is generally used to coat the surface of the welding interface area. The main problem of using tin-based brazing filler metal to weld graphite film thermal cables is that the two types of welding interfaces between graphite film layers and between the graphite film laminate and the metal end are relatively closed areas, and the flux and the gas generated by it are difficult to be completely discharged. Pores and flux residues are generated at the welding interface, resulting in defects such as reduced interface bonding strength and thermal conductivity. The residual solder flux will decompose and volatilize in a thermal vacuum environment, which will greatly increase the risk of graphite film thermal conductors being contaminated near precision optoelectronic devices. In addition, the thermal expansion coefficient of flexible graphite film materials is about 1×10 -6 / k, the thermal expansion coefficients of copper and aluminum alloys are both greater than 19×10-6 / k. The thermal expansion coefficients of graphite film and end metal are very different. Whether the low-strength tin-based solder weld can withstand the temperature alternating thermal stress of the weld area during service is also a potential interface failure factor.

[0006] In order to further improve the overall heat transfer efficiency and brazing connection strength of graphite film thermal cables, it is necessary to design the brazing system selection, brazing layer thickness, laying method, and thermal expansion mismatch and thermal stress relief between graphite film and metal to meet the urgent needs of satellite and spacecraft thermal control systems for high heat flux and long life graphite film thermal cables. Summary of the invention

[0007] The purpose of the present invention is to provide a brazing structure of a lightweight flexible graphite thermal cable, which can improve the overall heat transfer efficiency and brazing connection strength of the graphite film thermal cable and has the characteristics of high thermal conductivity, high strength and long life.

[0008] Another object of the present invention is to provide a method for brazing a graphite thermal cable, wherein the method adopts the brazing structure of the lightweight flexible graphite thermal cable.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] A brazing structure for a lightweight flexible graphite thermally conductive cable, comprising a graphite film stack composed of multiple layers of graphite films, and thermally conductive metal connecting ends located on both sides of the graphite film stack; the graphite film stack comprises a middle flexible section, connecting sections at both ends of the graphite film, and a gold-based solder layer is provided between two adjacent layers of graphite films in the connecting sections at both ends of the graphite film; sandwich structure composite solder layers are provided between the connecting sections at both ends of the graphite film and the thermally conductive metal connecting ends, respectively, and the sandwich structure composite solder layers are composed of a gold-based solder foil, a lead-based solder sheet, and a gold-based solder foil which are arranged in sequence.

[0011] Preferably, the thickness of the gold-based solder layer between two adjacent graphite film layers is preferably 5-10 μm.

[0012] Preferably, the total thickness of the sandwich structure composite solder layer is 190-380 μm, wherein the thickness of the gold-based soft solder foil is 20-40 μm, and the thickness of the lead-based soft solder sheet is 150-300 μm.

[0013] Preferably, a metallization layer is pre-set on the surface of the welding area of ​​the connecting sections at both ends of the graphite film, and the metallization layer is composed of a Ti film, a Mo film, and a Ni film deposited in sequence.

[0014] Preferably, the graphite film stack is composed of 10-50 layers of graphite films, and the thickness of a single layer of graphite film is 20-30 μm.

[0015] A method for brazing a graphite thermal cable, which adopts the brazing structure of the lightweight flexible graphite thermal cable, specifically comprises the following steps:

[0016] (1) Metallizing the surfaces of the welding areas of the connecting sections at both ends of the graphite film;

[0017] (2) depositing an ultra-thin gold-based soft solder layer on the metallized surface of the graphite film using a physical vapor deposition (PVD) method;

[0018] (3) preparing a gold-based soft solder foil and a lead-based soft solder sheet, and stacking them to form a sandwich structure composite solder sheet;

[0019] (4) Pretreatment and modification of the welding surface of the heat-conducting metal connection end;

[0020] (5) Assembling, stacking the multi-layer graphite film and butting and assembling them with the heat-conducting metal connecting end, and inserting the sandwich structure composite solder sheet obtained in step (3) into the gap between the connecting sections at both ends of the graphite film and the heat-conducting metal connecting end;

[0021] (6) placing the assembled graphite film thermal cable in a vacuum brazing furnace for vacuum brazing, and cooling after brazing to obtain a lightweight flexible graphite thermal cable integral welded part.

[0022] In the method for soldering the graphite thermal cable, the melting point of the gold-based solder cannot exceed 350°C, and the melting point of the lead-based solder in the sandwich structure composite solder layer must be 30-50°C higher than the melting point of the gold-based solder.

[0023] In the brazing method of the graphite thermal conductive cable, the metallization treatment method in step (1) is a PVD method, and the PVD method is ion plating or magnetron sputtering.

[0024] In the brazing method of the graphite thermal cable, a physical vapor deposition process is used to ultra-thin the solder layer on the metallized surface of the graphite film, so that the number of graphite film assemblies can be maximized within a limited assembly thickness to ensure heat transfer flux. Preferably, the PVD method in step (2) is magnetron sputtering.

[0025] In the brazing method of the graphite thermal cable, the material of the thermally conductive metal connecting end is aluminum alloy or oxygen-free copper, and the pretreatment and modification include alkaline degreasing, chemical cleaning and nickel electroplating treatment on the welding surface.

[0026] In the brazing method of the graphite thermal conductive wire, the vacuum degree of the vacuum brazing in the step (6) is higher than 10 - 3 Pa.m 3 / s, the holding time at the brazing temperature shall not exceed 10min, and the heating and cooling rate of the vacuum brazing furnace shall be lower than 30℃ / min.

[0027] The advantages of the present invention are:

[0028] The graphite film thermal cable brazing assembly structure of the present invention can achieve ultra-thin, dense, and 100% metallurgical bonding of the brazing interface between graphite film layers, greatly reducing the thermal resistance of the connection interface; at the same time, it can effectively alleviate the thermal stress generated by the thermal expansion mismatch between the graphite film stack and the end heat transfer metal, avoid deformation and cracking of the heterogeneous brazing interface, thereby improving the brazing connection strength and service safety and reliability.

[0029] The graphite film thermal cable prepared by the method of the present invention has small thermal resistance at the connection interface, large heat flux, high bonding strength between the flexible graphite segment and the end metal, resistance to mechanical vibration and temperature alternating shock, and high service safety and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the structure of graphite film thermal cable.

[0031] Figure 2 for Figure 1 Cross-sectional view taken along line AA.

[0032] Figure 3 It is a schematic diagram of a partial cross-section of the brazing structure of the lightweight flexible graphite film thermal cable of the present invention.

[0033] Reference numerals:

[0034] 1-heating device connection end, 2-heat sink connection end, 3, 11-graphite film stack, 12-heat conductive metal connection end, 13-gold-based solder layer, 14, 16-gold-based solder foil, 15-lead-based solder foil. DETAILED DESCRIPTION

[0035] The present invention is described in detail below with reference to the accompanying drawings and embodiments, but this does not necessarily limit the protection scope of the present invention.

[0036] like Figure 3 As shown, the brazing structure of the lightweight flexible graphite thermal cable of the present invention includes a graphite film stack 11 composed of multiple layers of graphite films and a thermally conductive metal connecting end 12; the graphite stack has a middle flexible section, connecting sections on both sides of the graphite film, and a gold-based solder layer 13 is provided between two adjacent layers of graphite films in the connecting sections on both sides of the graphite film; sandwich-structured composite solder layers are provided between the connecting sections on both sides of the graphite film and the thermally conductive metal connecting ends, and the sandwich-structured composite solder layers are composed of a gold-based solder foil 14, a lead-based solder foil 15, and a gold-based solder foil 16 which are arranged in sequence.

[0037] The present invention adopts an oxidation-resistant, flux-free gold-based soft solder to realize vacuum brazing connection between graphite film / graphite film and graphite film / metal. The gold-based soft solder has the highest thermal conductivity among low-temperature solders, which is beneficial to improving the overall heat transfer capacity of the graphite film thermal cable. The use of oxidation-resistant gold-based soft solder can realize flux-free vacuum brazing, further improve the quality of the weld and reduce various defects caused by the flux. In the present invention, the melting point of the gold-based soft solder is required to be no more than 350°C. The thickness of the gold-based soft solder layer between two adjacent graphite film layers is preferably 5-10μm to increase the effective heat transfer cross-sectional area. The present invention uses the gold-based soft solder to obtain a higher interface bonding strength of the flexible thermal cable, which can effectively improve the impact and mechanical vibration resistance of the graphite film thermal cable, and the performance is more reliable and the service life is longer.

[0038] The present invention uses a sandwich structure composite solder layer of "gold-based solder foil / lead-based solder sheet / gold-based solder foil" with good plasticity (ductility) as the sandwich layer, which can effectively alleviate the welding (thermal) stress caused by the thermal expansion mismatch between the graphite film stack and the end metal (aluminum alloy or copper alloy), and improve the thermal stability of the graphite film thermal conductive cable. The melting point of the lead-based solder in the sandwich structure composite solder layer must be 30 to 50°C higher than the melting point of the gold-based solder. Preferably, the total thickness of the sandwich structure composite solder layer is 190-380μm, of which the thickness of the gold-based solder foil is 20-40μm, and the thickness of the lead-based solder foil is 150-300μm.

[0039] In the present invention, a metallization layer is pre-set on the surface of the welding area of ​​the connecting sections at both ends of the graphite film, and the metallization layer is composed of a Ti film, a Mo film, and a Ni film deposited in sequence.

[0040] In the present invention, the number of graphite film layers in the graphite film stack can be determined according to design requirements and heat flux index requirements. Preferably, the graphite film stack is composed of 10-50 layers of graphite film, and the thickness of a single layer of graphite film is 20-30 μm.

[0041] As an embodiment of the present invention, a method for brazing a graphite thermal cable using the brazing structure of the lightweight flexible graphite thermal cable specifically comprises the following steps:

[0042] (1) metallizing the surfaces of the welding areas of the connecting sections at both ends of the graphite film, and sequentially depositing three layers of Ti, Mo, and Ni by ion plating or magnetron sputtering;

[0043] (2) depositing an ultra-thin gold-based soft solder layer with a thickness of 5-10 μm on the metallized surface of the graphite film by magnetron sputtering;

[0044] (3) preparing a gold-based solder foil and a lead-based solder foil, and stacking them to form a sandwich structure composite solder sheet;

[0045] (4) pre-treating and modifying the welding surface of the heat-conducting metal connection end, wherein the pre-treatment and modification include alkaline degreasing, chemical cleaning and nickel electroplating treatment of the welding surface;

[0046] (5) Assembling, stacking the multi-layer graphite film and butting and assembling them with the heat-conducting metal connecting end, and inserting the sandwich structure composite solder sheet obtained in step (3) into the gap between the connecting sections at both ends of the graphite film and the heat-conducting metal connecting end;

[0047] (6) Place the assembled graphite film thermal conductor in a vacuum brazing furnace for vacuum brazing. The vacuum degree of vacuum brazing is higher than 10 -3 Pa·m 3 / s, the holding time at the brazing temperature does not exceed 10min, and the heating and cooling rate of the vacuum brazing furnace is lower than 30℃ / min; after brazing, it is cooled to obtain a lightweight flexible graphite thermal conductive cable integral welded part.

[0048] Example 1: Preparation of flexible graphite thermal cable with aluminum alloy as the end thermal conductive metal

[0049] 1) According to the design drawings of graphite film thermal cable brazing assembly, the flexible graphite film material is cut. The flexible section is made of 30 layers of graphite film with a single layer thickness of 25 μm. The flexible section is 150 mm long and 20 mm wide.

[0050] 2) The surface and side surfaces of the welded areas at both ends of each layer of flexible graphite film are ion-plated to prepare a base layer (Ti), and then a solder barrier layer of molybdenum (Mo) and a surface layer of nickel (Ni) are deposited in sequence by a magnetron sputtering process to obtain an ultra-thin, high-bonding Ti\Mo\Ni metallized film layer, in which the titanium layer is about 0.1μm, the molybdenum layer is 0.3μm, and the nickel layer is about 1μm.

[0051] 3) A gold-based soft solder layer is deposited on the surface of the graphite film metallization layer using a magnetron sputtering process, and the thickness of the solder layer is controlled at 6-8 μm to ensure uniform thickness.

[0052] 4) Process the aluminum alloy parts according to the requirements of the thermal wire brazing assembly design drawings, perform alkaline degreasing, chemical cleaning, and surface nickel electroplating treatment.

[0053] 5) Prepare a 30 μm thick Au-20Sn solder foil and a 200 μm thick Pb-3Ag soft solder sheet, cut and stack them to form a sandwich composite solder sheet.

[0054] 6) Assembly: Use a specially designed fixed clamping tool to stack 30 layers of graphite film, and insert a "sandwich" composite brazing material stack into the contact gap between the graphite film stack and the aluminum alloy.

[0055] 7) Put the assembled graphite heat conducting wire and welding tooling into the vacuum brazing furnace, evacuate, heat, and keep warm at the brazing temperature of 320℃ for no more than 10 minutes, with a heating and cooling rate of less than 30℃ / min. During the whole process, ensure that the vacuum degree in the furnace is better than 10 -3 Pa·m 3 / s.

[0056] 8) The performance test of the flexible graphite thermal conductive cable with aluminum alloy at the end prepared in the embodiment was carried out, and it was found that the interface between the flexible graphite and the aluminum alloy was 100% metallurgical bonded, without defects such as cracks and holes, and the interface heat transfer coefficient was 10600 W / m 2 K, the tensile strength of the welding interface between the end aluminum alloy and the graphite film stack is 40.5MPa.

[0057] Example 2: Preparation of flexible graphite thermal cable with oxygen-free copper as the end thermal conductive metal

[0058] 1) According to the design drawings of the graphite film thermal cable brazing assembly, the flexible graphite film material is cut. The flexible section is made of 30 layers of graphite film. The flexible section is 150 mm long and 20 mm wide.

[0059] 2) The surface and side surfaces of the welded areas at both ends of each layer of flexible graphite film are ion-plated to prepare a base layer (Ti), and then a solder barrier layer of molybdenum (Mo) and a surface layer of nickel (Ni) are deposited in sequence by a magnetron sputtering process to obtain an ultra-thin, high-bonding Ti\Mo\Ni metallized film layer, in which the titanium layer is about 0.1μm, the molybdenum layer is 0.3μm, and the nickel layer is about 1μm.

[0060] 3) A gold-based soft solder layer is deposited on the surface of the graphite film metallization layer using a magnetron sputtering process, and the thickness of the solder layer is controlled at 5-6 μm to ensure uniform thickness.

[0061] 4) Process oxygen-free copper parts according to the requirements of the thermal cable brazing assembly design drawings, and perform alkaline degreasing and chemical cleaning on them.

[0062] 5) Prepare a 30 μm thick gold-based solder foil and a 200 μm thick lead-based solder sheet, cut and stack them to form a sandwich composite solder sheet.

[0063] 6) Assembly: Use a specially designed fixed clamping tool to stack 30 layers of graphite film, and insert a "sandwich" composite brazing material stack into the contact gap between the graphite film stack and the aluminum alloy.

[0064] 7) Put the assembled graphite heat conducting wire and welding tooling into the vacuum brazing furnace, evacuate, heat, and keep warm at the brazing temperature for no more than 10 minutes, with a heating and cooling rate of less than 30℃ / min. During the whole process, ensure that the vacuum degree in the furnace is better than 10 -3 Pa·m 3 / s.

[0065] 8) The performance test of the flexible graphite thermal conductive cable with aluminum alloy at the end prepared in the embodiment was carried out, and it was found that the interface between the flexible graphite and the oxygen-free copper was 100% metallurgical bonded, without defects such as cracks and holes, and the interface heat transfer coefficient was 11200 W / m 2 K, tensile strength of the welding interface between end oxygen-free copper and graphite film stack is 40.2MPa.

Claims

1. A brazing structure of a lightweight flexible graphite thermal cable, characterized in that: The soldering structure comprises a graphite film stack composed of multiple layers of graphite films, and heat-conducting metal connecting ends located on both sides of the graphite film stack; the graphite film stack comprises a middle flexible section, and connecting sections at both ends of the graphite film, and a gold-based solder layer is arranged between two adjacent layers of graphite films in the connecting sections at both ends of the graphite film; sandwich structure composite solder layers are respectively arranged between the connecting sections at both ends of the graphite film and the heat-conducting metal connecting ends, and the sandwich structure composite solder layers are composed of a gold-based solder foil, a lead-based solder sheet, and a gold-based solder foil arranged in sequence; The thickness of the gold-based solder layer between two adjacent graphite film layers is 5-10 μm; The total thickness of the sandwich structure composite solder layer is 190-380 μm, wherein the thickness of the gold-based soft solder foil is 20-40 μm, and the thickness of the lead-based soft solder welding sheet is 150-300 μm.

2. The brazing structure of the lightweight flexible graphite thermal cable according to claim 1, characterized in that: A metallization layer is pre-arranged on the surface of the welding area of ​​the connecting sections at both ends of the graphite film, and the metallization layer is composed of a Ti film, a Mo film, and a Ni film deposited in sequence.

3. The brazing structure of the lightweight flexible graphite thermal cable according to claim 1, characterized in that: The graphite film stack is composed of 10-50 layers of graphite films, and the thickness of a single layer of graphite film is 20-30 μm.

4. A method for brazing a graphite thermal cable, characterized in that: The brazing structure of the lightweight flexible graphite thermal conductive cable according to any one of claims 1 to 3 is adopted, and comprises the following steps: (1) Metallizing the surfaces of the welding areas of the connecting sections at both ends of the graphite film; (2) Using the PVD method to deposit an ultra-thin gold-based soft solder layer on the metallized surface of the graphite film; (3) preparing a gold-based soft solder foil and a lead-based soft solder sheet, and stacking them to form a sandwich structure composite solder sheet; (4) Pretreatment and modification of the welding surface of the thermally conductive metal connection end; (5) Assembling, stacking the multi-layer graphite film and butting them with the heat-conducting metal connecting end, and inserting the sandwich structure composite solder sheet obtained in step (3) into the gap between the connecting sections at both ends of the graphite film and the heat-conducting metal connecting end; (6) The assembled graphite film thermal conductor is placed in a vacuum brazing furnace for vacuum brazing. After the brazing is completed, the thermal conductor is cooled to obtain a lightweight flexible graphite thermal conductor integral welded part.

5. The method for brazing a graphite thermally conductive cable according to claim 4, characterized in that: The melting point of the gold-based soft solder does not exceed 350° C., and the melting point of the lead-based soft solder in the sandwich structure composite solder layer must be 30-50° C. higher than the melting point of the gold-based soft solder.

6. The method for brazing a graphite thermally conductive cable according to claim 4, characterized in that: In the step (1), the metallization treatment method is a PVD method, and the PVD method is ion plating or magnetron sputtering; the PVD method in the step (2) is magnetron sputtering.

7. The method for brazing a graphite thermal cable according to claim 4, characterized in that: In step (3), the material of the heat-conducting metal connecting end is aluminum alloy or oxygen-free copper; in step (4), the pretreatment and modification include alkaline degreasing, chemical cleaning and nickel electroplating treatment on the welding surface.

8. The method for brazing a graphite thermally conductive cable according to claim 4, characterized in that: In the step (6), the vacuum degree of vacuum brazing is higher than 10 -3 Pa·m 3 / s, the holding time at 320℃±10℃ brazing temperature shall not exceed 10min, and the heating and cooling rate of the vacuum brazing furnace shall be lower than 30℃ / min.

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