An electrically conductive material resistance spot welding process

Abstract

The application relates to a conductive material resistance spot welding process, which comprises the following steps: S01, coating nano silver glue on a welding surface of the conductive material; S02, adopting a resistance spot welding process to weld the conductive material, so that a dense and non-porous defect solid-phase sintered silver layer is formed on a welding point interface, and a porous solid-phase sintered silver layer is formed on a welding point surrounding interface, thereby realizing high-performance connection of the conductive material. Compared with the prior art, the application can greatly increase the effective connection area of the interface without melting the base material, avoids thermal damage of the material, and obtains excellent connection strength and conductive performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of electrical connection, and relates to a welding process for increasing the effective area of the connection interface of conductive material, reducing the thermal damage of the material, and thus obtaining a joint with high strength and high conductivity. BACKGROUND

[0002] In the integrated manufacturing of solar cells, a large number of conductive materials need to be connected to realize the transmission of electric energy. High-quality connection between conductive materials is an important guarantee for the efficient and stable operation of a solar cell system.

[0003] The conductive materials in a solar cell system are mainly micro-scale sheet or wire materials made of silver, gold or copper. The current process is mainly resistance spot welding. However, during the welding process, the welding heat is easy to cause thermal damage to the micro-scale materials, resulting in a decrease in the force and electrical performance of the joint. If the welding heat input is reduced, it is difficult to establish a larger effective connection area at the welding interface, and false welding is easy to occur, resulting in a decrease in the performance of the joint. These two contradictory problems seriously limit the improvement of the performance of the joint.

[0004] Patent application CN202210261007.3 discloses a method and structure for interconnecting battery strings, wherein the method for interconnecting battery strings comprises: providing a plurality of battery strings and arranging the plurality of battery strings side by side, the back surface of each battery string having a welding point; providing a conductive foil connecting strip, and placing the conductive foil connecting strip on the back surface of the plurality of battery strings and opposite to the welding points; and using a laser welding process to weld the conductive foil connecting strip and the welding points of the plurality of battery strings arranged side by side together. Although this method for interconnecting battery strings can reduce the series resistance, parallel resistance and shadow area, solve the problem of thermal stress between the battery string and the conductive foil connecting strip, and the problem of hidden cracking of the battery string caused by welding. However, it cannot solve the above-mentioned contradictory problems, especially cannot realize interface connection at low temperature. SUMMARY

[0005] The purpose of the present application is to overcome the defects of the prior art and provide a conductive material resistance spot welding process, which can greatly increase the effective connection area at the interface without damaging the base material and improve the performance of the joint. This is of great significance for reducing the electric energy loss of a solar cell system and improving the reliability of the solar cell system.

[0006] The purpose of the present application can be achieved by the following technical solution: conductive material resistance spot welding, comprising the following steps:

[0007] S01 applying nano-silver glue to the welding surface of the material to be welded (i.e. conductive material);

[0008] S02 adopts resistance spot welding process to weld the conductive material, a dense and non-porous defect-free solid-phase sintered silver layer is formed at the interface of the welding spot, a porous solid-phase sintered silver layer is formed at the interface around the welding spot, and high-performance connection of the conductive material is realized.

[0009] Further, the material to be welded is a conductive sheet (or wire) with a thickness (or diameter) of 1-500 microns, or a sheet plated with conductive material with a thickness of 1-500 microns, including solar cell sheets, interconnection sheets, wires and microelectronic devices.

[0010] Further, the silver particle size of the nano-silver glue is 1-500 nm.

[0011] Further, the gluing step is as follows: first, clean the surface of the material to be welded with alcohol or other cleaning agents, fix the cleaned material on the workbench to ensure that the sample surface to be processed is flat and free of warping, apply the nano-silver glue to the material to be welded by spot coating or printing, and then stack another piece of material on the glued material. For spot coating, the spreading of the glue layer can be completed by pre-pressing the electrode before welding; for printing, the thickness and area of the glue layer are directly controlled by the printing equipment and tooling. The thickness of the glue layer needs to be controlled within 1-100 microns, and the area of the glue layer should not be less than the size of the electrode end face.

[0012] Further, the resistance spot welding electrode arrangement is symmetrical on both sides or parallel on the same side.

[0013] Further, the resistance spot welding electrode material is pure molybdenum, molybdenum alloy, pure tungsten or tungsten alloy.

[0014] Further, the resistance spot welding electrode end section is circular or rectangular, and the diameter (circular) or length and width (rectangular) of the end section is 0.1-10 mm.

[0015] Further, the number of layers of material welded at one time is 2 or more.

[0016] Further, the specific steps of the resistance spot welding process are as follows: first, place the glued and assembled material between the electrodes (symmetrical electrode arrangement) or under the electrodes on the workbench (parallel electrode arrangement on the same side), drive the electrode head to press the welding area, apply direct current or alternating current for welding, and keep the pressure for a period of time after welding, then lift the electrode, and complete the welding. The electrode pressing force is 1-50 N, the resistance spot welding current is 10-2000 A, the welding time is 10 ms-10 s, and the pressure holding time is 10-1000 ms.

[0017] Compared with the prior art, the present application has the following advantages:

[0018] 1. The present application adopts the method of coating nano-silver glue on the surface of material to be welded and then performing resistance spot welding, utilizes the characteristics of low-temperature solid-phase sintering of nano-silver, realizes solid-phase connection of conductive material (i.e. the conductive material does not melt), thereby avoiding the problem of thermal damage of the material, and forms effective connection in the interface of the welding spot and the interface area around the welding spot, which can greatly improve the mechanical strength and conductive performance of the joint.

[0019] 2. The present application can realize non-damage and high-performance connection of the joint of conductive material by only adding nano-silver glue coating before the conventional welding process, which is easy to apply in production. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 Process diagram of the present application for coating nano-silver glue and welding process;

[0021] Figure 2 Welding process cross-sectional schematic diagram under the symmetrical arrangement of the two sides of the electrode;

[0022] Figure 3 Welded joint cross-sectional schematic diagram under the symmetrical arrangement of the two sides of the electrode;

[0023] Figure 4 Welding process cross-sectional schematic diagram under the parallel arrangement of the same side of the electrode;

[0024] Figure 5 Welded joint cross-sectional schematic diagram under the parallel arrangement of the same side of the electrode;

[0025] Figure 6 Microscopic morphology of the interface of the silver interconnection piece without nano-silver glue;

[0026] Figure 7 Microscopic morphology of the interface of the silver interconnection piece with added nano-silver glue;

[0027] Figure 8 Microscopic morphology of the interface around the silver interconnection piece with added nano-silver glue;

[0028] Figure 9 Tensile shear peak load of the silver interconnection piece welding joint without nano-silver glue and the silver interconnection piece welding joint with added nano-silver glue;

[0029] Figure 10 Resistance of the silver interconnection piece welding joint without nano-silver glue and the silver interconnection piece welding joint with added nano-silver glue. DETAILED DESCRIPTION

[0030] The present application will be described in detail below in combination with the drawings and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present application, and detailed implementation and specific operation process are given, but the protection scope of the present application is not limited to the following embodiments.

[0031] like Figure 1 As shown, the present invention provides a resistance spot welding process for conductive materials, comprising the following steps:

[0032] S01 When welding two layers of conductive materials, apply an appropriate amount of nano silver paste 2 to the surface of the conductive material 3 to be welded on one side using a dispensing needle 1, and place the area to be welded on the other side of the conductive material 3 at the application point to complete the pre-welding assembly. When welding multiple layers of conductive materials, nano silver paste needs to be applied to the multiple layers of materials to ensure that all interfaces of the areas to be welded are coated with nano silver paste after assembly.

[0033] S02 uses resistance spot welding to weld conductive materials, and the electrode arrangement can be selected. Figure 2 The symmetrical arrangement shown (i.e., the welding electrodes 4 are symmetrically arranged on both sides of the conductive material 3) or Figure 4 The parallel arrangement shown is on the same side (i.e., welding electrode 4 is located on the same side as conductive material 3). During spot welding, welding pressure 6 first drives welding electrode 4 to press down on the area to be welded. Then, welding current 5 flows in from one side of welding electrode 4, through conductive material 3 and nano-silver paste 2 at the interface, and finally flows out from the other side of welding electrode 4. At this time, welding electrode 4, conductive material, and nano-silver paste will all generate resistance heat, causing the weld area (material pressure area) to heat up rapidly. Under the action of pressure and high temperature, the nano-silver paste at the weld interface undergoes rapid solid-state sintering and forms a metallurgical bond with the conductive materials on both sides, ultimately forming a dense, pore-free silver interface layer 7. At the same time, the heat from the weld area is rapidly conducted to the surrounding area, causing it to heat up. The nano-silver paste at the interface around the weld is affected by this and undergoes pressureless solid-state sintering, forming a porous silver interface layer 8. The two connection interfaces together achieve high-performance connection of conductive materials.

[0034] The conductive materials to be soldered include solar cells, interconnects, wires, or microelectronic devices.

[0035] All equipment used is commercially available. All raw materials used are commercially available; for example, Daicel DNS351 can be used for nano-silver paste.

[0036] The following examples illustrate this point.

[0037] Example:

[0038] I. Welding Materials and Equipment

[0039] 1) The material to be soldered is a 50μm thick silver interconnect sheet, and the nano-silver paste has a particle size of 100-300nm;

[0040] 2) The dispensing equipment uses a pneumatic dispensing machine (a commercially available product, such as the Ruiyi 983AM fully automatic precision pneumatic dispensing machine);

[0041] 3) The welding equipment uses a high-frequency direct current resistance spot welding machine (a commercially available product, such as a high-frequency direct current resistance spot welding machine available from AVIO).

[0042] II. Dispensing process and assembly

[0043] First, the surface of the material 3 (silver interconnection sheet) to be treated is cleaned by wiping with alcohol to remove surface contaminants, and then the material 3 is fixed on the workbench to ensure flatness and no warping. The side of the material 3 (silver interconnection sheet) to be welded is dispensed with nano-silver glue using a dispensing machine. The dispensing process used in this example is shown in Figure 1 . The dispensing parameters are set as follows: time 20 ms, air pressure 50 psi, and dispensing amount about 0.06 mm 3 . After dispensing, the other side of the material 3 (silver interconnection sheet) to be welded is placed at the dispensing position to complete the pre-weld assembly.

[0044] III. Welding process

[0045] The welding process used in this example is resistance spot welding. The electrode tip is circular in cross-section with a diameter of 2 mm, and the electrode material is pure molybdenum. The electrodes are arranged symmetrically on both sides (see Figure 2 ). The assembled material to be welded is placed on the lower electrode end face, and the welding electrode 4 is pressed down to the material to be welded. A pre-pressing of 100 ms is performed to stabilize the welding pressure, and then a constant current welding current is applied for welding. The welding time is fixed at 150 ms, and different welding currents (600 A, 700 A, 800 A, and 850 A) are used for comparison to evaluate the joint performance under different welding heat inputs.

[0046] In addition, the same welding parameters are used to weld the 2-layer silver interconnection sheet without glue to compare the differences in joint performance and interface organization between the traditional process (without nano-silver glue resistance spot welding) and the process proposed in this application (with nano-silver glue resistance spot welding).

[0047] IV. Interface morphology of the welded joint

[0048] Figure 6 The welding current used for the silver interconnection sheet spot welded joint of the traditional process (without nano-silver glue resistance spot welding) is 850 A. It can be seen that although the upper limit of the welding current is selected, there are still a large number of gaps (i.e. unconnected areas) and hole defects in the interface, resulting in a small actual connection area of the interface. The reason is that the electrical resistivity of silver is low, and the heat generated during welding is small. The welding heat is mainly from the high-resistivity molybdenum electrode, so the high-temperature zone is located at the contact surface between the silver interconnection sheet and the molybdenum electrode, and the temperature is higher than that of the welding interface. If the actual connection area of the interface is increased by melting the welding interface by increasing the welding current, it will cause over-melting of the ultra-thin silver sheet, resulting in thermal damage (burning through), and adhesion between the material and the electrode.

[0049] Figure 7 For the silver interconnect sheet welding interface of the process (adding nano-silver glue resistance spot welding) proposed by the application, the welding current is 850 A. It can be seen that the nano-silver glue of the welding interface is converted into a dense silver layer without pore defects under the action of pressure and high temperature, and is well combined with the silver base materials on both sides. Figure 8 For the organizational morphology of the interface around the welding spot, due to the lack of welding pressure at this position, the nano-silver glue is solid-phase sintered under the action of high temperature, forms a porous structure, and is combined with the silver base materials on both sides. Therefore, the addition of nano-silver glue not only eliminates the interface gap and pore defects in the welding spot area, but also builds an interface layer in the area around the welding spot, greatly increasing the actual connection area of the interface, so that the joint obtains excellent mechanical and electrical properties.

[0050] Five, mechanical properties and electrical properties of the welded joint

[0051] Figure 9 The tensile shear strength of the silver interconnect sheet welding joint under different welding currents is compared between the traditional process (without nano-silver glue resistance spot welding) and the process (adding nano-silver glue resistance spot welding) proposed by the application. With the increase of the welding current, the strength of the welding spot under the traditional process gradually increases, which is because the increase of the welding current promotes the increase of the welding heat input, which is beneficial to the atomic interdiffusion behavior at the interface and the increase of the actual connection area at the interface. For the welded joint of the process proposed by the application, the increase of the welding current is also beneficial to the increase of the joint strength, mainly because the temperature rise is beneficial to the full combination of nano-silver particles. Compared with the traditional process, the welded joint of the process proposed by the application can obtain higher joint strength, for example, when the welding current is 850 A, the tensile shear strength of the joint of the process proposed by the application is ~ 30 N, while that under the traditional welding condition is only ~ 16 N.

[0052] Figure 10 The resistance of the welded joint obtained by the traditional process (without nano-silver glue resistance spot welding) and the process (adding nano-silver glue resistance spot welding) proposed by the application under different welding currents is compared. With the increase of the welding current, the resistance of the welding spot under the traditional process gradually decreases, which is because the increase of the actual connection area at the interface reduces the interface resistance. For the welded joint of the process proposed by the application, the increase of the welding current is also beneficial to the reduction of the joint resistance, mainly because the welding heat input makes the interface nano-silver particles combine more closely, thereby reducing the interface resistance. Overall, the joint resistance under the process proposed by the application is lower, for example, when the welding current is 850 A, the joint resistance under the process proposed by the application is only ~ 300 μΩ, while that under the traditional welding condition is ~ 450 μΩ.

[0053] Overall, the resistance spot welding process adding nano-silver glue proposed by the application can eliminate the interface gap and pore defects in the welding spot area, and form an interface layer in the area around the welding spot, greatly increase the actual connection area of the interface, so that the joint obtains excellent mechanical and electrical properties.

[0054] The foregoing description of the examples has been presented for the purposes of illustration and description. It is apparent to those skilled in the art that various modifications can be practiced as set forth in the foregoing description in conformance with the undertakings set forth in the disclosure of the application, without unduly departing from the spirit and scope of the application. Accordingly, the application is not limited to that described above, but is amenable to modification not only by those skilled in the art, but also by those who are not in the art.

Claims

1. An electrically conductive material resistance spot welding process characterized by, The method comprises the following steps: S01: applying nano-silver glue to the welding surface of the material to be welded; S02: using resistance spot welding process to weld the conductive material, so that a dense and non-porous defect-free solid-phase sintered silver layer is formed on the interface of the welding spot, and a porous solid-phase sintered silver layer is formed on the interface around the welding spot, thereby realizing high-performance connection of the conductive material; the specific steps of the resistance spot welding process are as follows: first, place the material coated with glue and assembled on the workbench between the electrodes or below the electrodes, drive the electrode head to press the welding area, use direct current or alternating current to perform welding, press for a period of time after welding, lift the electrode, and complete the welding; the electrode pressing force is 1-50 N, the resistance spot welding current is 10-2000 A, the welding time is 10 ms-10 s, and the holding time is 10-1000 ms.

2. The electrically conductive material resistance spot welding process of claim 1, wherein, The material to be welded is a conductive sheet material with a thickness of 1-500 μm, a conductive wire material with a diameter of 1-500 μm, or a sheet material plated with a conductive material with a thickness of 1-500 μm.

3. The conductive material resistance spot welding process of claim 1, wherein, The silver particle size of the nano-silver glue is 1-500 nm.

4. The conductive material resistance spot welding process of claim 1, wherein, The nano-silver glue is applied by spot coating or printing.

5. The conductive material resistance spot welding process of claim 1, wherein, The electrode arrangement mode of the resistance spot welding is symmetrical arrangement on both sides or parallel arrangement on the same side.

6. The conductive material resistance spot welding process of claim 1, wherein, The electrode material of the resistance spot welding is pure molybdenum, molybdenum alloy, pure tungsten, or tungsten alloy.

7. The conductive material resistance spot welding process of claim 1, wherein, The cross section of the electrode end of the resistance spot welding is circular, and the diameter of the end section is 0.1-10 mm.

8. The conductive material resistance spot welding process of claim 1, wherein, The cross section of the electrode end of the resistance spot welding is rectangular, and the length and width of the end section are 0.1-10 mm.

9. The conductive material resistance spot welding process of claim 1, wherein, The number of layers of the material welded at one time is 2 or more.

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