High power density plastic package structure of power module and its lead assembly method

Abstract

The application discloses a high-power-density plastic sealing structure of a power module and a pin leg assembling method thereof. The high-power-density plastic sealing structure of the power module comprises a module plastic sealing body and a pin end cover. The module plastic sealing body comprises a substrate layer and plastic sealing material layers. The plastic sealing material layers are arranged on opposite sides of the substrate layer to form a sandwich structure. The pin end cover comprises a pin and a pin plastic body fixedly connected with the pin. A laser resistance welding area is arranged on the pin. A space surrounded by the laser resistance welding area is provided with a pin pad. The pin pad is welded with a metal pad arranged on a side of the substrate layer. The pin end cover is fixedly connected with the substrate layer and the two plastic sealing material layers. The high-power-density plastic sealing power module realizes high-reliability connection of the pin end cover through reflow welding and bonding reinforcement process, realizes good electrical connection and high mechanical connection strength between the module plastic sealing body and the pin end cover, and has the advantages of small size, light weight and good heat dissipation.

Description

Technical Field

[0001] This invention relates to the field of power module molding technology, and in particular to a high power density molding structure for a power module and its lead assembly method. Background Technology

[0002] With the rapid development of electronic technology, the requirements for lightweight and miniaturized electronic equipment are increasing. DC / DC power modules are indispensable core components in electronic equipment and are widely used in airborne, missile-borne, radar, and aerospace fields. Output power, conversion efficiency, and power density are important technical indicators of modular power supplies. Electronic equipment has extremely stringent requirements for its size and weight, leaving very limited design space for secondary modular power supplies. Therefore, the size and weight of power modules must be controlled. In addition, modular power supplies are usually installed in enclosed environments with poor heat dissipation, which requires high efficiency, reduced heat loss, and long-term reliable operation. Therefore, power modules will inevitably develop towards higher power density and higher efficiency, continuously improving the output power per unit volume and high-reliability heat dissipation.

[0003] The achievement of high efficiency and high power density in modular power supplies relies on advanced packaging technology. Without the support of packaging technology, the power density and heat dissipation efficiency of the product cannot be further improved. The modular integrated molding process not only reduces the size and weight of the power module but also improves the product's heat dissipation efficiency. However, there is currently little research on advanced packaging—modular integrated molding—and most existing modular power supplies typically use open-panel or potted module structures, employing single-board or double-board PCB stacking methods. This involves adding a metal or plastic shell to the PCB substrate and then encapsulating it with thermally conductive silicone rubber inside the cavity. This type of packaging structure has disadvantages such as large size, high weight, and poor heat dissipation, failing to meet the technical requirements of high power density and high-reliability heat dissipation in modules. Summary of the Invention

[0004] Based on the technical problems existing in the background technology, the present invention proposes a high power density plastic package structure for a power module and its lead assembly method. The module plastic package and the lead end cover are connected to the lead end cover through reflow soldering and adhesive reinforcement processes, achieving a high reliability connection between the module plastic package and the lead end cover, realizing good electrical connection and high mechanical connection strength between them. It has the advantages of small size, light weight and good heat dissipation.

[0005] The present invention proposes a high power density plastic encapsulation structure for a power module, comprising a module plastic encapsulation body and pin end caps. The module plastic encapsulation body includes a substrate layer and a plastic encapsulation layer. The plastic encapsulation layer is disposed on opposite sides of the substrate layer to form a sandwich structure. The pin pads of the pin end caps are soldered to the metal pads of the substrate layer. The pin end caps are fixedly connected to the substrate layer and the two plastic encapsulation layers respectively.

[0006] Furthermore, the pin end cap includes a pin and a pin plastic body fixedly connected to the pin. The pin is provided with a laser solder resist area, and the space enclosed by the laser solder resist area is provided with a pin pad. The pin pad is soldered to a metal pad provided on the side of the substrate layer.

[0007] Furthermore, the lead pin plastic body is provided with protrusions, and the lead pin is provided with grooves. The protrusions are inserted into the grooves to fix them in place. Alternatively, the lead pin plastic body and the lead pin are integrally formed by injection molding. The material of the lead pin plastic body is PPS material, and the surface roughness is Ra1.2 to Ra1.6.

[0008] Furthermore, the lead plastic body is provided with a U-shaped glue groove cavity, which is located around the lead pad. The bottom of the U-shape of the glue groove cavity is bonded to one of the molding compound layers, and the opposite sides of the U-shape of the glue groove cavity are bonded to one molding compound layer, the substrate layer, and the other molding compound layer in sequence.

[0009] Furthermore, the material of the probe is oxygen-free copper, and the copper surface is successively plated with a nickel layer, a palladium layer, and a gold layer.

[0010] Furthermore, if the pin end cap is an n-pin end cap, n pins are set accordingly, and n metal pads are set in the metal pads set on the side of the substrate layer.

[0011] A method for assembling leads of a high power density plastic-encapsulated power module includes the following steps:

[0012] After the module molding compound is placed horizontally on a constant temperature hot plate at a set temperature and preheated for a set time, the metal pads on the side of the substrate layer and the pin pads on the pin end cap are tinned to obtain the tinned pads of the module molding compound and the tinned pads of the pin end cap.

[0013] Sn62Pb36Ag2 lead-based solder paste is applied to the surface of the tin-electrode pads of the module molding compound to obtain lead-based solder paste pads for the module molding compound.

[0014] The module molding body, the leaded solder paste pads of the module molding body and the tinned pads of the pin end cap are respectively placed into the welding fixture for docking and clamping to obtain the pre-processed power module.

[0015] The pre-processed power module is reflow soldered to obtain the soldered power module.

[0016] After welding, the module mold body and pin end caps in the power module are glued together and then placed in a constant temperature oven for baking and curing to obtain the glued and encapsulated power module.

[0017] Furthermore, the welding fixture includes a base and a pressing mechanism for pressing the pre-processing power module. The base has a cavity for placing the pre-processing power module. The pressing mechanism includes a push rod, a baffle, and a drive assembly for driving the push rod. One end of the push rod is connected to the output end of the drive assembly, and the other end is connected to the baffle. The baffle is disposed on one side of the pre-processing power module. The baffle abuts against and presses against the pre-processing power module as the push rod extends.

[0018] Furthermore, the drive assembly includes a handle and an arc-shaped rod, one end of the handle being rotatably connected to the base, one end of the arc-shaped rod being rotatably connected to the handle, and the other end being rotatably connected to the push rod at the end away from the baffle.

[0019] Alternatively, the drive assembly may include a linear module mounted on a base, with the push rod fixedly connected to a slider on the linear module at the end furthest from the baffle.

[0020] The advantages of the high power density plastic encapsulation structure and lead assembly method for a power module provided by this invention are as follows: The high power density plastic encapsulation structure and lead assembly method for a power module provided by this invention achieve a highly reliable connection between the module encapsulation body and the lead end caps through reflow soldering and adhesive reinforcement processes. This results in good electrical connection and high mechanical connection strength between the module encapsulation body and the lead end caps, offering advantages such as small size, light weight, and good heat dissipation. The laser solder resist area is used to prevent continuous short circuits and solder overflow defects between the lead pads and metal pads, ensuring accurate electrical connection of the lead end caps. The leads are made of oxygen-free copper with excellent electrical and thermal conductivity. Ni, Pd, and Au layers are electroplated on the surface of the oxygen-free copper to prevent metal oxidation and improve solderability. The adhesive cavity, reinforced by epoxy adhesive bonding, strengthens the bond between the module encapsulation body and the lead end caps, improving the mechanical connection strength of the lead end caps. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the present invention;

[0022] Figure 2 This is a schematic diagram of the module's molding compound.

[0023] Figure 3 A schematic diagram of a structure with metal pads on the side of the substrate layer;

[0024] Figure 4 This is a schematic diagram of the pin end cap structure;

[0025] Figure 5 This is a schematic diagram of the connection between the lead pin and the lead pin plastic body.

[0026] Figure 6 This is a schematic diagram of the welding fixture.

[0027] Among them, 1-module molding compound, 2-pin end cap, 3-welding fixture, 11-substrate layer, 12-molding compound layer, 13-metal pad, 21-lead, 22-lead plastic body, 23-laser solder resist area, 24-glue groove cavity, 25-lead pad, 31-base, 32-extrusion mechanism, 33-cavity, 221-protrusion, 222-groove, 321-push rod, 322-baffle, 323-drive assembly, 324-handle, 325-arc rod. Detailed Implementation

[0028] The technical solution of the present invention will now be described in detail through specific embodiments. Many specific details are set forth in the following description to provide a thorough understanding of the invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0029] like Figures 1 to 6 As shown, the present invention proposes a high power density molding compound structure for a power module, comprising a module molding compound 1 and pin end caps 2. The module molding compound 1 includes a substrate layer 11 and molding compound layers 12. The molding compound layers 12 are disposed on opposite sides of the substrate layer 11 to form a sandwich structure. The pin pads 25 on the pin end caps 2 are soldered to the metal pads 13 on the substrate layer 11. The pin end caps 2 are fixedly connected to the substrate layer 11 and the two molding compound layers 12 respectively.

[0030] The module molded body 1 is manufactured from a high-power-density PCBA product using mature processes such as double-sided integral molding, side cutting, and top and bottom surface grinding, resulting in a small size and excellent quality. The pin end cap 2 features a flexible design, simple construction, convenient assembly, controllable quality, and reliable connection. The module molded body 1 and pin end cap 2 achieve a highly reliable connection through reflow soldering and adhesive reinforcement processes, realizing good electrical connection and high mechanical connection strength between them. The specific design of the module molded body 1 and pin end cap 2 fills a research gap in the structural design and manufacturing process of molded power modules, and has significant guiding significance for improving the manufacturing process and reliable mass production capabilities of molded power modules.

[0031] Additionally, the pin end cap 2 can be configured with 2-pin end caps, 3-pin end caps, ..., n-pin end caps as needed. The number of pins corresponding to the pins 21 is set. The metal pads 13 on the side of the module molded body 1 are also configured with n metal pads. The size and spacing of the metal pads 13 and the spacing between adjacent metal pads 13 correspond to the pin pads 25 on the pins 21, so as to realize the electrical connection between the module molded body 1 and the pin end cap 2.

[0032] In this embodiment, as Figure 4 As shown, the pin end cap 2 includes a pin 21 and a pin plastic body 22 fixedly connected to the pin 21. A laser solder resist area 23 is provided on the pin 21. The laser solder resist area 23 is used to prevent continuous short circuits and solder overflow defects between the pin pad 25 and the metal pad 13, ensuring accurate electrical connection of the pin end cap 2. The space enclosed by the laser solder resist area 23 is used to set the pin pad 25, which is soldered to the metal pad 13 provided on the side of the substrate layer 11. The direct soldering between the metal pad 13 and the pin pad 25 forms an electrical connection, resulting in a power module with good heat dissipation. The pin plastic body 22 of the pin end cap 2 is made of PPS material with a surface roughness of Ra1.2 to Ra1.6. The surface of the pin plastic body 22 is smooth and complete, without defects such as shrinkage cavities, bubbles, and cracks. It is heat-resistant and has excellent comprehensive mechanical properties, fully meeting the requirements of high-temperature soldering and high structural strength of the pin end cap 2.

[0033] The lead pin 21 is made of oxygen-free copper with excellent electrical and thermal conductivity. Ni, Pd and Au layers are electroplated on the surface of the oxygen-free copper to prevent metal oxidation and improve the solderability of the welding.

[0034] Among them, such as Figure 5 As shown, the connection relationship between the lead pin 21 and the lead pin plastic body 22 can be as follows: One method involves a protrusion 221 on the lead pin plastic body 22 and a groove 222 on the lead pin 21. The protrusion 221 is inserted into the groove 222 for fixation. This direct fit between the protrusion 221 and the groove 222 can be achieved through an interference fit or by providing a small boss at the end of the protrusion 221, which is then embedded in the groove 222 for disassembly and fixation, thus achieving a fixed connection between the lead pin 21 and the lead pin plastic body 22. Another method involves injection molding the lead pin plastic body 22 and the lead pin 21 as a single unit. The lead pin 21 is embedded into the PPS material lead pin plastic body 22, and the integral structure of the lead pin 21 and the lead pin plastic body 22 is obtained through injection molding with liquid plastic.

[0035] In this embodiment, a U-shaped adhesive groove cavity 24 is provided on the lead plastic body 22. The adhesive groove cavity 24 is located around the lead pad 25. The bottom of the U-shape of the adhesive groove cavity 24 is bonded to one of the molding compound layers 12. The opposite sides of the U-shape of the adhesive groove cavity 24 are sequentially bonded to one of the molding compound layers 12, the substrate layer 11, and the other molding compound layer 12. The adhesive groove cavity 24 is a space for the flow and filling of adhesive. By using epoxy adhesive bonding for reinforcement, the bonding between the module molding body 1 and the lead end cap 2 can be strengthened, thereby improving the mechanical connection strength of the lead end cap 2.

[0036] A method for assembling leads of a high power density plastic-encapsulated power module includes the following steps:

[0037] A. After placing the module molding body 1 horizontally on a constant temperature hot plate at a set temperature for a set preheating time, perform a tinning operation on the metal pads 13 on the side of the substrate layer 11 and the pin pads 25 of the pin end cap 2 to obtain the tinned pads of the module molding body 1 and the tinned pads of the pin end cap 2.

[0038] Before soldering, the oxide layer and excess material on the metal pads 13 of the module molded body 1 are removed, which increases the solderability of the metal pads 13 and ensures a reliable connection between the module molded body 1 and the lead terminal 2.

[0039] In the solder pad tinning operation, the metal solder pads 13 on the module molding compound 1 and the pin solder pads 25 on the pin end cap 2 are tinned sequentially using a soldering iron, as detailed below:

[0040] Tinning of metal pads 13 on module molded body 1: After preheating the module molded body 1 horizontally on a constant temperature hot plate of 150 degrees Celsius for 8-10 minutes, first use W200 metallographic sandpaper to gently polish the metal pads 13 to remove the oxide layer and excess material on the surface. Then wipe the metal pads 13 clean with a lint-free cloth soaked in alcohol or acetone. After cleaning the metal pads 13, tinning should be performed within half an hour to prevent the metal pads 13 from oxidizing again. Use Φ1.5mm Sn63Pb37 solder wire to tin the metal pads 13 one by one by manually tinning with an electric soldering iron. The operation is convenient and the tinning effect is good. The soldering iron temperature is set to 330±10℃. The tinning operation time for a single metal pad 13 is 3-5 seconds until a bright and uniform thin layer of solder is covered on the surface of the metal pad 13.

[0041] Tinning of pin pads 25 on pin end cap 2: Place pin end cap 2 horizontally on a constant temperature hot plate at 150℃ and preheat for 3-5 minutes. After the overall temperature of pin end cap 2 reaches equilibrium, perform manual tinning of pin pads 25 using a soldering iron. First, wipe pin pads 25 clean with a lint-free cloth soaked in alcohol or acetone and let it air dry. Then, apply a small amount of even Sn62Pb36Ag2 lead-based solder paste to the center of pin pads 25 to ensure that the tinning of pin pads 25 is uniform and thin. Use a soldering iron to heat and tin the pin pads 25. The soldering iron temperature is 270±10℃, and the tinning time for a single pad is 2-3 seconds.

[0042] After tinning, the metal pads 13 and pin pads 25 should be cleaned promptly. Use a lint-free cloth soaked in alcohol or acetone to wipe the metal pads 13 and pin pads 25 clean. After cleaning, put the module molded body 1 and pin end cap 2 into a nitrogen cabinet for storage to prevent the tinned metal pads 13 and pin pads 25 from oxidizing again and affecting subsequent connections.

[0043] B. Coat the surface of the solder pads of module molded body 1 with Sn62Pb36Ag2 leaded solder paste to obtain the leaded solder paste pads of module molded body 1.

[0044] A uniform layer of Sn62Pb36Ag2 leaded solder paste, approximately 0.1–0.12 mm thick, is printed onto the solder pads on both sides of the module's plastic package 1 using a stencil printing method. This ensures reliable soldering and consistent strength of the pin end caps 2.

[0045] C. Place the module molding body 1 and the pin end cap 2, which have completed the above steps A and B, into the welding fixture 3 for docking and clamping to obtain the pre-processed power module.

[0046] Among them, such as Figure 6 As shown, the welding fixture 3 includes a base 31 and a pressing mechanism 32 for pressing the pre-processing power module. The base 31 has a cavity 33 for placing the pre-processing power module. The pressing mechanism 32 includes a push rod 321, a baffle 322 and a drive assembly 323 for driving the push rod 321. One end of the push rod 321 is connected to the output end of the drive assembly 323 and the other end is connected to the baffle 322. The baffle 322 is disposed on one side of the pre-processing power module. The baffle 322 extends with the push rod 321 and abuts against and presses against the pre-processing power module. The drive assembly 323 includes a handle 324 and an arc-shaped rod 325. One end of the handle 324 is rotatably connected to the base 31. One end of the arc-shaped rod 325 is rotatably connected to the handle 324, and the other end is rotatably connected to the push rod 321 at the end away from the baffle 322. At this time, the arc-shaped rod 325 makes arc-shaped movements, so that the push rod 321 can make extension and retraction movements in the horizontal direction, thereby driving the baffle 322 to move and abut against the pin end cover 2. Alternatively, the drive assembly 323 includes a linear module disposed on the base 31, and the end of the push rod 321 away from the baffle 322 is fixedly connected to the slider on the linear module.

[0047] The module molding compound 1 and the pin end caps 2 are placed into the cavity 33 of the welding fixture 3. A ceramic baffle is set in the cavity, and the ceramic baffle and the baffle 322 are positioned opposite each other and abut against the pin end caps 2 set on both sides of the module molding compound 1. The baffle 322 is set as a telescopic structure, so that the welding fixture 3 can be used for the packaging of power modules of different specifications, with a wide range of applications. When the lead solder paste pads of the module molding compound 1 and the tin-plated pads of the pin end caps 2 are placed in the cavity, the two pin end caps 2 are set on opposite sides of the module molding compound 1, one pin end cap 2 abuts against the ceramic baffle, and the other pin end cap 2 abuts against the telescopic baffle 322.

[0048] D. Apply horizontal pressure to the module molding body 1 and the pin end cap 2 using the welding fixture 3 to achieve the assembly of the module molding body 1 and the pin end cap 2;

[0049] After the module molding compound 1 and the lead end cap 2 are horizontally placed into the cavity 33 of the welding fixture 3, the handle 324 is pressed down from top to bottom. The arc rod 325, push rod 321 and ceramic baffle gradually apply horizontal pressure to the module molding compound 1 and the lead end cap 2. That is, the left and right baffles 322 and ceramic baffles apply gradually increasing clamping forces F1 to the right and F2 to the left to the module molding compound 1 and the lead end cap 2. The pressure is uniform and constant, so as to achieve relative fixation between the module molding compound 1 and the lead end cap 2. During subsequent reflow soldering, the connection of the lead end cap 2 can be ensured to be reliable.

[0050] When the handle 324 is pressed down until the base 31 is locked and fixed, the pin end cap 2 is subjected to the maximum clamping force in the horizontal direction. The pressure is constant and uniform, ensuring that the stress state of all pin end caps 2 is consistent during reflow soldering, and ensuring reliable connection between the module encapsulation body 1 and the pin end cap 2.

[0051] E. Reflow soldering is performed on the assembled module plastic package 1 and the pin end cap 2 to obtain the soldered power module;

[0052] The assembled module molded body 1 and the pin end cap 2 are placed in a vacuum reflow oven for reflow soldering. The temperature profile of the vacuum reflow oven is shown in the table below:

[0053]

[0054] The pin end caps 2 are brazed in a vacuum reflow oven, ensuring reliable welding. Temperature zones 6 and 7 are vacuum reflow zones with a vacuum level of 50–100 Pa. After reflow soldering, the metal pads 13 of the module molded body 1 and the pin pads 25 of the pin end caps 2 are brazed together. The module molded body 1 and the pin end caps 2 on both sides form the power module as a whole.

[0055] F. After welding, the substrate layer 11, the molding compound layer 12 and the U-shaped adhesive cavity 24 of the lead end cover 2 in the power module are glued and encapsulated, and then placed in a constant temperature oven for baking and curing to obtain the encapsulated power module.

[0056] Apply H907-HF epoxy adhesive to the glue tank cavity 24. Place the entire power module into a vacuum chamber for vacuuming at 50-100 Pa for 5 minutes. Then remove the product and apply more adhesive. Repeat the vacuuming-removal-adhesive application process 2-3 times to completely fill the glue tank cavity 24. Next, clean off any excess adhesive from the entire surface of the power module and then place it in a constant temperature oven for baking and curing at 125℃ for 1 hour. In the oven, the glue tank cavity 24 should be facing upwards and the pins 21 downwards. After baking, remove the product and allow it to cool naturally. The module pin ends are reliably bonded.

[0057] Electrical connection: The power module product that has completed the above steps was subjected to electrical performance testing. The results showed that the electrical connection of the product was normal, and accurate electrical connection was achieved between the module plastic package 1 and the pin end cap 2.

[0058] Mechanical connection strength: According to the relevant provisions of the lead-out strength-test condition A in Method 211 of the National Military Standard GJB360B-2009, the mechanical connection strength of the lead-out of the power module was tested. When a force of 25N was applied, the lead-out end structure was not damaged.

[0059] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high power density plastic encapsulation structure for a power module, characterized in that, The module includes a molding compound (1) and a pin cap (2). The molding compound (1) includes a substrate layer (11) and a molding compound layer (12). The molding compound layer (12) is disposed on opposite sides of the substrate layer (11) to form a sandwich structure. The pin pad (25) of the pin cap (2) is soldered to the metal pad (13) of the substrate layer (11). The pin cap (2) is fixedly connected to the substrate layer (11) and the two molding compound layers (12) respectively. The pin end cap (2) includes a pin (21) and a pin plastic body (22) fixedly connected to the pin (21). A laser solder resist area (23) is provided on the pin (21), and a pin pad (25) is provided in the space enclosed by the laser solder resist area (23). The lead plastic body (22) is provided with a U-shaped glue groove cavity (24). The glue groove cavity (24) is located around the lead pad (25). The bottom of the U-shaped glue groove cavity (24) is bonded to one of the molding compound layers (12). The opposite sides of the U-shaped glue groove cavity (24) are bonded to one of the molding compound layers (12), the substrate layer (11), and the other molding compound layer (12) in sequence.

2. The high power density molding structure of the power module according to claim 1, characterized in that, The lead pin plastic body (22) is provided with a protrusion (221), and the lead pin (21) is provided with a groove (222). The protrusion (221) is inserted into the groove (222) and fixedly disposed. Alternatively, the lead pin plastic body (22) and the lead pin (21) are integrally formed by injection molding. The material of the lead-in plastic body (22) is PPS material, and the surface roughness is Ra1.2~Ra1.

6.

3. The high power density molding structure of the power module according to claim 1, characterized in that, The material of the needle (21) is oxygen-free copper, and the surface of the oxygen-free copper is successively plated with a nickel layer, a palladium layer and a gold layer.

4. The high power density molding structure of the power module according to claim 1, characterized in that, If the pin end cap (2) is an n-pin end cap, n pins (21) are set accordingly, and n metal pads are set in the metal pads (13) set on the side of the substrate layer (11).

5. The method for assembling the leads of a high-power-density plastic-encapsulated power module according to any one of claims 1 to 4, characterized in that, Includes the following steps: After the module molding compound (1) is placed horizontally on a constant temperature hot plate at a set temperature for a set time, the metal pads (13) on the side of the substrate layer (11) and the pin pads (25) of the pin end cap (2) are tinned to obtain the tinned pads of the module molding compound (1) and the tinned pads of the pin end cap (2). Sn62Pb36Ag2 leaded solder paste was coated on the surface of the tin-plated pads of the module molding compound (1) to obtain the leaded solder paste pads of the module molding compound (1). The leaded solder paste pads of the module molding body (1) and the tinned solder pads of the pin end cap (2) are respectively placed into the welding fixture (3) for docking and clamping to obtain the pre-processed power module. The pre-processed power module is reflow soldered to obtain the soldered power module. After welding, the module mold body (1) and pin end cap (2) in the power module are glued together and then placed in a constant temperature oven for baking and curing to obtain the glued and encapsulated power module.

6. The leg assembly method according to claim 5, characterized in that, The welding fixture (3) includes a base (31) and a pressing mechanism (32) for pressing the pre-processing power module. A cavity (33) for placing the pre-processing power module is provided on the base (31). The pressing mechanism (32) includes a push rod (321), a baffle (322) and a drive assembly (323) for driving the push rod (321) to move. One end of the push rod (321) is connected to the output end of the drive assembly (323), and the other end is connected to the baffle (322). The baffle (322) is located on one side of the pre-processing power module. The baffle (322) extends with the push rod (321) and abuts against and presses against the pre-processing power module.

7. The leg assembly method according to claim 6, characterized in that, The drive assembly (323) includes a handle (324) and an arc rod (325). One end of the handle (324) is rotatably connected to the seat (31), and one end of the arc rod (325) is rotatably connected to the handle (324), while the other end is rotatably connected to the push rod (321) at the end away from the baffle (322). Alternatively, the drive assembly (323) includes a linear module mounted on the base (31), with the push rod (321) fixedly connected to a slider on the linear module at the end away from the baffle (322).

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