SnBi ultrasonic low-temperature thermal compression bonding method and structure based on porous metal framework enhancement

Abstract

This invention relates to the field of semiconductor manufacturing technology, and particularly to a method and structure for SnBi ultrasonic low-temperature hot-press bonding based on a porous metal framework. The method includes the following steps: S1: Preparing a porous metal buffer layer with interconnected micropores on the bonding surface of a first connector; S2: Mounting the first connector and a second connector, and placing Sn-Bi low-temperature solder between the bonding surfaces of the porous metal buffer layer and the second connector, aligning the solder and the porous metal buffer layer using optical calibration during mounting; S3: Performing ultrasonic-assisted variable pressure hot-press bonding on the mounted components, including a sequential ultrasonic-assisted wetting and activation stage and a hydrostatic forced infiltration and filling stage; S4: Rapidly cooling under pressure until the solder solidifies and forms a dense composite interconnect structure. This invention can significantly improve the mechanical toughness and electrical and thermal conductivity of the solder joints during low-temperature hot-press bonding with Sn-Bi-based low-temperature solder.

Description

Technical Field

[0001] This invention relates to the field of semiconductor manufacturing technology, and in particular to a method and structure for SnBi ultrasonic low-temperature hot pressing bonding based on a porous metal framework. Background Technology

[0002] As electronic devices evolve towards miniaturization and higher performance, packaging density continues to increase, leading to increasingly stringent thermal budget constraints in the packaging process. To protect heat-sensitive components and reduce defects such as warpage caused by coefficient of thermal expansion (CTE) mismatch, low-temperature bonding technology has become crucial for industry development. Sn-Bi solders, especially Sn58Bi eutectic solder, have become one of the most widely used low-temperature lead-free solders due to their low melting point (approximately 139°C), low cost, and good wettability.

[0003] However, existing SnBi bonding technologies face the following serious challenges in high-end applications: I. Intrinsic brittleness leading to reliability shortcomings: Bismuth (Bi) phase has a hard and brittle crystal structure and is prone to macroscopic segregation during solidification. When traditional SnBi solder joints are subjected to drop impact, cracks often rapidly propagate along the Bi phase interface and penetrate the entire solder joint (transgranular fracture), causing instantaneous device failure.

[0004] Second, insufficient electrical and thermal conductivity: The resistivity and thermal resistance of SnBi alloy are much higher than those of mainstream SAC305 or pure tin solder. In high-current power device applications, high resistance can lead to severe Joule heating.

[0005] 3. Electromigration risk: Under high current density, Bi atoms are prone to directional migration and agglomeration at the anode, resulting in Kirkendal voids on the cathode side, which further deteriorates mechanical properties.

[0006] Current improvement methods mainly focus on microalloying (adding Ag, In, etc.) or nanoparticle doping. While these methods can refine the grain size, they cannot fundamentally change the brittle fracture mode of the SnBi matrix, and nanoparticles are prone to agglomeration, making it difficult to ensure process consistency. In addition, the existing hot-press bonding (TCB) process only uses pressure to reduce voids and fails to strengthen the solder joints at the microstructure level.

[0007] Therefore, there is an urgent need for a new bonding method that can maintain the advantages of low-temperature processes while significantly improving the mechanical toughness, electrical conductivity, and thermal conductivity of the solder joints. Summary of the Invention

[0008] This invention provides a method and structure for ultrasonic low-temperature hot pressing bonding of SnBi based on a porous metal skeleton reinforcement, which can effectively solve the problems of intrinsic brittleness, easy transgranular fracture, and poor electrical and thermal conductivity of Sn-Bi-based low-temperature solders in the background art.

[0009] This invention provides a method for ultrasonic low-temperature hot-press bonding of SnBi based on a porous metal skeleton reinforcement, characterized by the following steps: S1: Prepare a porous metal buffer layer with a connected micropore structure on the bonding surface of the first connector; S2: Mount the first connector and the second connector, and place Sn-Bi-based low-temperature solder between the porous metal buffer layer and the bonding surface of the second connector. During mounting, use optical calibration to align the solder and the porous metal buffer layer. S3: The mounted components are subjected to ultrasonic-assisted variable pressure thermocompression bonding. This process includes an ultrasonic-assisted wetting and activation stage and a static pressure forced impregnation and filling stage performed sequentially. S4: Rapid cooling is performed while maintaining pressure until the solder solidifies and forms a dense composite interconnect structure.

[0010] Further, in step S1, the preparation of the porous metal buffer layer specifically involves: The porous metal buffer layer is determined to be made of one or more combinations of Cu, Ag, Ni, and Au. The porous metal buffer layer is prepared by either dealloying or low-temperature sintering. If the dealloying method is used, a Cu-Zn or Ag-Al precursor alloy layer is first electroplated on the substrate surface, and then the active metal components are selectively removed by chemical etching to form a nanoporous structure. If a low-temperature sintering method is used, a micron-scale porous structure is formed by printing nano-metal paste and pre-sintering it at a temperature below its melting point. The porous metal buffer layer has a porosity of 30% to 70%, an average pore size ranging from 100 nm to 5 μm, and a layer thickness ranging from 5 μm to 20 μm.

[0011] Furthermore, in step S1, after preparing the porous metal buffer layer, a step of cleaning the porous metal buffer layer is also included, specifically: the porous metal buffer layer is ultrasonically cleaned with deionized water and then dried with nitrogen gas.

[0012] Furthermore, in step S2, when setting the Sn-Bi-based low-temperature solder, the form of the solder includes preformed solder sheets, printed solder paste, or electroplated solder layers; the Sn-Bi-based solder is Sn58Bi eutectic solder or modified solder with trace amounts of Ag, In, and Sb added.

[0013] Furthermore, in step S2, during optical alignment and mounting, the first connector with the prepared porous metal buffer layer is first fixed; solder is placed at a designated position on the porous buffer layer surface of the first connector; the second connector is moved above the first connector; and the alignment marks of the second connector and the first connector are identified respectively through the optical system. Based on the recognition results, the position offset is calculated and the placement head is moved to complete the precise alignment in three dimensions. Then, the second connector is lowered so that the protrusion of the second connector contacts the solder to complete the placement.

[0014] Furthermore, the ultrasound-assisted wetting and activation stage specifically includes: The temperature of the heating head is raised above the melting point of the solder, a first pressure P1 is applied to the heating head, and ultrasonic vibration of a set frequency is applied to the heating head at the same time for a period of time until the solder and the oxide film on the porous layer surface are broken.

[0015] Furthermore, in step S3, the static pressure forced infiltration and filling stage specifically includes: Keep the heating head temperature above the melting point, stop applying ultrasonic vibration to the heating head, and rapidly increase the pressure on the heating head to a second pressure P2, where P2 > P1. Maintain this pressure for a period of time until the liquid solder completely fills the pores of the porous metal buffer layer.

[0016] Furthermore, in step S3, both the ultrasonic-assisted wetting and activation stage and the hydrostatic forced impregnation and filling stage are carried out in an inert gas protective atmosphere or in a vacuum.

[0017] Further, in step S4, the pressure holding and cooling process specifically involves: While maintaining the second pressure P2 applied to the heating head, the cooling system is activated to reduce the overall component temperature to below 100°C at a cooling rate greater than 5°C / s. Then, the pressure applied to the heating head is released, and the bonding is completed.

[0018] The present invention also provides an ultrasonic low-temperature hot-pressing bonding structure based on a porous metal skeleton reinforcement, which is prepared according to the method described above.

[0019] The technical solution of this invention can achieve the following technical effects: This invention introduces a porous metal skeleton with a porous metal buffer layer as a reinforcing phase and creatively employs a step-by-step coupling process of ultrasonic-assisted wetting (low pressure) + variable pressure forced infiltration (high pressure). First, the ultrasonic wave effect and cavitation are used to break the oxide film and reduce the surface tension. Then, the ultrasonic wave is stopped and high pressure is applied. This allows a composite structure of metal skeleton and SnBi interpenetrating network to be constructed at low temperature. The continuous tough skeleton is used to achieve crack trapping and electrothermal bypass transport. An interconnect structure with high reliability (impact resistance) and excellent electrothermal performance (low resistance) is successfully constructed at low temperature. The entire process is carried out at low temperature and is fully compatible with existing low-temperature packaging processes. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the process for the ultrasonic low-temperature hot pressing bonding method of SnBi based on porous metal skeleton reinforcement. Figure 2 This is a schematic diagram of the composite interconnect structure; Figure 3 This is a schematic diagram illustrating the structural characteristics of a composite interconnect structure and existing technologies when cracks occur; where, Figure 3 'a' is a schematic diagram of solid solder cracking in the prior art. Figure 3 b is a schematic diagram of the solder cracking in the composite interconnect structure. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0024] This invention relates to an ultrasonic low-temperature hot-press bonding method for Sn-Bi based on a porous metal framework reinforcement, which overcomes the defects of existing Sn-Bi-based low-temperature solders, such as intrinsic brittleness, susceptibility to transgranular fracture, and poor electrical and thermal conductivity. This method includes multiple steps S1 to S4, such as... Figure 1 As shown, a composite interconnect structure (bottom left of the figure) in which a metal skeleton and solder interpenetrate is constructed at low temperature by prefabricating a porous metal layer at the connection interface (top left of the figure) and employing a step-by-step coupling process of ultrasonic activation wetting (top right of the figure) and pressure-driven forced infiltration (bottom right of the figure). The specific implementation process is as follows: S1: On the surface of the bonding area of ​​the first connector (such as substrate pad, chip bump), a porous metal buffer layer with a three-dimensional interconnected pore structure is prefabricated. This layer serves as the physical framework for subsequent reinforcement, and its pore structure provides space for solder filling and mechanical interlocking. S2: Mount the first connector and the second connector (such as a flip chip), and place Sn-Bi-based low-temperature solder between the porous metal buffer layer and the bonding surface of the second connector. During mounting, use optical calibration to align the solder and the porous metal buffer layer. S3: Place the mounted components in the thermosetting bonding machine and perform an ultrasonic-assisted variable pressure thermosetting bonding process with two consecutive functions. This process includes an ultrasonic-assisted wetting and activation stage and a hydrostatic forced infiltration and filling stage, which are performed sequentially. The ultrasonic-assisted wetting and activation stage uses the physical effect of ultrasound to break down interface barriers and promote initial wetting, while the hydrostatic forced infiltration and filling stage uses high hydrostatic pressure to force the solder to penetrate and fill deep into the micropores. S4: Rapid cooling is performed while maintaining pressure until the solder solidifies and solidifies, allowing the formed interpenetrating network structure to take shape and form a dense composite interconnect structure. For example... Figure 2 As shown, the continuous skeleton (slanted lines) represents copper (Cu) or silver (Ag), which is continuous and serves to conduct electricity and provide support. The dotted fillers represent SnBi solder, which completely fills the gaps in the skeleton without voids. The IMC interface layer is a very thin layer of intermetallic compound (such as Cu6Sn5) drawn on the contact surface between the skeleton and the solder, indicating that the two have undergone chemical metallurgical bonding.

[0025] The final composite interconnect structure is as follows Figure 3 As shown, in the structure (b) of the present invention, with the cooperation of the skeleton and the solder, when a crack (dashed part) occurs, the crack is forced to change direction when it encounters the skeleton obstacle and eventually stops, which can effectively achieve crack deflection and thus increase the toughness of the material; while in the prior art, when the solid solder (a) cracks, the crack will instantly penetrate the solder joint in a straight line (brittle fracture), and the toughness of the material is obviously poor.

[0026] Preferably, in step S1, the preparation of the porous metal buffer layer specifically involves: First, the porous layer is determined to be made of a highly electrical and thermally conductive metal, selected from one or more combinations of copper (Cu), silver (Ag), nickel (Ni), or gold (Au). Second, the preparation method is selected, primarily dealloying or low-temperature sintering.

[0027] If the dealloying method is used, the specific operation process is as follows: on the clean copper pad surface of the substrate, a bimetallic precursor alloy layer (such as Cu-Zn or Ag-Al) of a specific thickness is formed by electroplating; then the substrate is immersed in a specific chemical etching solution (such as dilute hydrochloric acid) to dissolve the active metal components (such as Zn or Al) through selective etching, leaving a nanoscale sponge-like porous metal structure.

[0028] If the low-temperature sintering method is used, the specific operation process is as follows: the nano metal powder (such as silver powder) is mixed with an organic carrier to form a slurry, which is then coated on the surface of the pad to form a wet film using printing technology; after the solvent is removed by low-temperature drying, sintering is carried out in a protective atmosphere at a temperature lower than the melting point of the metal, so that the surface of the metal particles is melted and connected to form a micron-sized porous structure.

[0029] The final porous layer must meet the following requirements: porosity between 30% and 70%, average pore size between 100 nm and 5 μm, and layer thickness between 5 μm and 20 μm, to ensure that it has both good fillability and sufficient mechanical support strength.

[0030] Preferably, in step S1, after preparing the porous metal buffer layer, a step of cleaning the porous metal buffer layer is also included, specifically: ultrasonically cleaning the porous metal buffer layer with deionized water and then drying it with nitrogen gas to ensure that the pores are unobstructed and the surface is clean.

[0031] Preferably, in step S2, the solder and alignment mounting are set as follows: The Sn-Bi-based low-temperature solder is preferably Sn58Bi eutectic solder, or a modified solder with trace amounts of Ag, In, Sb, etc., to optimize some properties while maintaining a low melting point. The solder can be provided in various forms, including: preformed solder sheets, printed solder paste, or solder layers formed by direct electroplating on the bump surface.

[0032] Preferably, in step S2, during optical alignment and mounting, the specific operation is as follows: First, the first connector (substrate) with the prepared porous metal buffer layer is fixed on the substrate stage of the flip-chip soldering machine; then, a predetermined amount of solder (such as a Sn58Bi preformed solder sheet) is placed in the central area of ​​the porous layer of the substrate. Next, the machine vision system is activated to capture the bump array on the lower surface of the second connector (chip) and the corresponding alignment mark on the substrate. The system software automatically calculates the positional offset of the two in the X, Y and θ directions and drives the mounting head to make precise compensation movements until an alignment accuracy better than ±5μm is achieved. Finally, the mounting head is slowly lowered, allowing the chip bumps to make initial physical contact with the porous layer of the substrate through the intermediate solder, thus completing the mounting process.

[0033] Preferably, in step S3, the ultrasonic-assisted wetting and activation stage specifically includes: The heating head temperature of the hot-press bonding machine is set above the solder melting point, preferably in the range of 140°C to 160°C. After the heating head descends and contacts the back of the chip, a low initial pressure P1, ranging from 0.2 MPa to 2 MPa, is applied. Simultaneously, an ultrasonic generator is immediately activated, applying ultrasonic vibrations with a frequency range of 20 kHz to 80 kHz and a power range of, for example, 1-2 W, for a duration typically of 0.2 to 2 seconds. During this stage, the ultrasonic waves generate a strong cavitation effect in the molten solder, effectively breaking down the oxide film on the solder itself and the surface of the porous metal framework; at the same time, the acoustic flow effect significantly reduces the surface tension and viscosity of the liquid solder. These two factors work together to overcome the initial barriers preventing the liquid SnBi solder from wetting and penetrating the microporous structure, allowing it to spread well and cover the porous layer surface.

[0034] Preferably, in step S3, the specific operation and principle of switching from the ultrasonic activation stage to the static pressure filling stage are as follows: Immediately after the preset ultrasonic treatment time (e.g., 0.5 seconds) has elapsed, the ultrasonic generator should be shut off. Stopping the ultrasonic treatment is to prevent continuous high-frequency mechanical vibration from causing fatigue damage or collapse of the already partially wetted but still fragile nano / micro porous framework. After the ultrasonic treatment stops, the pressure control system must respond within a very short time (e.g., within 0.1 seconds) by rapidly increasing the pressure applied to the component from the first pressure P1 to a significantly higher second pressure P2.

[0035] Preferably, in step S3, the static pressure forced impregnation and filling stage specifically includes: Maintain a constant temperature in the bonding region above the melting point (e.g., 150°C). Rapidly increase the pressure from P1 (0.5 MPa) to a second pressure P2, where P2 > P1, and P2 ranges from 5 MPa to 30 MPa. Then, maintain P2 for a period ranging from 1 to 10 seconds. Under this high pressure, the hydrostatic pressure becomes the primary driving force, overcoming the significant capillary resistance encountered by the liquid solder flowing through the micropores, forcing the solder to flow deeper into the porous framework until all interconnected pores are completely filled. During this process, the liquid Sn and the metal (e.g., Cu) of the porous framework undergo rapid interdiffusion and reaction at the pore wall interface, forming a thin, continuous intermetallic compound layer in situ, thus achieving a strong metallurgical bond.

[0036] Preferably, in step S4, the pressure holding and cooling specifically includes: After the hydrostatic filling stage, the second pressure P2 (e.g., 15 MPa) is kept constant while the forced cooling system is activated. The cooling system needs to rapidly cool the temperature of the bonded components from above the solder melting point to below 100°C at a rate greater than 5°C / s. Throughout the cooling and solidification process, the continuously applied high pressure effectively suppresses the volume shrinkage that occurs when the solder transitions from liquid to solid, preventing internal microvoids or delamination between the solder and the skeleton interface due to shrinkage. Once the temperature drops to a safe range, the pressure system is unloaded, the pressure drops to zero, the heating head is raised, and a dense, uniform, and firmly bonded metal skeleton / SnBi composite interconnect structure is finally obtained.

[0037] This invention also relates to a system for implementing the above-described bonding method, comprising a hot-press bonding machine, an ultrasonic generator, a precision pressure control system, an optical alignment system, and a temperature control module. The hot-press bonding machine is used to support and heat the components; the ultrasonic generator is integrated into the bonding head to generate mechanical vibrations of specific frequency and power; the precision pressure control system can be independently programmed and quickly switch between different pressure values; the optical alignment system is used to achieve high-precision alignment between the chip and the substrate; and the temperature control module is used to precisely control the heating and cooling rates of the bonding area. These hardware modules work collaboratively under the program instructions of the control unit to execute the steps of the above-described method.

[0038] The present invention also relates to a SnBi ultrasonic low-temperature hot-press bonding structure based on a porous metal skeleton reinforcement, which is prepared according to the method described above.

[0039] The following example, using a nanoporous copper framework + Sn58Bi + variable pressure ultrasonic process, serves as Example 1 to illustrate this method in detail: (1) Preparation: The selected test chip measures 5mm × 5mm, features copper pillar bumps with a diameter of 50μm and a spacing of 100μm. The selected substrate is an FR-4PCB with bare copper pads. The selected solder is Sn-58Bi preform solder with a melting point of 139°C.

[0040] (2) Implementation steps: Preparation of S1 porous metal buffer layer: (a) Electroplating precursor: A Cu-Zn alloy layer with a thickness of 10 μm (Zn content of approximately 65 at.%) is electroplated on the copper pads of the substrate.

[0041] (b) Etching to create pores: Immerse the substrate in a 5wt% hydrochloric acid (HCl) solution and etch at room temperature for 20 minutes to remove Zn atoms.

[0042] (c) Cleaning: Ultrasonic cleaning with deionized water, followed by nitrogen drying. The final product is a nanoporous copper layer (porous metal buffer layer) with an average pore size of approximately 150 nm and a porosity of approximately 55%.

[0043] S2 mounting: Sn58Bi solder pads were pre-placed on top of the porous layer and optically aligned and mounted using a flip-chip soldering machine.

[0044] S3 Ultrasonic-Assisted Transformer Bonding: a (Immersion): Heat the bonding head to 150°C. Apply an initial contact pressure of 0.5 MPa. Turn on the ultrasonic wave at a frequency of 60 kHz and a power of 1.5 W for 0.5 seconds. At this time, the solder melts and spreads on the surface of the porous layer.

[0045] b (filling): Maintain a constant temperature of 150°C. Immediately stop the ultrasound. Increase the pressure to 15 MPa within 0.1 seconds and hold for 3 seconds. Use high pressure to force liquid SnBi into the nanopores.

[0046] S4 Cooling: Maintain a pressure of 15 MPa, initiate rapid cooling (rate 15°C / s), and unload the pressure after cooling to 90°C.

[0047] (3) Implementation results: A dense interconnect structure with a continuous copper skeleton was obtained, with an interface IMC layer thickness of less than 1 μm.

[0048] The following example, using a micron-sized porous silver framework + SnBiAg + transformer ultrasonic process, serves as Example 2 to illustrate this method in detail: (1) Preparation: The selected test chip is the same as in Example 1. The selected substrate is a DBC (Direct Copper Clad) ceramic substrate. The selected solder is Sn-57.6Bi-0.4Ag solder paste.

[0049] (2) Implementation steps: Preparation of S1 porous metal buffer layer: (a) Electroplating precursor: A Cu-Zn alloy layer with a thickness of 10 μm (Zn content of approximately 65 at.%) is electroplated on the copper pads of the substrate.

[0050] (b) Etching to create pores: Immerse the substrate in a 5wt% hydrochloric acid (HCl) solution and etch at room temperature for 20 minutes to remove Zn atoms.

[0051] (c) Cleaning: Ultrasonic cleaning with deionized water, followed by nitrogen drying. The final product is a nanoporous copper layer (porous metal buffer layer) with an average pore size of approximately 150 nm and a porosity of approximately 55%.

[0052] S2 mounting: Sn58Bi solder pads were pre-placed on top of the porous layer and optically aligned and mounted using a flip-chip soldering machine.

[0053] S3 Ultrasonic-Assisted Transformer Bonding: a (Immersion): Heat the bonding head to 150°C. Apply an initial contact pressure of 0.5 MPa. Turn on the ultrasonic wave at a frequency of 60 kHz and a power of 1.5 W for 0.5 seconds. At this time, the solder melts and spreads on the surface of the porous layer.

[0054] b (filling): Maintain a constant temperature of 150°C. Immediately stop the ultrasound. Increase the pressure to 15 MPa within 0.1 seconds and hold for 3 seconds. Use high pressure to force liquid SnBi into the nanopores.

[0055] S4 Cooling: Maintain a pressure of 15 MPa, initiate rapid cooling (rate 15°C / s), and unload the pressure after cooling to 90°C.

[0056] (3) Implementation results: The silver skeleton and SnBiAg solder form a good interpenetrating network, and the silver skeleton provides excellent conductive channels.

Claims

1. A method for ultrasonic low-temperature hot pressing bonding of SnBi based on porous metal skeleton reinforcement, characterized in that the steps include... include: S1: Prepare a porous metal buffer layer with a connected micropore structure on the bonding surface of the first connector; S2: Mount the first connector and the second connector, and place Sn-Bi-based low-temperature solder between the porous metal buffer layer and the bonding surface of the second connector. During mounting, use optical calibration to align the solder and the porous metal buffer layer. S3: The mounted components are subjected to ultrasonic-assisted variable pressure thermocompression bonding. This process includes an ultrasonic-assisted wetting and activation stage and a static pressure forced impregnation and filling stage performed sequentially. S4: Rapid cooling is performed while maintaining pressure until the solder solidifies and forms a dense composite interconnect structure.

2. The method for ultrasonic low-temperature hot pressing bonding of SnBi based on porous metal skeleton reinforcement according to claim 1, characterized in that, In step S1, the preparation of the porous metal buffer layer specifically involves: The porous metal buffer layer is determined to be made of one or more combinations of Cu, Ag, Ni, and Au. The porous metal buffer layer is prepared by either dealloying or low-temperature sintering. If the dealloying method is used, a Cu-Zn or Ag-Al precursor alloy layer is first electroplated on the substrate surface, and then the active metal components are selectively removed by chemical etching to form a nanoporous structure. If a low-temperature sintering method is used, a micron-scale porous structure is formed by printing nano-metal paste and pre-sintering it at a temperature below its melting point. The porous metal buffer layer has a porosity of 30% to 70%, an average pore size ranging from 100 nm to 5 μm, and a layer thickness ranging from 5 μm to 20 μm.

3. The method for ultrasonic low-temperature hot pressing bonding of SnBi based on porous metal skeleton reinforcement according to claim 2, characterized in that, In step S1, after preparing the porous metal buffer layer, a step of cleaning the porous metal buffer layer is also included, specifically: the porous metal buffer layer is ultrasonically cleaned with deionized water and then dried with nitrogen gas.

4. The method for ultrasonic low-temperature hot pressing bonding of SnBi based on porous metal skeleton reinforcement according to claim 1, characterized in that, In step S2, when setting Sn-Bi-based low-temperature solder, the form of the solder includes pre-formed solder sheet, printed solder paste, or electroplated solder layer; the Sn-Bi-based solder is Sn58Bi eutectic solder or modified solder with trace amounts of Ag, In, and Sb.

5. The method for ultrasonic low-temperature hot pressing bonding of SnBi based on porous metal skeleton reinforcement according to claim 4, characterized in that, In step S2, during optical alignment and mounting, the first connector with the prepared porous metal buffer layer is first fixed; solder is placed at a designated position on the porous buffer layer surface of the first connector; the second connector is moved above the first connector; and the alignment marks of the second connector and the first connector are identified respectively through the optical system. Based on the recognition results, the position offset is calculated and the placement head is moved to complete the precise alignment in three dimensions. Then, the second connector is lowered so that the protrusion of the second connector contacts the solder to complete the placement.

6. The method for ultrasonic low-temperature hot pressing bonding of SnBi based on porous metal skeleton reinforcement according to claim 1, characterized in that, The ultrasound-assisted wetting and activation stage specifically includes: The temperature of the heating head is raised above the melting point of the solder, a first pressure P1 is applied to the heating head, and ultrasonic vibration of a set frequency is applied to the heating head at the same time for a period of time until the solder and the oxide film on the porous layer surface are broken.

7. The method for ultrasonic low-temperature hot pressing bonding of SnBi based on porous metal skeleton reinforcement according to claim 6, characterized in that, In step S3, the static pressure forced infiltration and filling stage specifically includes: Keep the heating head temperature above the melting point, stop applying ultrasonic vibration to the heating head, and rapidly increase the pressure on the heating head to a second pressure P2, where P2 > P1. Maintain this pressure for a period of time until the liquid solder completely fills the pores of the porous metal buffer layer.

8. The method for ultrasonic low-temperature hot pressing bonding of SnBi based on porous metal skeleton reinforcement according to claim 1, characterized in that, In step S3, both the ultrasonic-assisted wetting and activation stage and the hydrostatic forced impregnation and filling stage are carried out in an inert gas protective atmosphere or in a vacuum.

9. The method for ultrasonic low-temperature hot pressing bonding of SnBi based on porous metal skeleton reinforcement according to claim 7, characterized in that, In step S4, the pressure holding and cooling process specifically involves: While maintaining the second pressure P2 applied to the heating head, the cooling system is activated to reduce the overall component temperature to below 100°C at a cooling rate greater than 5°C / s. Then, the pressure applied to the heating head is released, and the bonding is completed.

10. A SnBi ultrasonic cryogenic hot-pressing bonding structure based on a porous metal skeleton reinforcement, characterized in that: The SnBi ultrasonic low-temperature hot-press bonding structure based on a porous metal skeleton is prepared according to the method described in any one of claims 1 to 9.

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