Board-level patch type liquid metal interconnection structure

Abstract

The invention discloses a board-level patch type liquid metal interconnection structure, and relates to the technical field of electronic packaging interconnection. The structure comprises a PDMS (Polydimethylsiloxane) elastic accommodating body, a gallium-based liquid metal core material, a symmetrical one-way valve, a copper pin header cap, a PCB (Printed Circuit Board) with a concave bonding pad, a chip and a crimping packaging shell. A cavity for containing liquid metal is formed in the PDMS elastic containing body, and symmetrical one-way valves are integrated at the two ends to achieve filling and zero-leakage sealing of the liquid metal. One end of the copper pin header cap is crimped with the elastic accommodating body, and the other end of the copper pin header cap is crimped with the concave bonding pad of the PCB to form a conductive path. The design concept of mechanical bonding and electrical interconnection decoupling is adopted, traditional rigid welding spots are replaced with liquid metal, vibration stress is dispersed through flexible buffering of the PDMS elastomer, and the problem that reliability of the traditional welding spots is insufficient due to stress concentration in the vibration environment is solved. The structure has the advantages of being low in contact resistance, excellent in vibration resistance, compatible with an existing SMT production line and the like, and is suitable for a board-level vertical interconnection packaging scene.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging technology, specifically to a surface-mount liquid metal interconnect structure and its packaging method suitable for vertical interconnection at the PCB level, particularly suitable for packaging scenarios with millimeter-level integration and high reliability requirements in vibration environments. Background Technology

[0002] With the rapid development of high-performance computing, artificial intelligence, and 5G communication technologies, electronic devices are evolving towards higher integration, millimeter-level interconnects, and flexibility. Industry forecasts predict that by 2030, the number of interconnect pins in large-package products will exceed 15,000, and the package size will reach over 150×150mm, posing stringent challenges to board-level interconnect technologies.

[0003] Traditional board-level interconnect technologies, centered on rigid solders (such as tin-lead alloys) and wire bonding, suffer from several insurmountable drawbacks: First, the thermal expansion coefficient (CTE) of rigid solder joints mismatches with the PCB substrate, easily leading to stress concentration under random vibration loads and causing solder joint cracking and failure. Second, BGA packages are prone to substrate warping as the number of pins increases, reducing the yield of surface mount processes. Third, the parasitic inductance and resistance of wire bonding limit high-frequency signal transmission capabilities, failing to meet the performance requirements of next-generation electronic systems. Fourth, traditional solders are prone to oxidation, and contact resistance increases significantly over service life, affecting the long-term stability of electrical performance. The unique properties of room-temperature liquid metals offer a new approach to solving these problems. Compared to traditional metals and materials such as graphene, liquid metals possess high electrical conductivity, high thermal conductivity, fluidity, and self-healing capabilities at room temperature, along with excellent mechanical flexibility. They can buffer thermomechanical stress through deformation and fill micro-gaps at the contact interface through flow, fundamentally alleviating the "rigid coupling" problem of traditional interconnects.

[0004] In recent years, research on liquid metal interconnect technology has made some progress. For example, existing studies have achieved stable contact after multiple insertion and removal cycles of liquid metal socket technology and verified the application potential of liquid metal in flexible circuits. However, the practical application of PCB-level liquid metal interconnects still faces three core problems: First, insufficient sealing reliability. The fluidity of liquid metal can easily lead to leakage and short circuits, and existing sealing solutions are difficult to adapt to the spatial constraints of millimeter-scale arrays. Second, the multiphysics coupling mechanism is unclear. The fluid-structure interaction effect between liquid metal flow and elastic container deformation under vibration loads has not been clarified, which restricts reliability assessment. Third, poor compatibility in large-scale assembly. The requirements for small size and dense arrangement lead to a surge in design complexity, requiring simultaneous consideration of sealing reliability, electrical contact stability, and array synergy.

[0005] Therefore, there is an urgent need to provide a board-level surface-mount liquid metal interconnect structure that can overcome the above-mentioned technical bottlenecks. Summary of the Invention

[0006] This invention aims to provide a board-level surface-mount liquid metal interconnect structure that achieves synergistic optimization of mechanical vibration reliability and electrical performance through the design concept of "decoupling mechanical bonding and electrical interconnection", thereby solving the problems of sealing leakage, vibration failure and process compatibility in the prior art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A board-level surface-mount liquid metal interconnect structure includes an elastic container, a liquid metal core, a symmetrical one-way valve, a conductive connector, a PCB board, a chip, and a press-fit package shell.

[0008] The elastic container is made of PDMS material, and its interior forms a cavity for containing liquid metal, with openings at both ends. The outer diameter of the container is 2-3 mm, the inner diameter is 1-2 mm, and the length is 3-4 mm. The PDMS material has a Shore hardness of 40A-60A, a long-term operating temperature range of -40℃ to 200℃, and an elongation at break of ≥300%. It possesses excellent superelastic deformation capability, and can buffer mechanical stress through its own elastic deformation under harsh environments such as vibration, avoiding stress concentration failure caused by rigid packaging structures.

[0009] The liquid metal core material is filled within the cavity of the elastic container. It is a gallium-based liquid alloy, preferably a gallium-indium alloy (such as Ga-10In), with a melting point not exceeding 25°C, and remains liquid at room temperature. This core material exhibits low viscosity (approximately 2.4 mPa·s at 25°C), millimeter-scale density (approximately 6440 kg / m³), and high conductivity (conductivity ≥ 3.0 × 10⁻⁶). 6 With its S / m ratio and self-healing properties, it can serve as an excellent conductive medium to achieve stable electrical connections.

[0010] The symmetrical one-way valves are integrated into the openings at both ends of the elastic container. Each one-way valve contains a diaphragm integrally molded with PDMS, and its core function is to achieve one-way filling and sealing of liquid metal. During the liquid metal injection process, the diaphragm bends and opens under an injection pressure of 5~15N, allowing the liquid metal to smoothly fill the cavity. After injection, under back pressure, the diaphragm tightly seals the opening, achieving zero-leakage sealing of the liquid metal and effectively avoiding the risk of short circuits. The one-way valves at both ends have completely identical structures, allowing for bidirectional adaptation to the assembly requirements of "chip-PCB".

[0011] The conductive connector is a copper pin header cap made of copper with a purity of ≥90% and an elastic modulus of approximately 120 GPa. One end of the copper pin header cap is crimped into the opening of the elastic housing, with a gap of less than 0.05 mm between the two for seamless crimping; the other end is crimped into the pad on the PCB to form a conductive path. The crimping pressure is 15~25 N / mm², the contact angle at the contact interface is less than 20°, forming a stable electrical contact, and the crimping resistance fluctuation is less than 0.5 mΩ.

[0012] The PCB board has a concave pad structure with a depth of 0.1~0.3mm, which complements the convex cap structure of the copper pin header, achieving mechanical positioning while increasing the contact area. The PCB board consists of a copper-clad layer and an epoxy glass cloth substrate FR-4, with an anisotropic elastic modulus of approximately 15.9GPa in the x / y direction and approximately 3.2GPa in the z direction, providing a stable support foundation for the interconnect structure.

[0013] The chip is electrically connected to the PCB board through the aforementioned interconnect structure. Multiple chips are arranged in an array on the PCB board to form an array-type board-level system, meeting the requirements for millimeter-level interconnection.

[0014] The press-fit enclosure is used to fix the overall structure, integrating the PCB board, chip, and interconnect structure into one unit through mechanical pressing. After encapsulation, the structure exhibits excellent vibration resistance, with the maximum stress at frequencies of 5~2000Hz far below the fracture strength of PDMS under random vibration loads conforming to the MIL-STD-810H standard.

[0015] Furthermore, the outer diameter of the aforementioned elastic housing is set to 2-3 mm, the inner diameter to 1-2 mm, and the length to 3-4 mm, which is optimized based on the millimeter-level layout requirements of board-level interconnects. This size range can accommodate the millimeter-level pad spacing on the PCB board (typically 2-4 mm), avoiding signal interference between adjacent interconnect structures, while also providing sufficient space for the internal liquid metal core material to ensure that the cross-sectional area of ​​the conductive path meets the current transmission requirements. At the same time, the 3-4 mm length design matches the typical interconnect gap between PCB boards, without requiring additional adjustments to the existing packaging process.

[0016] Furthermore, the long-term operating temperature range of the elastic containment is -40℃ to 200℃, covering the service environment temperature range of most electronic devices, including industrial control (-40~85℃), automotive electronics (-40~125℃), and aerospace (-55~150℃) scenarios. PDMS material does not undergo glass transition or thermal decomposition within this temperature range, maintaining stable mechanical properties and chemical inertness, thus preventing seal failure due to temperature fluctuations.

[0017] Furthermore, the diaphragm material of the symmetrical one-way valve is the same as that of PDMS, integrally formed, and possesses resistance to liquid metal corrosion. During the injection stage, the thrust of the liquid metal causes the diaphragm to bend towards the inside of the cavity, forming a flow channel with a diameter greater than 1 mm, much larger than the particle size of the liquid metal, ensuring that the metal smoothly fills the cavity. During the sealing stage, after injection, the liquid metal in the cavity generates back pressure due to its own gravity and the elastic contraction of the PDMS container. Under the action of back pressure, the diaphragm tightly adheres to the sealing step at the opening edge, forming a sealing band that blocks the backflow path of the liquid metal. This "pressure-adaptive" sealing mechanism requires no additional locking structure and can dynamically adjust the sealing force according to changes in the pressure inside the cavity.

[0018] Furthermore, the crimping pressure is set to 15~25 N / mm², which is an optimized balance between contact reliability and structural protection. Too low a pressure will result in poor contact between the copper pin header cap and the PCB pad, and air or oxide layer may remain at the interface, increasing contact resistance; too high a pressure may cause deformation of the PCB pad or excessive compression of the PDMS container, leading to the risk of liquid metal leakage.

[0019] This pressure range ensures direct metal-to-metal contact while preventing structural damage. Furthermore, the contact angle is a key indicator of the wettability of liquid metal with copper pin caps and PCB pads. A smaller contact angle allows the liquid metal to easily fill the micro-gaps at the contact interface, forming a continuous conductive path. Experiments show that when the contact angle is less than 20°, the interfacial porosity can be reduced to below 0.5%, and the contact resistance is significantly lower than that of structures with a contact angle greater than 30°.

[0020] Furthermore, the concave structure of the PCB board's pads complements the convex caps of the copper pin headers: the concave depth of 0.1~0.3mm enables mechanical positioning, preventing excessive horizontal offset, while increasing the contact area by approximately 30% compared to planar pads. This design improves the alignment error tolerance during the crimping process from ±0.05mm to ±0.1mm, reducing assembly difficulty and increasing mass production yield.

[0021] Furthermore, the press-fit package integrates the PCB board, chip, and interconnect structure into a single unit through mechanical pressing force, preventing relative displacement of components during vibration. Compared to a shell-less design, this reduces the system resonance peak amplitude by approximately 40%, minimizing contact failures caused by component loosening.

[0022] Furthermore, the interconnect structure uses a press-fit assembly, eliminating the need for reflow soldering processes using traditional solder joints. This allows for direct compatibility with existing SMT production lines, reducing process modification costs.

[0023] Compared with the prior art, the present invention has the following advantages: (1) Excellent vibration resistance: Through the flexible buffer of the PDMS elastic container, the maximum stress of the structure under vibration is much lower than the fracture strength of PDMS. The stress concentration factor is reduced from 3.2 of the traditional weld to 1.1. Based on Miner's law, the vibration cycle survival rate is predicted to reach 92%, which is 2.7 times that of the traditional weld. (2) Excellent electrical performance: The direct contact between liquid metal and copper pins and the self-healing properties reduce the contact resistance by 47% compared to traditional solder joints, and the increase is only 11% after 1000 vibration cycles; (3) Reliable sealing: The symmetrical check valve achieves zero leakage of liquid metal, effectively solves the risk of short circuit, and adapts to the requirements of millimeter-level array arrangement; (4) Good process compatibility: The press-fit structure is compatible with existing SMT production lines, the array packaging yield can reach more than 98.7%, the single cavity cost is low, and it has the potential for large-scale application. (5) Innovative design concept: Through the "decoupling of mechanical bonding and electrical interconnection" design, the bottleneck of mutual constraint between mechanical strength and electrical performance in traditional interconnection technology has been broken, providing a new solution for millimeter-level board-level interconnection and 3D packaging scenarios. Attached Figure Description

[0024] Figure 1 This is a three-dimensional structural schematic diagram of the board-level surface-mount liquid metal interconnect device provided by the present invention.

[0025] Figure 2 This is a side view of the board-level surface-mount liquid metal interconnect device provided by the present invention.

[0026] Figure 3 This is a diagram showing the working state of the symmetrical one-way valve used in this invention: (a) open state during injection, and (b) closed state during sealing.

[0027] Figure 4 This is a flowchart of the molding process of the PDMS elastic accommodator used in this invention.

[0028] Figure 5 This is a diagram illustrating the liquid metal filling process and assembly used in this invention. Figure 6 This is a schematic diagram of the maximum stress of the provided interconnect structure under vibration.

[0029] Figure 7 This is a diagram showing the sixth-order modal deformation of the interconnect structure provided by the present invention.

[0030] Figure 8 This is a stress-strain fatigue cycle diagram of the PDMS elastic reservoir of the present invention.

[0031] In the figure, (1) PDMS elastic container; (2) Gallium indium alloy liquid metal core; (3) symmetrical one-way valve; (4) copper pin header cap; (5) PCB board with concave pads; (6) chip; (7) press-fit package shell. Detailed Implementation

[0032] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions will be clearly and completely described below in conjunction with the embodiments of this invention.

[0033] like Figure 1 As shown, the preparation process of the PDMS elastic containment (1) is as follows: PDMS material with a Shore hardness of 50A is used. The prepolymer and curing agent are mixed at a mass ratio of 10:1 and injection molded using a precision mold. The mold design matches the containment dimensions: outer diameter 2.5mm, inner diameter 1.5mm, and length 3.5mm. Stepped openings for installing one-way valves are reserved at both ends of the cavity. After injection molding, it is cured in an 80℃ oven for 2 hours to ensure complete cross-linking of PDMS and form a containment structure with superelasticity. Its elongation at break is tested to reach 320%, which meets the deformation buffering requirements under vibration environment.

[0034] Furthermore, the integration process of the symmetrical one-way valve (3) is as follows. The core component of the one-way valve is a diaphragm integrally formed with the PDMS elastic container (1), with a thickness of 0.1 mm, which is integrated into the openings at both ends of the PDMS elastic container (1) through an in-mold embedding process. The edge of the diaphragm fits tightly with the opening steps of the container, forming an initial sealing state. Tests show that the diaphragm begins to bend and open under an injection pressure of 5 N, and can form a flow channel with a diameter of 1.2 mm at a pressure of 10 N, which meets the filling requirements of liquid metal; after injection, it can fit tightly with the sealing steps under a back pressure of 0.5 N, achieving zero leakage.

[0035] Furthermore, the filling process of the gallium-indium alloy liquid metal core (2) is as follows: a gallium-indium alloy Ga-10In (indium content 10%) is selected, with a melting point of 22.7℃ and a viscosity of 2.4 mPa·s at 25℃. The liquid metal is injected through a precision injection pump (needle diameter 0.8 mm) via a symmetrical one-way valve (3) at one end of the PDMS elastic container (1). The injection pressure is set to 8 N to ensure complete filling of the cavity without any air bubbles. After injection, the mixture is left to stand for 10 minutes. The elastic contraction of the PDMS generates back pressure, causing the symmetrical one-way valves (3) at both ends to automatically seal. A 24-hour standing test shows no leakage.

[0036] Furthermore, the processing and assembly process of the copper pin header cap (4) is as follows: The pin header cap is processed using oxygen-free copper with a purity of 99.95%. One end is a boss (diameter 1.45mm, tolerance ±0.01mm) that matches the opening of the PDMS elastic container (1), and the other end is a hemispherical convex cap (radius of curvature 0.3mm) that matches the PCB pad. The copper pin header cap (4) is pressed together with the openings at both ends of the PDMS elastic container (1) using a precision crimping machine. The assembly gap is controlled within 0.03mm. During the crimping process, the pressure is gradually increased to 5N to ensure seamless fit and to avoid damaging the symmetrical one-way valve (3).

[0037] Furthermore, the pretreatment process of the PCB board (5) with recessed pads and the chip (6) is as follows: The PCB board (5) is made of FR-4 substrate with a copper layer thickness of 35μm. The pads are designed to be recessed (0.2mm depth, 2mm diameter) and are treated with immersion gold to reduce the risk of oxidation. The chip (6) is encapsulated in molding resin with dimensions of 5×5×0.8mm. A contact area matching the copper pin header cap (4) is reserved at the pins. The PCB board (5) and the chip (6) are baked at 120℃ for 2 hours to remove surface moisture and ensure a clean bonding interface.

[0038] The overall assembly process includes the following steps: 1. Interconnect unit pre-assembly: The PDMS elastic container (1) filled with gallium indium alloy liquid metal core material (2) and sealed is combined with copper pin header cap (4) to form an interconnect unit. The positioning is achieved by a fixture to ensure that the coaxiality error of the copper pin header cap (4) at both ends is ≤0.05mm.

[0039] II. Crimping of PCB board (5) and chip (6): Using SMT-compatible precision crimping equipment, one end of the interconnect unit's copper pin header cap (4) is crimped to the recessed pad of the PCB board (5) at a pressure of 20 N / mm², held for 10 seconds. The other end is crimped to the contact area of ​​the chip (6) at a pressure of 15 N / mm². After crimping, the contact angle is tested to be 18.5°, and the initial contact resistance is 8.2 mΩ, which meets the electrical connection requirements.

[0040] III. Array-type system integration: Fifteen interconnect units are arranged on the PCB board (5) at a 2mm spacing to form an array-type board-level system containing 15 chips (6). A vision alignment system is used to ensure that the alignment error between the chips (6) and the interconnect units is less than 0.1mm, thereby improving the overall assembly accuracy.

[0041] IV. Fixing of the Press-fit Encapsulation Shell (7): An aluminum alloy encapsulation shell with a thickness of 1mm is used. It is fixed to the PCB board (5) by 4 positioning posts. A total pressing force of 5N is applied to integrate all components into one unit. Tests show that the resonance peak amplitude of the encapsulated system is reduced by 42% compared with the design without an encapsulation, and the structural stability is significantly improved.

[0042] like Figure 3 As shown, the symmetrical one-way valve (3) in this invention has two core working states: (a) the open state during injection and (b) the closed state during sealing. The symmetrical one-way valve (3) is integrated into the two openings of the PDMS elastic container (1). The core component is the PDMS diaphragm, which is made of the same material as the PDMS elastic container and has the characteristics of being resistant to gallium indium alloy corrosion. Its "symmetrical" design means that the valve structures at both ends are completely identical, which can adapt to the injection and sealing requirements of liquid metal in both directions and is suitable for the assembly scenario of bidirectional pressing of "chip (6) - PCB board (5)". The operation of the one-way valve is based on the pressure adaptive mechanism. By switching the deformation state of the diaphragm under different pressure directions, the unidirectional flow and reverse sealing of liquid metal are realized.

[0043] Figure 3 (a) Open state during injection: When the gallium indium alloy liquid metal core material (2) is injected into the PDMS elastic container (1) through the syringe, the injection pressure (5~15N) acts on the inner side of the diaphragm. Under the push of the injection pressure, the diaphragm bends towards the inner side of the container cavity, forming a flow channel with a diameter greater than 1mm, which is much larger than the liquid metal particle size, ensuring that the gallium indium alloy is smoothly filled into the cavity and avoiding insufficient filling or air bubbles due to narrow channels. In this state, the one-way valve reduces the injection resistance of the liquid metal through active deformation, is compatible with room temperature injection process, does not require high temperature assistance, and avoids thermal damage to the chip (6) or PCB board (5).

[0044] Figure 3 (b) Closed state during sealing: After injection, the needle pressure is released, and the liquid metal in the cavity generates back pressure due to its own weight (density 6440 kg / m³) and the elastic contraction of the PDMS elastic container (1). Under the action of back pressure, the diaphragm tightly adheres to the sealing step at the edge of the opening, forming an annular sealing band, completely blocking the backflow path of the liquid metal. At this time, the contact pressure between the diaphragm and the opening of the container is dynamically adjusted with the back pressure, and the greater the pressure, the tighter the seal. This state achieves "zero leakage" sealing of the liquid metal, effectively avoiding the risk of short circuits between adjacent interconnect units in the high-density array at the board level. Experimental verification shows that the sealing performance can be stably maintained under conditions such as vibration and temperature cycling, with no liquid metal leakage.

[0045] like Figure 4 As shown, the PDMS elastic container serves as the core for supporting and sealing the liquid metal, and its molding quality directly affects the sealing reliability and flexible buffering performance of the structure. This invention employs a precision molding process to achieve the integrated fabrication of the PDMS elastic container and the internal diaphragm check valve. The core of this process lies in precisely controlling the degassing quality of the prepolymer and the pressure-assisted curing process to ensure the molding accuracy of the millimeter-level microstructure.

[0046] The specific process steps, parameters, and operating requirements are as follows: Step 1, Raw Material Proportioning and Mixing: PDMS prepolymer with a Shore A hardness of 50A and a matching curing agent are mixed at a mass ratio of 10:1. This ratio balances the elasticity and structural stiffness of PDMS, meeting the requirements for vibration damping and sealing support. The mixed PDMS prepolymer solution and curing agent solution are placed at room temperature (23°C) and thoroughly stirred for 5 minutes using a magnetic stirrer to ensure uniform mixing and avoid localized differences in the properties of the molded PDMS substrate due to uneven mixing.

[0047] Step 2, Vacuum Degassing: During the mixing process, a large number of tiny air bubbles are entrained. If these bubbles remain inside the PDMS, they will cause defects such as porosity and uneven membrane thickness in the molded container, affecting sealing performance. Therefore, the uniformly mixed PDMS mixture is transferred to a vacuum drying oven and degassed under a vacuum of less than 10 Pa for 30 minutes until no visible air bubbles remain in the mixture, ensuring a dense internal structure of the PDMS substrate.

[0048] Step 3, Mold Injection and Curing: A CNC-precision machined aluminum mold is used for molding. The mold is preheated to 60℃, a temperature that significantly improves the fluidity of the PDMS mixture, promoting its full filling of the mold cavity and the microstructure area of ​​the check valve diaphragm, avoiding problems such as insufficient material or incomplete molding. The degassed PDMS mixture is slowly injected into the preheated aluminum mold, followed by a holding pressure of 0.3MPa for 30 minutes to ensure complete PDMS impregnation of the mold cavity, especially the microstructure area of ​​the check valve diaphragm. After holding the pressure, the mold is placed at 23℃ for 24 hours to allow the PDMS to fully cross-link and cure, forming a smooth, non-stick, and highly elastic molded part.

[0049] Step 4, Demolding and Surface Treatment: After curing, gently demold using a specialized demolding tool, avoiding excessive force that could damage the PDMS container or check valve diaphragm. After demolding, perform plasma surface treatment on the PDMS container to improve the hydrophilicity and wettability of the PDMS surface, ensuring that the subsequent liquid metal can fully fill the container cavity, while also enhancing the adhesion between the liquid metal and the inner wall of the PDMS, further improving sealing reliability.

[0050] The aforementioned PDMS molding process, through precise control of key parameters such as proportioning, degassing, and curing, achieves integrated molding of the PDMS container and the check valve. This ensures both the dimensional accuracy of the structure and the mechanical and sealing properties of the PDMS material, laying the technological foundation for subsequent liquid metal filling and interconnection structure assembly.

[0051] like Figure 5As shown, the filling quality of the liquid metal is crucial to ensuring the conductive continuity of the interconnect structure. For an integrated check valve type PDMS housing, this invention employs a controllable filling process combining precision injection filling with axial pressing assembly of copper pin caps. To ensure uniform filling and stable response of the check valve diaphragm, the process parameters are set as follows: injection pressure 0.5 MPa, injection pump advance speed 2.5 mm / s. The filling process consists of the following three stages: The first stage is quantitative extraction: a precise syringe pump is used to extract a fixed amount of Ga-10In liquid metal from the reservoir at a uniform speed, ensuring that no air bubbles remain in the syringe.

[0052] The second stage is pressure filling: Under the driving force of filling pressure, the liquid metal overcomes the prestress of the diaphragm and drives the symmetrical diaphragm check valve to open automatically, and the liquid metal then fills the inner cavity of the PDMS container.

[0053] The third stage is in-situ self-sealing: After the injection pump head is removed, the diaphragm quickly resets and fits tightly against the valve seat by relying on the superelastic rebound force of PDMS and the reverse pressure of the fluid inside the cavity, thus achieving anti-backflow and pre-sealing of the liquid metal.

[0054] After the liquid metal is filled, the copper lead caps are axially pressed together using a directional thrust device. The pressing force is controlled within the range of 10~20 N / mm², uniformly pressing the copper lead caps into both ends of the PDMS housing to form a circumferential sealing interference of approximately 0.05 mm. This pressing force ensures sufficient contact between the leads and the liquid metal, promotes the rupture of the surface oxide layer to reduce contact resistance, and avoids tearing of the PDMS cavity or damage to the check valve structure due to excessive pressure. The native Ga₂O₃ oxide layer is inherently electrically insulating, so the unruptured oxide film will form an open circuit in the initial state. However, due to the extremely low mechanical strength of the dense Ga₂O₃ oxide film on the surface of the gallium indium alloy, these oxide layers are prone to local rupture under the action of pressing force, allowing direct metal-to-metal contact to be formed between the liquid metal, the copper lead caps, and the PCB pads. Multiple micro-contact points form parallel conductive paths, thereby enabling the rapid establishment of low and stable contact resistance.

[0055] like Figure 6 As shown, the maximum stress distribution contour map of the plate-level gallium-indium alloy liquid metal interconnect structure is obtained based on the fluid-structure interaction finite element model established by Ansys Workbench under random vibration loads (following the MIL-STD-810H standard, frequency range 5~2000Hz). The results show that the maximum stress of the structure is 31.847MPa, mainly concentrated in the contact interface region between the PDMS elastic container and the copper pin header. The stress gradually decreases from the contact interface to the center of the PDMS container, and the overall distribution is relatively uniform, with no obvious local stress sharpening phenomenon.

[0056] The yield strength of PDMS material is much higher than 31.847 MPa, indicating that the PDMS container did not undergo plastic deformation or fracture under random vibration loads and remained within the elastic safety range, verifying the mechanical reliability of the structure. Compared with traditional welded joint interconnections, this structure disperses vibration stress through the flexible deformation of PDMS, reducing the stress concentration factor from 3.2 in traditional welded joints to 1.1, effectively avoiding the welded joint cracking failure problem caused by stress concentration in rigid connections.

[0057] like Figure 7 As shown, the first six mode shapes and natural frequencies of the board-level liquid metal interconnect structure under prestressed modal analysis are as follows: the first natural frequency is 140.88Hz, which is lower than the lower limit of the typical resonance region (200~800Hz) of electronic devices, thus avoiding the concentrated area of ​​low-frequency vibration energy; the second to sixth natural frequencies are 239.38~370.28Hz, which, although falling into the resonance region, are concentrated in the 200~400Hz range, far from the first natural frequency of 820Hz of traditional solder joint interconnects, and there is a gap between them and the high-frequency strong vibration band, reducing the probability of resonance. In terms of deformation modes and energy dissipation, the mode shapes are mainly concentrated in the thin-walled area in the middle of the PDMS housing, and the vibration amplitude of the connection area between the copper pin header and the PCB board is less than 0.03mm, ensuring the stability of the electrical connection. The flexible deformation characteristics of PDMS enable it to dissipate vibration energy through elastic deformation, avoiding the rigid impact between traditional solder joints and the substrate. Comparative data shows that the vibration mode of traditional weld points is mainly rigid impact, and its displacement amplitude is 3.2 times that of liquid metal interconnect structure.

[0058] Modal analysis results provide a basis for lifetime prediction based on Miner's law. Combined with the maximum stress of 31.847 MPa, which is far lower than the fracture strength of PDMS, and the uniform stress distribution, the predicted vibration cycle survival rate can reach 92%, significantly higher than that of traditional welded joints, indicating that this structure has excellent fatigue resistance.

[0059] like Figure 8 As shown, clearly identifying the physical source and location of fatigue damage is crucial for ensuring that fatigue analysis aligns with the actual stress mechanism of the system. A detailed analysis of the stress-strain response in key structural regions reveals that, under the boundary conditions set in this invention, fatigue damage primarily originates from the periodic deformation of the PDMS and stress concentration at the interface between the PDMS elastic containment and the copper pin cap, rather than from changes in hydrostatic pressure within the liquid metal.

[0060] Figure 8 The stress-strain fatigue cycle plot extracted at node 8524 is shown, which corresponds to... Figure 6The location of the maximum peak stress was determined. Three representative cycles were selected: the 1st, 10th, and 100th cycles, all within the stable loading range of 0.5 s to 10 s. The consistent ring morphology in the closed stress-strain cycles indicates that the PDMS elastic containment possesses stable viscoelastic response characteristics and a consistent energy dissipation rate during cyclic loading. This result demonstrates that the observed fatigue behavior is dominated by the periodic deformation of the PDMS elastic containment, rather than by changes within the liquid metal.

[0061] Furthermore, the local von Mises stress peak at the interface between the PDMS elastic containment and the copper lead cap reached 31.847 MPa, corresponding to a stress concentration factor Kt of 8.5, which is within the expected range of stress at the heterogeneous material interface. The PDMS-copper lead cap interface is the main location of stress concentration and also the initiation region of fatigue damage. In contrast, the hydrostatic pressure in the Ga-In alloy liquid metal conductor is approximately 0.203 MPa, about two orders of magnitude lower than the peak stress of the PDMS elastic containment, and no quantifiable cyclic fluctuations were observed under vibration conditions. These results demonstrate that the elastic deformation of the PDMS containment can effectively withstand cyclic vibration loads, and no significant fatigue-related hydrostatic pressure fluctuations occur within the liquid metal, verifying the long-term reliability of this interconnect structure under vibration conditions.

[0062] Although the present invention has been described in detail by way of preferred embodiments, the invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A board-level surface-mount liquid metal interconnect structure, characterized in that, include: An elastic container, made of PDMS material, has an internal cavity for containing liquid metal, with openings at both ends of the cavity; a liquid metal core, made of gallium-indium alloy, is filled into the cavity of the elastic container; symmetrical one-way valves are integrated at the openings at both ends of the elastic container, each of the symmetrical one-way valves comprising a PDMS diaphragm, which bends and opens under injection pressure to allow liquid metal to fill the cavity, and then closes the opening under back pressure to seal the liquid metal; The conductive connector is a copper pin header cap, one end of which is press-fitted to the opening of the elastic receptacle, and the other end is press-fitted to the pad on the PCB to form a conductive path.

2. The board-level surface-mount liquid metal interconnect structure as described in claim 1, characterized in that, The liquid metal core material is a gallium-indium alloy with an indium content of 5% to 20%, a melting point not exceeding 25°C, and remains liquid at room temperature. It is electrically conductive with a conductivity ≥3.0 × 10⁻⁶. 6 S / m.

3. The board-level surface-mount liquid metal interconnect structure as described in claim 1, characterized in that, The elastic reservoir has an outer diameter of 2-3 mm, an inner diameter of 1-2 mm, and a length of 3-4 mm; the PDMS material has a Shore hardness of 40A-60A, a long-term working temperature range of -40℃ to 200℃, and an elongation at break of ≥300%.

4. The board-level surface-mount liquid metal interconnect structure as described in claim 1, characterized in that, The diaphragm of the symmetrical one-way valve bends and opens under an injection pressure of 5~15N, and then seals the opening under back pressure, achieving a zero-leakage seal for the liquid metal.

5. The board-level surface-mount liquid metal interconnect structure as described in claim 1, characterized in that, The crimping pressure between the copper pin header cap and the PCB board pad is 15~25N / mm², the contact angle at the contact interface is less than 20°, and the crimping resistance fluctuation is less than 0.5mΩ.

6. The board-level surface-mount liquid metal interconnect structure as described in claim 1, characterized in that, The assembly gap between the elastic housing and the copper pin header cap is less than 0.05 mm; the copper pin header cap is made of copper with a purity of ≥90%.

7. The board-level surface-mount liquid metal interconnect structure as described in claim 1, characterized in that, The PCB board has a concave pad structure with a concave depth of 0.1~0.3mm, which complements the convex cap structure of the copper pin header.

8. The board-level surface-mount liquid metal interconnect structure as described in claim 7, characterized in that, The PCB board consists of a copper clad layer and an epoxy glass cloth substrate. The copper clad layer has a thickness of 30~40μm, and the epoxy glass cloth substrate is made of FR-4 material.

9. The board-level surface-mount liquid metal interconnect structure as described in claim 1, characterized in that, It also includes chips, which are electrically connected to the PCB board through the conductive connectors, and multiple chips form an array-type board-level system on the PCB board.

10. The board-level surface-mount liquid metal interconnect structure as described in claim 9, characterized in that, It also includes a press-fit package housing, which is fixedly connected to the PCB board via positioning posts, integrating the PCB board, chip and interconnect structure into an integrated package structure.

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