A method of combining a stretchable circuit board and a flexible circuit board

Abstract

The application discloses a combination method of stretchable circuit board and flexible circuit board, and belongs to the technical field of electronic circuit manufacturing. The application improves the surface energy of the flexible circuit board by argon-oxygen mixed gas plasma activation, realizes the double smooth transition of the conductive performance and the mechanical performance by using the gradient conductive composite layer which is continuously graded from the high-conductive phase to the high-elasticity phase, forms the conductive interconnection structure by using the pulse laser induced in-situ solidification, and enhances the interface bonding strength by the micropore array mechanical interlocking, so that the interface peeling strength is more than 3 N / cm, the resistance change rate is less than 15% under the 30% tensile strain, the stretch-bending cycle can be supported for more than 10000 times, and the application is suitable for wearable devices, flexible robots, electronic skin and other fields.

Description

Technical Field

[0001] This invention relates to the field of circuit manufacturing technology, specifically a method for combining a stretchable circuit board and a flexible circuit board. Background Technology

[0002] Flexible printed circuit boards (FPCs) are highly reliable and extremely flexible printed circuit boards made with polyimide or polyester film as the substrate. They are characterized by high wiring density, light weight, thinness, and good bending properties, and are widely used in consumer electronics, automotive electronics, medical devices, and other fields. However, traditional flexible circuit boards have limited tensile deformation capabilities, typically only able to withstand less than 5% tensile strain, which is insufficient for applications requiring large deformation and stretching.

[0003] Stretchable circuit boards are a type of circuit carrier capable of withstanding significant tensile deformation (typically greater than 20%) while maintaining stable electrical performance. They achieve their stretchability through designs such as serpentine traces, island bridge structures, and liquid metal conductive channels. However, stretchable circuit boards have shortcomings in terms of fine circuit fabrication, high-density component integration, and compatibility with standard electronic components. Furthermore, they have higher manufacturing costs and greater process complexity.

[0004] In practical applications, it is often necessary to combine stretchable circuit boards with flexible circuit boards. The stretchable area is used for deformation adaptation, while the flexible circuit board area is used for high-density circuit integration and component mounting. In existing technologies, the combination of the two is usually achieved through conductive adhesive bonding, soldering, or connector mating. However, these methods have the following drawbacks: (1) Insufficient interfacial bonding force makes it prone to delamination and peeling under repeated stretching-bending cycles; (2) Poor reliability of conductive connection; the contact resistance increases sharply or even breaks the circuit when stretched. (3) Abrupt stiffness changes in the bonding area lead to stress concentration and accelerate fatigue failure; (4) The process steps are complicated and it is difficult to achieve large-scale production.

[0005] Therefore, there is an urgent need to develop a method for combining stretchable circuit boards and flexible circuit boards. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for combining stretchable circuit boards and flexible circuit boards, solving technical problems such as insufficient interface bonding, poor conductivity continuity, severe stress concentration, and complex processes in existing technologies.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for combining a stretchable circuit board and a flexible circuit board, comprising the following steps: S1. Interface pretreatment: Plasma surface activation treatment is performed on the area to be bonded on the flexible circuit board. The treatment power is 80-200W, the treatment time is 30-120s, and the treatment gas is a mixture of argon and oxygen with a volume ratio of (3-7):1. S2. Preparation of a gradient conductive layer: A gradient conductive composite layer is formed on the surface of the area to be bonded on the flexible circuit board by screen printing or scraping. The gradient conductive composite layer gradually transitions from a highly conductive phase near the flexible circuit board to a highly elastic phase near the stretchable circuit board. The highly conductive phase is a mixture of 70-85% by mass of silver-coated copper powder and epoxy resin, and the highly elastic phase is a mixture of 40-60% by mass of liquid metal droplets and thermoplastic polyurethane. S3. Preparation of stretchable circuit board substrate layer: A stretchable elastomer substrate is coated on the highly elastic phase side of the gradient conductive composite layer. The stretchable elastomer substrate is one of thermoplastic polyurethane, silicone rubber or polydimethylsiloxane, and the coating thickness is 50-300μm. S4. Laser-induced in-situ bonding: The bonding area is scanned and irradiated with a pulsed laser with a wavelength of 355nm or 532nm, a pulse width of 10-50ns, and a scanning speed of 50-200mm / s, so that the gradient conductive composite layer forms an in-situ solidified conductive interconnect structure between the stretchable elastomer substrate and the flexible circuit board. S5. Enhanced Interface Mechanical Interlock: During the laser bonding process, a micro-hole array is pre-set in the bonding area of ​​the flexible circuit board, allowing the molten gradient conductive composite layer to penetrate into the micro-holes to form a mechanical interlock structure. The pore diameter of the micro-hole array is 20-80μm, the pore depth is 10-50μm, and the pore spacing is 100-300μm. S6. Encapsulation and protection: A UV-curable protective adhesive is coated on the surface of the bonding area. The thickness of the UV-curable protective adhesive is 10-30μm. After curing by UV irradiation, a protective layer is formed.

[0008] Compared with the prior art, the technical solution of this application has the following beneficial effects: 1. This invention utilizes argon-oxygen mixed gas plasma activation treatment to reduce the water contact angle of the flexible circuit board surface to below 30°, significantly improving surface energy and providing a good wetting foundation for subsequent coatings. Simultaneously, by combining a pre-set micropore array with laser-induced melt penetration, a mechanical interlocking structure is formed at the bonding interface, enabling the interface peel strength to reach over 3N / cm, far exceeding the 0.5-1N / cm of traditional conductive adhesive bonding methods, effectively solving the problem of delamination and peeling under repeated stretching-bending cycles.

[0009] 2. This invention employs a gradient conductive composite layer that continuously transitions from a highly conductive phase to a highly elastic phase. In the highly conductive phase, silver-coated copper powder provides a low-resistance conductive pathway. In the highly elastic phase, liquid metal droplets undergo flow reconstruction during tensile deformation, maintaining the continuity of the conductive network. The intermediate gradient region achieves a smooth transition between the conductive filler concentration and the elastic matrix modulus, avoiding the abrupt change in resistance at the interface of traditional heterogeneous materials. Testing shows that the resistance change rate is less than 15% under 30% tensile strain, demonstrating significantly superior conductivity stability compared to existing technologies.

[0010] 3. The gradient conductive composite layer of this invention simultaneously achieves a dual gradual transition in conductivity and mechanical properties. The elastic modulus continuously transitions from approximately 2-3 GPa on the flexible circuit board side to approximately 5-20 MPa on the stretchable circuit board side, eliminating abrupt changes in stiffness in the bonding area and reducing the stress concentration factor from above 3.5 in traditional methods to below 1.8, significantly improving fatigue resistance. After more than 10,000 tensile-bending cycle tests, no cracks or open-circuit failures were observed in the samples. Attached Figure Description

[0011] Figure 1 This is a flowchart of the method for combining the stretchable circuit board and the flexible circuit board of the present invention; Figure 2 This is a partial schematic diagram of the micropore array mechanical interlocking structure of the present invention. Detailed Implementation

[0012] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0013] Example 1 A method for combining a stretchable circuit board and a flexible circuit board includes the following steps: S1. Interface pretreatment: Plasma surface activation treatment is performed on the area to be bonded on the flexible circuit board. The treatment power is 120W, the treatment time is 60s, and the treatment gas is a mixture of argon and oxygen with a volume ratio of argon to oxygen of 5:1. S2. Preparation of gradient conductive layer: A gradient conductive composite layer is formed on the surface of the area to be bonded on the flexible circuit board by screen printing. The gradient conductive composite layer gradually changes from a highly conductive phase near the flexible circuit board to a highly elastic phase near the stretchable circuit board. The highly conductive phase is a mixture of 78% by mass of silver-coated copper powder and epoxy resin, and the highly elastic phase is a mixture of 50% by mass of liquid metal droplets and thermoplastic polyurethane. S3. Preparation of stretchable circuit board substrate layer: Thermoplastic polyurethane is coated on the highly elastic phase side of the gradient conductive composite layer with a coating thickness of 150μm. S4. Laser-induced in-situ bonding: The bonding area is scanned and irradiated with a pulsed laser with a wavelength of 355nm, a pulse width of 20ns, and a scanning speed of 100mm / s, so that the gradient conductive composite layer forms an in-situ solidified conductive interconnect structure between the stretchable elastomer substrate and the flexible circuit board. S5. Enhanced Interface Mechanical Interlock: During the laser bonding process, a micro-hole array is pre-set in the bonding area of ​​the flexible circuit board, allowing the molten gradient conductive composite layer to penetrate into the micro-holes to form a mechanical interlock structure. The micro-hole array has a pore diameter of 50μm, a pore depth of 30μm, and a pore spacing of 200μm. S6. Encapsulation and protection: A UV-curable protective adhesive with a thickness of 20μm is coated on the surface of the bonding area and cured by UV irradiation to form a protective layer.

[0014] In this embodiment, in step S1, argon ions remove organic contaminants and oxide layers from the surface of the flexible circuit board through physical bombardment, while oxygen plasma introduces polar functional groups such as hydroxyl and carboxyl groups into the surface through chemical reactions. The synergistic effect of the two reduces the surface water contact angle from 75° before treatment to 25° and increases the surface energy from 38mN / m to 68mN / m, providing an excellent wetting and adhesion basis for the gradient conductive composite layer.

[0015] In step S2, by controlling the paste ratio and printing sequence of the screen printing, a highly conductive phase paste is printed first, followed by a highly elastic phase paste printed in a semi-dry state on its surface. The mutual penetration of the two pastes forms a continuous, gradual transition zone, avoiding obvious delamination. The silver-coated copper powder forms a tightly packed conductive network within the highly conductive phase, resulting in a volume resistivity as low as 3 × 10⁻⁶. -5 Ω·cm, liquid metal droplets exist as a dispersed phase in a highly elastic phase. When subjected to tensile strain, the droplets can form new conductive pathways through plastic flow, thereby maintaining conductive continuity.

[0016] In step S3, the thermoplastic polyurethane has good fluidity under heating, and can undergo molecular chain diffusion and entanglement with the thermoplastic polyurethane matrix in the highly elastic phase to form a strong interfacial bond.

[0017] In step S4, the 355nm ultraviolet pulsed laser is strongly absorbed by the silver-coated copper powder in the gradient conductive composite layer, generating local instantaneous high temperature (about 300-400℃), causing the epoxy resin to cross-link and cure, and the thermoplastic polyurethane to melt and re-solidify. At the same time, the photochemical effect of the laser promotes the bonding of interfacial molecules. Since the pulse width is only 20ns, the heat-affected zone is limited to within 50μm, which will not cause damage to the components on the flexible circuit board.

[0018] In step S5, the laser-induced molten gradient conductive composite material penetrates into the preset micropores under the combined action of capillary force and laser shock pressure. After solidification, it forms a mechanically interlocked structure with a pinning effect. The rough surface of the inner wall of the micropore further increases the contact area and friction, thereby increasing the interfacial peel strength to 3.5 N / cm.

[0019] In step S6, the UV-curable protective adhesive undergoes a free radical polymerization reaction under UV irradiation and can be cured within seconds. Its high elongation characteristic allows it to deform synchronously with the substrate, while also blocking moisture and oxygen erosion.

[0020] Example 2 A method for combining a stretchable circuit board and a flexible circuit board includes the following steps: S1. Interface pretreatment: Plasma surface activation treatment is performed on the area to be bonded on the flexible circuit board. The treatment power is 80W, the treatment time is 120s, and the treatment gas is a mixture of argon and oxygen with a volume ratio of argon to oxygen of 3:1. S2. Preparation of Gradient Transition Conductive Layer: A gradient conductive composite layer is formed on the surface of the area to be bonded on the flexible circuit board by a scraping method. The gradient conductive composite layer gradually transitions from a highly conductive phase near the flexible circuit board to a highly elastic phase near the stretchable circuit board. The highly conductive phase is a mixture of 70% by mass of silver-coated copper powder and epoxy resin, and the highly elastic phase is a mixture of 40% by mass of liquid metal droplets and thermoplastic polyurethane. The gradient conductive composite layer also contains 3% by mass of carbon nanotubes. S3. Preparation of stretchable circuit board substrate layer: Silicone rubber is coated on the highly elastic phase side of the gradient conductive composite layer with a coating thickness of 300μm. S4. Laser-induced in-situ bonding: The bonding area is scanned and irradiated with a pulsed laser with a wavelength of 532nm, a pulse width of 50ns, and a scanning speed of 50mm / s. The laser scanning adopts a cross-shaped scanning path, and the overlap rate of adjacent scanning lines is 40%, so that the gradient conductive composite layer forms an in-situ cured conductive interconnect structure between the stretchable elastomer substrate and the flexible circuit board. S5. Enhanced Interface Mechanical Interlock: During the laser bonding process, a micro-hole array is pre-set in the bonding area of ​​the flexible circuit board, allowing the molten gradient conductive composite layer to penetrate into the micro-holes to form a mechanical interlock structure. The micro-hole array has a pore diameter of 80μm, a pore depth of 50μm, and a pore spacing of 300μm. S6. Encapsulation and protection: A UV-curable protective adhesive with a thickness of 30μm is coated on the surface of the bonding area and cured by UV irradiation to form a protective layer.

[0021] The difference between this embodiment and Embodiment 1 is as follows: Step S1 uses lower power and a longer processing time, suitable for thinner flexible circuit board substrates, avoiding substrate damage caused by over-processing; the carbon nanotubes added in Step S2 form a one-dimensional conductive network in the gradient conductive composite layer, synergistically constructing a multi-dimensional conductive pathway with the zero-dimensional silver-coated copper powder and liquid metal microdroplets, further reducing resistance and improving conductivity stability under tension; Step S3 uses silicone rubber as the stretchable substrate, which has a wider temperature range (-50℃ to 200℃), suitable for high-temperature applications; Step S4 uses a 532nm green laser and a cross-scanning path, as green light has a higher absorption rate for copper, and the cross-scanning ensures uniform energy distribution and avoids uncured areas; Step S5 uses a micropore array with larger pore size and depth, resulting in stronger mechanical interlocking, suitable for applications requiring higher peel strength. Testing showed that the interface peel strength of the sample in this embodiment was 4.2 N / cm, the resistance change rate was 12% under 30% tensile strain, and the resistance change rate was 8% after 10,000 tensile-bending cycles.

[0022] Example 3 A method for combining a stretchable circuit board and a flexible circuit board includes the following steps: S1. Interface pretreatment: Plasma surface activation treatment is performed on the area to be bonded on the flexible circuit board. The treatment power is 200W, the treatment time is 30s, and the treatment gas is a mixture of argon and oxygen with a volume ratio of 7:1. S2. Preparation of gradient conductive layer: A gradient conductive composite layer is formed on the surface of the area to be bonded on the flexible circuit board by screen printing. The gradient conductive composite layer gradually changes from a highly conductive phase near the flexible circuit board to a highly elastic phase near the stretchable circuit board. The highly conductive phase is a mixture of 85% by mass of silver-coated copper powder and epoxy resin, and the highly elastic phase is a mixture of 60% by mass of liquid metal droplets and thermoplastic polyurethane. S3. Preparation of stretchable circuit board substrate layer: Polydimethylsiloxane is coated on the highly elastic phase side of the gradient conductive composite layer with a coating thickness of 50μm. 2% by mass graphene nanosheets are doped into the polydimethylsiloxane. S4. Laser-induced in-situ bonding: The bonding area is scanned and irradiated with a pulsed laser with a wavelength of 355nm, a pulse width of 10ns, and a scanning speed of 200mm / s, so that the gradient conductive composite layer forms an in-situ solidified conductive interconnect structure between the stretchable elastomer substrate and the flexible circuit board. S5. Enhanced Interface Mechanical Interlock: During the laser bonding process, a micro-hole array is pre-set in the bonding area of ​​the flexible circuit board, allowing the molten gradient conductive composite layer to penetrate into the micro-holes to form a mechanical interlock structure. The micro-hole array has a pore diameter of 20μm, a pore depth of 10μm, and a pore spacing of 100μm. S6. Encapsulation and protection: A UV-curable protective adhesive with a thickness of 10μm is coated on the surface of the bonding area and cured by UV irradiation to form a protective layer.

[0023] The difference between this embodiment and Embodiment 1 is as follows: Step S1 uses high-power, short-time processing, suitable for thick copper foil flexible circuit boards, which can quickly remove the oxide layer on the copper surface and roughen the surface; in Step S2, the mass fraction of the highly conductive silver-coated copper powder is increased to 85%, resulting in better conductivity, and the mass fraction of the highly elastic liquid metal is increased to 60%, resulting in better tensile conductivity stability; in Step S3, polydimethylsiloxane (PDMS) is used as a stretchable substrate, which has excellent biocompatibility and is suitable for medical wearable and implantable devices. The doped graphene nanosheets can improve the thermal conductivity of the substrate (from 0.16 W / m·K to 0.45 W / m·K), which is beneficial for circuit heat dissipation; in Step S4, a shorter pulse width and faster scanning speed are used, resulting in a smaller heat-affected zone (approximately 20 μm), suitable for fine circuit areas; in Step S5, a small-aperture, high-density micropore array is used, with a more refined and uniform interlocking structure, suitable for thin flexible circuit boards. The sample in this embodiment was tested and found to have an interfacial peel strength of 3.1 N / cm, a resistance change rate of 10% under 30% tensile strain, and a cytotoxicity level of 0, which meets the requirements for medical implant materials.

[0024] In summary, this invention employs mature processes such as screen printing, laser processing, and UV curing, making it compatible with existing roll-to-roll production lines for flexible circuit boards without requiring large-scale equipment modifications. Laser-induced in-situ bonding is a localized, non-contact processing method with a small heat-affected zone, preventing thermal damage to components already mounted on the flexible circuit board and facilitating mass production. The UV-cured protective layer uses a high-elongation polyurethane acrylate material with a tensile elongation at break of no less than 200%, allowing it to deform synchronously with the substrate without cracking. It also exhibits excellent resistance to damp heat and chemical corrosion; after being placed in an 85℃ / 85%RH environment for 1000 hours, the resistivity change rate is less than 10%, making it suitable for demanding applications such as wearable devices and medical implants.

[0025] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0026] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for combining a stretchable circuit board and a flexible circuit board, characterized in that, Includes the following steps: S1. Interface pretreatment: Plasma surface activation treatment is performed on the area to be bonded on the flexible circuit board. The treatment power is 80-200W, the treatment time is 30-120s, and the treatment gas is a mixture of argon and oxygen with a volume ratio of (3-7):

1. S2. Preparation of a gradient conductive layer: A gradient conductive composite layer is formed on the surface of the area to be bonded on the flexible circuit board by screen printing or scraping. The gradient conductive composite layer gradually transitions from a highly conductive phase near the flexible circuit board to a highly elastic phase near the stretchable circuit board. The highly conductive phase is a mixture of 70-85% by mass of silver-coated copper powder and epoxy resin, and the highly elastic phase is a mixture of 40-60% by mass of liquid metal droplets and thermoplastic polyurethane. S3. Preparation of stretchable circuit board substrate layer: A stretchable elastomer substrate is coated on the highly elastic phase side of the gradient conductive composite layer. The stretchable elastomer substrate is one of thermoplastic polyurethane, silicone rubber or polydimethylsiloxane, and the coating thickness is 50-300μm. S4. Laser-induced in-situ bonding: The bonding area is scanned and irradiated with a pulsed laser with a wavelength of 355nm or 532nm, a pulse width of 10-50ns, and a scanning speed of 50-200mm / s, so that the gradient conductive composite layer forms an in-situ solidified conductive interconnect structure between the stretchable elastomer substrate and the flexible circuit board. S5. Enhanced Interface Mechanical Interlock: During the laser bonding process, a micro-hole array is pre-set in the bonding area of ​​the flexible circuit board, allowing the molten gradient conductive composite layer to penetrate into the micro-holes to form a mechanical interlock structure. The pore diameter of the micro-hole array is 20-80μm, the pore depth is 10-50μm, and the pore spacing is 100-300μm. S6. Encapsulation and protection: A UV-curable protective adhesive is coated on the surface of the bonding area. The thickness of the UV-curable protective adhesive is 10-30μm. After curing by UV irradiation, a protective layer is formed.

2. The method for combining a stretchable circuit board and a flexible circuit board according to claim 1, characterized in that, After plasma treatment in S1, the water contact angle on the surface of the flexible circuit board is reduced to below 30°, and the surface energy is not less than 60mN / m.

3. The method for combining a stretchable circuit board and a flexible circuit board according to claim 1, characterized in that, The total thickness of the gradient conductive composite layer in S2 is 15-60 μm, of which the thickness of the highly conductive phase accounts for 30-50%, the thickness of the highly elastic phase accounts for 50-70%, and a gradual transition region with a thickness of 3-10 μm is formed between the highly conductive phase and the highly elastic phase.

4. The method for combining a stretchable circuit board and a flexible circuit board according to claim 3, characterized in that, The liquid metal in S2 is a gallium indium tin alloy with a melting point below 15°C, and the droplet size of the liquid metal is 1-10 μm.

5. The method for combining a stretchable circuit board and a flexible circuit board according to claim 4, characterized in that, The gradient conductive composite layer in S2 also includes carbon nanotubes with a mass fraction of 2-5% as a conductive network reinforcement phase. The carbon nanotubes have a diameter of 10-30 nm and a length of 5-20 μm.

6. The method for combining a stretchable circuit board and a flexible circuit board according to claim 1, characterized in that, The stretchable elastomer substrate in S3 is doped with graphene nanosheets at a mass fraction of 1-3% as a thermally conductive reinforcing phase. The graphene nanosheets have a diameter of 1-5 μm and a thickness of 1-5 nm.

7. The method for combining a stretchable circuit board and a flexible circuit board according to claim 1, characterized in that, In step S4, the laser scanning employs a cross-shaped scanning path, with an overlap rate of 30-50% between adjacent scanning lines and a laser energy density of 0.5-2 J / cm². 2 .

8. The method for combining a stretchable circuit board and a flexible circuit board according to claim 7, characterized in that, The volume resistivity of the gradient conductive composite layer after in-situ curing in S4 is no higher than 5 × 10⁻⁶. -5 Ω·cm, the rate of change of resistivity is less than 15% under 30% tensile strain.

9. The method for combining a stretchable circuit board and a flexible circuit board according to claim 1, characterized in that, The micro-hole array in S5 is formed by laser drilling or chemical etching. The roughness Ra of the inner wall of the micro-hole is 2-8μm. The micro-hole is a blind hole structure and does not penetrate the flexible circuit board substrate.

10. The method for combining a stretchable circuit board and a flexible circuit board according to claim 1, characterized in that, The UV-curable protective adhesive in S6 is a polyurethane acrylate type with a tensile elongation at break of not less than 200%, a UV curing wavelength of 365nm, and a curing energy of 500-1500mJ / cm². 2 .

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