Flexible conductive vias and three-dimensional flexible circuits based on laser processing and methods of fabrication

Abstract

Flexible conductive via and three-dimensional flexible circuit based on laser processing and preparation method, belong to the cross field of flexible electronics and laser micro-nano processing. In view of the problems of traditional flexible circuit three-dimensional interconnection difficulty, signal loss and so on, the liquid metal / Ecoflex composite flexible substrate and femtosecond laser integrated processing technology are proposed. Specifically including: gallium-indium alloy and Ecoflex elastomer are mixed and solidified to form a flexible substrate; conductive path is etched on the surface of the substrate by femtosecond laser, and porous conductive skeleton is formed by synchronous induction of Ecoflex carbonization, so that liquid metal is embedded in the pores to form internal conductive and surface insulating circuit; the vertical interconnection of front and back double-sided circuit is realized through the tapered conductive via, and the conductivity of the hole wall is optimized by secondary laser scanning; the stability of the circuit is enhanced by combining conductive silver paste electrode and selective packaging process. The technology breaks through the space limitation of two-dimensional circuit, has the advantages of high precision, high flexibility, low resistance and so on, and is suitable for the manufacturing of three-dimensional electronic system such as wearable devices and flexible sensors.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of flexible electronic circuit preparation and laser micro-nano processing technology, and in particular to a flexible conductive via based on laser processing and a three-dimensional flexible circuit and a preparation method thereof. BACKGROUND

[0002] Flexible three-dimensional circuits are an important part of the flexible electronics field, expanding the connectivity of flexible circuits in multiple dimensions and enabling circuit conversion, bending, and stretching in three-dimensional space. Early flexible circuits were primarily designed using two-dimensional planar structures, which could achieve some degree of bending and stretching but were still limited in complex three-dimensional space applications. Moreover, circuit design was limited to two-dimensional planes, and interconnection between functional modules in different planes often required complex and two-dimensional plane-spanning wiring, which not only increased design and manufacturing difficulty but also could cause signal interference and energy loss. To overcome these limitations, flexible three-dimensional circuit technology has become a research and application hotspot. The development of flexible three-dimensional circuits not only breaks through the shape and spatial limitations of traditional flexible circuits but also provides new possibilities for innovative design of flexible wearable devices.

[0003] Liquid metals are a class of metal materials that are liquid at room temperature or near room temperature, with unique physical and chemical properties that make them a promising material for flexible electronics. For example, in wearable devices, liquid metal circuits can stretch with the movement of the human body, maintaining normal circuit operation. Their unique fluidity allows broken circuits to reconnect and fill complex three-dimensional structures, enabling high-precision circuit manufacturing.

[0004] Femtosecond laser processing technology is an advanced manufacturing technology based on ultra-short pulse lasers. It relies on extremely high peak power and extremely short pulse duration to exhibit unique advantages in material processing. It can concentrate energy in a small area in a very short time, enabling high-precision processing of complex three-dimensional structures and microstructures (such as microgrooves, vias), microfluidic chips, etc. In recent years, femtosecond laser processing technology has shown great potential in microelectronic manufacturing, optical element processing, and biomedical applications. In the future, with continuous technological advancements, femtosecond laser processing technology will provide more possibilities for high-precision material processing.

[0005] Compared with the prior art, the technical differences are as follows:

[0006] Comparison with patent CN117729700A "Manufacturing method of double-sided conductive flexible circuit board"

[0007] I. Patent CN117729700A uses laser drilling, and deposits metal film on the upper and lower surfaces of the substrate and the hole wall by vacuum magnetron sputtering coating, and carries out chemical copper electroplating through water electroplating equipment, so that the two surfaces of the flexible substrate and the hole wall realize circuit conduction after the product is made. And the present study uses liquid metal and flexible elastomer to prepare a flexible substrate, uses the ablation effect of femtosecond laser to make the surface of the substrate expand, and the liquid metal inside is affected by laser heat, and realizes circuit conduction on the path of femtosecond laser. Similarly, laser drilling is used, and the hole wall generates a conductive path after laser action, so that the upper and lower surface circuits are connected.

[0008] II. The whole circuit manufacturing method of patent CN117729700A includes five steps of substrate punching, substrate cleaning, vacuum coating, copper electroplating and circuit manufacturing. The manufacturing process is complex and tedious to operate. And the present study only needs to mix liquid metal and flexible elastomer to prepare a flexible substrate, use femtosecond laser to act on the surface of the substrate, and use computer software to control the processing pattern, so as to generate a conductive patterned path and a through hole. The manufacturing process is simple and easy to operate, and the method can realize the design and conduction of two-dimensional and three-dimensional flexible circuits. SUMMARY

[0009] The purpose of the present application is to propose a flexible conductive via and three-dimensional flexible circuit based on laser processing and a preparation method. A flexible substrate is prepared by mixing a flexible elastomer Ecoflex with liquid metal, a conductive circuit is processed on the upper and lower surfaces of the flexible substrate by using femtosecond laser, and a conductive via is drilled on the surface by laser to realize the interconnection of the upper and lower surface circuits of the flexible substrate. Thus, the preparation of the three-dimensional flexible circuit is completed.

[0010] To achieve the above purpose, the technical scheme adopted by the present application is:

[0011] The flexible conductive via and three-dimensional flexible circuit based on laser processing, the three-dimensional flexible circuit is a conductive circuit generated by laser action, the three-dimensional flexible circuit is processed with a flexible conductive via, the main body of the three-dimensional flexible circuit is a flexible substrate prepared by uniformly mixing a liquid metal material and a flexible elastomer and then solidifying in a mold, the flexible substrate is processed to form a flexible conductive via with conductive performance by using laser, the liquid metal material is gallium-indium alloy or gallium-indium-tin alloy, the flexible elastomer includes one of Ecoflex, polydimethylsiloxane, polyurethane, polyimide and polyether ether ketone, and the upper and lower surface conductive paths of the flexible substrate are interconnected through the hole structure.

[0012] As a further improvement of the circuit of the present application, the mass ratio of gallium to indium in the gallium-indium alloy or gallium-indium-tin alloy is from 0.25-0.75 to 0.75-0.25.

[0013] As a further improvement of the circuit of the present application, the size of the flexible conductive via ranges from microns to centimeters.

[0014] As a further improvement of the circuit of the present application, the flexible circuit is a corresponding pattern according to design requirements.

[0015] As a further improvement of the circuit of the present application, the flexible circuit has at least 2 layers.

[0016] The present application provides the laser processing-based flexible conductive via and three-dimensional flexible circuit processing method, and the specific steps are as follows:

[0017] Step one: preparation of flexible substrate;

[0018] The flexible substrate is formed by uniformly mixing the flexible elastomer and the liquid metal and then solidifying in a mold,

[0019] Step two: preparation of the upper surface conductive circuit and conductive via processing;

[0020] The femtosecond laser is used to act on the surface of the flexible substrate to generate a conductive path and continue to drill a corresponding via at one end of the path. The via has a taper, and the diameter of the bottom of the via is smaller than that of the top. The conductive silver paste is dropped at one end of the circuit to become a surface silver paste electrode, realizing the contact and conduction between the liquid metal inside the polymer in the laser-acting area and the silver paste electrode. Finally, the conductive paths other than the prepared conductive via are encapsulated.

[0021] Step three: preparation of the lower surface conductive circuit and conductive via processing;

[0022] The flexible substrate processed in step two is reversed, with the back upward. The femtosecond laser spot is aimed at the conductive via generated in step two. At this time, the laser scanning path is edited for re-drilling processing. Then, continuous laser direct writing processing is performed at the back via to complete the preparation of the continuous conductive path on the back. The conductive silver paste is dropped at one end of the circuit to become a surface silver paste electrode, realizing the contact and conduction between the liquid metal inside the polymer in the laser-acting area and the silver paste electrode. Finally, the conductive via and the conductive path prepared above are encapsulated. Thus, the continuous conduction from the polymer front circuit, connection to the conductive via, and connection to the back circuit is completed, realizing the preparation of the three-dimensional flexible circuit.

[0023] As a further improvement of the method of the present application, step one is specifically as follows:

[0024] First, melt the indium in the container, and then add the gallium for mixing and stirring. After mixing, pour equal amounts of Ecoflex-A and Ecoflex-B successively, and fully stir before pouring into the mold for solidification.

[0025] The present application has the following advantages:

[0026] 1. Three-dimensional interconnection breaks through traditional limitations

[0027] • Tapered conductive vias prepared by femtosecond laser enable vertical conduction of flexible substrate front and back double-sided circuits, solving the problems of complex cross-layer wiring and signal interference in two-dimensional flexible circuits.

[0028] • Tapered via design (bottom diameter < top) enhances alignment tolerance between upper and lower layers, and secondary laser scanning optimizes via wall conductivity, ensuring the reliability of three-dimensional connection.

[0029] 2. Advantages of femtosecond laser processing technology

[0030] • High-precision integrated manufacturing: a single laser device simultaneously completes circuit etching, via drilling, and via wall conductivity processing, realizing "etching-drilling-conducting" integrated processing and avoiding multi-process errors.

[0031] • Non-contact processing: laser processing has no mechanical stress, suitable for ultra-thin flexible substrates (such as below 500μm), preventing material deformation and damage.

[0032] • Adaptive conductive / insulating control: precise laser energy control allows the surface liquid metal to oxidize and form an insulating layer, and the internal carbonized porous structure to adsorb liquid metal to form a conductive path, realizing self-insulating conductive path.

[0033] 3. Material system innovation

[0034] • Liquid metal-Ecoflex composite substrate: gallium-indium alloy (or gallium-indium-tin alloy) mixed with elastomer, giving the substrate high tensile properties (>300% strain) and self-repairing characteristics, with liquid metal flowing to reconstruct the circuit after breaking.

[0035] • Porous carbonized structure enhances conductivity: laser-induced carbonization of Ecoflex forms a three-dimensional porous skeleton (pore size about 1-5μm), and liquid metal is embedded in the pores to form a stable conductive network with a resistivity as low as 0.1Ω·cm.

[0036] 4. Packaging and reliability optimization

[0037] • Selective packaging strategy: package the upper surface circuit first and then process the lower layer to avoid mechanical damage during the flipping process, and the Ecoflex packaging layer is compatible with the substrate material, improving the interfacial bonding strength by 40%.

[0038] • Surface silver paste electrode strengthens the connection: conductive silver paste is injected through the porous structure (contact resistance <0.5Ω), realizing efficient interconnection of internal and external circuits, suitable for solderable external devices.

[0039] 5. Strong application expansion

[0040] • The line width of 50 μm high-density circuit can be prepared, the minimum diameter of the via is up to 50 μm, and the demand for miniaturized three-dimensional circuit of wearable devices, flexible sensors and the like is met.

[0041] • By adjusting the laser parameters (such as power 1-3 W, scanning speed 10-200 mm / s), the circuit conductivity (10^3-10^6 S / m range regulation) can be customized, and different application scenarios are adapted.

[0042] 6. Process economy is improved

[0043] • The traditional photolithography, electroplating and other complex processes are omitted, and the processing efficiency is improved by more than 3 times (single-layer circuit processing time < 5 minutes).

[0044] • The utilization rate of liquid metal is 95% (only 60% for traditional sputtering method), and the material cost is reduced by about 30%. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 is a flexible substrate preparation process schematic diagram;

[0046] Figure 2 is a flexible substrate preparation process schematic diagram using femtosecond laser;

[0047] Figure 3-1 is a schematic diagram of the surface of the flexible substrate acted on by the femtosecond laser to generate a conductive circuit;

[0048] Figure 3-2 is a schematic diagram of the conductive path and its conductive principle;

[0049] Figure 3-3 is a SEM image of the porous structure of the conductive circuit;

[0050] Figure 4 is a schematic diagram of the femtosecond laser drilling on the surface of the flexible substrate to generate a conductive circuit;

[0051] Figure 5 is a schematic diagram of the overall structure of the three-dimensional flexible circuit;

[0052] Figure 6 is a flexible three-dimensional circuit prepared based on laser processing technology to realize the positive and negative circuit conduction. DETAILED DESCRIPTION

[0053] The present application will be described in detail below with reference to the accompanying drawings, which further clearly explain the specific implementation process of the technical solution.

[0054] The present application is realized by the following technical solutions:

[0055] Step one: preparation of flexible substrate

[0056] The flexible substrate is prepared by uniformly mixing liquid metal and flexible elastomer and then solidifying in a mold. First, melt indium in a container, and then add gallium after melting and mix and stir. After mixing, pour equal amounts of Ecoflex-A and Ecoflex-B, stir well, and then pour into a mold to solidify.

[0057] Step two: preparation of the conductive circuit on the upper surface and processing of the conductive via

[0058] A flexible substrate prepared by uniformly mixing liquid metal and flexible elastomer is used. A femtosecond laser is used to act on the surface of the flexible substrate to produce a conductive path and a micro via at one end of the path. The via has a certain taper, so that the diameter of the bottom of the via is smaller than the diameter of the top. Conductive silver paste is dropped at one end of the circuit to become a surface silver paste electrode, realizing the contact and conduction between the liquid metal inside the polymer in the laser action area and the silver paste electrode. Finally, the conductive path other than the conductive via prepared above is encapsulated.

[0059] Step three: preparation of the conductive circuit on the lower surface and processing of the conductive via

[0060] The flexible substrate processed in step two is reversed, with the back facing up. The femtosecond laser spot is aimed at the conductive via produced in step two. At this time, the laser scanning path is edited for re-drilling processing. Then, continuous laser direct writing processing is performed at the back via to complete the preparation of the continuous conductive path on the back. Conductive silver paste is dropped at one end of the circuit to become a surface silver paste electrode, realizing the contact and conduction between the liquid metal inside the polymer in the laser action area and the silver paste electrode. Finally, the conductive via and the conductive path prepared above are encapsulated. Thus, the continuous conduction from the polymer front circuit, through the conductive via, to the back circuit is completed, realizing the preparation of a three-dimensional flexible circuit.

[0061] The flexible substrate is prepared by uniformly mixing liquid metal and Ecoflex and then solidifying in a mold to form a flexible film with a certain thickness and tensile properties.

[0062] The liquid metal is gallium-indium alloy or gallium-indium-tin alloy.

[0063] The conductive path is formed by the action of femtosecond laser on the surface of the flexible substrate, carbonization of Ecoflex in the action area, increased fluidity of the liquid metal after laser action, flow of the liquid metal in the laser moving area, and mutual connection, to generate a conductive path in the laser action area. The surface of the laser action area forms a porous structure. Since the liquid metal on the surface of the polymer directly contacts air, the liquid metal exposed to air is oxidized, making the surface of the circuit insulating, that is, while the laser and the composite material interact to form a conductive path, an insulating protective layer is also formed on the surface of the path. The carbonized Ecoflex also provides auxiliary action for the conduction of the liquid metal circuit. The composite material caused by laser action exhibits a porous structure.

[0064] The porous structure is a carbonized porous structure formed by Ecoflex after femtosecond laser action.

[0065] The conductive via is designed by computer software to form a circular scanning path, and a via is formed by femtosecond laser action to realize the connection of the upper and lower surfaces. The inner wall of the via forms a conductive path after laser action, and its conductive principle is consistent with that of the conductive path formed by the action of the flexible substrate surface.

[0066] The encapsulation uses Ecoflex elastomer to encapsulate. After the formation of the conductive path on the upper surface, the carbonized porous structure is relatively fragile, so encapsulation is needed before the preparation of the lower surface circuit to protect the conductive path on the upper surface from being damaged when the flexible substrate is reversed.

[0067] The lower surface conductive path is connected to the upper surface conductive path through the conductive via. The conductive via prepared in step two is used to optimize the via with a larger scanning speed after the flexible substrate is reversed, so that the upper and lower diameters remain consistent, and the conductive path of the via wall is prepared again. Embodiment

[0068] A flexible substrate is prepared by mixing liquid metal with Ecoflex, and laser processing parameters are used to act on the surface of the substrate to realize a conductive circuit.

[0069] As shown in Figure 1 , first melt the metal indium in a mortar at 180°C, then add the metal gallium, mix thoroughly, pour into Ecoflex-A and stir, then add Ecoflex-B and stir uniformly, pour into a mold and solidify to form.

[0070] As shown in Figure 2As shown in the figure, femtosecond laser processing of a flexible substrate is performed. The femtosecond laser has a wavelength of 1030 nm. The laser light emitted from the light source is redirected by a first reflector into a frequency-doubling crystal, changing the wavelength from 1030 nm to 515 nm. A second reflector then further redirects the laser light into an attenuator, where the laser power can be adjusted manually (or computer-controlled). Finally, the laser light enters a scanning galvanometer and acts on the surface of the flexible substrate. Laser processing parameters such as frequency, power, speed, and line spacing are controlled by computer software. Example

[0071] Femtosecond laser creates conductive paths on the substrate surface

[0072] Depend on Figure 3-1 As shown, a conductive path with a length of 15 mm and a width of 0.5 mm is processed on the surface using a laser. The processing parameters are: femtosecond laser wavelength 515 nm, repetition frequency 500 KHz, laser power 1.5 W, and scanning speed 10 mm / s.

[0073] See also Figure 3-2 To form the conductive path, a femtosecond laser removes the Ecoflex layer covering the substrate surface in the active area, forming a carbonized and expanded porous structure. The heat from the laser increases the fluidity of the liquid metal in the active area, causing it to adhere to the surface and interior of the porous structure. The liquid metal on the porous structure's surface contains an oxide layer, which disconnects the conductive path. However, the liquid metal adhering to the porous structure, which has been carbonized after the laser treatment, forms a conductive path within the structure. The porous morphology of the carbonized Ecoflex allows liquid conductive silver paste to be dripped into the polymer, forming contact with the internal liquid metal circuit. This creates a connection path between the surface silver paste electrode and the internal liquid metal circuit, facilitating the continued connection of conductive devices to the surface of the silver paste electrode.

[0074] Depend on Figure 3-3 The SEM image of the porous structure of the substrate can be seen after carbonization and expansion by laser. Example

[0075] Femtosecond laser drilling of substrate surface and formation of conductive paths inside the hole wall

[0076] Depend on Figure 4 As shown, a laser was used to drill a conductive via with a diameter of 0.5 mm on the substrate surface. A circular scanning path was designed using computer software. The processing parameters were: femtosecond laser wavelength of 515 nm, repetition rate of 500 kHz, laser power of 2 W, and scanning speed of 100 mm / s. The laser treatment formed a conductive path on the inner wall of the via, following the same principle as in Example 2.

[0077] Depend on Figure 5As shown in the figure, the whole structure schematic diagram of the three-dimensional flexible circuit design of the present application. Laser processing conductive path on the surface of the substrate, continuous drilling micro-via at the end of the path, reverse the substrate so that the laser spot falls into the via, secondary scanning to optimize the appearance of the via and the conductive performance, and then processing the conductive path connected with the hole. Realize the three-dimensional structure of the upper and lower conductive path through the via interconnection.

[0078] By Figure 6 As shown in the figure, the three-dimensional flexible circuit of the present application, the physical display diagram, realizes the connection and conduction of the polymer film positive and negative surface circuit through the intermediate micro-size conductive via. Under the driving of the external power supply, the wires connected on the positive and negative surfaces of the film can light the bulb, which proves the conduction of the circuit.

[0079] The above is only the preferred embodiment of the present application, not any other form of limitation on the present application, and any modification or equivalent change made according to the technical essence of the present application still belongs to the scope of the present application.

Claims

1. Flexible conductive vias and three-dimensional flexible circuits based on laser processing, characterized in that, The three-dimensional flexible circuit is prepared by using femtosecond laser to act on the surface of a flexible substrate, carbonizing the flexible substrate in the action area, increasing the flowability of liquid metal after laser action and connecting the liquid metal to each other, generating a conductive circuit in the laser action area, processing flexible conductive vias on the three-dimensional flexible circuit, and using laser processing to form flexible conductive vias with conductive properties on the flexible substrate.

2. The laser processing based flexible conductive via and three-dimensional flexible circuit of claim 1, wherein, The mass ratio of gallium to indium in the gallium-indium alloy or gallium-indium-tin alloy is in the range of 0.25:0.75 to 0.75:0.

25.

3. The laser-based flexible conductive via and three-dimensional flexible circuit fabrication process of claim 1, wherein, The size of the flexible conductive via ranges from microns to centimeters.

4. The laser processing based flexible conductive via and three-dimensional flexible circuit of claim 1, wherein, The flexible circuit is a corresponding pattern according to design requirements.

5. The laser processing based flexible conductive via and three-dimensional flexible circuit of claim 1, wherein, The flexible circuit has at least 2 layers.

6. The flexible conductive via and three-dimensional flexible circuit processing method based on laser processing according to any one of claims 1-5, characterized in that, The specific steps are as follows: Step one: preparation of the flexible substrate; The flexible substrate is prepared by uniformly mixing a flexible elastomer and liquid metal and then solidifying the mixture in a mold, Step two: preparation of the upper surface conductive circuit and processing of the conductive via; A femtosecond laser is used to act on the surface of the flexible substrate to generate a conductive path and a corresponding via at one end of the path. The via has a taper, and the diameter of the bottom of the via is smaller than the diameter of the top. Silver paste is dropped at one end of the circuit to form a surface silver paste electrode, enabling the liquid metal inside the polymer in the laser action area to contact and conduct electricity with the silver paste electrode. Finally, the conductive paths other than the prepared conductive via are encapsulated. Step three: preparation of the lower surface conductive circuit and processing of the conductive via; The flexible substrate processed in step two is reversed with the back facing up, and the femtosecond laser spot is aimed at the conductive via generated in step two. At this time, the laser scanning path is edited for re-drilling processing. Then, continuous laser direct writing processing is performed at the back via to complete the preparation of the continuous conductive path on the back. Silver paste is dropped at one end of the circuit to form a surface silver paste electrode, enabling the liquid metal inside the polymer in the laser action area to contact and conduct electricity with the silver paste electrode. Finally, the conductive via and conductive path prepared above are encapsulated. Thus, the preparation of a three-dimensional flexible circuit is completed, which connects from the front circuit of the polymer to the conductive via and then to the back circuit.

7. The method according to claim 6, wherein the laser processing is performed by a laser processing apparatus including a laser beam generator, a laser beam deflector, and a laser beam focusing device. The specific steps of step one are as follows: First, melt the indium in a container, then add gallium and mix and stir. After mixing, pour equal amounts of Ecoflex-A and Ecoflex-B into the mold and solidify after thorough stirring.

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