Welding method for LED chip of Mini / Micro LED

Abstract

The invention relates to the technical field of LED display, and discloses a welding method of an LED chip of a Mini / Micro LED. The welding method comprises the steps that a provided hydrophobic nano steel mesh with a plurality of steel holes is placed on a provided substrate with a plurality of bonding pads and aligned, so that each steel hole is aligned with each bonding pad correspondingly; adding the provided anisotropic conductive adhesive to the hydrophobic nano steel mesh, and uniformly blade-coating the anisotropic conductive adhesive on the hydrophobic nano steel mesh, so that the anisotropic conductive adhesive is printed on the bonding pad; stripping the hydrophobic nano steel mesh; mounting the provided LED chip on the anisotropic conductive adhesive on the bonding pad; and heating and pressurizing the LED chip positioned on the anisotropic conductive adhesive, so that the insulating film is longitudinally broken to expose the metal conductive particle balls wrapped in the insulating film, the metal conductive particle balls are respectively in contact with the LED chip and the bonding pad to form a vertical conductive path, and the insulating adhesive is preliminarily cured, so that the LED chip is welded on the substrate. The welding quality is stable, the maintenance difficulty is small, and the maintenance cost is low.

Description

Technical Field

[0001] The present invention relates to the technical field of LED display, and in particular to a welding method for a Mini / Micro LED LED chip. Background Art

[0002] Although the reflow process is widely used in electronic manufacturing, it also has some temperature-related disadvantages. The following are the disadvantages of the reflow process in Mini / Micro LED display welding: 1) During the reflow soldering process, the soldering temperature is usually high. During high-temperature soldering, the chip is easily damaged, resulting in unstable soldering quality.

[0003] 2) In addition, when a chip is defective, we need to heat the PCB or glass substrate to a higher temperature to melt the solder paste and remove the chip. However, due to the physical phenomena of heat diffusion and heat conduction, the high temperature will quickly transfer to the surrounding chips, making it difficult to accurately apply the heating area to the damaged chip, which will cause damage to the normal chips nearby, increasing the risk during the repair process.

[0004] 3) Since the ACF is a whole conductive film, if there is a poor connection or failure of individual chips, it is extremely difficult to repair. Because thousands of chips are already connected to the film, repair often requires replacing the entire conductive film, resulting in all chips on the ACF film being scrapped and high repair costs. Summary of the invention

[0005] The main purpose of the present invention is to provide a welding method for Mini / Micro LED LED chips, aiming to solve the technical problems of unstable welding quality, great maintenance difficulty and high maintenance cost of existing LED chips.

[0006] To achieve the above object, the present invention provides a welding method for a Mini / Micro LED LED chip, the welding method comprising: providing a hydrophobic nano-steel mesh having a plurality of steel holes and a substrate having a plurality of solder pads, placing the hydrophobic nano-steel mesh on the substrate, and aligning the substrate so that each of the steel holes is aligned with each of the solder pads, wherein the substrate is one of a circuit board or a glass substrate; Providing anisotropic conductive glue, adding the anisotropic conductive glue to the side of the hydrophobic nano-steel mesh away from the pad, and evenly scraping the anisotropic conductive glue on the hydrophobic nano-steel mesh by a scraping method, so that the anisotropic conductive glue is printed onto the pad through the steel hole; peeling off the hydrophobic nano-steel mesh; Providing an LED chip, and mounting the LED chip on the anisotropic conductive adhesive located on the pad; The LED chip on the anisotropic conductive adhesive is heated and pressurized by a hot pressing process, so that the insulating film of the anisotropic conductive adhesive is longitudinally ruptured to expose the metal conductive particle balls wrapped in the insulating film, so that the metal conductive particle balls are respectively in contact with the LED chip and the soldering pad to form a vertical conductive path, and the insulating glue of the anisotropic conductive adhesive is initially solidified, so that the LED chip is soldered on the substrate.

[0007] Further, in one embodiment, the step of providing anisotropic conductive adhesive is formed in three steps, including: The metal conductive particle balls are prepared by a mechanical separation process or a spraying process; A layer of insulating coating is applied on the surface of the metal conductive particle ball by a coating process, and air-dried to form an insulating film; An insulating glue is provided, and the metal conductive particle balls coated with the insulating film are placed in the insulating glue and stirred to form an anisotropic conductive glue.

[0008] Further, in one embodiment, the material of the insulating film is one or more of a resin, a polymer or a thermoplastic polymer; and / or, The material of the insulating glue is modified epoxy resin.

[0009] Furthermore, in one embodiment, the material of the metal conductive particle ball is one or more of tin powder, silver powder, nickel powder, gold powder, tin alloy powder, copper powder, and aluminum powder.

[0010] Further, in one embodiment, the diameter of the metal conductive particle ball is 10 μm-20 μm; and / or, The thickness of the insulating film is 3%-25% of the diameter of the metal conductive particle ball.

[0011] Further, in one embodiment, the step of providing an insulating glue comprises: Providing an insulating glue, placing the metal conductive particle balls coated with the insulating film into the insulating glue, and stirring at a low speed so that a plurality of the metal conductive particle balls are evenly distributed in the insulating glue to form an anisotropic conductive glue; or, An insulating glue is provided, and metal conductive particle balls coated with the insulating film are placed in the insulating glue, and stirred by a planetary mixer with a combined motion of rotation and revolution, so that a plurality of the metal conductive particle balls are evenly distributed in the insulating glue to form an anisotropic conductive glue.

[0012] Furthermore, in one embodiment, the step of coating a layer of insulating coating on the surface of the metal conductive particle ball by a coating process and air-drying the coating to form an insulating film comprises: The metal conductive particle balls are immersed in a chemical solution by chemical deposition, an insulating coating is formed on the surface of the metal conductive particle balls by chemical reaction, and the insulating coating is air-dried to form an insulating film; or, The material of the insulating coating is deposited onto the surface of the metal conductive particle ball by physical vapor deposition to form an insulating coating, and the insulating coating is air-dried to form an insulating film.

[0013] Further, in one embodiment, the step of providing a hydrophobic nano-steel mesh having a plurality of steel pores is formed in two steps, including: Providing a steel plate, and opening steel holes on the steel plate corresponding to the welding pads of the substrate to form a steel mesh; A layer of nano hydrophobic coating is coated on the steel mesh to obtain a hydrophobic nano steel mesh.

[0014] Furthermore, in one embodiment, after the LED chip on the anisotropic conductive adhesive is heated and pressurized by the hot pressing process, so that the insulating film of the anisotropic conductive adhesive is longitudinally ruptured to expose the metal conductive particle balls wrapped in the insulating film, so that the metal conductive particle balls are respectively in contact with the LED chip and the pad to form a vertical conductive path, and the insulating glue of the anisotropic conductive adhesive is initially cured, so that the LED chip is soldered on the substrate, the method further includes: Testing the LED chip; When the LED chip is defective, the LED chip is heated to melt the anisotropic conductive adhesive, and the defective LED chip is removed and replaced with a good LED chip.

[0015] Furthermore, in one embodiment, the step of providing an LED chip and mounting the LED chip on the anisotropic conductive adhesive located on the pad includes: The LED chip is mounted on the anisotropic conductive adhesive on the pad through a mass transfer process or a SMT process.

[0016] In the technical solution provided by the present invention, the hydrophobic nano steel mesh is first aligned with the substrate, and the steel holes of each hydrophobic nano steel mesh are aligned with each pad respectively, and then anisotropic conductive glue is added to the hydrophobic nano steel mesh, and the anisotropic conductive glue is evenly scraped on the surface of the hydrophobic nano steel mesh by scraping, so that the anisotropic conductive glue is printed on the pad; the LED chip is mounted on the anisotropic conductive glue located on the pad, and the insulating film of the anisotropic conductive glue is longitudinally (Z-axis direction) ruptured to expose the metal conductive particle balls wrapped in the insulating film, so that the metal conductive particle balls can contact the LED chip and the pad respectively to form a vertical The conductive path is formed, and the insulating glue of the anisotropic conductive glue is initially solidified, so that the LED chip is welded on the substrate. Since the anisotropic conductive glue contains multiple metal conductive particle balls, among the numerous metal conductive particle balls, as long as the metal conductive particle balls are turned on, the LED chip can be normally lit, and it is not necessary for all the metal conductive particles to be turned on at the same time, thereby reducing the difficulty of welding and improving the welding quality. The insulating glue has an open time of 4 hours to 8 hours, and the insulating glue can be melted by reheating, so that the defective LED chip can be easily removed without damaging the LED chip, and the good LED chip can be replaced, which reduces the difficulty of maintenance and the cost of maintenance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] One or more embodiments are exemplarily described by corresponding drawings, which do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings represent similar elements, and the figures in the drawings do not constitute proportional limitations unless otherwise stated.

[0018] Figure 1 This is a flow chart of a first embodiment of a welding method for a Mini / Micro LED LED chip in an embodiment of the present invention; Figure 2 This is a flow chart of a second embodiment of a welding method for a Mini / Micro LED chip in an embodiment of the present invention. DETAILED DESCRIPTION

[0019] In order to facilitate the understanding of the present invention, the present invention is described in more detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that when an element is described as "fixed to" another element, it can be directly on another element, or there can be one or more centered elements therebetween. When an element is described as "connected" to another element, it can be directly connected to another element, or there can be one or more centered elements therebetween. The terms "vertical", "horizontal", "left", "right", "inside", "outside" and similar expressions used in this specification are only for illustrative purposes. In the description of the present invention, the terms "first" and "second" are used only for descriptive purposes and cannot be understood as indicating relative importance or implicitly indicating the number of technical features indicated. Therefore, unless otherwise specified, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features; the meaning of "multiple" is two or more. The term "include" and any variation thereof means non-exclusive inclusion, and one or more other features, integers, steps, operations, units, components and / or combinations thereof may be present or added.

[0020] In addition, unless otherwise expressly specified and limited, the terms "installed", "connected" and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, an indirect connection through an intermediate medium, or a connection between two components. All technical and scientific terms used in this specification have the same meaning as those generally understood by technicians in the technical field of the present invention. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used in this specification includes any and all combinations of one or more related listed items.

[0021] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0022] For ease of understanding, the specific process of the embodiment of the present invention is described below. Figure 1 The first embodiment of the welding method of the Mini / Micro LED LED chip in the embodiment of the present invention includes: S1, providing a hydrophobic nano-steel mesh having a plurality of steel holes and a substrate having a plurality of pads, placing the hydrophobic nano-steel mesh on the substrate, and aligning the hydrophobic nano-steel mesh so that each of the steel holes is aligned with each of the pads, wherein the substrate is a circuit board or a glass substrate; In this step, a substrate with a plurality of pads is provided, and steel holes are opened on the steel plate according to the distribution of the pads of the substrate to form a steel mesh, and a hydrophobic nano coating is coated on the steel mesh to prevent the anisotropic conductive adhesive from adhering to the steel holes to form a hydrophobic nano steel mesh. After the hydrophobic nano steel mesh and the substrate are prepared, the substrate is first mounted on a fixture of a steel mesh printer, and the fixture has adjustable supports and positioning pins to ensure the accurate position of the substrate; then the hydrophobic nano steel mesh is fixed to the frame of the printer, and the positioning mark (Fiducial Mark) of the substrate is automatically detected by the visual positioning system of the printer for correction, so that the position of the substrate and the hydrophobic nano steel mesh are aligned, and each steel hole is aligned with each pad respectively, that is, each steel hole is located directly below each pad, and then the substrate is fixed on the printing platform by vacuum adsorption or mechanical clamping to prevent movement during the printing process.

[0023] S2, providing anisotropic conductive glue, adding the anisotropic conductive glue to the side of the hydrophobic nano-steel mesh away from the pad, and evenly scraping the anisotropic conductive glue on the hydrophobic nano-steel mesh by a scraping method, so that the anisotropic conductive glue is printed on the pad through the steel hole; In this step, anisotropic conductive glue is provided, and a scraper (usually made of metal or polyurethane material) in a steel screen printer is used to push the anisotropic conductive glue onto the surface of the hydrophobic nano steel mesh, and scrape along the hydrophobic nano steel mesh at a set speed and pressure; the scraper moves back and forth on the surface of the hydrophobic nano steel mesh, squeezes the anisotropic conductive glue through the steel holes on the hydrophobic nano steel mesh and prints it onto the pad of the substrate, and makes the height of the anisotropic conductive glue on the pad consistent, and the excess anisotropic conductive glue is collected by the scraper back to the top of the hydrophobic nano steel mesh. Among them, the scraper speed: 20~100 mm / s, the scraper pressure: 0.5~1.5 kg, the printing environment: the humidity is less than 60%, keep it clean and dry.

[0024] S3, peeling off the hydrophobic nano-steel mesh; In this step, after the anisotropic conductive adhesive is printed, the control system of the steel mesh printer slowly lifts the hydrophobic nano-steel mesh to gradually separate the hydrophobic nano-steel mesh from the substrate. Generally, the hydrophobic nano-steel mesh is gradually separated from the top to the bottom or from the bottom to the top to ensure that the anisotropic conductive adhesive filled on the pad remains intact and clear, which is convenient for the subsequent mounting of the LED chip on the anisotropic conductive adhesive.

[0025] S4, providing an LED chip, and mounting the LED chip on the anisotropic conductive adhesive located on the pad; In this step, the LED chip can be an unpackaged LED chip or a packaged LED chip. The unpackaged LED chip or the packaged LED chip is mounted on the anisotropic conductive adhesive on the pad through a mass transfer process or an SMT process. Since the anisotropic conductive adhesive is directly scraped onto the pad through a hydrophobic nano steel mesh, no separate glue dispensing process is required, which saves process steps and reduces costs.

[0026] S5. The LED chip on the anisotropic conductive adhesive is heated and pressurized by a hot pressing process, so that the insulating film of the anisotropic conductive adhesive is longitudinally ruptured to expose the metal conductive particle balls wrapped in the insulating film, so that the metal conductive particle balls are respectively in contact with the LED chip and the soldering pad to form a vertical conductive path, and the insulating glue of the anisotropic conductive adhesive is initially cured, so that the LED chip is soldered to the substrate.

[0027] In this step, the hot pressing process is to apply pressure while heating; specifically, the LED chip mounted on the anisotropic conductive adhesive on the pad is heated, and pressure is applied in the longitudinal direction of the LED chip, so that the excess insulating glue overflows and the insulating film is broken. Specifically, when heated to 120°C-250°C, the insulating film of the anisotropic conductive adhesive softens, and then a vertical uniform pressure of 30N·m-100N·m is applied to the LED chip through a flat pressure plate, so that the softened insulating film is broken in the Z-axis direction. Since the insulating film wraps the metal conductive particle balls, the metal conductive particle balls are exposed when the insulating film is broken. The exposed metal conductive particle balls are in contact with the LED chip and the soldering pad respectively to form a vertical conductive path. Since the insulating film is not broken in the X-axis direction and the Y-axis direction, and the insulating film is cured, it is prevented from being damaged by external force or environmental influence during use, and the contact between two adjacent metal conductive particle balls in the X-axis direction and the Y-axis direction to cause a short circuit is avoided, which plays a role in forming permanent insulation in the X-axis direction and the Y-axis direction. Under pressure, the LED chip, the metal conductive particle balls and the soldering pad are in closer contact, which improves the stability of the electrical connection. The insulating glue of the anisotropic conductive glue is initially cured, so that the LED chip is soldered on the substrate.

[0028] One LED chip is soldered on one pad, and the current driving a single LED chip is very small. The maximum driving current of each LED chip is usually less than 3-5mA. In normal operation, the driving current is even only a few tenths of a milliampere in many cases. The current driving a single LED chip is very small, and there are multiple metal conductive particle balls in the anisotropic conductive glue on a pad, and one LED chip is soldered on one pad. Therefore, as long as one or two of the multiple metal conductive particle balls are able to conduct, the LED chip can be lit normally, and it is not necessary for all the metal conductive particles to be conducted at the same time, which reduces the difficulty of welding and improves the welding quality.

[0029] In this embodiment, the hydrophobic nano steel mesh is first aligned with the substrate, and the steel holes of each hydrophobic nano steel mesh are aligned with each pad, and then anisotropic conductive glue is added to the hydrophobic nano steel mesh, and the anisotropic conductive glue is evenly scraped on the surface of the hydrophobic nano steel mesh by scraping, so that the anisotropic conductive glue is printed on the pad; the LED chip is mounted on the anisotropic conductive glue located on the pad, and the insulating film of the anisotropic conductive glue is longitudinally (Z-axis direction) ruptured to expose the metal conductive particle balls wrapped in the insulating film, so that the metal conductive particle balls can contact the LED chip and the pad to form a vertical conductive path. The anisotropic conductive glue is preliminarily solidified to weld the LED chip on the substrate. As the anisotropic conductive glue contains a plurality of metal conductive particle balls, the LED chip can be normally lit as long as one of the metal conductive particle balls is turned on. It is not necessary for all the metal conductive particles to be turned on at the same time, thereby reducing the difficulty of welding and improving the welding quality. The insulating glue has an open time of 4 to 8 hours. The insulating glue can be melted by reheating, so that the defective LED chip can be easily removed and replaced with a good LED chip without damaging the LED chip, thereby reducing the difficulty of repair and the cost of repair.

[0030] See also Figure 2 As shown, the embodiment of the present invention also discloses a second embodiment of a welding method for a Mini / Micro LED LED chip, comprising: S10, preparing metal conductive particle balls by mechanical separation process or spraying process; In this step, the material of the metal conductive particle ball is one or more of tin powder, silver powder, nickel powder, gold powder, tin alloy powder, copper powder, and aluminum powder, so that the metal conductive particle ball can conduct electricity.

[0031] Exemplary: When the material of the metal conductive particle ball is tin powder, the specific steps of preparing the metal conductive particle ball by mechanical separation process are: heating the tin powder to above the melting point to make it liquid; using a high-speed rotating disk or a vibrating nozzle to divide the molten tin liquid into tiny droplets, and controlling the diameter of the droplets to 10μm-20μm by controlling the rotation speed and vibration frequency of the rotating disk; the liquid tin droplets form a spherical shape under the action of centrifugal force or mechanical force, and due to the effect of surface tension, the molten tin naturally tends to form a spherical shape; ensure that the liquid tin droplets are quickly cooled in air or inert gas (such as nitrogen) and solidified into tin balls with a diameter of 10μm-20μm; use a screen or particle size classification equipment to screen out tin balls that meet the required size, remove unqualified tin balls, and finally obtain tin balls with a diameter of 10μm-20μm. The principle of preparing tin balls by mechanical separation process is to form tin balls by separating molten tin metal into fine particles. This method usually involves mechanical force or vibration to control the size and distribution of liquid tin droplets.

[0032] When the material of the metal conductive particle ball is tin powder, the specific steps of preparing the metal conductive particle ball by spraying process are as follows: heating the tin powder to above the melting point to make it completely melted; the molten tin is sprayed out through the nozzle, and the tin flow is dispersed into tiny droplets by high-pressure inert gas (such as nitrogen or argon); the molten tin is introduced into a high-speed rotating disk or spray cup, and the tin liquid is thrown out by centrifugal force to form tiny tin droplets; during the spraying process, the tin droplets are quickly cooled in the air or inert gas, and due to the effect of surface tension, the tin droplets naturally form a spherical shape and solidify; the cooled tin ball is recovered by a collection device, and then the tin ball with a diameter of 10μm-20μm is separated by screening or grading equipment. The principle of preparing tin balls by spraying process is to use high-speed airflow or centrifugal force to decompose the molten tin into tiny droplets, which form tin balls during the cooling process.

[0033] S11, coating a layer of insulating coating on the surface of the metal conductive particle ball by a coating process, and air-drying to form an insulating film; further, this step specifically includes the following two methods: 1) soaking the metal conductive particle ball in a chemical solution by chemical deposition, forming an insulating coating on the surface of the metal conductive particle ball by chemical reaction, and air-drying the insulating coating to form an insulating film; 2) depositing the insulating coating material onto the surface of the metal conductive particle ball by physical vapor deposition to form an insulating coating, and air-drying the insulating coating to form an insulating film; In this step, in this step, a layer of insulating coating with a thickness of 3%-25% of the diameter of the metal conductive particle ball is coated on the surface of the metal conductive particle ball through a chemical coating process or a physical coating process, and the insulating coating is air-dried at 50°C - 80°C, and the air-drying time is 10 minutes - 30 minutes, and the air flow speed is controlled at 1m / s - 2 m / s, so that the insulating coating is cured and forms an insulating film with uniform thickness, good adhesion and stable mechanical properties. At this time, the insulating film wraps the metal conductive particle ball.

[0034] Chemical deposition is a chemical reduction process that uses a suitable reducing agent to reduce metal ions in the plating solution and deposit them on the surface of the substrate. Specifically, the metal conductive particle balls are immersed in a chemical solution to reduce the metal ions in the insulating coating material and deposit them on the surface of the metal conductive particle balls to form an insulating coating with a thickness of 3%-25% of the diameter of the metal conductive particle balls, and the insulating coating is air-dried at 50°C - 80°C, and the air-drying time is 10 minutes - 30 minutes, and the air flow speed is controlled at 1m / s - 2m / s, so that the insulating coating is cured and forms an insulating film with uniform thickness, good adhesion and stable mechanical properties, and the insulating film wraps the metal conductive particle balls.

[0035] Physical vapor deposition is a method of vaporizing the coating material by physical methods (such as evaporation or sputtering) and depositing a film on the surface of the substrate. Specifically, in a vacuum environment, the insulating coating material is deposited on the surface of the metal conductive particle ball by evaporation or sputtering to form an insulating coating with a thickness of 3%-25% of the diameter of the metal conductive particle ball, and the insulating coating is air-dried at 50°C - 80°C, and the air-drying time is 10 minutes - 30 minutes, and the air flow speed is controlled at 1m / s - 2m / s, so that the insulating coating is cured and forms an insulating film with uniform thickness, good adhesion and stable mechanical properties, and the insulating film wraps the metal conductive particle ball.

[0036] The material of the insulating film is one or more of resin, polymer or thermoplastic polymer. For example, the resin can be epoxy resin, which has excellent mechanical strength, electrical insulation and heat resistance; the polymer can be polyimide, which has excellent heat resistance, electrical insulation and flexibility, and is suitable for high temperature process occasions; the thermoplastic polymer can be polyethylene terephthalate, polyethylene, polypropylene or ethylene-vinyl acetate copolymer, and the cost of polyethylene terephthalate is relatively low, and it has good electrical properties and mechanical strength. The insulating film made of the above materials can prevent the short circuit of the metal conductive particle ball, and make the metal conductive particle ball conduct electricity along the Z-axis direction under a specific pressure, and ensure that the conductor is protected from the influence of the external environment (such as oxidation); and it is in the position of ensuring the positioning and crimping effect of the conductor during the combination, that is, the stability during hot pressing.

[0037] If the thickness of the insulating film exceeds 25%, the vertical surface may not break during extrusion. If the insulating film does not break, it will still be insulated, and the current cannot pass through, and the metal conductive particle balls cannot be turned on. If the thickness of the insulating film is less than 3%, the insulating film is too thin, which may lead to insufficient mechanical strength of the insulating film. Therefore, the thickness of the insulating film is controlled to 3%-25% of the diameter of the metal conductive particle balls, so that under appropriate pressure and temperature conditions, the insulating film can be easily broken in the Z-axis direction to expose the metal conductive particle balls, so that current can pass through the metal conductive particle balls.

[0038] For example, when the insulating film material is a material with a melting or softening temperature of about 150°C, such as polyethylene (PE), polypropylene (PP) or ethylene-vinyl acetate copolymer (EVA), during the packaging process, when the temperature rises to about 150°C, the insulating film will soften, and because the insulating film is thin, the insulating film will rupture in the Z-axis direction under the action of vertical pressure.

[0039] It is worth noting that: the Z-axis direction is perpendicular to the surface of the substrate and consistent with the direction of the conductive path, that is, the direction in which the insulating film is broken and the metal conductive particle balls are exposed; the X-axis direction and the Y-axis direction are parallel to the surface of the substrate, which are the directions in which the metal conductive particle balls need to be kept insulated, that is, the part of the insulating film that is not broken.

[0040] The insulating film can not only make the metal conductive particle balls contact with the pads of the LED chip and the substrate respectively to form a vertical conductive path, but also prevent two adjacent metal conductive particle balls from being short-circuited in the X-axis and Y-axis directions, that is, the insulating effect. Since the two adjacent metal conductive particle balls are insulated in the X-axis and Y-axis directions, a plurality of LED chips as a collection will not cause a short circuit between two adjacent LED chips. Therefore, when designing the steel holes of the hydrophobic nano-steel mesh, the diameter of the steel holes can be designed to be larger than the size of a pad on the substrate (one pad corresponds to one LED chip). Specifically, the diameter of the steel holes can be designed to accommodate three pads. Since the diameter of the steel holes at this time is 2 times larger than the diameter of the steel holes accommodating one pad, when printing anisotropic conductive adhesive, the anisotropic conductive adhesive is easier to fill into the pad through the steel holes, making it more suitable for welding LED chips with small pixels and applied to ultra-high-resolution Mini / Micro LED display screen, of course, the diameter of the steel hole can be designed to accommodate the size of two, four, five, six welding pads, etc. This embodiment does not impose specific restrictions on the number of welding pads that the diameter of the steel hole can be designed to accommodate.

[0041] S12, providing an insulating glue, placing the metal conductive particle balls coated with the insulating film into the insulating glue, and stirring to form an anisotropic conductive glue; further, this step specifically includes the following two methods: 1) providing an insulating glue, placing the metal conductive particle balls coated with the insulating film into the insulating glue, and stirring at a low speed so that a plurality of the metal conductive particle balls are evenly distributed in the insulating glue to form an anisotropic conductive glue; 2) Providing an insulating glue, placing the metal conductive particle balls coated with the insulating film into the insulating glue, and stirring through the combined motion of rotation and revolution of a planetary mixer so that a plurality of the metal conductive particle balls are evenly distributed in the insulating glue to form an anisotropic conductive glue.

[0042] In this step, an insulating glue with a certain viscosity and consistency is provided, and the consistency of the insulating glue must ensure the bonding performance and make the metal conductive particle balls wrapped with insulating film easier to disperse; the metal conductive particle balls wrapped with insulating film are placed in the insulating glue and stirred by a low-speed stirring method. The low-speed stirring method can reduce the aggregation of the metal conductive particle balls wrapped with insulating film and the generation of bubbles, which helps to evenly distribute the metal conductive particle balls wrapped with insulating film in the insulating glue. A dispersant can also be added to the insulating glue. The dispersant can reduce the interaction between the metal conductive particle balls wrapped with insulating film and reduce agglomeration. The dispersant can be a surfactant or an organic solvent.

[0043] In addition to the low-speed stirring method, a planetary stirring method can also be used. The metal conductive particle balls wrapped with the insulating film are stirred by the combined motion of rotation and revolution of the planetary mixer, so that the metal conductive particle balls wrapped with the insulating film are more evenly distributed.

[0044] Since the current driving a single LED chip is very small, the maximum driving current of each LED chip is usually less than 3-5mA. In normal operation, the driving current is even only a few tenths of a milliampere in many cases. Therefore, as long as the metal conductive particle ball can touch the soldering pad of the LED chip and the soldering pad of the substrate at the same time, the current can be conducted and the LED chip can light up normally.

[0045] Therefore, in the case of low current demand, even if some solder balls are not fully in contact during the welding process, it will not affect the normal power-on and lighting of the chip. However, in order to ensure the stability and reliability of long-term operation, try to ensure that the multiple solder balls can contact the pads well. Therefore, due to the viscosity and consistency of the glue, the metal conductive particle balls wrapped with the insulating film have a small amount of adhesion, but this small amount of adhesion does not affect the basic working principle of the metal conductive particle balls.

[0046] S13, providing a hydrophobic nano-steel mesh having a plurality of steel holes and a substrate having a plurality of pads, placing the hydrophobic nano-steel mesh on the substrate, and aligning the hydrophobic nano-steel mesh so that each of the steel holes is aligned with each of the pads, wherein the substrate is a circuit board or a glass substrate; The specific steps of providing a hydrophobic nano steel mesh having a plurality of steel pores in this step are: 1) Providing a steel plate, and drilling steel holes on the steel plate corresponding to the welding pads of the substrate to form a steel mesh; 2) coating a layer of nano hydrophobic coating on the steel mesh to obtain a hydrophobic nano steel mesh; In this step, the substrate can be one of a circuit board or a glass substrate, and the printing of anisotropic conductive adhesive is completed by a steel screen printer. First, a hydrophobic nano steel mesh is prepared. Specifically, a steel plate is provided, and steel holes of the same size as the pads are opened on the steel plate corresponding to the pad positions of the substrate, and the distribution of the steel holes is the same as the distribution of the pads to obtain a steel mesh; a layer of nano hydrophobic coating is applied to the steel mesh to obtain a hydrophobic nano steel mesh. Among them, the nano hydrophobic coating can be a fluoride-based nano coating, a silane-based nano coating or a nano silica coating. Exemplarily, the fluoride-based nano coating material is polytetrafluoroethylene, a perfluoroalkyl silane compound, and the fluoride-based nano coating has extremely high hydrophobicity, and the water contact angle can reach 120°-150°, which keeps the surface of the hydrophobic nano steel mesh clean and improves printing accuracy; it has extremely low surface energy, prevents anisotropic conductive adhesive from adhering to the steel holes of the hydrophobic nano steel mesh, reduces clogging, and prolongs the service life of the hydrophobic nano steel mesh; it has excellent corrosion resistance and chemical stability. The material of silane-based nano coating is octyltriethoxysilane or methyltrichlorosilane, which has good hydrophobicity and a water contact angle of up to 110°-130°, preventing anisotropic conductive adhesive from adhering to the steel holes of hydrophobic nano steel mesh; it has strong chemical bonding, stable and wear-resistant coating; it is easy to process and prepare, has excellent adhesion, and is suitable for forming a long-term and effective anti-sticking layer on the surface of the steel mesh. Nano-silicon dioxide coating has good hydrophobicity, and its performance can be enhanced by chemical modification; it has good wear resistance and thermal stability; it has low cost and is suitable for surface treatment of steel mesh produced in large quantities.

[0047] After the hydrophobic nano-steel mesh and the substrate are prepared, the substrate is first mounted on the fixture of the stencil printer. The fixture has adjustable supports and positioning pins to ensure the accurate position of the substrate. The hydrophobic nano-steel mesh is then fixed to the frame of the printer. The visual positioning system of the printer automatically detects the positioning mark (FiducialMark) of the substrate for correction, so that the position of the substrate and the hydrophobic nano-steel mesh are aligned, and each steel hole is aligned with each pad, that is, each steel hole is located directly below each pad. The substrate is then fixed to the printing platform by vacuum adsorption or mechanical clamping to prevent movement during the printing process. The scraper in the steel screen printer (usually made of metal or polyurethane material) pushes the anisotropic conductive glue onto the surface of the hydrophobic nano steel mesh and scrapes it along the hydrophobic nano steel mesh at a set speed and pressure; the scraper moves back and forth on the surface of the hydrophobic nano steel mesh, squeezing the anisotropic conductive glue through the steel holes on the hydrophobic nano steel mesh and printing it on the pad of the substrate, so that the height of the anisotropic conductive glue on the pad is consistent, and the excess anisotropic conductive glue is collected by the scraper back to the top of the hydrophobic nano steel mesh; after the anisotropic conductive glue is filled, the control system of the steel screen printer slowly lifts the hydrophobic nano steel mesh to gradually separate the hydrophobic nano steel mesh from the substrate. Among them, the scraper speed: 20~100 mm / s, the scraper pressure: 0.5~1.5 kg, the printing environment: the humidity is less than 60%, keep it clean and dry.

[0048] S14, adding the anisotropic conductive adhesive to the side of the hydrophobic nano-steel mesh away from the pad, and evenly scraping the anisotropic conductive adhesive on the hydrophobic nano-steel mesh by a scraping method, so that the anisotropic conductive adhesive is printed onto the pad through the steel hole; In this step, the description of the above step S14 refers to S2 of the first embodiment, and this step will not be described in detail.

[0049] S15, peeling off the hydrophobic nano-steel mesh; In this step, the description of the above step S15 refers to S3 of the first embodiment, and this step will not be described in detail.

[0050] S16, mounting the LED chip onto the anisotropic conductive adhesive on the pad by a mass transfer process or a SMT process; In this step, a mass transfer process or a standard SMT process is used according to different mounting objects.

[0051] When the mounting object is an unpackaged LED chip, a mass transfer process can be used. Specifically, a transfer head with an elastic mold or an electromagnetic adsorption head is used to pick up LED chips in batches from a temporary carrier; the picked LED chips are accurately aligned with the anisotropic conductive adhesive on the pad through a visual recognition system; and the LED chips are released onto the anisotropic conductive adhesive. Among them, the alignment accuracy is: ±1μm~2μm, the release pressure is: 0.2MPa~1.0 MPa, and the temperature is: 80°C~150°C; the mass transfer process can transfer LED chips from the carrier of the LED chip in large quantities and accurately mount them on the anisotropic conductive adhesive on the pad, improving mounting efficiency, saving time, reducing costs, and ensuring high-quality electrical connections and mechanical strength.

[0052] When the mounting object is a packaged LED chip, the packaged LED chip is mounted on the anisotropic conductive adhesive on the pad using the standard SMT process. Specifically, a transfer head with an elastic mold or an electromagnetic adsorption head is used, and the packaged LED chip is picked up by the placement machine; the packaged LED chip is accurately aligned with the anisotropic conductive adhesive on the pad through the placement machine's visual system; the packaged LED chip is accurately placed on the anisotropic conductive adhesive to ensure high-quality electrical connection and mechanical strength.

[0053] S17, heating and pressurizing the LED chip on the anisotropic conductive adhesive by a hot pressing process, so that the insulating film of the anisotropic conductive adhesive is longitudinally ruptured to expose the metal conductive particle balls wrapped in the insulating film, so that the metal conductive particle balls are respectively in contact with the LED chip and the pad to form a vertical conductive path, and the insulating glue of the anisotropic conductive adhesive is initially cured, so that the LED chip is soldered on the substrate; In this step, the description of the above step S17 refers to S5 of the first embodiment, and this step will not be described in detail.

[0054] S18, testing the LED chip; In this step, since it takes 4 to 8 hours for the insulating glue to be completely cured, that is, the insulating glue is in a preliminary curing state during 4 to 8 hours, the insulating glue has an open time of 4 to 8 hours, and the LED chip can be tested during this time.

[0055] S19. When the LED chip is defective, the LED chip is heated to melt the anisotropic conductive adhesive, and the defective LED chip is removed and replaced with a good LED chip.

[0056] In this step, when it is detected that the LED chip is defective, the anisotropic conductive glue can be reactivated by heating to 120°C-250°C to melt the insulating glue, and the junction temperature of the LED chip is 125°C-150°C. Therefore, the heating temperature of 120°C-250°C can not only re-melt the insulating glue, but also will not damage the LED chip; after the insulating glue is melted, the defective LED chip can be easily removed and replaced with a good LED chip. The insulating glue has an open time of 4 hours to 8 hours, so that during maintenance, defective products can be repaired and replaced, which reduces the difficulty of maintenance, ensures that the product yield is close to 100%, avoids scrapping of the entire substrate or display module, and has low maintenance costs. The insulating glue has an open time of 4 to 8 hours, so that defective LED chips can be repaired by heating during the open time, ensuring that the product yield is close to 100% and avoiding the scrapping of the entire substrate or display module; if the problem is discovered after curing, the entire substrate or display module may be scrapped, and repair within the open time greatly reduces production waste and material loss; it enables the production line to flexibly handle quality inspection and repair, reducing the risk of batch scrapping.

[0057] The open time of the insulating glue can also be adjusted by adding specific chemical materials to the insulating glue; illustratively, the material that prolongs the open time can be a retardant or a plasticizer; the material that accelerates curing can be a catalyst or an initiator; it is also possible to use moisture-curing glue so that the insulating glue gradually cures by absorbing moisture (humidity) in the air.

[0058] Among them, the LED chip can be a blue light LED chip, a green light LED chip and a red light LED chip. The maximum junction temperature of the blue light LED chip and the green light LED chip is 150°C. At this temperature, even if the heating is continued for 30 minutes, it will not have a significant impact on the life of the blue light LED chip and the green light LED chip; the maximum junction temperature of the red light LED chip is 125°C. Laboratory tests show that hot pressing at 150°C for 30 minutes will not significantly affect the life of the red light LED chip; of course, the heating temperature and heating time are coordinated with each other. When heated to 250°C, the heating time is 1 second to 8 seconds; when heated to 150°C, the heating time is 30 minutes to ensure that the anisotropic conductive glue can be heated to reactivate the insulating glue, so that the LED chip is melted, and the LED chip is not damaged; but when the heating temperature exceeds 250°C and the heating time exceeds 10 seconds, the LED chip may be damaged.

[0059] In this embodiment, a metal conductive particle ball with a diameter of 10 μm-20 μm is prepared by a mechanical separation process or a spraying process, and a layer of insulating coating is coated on the surface of the metal conductive particle ball by a coating process to form an insulating film with a thickness of 3%-25% of the diameter of the metal conductive particle ball, so that the insulating film softens when heated to 120° C.-250° C. and ruptures under vertical pressure, and the metal conductive particles are exposed at the rupture. The metal conductive particle ball coated with the insulating film is placed in insulating glue and stirred to form an anisotropic conductive glue. The insulating glue fixes and connects the LED chip, the metal conductive particle ball and the pad to improve the stability of the electrical connection; the hydrophobic nano steel mesh is aligned with the substrate, and the steel holes of each hydrophobic nano steel mesh are aligned with each pad respectively, and then the anisotropic conductive glue is added to the hydrophobic nano steel mesh, and the anisotropic conductive glue is evenly scraped on the surface of the hydrophobic nano steel mesh by scraping, so that the anisotropic conductive glue is printed on the pad The LED chip is mounted on the anisotropic conductive adhesive on the pad, and the insulating film of the anisotropic conductive adhesive is ruptured longitudinally (in the Z-axis direction) to expose the metal conductive particle balls wrapped in the insulating film, so that the metal conductive particle balls can contact the LED chip and the pad respectively to form a vertical conductive path, and the insulating glue of the anisotropic conductive adhesive is initially solidified, so that the LED chip is welded on the substrate. Since the anisotropic conductive adhesive contains a plurality of metal conductive particle balls, as long as there is a metal conductive particle ball that is turned on among the plurality of metal conductive particle balls, the LED chip can be normally lit, and it is not necessary for all the metal conductive particles to be turned on at the same time, thereby reducing the difficulty of welding and improving the quality of welding. The insulating glue has an open time of 4 to 8 hours, and the insulating glue can be melted by reheating, so that the bad LED chip can be easily removed and replaced with a good LED chip without damaging the LED chip, thereby reducing the difficulty of repair and the cost of repair.

[0060] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Under the concept of the present invention, the technical features in the above embodiments or different embodiments may also be combined, the steps may be implemented in any order, and there are many other changes in different aspects of the present invention as described above, which are not provided in detail for the sake of simplicity. Although the present invention has been described in detail with reference to the above embodiments, a person of ordinary skill in the art should understand that the technical solutions described in the above embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for welding a Mini / Micro LED chip, characterized in that: The welding method comprises: Providing a hydrophobic nano-steel mesh having a plurality of steel holes and a substrate having a plurality of pads, placing the hydrophobic nano-steel mesh on the substrate, and aligning the hydrophobic nano-steel mesh so that each of the steel holes is aligned with each of the pads, wherein the substrate is a circuit board or a glass substrate; Providing anisotropic conductive glue, adding the anisotropic conductive glue to the side of the hydrophobic nano-steel mesh away from the pad, and evenly scraping the anisotropic conductive glue on the hydrophobic nano-steel mesh by a scraping method, so that the anisotropic conductive glue is printed onto the pad through the steel hole; peeling off the hydrophobic nano-steel mesh; Providing an LED chip, and mounting the LED chip on the anisotropic conductive adhesive located on the pad; The LED chip on the anisotropic conductive adhesive is heated and pressurized by a hot pressing process, so that the insulating film of the anisotropic conductive adhesive is longitudinally ruptured to expose the metal conductive particle balls wrapped in the insulating film, so that the metal conductive particle balls are respectively in contact with the LED chip and the soldering pad to form a vertical conductive path, and the insulating glue of the anisotropic conductive adhesive is initially solidified, so that the LED chip is soldered on the substrate.

2. The welding method according to claim 1, characterized in that: The step of providing anisotropic conductive adhesive is formed in three steps, including: The metal conductive particle balls are prepared by a mechanical separation process or a spraying process; A layer of insulating coating is applied on the surface of the metal conductive particle ball by a coating process, and air-dried to form an insulating film; An insulating glue is provided, and the metal conductive particle balls coated with the insulating film are placed in the insulating glue and stirred to form an anisotropic conductive glue.

3. The welding method according to claim 2, characterized in that: The material of the insulating film is one or more of a resin, a polymer or a thermoplastic polymer; and / or, The material of the insulating glue is modified epoxy resin.

4. The welding method according to claim 2, characterized in that: The material of the metal conductive particle ball is one or more of tin powder, silver powder, nickel powder, gold powder, tin alloy powder, copper powder and aluminum powder.

5. The welding method according to claim 2, characterized in that: The diameter of the metal conductive particle ball is 10 μm-20 μm; and / or the thickness of the insulating film is 3%-25% of the diameter of the metal conductive particle ball.

6. The welding method according to claim 2, characterized in that: The step of providing an insulating glue comprises: An insulating glue is provided, and metal conductive particle balls coated with the insulating film are placed in the insulating glue, and a plurality of the metal conductive particle balls are evenly distributed in the insulating glue by low-speed stirring to form an anisotropic conductive glue; or, an insulating glue is provided, and metal conductive particle balls coated with the insulating film are placed in the insulating glue, and a combination of rotation and revolution of a planetary mixer is used to stir the metal conductive particle balls so that a plurality of the metal conductive particle balls are evenly distributed in the insulating glue to form an anisotropic conductive glue.

7. The welding method according to claim 2, characterized in that: The step of coating a layer of insulating coating on the surface of the metal conductive particle ball by a coating process and air-drying the coating to form an insulating film comprises: The metal conductive particle ball is immersed in a chemical solution by a chemical deposition method, an insulating coating is formed on the surface of the metal conductive particle ball by a chemical reaction, and the insulating coating is air-dried to form an insulating film; or, the insulating coating material is deposited onto the surface of the metal conductive particle ball by a physical vapor deposition method to form an insulating coating, and the insulating coating is air-dried to form an insulating film.

8. The welding method according to claim 1, characterized in that: The step of providing a hydrophobic nano steel mesh having a plurality of steel pores is formed in two steps, including: Providing a steel plate, and opening steel holes on the steel plate corresponding to the welding pads of the substrate to form a steel mesh; A layer of nano hydrophobic coating is coated on the steel mesh to obtain a hydrophobic nano steel mesh.

9. The welding method according to claim 1, characterized in that: After the LED chip on the anisotropic conductive adhesive is heated and pressurized by the hot pressing process, so that the insulating film of the anisotropic conductive adhesive is longitudinally ruptured to expose the metal conductive particle balls wrapped in the insulating film, so that the metal conductive particle balls are respectively in contact with the LED chip and the pad to form a vertical conductive path, and the insulating glue of the anisotropic conductive adhesive is initially cured so that the LED chip is soldered on the substrate, the method further includes: Testing the LED chip; When the LED chip is defective, the LED chip is heated to melt the anisotropic conductive adhesive, and the defective LED chip is removed and replaced with a good LED chip.

10. The welding method according to claim 1, characterized in that: The step of providing an LED chip and mounting the LED chip on the anisotropic conductive adhesive on the pad comprises: The LED chip is mounted on the anisotropic conductive adhesive on the pad through a mass transfer process or a SMT process.

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