Halogen-free environmentally friendly soldering paste and preparation method thereof

Abstract

The present application relates to the field of welding material, disclose a kind of halogen-free environmental protection soldering paste and its preparation method.The soldering paste is composed of alloy powder and soldering paste, and the soldering paste contains photo / thermal dual-curing resin system (acrylate modified rosin and epoxy resin), activator, thixotropic agent, latent thermal curing agent and photoinitiator.Preparation includes resin modulation, functional component integration, light-protected addition of initiator and vacuum mixing forming.Through in-situ construction of interpenetrating polymer network structure, the present application converts residues into high-hardness, high-insulation protective layer, improves the electrical insulation and thermal mechanical reliability of electronic components in complex environment, and has excellent process compatibility.

Description

Technical Field

[0001] This invention belongs to the field of welding materials, specifically relating to a halogen-free environmentally friendly solder paste and its preparation method. Background Technology

[0002] Solder paste, as a core material for achieving electrical connections and mechanical support between electronic components and printed circuit boards, directly determines the yield rate, long-term reliability, and service life of electronic products. In recent years, halogen-free and environmentally friendly solder paste has become the leading direction in the research and development of electronic connection materials.

[0003] In existing halogen-free environmentally friendly solder paste systems, flux components typically consist of rosin resin, activators, thixotropic agents, and high-boiling-point solvents. Among these, rosin, as the main carrier resin, is widely used to protect the solder area metal from secondary oxidation and promote solder wetting and spreading due to its excellent fluxing activity and film-forming properties at high temperatures, as well as its superior electrical insulation properties at room temperature.

[0004] However, the residue left by traditional flux after soldering is essentially a thermoplastic mixture mainly composed of amorphous rosin. Rosin-based residues typically appear as a relatively soft and somewhat viscous film at room temperature. This inherent stickiness, especially in circuit designs that tend towards micro-pitch and high-density designs, significantly reduces the surface insulation resistance between circuits due to the adsorption of contaminants. Particularly under high humidity and heat conditions, the active ions in the residues readily migrate, leading to severe electrochemical migration and dendrite growth, ultimately causing leakage or short-circuit failures in precision circuits.

[0005] Because rosin residue films have low cohesive strength and their chemical structure differs from commonly used conformal coatings and encapsulating adhesives in terms of solubility parameters, the interfacial bonding between them is extremely weak. Under the influence of temperature cycling stress or mechanical vibration, this extremely weak interfacial compatibility often leads to large-area peeling or delamination of the coating, not only causing the conformal coating to fail but also potentially causing secondary failure due to moisture accumulation at the delamination points.

[0006] Meanwhile, traditional rosin has a low glass transition temperature and undergoes significant physical softening under high-temperature operating conditions. This phase instability prevents it from providing effective structural reinforcement for solder joints. For micro-pitch devices such as BGA and CSP, when solder joints are subjected to external mechanical impacts such as drops or bending, the thin rosin residue layer cannot provide stress buffering, making the root of the solder joint highly susceptible to fatigue cracks. Summary of the Invention

[0007] The purpose of this invention is to overcome the technical defects of existing halogen-free environmentally friendly solder paste, such as high viscosity of soldering residue, easy dust attraction, high risk of electrochemical migration, and poor compatibility with subsequent coating processes. This invention provides a halogen-free environmentally friendly solder paste and its preparation method. By constructing an interpenetrating polymer network system composed of photocurable acrylate-modified rosin and thermosetting epoxy resin, this invention achieves a fundamental transformation of soldering residue from a thermoplastic rosin film to a functionalized protective layer with high hardness, high chemical stability, and in-situ bottom filling capability. This improves the electrical insulation reliability and thermomechanical reliability of electronic components under complex service environments.

[0008] To achieve the above-mentioned objectives, this invention provides a halogen-free environmentally friendly solder paste, comprising the following components by weight percentage: 85% to 90% alloy powder and 10% to 15% flux. The flux comprises the following components by weight percentage: 45% to 60% photo / thermal dual-curing resin system, 3% to 8% organic activator system, 2% to 6% thixotropic agent, 1% to 3% latent thermosetting agent, 0.5% to 2.0% photoinitiator, 0.2% to 1.0% leveling enhancer, with the balance being a high-boiling-point organic solvent.

[0009] The photocurable / thermal dual-curing resin system is the core film-forming material of this invention, which is composed of photocurable acrylate-modified rosin and thermocurable epoxy resin mixed in a mass ratio of 2:1 to 4:1. The preparation process of the photocurable acrylate-modified rosin involves a ring-opening esterification reaction between refined hydrogenated rosin and acrylate monomers containing active epoxy groups under the action of a catalyst. This allows the carboxyl groups on the rosin molecular chain to combine with the epoxy groups, thereby introducing photosensitive acrylate functional groups onto the rosin backbone. This modified rosin retains the excellent fluxing properties and spreadability of natural rosin at high temperatures, while also endowing it with the ability to undergo free radical polymerization under ultraviolet light irradiation. The thermocurable epoxy resin is a multifunctional epoxy resin, selected from one or more of bisphenol A type epoxy resin, bisphenol F type epoxy resin, or o-cresyl formaldehyde epoxy resin. During the high-temperature stage of welding reflow, the epoxy resin component undergoes a cross-linking reaction with the residual organic acid carboxyl groups, hydroxyl groups, or added latent curing agents in the system through its epoxy groups, thereby constructing a preliminary thermosetting three-dimensional network.

[0010] The organic activator system employs a multi-stage activation release design, comprising a dibasic organic acid, an organic amine salt, and an auxiliary activator. The dibasic organic acid is selected from at least three of succinic acid, glutaric acid, adipic acid, sebacic acid, or dodecanoic acid. Through the combination of acids with different carbon chain lengths, continuous activation is achieved within a temperature range of 150°C to 250°C, effectively removing the oxide film from the alloy powder and the surface of the solder pads. The organic amine salt is selected from triethanolamine hydrochloride or cyclohexylamine hydrobromide, used to enhance the instantaneous wetting ability before soldering.

[0011] The latent thermosetting agent is a chemical substance that does not react with epoxy resin at room temperature but releases active functional groups at a specific activation temperature (above 150 degrees Celsius). Specifically, it is selected from dicyandiamide and its derivatives, modified imidazole compounds, or organic hydrazides. It is activated in the high-temperature zone of reflow soldering, guiding the epoxy resin to undergo thermal cross-linking, ensuring that the residue possesses preliminary structural strength and low residual tack in the first stage after soldering is completed.

[0012] The photoinitiator is selected from free radical photoinitiators with high initiation efficiency and deep curing capability, such as 2-hydroxy-2-methyl-1-phenylpropanone, 1-hydroxycyclohexylphenyl ketone, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, or 2-isopropylthioxanthraphenone. The absorption spectra of these photoinitiators match the emission spectra of industrial-grade UV curing equipment, ensuring that the acrylate-modified rosin can be rapidly polymerized in the subsequent UV irradiation process, ultimately forming a tight interpenetrating polymer network structure around the solder joint.

[0013] The alloy powder consists of spherical particles with a particle size distribution conforming to the IPC standard for powder No. 3, No. 4, or No. 5. The alloy composition is selected from lead-free solders based on tin-silver-copper (Sn-Ag-Cu), tin-bismuth (Sn-Bi), or tin-bismuth-silver (Sn-Bi-Ag).

[0014] This invention also provides a method for preparing the above-mentioned halogen-free environmentally friendly solder paste, which includes the following steps: Step 1, pre-mixing of the flux base. Under nitrogen protection, a high-boiling-point organic solvent is heated to 120°C to 150°C, followed by the sequential addition of photocurable acrylate-modified rosin, thermosetting epoxy resin, and thixotropic agent. The mixture is stirred at 500 to 1000 rpm until completely dissolved, forming a uniform resin carrier solution. Step 2, integration of functional components. The solution is cooled to 60°C to 80°C, and an organic activator system, a latent thermosetting agent, and a leveling synergist are added. Stirring continues for 60 to 90 minutes to ensure that all solid components are uniformly dispersed at the molecular level. Step 3, addition of photosensitive components in the dark. The mixture is further cooled to below 40°C, and a photoinitiator is added in a dark environment. Vacuum degassing is performed, with the vacuum degree controlled below -0.09 MPa, for a processing time greater than 30 minutes, to obtain the finished flux paste. Step 4, mixing and molding of the solder paste. The flux and alloy powder are placed in a planetary power mixer according to a predetermined ratio and stirred at low speed under vacuum and cooling circulation conditions to avoid oxidation of the alloy powder and generation of bubbles, ultimately yielding halogen-free environmentally friendly solder paste.

[0015] In the photo / thermal dual-curing resin system described in this invention, the introduction of acrylate-modified rosin is crucial. While traditional hydrogenated rosin possesses good fluxing properties, its molecular weight after soldering is low, and it contains a large number of reactive carboxyl groups, which is the root cause of the hygroscopic and corrosive residues. This invention, through chemical modification, converts most of the carboxyl groups into stable ester groups and introduces crosslinking points. This modified rosin exhibits a lower high-temperature viscosity than natural rosin during reflow soldering, which is beneficial for rapid flux spreading and oxide film removal. Furthermore, under UV light initiation, the rapid polymerization of its double bonds causes the residue to lose its stickiness within seconds.

[0016] The thermosetting epoxy resin used is a multifunctional o-cresolaldehyde epoxy resin, which provides extremely high crosslinking density. In the high-temperature environment of reflow soldering, the epoxy groups react not only with the curing agent but also with organic acids in the flux that have not yet fully volatilized. This scavenging effect further reduces the ionic activity of residues. Simultaneously, the epoxy resin exhibits excellent wetting and adhesion capabilities to the printed circuit board substrate, ensuring that the final interpenetrating polymer network film can be anchored to the substrate surface like tree roots.

[0017] The multi-stage release mechanism of the organic activator system is designed to complement the reaction process of the dual-curing system. Short-chain dicarboxylic acids (such as succinic acid) play a primary role in the early preheating stage, removing surface oxides; medium- and long-chain dicarboxylic acids (such as sebacic acid) provide sustained reducing power near the melting point, ensuring the alloy powder has minimal surface tension during melting and agglomeration. This gradual release of activity prevents the gases generated by premature decomposition of the activator from being trapped in the gradually curing resin system and forming pores, thus ensuring the compactness of the residual film.

[0018] The leveling enhancer plays a key role in regulating the morphology of the residue in this invention. It is typically selected from modified polysiloxane or polyacrylate leveling agents, and is designed to reduce the interfacial tension between the flux and the substrate, so that the residue after soldering forms a uniform, smooth, and non-aggregated film profile around the solder joint. This is crucial for the uniform absorption of ultraviolet light during the photocuring stage.

[0019] The precise control of temperature and vacuum in the preparation method described in this invention is a technical guarantee for ensuring the storage stability of solder paste. Since the system contains both a thermosensitive latent curing agent and a photosensitive initiator, the entire preparation process must be controlled under light-protected and low-temperature conditions. The vacuum degassing step not only eliminates physical air bubbles, but more importantly, removes trace amounts of water and oxygen dissolved in the system, preventing early hydrolysis of the epoxy resin or oxygen inhibition of the acrylate.

[0020] The halogen-free, environmentally friendly solder paste obtained by this invention exhibits excellent process applicability in practical applications. During printing, it demonstrates superior thixotropic index and release properties, enabling precise printing of micro-devices with a pitch of 0.3 mm or less. After reflow soldering, the residue can be modified by treating it with an online UV curing device at an irradiation energy of 500 to 2000 mJ / cm².

[0021] In a preferred embodiment of the present invention, the proportions of the components in the flux paste are synergistically optimized to balance flux activity and residue strength. For example, when the mass ratio of acrylate-modified rosin to epoxy resin in the photo / thermal dual-curing resin system is 3:1, the system can form an interpenetrating polymer network with optimal balance between toughness and hardness while ensuring sufficient flux wetting power. If the epoxy resin content is too high, although the hardness increases, it may lead to a decrease in flux activity and an increase in the brittleness of post-soldering residue; conversely, if the proportion of modified rosin is too high, it may lead to insufficient thermal stability of the film and a decrease in the glass transition temperature.

[0022] Another advantage of the halogen-free environmentally friendly solder paste described in this invention lies in its ability to inhibit the migration of minute residues. In high-density assemblies, tiny solder splatters or trace amounts of flux migration often lead to short circuits between adjacent circuits. In the system of this invention, the resin component undergoes a rapid viscosity increase and initial curing during heating, and this physical state transition limits the diffusion coefficient of active ions. Even if minute residues exist, they will rapidly cure into an insulator after ultraviolet light irradiation, losing the possibility of forming conductive channels.

[0023] In terms of mechanical properties, this invention achieves elastic support for the solder joint through an interpenetrating polymer network structure. Unlike the brittle cracking of traditional rosin residues, the long-chain molecules in the interpenetrating polymer network possess a certain degree of flexibility, enabling them to dissipate energy through conformational changes in their molecular chains when subjected to external stress. This macroscopic toughening effect results in a more uniform stress distribution at the solder joint when subjected to high-frequency vibration or rapid temperature shock, effectively extending fatigue life.

[0024] Regarding the molecular structure design of the aforementioned photocurable acrylate-modified rosin, this invention employs a synthetic route with controllable reaction rates. During synthesis, the molar ratio of epoxy acrylate monomers to rosin acid is strictly controlled to ensure that the carboxyl group conversion rate on the rosin backbone is between 70% and 90%. This incomplete conversion design retains approximately 10% to 30% of free carboxyl groups. The purpose is to utilize these residual carboxyl groups to provide the necessary chemical activity in the early stages of reflow soldering, assisting the activator in removing the cuprous oxide layer on the copper pad surface. In the subsequent curing stage, these free carboxyl groups can also serve as active sites for the thermosetting of epoxy resin, participating in the thermocrosslinking reaction and thus being anchored in the network structure.

[0025] Further analysis was conducted on the role of the thermosetting epoxy resin in the formation of the interpenetrating polymer network. Due to its high molecular chain rigidity and good chemical resistance, bisphenol A epoxy resin enhances the bulk strength of the film in the system. Meanwhile, o-cresol epoxy resin, through its multifunctional structure, increases the crosslinking density and glass transition temperature of the residue. Experiments show that when an epoxy resin containing biphenyl structures is introduced into the system, the water vapor permeability of the residual film further decreases due to the hydrophobicity of the biphenyl groups. This is of significant protective value for electronic products operating in extreme high-humidity environments such as marine climates.

[0026] In terms of the rheological design of the solder paste, the thixotropic agent system used in this invention is a composite system composed of modified hydrogenated castor oil and polyamide wax. This composite thixotropic agent forms a hydrogen bond network in the solvent, exhibiting high shear-thinning properties and rapid viscosity recovery. During the printing process, when the solder paste is squeezed by the squeegee, the hydrogen bonds break, and the viscosity rapidly decreases to achieve good filling and demolding. After printing, the hydrogen bonds are rapidly rebuilt, preventing the solder paste from collapsing and ensuring the shape retention of the printed pattern. This precise rheological control, combined with the subsequent curing reaction, ensures that the cured interpenetrating polymer network film can accurately cover the bottom and surrounding area of ​​the solder joint, preventing disordered flow.

[0027] The selection of the high-boiling-point organic solvent is crucial for controlling the volatilization gradient during the welding process. This invention preferably uses a combination of diethylene glycol monohexyl ether, tripropylene glycol methyl ether, and aromatic hydrocarbon solvents, with a boiling point gradient between 180°C and 260°C. This stepped volatilization design ensures that the solvent evaporates smoothly during the preheating stage, avoiding solder paste splattering and porosity caused by instantaneous vaporization. Simultaneously, a small amount of residual solvent acts to regulate resin flowability before curing, promoting the micro-orientation of the interpenetrating polymer network in the early stages of curing and reducing internal stress.

[0028] Regarding the synergistic effect of photoinitiators, this invention discovers that a composite system of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and 1-hydroxycyclohexylphenyl ketone exhibits excellent deep curing performance. Because 2,4,6-trimethylbenzoyl-diphenylphosphine oxide has strong absorption in the long-wave ultraviolet region, it can penetrate residues containing pigments or thicker layers, initiating deep resin polymerization; while 1-hydroxycyclohexylphenyl ketone is primarily responsible for rapid surface debinding, preventing surface stickiness caused by oxygen inhibition. This consistent curing effect ensures uniform mechanical properties of the residual film across its entire thickness range.

[0029] The vacuum treatment step in the preparation process of this invention is crucial for its engineering application. During the flux preparation process, staged vacuum degassing (above -0.095 MPa) effectively removes microbubbles entangled during mixing. If these bubbles are not removed, they will expand dramatically at the high temperatures of reflow soldering, leading to large voids inside the solder joint, severely weakening its mechanical strength, and potentially forming weak points in the interpenetrating polymer network film, thus reducing its insulation performance.

[0030] In the final solder paste mixing process, this invention emphasizes the importance of cooling circulation. Since the mixing of alloy powder and flux is a high-shear process, frictional heat can cause localized temperature increases. If the temperature exceeds 40 degrees Celsius, the latent curing agent may undergo a slight reaction, leading to an increase in solder paste viscosity and a shortened shelf life at room temperature. Therefore, strictly limiting the mixing temperature to 20 to 25 degrees Celsius using the cooling jacket of the planetary mixer is essential to ensure stable printability of the solder paste.

[0031] Furthermore, the solvent resistance of the residual film is significantly improved. After conventional industrial cleaning processes (such as ultrasonic cleaning with alcohol and isopropanol), the interpenetrating polymer network film remains intact without dissolving or swelling. This characteristic allows the film layer to exist as a stable interface layer in subsequent processes such as conformal coating and underfill injection, enhancing the structural stability of the entire encapsulation system.

[0032] As a further refinement of the preparation method of this invention, the surface treatment of the alloy powder is also an important step in improving system compatibility. Before mixing, the alloy powder can be subjected to a small amount of organic acid passivation treatment to reduce the early autocatalytic reaction between the metal surface atoms and the resin groups in the flux, thereby extending the room temperature storage period of the solder paste to more than 6 months and eliminating the need for refrigerated transportation, thus reducing logistics costs.

[0033] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention constructs an interpenetrating polymer network structure in situ in the post-weld residue through a photo / thermal dual curing mechanism, reducing residual stickiness and forming a dense, tough, high-modulus composite thin film layer; 2. This invention realizes the functional transformation of welding residue, turning it from process waste into a permanent protective layer, and improving its glass transition temperature and surface insulation resistance; under high temperature and high humidity cycling environment, the film layer will not soften or absorb moisture, blocking the penetration of moisture and corrosive ions into the root of the weld joint. 3. This invention enhances the thermomechanical reliability of solder joints. The resulting interpenetrating polymer network residual film tightly wraps around the interface between the solder joint and the printed circuit board pad, significantly improving the drop reliability of micro-pitch devices. 4. The interpenetrating polymer network residual film has excellent chemical compatibility with commonly used conformal coatings and encapsulating adhesives, eliminating the risk of interface peeling and delamination, and ensuring the integrity of the overall packaging of electronic components. Detailed Implementation

[0034] This invention provides a halogen-free environmentally friendly solder paste and its preparation method. Its core structure consists of an alloy powder and a high-performance flux system, wherein the alloy powder accounts for 85% to 90% by weight, and the flux accounts for 10% to 15% by weight. This ratio ensures sufficient rheological stability of the solder paste during the printing stage, while providing sufficient chemical activation energy and a physical protective layer during the reflow soldering stage. The flux, as the core of the invention's chemical function, integrates a precisely designed photo / thermal dual-curing resin system, which accounts for 45% to 60% by weight in the flux. To facilitate the cross-linking reaction of this resin system and achieve an efficient soldering deoxidation process, the flux further includes 3% to 8% of an organic activator system, 2% to 6% of a thixotropic agent, 1% to 3% of a latent thermosetting agent, 0.5% to 2.0% of a photoinitiator, and 0.2% to 1.0% of a leveling enhancer. The remainder is supplemented by a high-boiling-point organic solvent to adjust the final viscosity and volatility gradient of the system.

[0035] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0036] Example 1: Alloy powder 88% (tin-silver-copper system, No. 4 powder, oxygen content 85ppm); 12% flux (52% light / heat curing dual-curing resin system, acrylate modified rosin: thermosetting epoxy resin = 3:1); The organic activator system is 5% succinic acid: glutaric acid: sebacic acid: triethanolamine hydrochloride = 3:2:2:1; Thixotropic agent 4%, modified hydrogenated castor oil: polyamide wax = 1:1; Latent thermosetting agent 2%, dicyandiamide derivative; Photoinitiator 1.2%, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide: 1-hydroxycyclohexylphenyl ketone = 1:1; 0.5% leveling synergist, modified polysiloxane; High-boiling-point organic solvent balance, diethylene glycol monohexyl ether: tripropylene glycol methyl ether = 2:1); Preparation steps: S1: Pre-mix the flux base. Under nitrogen protection, heat the high-boiling-point organic solvent to 135°C, and add acrylate-modified rosin, thermosetting epoxy resin, and thixotropic agent in sequence. Stir at 800 rpm until completely dissolved to form a resin carrier solution. S2: Functional component integration, cool the resin carrier solution to 70℃, add organic activator system, latent thermosetting agent, leveling synergist, stir for 75 minutes until molecular-level dispersion; S3: The photosensitive component is added in the dark, the mixture is cooled to 35°C, the photoinitiator is added in the dark environment, and the mixture is degassed under vacuum at -0.092MPa for 40 minutes to obtain the finished flux paste. S4: Solder paste mixing and molding. The flux paste and alloy powder are placed in a planetary power mixer at a weight ratio of 88:12 and stirred under vacuum cooling circulation conditions. The material temperature is controlled at 22℃ to obtain halogen-free environmentally friendly solder paste.

[0037] Example 2: 85% alloy powder, 15% flux, and the remaining components and proportions are the same as in Example 1; Preparation steps: Same as in Example 1.

[0038] Example 3: 90% alloy powder, 10% flux, and the remaining components and proportions are the same as in Example 1; Preparation steps: Same as in Example 1.

[0039] Example 4: Acrylic ester modified rosin: thermosetting epoxy resin = 2:1, other components and proportions are the same as in Example 1; Preparation steps: Same as in Example 1.

[0040] Example 5: Acrylic ester modified rosin: thermosetting epoxy resin = 4:1, other components and proportions are the same as in Example 1; Preparation steps: Same as in Example 1.

[0041] Example 6: 3% organic activator system, with the remaining components and proportions the same as in Example 1; Preparation steps: Same as in Example 1.

[0042] Example 7: 8% organic activator system, with the remaining components and proportions the same as in Example 1; Preparation steps: Same as in Example 1.

[0043] Example 8: The photoinitiator was replaced with 2-hydroxy-2-methyl-1-phenylpropanone: 2-isopropylthioxanthrone = 1:1, and the other components and proportions were the same as in Example 1; Preparation steps: Same as in Example 1.

[0044] Comparative Example 1: The resin system in the flux is only refined hydrogenated rosin, without acrylate modification and epoxy resin, and the other components are the same as in Example 1; Preparation steps: The photoinitiator addition and UV curing steps are omitted, and the remaining process parameters and steps are the same as in Example 1.

[0045] Comparative Example 2: The flux contains only photocurable acrylate-modified rosin, without thermocurable epoxy resin and latent thermocuring agent, and the other components are the same as in Example 1; Preparation steps: Same as in Example 1, except for the UV curing step.

[0046] Test method: Welding performance testing: Measure wetting angle and solder spread rate to evaluate welding wetting effect; Residue performance testing: Measure the surface hardness (pencil hardness) and surface insulation resistance (85℃ / 85% relative humidity) of the cured residue. Residue compatibility test: Evaluate the interfacial adhesion with conformal coating and determine the peel rate; Electrochemical migration test: The dendrite growth time was measured by applying a bias voltage at 85℃ / 85% relative humidity; Storage stability test: After 6 months of storage at room temperature, the viscosity change rate and printing performance of the solder paste were measured. Mechanical reliability testing: Conduct drop tests to determine the failure rate of solder joints.

[0047] The test data comparisons are shown in Table 1 and Table 2.

[0048] Table 1. Comparison of wetting angle, residual hardness, surface insulation resistance at 85℃ / 85% and conformal coating peeling rate. Table 2 Comparison of viscosity change rate, drop test failure rate, and dendrite growth time after 6 months of storage Examples 1 to 8 utilize photocuring to achieve rapid debonding of residues, followed by thermocuring to construct a cross-linked framework, forming a dense and tough protective layer. Multi-stage release of organic activators ensures weld wetting, while thixotropic agents and leveling agents optimize process performance. Comparative Example 1, lacking a dual-curing system, exhibits residues with high viscosity and poor insulation; Comparative Example 2, lacking a thermocuring network, suffers from insufficient hardness and stability of its residues.

[0049] When the alloy powder content is 88% to 90%, the resin ratio is 3:1 to 4:1, and the activator content is 5% to 8%, the soldering wetting and residue protection effects are better. Among these, the alloy powder ratio determines the soldering quality, the resin ratio affects the network cross-linking density, and the activator content balances the flux activity and the residual ion content. The three factors work together to ensure the overall performance of the solder paste.

[0050] Compared to Comparative Example 1 without a dual-curing system, the wettability angle of the embodiment is reduced by more than 45%, the surface insulation resistance is increased by more than 6 orders of magnitude, and the drop failure rate is reduced by more than 82%. Compared to Comparative Example 2 without a thermosetting network, the hardness of the residue is increased by more than 2 times, the peeling rate of the conformal coating is reduced by more than 93%, and the dendrite growth time is increased by more than 240%, meeting the high reliability requirements of high-density electronic packaging.

[0051] In summary, this invention achieves simultaneous improvement in efficient soldering and high-performance residue protection by coupling the construction of a photothermal dual-curing system with optimized process parameters. It exhibits excellent environmental stability and process compatibility, solves the core pain points of traditional halogen-free solder paste, is suitable for high-end electronic manufacturing, and has good potential for industrialization.

[0052] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A halogen-free environmentally friendly solder paste, characterized in that, It consists of 85% to 90% alloy powder and 10% to 15% flux by weight. The flux paste comprises the following components by weight percentage: 45% to 60% of the light / heat curing resin system; Organic activator system 3% to 8%; Thixotropic agent 2% to 6%; Latent thermosetting agent 1% to 3%; Photoinitiator 0.5% to 2.0%; Leveling synergist 0.2% to 1.0%; The remainder is a high-boiling-point organic solvent.

2. The halogen-free environmentally friendly solder paste according to claim 1, characterized in that, The light / heat dual-curing resin system is composed of light-curing acrylate-modified rosin and thermosetting epoxy resin mixed in a mass ratio of 2:1 to 4:

1.

3. The halogen-free environmentally friendly solder paste according to claim 2, characterized in that, The photocurable acrylate-modified rosin is prepared by ring-opening esterification reaction of refined hydrogenated rosin and acrylate monomers containing active epoxy groups under the action of a catalyst.

4. The halogen-free environmentally friendly solder paste according to claim 3, characterized in that, In the preparation process of the photocurable acrylate-modified rosin, by controlling the molar ratio of the acrylate monomer containing active epoxy groups to the rosin acid in the refined hydrogenated rosin, the carboxyl group conversion rate on the skeleton of the refined hydrogenated rosin is made to be 70% to 90%, thereby retaining 10% to 30% of free carboxyl groups in the molecular chain of the photocurable acrylate-modified rosin.

5. The halogen-free environmentally friendly solder paste according to claim 3, characterized in that, The acrylate monomer containing the active epoxy group is selected from at least one of glycidyl acrylate or glycidyl methacrylate.

6. The halogen-free environmentally friendly solder paste according to claim 2, characterized in that, The thermosetting epoxy resin is a multifunctional epoxy resin, selected from at least one of bisphenol A type epoxy resin, bisphenol F type epoxy resin, o-cresol epoxy resin, or epoxy resin containing a biphenyl structure.

7. The halogen-free environmentally friendly solder paste according to claim 1, characterized in that, The organic activator system consists of dibasic organic acids, organic amine salts, and auxiliary activators.

8. The halogen-free environmentally friendly solder paste according to claim 7, characterized in that, The dicarboxylic acid adopts a multi-stage active release composite formula and is selected from at least three of succinic acid, glutaric acid, adipic acid, sebacic acid, or dodecanoic acid.

9. The halogen-free environmentally friendly solder paste according to claim 7, characterized in that, The organic amine salt is selected from at least one of triethanolamine hydrochloride or cyclohexylamine hydrobromide.

10. The halogen-free environmentally friendly solder paste according to claim 1, characterized in that, The latent thermosetting agent is selected from at least one of dicyandiamide, dicyandiamide derivatives, modified imidazole compounds, or organic hydrazides.

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