Multi-stage thermal spectrum regulation low porosity solder paste and preparation method thereof

Abstract

The application discloses a multi-stage thermal spectrum regulated low-cavity soldering tin paste and a preparation method thereof, relates to the technical field of electronic assembly materials, and the tin paste comprises 880-896 mass parts of tin-silver-copper lead-free spherical solder powder and 104-120 mass parts of flux mother glue, the solder powder is composed of type 4 powder and type 5 powder, the flux mother glue contains a resin phase, a solvent phase, an acid-amine-imidazole-anhydride system and a thixotropic microstructure, and the liquidus front weight loss ratio before the liquidus, the liquid phase retention stage weight loss ratio and the instantaneous weight loss rate are limited by taking the solder liquidus as a boundary. The tin paste is prepared through drying, nitrogen replacement, segmented compatibilization, three-roll grinding, segmented powder adding and vacuum slope defoaming. The tin paste makes the volatile release occur before the solder forms a continuous molten pool, reduces the residual gas retention in the liquid phase stage, and is suitable for low-gap and large-pad mounting and welding applications.

Description

Technical Field

[0001] This invention relates to the field of electronic assembly materials technology, specifically to multi-segment thermal spectrum-controlled low-voidity solder paste and its preparation method. Background Technology

[0002] As automotive electronics, industrial control, server power supplies, and RF communication modules evolve towards miniaturization, high power density, and high reliability, the application of quad flat no-lead packages, bottom-mount packages, and ball grid array packages in surface mount technology (SMAP) is continuously increasing. These soldering modules often feature exposed heat sinks, bottom metallized terminals, low solder joint support height, and large pad coverage areas, making continuous venting from the side during reflow difficult. For heat sinks or large grounding pads, stencil openings, mounting pressure, solder paste volume, reflow profile, and substrate moisture content all affect the gas migration path. Currently, common lead-free no-clean solder pastes mainly consist of tin-silver-copper solder powder, rosin resin, solvents, activators, and thixotropic systems. In terms of processing, methods such as segmented openings, adjusted preheating zones, extended liquid phase holding time, or vacuum reflow are generally used to address solder joint voids.

[0003] The above measures can improve exhaust conditions to some extent, but for solder paste with low gaps, large pads, and high solids content, flux volatiles, oxide film reaction products, and trace amounts of moisture introduced by raw materials or the environment may still continue to precipitate after the solder begins to melt. The main mechanism is as follows: after solder particles aggregate and form a continuous molten pool, the gap between the pad edge and the bottom of the device is gradually sealed by the liquid solder. The gas generated subsequently must overcome the surface tension of the liquid solder and the resistance of the narrow channel to escape. If the gas generation rate and the escape rate do not match, the bubbles will grow and merge in the molten pool and remain as internal cavities before the solder solidifies. The cavities reduce the effective metal contact area of ​​the solder joint, increase the discontinuity of heat flow and current paths, and cause local stress concentration during thermal cycling, power cycling, or mechanical vibration. Moreover, for devices that use heat dissipation pads to perform functions such as heat conduction, electrical grounding, or shielding, it may also cause local overheating, unstable contact, increased inspection rework rate, and decreased long-term reliability. In particular, if only the total volatile amount, room temperature viscosity, slump, or post-soldering residue are considered in the development of solder paste formulations, without fully distinguishing the volatile sequence of the preheating stage, the vicinity of the liquidus line, and the liquid phase holding stage, even if the total weight loss or normal printing meets the requirements, residual gas tailing in the liquid phase stage may occur, making it difficult to control solder joint voids.

[0004] Therefore, the current problem to be solved is that when tin-silver-copper lead-free solder powder and no-clean flux masterbatch are used to make high-solids solder paste for surface mount technology with low gaps and large pads, a small amount of residual gas will be trapped inside the molten solder during the liquid phase of the solder, which will cause the gas to form voids. Summary of the Invention

[0005] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a multi-segment thermal spectrum-controlled low-voidity solder paste and its preparation method. The solder paste comprises 880 to 896 parts by weight of tin-silver-copper lead-free spherical solder powder and 104 to 120 parts by weight of flux masterbatch. The solder powder is composed of type 4 and type 5 powders, and the flux masterbatch contains a resin phase, a solvent phase, an acid-amine-imidazolium-anhydride system, and a thixotropic microstructure. The weight loss ratio before the liquidus line, the weight loss ratio during the liquidus holding stage, and the instantaneous weight loss rate are defined by the solder liquidus line as the boundary. The paste is prepared through drying, nitrogen purging, segmented compatibility, three-roll milling, segmented powder addition, and vacuum slope degassing. This solder paste allows volatilization to occur before the solder forms a continuous molten pool, reducing residual gas retention during the liquidus stage, and is suitable for low-gap, large-pad surface mount soldering applications.

[0006] (II) Technical Solution To achieve the above objectives, the present invention provides the following technical solution: This multi-segment thermal spectrum controlled low-voidity solder paste comprises: 880–896 parts by weight of tin-silver-copper lead-free spherical solder powder and 104–120 parts by weight of flux masterbatch; in the solder powder, type IV powder accounts for 78–85% by weight and type V powder accounts for 15–22% by weight; the flux masterbatch includes hydrogenated rosin, polymerized rosin, rosin pentaerythritol ester, 2-ethyl-1,3-hexanediol, diethylene glycol monohexyl ether, succinic acid, adipic acid, and nonanedioic acid. Acids, N-hydroxyethylimidazole, 2-ethylimidazole, triethanolamine, succinic anhydride, glutaric anhydride, polyamide wax micropowder, and hydrogenated castor oil derivatives; the flux masterbatch was subjected to thermogravimetric analysis, with the liquidus line of the tin-silver-copper lead-free solder powder measured by differential scanning calorimetry as the boundary. The weight loss ratio before the liquidus line was 80-88%, the weight loss ratio during the liquidus holding stage was 6-13%, and the maximum instantaneous weight loss rate during the liquidus holding stage was not higher than 0.025 mass percentage / second.

[0007] Furthermore, the flux masterbatch comprises 26–30 parts by weight of hydrogenated rosin, 14–17 parts by weight of polymerized rosin, 11–14 parts by weight of rosin pentaerythritol ester, 4.5–6.5 parts by weight of 2-ethyl-1,3-hexanediol, 21–25 parts by weight of diethylene glycol monohexyl ether, 7.0–9.5 parts by weight of isoborneol cyclohexanol, 3.8–5.2 parts by weight of dibutyl sebacate, 1.0–1.5 parts by weight of succinic acid, 1.4–1.9 parts by weight of adipic acid, 1.3–1.8 parts by weight of azelaic acid, 0.7–1.1 parts by weight of N-hydroxyethylimidazolium, and 2 -Ethylimidazole 0.35–0.60 parts by weight, triethanolamine 0.15–0.35 parts by weight, succinic anhydride 0.7–1.1 parts by weight, glutaric anhydride 0.25–0.45 parts by weight, polyamide wax micro powder 3.3–4.4 parts by weight, hydrogenated castor oil derivative 2.5–3.4 parts by weight, non-fluorinated polyether modified surface tension modifier 0.20–0.40 parts by weight, hindered phenolic antioxidant 0.05–0.15 parts by weight, phosphite antioxidant 0.04–0.08 parts by weight, and oleamide rheology stabilizer 0.45–0.70 parts by weight.

[0008] Furthermore, the flux masterbatch also includes 0.05–0.15 parts by weight of a reactive, detectable active ingredient, which is 9-anthramethylsuccinic acid half ester-imidazolium salt.

[0009] Furthermore, the median particle size of the tin-silver-copper lead-free spherical solder powder is 24–30 micrometers, the 90% cumulative distribution particle size is 34–38 micrometers, the total oxygen content of the powder is 350–700 parts per million, the proportion of satellite powder is not higher than 2.0%, and the moisture content of the powder is not higher than 250 parts per million.

[0010] Furthermore, the peak area ratio of the volatiles in the later stage of the solder paste, as determined by headspace gas chromatography-mass spectrometry, is not higher than 0.20; the apparent vapor pressure safety ratio of the solder paste is 45–60, and the mass loss of the solder paste before and after degassing is not higher than 0.35% by mass; the apparent vapor pressure safety ratio is the ratio of the absolute pressure after degassing to the apparent mixed vapor pressure at 25°C.

[0011] A method for preparing low-voidity solder paste with multi-segment thermal spectrum regulation, including: Under nitrogen protection, 85–90% by mass of diethylene glycol monohexyl ether, isobornylcyclohexanol, dibutyl sebacate, and 2-ethyl-1,3-hexanediol are heated to 76–80°C, and hydrogenated rosin, polymerized rosin, and pentaerythritol rosin are added in batches. The temperature is then lowered to 70–73°C, and succinic acid, adipic acid, and azelaic acid are added. At 67–70°C, a premix formed by the remainder of diethylene glycol monohexyl ether, N-hydroxyethylimidazolium, 2-ethylimidazolium, and triethanolamine is added dropwise. Succinic anhydride and glutaric anhydride are added at 58–63°C. Polyamide wax micropowder and hydrogenated castor oil derivatives are dispersed at 74–78°C to obtain flux masterbatch. Tin-silver-copper lead-free spherical solder powder is then added in stages and degassed under vacuum slope.

[0012] Furthermore, 10–14 hours before operation, succinic acid, adipic acid, azelaic acid, succinic anhydride, glutaric anhydride, polyamide wax micro powder, and hydrogenated castor oil derivatives are respectively placed in a vacuum dryer at 42–48°C and an absolute pressure of 6–10 kPa for 3–5 hours, cooled to 20–30°C, and then sealed for later use. The reactor body is subjected to vacuuming at 15–25 kPa and nitrogen backfilling circulation 2–4 times, and purged until the oxygen content at the top of the reactor is not higher than 1,000,000 parts per million and the dew point is not higher than -35°C.

[0013] Furthermore, the premixed solution is added via a submerged dropper, with the outlet of the dropper located 12–18 mm below the liquid surface. The dropping rate is 0.35–0.45 mL / min, and the system temperature is maintained at 67.5–70.0 °C during the dropping process. After the dropping is completed, stirring continues until the torque drops back to 85–90% of its peak value. Succinic anhydride and glutaric anhydride are added at 0.18–0.35 parts by mass / min, and the moisture content of the flux masterbatch after addition is not higher than 1 / 350 million.

[0014] Furthermore, after the flux masterbatch is left to stand for 10–14 hours, it is ground by three rollers at a roller temperature of 22–28°C. The roller gaps are 35–45 micrometers, 20–30 micrometers, and 12–20 micrometers respectively, and the discharge temperature of the third pass does not exceed 45.0°C. The tin-silver-copper lead-free spherical solder powder is added to the flux masterbatch in 3–5 stages in a low-humidity nitrogen environment. Each stage is 20–35% of the total solder powder, and each stage is mixed for 120–180 seconds after adding the powder.

[0015] Furthermore, vacuum ramp degassing is performed at an orbital speed of 6–10 rpm and a rotational speed of 3–6 rpm. The vacuum is reduced to an absolute pressure of 14–20 kPa at a descent rate of 12–20 kPa and maintained for 5–8 minutes. The apparent vapor pressure safety ratio is 45–60. The mass loss of solder paste before and after degassing is no more than 0.35% by mass. After degassing, the pressure is backfilled to atmospheric pressure within 90–150 seconds with dry nitrogen.

[0016] (III) Beneficial Effects This invention provides a multi-segment thermal spectrum-controlled low-voidity solder paste and its preparation method, which has the following beneficial effects:

[0017] By establishing a matching relationship between tin-silver-copper lead-free spherical solder powder and flux masterbatch, and controlling the particle size distribution of type IV and type V powders, the resin phase, solvent phase, acid-amine-imidazolium-anhydride system, and thixotropic microstructure of flux masterbatch as a whole, the solder paste can maintain relatively stable paste uniformity during printing, mounting, and reflow.

[0018] The flux masterbatch used to assist in battery fabrication is bounded by the liquidus line determined by thermogravimetric analysis and differential scanning calorimetry. The volatilization sequence is jointly defined by the weight loss ratio before the liquidus line, the weight loss ratio during the liquidus holding stage, and the maximum instantaneous weight loss rate during the liquidus holding stage. This ensures that the main volatilization and release occur before the solder forms a continuous molten pool, reducing the possibility of residual gas generation during the liquidus holding stage remaining in the low-gap, large-pad structure.

[0019] The headspace-gas chromatography-mass spectrometry (HS-GC-MS) analysis of the peak area ratio of volatiles in the later stage, the apparent vapor pressure safety ratio during vacuum ramp degassing, and the mass loss of solder paste before and after degassing are coordinated to avoid damage to the thermal spectral window caused by insufficient degassing, excessive extraction, and secondary bubble trapping. Therefore, the solder paste described in this paper is beneficial for surface mount applications such as quad flat leadless packages and ball grid array packages, balancing venting controllability, print release, wetting continuity, and post-soldering residue stability, reducing the risk of void formation inside solder joints, and improving consistency in batch preparation and reflow applications. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the overall process flow for the preparation and thermal spectrum release of the low-void solder paste of the present invention. Figure 2 This is a longitudinal cross-sectional view of the jacketed flux preparation vessel in this invention. Figure 3 This is a schematic diagram illustrating the state evolution of the flux masterbatch segmented compatibility and thixotropic microstructure formation process in this invention. Figure 4 This is a schematic diagram of the thermal fingerprint window corresponding to multi-segment thermogravimetric analysis and differential scanning calorimetry in this invention; Figure 5 This is a schematic diagram of the process in which bimodal solder powder is added in stages and compounded with flux masterbatch to form solder paste in this invention; Figure 6 This is a schematic diagram comparing the safety window and the out-of-bounds degassing state in the vacuum ramp degassing of this invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Please see Figures 1-6 This invention provides a multi-segment thermal spectrum-controlled low-voidity solder paste and its preparation method, comprising: The present invention will be further described below with reference to specific embodiments. Unless otherwise specified, all percentage contents are mass percentage contents, and all temperatures are actual temperatures measured by the material or equipment. Liquid phase holding time refers to the time the solder remains above the liquidus line; the liquidus line of tin-silver-copper lead-free solder is based on the actual measurement results of differential scanning calorimetry, specifically using the extrapolated endpoint temperature of the last main melting endothermic peak as the liquidus line boundary; if the differential scanning calorimetry analysis curve has multiple endothermic peaks or shoulder peaks, the same batch of samples should be tested at least 3 times and the average value should be taken as the liquidus line temperature. Thermogravimetric analysis, differential scanning calorimetry, headspace-gas chromatography-mass spectrometry analysis, and temperature-rheological testing of flux are the internal control characterization methods of the present invention, used to limit the flux volatilization sequence, residual gas generation in the liquid phase, and exhaust window in the preheating section.

[0023] In a preferred embodiment, the core control parameters that can be used for refining the claims include: a weight loss ratio of 80–88% before the liquidus phase; a weight loss ratio of 6–13% during the liquid phase holding stage; a maximum instantaneous weight loss rate of no more than 0.025 mass percentage / second during the liquid phase holding stage; a peak area ratio of volatiles in the later stage of headspace-gas chromatography-mass spectrometry analysis no more than 0.20; a safe vapor pressure ratio of 45–60 for vacuum degassing; and a solder paste mass loss of no more than 0.35 mass percentage before and after degassing. These parameters are verified by thermogravimetric analysis, headspace-gas chromatography-mass spectrometry analysis, static headspace method or pressure sensing method, and weighing method, respectively. Example 1: Preparation of low-voidity solder paste

[0024] This embodiment is based on the preparation of a batch of low-voidity lead-free solder paste. Unless otherwise specified, the formulation amounts are by mass parts. The solder paste comprises 880–896 parts by mass of tin-silver-copper lead-free spherical solder powder and 104–120 parts by mass of flux masterbatch. In the tin-silver-copper lead-free spherical solder powder, type IV powder accounts for 78–85% by mass of the total solder powder, and type V powder accounts for 15–22% by mass of the total solder powder, the total of which is the total amount of solder powder. The components of the flux masterbatch are selected within the following ranges, and the total amount of flux masterbatch is normalized to 104–120 parts by mass by adjusting the amount of resin matrix, solvent pack, or oleamide rheology stabilizer.

[0025] The solder powder is a tin-silver-copper lead-free spherical solder powder, with a total weight of 880–896 parts by mass. Type IV powder accounts for 78–85% of the total solder powder weight, and Type V powder accounts for 15–22% of the total solder powder weight, the sum of which constitutes the total solder powder weight. The median particle size of the solder powder is 24–30 micrometers, with a 90% cumulative distribution particle size of 34–38 micrometers; the total oxygen content of the powder is 350–700 parts per million, the satellite powder ratio is not higher than 2.0%, and the moisture content of the powder is not higher than 250 parts per million.

[0026] The following flux masterbatch components are selected within the indicated ranges; unless otherwise specified, the total amount of flux masterbatch is normalized to 104–120 parts by mass by adjusting the amount of resin matrix, solvent pack, or oleamide rheology stabilizer. The reactive, detectable active component is optional, preferably 9-anthramethylsuccinate half-ester-imidazolium salt or its equivalent anthracene-carboxylic acid imidazolium salt; in embodiments without this component, the mass can be made up with an equal amount of oleamide rheology stabilizer or a solvent compatible with the flux masterbatch.

[0027] The total amount of flux masterbatch is 104–120 parts by weight. The flux masterbatch comprises 26–30 parts by weight of hydrogenated rosin, 14–17 parts by weight of polymerized rosin, 11–14 parts by weight of pentaerythritol rosin, 4.5–6.5 parts by weight of 2-ethyl-1,3-hexanediol, 21–25 parts by weight of diethylene glycol monohexyl ether, 7.0–9.5 parts by weight of isoborneol cyclohexanol, 3.8–5.2 parts by weight of dibutyl sebacate, 1.0–1.5 parts by weight of succinic acid, 1.4–1.9 parts by weight of adipic acid, 1.3–1.8 parts by weight of azelaic acid, 0.7–1.1 parts by weight of N-hydroxyethylimidazole, and 0.35–0.60 parts by weight of 2-ethylimidazole. The composition includes, by weight: triethanolamine 0.15–0.35 parts by weight, succinic anhydride 0.7–1.1 parts by weight, glutaric anhydride 0.25–0.45 parts by weight, polyamide wax micropowder 3.3–4.4 parts by weight, hydrogenated castor oil derivative 2.5–3.4 parts by weight, non-fluorinated polyether modified surface tension modifier 0.20–0.40 parts by weight, hindered phenolic antioxidant 0.05–0.15 parts by weight, phosphite antioxidant 0.04–0.08 parts by weight, reactive detectable active ingredient 0.05–0.15 parts by weight, and oleamide rheology stabilizer 0.45–0.70 parts by weight.

[0028] A 1.5–3.0 liter jacketed 316L stainless steel flux preparation vessel is used. The vessel's inner diameter is 110–125 mm, with an effective working volume of 1.0–1.4 liters and a filling rate controlled at 45–55%. The vessel is equipped with a 55–70 mm diameter four-bladed oblique impeller, a 32–45 mm diameter liftable high-speed dispersion disc, and PTFE wall scrapers, with the gap between the scrapers and the vessel wall controlled at 0.8–1.2 mm. The vessel lid is equipped with a vacuum port, a nitrogen inlet, a dew point probe, an oxygen content probe, an insertion thermocouple, and a torque sensor. The oxygen content probe is installed in the gas phase space of the vessel lid, and the thermocouple is inserted approximately 18–25 mm below the liquid surface, ensuring it is not close to the vessel wall to avoid falsely high or low readings.

[0029] Subsequent compounding was performed using a vacuum planetary solder paste mixer, and three-roll milling was conducted using ceramic or carbide rollers. Filling was carried out in a low-humidity nitrogen glove box. The preparation environment temperature was 20–30℃, and the relative humidity was not higher than 20%. The dew point of the filling glove box was not higher than -40℃, preferably not higher than -45℃, and the oxygen content was not higher than 1 / 1000 million, preferably not higher than 1 / 800 million. Preparation steps

[0030] Step 1: Raw material drying and low-humidity equilibrium 10–14 hours before the start of the operation, succinic acid, adipic acid, azelaic acid, succinic anhydride, glutaric anhydride, polyamide wax micropowder, and hydrogenated castor oil derivatives are separately spread in shallow dishes and dried in a vacuum drying oven at 42–48°C and an absolute pressure of 6–10 kPa for 3–5 hours. After drying, cool to 20–30°C in a nitrogen-purging chamber, and then transfer to sealed brown bottles for later use. Under qualified conditions, the organic acid crystals are dry, loose, and finely crystalline; there should be no mist or condensation at the mouth of the anhydride bottle.

[0031] Step 2: Reactor body replacement and establishment of solvent-resin clarifying phase After closing the vessel lid, use a diaphragm vacuum pump to reduce the pressure inside the vessel to 15–25 kPa absolute pressure and maintain this pressure for 30–60 seconds. Then, backfill with dry nitrogen gas with a dew point not higher than -50°C to 100–110 kPa. Repeat this vacuum-nitrogen-purging cycle 2–4 times. After the last backfill, continue purging with dry nitrogen gas at a rate of 600–1000 mL / min for 8–15 minutes until the oxygen content at the top of the vessel is not higher than 1 / 1000 million and the dew point is not higher than -35°C. If the oxygen content cannot be reduced to below 1 / 1000 million within 8–15 minutes after the last backfill, the vessel lid sealing ring, vacuum connector, and sampling port should be checked. Extending the stirring time should not be used as a substitute for determining whether the atmosphere is qualified. Subsequently, 85–90% by mass of diethylene glycol monohexyl ether, 7.0–9.5 parts by mass of isoborneol cyclohexanol, 3.8–5.2 parts by mass of dibutyl sebacate, and 4.5–6.5 parts by mass of early exhaust solvent were added to the reactor in sequence; the remaining 10–15% by mass of diethylene glycol monohexyl ether was reserved as a solvent for the subsequent complexing agent premix.

[0032] In this embodiment, the early exhaust solvent can be 2-ethyl-1,3-hexanediol, or an alcohol ether, glycol, or ester alcohol solvent whose 50% weight loss temperature is located at 135–185℃ as determined by the thermogravimetric analysis program of this invention in the flux masterbatch system; diethylene glycol monohexyl ether serves as the mid-stage moisturizing / active release solvent, with its main weight loss peak located in the range of 170–215℃; isobornylcyclohexanol and dibutyl sebacate serve as resin solvation components, mainly adjusting the open time and residual film after reflux.

[0033] Start the slanted paddle agitator at 80–100 rpm to create a low and wide annular vortex at the bottom of the vessel, ensuring no deep vortex forms at the center of the liquid surface. Increase the jacket temperature to 76–80°C at a rate of 1.0–1.5°C / min. Once the temperature exceeds 52–58°C, the solvent layer changes from a clear, watery layer to an oily phase with distinct refractive properties. Then, add hydrogenated rosin, polymerized rosin, and pentaerythritol rosin ester in 5–7 batches, with an interval of 60–120 seconds between each batch. After entering the vessel, the resin softens into translucent flakes, then is drawn into long filaments by the vortex and gradually disappears. If resin clumps adhere to the vessel wall for more than 3 minutes, activate the wall scraper and reduce the feeding rate; do not directly increase the temperature to harden the resin.

[0034] After all the resin was added, the mixture was kept at 76.5–79.0℃ for 18–24 minutes to obtain a light amber clear viscous liquid. The liquid continuously flowed down the vessel wall without any particle shadows, and the torque remained stable within ±4% of the average value of the first 4–6 minutes.

[0035] Step 3: Forming an acid-resin compatible state Reduce the material temperature to 70–73°C and maintain inclined paddle stirring. Mix 1.0–1.5 parts by weight of succinic acid, 1.4–1.9 parts by weight of adipic acid, and 1.3–1.8 parts by weight of azelaic acid, and add the mixture tangentially to the reactor wall using a vibrating micro-powder feeder at an average rate of 0.30–0.50 parts by weight / minute. A brief, pale milky-white hazy band is permissible within 60–120 seconds after acid addition; after continuing stirring for approximately 5–9 minutes, the hazy band should largely disappear, and the system should return to a clear, light amber color, with the torque increasing by 6–13% compared to before acid addition.

[0036] Step 4: Submerged starvation method for adding nitrogen-containing complexing agent. In a dry brown bottle, 2.3–3.2 parts by weight of the retained diethylene glycol monohexyl ether, 0.7–1.1 parts by weight of N-hydroxyethylimidazole, 0.35–0.60 parts by weight of 2-ethylimidazole, and 0.15–0.35 parts by weight of triethanolamine are mixed. After purging with nitrogen for 1–3 minutes, the mixture is shaken to form a light yellow, transparent premix. The reactor temperature is controlled at 67–70°C. The premix is ​​added through a submerged dropper, with the dropper outlet 12–18 mm below the liquid surface, at a dropping rate of 0.35–0.45 mL / min. When approximately 35–45% of the mixture has been added, a reversible torque peak appears, with the torque increasing by 12–20% and the material temperature rising by approximately 1.0–2.5°C. The system is then cooled using a jacket to maintain the temperature at 67.5–70.0°C. After the addition is complete, stirring continues until the torque drops back to 85–90% of its peak value.

[0037] Step 5: Low-temperature embedding of acid anhydride moisture trap Cool the material to 58–63°C and maintain the anchor stirring speed at 100–140 rpm. Mix 0.7–1.1 parts by weight of succinic anhydride and 0.25–0.45 parts by weight of glutaric anhydride, and add the mixture along the vessel wall at a rate of 0.18–0.35 parts by weight / min. During the addition process, only a fine refractive band is allowed on the liquid surface; continuous bubbles or white flocculation should not occur. Take a sample for Karl Fischer moisture testing approximately 4–6 minutes after the addition is complete. The moisture content of the flux masterbatch should not exceed 1 / 350,000,000.

[0038] Step Six: Dispersion of Thixotropic Microstructures Heat the system to 74–78°C, switch to a high-speed dispersion disc, and control the disc circumferential velocity at 2.4–3.2 m / s. Premix 3.3–4.4 parts by weight of polyamide wax powder with 2.5–3.4 parts by weight of hydrogenated castor oil derivative, and add the mixture in 5–7 portions over 4–6 minutes. After the third addition, the system begins to exhibit a soft, milky white sheen; after all the mixture has been added, the system changes from a clear amber state to a translucent amber thixotropic paste with a slight opalescence, and the liquid hanging on the vessel wall changes from continuous dripping to slow retraction. Continue dispersing at 74–78°C for 12–18 minutes.

[0039] Step 7: Add interface modifiers, antioxidants, and optional reactive detectable active ingredients. Stop high-speed dispersion and switch to anchor stirring at 80–110 rpm, cooling to 67–70°C at a rate of 0.8–1.2°C / min. Then, sequentially add 0.20–0.40 parts by weight of non-fluorinated polyether modified surface tension modifier, 0.05–0.15 parts by weight of hindered phenolic antioxidant, 0.04–0.08 parts by weight of phosphite antioxidant, 0.05–0.15 parts by weight of reactive detectable active ingredient (optional, preferably reactive detectable active ingredient), and 0.45–0.70 parts by weight of oleamide rheology stabilizer.

[0040] When the reactive active component is added, it forms a very faint golden-yellow thread-like trace in the amber masterbatch, which disappears completely after about 2–4 minutes. Continue stirring for 6–10 minutes, then cool to 38–45°C under nitrogen protection, discharge into a dry polypropylene transfer cup, seal and let stand for 10–14 hours to obtain flux masterbatch.

[0041] Step 8: Three-roll milling and masterbatch thermal spectrum pre-release After settling, the masterbatch is processed using a three-roll mill with a roller diameter of at least 80 mm, a roller speed ratio of approximately 1:3:9, and a roller temperature controlled at 22–28°C. The roller gaps for the three passes are 35–45 micrometers, 20–30 micrometers, and 12–20 micrometers, respectively. The temperature of the third pass should not exceed 45.0°C. After the three passes, the masterbatch should be a uniform light amber paste, forming short, blunt peaks when lifted with a glass rod, without long filaments or bright particles. If the discharged material has a noticeable solvent odor, or if the weight loss before the liquidus line in thermogravimetric analysis exceeds 88%, it indicates that the shear heat from the three rolls has damaged the thermometric window, and this should be considered an anomaly.

[0042] Take 18–22 mg of masterbatch for thermogravimetric analysis and pre-release testing. The test procedure is as follows: equilibrate at 30°C for 5 minutes, increase to 150°C at 50°C / min, hold at 150°C for 60 seconds, increase to 180°C at 30°C / min, hold at 180°C for 60 seconds, increase to 5°C below the solder liquidus line at 20°C / min, and then increase to 245°C at 20°C / min and hold for 70 seconds.

[0043] Step 9: Add solder powder in stages Transfer the vacuum planetary mixing cup to a low-humidity nitrogen environment, with a dew point no higher than -40°C, preferably no higher than -45°C, and an oxygen content no higher than 1 / 1000 million, preferably no higher than 1 / 800 million. First, add 104–120 parts by weight of flux masterbatch, level it, and pre-stir for 45–90 seconds, rotating at 10–15 rpm and 6–10 rpm to spread the masterbatch into a uniform, thick paste layer. Then, add tin-silver-copper lead-free solder powder in 3–5 stages, each stage being 20–35% of the total solder powder, with each stage adding powder for 80–120 seconds. The powder is slowly sprinkled into the cup along the wall through a narrow-mouthed dry powder funnel, not directly piled into the center. Mix for 120–180 seconds after each stage of powder is added. When the torque change rate is less than 3% / 90 seconds and there is no dry powder residue on the cup wall, proceed to the next stage. The final paste should be a uniform matte silver-gray color, with no dry powder spots or oily spots on the surface.

[0044] Step 10: Vacuum slope degassing After adding the powder, maintain low-speed stirring at 6–10 rpm for revolution and 3–6 rpm for rotation. Reduce the vacuum from atmospheric pressure to 14–20 kPa absolute pressure at a rate of 12–20 kPa / min, a process that takes approximately 4–6 minutes; maintain the target absolute pressure for 5–8 minutes. This step uses the apparent vapor pressure safety ratio to verify the degassing window. This apparent vapor pressure safety ratio is the ratio of the degassing absolute pressure to the apparent mixed vapor pressure at 25°C. The apparent mixed vapor pressure at 25°C is the apparent mixed vapor pressure of the flux solvent package measured at 25°C using a static headspace method or an equivalent method. This can be achieved using a 20 mL headspace vial, adding 1.00 ± 0.02 g of flux solvent package or flux masterbatch extract, equilibrating at 25.0 ± 0.5°C for 30 minutes, and then measuring using headspace gas chromatography or a pressure sensing method. Perform three parallel tests on the same sample and take the average value. In this embodiment, the apparent vapor pressure safety ratio is controlled at approximately 45–60.

[0045] During degassing, sparse, pinpoint-like bubbles are permissible, which gradually shrink and disappear. If continuous foam blankets, crater-like bubbling, or paste spraying occurs, the pressure should be immediately reversed to 40 kPa, and the batch should be deemed degassing abnormal. The mass loss of solder paste before and after degassing should not exceed 0.35% by mass, preferably 0.18–0.28% by mass. If it exceeds 0.35% by mass, it indicates that low- or medium-boiling volatile components have been excessively removed, and the thermogravimetric analysis (TGA) chromatogram has undergone irreversible shift. After degassing, the pressure should be slowly reversed to atmospheric pressure using dry nitrogen for 90–150 seconds.

[0046] Step 11: Filling and Sealing Solder paste is filled into 50–200 g polypropylene cans in a glove box with a dew point not exceeding -40°C, preferably not exceeding -45°C, and an oxygen content not exceeding 1 / 1000 million, preferably not exceeding 1 / 800 million. The net weight of each can is controlled according to packaging specifications, with a head space of 2.5–3.5 ml and a capping torque of 0.6–1.0 N·m. After filling, it is refrigerated at 0–10°C and warmed to 20–30°C in a sealed state before use. IV. Finished Product Inspection and Implementation Results

[0047] The solder paste prepared according to the above method shall be subjected to the following tests and judgments.

[0048] For thermogravimetric analysis (TGA) testing, flux masterbatch is preferred. If flux extraction from finished solder paste is necessary, a low-temperature extraction method that does not alter the proportion of low-boiling components should be used at a temperature not exceeding 30°C, and the extraction recovery rate should be recorded. The sample mass should be 18–22 mg, using an open aluminum crucible or an equivalent low-holding-time crucible, with a nitrogen flow rate of 45–55 mL / min. The liquidus temperature for differential scanning calorimetry (DSC) is the extrapolated endpoint temperature of the last major melting endothermic peak of the tin-silver-copper lead-free solder powder. If the DSC curve contains multiple endothermic peaks or shoulders, the extrapolated endpoint temperature of the last major melting endothermic peak should be used as the liquidus temperature boundary. The same batch of samples should be tested at least three times, and the average value should be taken as the liquidus temperature. The liquidus holding phase is defined by the measured liquidus temperature.

[0049] In the standard Quad Flat No-Leader Package (QF No-Leader) board, the thermal pads are divided by 85%, and the total solder paste coverage of a single thermal pad is 83–87%. Each group should contain at least 50 QF No-Leader Package thermal pads. For Ball Grid Array (BGA) packages, each group should contain at least 200 solder joints. The void ratio is calculated based on the two-dimensional projection area of ​​X-ray imaging, i.e., the sum of the projected areas of voids within the solder joint divided by the effective projected area of ​​the solder joint; the maximum single void ratio is the projected area of ​​the largest single void divided by the effective projected area of ​​that solder joint.

[0050] When verified on the unified quad flat no-lead package standard board and ball grid array package standard board, the average void ratio of the quad flat no-lead package standard board was 3.18±0.86%, the average void ratio of the ball grid array package standard board was 1.42±0.46%, and the maximum single void ratio was 5.82±1.75%; the surface insulation resistance logarithmic value of base 10 under 85℃ / 85% relative humidity conditions was 10.31±0.22 ohms.

[0051] The solder paste prepared by the above method preferably meets the following finished product testing indicators: the solder paste metal content is 88.8±0.3% by mass; the apparent viscosity at 25°C and 10 seconds⁻¹ is 950±150 Pa·s; the thixotropic index is 4.6±0.6; the ratio of viscosity at 160°C to viscosity at 25°C is 0.16±0.04; the total weight loss from thermogravimetric analysis is approximately 16.9% by mass; the weight loss ratio before the liquidus is 80–88%, and in Example 1 it is 84.7±2.1%; the weight loss ratio during the liquidus holding stage is 6%. –13%, Example 1: 9.6±1.3%; apparent vapor pressure safety ratio: 45–60; defoaming mass loss: not higher than 0.35% by mass, preferably 0.18–0.28% by mass; maximum instantaneous weight loss rate during liquid phase holding: not higher than 0.025% by mass / second, Example 1: 0.019±0.004% by mass / second; peak area ratio of volatiles in the latter part of headspace-gas chromatography-mass spectrometry analysis: not higher than 0.20, Example 1: 0.145±0.038.

[0052] In this embodiment, the flux volatilization sequence is defined by the pre-liquid phase weight loss ratio, the liquid phase holding phase weight loss ratio, and the maximum instantaneous weight loss rate during the liquid phase holding phase in thermogravimetric analysis; the liquid phase volatiles are defined by the peak area ratio of the volatiles in the latter stage in headspace-gas chromatography-mass spectrometry analysis; the defoaming window for the paste is defined by the apparent vapor pressure safety ratio and the defoaming mass loss. The formation process of the acid-amine-imidazole-anhydride system is verified by the torque peak during the dropping stage, Karl Fischer moisture content, surface insulation resistance, and residual glass transition temperature after reflux as auxiliary verification indicators. Compared with systems that do not employ thermogravimetric shifting, segmented feeding, or a suitable vacuum window, the low-void performance of this embodiment is significantly improved; correspondingly, its printing open time can be slightly shortened compared to post-boiling volatilization systems. This performance trade-off is jointly defined by thermogravimetric analysis, temperature-rheological analysis, and X-ray imaging void statistics. Preparation of reactive detectable active components

[0053] If the reactive detectable active ingredient is not commercially available, it can be prepared as follows: Dissolve 9-anthracene methanol and succinic anhydride in anhydrous ethyl acetate at a molar ratio of 1.00:(1.03–1.08), add 0.01–0.05 equivalents of 4-dimethylaminopyridine, and react at 40–50°C for 3–5 hours to obtain 9-anthracene methyl succinate half ester; after washing, drying, and desolvating under reduced pressure, add 0.95–1.05 equivalents of N-hydroxyethyl imidazole to neutralize, reducing the acid value to 95±5% of the theoretical value, to obtain the preform of the reactive detectable active ingredient. The moisture content of the obtained preform should not exceed 1 / 500 million, and the free imidazole content should not exceed 0.2% by mass. The reactive detectable active component can be used as an optional residual detectable component, the presence of which is beneficial for the analysis of residuals after reflux by liquid chromatography-high resolution mass spectrometry, Fourier transform infrared spectroscopy or fluorescence detection; in embodiments where residual marker detection is not required, the mass can be made up with an equal amount of oleamide rheology stabilizer or a compatible solvent.

[0054] This section includes Experiment 1 (thermal spectrum window verification), Experiment 2 (void rate verification), Experiment 3 (surface insulation resistance and residual reliability verification), and Comparative Examples 1–5 (missing element verification). The following data are the test results for the corresponding samples, as shown in the table; all test data listed in the table correspond to the original test records or inspection reports. Original thermogravimetric analysis, differential scanning calorimetry, gas chromatography-mass spectrometry analysis, X-ray imaging, surface insulation resistance, and rheological test records are retained for future reference. I. Experiment Example 1: Validation of Thermal Spectrum Window and Sample Preparation Matrix 1.1 Sample Design Principles

[0055] The following experimental examples are single-point experimental conditions falling within the scope of the above-described specific embodiments, used to illustrate the technical effects of the present invention. Unless otherwise specified, the weight, temperature, pressure, time, and rotation speed in the experimental examples are not intended to limit the scope of protection of the present invention.

[0056] All samples were designed at 1000.0 g / batch, and the target metal content was controlled at around 88.8% by mass to eliminate the interference of metal content differences on the void ratio; the core differences consist of the following five variables.

[0057] Specifically, this includes: whether the flux has a pre-liquid phase window of primary weight loss; whether the acid-amine-imidazolium-anhydride system forms a detectable compatibility state; whether it forms a thixotropic microstructure that can be verified by torque peaks, Karl Fischer moisture content, thermogravimetric analysis, and rheological indices in a specific order; whether vacuum degassing falls within the anti-boiling critical window; and whether the paste preparation process can retain the preset thermogravimetric analysis and differential scanning calorimetry (DSC) thermal fingerprints.

[0058] In the experiment, the solder powder type was uniformly designed according to the four-type / five-type system to compare the effects of particle size distribution, thermal spectrum window, compatibility active system and degassing parameters on low void ratio and printing performance. 1.2 Comparative Examples 1–5

[0059] Comparative Example 5 was used to examine the effect of exceeding the vacuum parameter limit. Its material composition, feeding sequence, temperature, shear rate and number of powder addition stages were the same as those in Example 1. The only difference was that the vacuum degassing absolute pressure was changed from 16.0 kPa to 4.8 kPa, which is lower than the lower limit of the safety window in this case.

[0060] Comparative Example 4 was used to investigate the effect of the same components but with an incorrect order of addition. The total amount of components was the same as in Example 1, but the acid, amine, imidazole, acid anhydride, reactive detectable active component and thixotropic agent were added at high temperature all at once, eliminating the submerged starvation drop addition and segmented compatibility process.

[0061] Comparative Example 3 was used to investigate the effect of the absence of dynamic active / moisture capture system. It retained the ternary thermal spectroscopy solvent package, type IV / V bimodal powder, segmented powder addition and 16.0 kPa vacuum degassing, but removed imidazole / amine salt, acid anhydride and reactive detectable active components, and replaced it with free dicarboxylic acid with equal acid value.

[0062] Comparative Example 2 was used to investigate the effect of the missing liquid phase line pre-main weight loss thermography design. It retained the acid-amine-imidazolium-anhydride compatibility active system, bimodal powder and segmented powder addition, but removed the early degassing solvent and made up the mass with high-boiling resin solvation components.

[0063] Comparative Example 1 serves as the industry baseline and closest to conventional technology, employing conventional no-clean tin-silver-copper lead-free solder paste, specifically using a type IV single-peak powder, a common rosin / high-boiling solvent / free organic acid system, a one-time powder addition, and a rapid fixation vacuum process.

[0064] Example 1 is the optimal embodiment that fully retains the five technical elements of this case. It adopts multi-segment thermogravimetric analysis and differential scanning calorimetry, acid-amine-imidazole-acid anhydride compatibility activation, type IV / V bimodal powder, low-shear segmented powder addition, and 16.0 kPa vacuum ramp degassing. 1.3 Example 1: Perfectly Reproducible Process Flow and Benign Physical Evolution

[0065] Example 1 followed the aforementioned standard operating procedure exactly: 888.000 grams of tin-silver-copper lead-free bimodal solder powder was used, including 728.160 grams of type IV powder and 159.840 grams of type V powder; 112.000 grams of flux masterbatch was also used. The powder oxygen content was set at 1 / 480 million, the median particle size was 27.0 micrometers, the 90% cumulative distribution particle size was 36.0 micrometers, and the satellite powder ratio was approximately 0.9%.

[0066] During the flux preparation process, the three most critical "observable intermediate states" all appeared successfully.

[0067] First, during the resin-mixed solvent setup stage, hydrogenated rosin, polymerized rosin, and pentaerythritol rosin ester were gradually dissolved into the ternary solvent package at 78.5°C. Initially, the contents of the reactor were a light yellow oil phase with distinct refractive properties; after the resin flakes entered the vortex, they were first stretched into soft ribbon-like shapes and then disappeared completely. After holding at this temperature for 18.0 minutes, the system became a clear, light amber-colored viscous liquid, with no particles or white mist on the reactor walls, and the torque stabilized at approximately 1.18 times the initial value.

[0068] Second, after the diacid was slowly added, the system briefly exhibited a pale milky white to amber transition state. After approximately 6.5 minutes, the turbidity disappeared, the system became clear again, and the torque increased gently by 8–12%. This indicates that the diacid had entered the resin-solvent microdomain and formed an acid-resin compatible state; this state was verified by the short-term turbidity reduction, the magnitude of the torque increase, and the stability of the subsequent thermogravimetric analysis weight loss curve, rather than by the diacid remaining suspended in crystalline particles.

[0069] Third, the premixed solution of N-hydroxyethylimidazole, 2-ethylimidazole, and triethanolamine was added dropwise at a starvation rate of 0.420 mL / min. When the addition reached approximately 35–45%, a very narrow "small peak" appeared in the online torque: the system changed from a smooth amber syrup state to a transparent honey state with slight stringiness, and a slight exothermic peak appeared at the vessel temperature of approximately 1.7 °C; the jacket cooling forced the temperature back down to 68.5 ± 0.5 °C. Subsequently, the torque naturally decreased, indicating that the acid-amine-imidazole compatibility state had formed and that irreversible thickening had not occurred; the objective indicators for this were the torque peak during the addition stage, the temperature rise, and the percentage decrease in torque after the addition was completed.

[0070] After the thixotropic agent was dispersed at 76.5℃ and a local shear rate of approximately 900 seconds⁻¹, the system exhibited a soft microemulsion without producing any bright specks. The outlet temperature of the three-roll mill was maintained below 42.5℃. During the four-stage low-shear powder addition, each stage of powder resembled a thin layer of silver-gray snow covering the amber masterbatch, undergoing a continuous transformation from "wet sand state - clumped state - homogeneous silver-gray paste"; each stage ended with a torque change rate of <3% / 90 seconds. Finally, under a sloping degassing pressure of 16.0 kPa, only a few pinhead bubbles appeared on the surface of the paste, without any crater-like bubbling. The finished product was a matte silver-gray short-peak paste; after being lifted with a spatula, the peak tip slowly bent back within 10 seconds without collapsing.

[0071] This group forms a stable thermal spectrum window: approximately 85% weight loss occurs before the liquidus line, and approximately 9–10% weight loss occurs during the liquidus holding stage. This allows the main gas release peak to avoid the solder liquidus holding stage determined by the differential scanning calorimetry liquidus line and reflux profile. To achieve low void performance, the volatilization peaks in the preheating stage are more concentrated, resulting in a slightly shorter printing open time compared to the volatilization system after high boiling, but still meeting the process window requirements. 1.4 Comparative Example 1:

[0072] Controlled variables: Adopting the current industry standard for no-clean tin-silver-copper lead-free solder paste, the multi-segment thermal spectrum solvent design, acid-amine-imidazole-anhydride compatibility active system, bimodal powder, and segmented powder addition methods of this design were eliminated. This group serves as the reference system closest to the existing technology.

[0073] This group used 888,000 grams of tin-silver-copper lead-free solder type IV single-peak powder, with a median particle size of approximately 30.2 micrometers and an oxygen content of approximately 1 / 620 million. The flux was a common rosin-high-boiling solvent-free dicarboxylic acid system: the resin ratio was relatively high, and the solvents were mainly diethylene glycol monohexyl ether, sebacic acid ester, and terpene alcohols. No early degassing solvent was set, nor were acid anhydride water-trapping and imidazole complex networks set.

[0074] During preparation, workers followed standard procedure by directly stirring the resin, solvent, and free organic acid at 80.0°C. Initially, the system formed a light amber-colored fluid, but after the addition of the dicarboxylic acid, the detectable torque peak and compatibility characteristic of Example 1 ("short torque peak-and-fallback") did not appear. Instead, a persistent, slightly turbid tail appeared. White streaks formed by tiny acid crystals being dragged by the viscous liquid could be seen on the vessel wall near the liquid surface. Although the macroscopic appearance was uniform after continued heating, scattered bright spots were still visible under polarized light when the sample tablets were pressed, indicating that the free acid had not completely entered the stable micro-region.

[0075] When the thixotropic agent is added at 65.0℃, due to the high viscosity of the resin phase, the polyamide wax initially forms small, "fish-eye" gels in localized areas. After three-roll mixing, these small gels are mechanically crushed but do not completely disappear; they simply become finer rheologically inhomogeneous points. Subsequently, all the solder powder is added at once, forming a grayish-white dry mass in the center of the cup. It takes about 7 minutes for the agitator to pull the dry mass into the masterbatch. Localized "dry powder islands" appear in the early stages of mixing, followed by shiny spots of localized flux enrichment in the later stages.

[0076] When rapidly evacuated to approximately 10 kPa absolute pressure, a distinct continuous bubble band appeared on the surface of the solder paste. The bubbles did not disappear uniformly; instead, they first expanded, ruptured, and then formed small pits in situ. Although the surface of the finished solder paste could be smoothed, microscopic observation revealed internal microbubbles ranging from 20 to 80 micrometers. The core problem for this group was that the gas source and liquid phase window were not staggered during preparation. During reflow, volatiles continued to be released in the latter part of the liquid phase holding stage, easily leading to large voids forming in the center of the thermal pads of quad flat leadless packages / bottom terminal components. 1.5 Comparative Example 2:

[0077] Controlled variables: Based on Example 1, the acid-amine-imidazolium-anhydride compatibility active system, bimodal powder, and segmented powder addition were retained; however, the early degassing solvent step one was omitted. Specifically, 5.376 grams of 2-ethyl-1,3-hexanediol were removed, and the mass was replenished with low-volatile resin solubilization components such as isobornylcyclohexanol and dibutyl sebacate.

[0078] The first half of the preparation process appeared very "beautiful." The resin dissolved more smoothly in the high-boiling solvent, and the system remained glossy and transparent throughout, with almost no brief microemulsification window as seen in Example 1. A torque peak also appeared during the addition of the acid-amine-imidazole, but the peak shape broadened and the decline rate slowed down, indicating that the active network was more strongly solvated and bound by the high-boiling solvent phase.

[0079] The real problem emerged during thermogravimetric analysis (TGA) pre-release and reflow simulation. The three-roll masterbatch lost weight very slowly in the 150–180°C range, as if it were being "held down" and not released; after rising close to the liquidus line of tin-silver-copper lead-free solder, the weight loss curve suddenly tailed. Under a hot-stage microscope, when the solder paste particles began to drip around 217°C, tiny bright bubbles could still be seen emerging from the powder gaps in the flux phase. These bubbles could escape on open glass slides, but in the low-gap, high-height structure of quad flat no-leads packages, they would be sealed by the edge molten pool.

[0080] The ointment preparation process itself did not present any significant problems. This study showed that, following the conventional approach that "the more stable the preparation and the less volatile the solvent, the better," the void ratio actually worsened significantly. This phenomenon indicates that the total volatile amount and the volatile sequence do not have the same impact on the void ratio. 1.6 Comparative Example 3:

[0081] Controlled variables: Based on Example 1, the ternary thermal spectrum solvent package, type IV / V bimodal powder, segmented powder addition, and 16.0 kPa vacuum degassing were retained; however, N-hydroxyethyl imidazole, 2-ethylimidazole, triethanolamine, succinic anhydride, glutaric anhydride, and reactive detectable active components were removed and replaced with free succinic acid / adipic acid / azelic acid of equal mass and approximately equal acid value.

[0082] This group can still form a clear masterbatch during the resin-solvent setup stage, but the physical state of the system changes significantly after the addition of the dicarboxylic acid: the acid crystals are more difficult to completely "absorb" into the resin phase at 72°C, resulting in a ring of fine white powdery mist adhering to the reactor wall. Because no amine / imidazole forms an acid-amine-imidazole compatibility state, the torque after adding free acid does not show a narrow peak and then decline, but rather increases slowly and monotonically. This increase is dangerous in the process: superficially it only appears as an increase in viscosity, but in reality it indicates an excessively strong interaction between the free acid and resin, and the free acid will be prematurely consumed when it subsequently contacts the powdered oxide film.

[0083] The absence of anhydride renders moisture management ineffective. Even if all raw materials are dried under the same conditions, after the masterbatch has cooled and stood for 12 hours, the Karl Fischer moisture content is still expected to rise to approximately 650–750 parts per million. This water is not simply free water, but rather bound water adsorbed by free diacid and the polar microdomains of the resin. Upon subsequent addition of powder, the solder paste begins to exhibit a "false thickness": the paste clumps together in the mixing cup, but when scraped, the surface is dark, slightly rough, and shows localized fine cracks.

[0084] During reflow, two types of failures coexist in this group: one is small bubbles generated by moisture / free acid, and the other is interface-type voids caused by insufficient removal of the oxide film. More serious is the surface insulation resistance. The free acid is not salinized and the residual network is not passivated. After reflow, it will exhibit a significant leakage risk when exposed to an 85℃ / 85% relative humidity bias environment. 1.7 Comparative Example 4:

[0085] Controlled variables: The total amount of components is exactly the same as in Example 1, but the key order of addition is eliminated. The dicarboxylic acid, imidazole / amine, acid anhydride, reactive detectable active component and thixotropic agent are added to the resin-solvent phase at 78.5°C at one time, and then dispersed by high shear, in order to replace the starvation drop-down and segmented compatibility of this case with "strong mixing".

[0086] This group is the best example to illustrate that "the same ingredients do not equal the same invention".

[0087] At the start of the operation, the resin-solvent phase remained clear. However, upon the simultaneous addition of the mixed acid / amine / anhydride / reactive detectable active component powder to the heated reactor, an uneven, light yellow cloud immediately appeared on the surface. Approximately 40 seconds later, visible flocculent bright spots, resembling tiny fish eyes floating in amber syrup, appeared in localized areas within the reactor. The torque curve did not exhibit the narrow peak of Example 1, but rather a stepped increase: initially rising rapidly by about 20%, then continuing to climb over the next 3 minutes. The reactor temperature also showed a wider exothermic shoulder; although the jacket controlled the overall temperature, irreversible acid-base-salt agglomeration had already occurred in localized micro-regions within the reactor.

[0088] Worse still, the acid anhydride undergoes rapid ring-opening under high temperature and localized high amine concentration, prematurely transforming into a highly polar salt / acid mixture; these microregions cannot be completely eliminated by the three-roller. When the thixotropic agent is also present, the polyamide wax fixes these polar agglomerates into the physical network, forming a masterbatch that "appears delicate but is actually microphase heterogeneous."

[0089] With the three-roller method, the outlet temperature of this group more easily reaches 47–50℃, and the surface of the discharged paste has a slightly granular feel. After adding powder, the yield stress of the paste increases significantly, resulting in poor release during printing with small openings. Even after preheating to 160–180℃, these microphase agglomerates still exhibit microphase aggregates, which can be verified by the particle highlights after the three-roller method, the yield stress at 180℃, and the transfer efficiency with small openings. This group shows that the same component will form different microphase structures under different feeding sequences, thus affecting printing release and recirculation venting. 1.8 Comparative Example 5:

[0090] Controlled variables: The material composition, feeding sequence, temperature, shear rate, and number of powder addition stages were the same as in Example 1, except that the final absolute pressure of vacuum degassing was changed from 16.0 kPa to 4.8 kPa, which is lower than the aforementioned lower limit of the process protection window.

[0091] This group exhibited almost identical performance to Example 1 in the flux masterbatch preparation and staged powder addition phases, with all early process phenomena showing good results: amber clear phase, torque peak during the dropping phase, thixotropic microemulsification, and a uniform silver-gray paste were all observed. Therefore, it is highly suitable for demonstrating the critical significance of vacuum parameters.

[0092] The anomaly occurred in the latter half of the vacuum ramp. When the pressure dropped to approximately 8–7 kPa, dense, silvery-white bubbles suddenly appeared on the surface of the paste, which previously only had pinpoint bubbles, spreading out like a very thin foam blanket from the area swept by the agitator. As the pressure continued to drop to 4.8 kPa, the local bubbles did not slowly rise and burst, but instead repeatedly "bulged-collapsed-bulged again." This phenomenon should not be simply attributed to the overall boiling of the high-boiling solvent; a more reasonable explanation is a combined failure caused by the rapid expansion of entrained gas according to Boyle's law, the evaporation of localized low-boiling / moisture micro-areas, and the bubble trapping effect of the high-solids-content powder network.

[0093] After 7.0 minutes, the surface foam may appear to disappear, but two types of irreversible changes have occurred in the system: The solvent was excessively removed during early degassing, and the weight loss ratio before the liquidus line in the thermogravimetric analysis curve was pushed to an excessively high range, making the solder paste prone to drying before reflow. The microbubbles formed during the sudden boiling are captured by the high solids content powder network, shrink into closed pores invisible to the naked eye after back pressure, and then grow again at the center of the solder joint after printing.

[0094] The finished product surface was drier and "harder" than in Example 1, the spatula peak height was higher but the fall was slower, and the small opening transfer efficiency decreased. This group shows that deeper vacuum is not necessarily better; below the critical absolute pressure, the degassing process will turn into a secondary bubble trapping process. II. Experimental Examples 1-3: Detection Items and Detection Basis

[0095] The testing system covers two categories of indicators: one is process / engineering kinetic indicators, such as thermogravimetric analysis weight loss time, downstream volatiles, rheological window, and printing transfer efficiency; the other is end-material / welding indicators, such as quad flat leadless package / ball grid array package voids, maximum single void, surface insulation resistance, residual glass transition temperature, and residual marker detection.

[0096] Solder joint voids were detected using two-dimensional X-ray imaging or three-dimensional computed tomography (CT) to measure the average void rate, 95th percentile void rate, and maximum single void rate of quad flat leadless packages, as well as the average void rate of ball grid array packages. Residual reliability was tested using a surface insulation resistance testing system and a comb plate under 85°C / 85% relative humidity and bias conditions; the residual glass transition temperature after reflow could be determined using differential scanning calorimetry or dynamic mechanical analysis. Residual markers were detected using liquid chromatography-high resolution mass spectrometry (LC-HPLC), Fourier transform infrared spectroscopy (FTIR), or fluorescence spectroscopy as internal control methods, not as industry-standard release criteria.

[0097] Slump performance was tested using stencil printing, hot air oven, and microscope to measure cold slump and hot slump bridging distance; solder ball and wetting performance were tested using reflow oven, optical microscope, or wetting balancer to measure solder ball grade and spread rate; small opening printing performance was tested using solder paste printing inspection instrument and three-dimensional automatic optical inspection instrument to measure the transfer efficiency and process capability index of openings with an area ratio of 0.66.

[0098] The volatiles in the later stage were analyzed using headspace gas chromatography-mass spectrometry (GC-MS) or pyrolysis-GC-MS to determine the peak area ratio of the volatiles and the peak area of ​​characteristic solvents or pyrolysis products. This method corresponds to the segmented sampling procedure of thermogravimetric analysis and is used to determine whether the gas generation in the later stage of the liquid phase is controlled. The viscosity of the solder paste was measured using a spiral pump viscometer or a crossbar rotor viscometer, and the apparent viscosity and thixotropic index at 25°C were tested. Thermodynamic analysis was performed using a rotational rheometer to determine the viscosity ratio at 160°C / 25°C and the yield stress at 180°C. During the test, the fixtures, nitrogen hood, heating rate, and sample anti-volatilization boundary should be clearly defined.

[0099] Thermogravimetric analysis (TGA) of weight loss includes total weight loss, weight loss percentage before the liquidus line, weight loss percentage during the liquidus holding phase, and maximum weight loss rate during the liquidus holding phase. This is performed using a thermogravimetric analyzer, and general TGA testing methods can be referenced. The solder liquidus window is measured using differential scanning calorimetry (DSC). The main melting peak, liquidus line, or endothermic peak endpoint of tin-silver-copper lead-free solder is recorded, and general methods for melting and crystallization temperature testing can be referenced. III. Experimental Examples 1–3: Performance and Engineering Test Results

[0100] The table below lists the results of thermodynamic weight loss, volatile matter, rheology, printing, void ratio, residual reliability, and marker detection for Example 1 and five comparative examples. Each sample group was tested in three independently prepared batches, with each batch containing at least 50 Quadrature Flat No-Leader (QFLOP) solder joints and 200 Ball Grid Array (BGA) solder joints. The QFLOP standard board used a uniform thermal pad and an 85% split opening, with a total solder paste coverage of 83–87% for a single thermal pad; reflow was performed in an air atmosphere, with a peak temperature of 242–246°C and a liquid phase hold time of 62–78 seconds; all void ratios were calculated as a percentage of the two-dimensional projected area in X-ray imaging.

[0101] Flux total weight loss / mass percentage at 30–260°C 16.92±0.74 18.41±1.05 17.36±0.92 17.28±0.86 17.71±1.18 13.84±0.97 Weight loss percentage before the liquidus 84.7±2.1 64.9±3.6 58.6±4.1 82.2±2.8 77.1±4.4 91.1±3.2 Weight loss percentage during liquid phase holding phase 9.6±1.3 27.4±3.2 33.2±4.5 12.6±2.1 18.9±3.6 4.7±1.0 Maximum rate of weight loss per second during liquid phase holding phase / percentage of mass 0.019±0.004 0.051±0.009 .060±0.012 0.026±0.006 0.044±0.010 0.010±0.003 Peak area ratio of volatiles in the latter part of the spectrum (headspace-gas chromatography-mass spectrometry analysis) 0.145±0.038 0.43±0.09 0.61±0.12 0.22±0.05 0.36±0.08 0.078±0.026 Vacuum degassing mass loss / mass percentage 0.26±0.07 0.61±0.18 0.21±0.06 0.30±0.08 0.44±0.15 1.18±0.31 Karl Fischer moisture content / 1 / million 286±54 648±91 301±57 772±115 436±82 268±62 Apparent viscosity at 25°C and 10 seconds⁻¹ (Pa·s) 1035±118 1168±164 912±103 1326±181 1715±246 1452±209 Viscosity ratio at 160℃ / 25℃ 0.19±0.03 0.33±0.05 0.24±0.04 0.27±0.05 0.42±0.08 0.11±0.03 Yield stress at 180℃ / Pa 58±11 138±27 83±18 101±23 176±39 46±14 Final Mixed 90-Second Torque Drift / Percentage 2.9±0.7 11.6±2.4 4.8±1.1 9.7±2.1 20.8±3.5 6.2±1.6 Area ratio 0.66 smaller opening transfer efficiency / percentage 90.8±3.4 83.9±5.1 88.1±4.0 79.6±6.3 66.8±7.9 74.5±6.8 Printing open time to transfer efficiency <85% / hour 10.6±1.1 11.7±1.4 12.3±1.2 8.2±1.0 6.4±0.9 7.9±1.3 Thermal collapse bridge distance / mm 0.11±0.04 0.23±0.07 0.17±0.05 0.21±0.08 0.07±0.03 0.15±0.05 Wetting and spreading rate / percentage 86.9±3.0 79.6±4.2 83.1±3.8 70.8±5.7 69.4±6.4 80.3±4.5 Average void ratio / percentage of quad flat no-leads thermal pads 3.18±0.86 12.10±2.95 15.44±3.88 8.72±2.46 11.38±3.42 7.64±2.18 Quad flat no-leads package with 95% void ratio / percentage 5.24±1.43 19.62±4.84 25.18±6.21 14.36±3.97 18.11±5.36 12.42±3.30 Quadrilateral flat no-leads package maximum single void ratio / percentage 5.82±1.75 18.77±5.21 24.63±6.88 13.94±4.12 17.85±5.44 12.06±3.68 Average void ratio / percentage of ball grid array package 1.42±0.46 6.28±1.61 8.10±2.14 4.72±1.39 5.71±1.64 3.92±1.21 Residual glass transition temperature after reflux / °C 56.9±2.8 43.6±3.6 50.4±3.2 33.1±4.8 45.9±4.2 63.7±3.5 Surface insulation resistance, logarithmic value with base 10 ohms 10.31±0.22 10.02±0.31 10.11±0.29 8.88±0.42 9.47±0.38 10.19±0.26 Probeability after reflux, contact anomaly rate / percentage 1.8±0.7 2.4±1.0 2.1±0.9 5.8±1.9 8.9±2.6 3.6±1.4 Reactive detection can detect active components or equivalent residues. Detectable, signal-to-noise ratio 18–42 Not detected Detectable, signal-to-noise ratio 16–38 Not detected Detectable but with peak-shaped tail Detectable, signal-to-noise ratio 14–35 Comprehensive judgment Core performance passed, but open time was slightly shorter than baseline. Gas production in the latter part of the liquid phase holding stage Liquid phase evaporation tailing Moisture / Free Acid / Surface Insulation Resistance Failure Micro-phase aggregation, poor release, and obstructed exhaust. Low-pressure expansion into bubbles, solvent loss IV. Data Analysis Conclusions of Experimental Examples 1–3 and Comparative Examples 1–5 4.1 Key Performance Improvements

[0102] Compared to the industry baseline of Comparative Example 1, Example 1 shows a significant improvement in low void performance, while the data retains the performance trade-offs that industrial materials should have: the print open time of Example 1 is slightly lower than that of Comparative Example 1 and Comparative Example 2, but the void ratio and surface insulation resistance reliability of Quad Flat No-Leader Package / Ball Grid Array Package are significantly better than those of the comparative examples.

[0103] The average void fraction of the quad flat no-lead package decreased from 12.10% to 3.18%, a reduction of approximately 73.7%.

[0104] The maximum single void ratio of the quad flat no-lead package decreased from 18.77% to 5.82%, a reduction of approximately 69.0%.

[0105] The average void fraction of the ball grid array package decreased from 6.28% to 1.42%, a reduction of approximately 77.4%.

[0106] The weight loss rate during the liquid phase retention stage decreased from 27.4% to 9.6%, a reduction of approximately 65.0%.

[0107] The peak area ratio of the volatiles in the latter part of the sample decreased from 0.43 to 0.145, a decrease of approximately 66.3%.

[0108] The transfer efficiency of the smaller opening with an area ratio of 0.66 increased from 83.9% to 90.8%, an increase of about 6.9 percentage points; however, the printing open time decreased from 11.7 hours in Comparative Example 1 to 10.6 hours, reflecting the reasonable process cost brought about by the forward shift of the low-void thermal spectrum.

[0109] Compared to Comparative Example 3, the surface insulation resistance of Example 1 increased from 8.88 ohms to 10.31 ohms, corresponding to an increase of approximately 26.9 times in geometric mean.

[0110] These data indicate that this case is not a "single-index optimization of low-voidity," but rather an engineering-acceptable comprehensive balance among low-voidity, small-aperture printing, wetting, residual reliability, and residual detectability. 4.2 Conclusions of Data Analysis of Five Comparative Proportions

[0111] Comparative Example 1 demonstrates that this is not a linear optimization of ordinary no-clean low-void solder paste. Comparative Example 1 uses a conventional rosin / high-boiling solvent / free acid system, and its preparation process is close to the traditional solder paste production logic. It did not have any obvious operational problems during the preparation stage, but the weight loss rate in the final liquid phase holding stage was as high as 27.4%, and the average void ratio of the quad flat no-lead package reached 12.10%. This indicates that the main defect of the conventional system is not "inability to make solder paste," but rather the inability to control residual gas generation in the liquid phase.

[0112] Comparative Example 2 demonstrates that the key factor is a forward shift in the thermal spectrum, not simply lower solvent volatility. In Comparative Example 2, after removing the early degassing solvent, the preparation process was actually more stable and clearer, and the printing open time was also longer. However, the average void ratio of the quad flat no-lead package increased to 15.44%, and the maximum single void reached 24.63%. This group shows that low void ratio does not mean minimizing flux volatilization, but rather that the main volatiles must be released before the solder liquidus line.

[0113] Comparative Example 3 demonstrates that the compatibility-active / water-trapping system is not a dispensable adjuvant combination. Comparative Example 3 retained the thermal spectrum solvent package, but after removing imidazole / amine salt / acid anhydride, although the void ratio was lower than the industry baseline, wetting decreased and surface insulation resistance deteriorated. This group indicates that simply shifting the thermal spectrum forward cannot solve all the problems. The relevant system still needs to be verified by objective indicators such as the torque peak during the dropping stage, Karl Fischer moisture content, wetting and spreading rate, and surface insulation resistance to confirm whether oxide film removal, moisture capture, and low ion residue are simultaneously satisfied.

[0114] Comparative Example 4 demonstrates that the same components do not necessarily equate to the same technical solutions. Comparative Example 4 used the exact same components as Example 1, but scrambled the feeding sequence, omitting the starvation dripping and segmented compatibility processes. During its preparation, fish-eye-like microphase agglomeration, stepped torque increases, three-roll outlet temperature rise, and poor printing release occurred. Ultimately, the average void ratio of the quad flat leadless package was still 11.38%. This demonstrates that identical formulation components do not necessarily yield the same process results; the feeding sequence and the submerged dripping compatibility process significantly affect the microphase structure and venting behavior.

[0115] Comparative example 5 demonstrates that absolute vacuum is the critical line between life and death, and that deeper is not necessarily better. Comparative Example 5 changed the vacuum absolute pressure from 16.0 kPa to 4.8 kPa, keeping everything else unchanged. The result was low-pressure expansion and foaming, as well as solvent loss. The weight loss ratio before the liquidus line was pushed up to 91.1%, the small-aperture transfer efficiency dropped to 74.5%, and the average void ratio of the quad flat leadless package rebounded to 7.64%. This group demonstrates that the vacuum window has a clear critical significance: too shallow and degassing is insufficient; too deep and secondary foaming may occur due to the combined effects of entrained gas expansion, localized volatile micro-regions, and powder network bubble trapping.

[0116] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0117] Those skilled in the art will understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0118] In the several embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between apparatuses or units may be electrical, mechanical, or other forms.

[0119] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0120] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A multi-segment thermal spectrum-controlled low-voidity solder paste, characterized in that: include: The flux masterbatch comprises 880–896 parts by weight of tin-silver-copper lead-free spherical solder powder and 104–120 parts by weight of flux masterbatch; in the solder powder, type IV powder accounts for 78–85% by weight and type V powder accounts for 15–22% by weight; the flux masterbatch includes hydrogenated rosin, polymerized rosin, rosin pentaerythritol ester, 2-ethyl-1,3-hexanediol, diethylene glycol monohexyl ether, succinic acid, adipic acid, azelaic acid, N-hydroxyethylimidazolium, 2-ethylimidazolium, triethanolamine, succinic anhydride, glutaric anhydride, polyamide wax micropowder, and hydrogenated castor oil derivatives; the flux masterbatch is subjected to thermogravimetric analysis, with the liquidus line of the tin-silver-copper lead-free solder powder measured by differential scanning calorimetry as the boundary, the weight loss ratio before the liquidus line is 80–88%, the weight loss ratio during the liquidus holding stage is 6–13%, and the maximum instantaneous weight loss rate during the liquidus holding stage is not higher than 0.025 mass percentage / second.

2. The multi-segment thermal spectrum controlled low-voidity solder paste according to claim 1, characterized in that: The flux masterbatch comprises 26–30 parts by weight of hydrogenated rosin, 14–17 parts by weight of polymerized rosin, 11–14 parts by weight of rosin pentaerythritol ester, 4.5–6.5 parts by weight of 2-ethyl-1,3-hexanediol, 21–25 parts by weight of diethylene glycol monohexyl ether, 7.0–9.5 parts by weight of isoborneol cyclohexanol, 3.8–5.2 parts by weight of dibutyl sebacate, 1.0–1.5 parts by weight of succinic acid, 1.4–1.9 parts by weight of adipic acid, 1.3–1.8 parts by weight of azelaic acid, 0.7–1.1 parts by weight of N-hydroxyethylimidazolium, and 2-ethyl Imidazole 0.35–0.60 parts by weight, triethanolamine 0.15–0.35 parts by weight, succinic anhydride 0.7–1.1 parts by weight, glutaric anhydride 0.25–0.45 parts by weight, polyamide wax micro powder 3.3–4.4 parts by weight, hydrogenated castor oil derivative 2.5–3.4 parts by weight, non-fluorinated polyether modified surface tension modifier 0.20–0.40 parts by weight, hindered phenolic antioxidant 0.05–0.15 parts by weight, phosphite antioxidant 0.04–0.08 parts by weight, and oleamide rheology stabilizer 0.45–0.70 parts by weight.

3. The multi-segment thermal spectrum controlled low-voidity solder paste according to claim 2, characterized in that: The flux masterbatch also includes 0.05–0.15 parts by weight of a reactive, detectable active ingredient, which is 9-anthramethylsuccinic acid half ester-imidazolium salt.

4. The multi-segment thermal spectrum controlled low-voidity solder paste according to claim 1, characterized in that: The median particle size of the tin-silver-copper lead-free spherical solder powder is 24–30 micrometers, the 90% cumulative distribution particle size is 34–38 micrometers, the total oxygen content of the powder is 350–700 parts per million, the proportion of satellite powder is not higher than 2.0%, and the moisture content of the powder is not higher than 250 parts per million.

5. The multi-segment thermal spectrum controlled low-voidity solder paste according to claim 1, characterized in that: The peak area ratio of the volatiles in the later stage of the solder paste, as determined by headspace gas chromatography-mass spectrometry, is not higher than 0.20; the apparent vapor pressure safety ratio of the solder paste is 45–60, and the mass loss of the solder paste before and after degassing is not higher than 0.35% by mass; the apparent vapor pressure safety ratio is the ratio of the absolute pressure after degassing to the apparent mixed vapor pressure at 25°C.

6. A method for preparing low-voidity solder paste with multi-segment thermal spectrum regulation, characterized in that: include, Under nitrogen protection, 85–90% by mass of diethylene glycol monohexyl ether, isobornylcyclohexanol, dibutyl sebacate, and 2-ethyl-1,3-hexanediol are heated to 76–80°C, and hydrogenated rosin, polymerized rosin, and pentaerythritol rosin are added in batches. The temperature is then lowered to 70–73°C, and succinic acid, adipic acid, and azelaic acid are added. At 67–70°C, a premix formed by the remainder of diethylene glycol monohexyl ether, N-hydroxyethylimidazolium, 2-ethylimidazolium, and triethanolamine is added dropwise. Succinic anhydride and glutaric anhydride are added at 58–63°C. Polyamide wax micropowder and hydrogenated castor oil derivatives are dispersed at 74–78°C to obtain flux masterbatch. Tin-silver-copper lead-free spherical solder powder is then added in stages and degassed under vacuum slope.

7. The preparation method according to claim 6, characterized in that: 10–14 hours before operation, succinic acid, adipic acid, azelaic acid, succinic anhydride, glutaric anhydride, polyamide wax micro powder, and hydrogenated castor oil derivatives are vacuum dried at 42–48°C and absolute pressure of 6–10 kPa for 3–5 hours, cooled to 20–30°C, and then sealed for later use. The reactor body is vacuumed at 15–25 kPa and filled with dry nitrogen 2–4 times, and then purged until the oxygen content at the top of the reactor is not higher than 1,000 parts per million and the dew point is not higher than -35°C.

8. The preparation method according to claim 6, characterized in that: The premixed solution is added via a submerged dropper, with the outlet of the dropper located 12–18 mm below the liquid surface. The dropping rate is 0.35–0.45 mL / min. During the dropping process, the system temperature is maintained at 67.5–70.0 °C. After the dropping is completed, stirring continues until the torque drops back to 85–90% of its peak value. Succinic anhydride and glutaric anhydride are added at 0.18–0.35 parts by mass / min. After addition, the moisture content of the flux masterbatch is not higher than 1 / 350 million.

9. The preparation method according to claim 6, characterized in that: After the flux masterbatch is left to stand for 10–14 hours, it is ground by three rollers at a roller temperature of 22–28°C. The roller gaps are 35–45 micrometers, 20–30 micrometers, and 12–20 micrometers respectively. The discharge temperature of the third pass does not exceed 45.0°C. The tin-silver-copper lead-free spherical solder powder is added to the flux masterbatch in 3–5 stages in a low-humidity nitrogen environment. Each stage is 20–35% of the total solder powder. After each stage is added, the mixture is mixed for 120–180 seconds.

10. The preparation method according to claim 6, characterized in that: Vacuum ramp degassing is performed at an orbital speed of 6–10 rpm and a rotational speed of 3–6 rpm. The vacuum is reduced to an absolute pressure of 14–20 kPa at a descent rate of 12–20 kPa and maintained for 5–8 minutes. The apparent vapor pressure safety ratio is 45–60. The mass loss of solder paste before and after degassing is no more than 0.35% by mass. After degassing, the pressure is backfilled to atmospheric pressure within 90–150 seconds with dry nitrogen.

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