Halogen-free flux for lead-free solder wire, method for preparing the same, and tin-based flux-cored wire having the same

Abstract

The application provides a halogen-free flux for lead-free solder wire, a preparation method thereof and a tin-based flux-cored wire with the same, and relates to the technical field of fluxes. The halogen-free flux comprises the following components in parts by weight: 90-100 parts of rosin, 4.5-7.5 parts of active agent, 3.5-5.0 parts of tetrahydrofurfuryl alcohol, 0.4-0.8 parts of surfactant and 0.1-0.3 parts of benzotriazole; the active agent comprises at least one of organic acid active agent, amine active agent and ester active agent. The preparation method comprises the following steps: heating rosin to complete melting, adding other components when the temperature is lowered to 140-150 DEG C, and stirring. The tin-based flux-cored wire comprises a tin-based solder wire and the halogen-free flux filled in the tin-based solder wire. According to the application, the halogen-free organic acid, amine and ester active agent are adopted, corrosive hydrogen halide gas is avoided from being released during welding from the raw material end, and the application meets the environmental protection regulations such as RoHS.

Description

Technical Field

[0001] This application relates to the field of flux technology, and in particular to a halogen-free flux for lead-free solder wire, its preparation method, and a tin-based flux-cored wire having the same. Background Technology

[0002] Flux plays an indispensable role in the soldering process. Because the surface of the metal workpiece being soldered contains oxides, dust, and other contaminants, it easily hinders the atomic diffusion between the base metal and the solder. Therefore, it is necessary to remove these contaminants to expose the metal substrate. However, the atoms on the cleaned metal substrate surface are immediately oxidized in the atmosphere, especially at the high temperatures of soldering, where the oxidation rate is even faster. Therefore, it is necessary to add flux during the soldering process to help provide an oxide-free metal surface and maintain this oxide-free state until the solder and metal surface are successfully soldered. Simultaneously, through the chemical action of the flux, it combines with the oxides on the surface of the metal being soldered, forming a liquid compound at the soldering temperature. This allows the metal atoms on the surface of the metal being soldered to diffuse with the atoms of the molten solder, achieving the purpose of solder bonding. During the soldering process, flux also promotes the flow and diffusion of solder, influencing the balance of surface tension in the solder diffusion direction by reducing surface roughness.

[0003] In the electronic packaging industry, most widely used fluxes are rosin-based fluxes containing halogens. These fluxes do not meet the EU RoHS standard. Even halogen-free fluxes have problems such as insufficient activity, excessive smoke, and severe spatter. Summary of the Invention

[0004] In order to improve the problems of insufficient activity, excessive smoke, and severe spatter in existing halogen-free fluxes, this application provides a halogen-free flux for lead-free solder wire, its preparation method, and a tin-based flux-cored wire having the same.

[0005] In the first aspect, this application provides a halogen-free flux for lead-free solder wire, employing the following technical solution: A halogen-free flux for lead-free solder wire, characterized in that it comprises the following components in parts by weight: 90-100 parts rosin, 4.5-7.5 parts activator, 3.5-5.0 parts tetrahydrofurfuryl alcohol, 0.4-0.8 parts surfactant, and 0.1-0.3 parts benzotriazole; The surfactant includes at least one of organic acid surfactants, amine surfactants, and ester surfactants.

[0006] This application uses a high proportion of rosin as the main matrix of the halogen-free flux. After melting, it forms a continuous and dense protective film that covers the metal surface, isolating it from oxygen to inhibit re-oxidation during the soldering process. A small amount of surfactant reduces the surface tension of the flux, improves the wettability of the solder on the substrate surface, and enhances the fullness of the solder joint. Tetrahydrofurfuryl alcohol, as a co-solvent, adjusts the overall viscosity of the flux, ensuring activity while reducing post-soldering residue. Benzotriazole, as a corrosion inhibitor, forms a coordination film with the metal surface through nitrogen heterocycles, inhibiting post-soldering oxidation and corrosion of the metal substrate.

[0007] This application utilizes halogen-free organic acids, amines, and esters as activators to replace traditional halogen-containing activators, thus avoiding the release of corrosive hydrogen halide gases during soldering from the raw material stage. This complies with RoHS and other environmental regulations, reducing the risk of corrosion to the respiratory tract of operators and to welding equipment. The added activators effectively decompose metal oxide pools, improving the adhesion between the solder and the substrate. Furthermore, they facilitate a wider activation temperature range for the halogen-free flux, effectively enhancing the wetting ability of the lead-free solder on the substrate.

[0008] Among them, organic acid surfactants can react with metal oxides through acid-base reactions of carboxyl groups to generate soluble metal salts, thereby quickly removing the oxide layer at the welding interface; the amino groups in amine surfactants can form active salts with organic acids, preventing the rapid decomposition and failure of organic acids at high temperatures and extending the effective action time of organic acid surfactants; ester surfactants undergo hydrolysis at welding temperatures, slowly releasing carboxyl groups, thus providing continuous deoxidation capabilities.

[0009] Optionally, the ester activator is triethyl citrate or acetylated triethyl citrate.

[0010] During the welding heating process, triethyl citrate undergoes thermal decomposition, releasing citric acid. This citric acid gently dissolves and removes oxides from the surface of the substrate being welded, as well as the oxide film on the solder powder surface. Furthermore, the decomposition products of triethyl citrate are primarily citric acid and ethanol, which not only have low corrosiveness but also possess chelating ability, forming complexes with metal ions to further reduce the risk of corrosion from free metal ions. In addition, as an ester, triethyl citrate has a weak acidity, which helps improve the storage stability of the flux and reduces deterioration problems caused by excessive acidity during storage.

[0011] As an acetylated derivative of triethyl citrate, triethyl acetylacetate (AAC) has an acetyl group in its molecular structure that replaces the hydrogen atom of the terminal hydroxyl group of triethyl citrate through esterification. The introduction of the acetyl group blocks the hydroxyl group of triethyl citrate, forming a more stable ester bond. Therefore, AAC exhibits high high-temperature stability and will not decompose prematurely during flux storage or preheating, thus preventing a decrease in activation efficiency. Furthermore, AAC undergoes thermal decomposition during soldering heating, releasing citric anhydride, which can gently dissolve and remove oxides from the surface of the substrate being soldered and the oxide film on the surface of the solder powder. Compared to triethyl citrate, the decomposition product citric anhydride of AAC contains fewer free carboxyl groups, thus further reducing the corrosiveness of the residue.

[0012] Optionally, the organic acid activator is composed of adipic acid and octanoic acid mixed in equal masses.

[0013] Adipic acid and octanoic acid, as dicarboxylic acids with different carbon chain lengths, can undergo acid-base reactions with metal oxides to form soluble metal salts, thus removing the oxide layer on the surface of the metal substrate. Furthermore, adipic acid, with its shorter carbon chain, exhibits higher reactivity and can rapidly initiate the deoxidation process, while octanoic acid, with its longer carbon chain, demonstrates better thermal stability and can prolong the deoxidation time. The combination of these two can extend the effective action time of organic acid activators, achieving a rapid initiation and sustained gradient deoxidation effect, thereby improving welding efficiency.

[0014] Optionally, the amine activator is amine succinate.

[0015] The amino group in amine succinate can react with organic acid activators to form organic amine salts. These salts slowly decompose at the high welding temperatures, releasing organic acids and amines, thus prolonging the effective action time of the activator and preventing the organic acids from rapidly volatilizing or decomposing at high temperatures. Simultaneously, the amino group in amine succinate can neutralize acidic byproducts that may be generated during welding, reducing the risk of corrosion to the welding materials.

[0016] Optionally, the mass ratio of the organic acid surfactant, the amine surfactant, and the ester surfactant is 16-22:1-3:1.6-3.6.

[0017] This application limits the amount of organic acid activators, amine activators, and ester activators, which helps the flux to obtain a wider active temperature range and effectively improves the wetting ability of lead-free solder to the substrate. Under the above ratio conditions, the flux has high overall activity, no obvious residue after soldering, and no obvious corrosiveness to the substrate.

[0018] Optionally, the rosin is a mixture of aqueous white rosin and fully hydrogenated rosin, wherein the mass ratio of aqueous white rosin to fully hydrogenated rosin is 1.5 to 2:3.

[0019] This application uses a blend of aqueous white rosin and fully hydrogenated rosin as a carrier. Aqueous white rosin produces less smoke and leaves a transparent residue, while fully hydrogenated rosin exhibits good activity and excellent heat resistance. When the two are blended in an appropriate ratio, the requirements for activity, smoke, and heat resistance can be met to the greatest extent.

[0020] Secondly, this application also provides a method for preparing a halogen-free flux for lead-free solder wire, comprising the following steps: Add rosin to a container and heat until completely melted to obtain rosin melt; When the rosin melt cools to 140-150°C, add other components and stir continuously for 30 minutes to obtain the final product. Keep it warm for later use.

[0021] Thirdly, this application also provides a tin-based flux-cored solder wire, comprising a tin-based solder wire and a flux core, wherein the flux core is filled inside the tin-based solder wire, and the flux core is a halogen-free flux for lead-free solder wire.

[0022] Flux is used as a core filler inside the tin-based solder wire. During soldering, the flux melts before the solder, removing the oxide layer at the soldering interface in advance. After the solder melts, it quickly wets the substrate under the spreading effect of the flux, forming a strong solder joint. The rosin film left by the flux wraps the solder joint, preventing post-soldering oxidation and improving the long-term reliability of the solder joint.

[0023] Optionally, the lead-free solder wire uses halogen-free flux at a weight percentage of 2% to 3% of the tin-based flux-cored wire.

[0024] This application limits the amount of flux used in tin-based flux-cored wire, ensuring sufficient coverage of the welding interface without leaving excessive post-weld residue. Furthermore, the flux release rate matches the melting rate of the tin-based wire, ensuring a stable welding process.

[0025] In summary, this application includes at least one of the following beneficial effects: 1. This application uses halogen-free organic acids, amines, and esters as surfactants to replace traditional halogen-containing surfactants, thereby avoiding the release of corrosive hydrogen halide gas during welding from the raw material end, complying with RoHS and other environmental regulations, and reducing the risk of corrosion to the respiratory tract of operators and welding equipment.

[0026] 2. The halogen-free flux for lead-free solder wire provided in this application has the advantages of high activity, high corrosion resistance, low spatter, and low smoke emission. Attached Figure Description

[0027] Figure 1 This is a comparison chart of welding activity test results; Figure 2 This is a comparison chart of the corrosion test results of the circuit board after soldering. Figure 3 This is a comparison chart of the corrosion test results of photovoltaic panels after welding; Figure 4 This is a comparison chart of smoke performance results; Figure 5 This is a comparison chart of the splash performance test results. Detailed Implementation

[0028] The technical solution of the present invention will be further described in detail below through specific embodiments and comparative examples; however, the present invention is not limited to these embodiments.

[0029] Example 1 Example 1 provides a tin-based flux-cored solder wire. First, a halogen-free flux for lead-free solder wire is prepared. Its raw materials include rosin, activator, tetrahydrofurfuryl alcohol, surfactant, and benzotriazole. The surfactant used is FSN-100, and the specific dosage is shown in Table 1. The preparation method is as follows: Add rosin to a container and heat until completely melted to obtain rosin melt; When the rosin melt cools to 150°C, add the other components and stir continuously for 30 minutes to obtain the final product. Keep it warm for later use.

[0030] Then, using the prepared lead-free solder wire, flux-cored solder wire is prepared with halogen-free flux. The specific preparation process is as follows: First, pure copper and pure tin are heated to melt them. During the process of raising the temperature to 400°C, rosin, sulfur, sawdust, etc. are added to cover the tin surface. Then, the temperature is lowered to about 350°C, and Sn-P antioxidant alloy is added to obtain an intermediate mixture. The intermediate mixture was continuously stirred at 320℃ for 1 hour. The state of the tin surface was observed and samples were taken for testing. After passing the test, the mixture was cast to obtain Sn-0.7Cu alloy. The flux is heated to 110°C and stirred continuously for 30 minutes. Then, a vacuum is drawn to remove the exhaust gas, and high-pressure gas is introduced to fill the core. The solid ingot is extruded into a cored tin wire by a crystal hydraulic press. The cored tin wire is drawn to a wire diameter of 1.0 mm by multiple wire drawing machines to obtain tin-based flux-cored solder wire, in which the mass percentage of halogen-free flux is 2%.

[0031] Examples 2-4 Examples 2-4 are basically the same as Example 1, except that the amount and type of some raw materials are different. For specific amounts, please refer to Table 1. The preparation method is the same as in Example 1.

[0032] Comparative Example 1 Comparative Example 1 is basically the same as Example 1, except that the amount and type of some raw materials are different. For specific amounts, please refer to Table 1. The preparation method is the same as that of Example 1.

[0033] Table 1. Raw materials for each embodiment and comparative example. Performance testing Test 1: Welding Activity Test The tin-based flux-cored solder wires prepared in Examples 1-2 and Comparative Example 1 were subjected to welding tests at 300°C. The appearance and residue of the solder joints were observed. The results are shown in [reference needed]. Figure 1 Where a1 and a2 are solder joint images of the tin-based flux-cored wire corresponding to Example 1, b1 and b2 are solder joint images of the tin-based flux-cored wire corresponding to Example 2, and c1 and c2 are solder joint images of the tin-based flux-cored wire corresponding to Comparative Example 1.

[0034] from Figure 1 It can be seen that the solder joints welded by the tin-based flux-cored wire in Example 1 are bright and full, with regular shapes, relatively uniform distribution of multiple solder joints, relatively smooth surfaces, good overall appearance consistency, and relatively little residue, with relatively clean surroundings. The solder joints welded by the tin-based flux-cored wire in Example 2 have irregular shapes, obvious flow marks, and a tendency to fuse between multiple solder joints. The surface is not smooth enough, therefore, its appearance is slightly inferior to Example 1. The solder joints welded by the tin-based flux-cored wire in Comparative Example 1 show obvious diffusion marks around the solder joints, unclear solder joint boundaries, many unevenly colored areas inside the solder joints, and unclear boundaries and irregular shapes among multiple solder joints, resulting in poor appearance quality. Moreover, with the same quality of solder, under the action of different fluxes, the solder joint area welded by the tin-based flux-cored wire in Example 1 is larger, indicating better solder spread.

[0035] Test 2: Circuit Board Corrosion Test The tin-based flux-cored solder wires prepared in Examples 1-2 and Comparative Example 1 were used for circuit board soldering, and corrosion tests were performed. The soldered boards were placed in a constant temperature and humidity chamber at 40°C and 90% humidity for 72 hours for corrosion testing. The test results are shown below. Figure 2 Wherein, a1, b1, and c1 are the solder joint states of the tin-based flux-cored wires corresponding to Example 1, Example 2, and Comparative Example 1, respectively, when manually soldered at 360°C; and a2, b2, and c2 are the solder joint states of the aforementioned three after corrosion testing.

[0036] from Figure 2 It can be seen that the solder joints of the tin-based flux-cored wires corresponding to Examples 1, 2 and Comparative Example 1 after manual soldering are relatively uniform and full, with transparent and non-sticky residue. The tin-based flux-cored wire corresponding to Example 1 has less residue after soldering. After corrosion, the solder joints are basically similar, and none of them corrode the substrate.

[0037] Test 3: Corrosion Test of Photovoltaic Panels The tin-based flux-cored welding wires prepared in Examples 1-2 and Comparative Example 1 were used for photovoltaic panel welding, and then corrosion tests were performed. The welded specimens were placed in a constant temperature and humidity chamber, with one cycle consisting of 85°C and 85% humidity for 20 hours and then -40°C for 1 hour. A total of 7 cycles were performed, and the test results are shown below. Figure 3 Wherein, a1, b1, and c1 are the solder joint states of the tin-based flux-cored solder wires corresponding to Example 1, Example 2, and Comparative Example 1, respectively, when manually soldered with a soldering iron at 360°C; and a2, b2, and c2 are the solder joint states of the aforementioned three after corrosion testing.

[0038] from Figure 3 It can be seen that the solder joints of the tin-based flux-cored wires corresponding to Examples 1, 2, and Comparative Example 1 all exhibited the expected metallic luster after manual soldering. However, the solder joints of Examples 1 and 2 were more regular in shape and more evenly distributed, while the shape and distribution of the solder joints in Comparative Example 1 were slightly worse. After corrosion, the solder joints of Examples 1 and 2 still maintained good integrity, while the solder joints of Comparative Example 1 were in significantly worse condition, with rust and corrosion products appearing on the surface. The integrity of the solder joints was greatly affected, and the metallic luster was significantly weakened, indicating that its corrosion resistance was not as good as that of Examples 1 and 2.

[0039] Test 4: Smoke Performance Test The tin-based flux-cored solder wires prepared in Examples 1-2 and Comparative Example 1 were soldered using a 360℃ soldering iron. At the same feed length, smoke images were taken every 3 seconds from the moment the solder wire contacted the soldering iron tip. The results are as follows: Figure 4 As shown in the figure. Among them, a1-a5 are smoke images of the tin-based flux-cored wire corresponding to Example 1, b1-b5 are smoke images of the tin-based flux-cored wire corresponding to Example 2, and c1-c5 are smoke images of the tin-based flux-cored wire corresponding to Comparative Example 1.

[0040] from Figure 4 It can be seen that the tin-based flux-cored wire corresponding to Example 1 produced relatively stable and low levels of smoke throughout the welding process. The tin-based flux-cored wire corresponding to Example 2 produced more smoke than Example 1, and was slightly inferior to Example 1 in terms of smoke control. The tin-based flux-cored wire corresponding to Comparative Example 1 produced significantly more smoke than Examples 1 and 2, with dense smoke and a relatively large diffusion range.

[0041] Test 5: Splash Performance Test Place a thermal paper with a central hole on a flat surface 15mm from the bottom of the soldering iron tip. There are 18 concentric circles with a radius increasing by 10mm around the central hole. During the soldering process, hot flux droplets or solder balls will leave black spots on the surface of the thermal paper after they come into contact with it. Figure 5The figures show the spatter test results of the tin-based flux-cored wires prepared in Examples 1-2 and Comparative Example 1. Among them, a1-a3 are thermal paper characterization images of the tin-based flux-cored wire corresponding to Example 1 at 360℃, 380℃, and 400℃, respectively; b1-b3 are thermal paper characterization images of the tin-based flux-cored wire corresponding to Example 2 at 360℃, 380℃, and 400℃, respectively; and c1-c3 are thermal paper characterization images of the tin-based flux-cored wire corresponding to Comparative Example 1 at 360℃, 380℃, and 400℃, respectively.

[0042] from Figure 5 It can be seen that, at the same temperature, the tin-based flux-cored wire corresponding to Example 1 exhibits the least spatter, followed by Example 2, while Comparative Example 1 shows the most spatter. Moreover, the spattering significantly intensifies with increasing temperature.

[0043] In summary, the halogen-free flux for lead-free solder wire provided in Examples 1 and 2 of this application has the advantages of high activity, low spatter, and low smoke.

[0044] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A halogen-free flux for lead-free solder wire, characterized in that, It comprises the following components in parts by weight: 90-100 parts rosin, 4.5-7.5 parts activator, 3.5-5.0 parts tetrahydrofurfuryl alcohol, 0.4-0.8 parts surfactant, and 0.1-0.3 parts benzotriazole; The surfactant includes at least one of organic acid surfactants, amine surfactants, and ester surfactants; The ester activator is triethyl citrate or acetylated triethyl citrate; The organic acid surfactant is composed of adipic acid and octanoic acid mixed in equal mass. The amine activator is amine succinate; The mass ratio of the organic acid surfactant, the amine surfactant, and the ester surfactant is 16~22:1~3:1.6~3.

6.

2. The halogen-free flux for lead-free solder wire according to claim 1, characterized in that, The rosin is a mixture of aqueous white rosin and fully hydrogenated rosin, with a mass ratio of aqueous white rosin to fully hydrogenated rosin of 1.5 to 2:

3.

3. The method for preparing halogen-free flux for lead-free solder wire according to claim 1 or 2, characterized in that, Includes the following steps: Add rosin to a container and heat until completely melted to obtain rosin melt; When the rosin melt cools to 140~150℃, add other components and stir continuously for 30 minutes to obtain the final product. Keep it warm for later use.

4. A tin-based flux-cored solder wire, characterized in that, It includes a tin-based solder wire and a flux core, wherein the flux core is filled inside the tin-based solder wire, and the flux core is a halogen-free flux for lead-free solder wire as described in claim 1 or 2.

5. The tin-based flux-cored wire according to claim 4, characterized in that, The halogen-free flux used in the lead-free solder wire accounts for 2% to 3% of the total weight of the tin-based flux-cored solder wire.

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