Tin-zinc alloy brazing material for welding thermal tripping device of piezoresistor type surge protection device and preparation method of tin-zinc alloy brazing material

Abstract

The invention discloses a tin-zinc alloy brazing material for welding a thermal tripping device of a piezoresistor type surge protection device and a preparation method of the tin-zinc alloy brazing material, the tin-zinc alloy brazing material comprises the following raw material components in percentage by weight: 9% of Sn, 5%-7% of Zn, 1%-8% of In and 1%-8% of Bi; and the balance of Sn and inevitable impurities. On the premise that the welding quality is guaranteed, the brazing alloy has the excellent thermal response characteristic, can resist a 8 / 20 microseconds waveform surge current test with the peak value being 50 kA without losing efficacy, can smoothly pass a 500 A short-circuit current test, and achieves balance between surge impact resistance and fusing reliability. While a low fusing threshold value (less than or equal to 190 DEG C) is maintained, the upper tolerance limit of the working temperature of the device is increased to 160 DEG C, and the device is suitable for a power system surge protection device welding scene requiring high reliability.

Description

Technical Field

[0001] The present invention relates to an alloy solder, in particular to a Sn–Zn–In–Bi quaternary alloy solder; specifically, it relates to a tin-zinc-based alloy solder for welding the thermal tripping device of a varistor type surge protection device and a preparation method thereof. Background Art

[0002] A surge protection device (SPD) is a device used to protect electronic equipment from surge current and transient voltage interference. The SPD limits the amplitude of the transient voltage by introducing additional resistance or capacitance into the circuit, thereby protecting the electronic equipment from damage. Common surge protection devices include transient voltage suppressors, electrostatic protection tubes, varistors, etc. At present, the main type of surge protection device selected is a varistor, which mainly consists of a varistor (MOV) and a thermal tripping device. When a transient voltage appears in the circuit, the varistor will quickly respond and limit the transient voltage within a safe range, thereby protecting the electronic components in the circuit. With the continuous increase in the number of electronic devices, the demand for surge protection devices is increasing, and higher requirements are also put forward for their protection performance.

[0003] Generally speaking, a surge protection device needs to be equipped with an internal thermal tripping device, which is connected to the varistor body through a solder. When the voltage borne by the MOV exceeds its rated value or a short-circuit current is generated due to an SPD failure, the varistor generates a large amount of heat and melts the solder, causing the thermal tripping device to act in time, disconnect the circuit, and prevent the failure from spreading to other devices or the entire electrical system.

[0004] Currently, Sn–58Bi and SAC305 are mainly used as two solders for welding the thermal tripping device and the varistor in the industry. However, when the application conditions are harsh and higher high-surge-current tolerance is required for the SPD, such as in a surge current test with a peak value of 50 kA for an 8 / 20 μs current wave, the thermal tripping device welded with the Sn–58Bi solder often cannot withstand the current impact and disconnects, resulting in misoperation of the thermal tripping device and the device being unable to provide effective lightning protection, that is, the Sn–58Bi solder joint will fail under the impact of a 50 kA surge current. At the same time, when welding with SAC305, although its surge current tolerance is strong, due to its high melting point and high strength characteristics, in a 500 A short-circuit current test, SAC305 cannot be melted in time and easily causes the varistor to be broken down by voltage, leading to device combustion and causing serious safety accidents.

[0005] Chinese invention patent application CN114346520A discloses a Sn-Zn-Bi-In lead-free solder and its preparation method, wherein each component includes Zn8%, Bi0-8%, In0.5% by mass percentage, and the sum of the mass percentages of the above components is 100%; its preparation method comprises weighing each raw material by mass percentage, and cleaning each component after weighing; putting the weighed Sn and Zn into a quartz crucible for melting, and pouring it into a mold to obtain alloy 1; melting alloy 1 with Bi and In, and pouring it into a mold to obtain alloy 2; pouring the smelted alloy 2 into the mold for cooling, and obtaining Sn-Zn-Bi-In lead-free solder. However, the Zn content of this technology deviates from the eutectic point of 9%, resulting in a large melting range of the solder and poor processing performance. At the same time, the In content of this technology is obviously too low, only 0.5%. The too low In content makes the solder insufficiently conductive. During the high peak surge current test, the current concentrated heating occurs in the local resistance too high area, causing the solder joint to fail and trip, and the test cannot be passed.

[0006] Chinese invention patent CN113146092B discloses a Sn-Bi-In-Zn alloy lead-free solder and its preparation method and application. The composition of Sn-Bi-In-Zn lead-free solder is as follows: Sn45.00-50.00%, Bi15.00-17.00%, In31.00-33.00%, Zn3.00-7.00%. This technology benefits from the interaction of four low-melting-point metal elements in the solder to form three different "phases". 0.2 Sn 0.8 , BiIn2, Zn, among which the BiIn2 phase with a relatively low melting point is formed, making its melting point relatively low, so that the Sn-Bi-In-Zn lead-free solder has good wetting properties, conductivity and soldering properties, and is suitable for 3DIC soldering processes, such as screen printing to form micro bumps, BGA, C-4 solder balls, reflow soldering and SMT assembly, etc. However, the In content of this technology is too high, which will lead to a sharp increase in costs on the one hand, and greatly reduce the melting point of the solder on the other hand; and the Bi content of this technology is also high, making the solder brittle, which is extremely unfavorable for impact experiments such as surge testing. Summary of the invention

[0007] The present invention aims to provide a solder joint of a tin-zinc alloy solder material which can be melted and tripped within 300ms under the heat transfer of a varistor, and has an electrical conductivity greater than 9.10×10 6 S / m, after a 50kA surge current impact test, the solder joints have no cracks, no melting, good mechanical properties and reliable connection. The invention relates to a tin-zinc alloy brazing material for welding a thermal release device of a varistor type surge protection device and a preparation method thereof.

[0008] The purpose of the present invention is achieved through the following technical solutions:

[0009] The invention discloses a tin-zinc alloy brazing material for welding a thermal release device of a varistor type surge protection device. The raw material components are composed of Sn, Zn, In and Bi. The weight percentages of Zn, In and Bi in the brazing material raw material are 9%, 5% to 7% and 1% to 8% respectively; the rest are Sn and unavoidable impurities.

[0010] To further achieve the purpose of the present invention, preferably, the melting temperature range of the tin-zinc alloy solder is 165°C to 190°C; the tin-zinc alloy solder is used to connect the electrode pins between the varistor and the thermal release device.

[0011] Preferably, the tin-zinc alloy solder is in the form of flakes, wires or powder.

[0012] Preferably, the powdered tin-zinc alloy solder has a particle size of 20-45 μm.

[0013] The preparation method of the tin-zinc alloy brazing material for welding the thermal release device of the varistor type surge protection device is as follows: weighing Sn, Bi, In, Zn block pure metals respectively according to the raw material formula; preheating the furnace and the container, adding Sn, Bi, In, Zn block pure metals for stirring and melting, controlling the melting temperature to be 300°C to 500°C, and the melting time to be 20 to 30 minutes, and forming the molten alloy after melting by atomization to obtain the brazing alloy powder; or pouring the molten alloy after melting into a preheated forming mold, cooling to obtain the brazing alloy ingot, and then rolling the brazing alloy ingot into a sheet, or then extruding the brazing alloy ingot and drawing it into a wire.

[0014] Preferably, in the raw materials, the purity of the Zn, In, Bi and Sn is greater than 99.9%; the bulk pure metals of Sn, Bi, In and Zn are added in sequence.

[0015] Preferably, the atomization molding of the molten alloy to obtain the solder alloy powder is performed by spraying the molten alloy liquid through a nozzle by atomization, and simultaneously impacting and breaking the sprayed alloy liquid flow with a high-speed airflow or a high-pressure water flow to disperse the alloy liquid into fine droplets, and obtaining the alloy solder powder after solidification.

[0016] Preferably, the rolling and processing of the solder alloy ingot into a sheet shape is to mechanically roll the alloy ingot through a rolling mill and then cut it into a sheet shape;

[0017] The method of drawing the solder alloy into a wire shape after casting and extrusion is to extrude the ingot from the die hole of the extrusion die and then draw the ingot into the wire shape for multiple times.

[0018] Preferably, for sheet and filamentous tin-zinc-based alloy solders, during melting, 20-25 g of eutectic salt is added to every 100 g of solder to isolate oxygen, and the eutectic salt is composed of potassium chloride and lithium chloride mixed in a mass ratio of 1.3-1.5:1.

[0019] Preferably, a vacuum melting furnace is used to prepare the solder alloy powder by the atomization method; when preparing sheet and filamentous solders, the furnace is a titanium-tin furnace or a crucible.

[0020] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0021] 1. The Sn–Zn–In–Bi quaternary alloy solder provided by the present invention has a conductivity greater than 9.10×10 6 S / m. After undergoing a 50 kA surge current impact test, the solder joints have no cracks, no melting, good mechanical properties, and reliable connection.

[0022] 2. The Sn–Zn–In–Bi quaternary alloy solder provided by the present invention has a melting range controlled at 164-190 °C. During the 500 A short-circuit current test (SCCR) required by UL1449 and the National Electrical Code (NEC), the solder joints formed by the solder of the present invention can be fused and tripped within 300 ms under the heat transfer action of the varistor. Under the same test conditions, the tripping time of the above solder is shorter than that of SAC305, and the safety is greatly improved.

[0023] 3. The Sn–Zn–In–Bi quaternary alloy solder provided by the present invention adds appropriate amounts of In and Bi to the Sn–Zn solder system. Adding the In element can reduce the melting point, improve fluidity and wettability, and promote the solder to quickly wet and flow to the surface of the welded joint during welding to form a uniform weld, which can improve the ductility of the solder joint; at the same time, In can form a dense oxide film on the surface of the solder, improve the oxidation resistance of the solder, and ensure the stability of the solder performance; adding the Bi element can improve the wettability of the solder, reduce the melting point of the solder, and enhance the mechanical properties of the solder joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a differential scanning calorimetry (DSC) curve of the solder alloy in Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] The following further describes the present invention in detail with specific embodiments, but the implementation manners of the present invention are not limited thereto. During the specific implementation process,

[0026] According to the purpose of the present invention, the solder alloy applicable to the thermal tripping device of the varistor type surge protection device needs to meet the following requirements: it can resist the 8 / 20 μs waveform surge current test with a peak value of 50 kA without failure, so as to ensure that the device can remain normal and not fail to disconnect under the impact of the working surge current; when dealing with the short-circuit current, it can successfully pass the short-circuit current test of 500 A to ensure rapid fusing when the circuit suffers from the short-circuit current. Since there is a certain antagonism between the performance requirements of the surge current test and the short-circuit test. Therefore, it is relatively difficult to find a solder that can be compatible with both the surge test and the short-circuit test. However, the present invention discovers that there is a relatively narrow regulation range between meeting the performance test requirements of these two seemingly conflicting tests, and a solder can be realized that can be timely fused and tripped when a short-circuit current is generated due to a line fault, and will not fail to trip under the impact of a large surge current, thus solving the technical problems in the field of lightning protection devices.

[0027] The present invention discovers that in the Sn–Zn–Bi–In solder system, based on Sn–9Zn, the electrical conductivity, melting point, and thermal strength of the solder, the three main solderability properties, are dynamically adjusted by Bi and In elements to be in a balanced range, simultaneously meeting the surge current test and the short-circuit test. The solder based on the Sn–9Zn composition is a eutectic solder, and its melting point is 198 °C, between Sn–58Bi and SAC305. On this basis, adding Bi and In elements can ensure that the melting range is not too large, making it easier to prepare. Therefore, the tin-zinc-based alloy solder for welding the thermal tripping device of the varistor type surge protection device provided by the present invention has raw material components composed of Sn, Zn, In, and Bi: Zn, In, and Bi respectively account for 9%, 5% - 7%, and 1% - 8% of the weight of the solder raw materials; the rest is Sn and inevitable impurities. In the present invention, the addition of Bi element mainly reduces the melting temperature range of the solder, which is beneficial to passing the 500 A short-circuit current test, but the addition of Bi element will reduce the electrical conductivity of the solder, which is not conducive to passing the 50 kA surge current test. The addition of In element mainly improves the electrical conductivity of the solder, balancing the decrease in electrical conductivity caused by the addition of Bi element, thus achieving a performance balance between the surge current test and the short-circuit test, and at the same time, it can also slightly reduce the melting temperature range of the solder and appropriately improve the wettability of the solder.

[0028] Existing technologies such as Sn–58Bi and SAC305 can only meet one of the surge current test and the short-circuit test. The melting point of Sn–58Bi (138 °C) is relatively low and its electrical conductivity is poor (2.33×10 6S / m), in the 50kA surge current test, the device temperature will reach above 130°C. At the same time, due to its poor conductivity, the current concentration in the local area of ​​the solder will make the temperature far exceed its melting point, so it will melt and trip and fail. However, due to the high melting point (217°C) and high strength of SAC305, it takes a longer time to reach its melting point and melt and trip during the short-circuit current test. The test found that if this time is too long, the device will collapse and catch fire before tripping. Compared with Sn-58Bi and SAC305, the Sn-Zn-Bi-In solder proposed in the present invention controls the melting range of the solder at 164~190°C through the two elements Bi and In, and the conductivity is controlled at 9.10×10 6 S / m or above, and the thermal strength softening point is controlled at about 150°C, and finally the solder of the present invention can meet both the surge current test and the short circuit test.

[0029] It should be noted that, although the Sn-Zn-Bi-In system has been disclosed in the prior art, these technologies may have obvious differences in the proportion of Sn-Zn-Bi-In due to different specific uses, and such a ratio and use are greatly different from the requirements of the MOV electrode interconnection solder joints of the surge protection device of the present invention, and cannot meet the requirements of the present invention. Based on the discovery of the present invention, it is found that the Sn-Zn-Bi-In system lead-free solder of the Chinese invention patent application CN114346520A includes Zn8%, Bi0~8%, In0.5% by mass percentage. When the sum of the mass percentages of the above components is 100%, the In content of 0.5% is too low, which makes the solder conductivity insufficient. During the high peak surge current test, the current concentrated heating occurs in the local resistance too high area, causing the solder joint to fail and trip, and fail to pass the test. Another example is the Sn-Bi-In-Zn alloy lead-free solder in China's invention patent CN113146092B, which contains 45.00-50.00% Sn, 15.00-17.00% Bi, 31.00-33.00% In, and 3.00-7.00% Zn. The In content is too high, which leads to a sharp increase in cost and greatly reduces the melting point of the solder, making it unable to meet the high-temperature storage test. In addition, during the high-peak surge current test, the device heats up more than the melting point of the solder and fails the test. Moreover, the excessive Bi content makes the solder brittle, which is extremely disadvantageous for impact tests such as surge tests.

[0030] The melting temperature range of the solder prepared by the present invention can be controlled within the range of 164°C to 190°C by adjusting the composition, and the brazing temperature range is 230°C to 270°C. The solder alloy provided by the present invention has excellent thermal response characteristics while ensuring the welding quality. The alloy system is optimized by element ratio, and can resist the 8 / 20μs waveform surge current test with a peak value of 50kA without failure, and can smoothly pass the 500A short-circuit current test, achieving a balance between surge impact resistance and fuse reliability. Compared with conventional low-melting point solders, this solder increases the upper limit of the device's operating temperature tolerance to 160°C while maintaining a low fuse threshold (≤190°C), and is suitable for welding scenarios of power system surge protection devices that require high reliability. Moreover, on the basis of ensuring the above two key performances, the present invention also has the same or similar mechanical properties as other solders.

[0031] It can be seen that the solder of the present invention has excellent electrical conductivity, suitable melting range and good thermal stability. It has higher strength, thermal stability and electrical conductivity than the existing Sn-58Bi solder, and can show excellent reliability under 50kA surge current impact. At the same time, it has a lower melting temperature than SAC305, and has a rapid thermal response time and a shorter fusing time under 500A short-circuit current test conditions.

[0032] The tin-zinc alloy solder of the present invention is mainly used for connecting the electrode pins between the varistor and the thermal release device; after testing, the melting temperature range of the tin-zinc alloy solder is 164°C to 190°C; the shape of the tin-zinc alloy solder is mainly in the form of flakes, filaments or powders. The particle size of the powdered tin-zinc alloy solder is preferably 20-45 μm.

[0033] For tin-zinc alloy brazing materials, the main technical indicators are the formula. As for the preparation method, although there are certain differences in the preparation of different formulas, the main methods and equipment are all available for reference in the prior art, especially the Sn, Bi, In, Zn formula system of the present invention. The prior art already has the same raw materials, but the dosage formula is different. Based on the prior art and combined with the formula system of the present invention, the preparation method of the tin-zinc alloy brazing material for welding the thermal release device of the varistor type surge protection device of the present invention is as follows: weigh Sn, Bi, In, Zn bulk pure metals respectively according to the raw material formula; preheat the furnace and container, add Sn, Bi, In, Zn bulk pure metals for stirring and melting, control the melting temperature to 300℃~500℃, the melting time to 20~30min, and the molten alloy after melting is atomized to obtain the solder alloy powder; or pour the molten alloy after melting into a preheated molding mold, and obtain the solder alloy ingot after cooling, and then roll the solder alloy ingot into a sheet, or cast and extrude the solder alloy and then draw it into a wire.

[0034] In the above preparation method, the raw materials are required to have a purity of Zn, In, Bi, and Sn all greater than 99.9%; preferably, the bulk pure metals of Sn, Bi, In, and Zn are added in sequence. During melting, they are added in sequence according to the order of elemental metal Sn, elemental metal Bi, elemental metal In, and elemental metal Zn.

[0035] For the sheet solder, in the above measures, the alloy ingot of the solder is rolled and processed into sheets by mechanically rolling the alloy ingot through a rolling mill and then cutting it into sheets.

[0036] For the wire solder, in the above measures, the alloy ingot of the solder is extruded and drawn into wires by extruding the ingot through the die hole of an extrusion die and then undergoing multiple drawing processes to form wires.

[0037] In the above measures, for the sheet and wire tin-zinc-based alloy solders, during melting, 20 - 25 g of eutectic salt is added per 100 g of solder to isolate oxygen. The eutectic salt is composed of potassium chloride and lithium chloride mixed in a mass ratio of 1.3 - 1.5:1. Due to the density difference, the alloy liquid will sink to the bottom of the titanium-tin furnace, and the molten eutectic salt will float on the surface to isolate oxygen. After the alloy ingot is formed, the eutectic salt will re-solidify on the upper surface of the ingot, and separation from the alloy ingot can be achieved.

[0038] For the powder solder, in the above measures, the molten alloy is formed into solder alloy powder by atomization. The molten alloy liquid is sprayed through a nozzle by atomization method, and at the same time, a high-speed gas flow or high-pressure water flow impacts and breaks the ejected alloy liquid stream to disperse the alloy liquid into fine droplets, and alloy solder powder is obtained after solidification. When preparing alloy powder by atomization method, a vacuum melting furnace is used, and no additional covering agent is required to isolate oxygen.

[0039] When preparing solder alloy powder by atomization method, a vacuum melting furnace is used; when preparing sheet and wire solders, the furnace used is a titanium-tin furnace or a crucible.

[0040] As a preferred technical solution of the present invention, a method for preparing sheet and wire-shaped tin-zinc alloy brazing materials for welding thermal release devices of varistor-type surge protection devices is provided: Sn, Bi, In, and Zn bulk pure metals are weighed respectively according to the raw material formula; a furnace and a container are preheated, a eutectic salt of potassium chloride and lithium chloride is added to the container, the temperature is increased to melt the eutectic salt of potassium chloride and lithium chloride, Sn, Bi, In, and Zn bulk pure metals are added, stirred and smelted, the smelting temperature is controlled to be 300°C to 500°C, the smelting time is 20 to 30 minutes, the liquid metal is poured into a preheated molding mold, and a brazing alloy ingot is obtained after cooling; the brazing alloy ingot is processed into a sheet or wire-shaped tin-zinc alloy brazing material. The solder alloy is melted in a titanium-tin furnace. When the alloy mass is less than 500g in small batches, a corundum crucible is used as the container. When the alloy mass is greater than 500g, a titanium-tin furnace filled with tin-free liquid can be used as the container. The heating is divided into three stages: first, the titanium-tin furnace is heated to 300°C, and sufficient eutectic salt is added to the crucible or titanium-tin furnace; the temperature is further raised to 500°C to melt the eutectic salt, and after the eutectic salt is completely melted, the single metals Sn, Bi, In, and Zn are added in sequence. When preparing sheet and wire alloy solder, a lead-free titanium-tin furnace is used to melt the alloy ingot, and the air is isolated by the molten eutectic salt of lithium chloride and potassium chloride. Finally, the alloy ingot is mechanically rolled into a sheet or the solder alloy wire is obtained by drawing and multiple extrusions.

[0041] As another preferred technical solution, the present invention directly obtains alloy brazing material powder by atomizing the liquid alloy in the vacuum melting furnace during the preparation of alloy powder. Specifically, during smelting, Sn, Bi, In, and Zn elements are introduced in the form of metal single substances, and a metal block with a mass purity of not less than 99.9% is used as a raw material. The materials are prepared according to the nominal composition of the alloy. During smelting, the temperature of melting and mixing each metal single substance is 300°C to 500°C, and the mixture is kept warm and stirred for 20 to 30 minutes. When preparing alloy powder, a vacuum melting furnace is used for smelting, and the molten alloy liquid is sprayed out through a nozzle by atomization. At the same time, a high-speed airflow or a high-pressure water flow is used to impact and break the sprayed alloy liquid flow to disperse it into fine droplets, and the alloy brazing material powder is obtained after solidification.

[0042] The test method is described as follows:

[0043] Peeling force at room temperature: Use the prepared solder to weld two copper plates together to form a lap specimen. Use a tensile tester to break the lap specimen and test the peeling force when the solder joint breaks.

[0044] Conductivity test: Prepare the solder sample into a suitable shape and size, place it on the sample stage of the four-probe tester, and ensure that the probe is in good contact with the sample. The conductivity of the solder can be obtained through the four-probe tester.

[0045] 50kA Surge Test: Use a surge generator with a maximum surge current exceeding 50kA. First, connect the lightning protection device to the output terminal of the surge generator. By setting the parameters of the generator, make it output a single 50kA surge current with an 8 / 20μs waveform and apply it to the lightning protection device of the thermal trip device soldered with the solder developed by the present invention. If the welding area breaks and trips after the test, the test fails; if the solder joint is undamaged, the test passes.

[0046] 500A Short - circuit Current Test:

[0047] Use a surge generator that can provide a maximum short - circuit current exceeding 500A. Adjust the working model to a continuous - output alternating - current mode. Set the short - circuit current to 500A by adjusting the resistance box. Then connect the lightning protection device of the thermal trip device soldered with the developed solder to the output terminal of the surge generator. Click the start button of the surge generator to release the instantaneous current and voltage, so that the lightning protection device bears a continuous voltage. Continuously passing current will cause the device to heat up rapidly and melt the solder joint, making the thermal trip device act to cut off the circuit. The oscilloscope synchronously collects the waveform after the test, and the current duration and melting time can be recorded through the waveform record. If the device has a thermal breakdown and catches fire before the thermal trip, the test fails; the passing state of the test is that the solder joint melts and trips in time and the device does not catch fire.

[0048] Example 1

[0049] Use an analytical balance to weigh the following block - shaped pure metals in weight percentages for formulation: Zn 9%, In 7%, Bi 1%, and the rest is Sn. Among them, the purities of metals Zn, In, Bi, and Sn are all greater than 99.9%.

[0050] Prepare an ingot: Taking the preparation of 100g of solder as an example, place the alumina ceramic crucible and the ingot - forming mold in a 200°C oven for preheating. At the same time, set the temperature of the titanium - tin furnace to 200°C. After the actual furnace temperature reaches the set temperature, place the crucible on the titanium - tin furnace and set the temperature to 300°C. After the temperature reaches, add 50g of eutectic salt composed of potassium chloride and lithium chloride and set the furnace temperature to 500°C. At this time, the eutectic salt gradually melts into a molten state. Add the weighed pure metal blocks to the crucible in the order of Sn, Bi, In, Zn, and continuously stir with a glass rod to make the liquid alloy fully mixed evenly and expel the internal gas. After continuously stirring for 20 minutes, pour the liquid metal into the preheated forming mold. After it cools, remove the eutectic salt solid to obtain the solder alloy ingot.

[0051] Roll into sheets: Set the temperature of the two-roll mill to 100 °C and preheat for 30 minutes. Then, pass the alloy ingot through the mill and roll it repeatedly to obtain a sheet-shaped brazing filler metal with a thickness of 0.3 mm. During the rolling process, the rolling direction needs to be adjusted each time, making it perpendicular to the previous rolling direction. If the rolled sample is bent, the sample can be turned over and rolled one or more times. If cracks or serrated edges appear on the rolled sample, it needs to be trimmed to prevent crack propagation, and then continue rolling after trimming. The rolling speed is controlled at 2–4 r / min.

[0052] Cut the preformed solder sheet rolled to the target thickness into appropriate sizes. After thinly coating a layer of rosin-based flux on both sides, it can be used for welding between the varistor and the thermal trip device. After testing, the melting temperature range of this brazing filler metal is 164 °C to 187 °C, and the conductivity is 9.22×10 6 S / m, and it has good wetting and spreading properties on the base metal Cu. The surge protection device welded with this brazing filler metal can achieve timely fusing and tripping when a short-circuit current is generated due to a device failure, and at the same time, it will not fail to trip under the impact of a large surge current.

[0053] Figure 1 is the differential scanning calorimetry (DSC) curve of the solder alloy in Example 1. In the figure, the abscissa Temperature is the temperature (°C), and the ordinate Heat flow is the heat flow rate (W / g). The principle of DSC is to heat or cool the sample at a constant heating or cooling rate, and at the same time, a high-precision sensor measures the small heat changes of the sample during the heating or cooling process. If a heat-related phase change occurs in the sample, this process can be measured in real time. The melting range of the solder alloy can be obtained through DSC testing. From Figure 1 it can be seen that the melting range of the solder of the present invention is 164–187 °C, and this melting range can take into account both the surge current test and the short-circuit current test.

[0054] Example 2

[0055] Use an analytical balance to weigh the following bulk pure metals in weight percentages for formulation: Zn 9%, In 5%, Bi 3%, and the rest is Sn. Among them, the purities of the metals Zn, In, Bi, and Sn are all greater than 99.9%.

[0056] Prepare the ingot melt: Heat the vacuum induction melting furnace (or an electric resistance furnace) to 150 °C for preheating treatment to remove the moisture inside the equipment. Add the weighed pure metal blocks to the melting furnace in the order of Sn, Bi, In, and Zn, melt at 250 °C, and at the same time make the alloy composition uniform through a stirring device. The melting time is controlled within 25 minutes (it can be any time between 20 and 30 minutes). Since the atomization method requires an alloy melt, there is no need to cast and form when using the atomization method.

[0057] Preparation before atomization: Transfer the melted alloy melt to the tundish. Install a diversion tube at the bottom of the tundish and adjust the position and angle of the diversion tube. Turn on the atomizing gas (such as nitrogen, argon) supply system, stabilize the gas pressure at 0.5 - 1.5 MPa, and preheat to 150 - 250 °C.

[0058] Atomization process: Let the alloy melt flow out through the diversion tube at a stable flow rate and break into fine droplets under the high-speed impact of the atomizing gas. Control the matching relationship between the melt outflow rate and the gas pressure and flow rate to ensure the atomization effect.

[0059] Cooling and solidification: The atomized droplets are rapidly cooled and solidified in an inert gas atmosphere to form alloy powder, and the cooling rate is controlled at 10 3 ~10 6 °C / s.

[0060] Powder collection: Classify and collect the atomized alloy powder through equipment such as a cyclone separator and a bag filter, and separate powders with different particle size ranges.

[0061] Sieving treatment: Use a vibrating screen to sieve the collected powder, remove powder particles that do not meet the particle size requirements, and the target particle size range is usually controlled at 20 - 150 μm.

[0062] Mix the solder powder with the target particle size and the rosin-based flux evenly at a mass ratio of 88:12 to obtain solder paste, which can then be used for welding between the varistor and the thermal trip device. Tested by the test method of Example 1, the melting temperature range of this brazing solder is 176 °C - 189 °C, the conductivity is 9.41×10 6 S / m, and it has good wetting and spreading properties on the base metal Cu. The surge protection device welded with this brazing solder can achieve timely fusing and tripping when a short-circuit current is generated due to device failure, and at the same time, it does not fail to trip under the impact of a large surge current.

[0063] Example 3

[0064] Use an analytical balance to weigh the following weight percentages of bulk pure metals for proportioning to prepare 5 kg of alloy solder: Zn 9%, In 7%, Bi 8%, and the rest is Sn. Among them, the purities of the metals Zn, In, Bi, and Sn are all greater than 99.9%.

[0065] Preparation of ingot: Select a titanium-tin furnace filled with Wuxi liquid, first add a sufficient amount of eutectic salt covering agent, heat up to 450 °C, wait for the eutectic salt to melt, then add the weighed pure metal blocks in the order of Sn, Bi, In, Zn, and melt at 450 °C. At the same time, use a stirring device to make the alloy composition uniform, and control the melting time at 20 - 30 minutes.

[0066] Mold Preparation: Select a suitable ingot mold (such as a graphite mold or a metal mold), preheat the mold (control the temperature at 150°C), and spray a release agent on the inner wall of the mold to prevent the alloy ingot from sticking to the mold.

[0067] Casting Process: Slowly pour the melted alloy melt into the preheated mold through the casting port, control the casting speed to avoid splashing and porosity, and let it stand and cool after casting to solidify the alloy melt into an alloy ingot in the mold.

[0068] Demolding Process: After the alloy ingot cools to room temperature, remove the eutectic salt solidified on the top of the alloy ingot and then perform the demolding operation. Check the surface quality of the alloy ingot and remove surface defects such as burrs and flash.

[0069] Drawing Forming: According to the diameter of the target solder alloy wire, select a suitable drawing die and drawing equipment, adjust the drawing speed (generally 0.5 - 1.5 m / min) and tension to ensure a stable drawing process.

[0070] Multi - stage Drawing: Adopt a multi - stage drawing process to gradually draw the alloy rod to the required wire diameter size. Control the deformation amount of each stage below 20% to avoid wire breakage or performance degradation caused by excessive single - stage deformation. During the drawing process, regularly check the diameter and surface quality of the wire and adjust the process parameters in a timely manner. A hollow alloy wire is formed during the drawing process and flux is injected into it, and finally a solder alloy wire with a diameter of 1 mm is obtained.

[0071] The solder alloy wire can be directly used for welding between the varistor and the thermal trip device. After testing, the melting temperature range of this brazing solder is 164°C - 178°C, the electrical conductivity is 9.12×10 6 S / m, and it has good wetting and spreading properties on the base metal Cu. The surge protection device welded with this brazing solder can achieve timely fusing and tripping when a short - circuit current is generated due to device failure, and will not fail to trip under the impact of a large surge current.

[0072] Comparative Example 1

[0073] By weight percentage, the raw materials are composed of: Bi: 58%, Sn: 42%. Its preparation method includes the following steps:

[0074] Melt 42 g of Sn raw material at 500°C in a lead - free titanium - tin furnace, then add 58 g of Bi raw material, stir and keep warm for 30 minutes to obtain Sn - 58Bi solder, which is one of the commonly used solders in the prior art.

[0075] Comparative Example 2

[0076] The raw materials, by weight percentage, are composed of: Ag: 3%, Cu: 0.5%, Sn: 96.5%. Its preparation method includes the following steps:

[0077] In a lead-free titanium-tin furnace, 96.5 g of Sn raw material is melted at a temperature of 500 °C. Subsequently, 3 g of Ag and 0.5 g of Cu raw materials are added and stirred for heat preservation for 30 minutes to obtain SAC305 solder, which is one of the commonly used solders in the prior art.

[0078] Comparative Example 3

[0079] This comparative example is provided entirely in accordance with Chinese Patent Application CN114346520A.

[0080] The raw materials, by weight percentage, are composed of: Zn: 8%, Bi: 4%, In: 0.5%, Sn: 87.5%. Its preparation method includes the following steps:

[0081] In a lead-free titanium-tin furnace, 87.5 g of Sn raw material is melted at a temperature of 500 °C. Subsequently, 8 g of Zn, 4 g of Bi and 0.5 g of In raw materials are added and stirred for heat preservation for 30 minutes to obtain the solder Sn–8Zn–0.5In–4Bi described in Chinese Patent Application CN114346520A.

[0082] Comparative Example 4

[0083] This comparative example is provided entirely in accordance with Chinese Patent CN113146092B.

[0084] The raw materials, by weight percentage, are composed of: Zn: 5%, Bi: 36%, In: 32%, Sn: 27%. Its preparation method includes the following steps:

[0085] In a lead-free titanium-tin furnace, 27 g of Sn raw material is melted at a temperature of 500 °C. Subsequently, 5 g of Zn, 36 g of Bi and 32 g of In raw materials are added and stirred for heat preservation for 30 minutes to obtain the solder Sn–5Zn–32In–36Bi described in Chinese Patent CN113146092B.

[0086] The results of the solder composition ratios, basic solder performance tests, and solder surge tests in the examples and comparative examples of the present invention are shown in Tables 1, 2, and 3 below:

[0087] Table 1 Alloy compositions of Examples 1 to 3 and Comparative Examples 1 to 4

[0088] Solder alloy Composition Example 1 Sn–9Zn–7In–1Bi Example 2 Sn–9Zn–5In–3Bi Example 3 Sn–9Zn–7In–8Bi Comparative example 1 Sn–58Bi Comparative example 2 SAC305 Comparative example 3 Sn–8Zn–0.5In–4Bi Comparative example 4 Sn–5Zn–32In–36Bi

[0089] Table 2 Solder alloy evaluation results of Examples 1 to 3 and Comparative Examples 1 to 4

[0090] Solder alloy Room temperature peel force (N) Melting range (°C) Conductivity (S / m) Example 1 161 179～189 9.22 Example 2 180 176～189 9.41 Example 3 125 164～178 9.12 Comparative example 1 99 138 2.33 Comparative example 2 276 217 8.85 Comparative example 3 105 175～193 9.01 Comparative example 4 77 71～99 5.09

[0091] As can be seen from Table 2, under the same test conditions, the peel strength of the SnZn-based lead-free solder for the thermal protection varistor in the embodiments of the present invention is higher than that of the commonly used Sn–58Bi solder. The conductivity is about four times that of Sn–58Bi, slightly higher than that of SAC305 and Comparative Example 3, significantly higher than that of Comparative Example 4, and the melting temperature range is between Sn–58Bi and SAC305. It can simultaneously meet the requirements of high conductivity, a melting range of 164–190 °C, and strength between Sn–58Bi and SAC305. It can pass both the surge current test and the short-circuit current test, and is suitable for welding varistor type surge protection devices and other temperature-sensitive devices.

[0092] Table 3 Evaluation results of solder alloys for Examples 1 to 3 and Comparative Examples 1 to 4

[0093]

[0094] As can be seen from Table 3, under the same test conditions, the performance of the SnZn-based lead-free solder for the thermal protection varistor in the embodiments of the present invention in the surge test is much higher than that of the commonly used Sn–58Bi solder on the current market, and the performance in the short-circuit current test is much better than that of SAC305. The comprehensive test results are better than those of Comparative Example 3 and Comparative Example 4. The average fusing time can reflect the fusing performance of the solder. The fusing time of Comparative Example 2 is about 100 ms higher than that of other examples and comparative examples. Therefore, the passing rate of its short-circuit current test is relatively low. The average fusing time of the solder in the examples is all lower than 300 ms. Tripping within this time is safe and reliable for the device. The melting point of Comparative Example 3 is too high and the conductivity is too low, and the passing rate of the test cannot reach 100%. While the melting point of Comparative Example 4 is too low and no sample passes the high-peak surge current test.

[0095] After testing, the melting temperature range of the brazing solder obtained by the technical solution of the present invention is 164 °C to 190 °C; the brazing temperature range is 230 °C to 270 °C; different forms of solder are all suitable for welding between the varistor and the thermal trip device, and can withstand the impact of a 50 kA 8 / 20 μs surge current without failure; it can be smoothly disconnected in the 500 A short-circuit current test, achieving a balance between the anti-surge impact performance and the fusing reliability. While maintaining a low fusing threshold (≤190 °C), the present invention raises the upper limit of the device operating temperature tolerance to 160 °C, which is suitable for the welding scenario of high-reliability power system surge protection devices. Under a pressure ratio of 1.1, the average response time is 270 ms. The conductivity of the brazing solder is 9.19×10 6S / m, significantly higher than Sn–58Bi and SAC305. The tensile test results of the solder are stable, and its thermal strength is significantly higher than that of Sn–58Bi and only slightly lower than that of SAC305. Generally speaking, the present invention provides a multi-component lead-free tin-zinc-based solder alloy with high strength, high thermal stability, strong surge current impact resistance and appropriate melting range. The solder joints of the tin-zinc-based alloy brazing solder can be fused and tripped within 300 ms under the heat transfer action of the varistor, and the conductivity is greater than 9.10×10 6 S / m. After undergoing a 50 kA surge current impact test, the solder joints have no cracks, no melting, good mechanical properties and reliable connection. This tin-zinc-based alloy brazing solder is very suitable for connecting the electrode pins between the varistor and the thermal tripping device.

[0096] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: the technical solutions described in the foregoing embodiments can still be modified, or some or all of the technical features can be equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A tin-zinc-based alloy solder for welding the thermal trip device of a varistor type surge protection device, the raw material components are composed of Sn, Zn, In and Bi, and it is characterized in that: The weight percentages of Zn, In and Bi in the brazing material raw material are 9%, 5% to 7% and 1% to 8% respectively; the rest are Sn and inevitable impurities.

2. The tin-zinc-based alloy soldering flux for welding the thermal trip device of the varistor type surge protection device according to claim 1, characterized in that: The melting temperature range of the tin-zinc alloy solder is 164° C. to 190° C.; the tin-zinc alloy solder is used for connecting the electrode pins between the varistor and the thermal release device.

3. The tin-zinc-based alloy brazing filler metal for soldering the thermal trip device of the varistor type surge protection device according to claim 1, characterized in that: The tin-zinc alloy solder is in the form of flakes, wires or powders.

4. The tin-zinc-based alloy brazing filler metal for welding the thermal trip device of the varistor type surge protection device according to claim 3, characterized in that: The particle size of the powdered tin-zinc alloy solder is 20-45 μm.

5. The preparation method of the tin-zinc-based alloy brazing filler metal for welding the thermal trip device of the varistor type surge protection device according to claim 1, characterized in that: According to the raw material formula, Sn, Bi, In, and Zn bulk pure metals are weighed respectively; a furnace and a container are preheated, and Sn, Bi, In, and Zn bulk pure metals are added for stirring and melting, and the melting temperature is controlled to be 300° C. to 500° C. and the melting time is 20 to 30 minutes, and the molten alloy after melting is formed by atomization to obtain solder alloy powder; or the molten alloy after melting is poured into a preheated forming mold, and a solder alloy ingot is obtained after cooling, and then the solder alloy ingot is rolled into a sheet, or the solder alloy is cast and extruded and then drawn into a wire.

6. The preparation method of the tin-zinc-based alloy soldering flux for welding the thermal trip device of the varistor type surge protection device according to claim 5, characterized in that: Among the raw materials, the purity of Zn, In, Bi and Sn is greater than 99.9%; the bulk pure metals of Sn, Bi, In and Zn are added in sequence.

7. A method for preparing a tin-zinc-based alloy brazing filler metal for soldering a thermal tripping device of a varistor type surge protection device according to claim 5, characterized in that: The method of atomizing the molten alloy to obtain the solder alloy powder is to spray the molten alloy liquid through a nozzle by atomization, and at the same time use a high-speed air flow or a high-pressure water flow to impact and break the sprayed alloy liquid flow to disperse the alloy liquid into fine droplets, and obtain the alloy solder powder after solidification.

8. The preparation method of the tin-zinc-based alloy brazing filler metal for welding the thermal tripping device of the varistor type surge protection device according to claim 5, characterized in that: The rolling and processing of the solder alloy ingot into a sheet shape is to mechanically roll the alloy ingot through a rolling mill and then cut it into a sheet shape; The method of drawing the solder alloy into a wire shape after casting and extrusion is to extrude the ingot from the die hole of the extrusion die and then draw the ingot into the wire shape for multiple times.

9. The preparation method of the tin-zinc-based alloy brazing filler metal for welding the thermal tripping device of the varistor type surge protection device according to claim 8, characterized in that: For sheet and wire tin-zinc alloy brazing materials, 20-25 g of eutectic salt is added to every 100 g of solder during smelting. The eutectic salt is prepared by mixing potassium chloride and lithium chloride in a mass ratio of 1.3 to 1.5:

1.

10. The preparation method of a tin-zinc-based alloy solder for welding a thermal trip device of a varistor type surge protection device according to claim 5, characterized in that: When the solder alloy powder is prepared by the atomization method, a vacuum melting furnace is used for preparation; when the sheet and wire solder are prepared, the furnace is a titanium tin furnace or a crucible.

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