A pre-tin coating process for laser welding

Abstract

This invention provides a pre-plated tin process for laser welding, comprising the following steps: Step 1: Inserting the substrate into the corresponding pad, then preheating the solder wire to a specified temperature. A pre-drilled hole is formed on the top of the pad, and pre-plated tin is placed inside the hole. A measured amount of flux is added to the surface of the pre-plated tin. Step 2: Transporting the pad to the welding station, using a welding torch to aim at the pad and activating the laser. Different preheating times and temperatures are used for pads of different sizes. Step 3: Continuously heating to raise the temperature of the pad and the pre-plated tin to the specified welding temperature. Step 4: After the pre-plated tin melts, controlling the feed speed of the solder wire to ensure it contacts the molten pre-plated tin. Simultaneously, the laser also melts the fed solder wire. Feeding is stopped after the solder wire reaches a specified length. Step 5: The molten solder wire fuses with the pad. Step 6: Insulating the weld joint. This method solves the problem of easily burning the pad in traditional welding processes, improving product quality.

Description

Technical Field

[0001] This invention belongs to the field of laser welding technology, and more specifically, relates to a pre-tin coating process for laser welding. Background Technology

[0002] Laser welding is a high-precision, non-contact welding method widely used in manufacturing. Welding power panels in photovoltaic projects is a crucial process that affects the performance and lifespan of solar panels. Traditional soldering, on the other hand, is a contact-based process that requires direct contact with the workpiece, easily leading to scratches. Furthermore, soldering requires frequent maintenance, increasing production costs.

[0003] For example, Chinese invention patent 202110660570.3 provides a welding device and welding process. This welding device, through the arrangement of a welding torch, a cooling chamber, and a rolling device, performs laser welding on the workpiece using the welding torch. Then, the rolling device compresses the molten pool produced during welding to increase the weld strength. Finally, the workpiece is transported to the cooling chamber for cooling, completing the welding operation. In existing technologies, traditional welding processes generally use a welding torch to perform laser welding on the workpiece. However, current laser welding technology is not mature enough. The laser energy emitted by the welding torch is too high, which can easily cause the welding pads of the workpiece to be unable to withstand the sudden increase in energy, resulting in the burning of the welding pads and the scrapping of the welded workpiece, thus reducing production efficiency. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a pre-tin coating process for laser welding, which solves the technical problems in the prior art where traditional welding processes are not mature enough, cannot accurately control the energy of the welding laser, easily lead to the burning and scrapping of solder pads, increase production costs, and reduce production efficiency.

[0005] The purpose and effect of the pre-coating tin process for laser welding of the present invention are achieved by the following specific technical means:

[0006] A pre-tin coating process for laser welding includes the following steps:

[0007] Step 1: Place the substrate on the corresponding pad on the PCB circuit board, prepare the solder wire, preheat it to the specified temperature, open multiple sets of pre-made holes on the top of the pad, set the pre-made solder coating in the multiple sets of pre-made holes, and add flux to the surface of the pad and the pre-made solder.

[0008] Step 2: Place the pre-tinned PCB board into the laser soldering station and align the soldering gun with the target pads. Activate the laser on the soldering gun and preheat the pads of different sizes using different preheating times and temperatures to ensure that the pads and the pre-tinned layer are preheated to a temperature below the melting point.

[0009] Step 3: Continuously heat the pads and pre-coated solder with a laser to raise their temperature to the specified soldering temperature, causing the pre-coated solder to completely melt into a liquid state and fuse with the pad surface. At this point, the required soldering temperature conditions have been met.

[0010] Step 4: After the pre-coated tin has melted, control the feeding speed of the tin wire to make it contact the molten pre-coated tin. At the same time, continuous laser irradiation also melts part of the fed tin wire. Stop feeding the tin wire after it has been fed to the specified length.

[0011] Step 5: The molten solder wire will mix and spread with the molten pre-coated solder on the pad, completing the soldering connection at the pad position;

[0012] Step Six: After the laser welding is completed, maintain a constant temperature to allow the weld to cool and solidify while keeping it fused.

[0013] As a further aspect of the present invention, in step one, the specified temperature of the preheated solder wire is 75-100°C.

[0014] As a further aspect of the present invention, in step one, the flux added to the solder pad and the pre-formed tin surface is 3.5 to 4 mg / drop.

[0015] As a further embodiment of the present invention, in step two, the pad is a double copper layer pad, the preheating time is 300-500ms, and the preheating temperature is 300-360℃.

[0016] As a further embodiment of the present invention, in step two, the pad is a four-copper-layer pad, the preheating time is 300-500ms, and the preheating temperature is 300-360℃.

[0017] As a further aspect of the present invention, in step two, the pad is a zero-copper-layer pad, the preheating time is 450-550ms, and the preheating temperature is 260-300℃.

[0018] As a further aspect of the present invention, in step three, the specified temperature for heating the solder pad and the pre-coated tin is 70–90°C.

[0019] As a further aspect of the present invention, in step four, the feeding speed of the tin wire is 15-20 mm / s.

[0020] As a further aspect of the present invention, in step four, the specified length for when the solder wire stops feeding is 12 to 17 mm.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. Compared with existing technologies, this method involves pre-drilling multiple pre-drilled holes on the top of the pads and pre-filling each hole with fusible pre-tin. The pre-tin is then pre-fused to the bottom surface of the pads using the heat of reflow soldering. This pre-treatment significantly enhances the heat conduction between the pads and the pre-tin, while also acting as a buffer and dispersing agent. The pre-tinned PCB is then placed in a laser soldering station, where a laser torch heats the surface for soldering. The pre-tin absorbs the high energy emitted by the laser, reducing the actual laser power received by the pads and preventing them from being burned by the high-energy laser. This solves the problems of traditional soldering processes being immature and unable to precisely control the laser energy, easily leading to pad burnout and scrap. It reduces scrap, lowers production costs, and minimizes resource waste.

[0023] 2. By incorporating flux, the laser soldering process facilitates heat conduction between the pads, pre-coated tin, and substrate, reducing thermal resistance and accelerating the temperature rise of both surfaces, thus promoting solder melting and flow. Secondly, the active ingredients in the flux actively remove oxides from the pads and pre-coated tin surfaces, ensuring metal-to-metal contact and preventing oxide films from affecting soldering quality. Finally, after soldering, residual flux reduces surface tension at the solder joint, removes surface oil, expands the solder area, and prevents re-oxidation, further improving solder joint quality and reliability.

[0024] 3. The quality of laser welding was further optimized by setting the solder wire preheating, solder feed speed, and solder feed length. First, preheating the solder wire to a certain temperature before laser welding reduces the temperature difference between the solder wire and the welding laser, avoiding thermal stress and solder ball formation caused by cold solder wire. Preheating also prevents wire blockage. Second, by setting the solder wire feed speed to match the liquid diffusion rate of the pre-coated solder, wire blockage is prevented. The wire feed speed also affects the welding yield. Finally, by setting the solder wire feed length, the coverage of the solder pads is ensured, preventing the welding laser from heating the pads for too long and burning them. The wire feed length also determines the processing time for a single solder joint, achieving a highly efficient and stable laser welding process.

[0025] 4. Preheating temperature and time are crucial for ensuring soldering quality. Excessive preheating temperature or time can overheat the pads and pre-coated solder, causing damage. Overheating also slows down the soldering cycle and reduces production efficiency. Conversely, insufficient preheating temperature or time will result in incomplete melting of the pads and pre-coated solder, leading to cold solder joints and lower solder joint quality. Therefore, based on the heat capacity characteristics of different pad sizes, a suitable preheating temperature and time are set to ensure the pads and pre-coated solder are fully molten, preventing both pad damage and cold solder joints, thus guaranteeing solder joint quality.

[0026] 5. Laser welding using a welding torch allows the laser beam to accurately irradiate the localized area of ​​the solder pads without affecting other components on the circuit board, avoiding potential damage from overheating. Furthermore, laser welding offers a fast response time, short single-point welding time, and high overall welding output and efficiency. As a non-contact process, the welding torch does not directly contact the substrate, avoiding scratches on the substrate surface caused by mechanical contact and improving product quality. Additionally, the small laser focus area of ​​the welding torch allows welding in confined spaces, expanding its applicability. Moreover, laser welding eliminates the need to replace soldering iron tips and other soldering materials, saving production costs. Attached Figure Description

[0027] Figure 1 This is a process flow diagram of a laser welding pre-coating tin process according to the present invention. Detailed Implementation

[0028] To further understand the present invention, preferred embodiments of the present invention will be described below in conjunction with the accompanying drawings and examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and are not intended to limit the scope of the claims of the present invention.

[0029] In the description of this invention, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," "outer," "front end," "rear end," "head," "tail," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0030] Example 1:

[0031] Please see as follows Figure 1As shown, the present invention provides a pre-tin coating process for laser welding, comprising the following steps:

[0032] Step 1: Place the substrate to be soldered onto the corresponding pad on the PCB board to complete the positioning. Prepare the solder wire and preheat it to the specified temperature. Preheating shortens the subsequent laser heating time and improves soldering efficiency. Multiple pre-drilled holes are made on the top of the pads; these holes serve as reserved solder storage areas. Prepare fusible solder into a paste and fill each pre-drilled hole to form a pre-formed solder coating. Apply an appropriate amount of flux to the pad surface and the pre-formed solder coating using a fine needle. The flux improves laser absorption and removes the oxide layer.

[0033] Step Two: The pre-tinned PCB board is placed in the laser soldering station. At the laser soldering station, a soldering gun with a precision positioning system is used, and its laser head is accurately aligned with the target pad. Then, the laser output of the soldering gun is activated, and the emitted laser light irradiates the pad and its surrounding pre-tin coating area for preheating. Depending on the pad size, pre-programmed matching parameters are used to set different preheating times and temperatures. This targeted preheating increases the temperature of the pad and pre-tin coating, fully considering the differences in heat capacity between different pads, making preheating more precise and effective.

[0034] Step 3: After preheating the pads and pre-molded solder for a certain period, laser irradiation will continue to further increase their temperature. As the laser continues to heat, the temperature of the pads and pre-molded solder will gradually rise, eventually reaching the pre-configured precise soldering temperature. At this temperature, the pre-molded solder layer will completely melt into a liquid state, increasing its fluidity. Simultaneously, the surface of the pads will also acquire sufficient molten state, fully fusing with the liquid pre-molded solder layer. At this point, the pads and pre-molded solder layer have completely transformed into a molten state, mixed uniformly together, and as a whole, have achieved the temperature conditions and physical state required for laser soldering.

[0035] Step 4: After the pre-coated solder is completely melted, the feeding speed of the solder wire needs to be precisely controlled to ensure it contacts the molten pre-coated solder. Simultaneously, laser irradiation continues during the solder wire feeding process to partially melt the solder wire and ensure a strong bond between the two. Stop feeding the solder wire once it reaches the designated length.

[0036] Step 5: When the molten solder wire comes into contact with the pre-coated solder, the materials begin to fuse and diffuse due to the temperature. This fusion and diffusion process allows the solder wire and pre-coated solder to mix thoroughly, forming a uniform solder. This uniform solder can fill the gap between the pad and the solder wire, and bond tightly to both, enabling it to withstand higher mechanical and thermal stresses and completing the soldering connection at the pad location.

[0037] Step Six: After laser welding is completed, temperature control is required. This keeps the weld joint within a suitable temperature range, allowing it to fully solidify. Temperature control reduces thermal stress and deformation during welding, thereby improving weld quality and reliability and ensuring the successful implementation of the welding process.

[0038] In step one, reducing the temperature difference between the solder wire and the welding laser avoids thermal stress and solder ball formation caused by cold solder. Preheating also prevents wire blockage. Wire blockage occurs when the solder wire, affected by laser energy, melts at the needle tip and jams the tip, preventing subsequent solder wire from being fed smoothly, thus triggering a "wire blockage" alarm. Through verification of the solder wire temperature, we ultimately adopted a preheating temperature range of 75–100℃. The effects of different preheating temperatures on solder ball formation and whether or not wire blockage occurs are detailed in Table 1 below.

[0039] Table 1 Verification of the effect of solder wire temperature on solder ball formation after soldering

[0040] Solder wire temperature (°C) Production of solder balls Is the solder wire clogged? normal temperature It produces many solder beads, with a diameter between 0.1 and 0.5 mm. no 25～50 A relatively large number of solder beads are produced, with a diameter between 0.1 and 0.5 mm. no 50～75 Few solder balls are produced, with diameters ranging from 0.1 to 0.35 mm. no 75～100 Almost no solder balls Almost no clogging 100～125 No solder balls Easy to clog wires

[0041] In step one, adding flux facilitates heat conduction between the pads, pre-coated tin, and substrate, accelerating the temperature rise of both. Secondly, flux removes oxides from the pads and pre-coated tin surfaces, preventing oxide films from affecting soldering quality. After soldering, residual flux reduces surface tension at the solder joint, removes surface oil, expands the soldering area, and prevents re-oxidation. Through flux validation, we ultimately adopted a dosage range of 3.5–4 mg / drop. Optimal results are achieved within this range. See Table 2 below for a comparison of the effects of different flux dosages on soldering.

[0042] Table 2 Verification of the effect of solder wire temperature on solder ball formation after soldering

[0043]

[0044] In step two, the pad is a double copper layer pad, the preheating time is 300-500ms, and the preheating temperature is 300-360℃.

[0045] In step two, the pad is a four-copper-layer pad, the preheating time is 300-500ms, and the preheating temperature is 300-360℃.

[0046] In step two, the pad is a zero-copper-layer pad, the preheating time is 450-550ms, and the preheating temperature is 260-300℃.

[0047] Different preheating times and temperatures were set for different specifications and models of solder pads to prevent pad damage and cold soldering, thereby ensuring solder joint quality. See Table 3 below for a detailed comparison of the effects of preheating time and temperature on soldering.

[0048] Table 3 Verification of the effects of preheating temperature and preheating time on pads and solder.

[0049]

[0050] In step three, to maintain the flux's activity and reduce the temperature difference between the electronic board and the welding laser, thus facilitating welding, we ultimately adopted an electronic board temperature range of 70–90℃ after verifying the temperature. The effects of different electronic board temperatures on welding are detailed in Table 4 below.

[0051] Table 4 Verification of the effect of electronic board temperature on welding

[0052]

[0053]

[0054] In step four, the wire feeding speed is adjusted to match the liquid diffusion rate of the pre-coated molten solder, preventing wire blockage. The feeding speed also affects the soldering yield. The feeding speed is influenced by the cylinder extension speed and the feeding speed of the solder breaking mechanism. If the feeding speed of the solder breaking mechanism exceeds 20 mm / s, slippage can easily occur due to the pulling force of the solder wire coil and the resistance of the already fed solder wire, thus affecting the amount of solder fed. Through verification of the feeding speed, we ultimately adopted a feeding speed in the range of 15–20 mm / s with the assistance of a speed control valve. The effects of different feeding speeds on solder ball formation and whether or not the wire is blocked are detailed in Table 5 below.

[0055] Table 5 Verification of the effect of solder wire temperature on solder ball formation after soldering

[0056]

[0057]

[0058] In step four, by setting the solder wire feed length, we can ensure the coverage of the solder pads and prevent them from being burned by the soldering laser due to prolonged laser heating. The feed length also determines the processing time for a single solder joint, achieving a highly efficient and stable laser soldering process. Through verification of the solder feed length, we determined the range to be 12–17 mm, which is beneficial for both solder yield and UPH (Uptime Per Hour). For a detailed comparison of the effects of other solder feed lengths on soldering, please refer to Table 6 below.

[0059] Table 6 Verification of the effect of solder feed length on soldering

[0060]

[0061] This embodiment improves production efficiency by setting the flux dosage, solder feeding speed, preheating time, and solder feeding length. See Table 7 below for specific optimization parameters.

[0062] Table 7 Optimized Welding Parameters

[0063]

[0064]

[0065] Example 2:

[0066] Based on the first embodiment of this application providing a pre-plating tin process for laser welding, the second embodiment of this application proposes a pre-plating tin process for laser welding. The second embodiment is merely a preferred embodiment of the first embodiment, and its implementation will not affect the separate implementation of the first embodiment. The second embodiment of the present invention will be further described below.

[0067] In step two, the pad is a four-copper-layer pad, the preheating time is 300-500ms, and the preheating temperature is 300-360℃. For the specific effects of preheating time and preheating temperature, please refer to Table 3.

[0068] The difference between Embodiment 2 and Embodiment 1 lies in replacing the two-copper-layer pads with four-copper-layer pads. In Embodiment 1, the pads used a common two-layer copper structure, with the first layer being immersion copper and the second layer being solder mask. In Embodiment 2, the copper layer of the pads is changed to a four-layer structure, namely, from bottom to top: immersion copper layer, solder mask layer, signal layer, and solder mask layer again. With the four-layer copper structure, an additional signal layer is placed in the middle. This independent signal layer can reduce the thickness of the immersion copper layer and lower the heat capacity of the pads. At the same time, it also increases the adhesion strength between the pads and the PCB board. Therefore, the application of four-layer copper pads can accelerate the heat conduction rate of the pads and reduce the laser power requirements of the pads. Compared with two-layer copper pads, four-layer copper pads can achieve more precise and efficient laser welding, which is beneficial for improving production volume and welding quality. This is the main difference and advantage compared to Embodiment 1.

[0069] Example 3:

[0070] In this embodiment, the pad is a zero-copper-layer pad, the preheating time is 450–550 ms, and the preheating temperature is 260–300 °C. For the specific effects of preheating time and preheating temperature, please refer to Table 3.

[0071] The difference between Example 3 and Example 1 is that, while ensuring sufficient preheating, the number of copper layers on the pads is changed from a double-layer structure to a zero-layer structure, i.e., the copper layers on the pads are removed. In Example 1, a common double-copper-layer pad was used, including a copper plating layer and a solder mask layer. However, through optimized preheating and control parameters, good laser welding quality can be obtained even with the copper layers removed. Therefore, in Example 3, under the premise of strictly controlling the preheating temperature, time, and energy density, a copper-free epoxy board is used as the pad, in contrast to Example 1. This copper-free pad has a simple structure, and removing the copper layer also reduces the heat capacity of the pad. Using a copper-free pad is beneficial to improving the utilization efficiency of laser power and shortening the single-point welding time, thereby further improving production efficiency. The remaining conditions are the same as in Example 1, so they will not be repeated in this example.

[0072] Since the optimized preheating temperature and time are sufficient to achieve good welding quality without a copper layer, replacing the double-layer copper pads with zero-copper epoxy board pads reduces the pad's heat capacity, shortens single-point welding time, and improves production efficiency. This copper-free pad design optimizes the pad structure and process flow without compromising welding quality, thus reducing manufacturing costs. It demonstrates that further simplification of structure and process can be achieved simply through precise optimization of process control parameters. The application of this copper-free pad proves that for laser welding, welding quality can be guaranteed solely through precise control of preheating and other parameters. This optimized structural and process design improves the economic practicality of this laser welding process without affecting the welding effect.

[0073] The embodiments of the present invention are given for illustrative and descriptive purposes only, and are not intended to be exhaustive or to limit the invention to the forms disclosed. Many modifications and variations will be apparent to those skilled in the art. The embodiments were chosen and described in order to better illustrate the principles and practical application of the invention, and to enable those skilled in the art to understand the invention and to design various embodiments with various modifications suitable for a particular purpose.

Claims

1. A process for laser welding pre-tinned tin processes, characterized by: The steps include the following: Step 1: Place the substrate on the corresponding pad on the PCB circuit board, prepare the solder wire, preheat it to the specified temperature, open multiple sets of pre-made holes on the top of the pad, set the pre-made solder coating in the multiple sets of pre-made holes, and add flux to the surface of the pad and the pre-made solder. Step 2: Send the pre-tin-coated PCB board into the laser welding station, use the welding gun to aim at the target pad position, start the laser of the welding gun, and use the laser to preheat the pads of different sizes with different preheating times and temperatures, so that the pads and the pre-tin coating reach a preheated state below the melting temperature. Step 3: Continuously heat the pads and pre-coated tin with a laser to raise their temperature to the specified soldering temperature, so that the pre-coated tin completely melts into a liquid state and fuses with the surface of the pads. At this point, the required temperature conditions for soldering are achieved. Step 4: After the pre-coated tin is completely melted, control the feeding speed of the tin wire to make it contact the molten pre-coated tin. At the same time, continuous laser irradiation also melts part of the fed tin wire. Stop feeding the tin wire after it has been fed to the specified length. Step 5: The molten solder wire will mix and spread thoroughly with the molten pre-coated solder on the pads, completing the soldering connection at the pad positions; Step Six: After the laser welding is completed, maintain a constant temperature to allow the weld to cool and solidify, thus preserving the fusion. The pad is a double copper layer pad, the preheating time is 300-500ms, and the preheating temperature is 300-360℃; The pad is a four-copper-layer pad, the preheating time is 300-500ms, and the preheating temperature is 300-360℃; The pad is a zero-copper-layer pad, the preheating time is 450-550ms, and the preheating temperature is 260-300℃.

2. A process for laser welding pre-tinned tin process as claimed in claim 1 wherein: In step one, the specified temperature for the preheated solder wire is 75–100°C.

3. The pre-tin coating process for laser welding according to claim 2, characterized in that: In step one, the flux added to the solder pad and the pre-formed tin surface is 3.5 to 4 mg / drop.

4. The pre-tin coating process for laser welding according to claim 1, characterized in that: In step three, the specified temperature for heating the solder pads and the pre-coated tin is 70–90°C.

5. The pre-tin coating process for laser welding according to claim 1, characterized in that: In step four, the feeding speed of the tin wire is 15-20 mm / s.

6. The pre-tin coating process for laser welding according to claim 5, characterized in that: In step four, the specified length for stopping the solder feeding of the solder wire is 12 to 17 mm.

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