1. Introduction
In recent years, with the continuous growth of the global demand for clean energy, the solar energy industry has developed rapidly. Solar panels, as the core components of solar power generation systems, their production technology and quality have always been the focus of attention. The welding quality of the junction box lead wires of solar panels directly affects the performance and reliability of the entire solar panel. Traditional welding methods have some limitations, and laser welding technology, with its unique advantages, has gradually become a research hotspot in the field of solar panel production. This article will focus on the application and analysis of laser welding technology in the lead wires of solar panel junction boxes, aiming to provide a reference for improving the production efficiency and quality of solar panels.
2. Traditional Welding Methods and Their Defects
2.1 Traditional Welding Methods
Traditional solar panel junction box lead wire welding mainly adopts hot melt pressure welding (hereinafter referred to as hot pressure welding). Hot pressure welding is mainly divided into manual welding and automatic hot pressure welding. The principle is the same. The copper wire covered with tin is pressed on the welding surface, and the soldering iron or heating block is used to heat the tin wire or the tin block on the copper lead wire. The melted tin wraps the copper wire and the conductive connection surface to complete the welding. Manual welding requires manually welding the positive and negative leads respectively, mainly relying on the workers’ own judgment. The technical level of the personnel directly affects the welding quality. Manual welding cannot accurately control the melting time and temperature of tin, and the efficiency is relatively low. Automatic hot pressure welding adopts an automated welding method. The equipment usually consists of a positioning and pressing mechanism, a heating system, a temperature control and cooling system, a visual judgment system, etc. Compared with manual welding, the welding position is accurate and the welding effect is stable. At the same time, there are also some derivatives of hot pressure welding, such as automatic tin wire feeding welding equipment and solder paste clamping welding equipment. Usually, additional mechanisms need to be added on the side of the soldering iron or heating block. This kind of equipment can use the melted tin to well cover the conductive copper wire.
2.2 Defects of Traditional Welding Methods
Hot pressure welding has corresponding defects. The main one is that welding requires a long heating and cooling time. Secondly, during welding, the welding head needs to contact the product, which may touch and damage the product, resulting in residual stress and thermal deformation. At the same time, after the tin melts, the contact area with the heating block (or soldering iron) is relatively large, and a lot of residual tin will stick to the heating head. In the non-welding area, it may fall on the product and contaminate the product. Therefore, the contact surface of the heating head needs to be cleaned and maintained frequently. Usually, the hot pressure welding of the solar panel junction box requires 6 independent mechanisms to be compatible with different states of the leads and weld simultaneously to achieve the highest efficiency, and the equipment structure cost is relatively high. The following table summarizes the comparison between manual welding and automatic hot pressure welding:
| Welding Method | Advantages | Disadvantages |
|---|---|---|
| Manual Welding | Simple operation, low equipment cost | Low welding quality, low efficiency, affected by personnel skills |
| Automatic Hot Pressure Welding | High welding quality, stable welding effect | High equipment cost, long heating and cooling time, easy to damage the product |
3. Laser Welding Process Analysis and Equipment Introduction
3.1 Welding Product Process Requirements
At present, the laser welding of the lead wires of the solar panel junction box has gradually formed some basic requirements at the client side, such as the accurate position of the junction box, the position of the lead wires inside the junction box after welding, the shape and surface quality of the welding, and the tensile test after welding. The welding and testing requirements are shown in the following figure:
| Requirement Stage | Requirements |
|---|---|
| Before Welding | Junction box position accuracy, lead wire length and bending |
| During Welding | Laser power stability, no welding explosion points |
| After Welding | Weld seam angle and length, welding tensile strength |
3.2 Laser Welding Equipment Composition
Laser welding, also known as near-infrared welding, focuses light energy on the welding area through optical fibers and lenses. This equipment is mainly composed of a control system, a laser generator, a temperature control system, a visual light source module, a welding module, a dust removal system, and a product conveying and positioning mechanism. When the product reaches this equipment on the production line, the conveying and positioning mechanism positions the product assembly. At the same time, confirm and analyze the position deviation of the junction box on the assembly. Then, the junction box positioning mechanism performs positioning and deviation compensation. Next, the system controls the welding module to move to complete the welding of all the lead wires of the junction box. The positioning mechanism resets, the camera performs post-welding detection and comparison, and the product is output at the same time. The following figure shows the schematic diagram of the laser welding equipment:
3.3 Feasibility Analysis of Laser Welding
Laser has been widely used in marking, engraving, welding and other fields. The width of the solar panel lead wire is usually only about 6mm, and the thickness is about 3.5mm. Using pulsed or continuous laser beams can achieve accurate and fast welding. The light spot can cover an area with a diameter of φ170mm, which can be compatible with the lead wires on both sides of the same junction box, so that the laser can complete the welding on both sides of a junction box without moving. The experimental test shows that the single junction box lead wire welding time is about 0.3s. The welding mechanism movement and welding can be controlled within 6s. Including vision and material handling, the theoretical CT should be about 12s. The following table shows the comparison between laser welding and traditional welding methods in terms of welding time:
| Welding Method | Welding Time per Junction Box (s) | Equipment CT Time (s) |
|---|---|---|
| Traditional Welding | > 6 (usually requires 6 sets of mechanisms to work simultaneously) | > 6 |
| Laser Welding | ≤ 6 (including welding and mechanism movement) | ≈ 12 (including vision and material handling) |
3.4 Experimental Results and Analysis
The actual test equipment adopts a 1500W single-mode laser generator, an optical fiber core diameter of 20mm, and a galvanometer with a focal length of 254mm for polarization. The welding track length is 6mm, the fusion zone diameter is less than 0.5mm, and the scanning track is a multi-turn circular shape with a diameter of φ1mm. There are 4 weld seams on a single lead wire. However, the actual test results of laser welding are not ideal. The most significant problem is virtual welding. Only through visual judgment, there is no difference in the appearance between virtual welding and perfect welding parts. As shown in the following figure (left is virtual welding, right is perfect welding), there is no obvious difference on the upper surface of the welding, but the lower surface of the lead wire can be pried open and separated with a flat-blade screwdriver, while the perfect welding can still ensure that the lead wire is fixed on the welding surface even if the lead wire is damaged.
Considering that the bottom of the virtual welding surface may not be melted and penetrated, which may be related to insufficient laser energy. The energy E required for the laser to penetrate the same material with the same penetration depth is affected by different factors. By readjusting the focal length of the laser, that is, the spot area A (mm), power P (kW), scanning frequency f (Hz), and lead wire thickness t (mm), and satisfying the formula:
E=(P•V)n/A
(where the lead wire thickness t (mm) is provided by the user and cannot be changed. Increasing the welding times n will affect the equipment CT time, but the effect is ideal). After batch DOE comparison tests, good welding results can be obtained under the conditions of a power of 900 ± 200W and a scanning frequency of 12000Hz. However, the adjustment of the scanning frequency has little impact on the actual effect optimization, and the change of power has a greater impact. The change of power is mainly manifested as virtual welding at low power and blackening of the welding surface at high power. Although the welding effect basically meets the requirements under these parameters, the problem of virtual welding still exists and is random. Considering that during the welding process, as the etching depth of the laser on the lead wire surface increases, the focus moves down beyond the defocus range, resulting in a decrease in the energy density at the bottom of the lead wire fusion pit and a decrease in welding stability. The following table shows the relationship between different power and scanning frequency combinations and the welding quality:
| Power (W) | Scanning Frequency (Hz) | Qualified Quantity (points) | Qualified Rate (%) |
|---|---|---|---|
| 40 | 100 | 24/50 – 4 | 000 |
| 45 | 500 | 71/50 – 4 | 23.67 |
| 50 | 1000 | 216/50 – 6 | 72.00 |
| 55 | 5000 | 223/80 – 6 | 74.30 |
| 80 | 30000 | 796/60 – 6 | 97.06 |
To increase the welding stability, it is necessary to adjust the defocus distance to increase the compatible range. The equipment uses a single-mode laser, which should satisfy the formula:
f=π✖D✖d/4λM^2
(where f is the focal length; D is the optical fiber core diameter; d is the focus spot diameter; λ is the laser wavelength (1030 – 1090nm); M^2 is the beam quality). Experiments have proved that the power and frequency can meet the requirements. After investigation, the beam quality M^2 has a great influence on the penetration depth, but it is difficult to adjust the actual mechanism. The influence of the laser wavelength λ on the focal length change is small and is not adjusted. The diameter of the laser fiber core D has various sizes (usually 15 – 50μm), and the field lens focal length L also has various specifications (usually 254mm, 330mm, 420mm). The field lens can be replaced to adjust the Z-axis distance to achieve, which is more operable to control the positive and negative defocus distances. Reducing the core diameter or replacing the galvanometer to increase the focal length can make the defocus distance in the welding range more compatible and control the energy more concentrated.
3.5 Analysis of the Influence of Equipment on Welding Quality
The quality of laser welding of solar panel lead wires is mainly affected by three aspects: (1) The influence of the power and scanning frequency of the laser itself. (2) The influence of the defocus amount and coverage range of the laser to the product. (3) The influence of the product positioning position detection. In addition to the adjustment of the laser parameters, the installation deviation of the product, the change of the product’s own weight, and the bending caused by the mechanism positioning will cause the product to deviate from the focus. The inclusion range of the defocus amount has a great influence on the stability of the welding effect. It is necessary to appropriately increase the defocus distance within a reasonable range to ensure the stability of the equipment. At the same time, supplemented by accurate visual judgment control and temperature sensor monitoring of the welding process and quality, the quality can be effectively controlled from the three aspects of before welding, during welding, and after welding, and the NG products can be output.
4. Conclusion
Laser welding is an efficient and high-quality welding method, which is suitable for the welding of the lead wires of the solar panel junction box. Through reasonable welding process parameters, excellent welding results can be obtained. With a reasonable control and monitoring method, the welding quality and efficiency can be greatly improved. With the in-depth and expansion of the laser welding equipment for the junction box lead wires, laser welding can play a more important role in the production of solar panels. In the future, further research and development can be carried out in the following aspects: (1) Optimization of laser welding process parameters to further improve welding quality and stability. (2) Development of more intelligent control and monitoring systems to improve the automation and reliability of the equipment. (3) Exploration of the application of new laser welding technologies and materials to meet the development needs of the solar energy industry.