Detailed explanation of the entire process of reflow soldering: steps, key points, and precautions

Reflow soldering is a process technology used to solder electronic components onto PCB boards, named after its heating method similar to river reflow. This process mainly transfers heat to the solder through thermal conduction, melting the solder and metallurgical bonding it with the component pins and copper foil on the PCB board, achieving reliable connection between the components and the PCB board. Reflow soldering technology has the advantages of high automation, stable and reliable welding quality, and is widely used in the field of electronic manufacturing. This article will provide a detailed introduction to the process flow of reflow soldering.

Reflow soldering process flow

Preparation phase

In the preparation stage, the operator needs to debug the reflow soldering equipment to ensure its normal operation. At the same time, it is necessary to prepare the PCB board, electronic components, solder paste and other materials to be soldered, and clean the PCB board to remove surface impurities and oxides to ensure welding quality.

Printing solder paste

Printing solder paste on PCB boards is one of the important steps in reflow soldering process. Operators use methods such as screen printing or spot coating to evenly apply solder paste to the solder pads of the PCB board. The main components of solder paste are solder alloy powder and flux, which are used to connect component pins and copper foil on the PCB board.

Mount components

After printing solder paste, the operator places the electronic components in the corresponding positions on the PCB board. This step is usually completed by an automatic SMT machine to ensure the accuracy and consistency of component placement. When placing components, attention should be paid to their polarity and orientation to ensure they match the design on the PCB board.

Reflow soldering

After completing the installation, place the PCB board into the reflow soldering equipment for heating and soldering. During this process, the solder powder in the solder paste melts and diffuses between the component pins and the copper foil on the PCB board, forming solder joints. Meanwhile, the volatilization or residue of flux helps to enhance the reliability and durability of welding points. The temperature curve (heating and cooling process) of reflow soldering is crucial for welding quality. The common temperature curve includes four stages: preheating, welding, thermal cycling, and cooling. The purpose of preheating is to gradually raise the PCB board to a suitable welding temperature, in order to avoid component damage or poor welding caused by temperature fluctuations. The welding stage is the process of melting the solder, which usually takes several minutes between 220 and 240 ℃. The thermal cycle stage is to cool the welded PCB board to promote the crystallization of solder joints and enhance mechanical strength. Finally, there is the cooling stage, during which the solder joints gradually cool and harden. During the entire reflow soldering process, the operator needs to monitor the temperature curve of the equipment to ensure that it meets process requirements and promptly handle any abnormal situations.

Cooling and inspection

After completing the heating process, cool the PCB board naturally or quickly through the cooling zone. After the temperature drops to room temperature, the operator needs to visually inspect the welded PCB board or use relevant testing equipment to confirm whether the welding quality meets the requirements. Common detection methods include visual inspection, X-ray inspection, and flying needle inspection. For unqualified welding points, repair or reprocessing is required.

Subsequent processing and testing

Finally, perform necessary follow-up processing and testing on the soldered PCB board. This includes cleaning, labeling, testing functions, and performance. The effectiveness of reflow soldering process can be verified through testing, ensuring reliable welding quality and meeting design requirements. Meanwhile, the test results can also provide improvement basis and experience summary for subsequent production.

Precautions for reflow soldering process

Temperature control: The key to reflow soldering process is to control the temperature curve to ensure that the temperature is moderate and not too high or too low. Excessive temperature may cause component damage or poor soldering, while low temperature may lead to weak soldering or the formation of cold solder joints. Therefore, operators need to closely monitor the temperature curve of the equipment and take corresponding measures to adjust it.

Solder paste selection: The selection of solder paste is crucial for the reflow soldering process. Different components of solder paste have different parameters such as melting point, viscosity, and flux properties, and need to be selected according to actual needs. At the same time, attention should be paid to the storage and use conditions of the solder paste to avoid performance degradation due to moisture or expiration.

PCB board cleaning: In the reflow soldering process, the cleanliness of the PCB board has a significant impact on the soldering quality. If there are impurities such as dirt, oxides, or residues on the surface of the PCB board, it can lead to poor soldering or the formation of defects such as virtual soldering. Therefore, operators need to regularly clean and maintain the equipment, and carefully inspect and clean the printed solder paste PCB board.

Component placement and mounting: When mounting components, attention should be paid to factors such as polarity, direction, and position of the components to ensure they match the design of the PCB board. At the same time, operators need to take corresponding measures to improve the accuracy and efficiency of mounting, such as using high-precision mounting machines and automated production lines and other equipment and technical means.

Safe operation: Reflow soldering equipment can generate hazardous factors such as high temperature and toxic gases during use, and operators need to comply with safety operating procedures.

To improve the welding quality of reflow soldering process, attention can be paid to the following aspects:

Optimizing temperature curve: The temperature curve of reflow soldering has a significant impact on welding quality. By precisely controlling the temperature and time of preheating, reflux, cooling, and other stages, it is possible to ensure sufficient melting and good crystallization of the solder, thereby improving welding quality.

Choosing appropriate solder and flux: The selection of solder and flux is also an important factor affecting welding quality. Suitable welding materials and fluxes should be selected based on specific process requirements and product characteristics to ensure the reliability and stability of welding.

Improving component mounting accuracy: The mounting accuracy of components directly affects the welding quality. Using high-precision mounting equipment and processes to ensure accurate placement of components can reduce welding defects and improve welding quality.

Control welding environment: Factors such as temperature, humidity, and cleanliness in the welding environment can have an impact on welding quality. Ensure that the welding environment meets the process requirements and reduce the adverse effects of environmental factors on welding quality.

Strengthening equipment maintenance and upkeep: The condition of reflow soldering equipment also has a significant impact on welding quality. Regular maintenance and upkeep of equipment to ensure its normal operation and good condition can improve the stability and consistency of welding quality.

Strengthening process monitoring and inspection: By strengthening process monitoring and inspection, problems during the welding process can be detected and dealt with in a timely manner, effectively avoiding the occurrence of welding defects and improving welding quality.

Continuous improvement and innovation: With the development of technology and market, new welding technologies and methods continue to emerge. By continuously improving and innovating, introducing new technologies and methods, the welding quality of reflow soldering process can be further improved.

In summary, improving the welding quality of reflow soldering process requires starting from multiple aspects, including optimizing temperature curve, selecting appropriate solder and flux, improving component assembly accuracy, controlling welding environment, strengthening equipment maintenance and upkeep, strengthening process monitoring and inspection, and continuous improvement and innovation.