China Topscom Deliverd Speech About Research on SMT pcb boards assembly Technology and Technique On Shanhai 2019 china electronic manufacturing Nepcon Fair.
 
Abstract: with the development of social science technology and the continuous reconstruction and development of microelectronics industry and computer technology, patch components in electronic assembly have been widely used in the industry. This paper discusses the application of SMT patch technology in the electronics industry, and makes a detailed analysis of the process links of the patch from the reality. the article is posted by Mr. Wangjun,director of Topscom SMT Workshop Department,china Topscom provide high difficulty density & printed circuit boards PCB design & layout, mass manufacturing & fabrication,pcba boards assembly,turnkey full systems integration electronic contract manufacturing services.
 
Key words: china SMT Industry,pcb SMT assembly technology; solder joint; patch process.electronic manufacturing services.



In recent years, China's rapid economic development has also led to the rapid rise of most industries. More and more attention has been paid to the development and growth of electronic industry and communication technology in the society. Under the premise of meeting the requirements of advanced science and technology, the area of products in the electronics industry is also gradually shrinking, which is convenient for workers to operate or users to carry. Traditional electronic products are much larger than those developed by modern science and technology in terms of size and area. With the continuous improvement of people's living standards, the requirements of users are also improving, so the traditional electronic products can no longer meet the requirements of the development of modern society. SMT technology plays a very important role in the welding of electronic products, and the quality and performance of electronic products are closely related to SMT technology.
 
In the research results of modern science and technology, surface mount technology has developed into a more reliable, automatic, the overall assembly structure process is also decreasing and the manufacturing cost is more reasonable. SMT technology is widely used in the process of surface placement of communication and computer products, which plays an important role in the electronic industry.
 
I. Application of SMT technology surface mount technology.
 
SMT technology includes three processes: screen printing, mounting components and reflow soldering.
 
(1) screen printing.
The first step of screen printing is to stir the solder paste, and the viscosity and uniformity of the solder paste should be paid attention to in the stirring process, in which the viscosity of the solder paste has an important influence on the printing quality. Viscosity control is also in accordance with the printing standards for stirring, stirring viscosity is too large or too small will affect the printing quality. In general, it is more suitable for solder paste to be preserved at the temperature of 0.5 ℃. In order to ensure that the solder paste does not change chemically, it is necessary to keep the temperature of the solder paste at the natural temperature when in use.
In the process of screen printing, the solder paste will be missed on the PCB pad, in the front end of SMT technology production, but also to do a full and accurate work for the welding of components. In the printing process, the pressure of the scraper will promote the distribution of solder paste to the pad, and the final thickness of the screen should be controlled at 0.15mm. In the process of practical application, the quality of screen printing solder paste is not only improved, but also the amount of tin solder paste on PCB pad can reach a certain fullness.
 
(2) component mounting.
The main function of the mounting component is to install the assembly components to the PCB position, and there are different characteristics in the form and position of the mounting components in the printing production process, so it is necessary to program them when they are accurately installed to the PCB location. The premise of accurate installation to the PCB position is to ensure that the mounting components do not have any errors in the programming process. If the inspection is not careful enough in the production of printed boards, a large number of printed boards will be discarded and can not be used in normal production printing. And when the mounting components are compiled, the program should be written according to the relatively simple structure, and then the components of the chip class with complex structure can be programmed, and the patch production can be carried out in the case of confirmation. The whole process of production belongs to automatic program production. When the position adjustment and direction judgment are carried out after the placement is completed, the effective methods adopted are laser recognition and camera recognition. Compared with the two, the ability of camera recognition is weaker than that of laser recognition, but it has a certain influence on the mechanical structure. Laser recognition is widely used in aircraft flight, but it can not be used in BGA components.

 

(3) Reflow soldering.
Before putting the printed board into reflow soldering, the position and direction of the components should be carefully checked. Temperature control is very important in the process of reflow soldering. Reflow soldering can be completed through four steps, including preheating, heat preservation, reflow and cooling. The main reason for preheating in the process of reflow soldering is to keep the temperature of the greenhouse at 180 ℃ and reduce the temperature difference in order to ensure the balance and stability of the temperature. Among them, keeping humidity is also very important, and the general humidity control is 40%, 60% of the condition, which is more suitable. In the heating process, the maximum temperature of the heater is 245 ℃, and the melting point of the solder paste is 183 ℃. The temperature of the PCB plate will cool slowly after it is transferred out of the reflow soldering furnace, so that the solder joint can achieve the best effect.
 
2: Visual processing system of printing process.
 
After loading the substrate in the printing process, it is necessary to strictly control its operating procedures, and make a real and accurate choice about the operation and supporting force of the substrate. Among them, the positioning is very important in the printing process, and the optical positioning is more suitable in optical positioning and mechanical positioning, but it will cause delay in the process of operation. Therefore, the use of mechanical positioning can ensure the accuracy of its supporting structure and positioning. Visual processing is very important, and the error caused by the visual light source to the substrate reference line should be avoided as much as possible. Among them, the installation position of the camera is also very important, if the installation position is not accurate enough, it will have a certain impact on the printing time. the article is posted by Mr. Wangjun,director of Topscom SMT Workshop Department,china Topscom provide high difficulty density & printed circuit boards PCB design & layout, mass manufacturing & fabrication,pcba boards assembly,turnkey full systems integration electronic contract manufacturing services.
 
 
3, dispensing technology.
The size of the mounting component is not fixed, so it depends on the actual situation in the mounting process. For example, large components are more suitable for integrated circuits and can play a good fixed role. However, large components to ensure their fixation characteristics in the selection of glue should pay attention to, special glue, glue with high viscosity has a good effect on the mounting and fixing of large components. In the process of making double panels, dispensing is needed to prevent parts from falling. Among them, red glue is widely used, in general, red glue will be refrigerated, so it should be warmed up before use.
 
4. Testing.
Under a series of technological operation, there are still several shortcomings after testing: poor bridge, missing welding, missing welding and insufficient solder joints. The main reason for poor bridging is that the solder paste does not meet the required standard in the mixing process and the viscosity is not enough. Because the pore diameter of the screen is too large, a large amount of solder paste will be infiltrated into the solder paste. In the heating process to maintain a certain speed, not too fast, too fast will also cause poor bridging. Usually insufficient solder paste, differences in the performance of pads and components, and too short re-soldering time will lead to missing solder, missing solder or insufficient solder joints.
 
5. Summary.
In the process of printing board, the prevention of static electricity is very important, the unreasonable requirements of static electricity prevention will lead to the failure of welding quality and the failure of the function of debugging test. According to the above process standard welding, and the detected problems are decomposed in detail, to develop a reasonable teaching plan or measures can further improve the quality of PCB welding. The existing problems are caused by the problems in the design process, so the accuracy of the design parameters is very important. If the existing problems are solved reasonably, the quality of the whole batch of printing plates can be maintained and the welding accuracy and efficiency can be improved.