PCB is becoming more and more important, and the reliability of assembly has become an important embodiment of the competitiveness of electronic products.
1. Introduction.
With the rapid development of information technology, especially the content and status in modern weapon systems have become the key factors that determine the overall strength of weapons and equipment, and the quality of electronic products directly determines the effectiveness of weapons and equipment on the battlefield. Therefore, it is particularly urgent to improve the assembly quality of electronic products, especially the reliability of PCB board assembly. This paper explains how to improve the reliability of PCB board assembly from five aspects: reasonable selection and design of components, selection and design of substrate, layout and direction design of components, printing of SMT solder paste and quality control of reflow soldering. .
2. Reasonable selection and design of components.
The reasonable selection and design of components is a key link in the board-level assembly of PCB. According to the requirements of process, equipment and overall design, the packaging form and structure of SMC/SMD are selected according to the electrical performance and function of determined components, which plays a decisive role in circuit design density, productivity, testability and reliability . At present, there are many specifications and different structures of SMT components, and there may be a variety of packaging forms for integrated circuits that achieve the same function; in the design of circuit PCB, reasonable choices should be made according to the specifications of components provided by market suppliers and the capacity and precision of existing production equipment.
3.Selection and Design of PCB substrate.
The performance of the substrate is an important part of the PCB module, which will greatly affect the electrical performance, mechanical performance and reliability of the electronic component, so it must be carefully selected.
3.1 substrate material.
It is generally required that the thermal expansion coefficient (CTE) should be as small as possible and the consistency is good, and the substrate must have the heat resistance of 260C / 50s. For single and double panels with lower general requirements, FR-4 copper-clad epoxy glass cloth laminate can be used, which is suitable for plug-in and paste mixed products. When installing fine pitch IC with high power and density, copper clad polyimide glass cloth laminate can be used, which is common in multilayer, double-sided reflow soldering process or electronic products requiring high reliability.
3.2 basic process requirements for SMT printed circuit boards.
The warping requirement of SMT PCB is more stringent than that of traditional PCB. The maximum value of upwarping is 0.5mm and that of downward warping is 1.2mm. In terms of process side, according to the maximum value of SMB manufacturing and installation workers, the long edge of PCB is generally within 5mm. In order to ensure the smooth transmission of PCB in the automatic production equipment of SMT, the four corners of PCB should be arc-shaped (< the diameter of 10.0mm). From reinspection to assembly, the vacuum package of PCB board is removed and exposed in the air for a long time, and the pad of PCB board is oxidized in air, which reduces the weldability of PCB board and is easy to cause virtual welding. vacuum packaging should be maintained before assembly.
