Surface mounting technology (SMT) is a relatively complex and constantly evolving technology. From lead-based processes to lead-free processes, from large pad soldering to micro-pad soldering, there are constant challenges. However, its basic principles have not changed, and the process The mission of the work has not changed (issues of process realization and process stability). Focusing on mastering the process essentials, engineering knowledge, and common welding defective phenomena and treatment strategies of SMT is of great practical significance for establishing an effective process control system and quickly solving production process problems.
 

1. Process essentials

       Process essentials, as the name suggests, refers to the key points of process technology or process methods and requirements. Mastering these key points is equivalent to grasping the "soul" of process technology. When encountering ever-changing adverse phenomena, you can analyze and solve them in the right direction. For example, if you don't understand the two microscopic physical processes of "two collapses" and "deformation" that BGA welding itself goes through, it will be difficult to understand the significance of the peak temperature and welding time of BGA welding. For another example, if you don't understand that soldering lead-free BGA with lead solder paste will change the melting point and component characteristics of the solder joints, it will be difficult to understand the complexity of the mixed assembly process. Therefore, when learning process knowledge, it is very important to master the essentials. It is the basis for analyzing and solving difficult process problems.

2. Engineering knowledge

       As an SMT engineer, if you only stay at the level of understanding book knowledge, you will definitely not be qualified. What is needed at the production site is someone with basic engineering knowledge. For the assembly process, engineering knowledge includes process windows, benchmark process parameters and basic process methods, such as steel mesh window opening, for a specific package, how thick the steel mesh should be, what shape and size of window should be opened, This specific, usable practical knowledge is generally obtained based on experimentation or experience.

3. The causes and countermeasures of common welding defects

       If we do not understand the welding problems that are prone to occur in each type of component and the causes, then we cannot effectively prevent them. The reason is very simple, if you don't think of it, you can never do it. The most fundamental way to master the occurrence mechanism and treatment countermeasures of common welding defects is to apply the theoretical knowledge learned in practice, analyze and solve problems, and transform theoretical knowledge into the ability to deal with problems. In the final analysis, craftsmanship is a highly practical knowledge that relies on the accumulation of experience. Just like a doctor seeing a doctor, the more patients he sees, the richer his experience will be. In practice, we often encounter such situations, such as what is wicking phenomenon? I believe that most engineers can answer it, but when encountering problems caused by wicking, they often do not think of wicking. This is because they do not have the ability to convert theoretical knowledge into solving problems. Japanese electronic products are world-famous for their quality, and an important rule of thumb is to “learn faults and eliminate expected faults.” It is a very important method to learn from practice and reuse the experience to guide practice.

4. The assembly process is a system engineering issue

       The quality of assembly process involves five major aspects: people, machines, materials, environment and methods. If the quality of these "entrances" fluctuates greatly, then establishing a high-quality, repeatable process is an empty promise. In order to reduce procurement costs and avoid risks, many companies use multiple brands of materials, which is a major hidden danger for the process. Different brands of materials, especially materials with a relatively low degree of standardization, often have different qualities and lead widths. These are often factors that lead to process instability. Therefore, to create first-class technology, we must think systematically and control it from aspects such as material selection, process design, process trial production, process optimization, and quality monitoring.

      In view of the above understanding, 65 core process issues will be screened from an application perspective, summarized and analyzed, and the essentials pointed out. At the same time, 134 typical cases will be carefully selected to introduce the characteristics of defects, common causes and improvement measures with pictures and texts. An article will be updated every day. Today we will explain the overview of SMT.

 

       SMT Overview

      Surface mounting technology, the English name is Surface Mount Technology, abbreviated as SMT, is a board-level assembly technology that installs surface mounting components (SMD) onto a printed circuit board (PCB). It is the core of modern electronic assembly technology 

 

        Surface assembly technology is also called "surface mounting technology" and "surface assembly technology" in the electronic engineering industry. It originated from the thick-film circuit external component technology in the 1960s. It developed rapidly in the 1980s with the mass production of color TV electronic tuners. By the mid-1990s, it was basically mature and became a modern technology. Mainstream electronic assembly technology.

 

1. Advantages of SMT

 

      Compared with THT (plug-in technology), SMT brings four major advantages to electronic products:

 

(1) High density. Since surface mount components adopt packaging technologies such as leadless or short leads, I/O terminal array layout, the size of the components is greatly reduced, and the I/O terminals are greatly increased, thereby greatly increasing the assembly density of the PCB. .

 

(2) High performance. The characteristics of surface-mounted components without leads or short leads reduce the parasitic inductance and capacitance of the leads, improve the high-frequency and high-speed performance of the circuit and the heat dissipation efficiency of the device.

 

(3) Low cost. Due to the standardization and hole-free installation characteristics of surface-mounted component packaging, it is particularly suitable for automated assembly and greatly reduces manufacturing costs.

 

(4) High reliability. Automated production technology ensures reliable connection of each solder joint, thereby improving the reliability of electronic products.

 

      It is precisely because of these four advantages of SMT that it promotes its wide application, which in turn also promotes the continuous development of SMT itself.

2. Technical composition of SMT

     SMT is a system engineering technology, including process technology, process equipment, process materials and detection technology

 

       It should be pointed out that although we regard SMD and PCB as surface assembly objects and substrates respectively, the packaging structure of SMD and the manufacturing quality of PCB are directly and closely related to the throughput rate of surface assembly. From the perspective of controlling SMT welding quality, SMT in a broad sense should include packaging technology of electronic components and manufacturing technology of PCB. This is why many monographs on SMT list packaging technology of electronic components and PCB manufacturing technology. content reasons.

 

3.The core of SMT

      As the saying goes, "Experts watch the door, while laymen watch the excitement." As far as SMT is concerned, what is the core technology? Is it equipment or process? At domestic academic exchange meetings, equipment is often discussed the most, and equipment is compared; in fact, equipment is just a means to achieve technology, and the real core is technology, which is the guarantee for achieving high-quality production.

       The goal of SMT work is to manufacture qualified solder joints. The formation of good solder joints depends on appropriate pad design, appropriate solder paste volume, and appropriate reflow soldering temperature curve. These are process conditions. Using the same equipment, some manufacturers have higher throughput welding rates, while others have lower welding rates. The difference lies in the different processes, which are reflected in "scientific, refined and standardized", such as the thickness of the steel mesh and the design and printing of the windows. Support and parameter adjustment, patch program settings, temperature curve settings, furnace entry intervals, tooling equipment during assembly, etc. These often require companies to spend a long time exploring, accumulating and standardizing, and these have been verified and The solidified technical documents, process methods, and tooling design are the "process" and are the core of SMT.