About PCB/PCBA SMT Surface Mount Components

2019-08-18 10:14:31 395

SMT is a surface mount technology (surface mount technology), the English full name of Surface Mount Technology, is currently the most popular technology and process in the electronics assembly industry.

It is a type of leadless or short lead surface mount component (referred to as SMC/SMD, Chinese called chip component) mounted on the surface of a printed circuit board (PCB) or other substrate surface. Circuit mounting technology for soldering and assembly by means of reflow soldering or dip soldering.

1. China's SMT industry is still mainly concentrated in the Pearl River Delta region and the Yangtze River Delta region. The industrial sales revenue of these two regions accounted for more than 90% of the overall industrial scale, of which only the Pearl River Delta region accounted for 67.5% of the overall proportion. In addition, the sales of the SMT industry in the Bohai Rim region also reached 310 million yuan, accounting for 7.6% of the total industry.

2. At the same time, we expect that China's SMT industry will continue to be concentrated in the Yangtze River Delta, the Pearl River Delta and the Bohai Rim region in the next five years. However, the proportion of the Yangtze River Delta region in China's SMT industry will rise rapidly from 2007, reaching 43.9% in 2009. Although the proportion of the Pearl River Delta region has dropped to 40.7%, it still holds the top position. In addition, the SMT industry in the Bohai Rim region has also developed rapidly. The rapid growth of the SMT industry in the Yangtze River Delta region is mainly due to the transfer of the global SMT industry, especially the shift of placement machine production. Historically, the foundation for the development of equipment manufacturing in the Yangtze River Delta is relatively strong. At the same time, the Yangtze River Delta region, such as notebooks, mobile phones and other high-end electronic products manufacturing industry is relatively developed, coupled with the unique geographical advantages of the Yangtze River Delta region, so in the process of the global SMT industry in 2007, the Yangtze River Delta region will Undertake a considerable proportion. However, for the Pearl River Delta region, due to the development of the past few years, its SMT industry has formed a relatively complete industrial chain and industrial supporting environment, so the Pearl River Delta region also has obvious advantages in undertaking industrial transfer.

3. From the perspective of the industry's own development cycle, although China's SMT industry is still in the early stage of development, it has already shown its vitality. At the same time, the SMT industry is an important basic industry, which is of great significance for promoting China's electronic information industry manufacturing restructuring and industrial upgrading. Promoting the rapid and healthy development of China's SMT industry requires the cooperation of all aspects of the upstream and downstream industries.

4. For the government, we recommend doing two things. The first is to increase support for the SMT industry, especially to strengthen investment in basic research in the fields of basic materials and precision instruments. Second, the government, as an important market supervision department, must strengthen the protection of intellectual property rights, actively guide the formulation of China's SMT industry standards, and actively promote and implement them.

5. For the company, we also recommend doing five things. The first is to change the concept and fully understand the importance of production technology for the development of SMT equipment. Only when you are completely familiar with the actual production process and understand the adjustment of process technology parameters in actual production can you really design an SMT device that meets the actual production requirements. The second is to comply with the trend of lead-free, break through key technologies and achieve serialization of key equipment products. The third is to strengthen sales services and develop new models suitable for Chinese enterprises. The fourth is to use training certification to form a new model for equipment marketing. According to the requirements of the Ministry of Labor and Social Security and the Ministry of Information Industry, employees engaged in the surface mount industry must be certified before 2006, which also gives domestic enterprises the opportunity to promote their SMT equipment products through professional training and certification methods. Expansion capacity. The fifth is to pay full attention to the cultivation of self-cultivation and achieve technological innovation and healthy development.

Process composition

Printing (red glue / solder paste) --> Inspection (optional AOI automatic or visual inspection) --> Mounting (first paste small device and then paste large devices: high-speed patch and integrated circuit placement) -- >Detection (optional AOI optical/visual inspection)-->welding (welding by hot air reflow) --> Inspection (can be divided into AOI optical inspection appearance and functional test) --> Maintenance (using tools: welding Table and hot air desoldering station, etc.) --> Sub-board (manual or split machine for cutting board)

The process flow is simplified to: printing ------- patch ------- welding ------- overhaul (testing can be added to each process to control quality)

Folded solder paste printing

Its function is to use a scraper to print the solder paste at a 45-degree angle to the pad of the PCB to prepare for the soldering of the components. The equipment used is a printing press (solder paste press) at the forefront of the SMT line.

Folded parts placement

Its role is to accurately mount the surface-mounted components to a fixed position on the PCB. The equipment used is a placement machine, which is located behind the printing machine in the SMT production line. Generally, the high-speed machine and the general-purpose machine are used in accordance with production requirements.

Folding reflow soldering

Its function is to melt the solder paste and firmly solder the surface-assembled components to the PCB. The equipment used is a reflow oven, which is located behind the placement machine in the SMT production line. The temperature requirements are quite strict, and the temperature measurement needs to be performed in real time. The measured temperature is expressed in the form of a profile.

Folding AOI optical inspection

Its role is to test the quality of the welded PCB. The equipment used is an automatic optical inspection machine (AOI), and the position can be placed in a suitable place on the production line according to the needs of the inspection. Some are before reflow soldering, and some are after reflow soldering.

Folding repair

Its role is to repair the defective PCB. The tools used are soldering irons, rework stations, and the like. Configured after AOI optical inspection.

Folding board

Its function is to divide the multiple-connected PCBA into separate individuals, generally using V-cut and machine cutting.

Folding basics

Solder paste is a paste obtained by mixing solder powder with a paste flux having a flux function. Usually, the solder powder accounts for about 90%, and the rest is a chemical component.

We refer to an object that can change shape or arbitrarily divide into a fluid. The science that studies the law and characteristics of deformation and flow behavior caused by external forces is called rheology. However, in engineering, the concept of viscosity is used to characterize the viscosity of a fluid.

Rheological behavior of solder paste

The solder paste is mixed with a certain amount of thixotropic agent and has pseudoplastic fluid properties. When the solder paste is printed, it is subjected to the thrust of the scraper, and its viscosity is lowered. When the template window is reached, the viscosity is the lowest, so that it can smoothly settle through the window onto the pad of the PCB. As the external force stops, the solder paste has a rapid viscosity. Pick up, so that there will be no collapse and flow of the printed graphics, and good printing results.

Factors affecting solder paste viscosity: solder powder content; solder powder particle size; temperature; shear rate.

1, solder powder content

An increase in solder powder in the solder paste causes an increase in viscosity.

2, solder powder particle size

The solder powder has an increased particle size and a reduced viscosity.

3, temperature

The temperature rises and the viscosity decreases. The optimum ambient temperature for printing is 23 ± 3 degrees.

Reflow soldering

Reflow soldering, also known as "reflow soldering" or "reflow soldering" or "reflow oven" (Reflow Oven), provides a heating environment that allows the solder paste to be melted by heat to allow surface mount components and PCB soldering. The device is reliably bonded together by solder paste alloy. According to the development of technology, it is divided into: vapor phase reflow soldering, infrared reflow soldering, far infrared reflow soldering, infrared heating wind reflow soldering and full hot air reflow soldering. In addition, according to the special needs of welding, a reflow oven containing nitrogen is included. Most popular and practical are far infrared reflow soldering, infrared heating wind reflow soldering and full hot air reflow soldering.

Process flow

1. Compatibility with SMB, including wettability of the pad and heat resistance of the SMB;

2. The quality of the solder joints and the tensile strength of the solder joints;

3. Welding work curve:

Preheating zone: The heating rate is 1.3~1.5 degrees/s, and the temperature rises to 150 degrees within 90~100s.

Insulation zone: temperature is 150~180 degrees, time is 40~60s.

Reflow zone: from 180 to the maximum temperature of 250 degrees, it takes 10~15s, and returns to the insulation zone for about 30s for rapid cooling.

Lead-free soldering temperature (tin-silver-copper) 217 degrees.

4, Flip Chip reflow soldering technology F.C.