Introduction to Surface Mount Technology

Learn more about surface mount technology

SMT is a surface mount technology, one of the most popular technologies and processes in the electronics assembly industry. It is a kind of surface mount components without leads or short leads to be mounted on the surface of a printed circuit board or other substrates. Circuit assembly technology in which reflow soldering or dip soldering is used to solder and assemble. Under normal circumstances, the electronic products we use are designed by PCB plus various capacitors, resistors and other electronic components according to the designed circuit diagram. Therefore, all kinds of electrical appliances need various SMT chip processing techniques to process.

The basic process components of surface mount technology

Silkscreen: Its function is to print solder paste or patch glue onto the PCB pads to prepare for the soldering of components. The equipment used is a screen printing machine, located at the forefront of the SMT production line.

Dispensing: It is to drip glue onto the fixed position of the PCB board. Its main function is to fix the components on the PCB board. The equipment used is a glue dispenser, located at the forefront of the SMT production line or behind the testing equipment.

Mounting: Its function is to accurately mount the surface mount components to the fixed position of the PCB. The equipment used is a placement machine, located behind the screen printing machine in the SMT production line.

Curing: Its function is to melt the patch glue. So that the surface mount components and the PCB board are firmly bonded together. The equipment used is a curing oven, located behind the placement machine in the SMT production line.

Reflow soldering: Its function is to melt the solder paste. So that the surface mount components and the PCB board are firmly bonded together. The equipment used is a reflow oven, located behind the placement machine in the SMT production line.

Cleaning: Its function is to remove the harmful welding residues such as flux on the assembled PCB board. The equipment used is a washing machine, the location can be not fixed, it can be online or offline.

Advantages of surface mount technology

High assembly density

The area and weight of SMT chip components are greatly reduced compared with traditional perforated components. Generally speaking, the use of SMT can reduce the volume of electronic products by 60% and the weight by 75%. Through-hole mounting technology components, they install components according to a 2.54mm grid. The SMT assembly component grid has developed from 1.27mm to the current 0.63mm grid, and the mounting components with a 0.5mm grid have a higher density.

High reliability

Because the chip components are small and light, have strong anti-vibration ability and high degree of automated production, the placement reliability is high. Generally, the bad solder joint rate is less than 10%, which is an order of magnitude lower than that of through-hole plug-in component wave connection technology. The mean time between failures (MTBF) of electronic products assembled with SMT is 2.5×105h. At present, almost 90% of electronic products adopt SMT process.

Good high frequency characteristics

Because the chip components are firmly mounted, the devices are usually leadless or short leads. The effect of parasitic capacitance is reduced and the high-frequency characteristics of the circuit are improved. The highest rate of circuits designed with chip components is 3GHZ. The use of through-hole components is only 500mhZ. The use of SMT can also shorten the transmission delay time and can be used in circuits with a clock frequency of 16MHz or more. If the multi-core module MCM technology is used, the end clock of the computer workstation can reach 100MHZ, and the additional power consumption caused by parasitic reactance can be reduced by 2-3 times.

Cut costs

• The printing area is reduced, and the area is 1/12 of the area of the through-hole. If the CSP is used for installation, the area can be greatly reduced.
• The frequency characteristic is improved, and the circuit debugging cost is reduced.
• The chip element is small in size and light in weight, which reduces packaging, transportation and storage costs.
• Chip components are developing rapidly and their costs are falling rapidly.

Convenient for automated production

At present, if the perforated printed board is to be fully automated, it is necessary to expand the area of the original printed board by 40%. In this way, the insertion head of the automatic plug-in can insert the components. If there is not enough space, the parts will be damaged. The automatic placement machine uses vacuum nozzles to pick and place components. The vacuum nozzle is smaller than the component shape, which can increase the installation density. In fact, small components and fine-pitch devices are produced by automatic placement machines, and the entire line is automated.

Of course, there are also some problems in SMT mass production.

• The nominal values on the components are not clearly visible, and the maintenance work is difficult.
• It is difficult to maintain and exchange components, and special tools are required.
• The coefficient of thermal expansion (CTE) consistency between the components and the printed board is poor.
• The initial investment is large, the structure of the production equipment is complex, the technology involved is wide, and the cost is expensive.

With the emergence of special disassembly and assembly and new types of low expansion coefficient printed boards, they no longer become obstacles to the further development of SMT.

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