Evolution of THT plug-in process and SMT process_
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Evolution of THT plug-in process and SMT process

Since the introduction of SMT production lines in 1985, China's SMT technology has been developed for more than 30 years. According to incomplete statistics, there are currently about 50,000 SMT production lines in China, and the total number of placement machines exceeds 100,000, accounting for 40% of the world. It is the world's largest and most important SMT market.

 

First. The characteristics of SMT process technology

The characteristics of SMT process technology can be compared with the traditional through-hole insertion technology (THT). From the perspective of assembly process technology, the fundamental difference between SMT and THT is "sticking" and "inserting." The difference between the two is also reflected in all aspects of the substrate, components, solder joint shape and assembly process method.

 

THT uses leaded components. The circuit connection wires and mounting holes are designed on the printed board. The component leads are inserted into the pre-drilled through holes on the PCB, and then temporarily fixed, wave soldering is used on the other side of PCB. The wave soldering technology performs welding to form reliable solder joints and establish long-term mechanical and electrical connections. The main components and solder joints of the components are respectively distributed on both sides of the PCB. With this method, since the components have leads, when the circuit is dense to a certain extent, the problem of reducing the volume cannot be solved. At the same time, the faults caused by the proximity of the leads and the interference caused by the lead length are also difficult to eliminate.

 

DIP package (DualIn-linePackage) is a part package in the THT plug-in process, also called dual in-line package technology, which is the simplest package method. Refers to integrated circuit chips packaged in dual in-line form. Most small and medium-sized integrated circuits use this form of packaging, and the number of pins generally does not exceed 100. The DIP packaged CPU chip has two rows of pins, which need to be inserted into the chip socket with the DIP structure.

 

On the traditional THT printed circuit board, the components and solder joints are located on each sides of the board; while on the SMT circuit board, the solder joints and components are on the same side of the board. Therefore, on the SMT printed circuit board, the through holes are only used to connect the wires PTH on both sides of the circuit board, the number of holes is much smaller, and the diameter of the holes is much smaller. In this way, the assembly density of the circuit board can be greatly improved.

 

Second, the evolution of THT plug-in technology and SMT technology

The surface mount technology (SMT) is developed from the traditional through hole insertion technology (THT), but it is different from the traditional THT. THT technology is the traditional electronic assembly process, wave soldering technology is generally used for the soldering of the through-hole plug-in component (THD) printed board assembly. However, wave soldering has many shortcomings: high-density, fine-pitch SMD components cannot be distributed on the soldering surface; bridging and missing soldering are easy to happen; flux spraying is required; printed boards are warped and deformed by greater thermal shock.

 

Due to the increasing density of current circuit assembly, high-density, fine-pitch SMD components will inevitably be distributed on the soldering surface. The traditional wave soldering process has been unable to do this. The soldering surface SMD components can only be reflowed firstly, And then manually solder the remaining plug-in solder joints, but there is a problem of poor solder joint quality consistency.

 

Third, the characteristics of SMT surface assembly technology

1. SMT finished PCBA, high assembly density, small size and light weight: The volume and weight of SMD components are only about 1/10 of that of traditional DIP through-hole insert components. After adopting SMT, the volume of electronic products is reduced by 40%~60%. 60%~80% weight reduction;

2. The PCBA quality performance of SMT welding is stable: the solder joints are firm and reliable, the anti-vibration ability is strong, and the solder joint defect rate is low;

PCBA soldered by SMT has stable electrical properties and low energy consumption: short parts and wiring, fast transmission and low energy consumption, reduced electromagnetic and radio frequency interference, with stable and reliable high-frequency characteristics;

3. SMT is easy to realize automation, improves production efficiency, saves materials, energy, equipment, manpower and time, and reduces costs by 30% to 50%.

Therefore, SMT will inevitably advance with the times, and it will continue to achieve rapid development for a long time in the future.