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What are SMT PCB assembly and through hole assembly?

What is SMT PCB Assembly?

SMT PCB assembly (Surface mount technology) represents a method to assemble electronic components to printed circuit boards. In the 1960s, it was developed by IBM in the United States, now is widely used in China PCB assembly industry because of its forward-looking and practical nature.

To understand how SMT PCB assembly works, we first need to know SMD (Surface Mounted Devices), which is a kind of electronic components used for SMT PCB assembly. SMD simplifies the construction of components and integrates pins, so there is no need to reserve through holes.

SMT PCB assembly

At the begining of SMT PCB assembly, solder paste will be printed at the specified position on the metal surfaces of PCB (for better fixing, we usually add little special chemical glue), then place SMD components on the board by machine, the pin-surfaces must be put upon the solder paste at the accurate positions. After passing through the oven, the solder paste will be completely cured to form electrical connection.

SMT PCB Assembly Processes     Solder paste for SMT PCB assembly


Advantages over THT

1.Smaller components (The minimum SMD we’re able to process is 01005.)

2.Higher density of connection

3.No need drilling

4.Smaller finished products, good for advanced fileds

5.Simpler and faster automated assembly, good for mass production

6.Better EMC performance because of the smaller package

7.Better mechanical performance under shock and vibration conditions


1.Not good for large, high-power, or high-voltage parts

2.Component-level repair is more difficult

3.High solder void rate

4.SMD is too small that is difficult to read the mark


What is Through Hole PCB Assembly?

THT (Through-hole technology) appeared before SMT PCB assembly and was developed on the basis of some more primitive soldering methods. It is to insert the pins of components into the existing through holes on printed circuit boards by manual assembly or automatic machine, then soldering the components. Unlike SMT PCB assembly, components and solder joints are located on two different surfaces of the PCB.

Now THT is still the mainstream method because there are some large components or connectors that must be plugged in the through holes, also it has some advantages over SMT, such as, high fault tolerance, easy repair and good for some high-power equipment and flexible parts.

Through hole PCB assembly

Now SMT PCB assembly and THT assembly are needed to cooperate to meet the higher hardware requirements, and the AI and smart medical era is around the corner, which will make the component types more and more complex, designers must consider components combination more fully, it also requires us to continually optimize the technologies and processes.

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