The characteristics of SMT technology can be compared with traditional through-hole insertion technology. From the point of view of assembly technology, the fundamental difference between SMT and THT is "paste" and "insert". The differences between the two are also reflected in the substrate, components, components, solder joint morphology and assembly process methods.
THT adopts leaded components to design circuit connecting conductors and mounting holes on the printed board. By inserting the leaded components into the pre-drilled through-holes on the PCB, the components are temporarily fixed and soldered on the other side of the substrate by soft brazing techniques such as wave soldering, to form reliable solder joints and establish long-term mechanical and electrical connections. The component body and solder joints are respectively distributed on both sides of the substrate. With this approach, because the components have leads, when the circuit is dense enough, the problem of reducing the size cannot be solved. At the same time, it is difficult to eliminate the faults caused by the proximity of leads and the interference caused by the length of leads.
The so-called surface assembly technology refers to the assembly technology of electronic components with certain functions by placing the components with sheet structure or the miniaturized components suitable for surface assembly on the surface of printed board according to the requirements of the circuit and assembling them with the soldering process such as pick and place machine, reflow oven or wave soldering. On traditional THT printed circuit boards, the components and solder joints are located on both sides of the board. On an SMT circuit board, solder joints and components are on the same side of the board. Therefore, on an SMT printed circuit board, through holes are used only to connect the wires on both sides of the board, with a much smaller number of holes and a much smaller diameter. In this way, the assembly density of PCB can be greatly improved.
