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PCB Through Hole vs Surface Mount

PCB Through Hole vs Surface Mount THT (Through Hole PCB Assembly Technology) vs SMT (Surface Mount PCB Assembly Technology)-select the suitable assembly scheme for your high reliability application.
The mainstream mounting methods of components on printed circuit board (PCB) are through hole PCB assembly technology and surface mount technology (SMT). SMT is a very popular installation method in recent decades, so application designers often set it as the default method. But in fact, these two schemes have their own merits, so we need to choose them carefully in high reliability application design.

THT(Through Hole PCB Assembly Technology), also known as through-hole mounting (THM), is a mounting scheme in which leads of electronic components are inserted into pre-drilled holes on PCB-the leads pass through the through-holes and are soldered to the back of the circuit board. At present, through hole PCB assembly is mainly divided into two types: axial and radial. The axial lead goes straight through the component along the central axis, and its two ends are connected to another component respectively, so it has a small structure. The radial leads extend from one side of the component and are vertically assembled on the circuit board, which takes up less space for the circuit board. The above differences are very important when evaluating the geometric requirements of circuit boards and the size limitations of applications.
At one time, most components on PCB were installed by Through Hole PCB Assembly Technology. Until SMT became popular in recent decades, there was another mainstream installation method in the industry. SMT is a surface mounting technology, that is, electronic components are directly mounted on the surface of PCB without pre-opening. SMT was widely popularized as a simpler manufacturing automation scheme in 1980s. Compared with Through Hole PCB Assembly, SMT often takes up much less space, and it is compatible with dual panels. With the development of circuit boards from single-sided to multi-layered, this has become a major advantage of SMT. In order to meet the requirements of SMT, many components are designed with leads (pins) on the bottom or around the package. Take multilayer ceramic capacitor (MLCC) as an example, its end is plated with nickel or copper protective layer so as to add leads or strips to adapt to SMT.
Advantages of SMT

The most obvious advantage of SMT is that there is no need to drill holes in PCB, and the components are fixed by pads on the surface of PCB. As mentioned earlier, compared with axial or radial Through Hole PCB Assembly, SMT often takes up less space and can be installed on both sides of MDF. With the shrinking appearance of electronic products, and designers' increasing emphasis on size, weight and power (SWaP), the space efficiency of SMT has been more reflected. Furthermore, SMT welding requirements are simpler-only one side of PCB needs to be welded, while Through Hole PCB Assembly needs to be welded on both sides. In addition, SMT uses reflow soldering instead of wave soldering. Wave soldering is more common for Through Hole PCB Assembly, which is a more complex technology and needs to control environmental factors such as temperature.

Advantages of Through Hole PCB Assembly
Components assembled by SMT are only fixed on the surface of the circuit board, while Through Hole PCB Assembly's through-hole insertion method establishes a stronger connection between components and PCB.

In addition, the leads of Through Hole PCB Assembly can directly absorb thermal stress and mechanical stress, which can effectively protect electronic components, but the protection that SMT can give is very limited. Finally, regardless of welding requirements, Through Hole PCB Assembly is more advantageous from the perspective of testing and prototyping, and it is more convenient to manually remove and replace components on PCB.
SMT and Through Hole PCB Assembly, both of which are indispensable.
Although SMT is a newer assembly method, it can't completely replace Through Hole PCB Assembly. For users, these two methods have their own strengths-generally speaking, SMT has higher space and cost-effectiveness; Through Hole PCB Assembly has more advantages in resisting mechanical stress, electrical stress and thermal stress.

For example, components such as connectors and transformers may need to withstand greater mechanical impact. On the same circuit board, it is often necessary to use these two different assembly methods at the same time. For example, components that need to be connected with other components can be assembled by Through Hole PCB Assembly, while other components on the circuit board can be assembled by SMT.
Through Hole PCB Assembly and SMT components
Through hole PCB assembly and SMT components include active and passive components. An obvious difference between Through hole PCB assembly and surface mount components is the size. Through hole PCB assembly is usually larger, while some SMD components are very small (for example, the size of 0402 resistor is only 1 mm x 0.5 mm). However, this does not necessarily mean that they need to occupy more surface space on the circuit board. For example, compared with similar SMT capacitors, Through hole PCB assembly capacitors are usually much taller and require more space in the vertical direction.
Ths without exception need more space in the board, because they need plated through-hole (PTH) vias that extend from the top surface to the bottom (or back) of the board. On the other hand, SMD may or may not need vias. When vias are needed—such as ball grid array (BGA) and QFN (quad flat no-leads) parts—these vias are either micro-vias, blind-vias, or punched holes on pads (this is the most difficult type of vias for most contract manufacturers). Other SMD packages, such as small outline packages (SOP) and small outline ics (SOIC), have pins protruding from the parts to the surface of the circuit board. Depending on the layout, vias may or may not be needed.

Depending on whether the design chooses to use Through hole PCB assembly or SMT, a major consideration is how the circuit board will be assembled.

PCB assembly process

PCB assembly is the key final process of manufacturing circuit boards, so it is the responsibility of the assembler to ensure that the finished products meet the design requirements. This manufacturing stage includes a lot of inspection, verification and the following basic steps:

1. Apply solder paste to prepare for welding.
2. Pick the right parts.
3. Put the components on the footprint accurately.
4. Weld the components in place.
5. Check the welding quality and make corrections if necessary.
6. Clean up the excess debris on the circuit board.
7. Disassemble the circuit board into a single unit for packaging and transportation.

The above steps are applicable to all circuit boards, regardless of the components used. The main difference lies in the welding process implemented by the contract manufacturer. For Through Hole PCB Assembly, wave soldering is almost always used. However, for SMT components, this process is more complicated.
How does SMD affect circuit board assembly?
When soldering surface-mounted components, reflow oven is usually used for wave soldering, which may not be able to accurately cover small-sized and fine-pitch components. In the reflow process, enough time will be provided for the solder to reach all necessary areas and the temperature will be controlled. Many SMT packages need to use vias, which also means internal circuit board inspection, which is not particularly important for Through Hole PCB Assembly components. SMD will also have solder bridging, that is, one side of the component falls off and needs to be reworked.
Another important factor that affects the choice of component technology is the type of design. For example, for high power consumption applications, Through Hole PCB Assembly is recommended, because these devices tend to be heavier and require stronger adhesion, and Through Hole PCB Assembly is just enough. On the other hand, SMT components are lighter and only connected to the surface of the circuit board. This means that they are more likely to be displaced in an environment with vibration and/or continuous motion. In these cases, it may be necessary to add supports to ensure that the SMD is maximally fixed in place.
What is the difference between PCB SMT and through hole PCB assembly?
Chip assembly (SMT and through hole PCB assembly) refers to placing chip-like components or miniaturized components suitable for surface assembly on the surface of printed boards according to the requirements of circuits, and assembling them by soldering processes such as reflow soldering or wave soldering. And then used in the production of finished products.



SMT is a surface mount technology, and through hole PCB assembly is a traditional through-hole insertion technology. From the perspective of assembly technology, the fundamental difference between SMT and through hole PCB assembly is "sticking" and "inserting". On the through hole PCB assembly circuit board, components and solder joints are located on both sides of the board respectively; On SMT circuit board, solder joints and components are all on the same side of the board. Therefore, on SMT circuit boards, through holes are only used to connect the wires on both sides of the circuit board, so the number of holes is much less and the diameter of holes is much smaller. In this way, the assembly density of the circuit board can be greatly improved. Second, compared with through hole PCB assembly, SMT has the following advantages:
1.Realize Miniaturization
SMT electronic components, its geometric size and occupied space volume are much smaller than those of through-hole plug-in components, which can generally be reduced by 60% ~ 70%; The weight is reduced by 60%-90%.
2. The Signal Transmission Speed is High
SMT has compact structure and high assembly density, which can reach 5.5 ~ 20 solder joints/／cm；; Due to the short connection and small delay, high-speed signal transmission can be realized; At the same time, it is more resistant to vibration and impact. This is of great significance for the ultra-high speed operation of electronic equipment.
3. Good High Frequency Characteristics.
Because the components have no leads or short leads, the distribution parameters of the circuit are naturally reduced, and the radio frequency interference is reduced.
4. It is Conducive to Automated Production.
Improve the yield and production efficiency.
5.The Material Cost is Low
The packaging cost of most SMT components is lower than that of iFHT components of the same type and function. As a result, the selling price of SMT components is lower than through hole PCB assembly components.
6. Improve the Production Efficiency
The packaging cost of most SMT components is lower than through hole PCB assembly components of the same type and function. As a result, the selling price of SMT components is lower than through hole PCB assembly components.

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