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Basic principles of PCB assembly: through-hole and surface mount
There are two methods for placing components on a printed circuit board (PCB). One is through-hole mounting (THM), and the other is called surface mount technology (SMT). At EPR, we use both SMT and THM. Both methods have their own advantages and disadvantages. This blog will explain the differences between SMT and THM, as well as the advantages and disadvantages of both methods.
There are two methods for placing components on a printed circuit board (PCB). One is through-hole mounting (THM), and the other is called surface mount technology (SMT). At EPR, we use both SMT and THM. Both methods have their advantages and disadvantages. This blog will explain the differences between SMT and THM and the advantages and disadvantages of both methods.
Through-Hole Mounting (THM)
Through-hole mounting is the predecessor of surface mount technology. Through-hole technology has been in use since the introduction of second-generation computers. Through-hole mounting replaced the old point-to-point construction.
Back in the 1950s, PCBs only had tracks printed on one side of the board. Holes were added to the board, into which the leads of the components were inserted. In this way, the components were placed on the blank side of the board and soldered to the copper on the side of the board with tracks. Later, double-sided boards with tracks on both sides were introduced, as well as PCBs with tracks on the inner layers. With this introduction, the introduction of plated through-holes (PTH) also became a reality. This technology made it possible to connect components to tracks on the inner layers of the PCB.
Axial Leads vs. Radial Leads
There are two types of through-hole mounting components: so-called axial and radial lead components. Axial lead components have leads at both ends of the component. Radial components have leads at one end of the component.
Advantages and Disadvantages of THM
THM of course has its advantages and disadvantages:
Advantages
Through-hole mounting technology is more reliable than surface mounting technology due to the stronger connection between the component and the PCB. Therefore, most connectors are still using THM. This is also why THM is still heavily used in military applications and applications with a lot of stress or rapid acceleration on the PCB.
Testing or prototyping with through-hole components is easier than with surface mount components because through-hole components are easier to replace. Combining a breadboard with through-hole components is also an ideal combination for application prototyping.
Disadvantages
Through-hole components require a lot of space on the PCB compared to surface mount components.
Most through-hole mounting components require manual placement.
Surface Mount Technology (SMT)
Surface mount technology was introduced around 1960 and became widely used in the mid-1980s. In its early days, the technology was called "planar mounting." Components used for surface mount technology have so-called surface mount packages (SMD). The leads of these components are located under or around the package. The big difference compared to through-hole mounting is that it is not necessary to drill holes in the PCB to establish a connection between the tracks and the components on the PCB. The leads of the components will make direct contact with the so-called PADs on the PCB. A stencil is used to place solder paste on the empty PADs. A pick-and-place machine places the components on the PADs covered with solder paste. When all components are placed, the PCB and components are passed through a reflow oven or (in our case, vapor phase) to solder to the PCB.
There are many different types of SMD packages, varying in shape and material. These types of packages are divided into different categories. The "Rectangular Passive Components" category mainly includes standard SMD resistors and capacitors.
Advantages and Disadvantages of SMT
Like through-hole mounting technology, surface mount technology also has its advantages and disadvantages:
Advantages
SMD components are much smaller than THM components. This will significantly increase the overall density of the circuit board.
SMD components can be placed on both sides of the PCB.
Using a pick-and-place machine to place components will reduce production time.
SMD components are generally cheaper than THM components.
Disadvantages
Most SMD components are not suitable for high-power applications.
SMD components are not suitable for prototyping or testing small circuits.
As you can see, both technologies have their advantages and disadvantages. The choice of specific technology depends on the application and use of the product.
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