Surface mount components – sometimes referred to as SMT or SMD components – are components which sit on the surface of a PCB, bridging pads to form an electrical connection. Such components are the most popular form of components for electronics as they enable faster assembly when using automated equipment such as pick and place machines. Almost any major component can be purchased in an SMT format, from capacitors and resistors to microcontrollers, drive chips and connectors.
Through-hole (TH) components are components which are leaded and in which the leads poke through holes in the PCB and are soldered on the opposite side of the board. These components were the first type of electronic components that were developed and most people have some experience of them, with small electronics projects at school or college. As with SMT components, it is almost always possible to get components in TH format but the assembly process for these tends to be slower than with SMT components. That said, equipment such as wave solder machines or selective soldering machines can be used to speed the assembly process up significantly.
As with virtually everything in engineering, it is not possible to say which is better overall. Rather it is important to consider the pros and cons of each type of component in order to understand which is the best option for a particular project. As an example, surface mount connectors, once they go over a certain size, can be prone to breaking off a board because the physical connection to the board (rather than the electrical connection) is not as strong as with a TH component. This is why a common option is for the bulk of the components on a board to be SMT, but the connectors to all be TH. By having the lead going through the board and being soldered on the underside of the board it is possible to get a much stronger physical connection to the board, reducing the risk of connectors being inadvertently pulled off through vibration or shock.
The same applies for naturally larger components such as high capacitance capacitors. SMT versions can be vulnerable to damage, whereas TH versions will be much better fixed. Depending on the application and the environment that a component is going into, it may not be necessary to use TH, but in other cases it will be important that these are used in order to ensure the board meets the minimum life targets of the customer.
Likewise, there are component choices where SMT would be the obvious choice – for example small capacitors and resistors. Such components can be quickly mounted on the board using a pick and place machine, meaning that production times are reduced significantly. Equally, SMT versions do not require leads and therefore use less material than a TH part, meaning their environmental impact is significantly reduced.
Choosing the right type of components ultimately boils down to an understanding of the application that a controller is being used in, the quantities it is required in and what components are available. A single prototype will nearly always be a faster build with TH than SMT. Equally, a production run will be faster with SMT than TH. However, by far the most common solution on any power electronics is a combination of the two, balancing the pros and cons of each and using them in roles where the pros come to the fore.