There are two main methods when mounting components on a PCB. These are Through-Hole Mounting (THM) and Surface Mounting (SMT).
Through-hole mounting involves leads being placed into holes that have been drilled into the PCB board. With the advent of SMT, it was widely presumed that THM would be phased out. However, manufacturers still make THM PCBs, as there are clients who still prefer, or require boards that use the older board design. The main reason is that THM boards have greater reliability.
The reasons for increased reliability are evident. SMT components are secured to the board by solder. THM components are secured by leads running through the board. The result is that the THM boards can hold up in more stressful environments. As a result, boards used in military or aerospace situations often specify THM boards, as the conditions they are used in may include very high or very low temperatures, or rapid acceleration.
There are other reasons that THM boards continue to be used. A common use of THM boards is in prototype testing, as the board structure allows for manual adjustments. Not all components are available for surface mount situations. And in some instances, THM components are less expensive than the equivalent SMT component.
However, the benefit of lower cost components needs to be balanced with the higher cost of preparing the THM. The THM board needs to have holes drilled, which can be an expensive process, as well as being time consuming.
Another issue with THM boards is layering. Drilled holes must pass through all layers of the PCB. Therefore, routing on multilayer boards is more challenging for THM.
Manufacturing of THM is a more complex process. SMT soldering is completed using reflow ovens, while THM requires hand-soldering, and needs soldering on both sides of the board. SMT rarely requires soldering on both sides of the board. As a result, components can only be placed at the rate of hundreds per hour, so manufacture is relatively slow compared to SMT boards.
In contrast, SMT has components mounted directly onto the surface of the PCB. Virtually all components are now manufactured using SMT. The process is considerably more cost effective for most requirements. The component leads in THM are replaced by vias, which are small components designed to allow a conductive connection between layers. SMT allows used of components such as BGAs that allow for shorter leads and more interconnection pins. They can deliver higher speed processing.
In summary, in most circumstances, SMT will provide a more cost effective and more reliable board, and be suitable for high production runs. In contrast, THM boards have become a specialist product, providing a valuable tool in specific situations such as high temperatures, or situations that may involve mechanical stress.