In the early stage of development, circuit components used to be connected using wires. Later, wood was used a substrate with copper foils. Technology slowly paced towards the use of printed circuit boards (PCB). As the name goes, PCBs are just the insulated substrate board used for only printing the circuit leaving space for the components to be placed. Initially, the components used to come leaded, which required holes to be made on the PCB during designing. This is through hole technology. Later, with the advancement of technology on the need basis, components started to come in market without leads. This led to the evolution of surface mount technology.
Through hole technology
Electronic components used to come in leaded form in the early states of development of PCBs. Hence, while designing circuits, space had to be left for the components and holes had to be drilled on the board. The lead of the components were put in the holes and soldered on the other side of the board. This rendered other side of the board unsuitable for designing. Later, when multilayered boards came into picture, holes used to be drilled through plates with an intention for interconnection of the plates and not for the purpose of soldering components.
The components used to come in two configurations namely, radial and axial based on the position of leads. The axial leads had lead coming out of two opposite sides of the component with a straight angle between them. They were marked with less height and more length. When used in a circuit, the circuit tended to be more in length with less in height.
The radial components have lead in the same surface and are parallel to each other. They offer height but have less circuit length. So the circuit designed with these components have more height and less length.
When radial components were required, axial components were used by bending one of the leaded parts.
The bonding of soldering is very strong and the circuit is regarded for their mechanical strength and resistance to shocks and jerks.
Making them useable with breadboard sockets for testing makes it preferable. Drilling holes on the board makes the process expensive and the other side of the board is rendered useless due to soldering.
Surface mount technology
It differs from the through hole technology in the sense that the components are places and soldered on the same side of the PCB. Although for the sake of electrical conductivity between the surfaces, holes have to be drilled on the board.
The surface mount technology became popular in 1980s due to the components getting manufactured without leads. The components have two metallic ends, which can be put on metallic beds and be soldered. This results in flat profile of the resultant circuit. Since the components were placed and soldered on the same side, the other side could be also designed and better economy in production could be achieved. The smaller component size resulted in high density circuits resulting in overall reduction in size. High data speeds were one of the biggest merits of the surface mount technology.
Besides these merits, this technology has demerits such as unreliable solder joints which are not resistant to chemicals and shocks and jerks. They are not compatible with breadboard sockets for testing as well.