A diode is a two-lead electronic device that allows electrical signals to pass in one direction but not in the other. While most diodes have two wires with which to connect them to a circuit board, a surface mount diode has no such wires. Instead, each end of the diode is metallic and solders directly onto a small pad on the circuit board.
Diodes find heavy use in electronics to control signal flow and to build various types of voltage regulators and converters. Traditionally, these devices typically had wires that extended through a hole and metal eyelet in a circuit board, to which they were soldered into place. This not only attached the diode electrically, but also affixed the diode to the board, holding it in place and resisting any force that might dislodge it.
As electronics advanced, circuits became smaller and smaller as they were designed more to handle digital signals than analog signals. While diodes serve the same purpose in both types of circuits, digital signals typically required much less current flow. This resulted in the use of smaller and smaller diodes.
Eventually, engineers began to realize that one of the most significant aspects of wire-lead type diodes, the wire holding the diode to the circuit board, was no longer of any importance in many cases. In the 1960s, work began on creating a surface mount diode as well as other surface mounted components that did not rely on wire leads to hold them in place. The result was surface mount technology.
Though surface mount technology had its proponents, it did not see widespread use until the 1980s. The technology required specific parameters for a surface mount diode. It had to be constructed so it would be lightweight, easily mounted, and had as low a profile on the circuit board as possible. It also needed to have metallic ends in place of the wire leads.
Since the early days, surface mount technology has become standardized, though it still requires different equipment than used for mounting wire-lead devices. The primary reason for this is that a device such as a surface mount diode has no means to hold it in place while soldering. The production equipment itself now performs this function automatically for all surface mount components.
Removal of the wire leads and reducing the size of surface mount diodes resulted in production savings. In addition, the lack of holes in printed circuit boards and full automation of circuit board assembly further reduced costs. There are times, however, when a surface mount diode is simply not big enough for the job. In these instances, standard wire-lead diodes are still the device of choice.