SMD: is an abbreviation for Surface Mount Device. These devices include active, passive and electromechanical components. The SMDs are external mountings which are mounted on PCBs which stands for Printed Circuit Board by means of soldering.
These tiny little devices have no extra leads but do comprise of metallic legs which are firmly soldered to the PCB during the mounting process. Certain SMDs neither have leads nor metallic legs and are thus soldered onto the PCB using solder balls.

SMDs are used in almost about all the electronic components today. Due to their tiny size and high efficiency these devices are mounted with extra care using a compatible technology more commonly known as the SURFACE MOUNT TECHNOLOGY (SMT). SMT is a method of production of electronic circuits that allow external electronic components to be mounted on it. This method is used in mass manufacturing of electronic circuits in a highly mechanized manner with great precision to maintain quality and low cost of manufacture. It is an assembling or mounting technology. SMT reduces the issues related to heavy wiring on a circuit board as it incorporated the SMD leads or legs onto an electronic circuit. An SMD can typically be categorized as:

Passive SMDs: There exists a large number of sub packages for this kind of SMDs. The more commonly used passive SMDs are resistors and capacitors which are very much standardized. Other components are crystals, coils and oscillators which have their own package requirements and are thus individually used. Resistors and capacitor packages comprise of 1812, 0402, 0805 like designations. The comparatively larger sizes such as 1812 and 1206 are some of the early SMD produces and are now not in much use as the tinier the size the more widespread is its application. These SMDs are soldered on metallized areas on the end of their packages.

Transistors and Diodes (Active SMDs): These are some of the electronics components which are an example for SMDs bearing leads. The leads emerge out of the tiny little package and are bent as required so as to solder or fit it to the PCB. These are tiny components that are used all across the globe and are in the form of a tiny plastic package.

Integrated Circuits (IC): There are a lot of ICs available to be integrated with a PCB. They have multiple packages to choose from are also standardized as well. An IC is basically a small chip that incorporates and entire complex OCB circuitry in it. They are made very compact and can hold upto several million transistors all in the size of a nail. They are much smaller when compared to discrete circuits and are more inexpensive with very high efficiency. Packages in ICs are required based on the interconnectivity requirements. Some ICs require 14/16 pins while other VLSI ICs may even go up to 200 pins. 7404, 7400, 7486 are some examples of ICs and are mounted on an electronic circuit using SMT. These ICs contain legs that are soldered firmly.

There are numerous advantages to SURFACE MOUNT DEVICES when compared to discrete electronic components. Smaller components, Low cost of production, lesser work needs to be done for the mounting process, easily replaceable and provides the best performance on extreme conditions are some of the features that make it a standout from its counterparts.