SMD means Surface Mounted Device. An SMD is any electronic component that is made to use with Surface Mount Technology (SMT). SMT was developed to meet the ongoing desire for printed circuit board manufacture to use smaller components and be faster, more efficient, and cheaper.
 
SMD resistors are divided into different packages. The term package refers to the size, shape and/or lead configuration of an electronic component. For instance, an IC chip that is has leads in two rows down opposite sides of the chip is called a Dual Inline Package (DIP) chip. In SMD resistors, resistor package designators tell the length and width of the resistor.


SMDs are smaller than their traditional counterparts. They are often square, rectangular or oval in shape, with very low profiles. Instead of wire leads that go through the PCB, SMD’s have small leads or pins that are soldered to pads on the surface of the board. This eliminates the need for holes in the board, and lets both sides of the board be more fully used.
  Small pads of silver or gold plate or tin-lead are placed on the board for attaching the components. Solder paste, a mixture of flux and small balls of solder, is then applied to the mounting pads by a machine similar to a computer printer. Once the PCB is prepared, SMDs are placed on it using a machine called a pick-and-place machine. The components are fed to the machine in long tubes, on rolls of tape or in trays. These machines can attach thousands of components per hour.
 
The board is then sent through a reflow soldering oven. In this oven, the board is slowly brought up to a temperature that will melt the solder. Once cooled, the board is cleaned to remove solder flux residue and stray solder particles. A visual inspection checks for missing or out-of-position parts and that the board is clean.