What are SMD Resistors? [caption id="attachment_944" align="alignright" width="103"] SMD resistors on a circuit board from a USB memory stick[/caption] SMD stands for Surface Mounted Device. An SMD is any electronic component that is made to use with SMT, or Surface Mount Technology. SMT was developed to meet the ongoing desire for printed circuit board manufacture to use smaller components and be faster, more efficient, and cheaper. 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. The manufacture of PCBs using SMT is similar to that for components with leads. 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; one manufacturer advertises a rate as high as 60,000cph. 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. SMD...
Resistors are available in a large amount of different packages. Nowadays the most used are the rectangular surface mount resistors, but also the good old axial resistor is still used extensively in through hole designs. This page will inform you about the dimensions of SMD, axial and MELF packages and about the required land patterns for SMD components. SMD resistor sizes The shape and size of surface mount resistors are standardized, most manufacturers use the JEDEC standards. The size of SMD resistors is indicated by a numerical code, such as 0603. This code contains the width and height of the package. So in the example of 0603 Imperial code, this indicates a length of 0.060" and a width of 0.030". This code can be given in Imperial or Metric units, in general the Imperial code is used more often to indicate the package size. On the contrary in modern PCB design metric units (mm) are more often used, this can be confusing. In general you can assume the code is in imperial units, but the dimensions used are in mm. The SMD resistor size depends mainly on the required power rating. The following table lists the dimensions and specifications of commonly used surface mount packages. Code Length (l) Width (w) Height (h) Power Imperial Metric inch mm inch mm inch mm Watt 0201 0603 0.024 0.6 0.012 0.3 0.01 0.25 1/20 (0.05) 0402 1005 0.04 1.0 0.02 0.5 0.014 0.35 1/16 (0.062) 0603 1608 0.06 1.55 0.03 0.85 0.018 0.45 1/10 (0.10) 0805 2012 0.08 2.0 0.05 1.2 0.018 0.45 1/8 (0.125) 1206 3216 0.12 3.2 0.06 1.6 0.022 0.55 1/4 (0.25) 1210 3225 0.12 3.2 0.10 2.5 0.022 0.55 1/2 (0.50) 1812 3246 0.12 3.2 0.18 4.6 0.022 0.55 1 2010 5025 0.20 5.0 0.10 2.5 0.024 0.6 3/4...