What are SMD Resistors?
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 resistor calculator
This calculator helps you to find the resistance value of surface mount resistors. Just fill in the code written on the resistor and the value displays underneath.
The calculator can be used for 3 and 4 digit codes, EIA values and codes using ‘R’ or ‘m’. Although we did our best to check the calculator function for all different codes we cannot guarantee that it calculates the correct value for all resistors. Sometimes manufacturers may use custom codes. To be absolutely sure about the value it is best to measure the resistance with a multimeter.
SMD Resistor 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. SMD packages may be given in inches as well as in milimeters. It is therefore important to check the manufacturer’s documentation. In the table below the most common packages are given in imperial units with the metric equivalent. Furthermore an approximation is given for the typical power ratings.
Resistor SMD Code
Because of the small size of SMD resistors, there is often not room for the traditional color band code to be printed on them. Therefore, new resistor SMD codes were developed. The most commonly seen codes are the three and four digit system and an Electronic Industries Alliance (EIA) system called EIA-96.
The Three and Four Digit System
In this system the first two or three digits indicate the numerical resistance value of the resistor and the last digit gives a multiplier. The number of the last digit indicates the power of ten by which to multiply the given resistor value. Here are some examples of values under this system:
- 450 = 45Ω x 100 is 45Ω
- 273 = 27Ω x 103 is 27,000Ω (27kΩ)
- 7992 = 799Ω x 102 is 79,900Ω (79.9kΩ)
- 1733 = 173Ω x 103 is 173,000Ω (173kΩ)
The letter “R” is used to indicate the position of a decimal point for resistance values lower than 10 ohms. Thus, 0R5 would be 0.5Ω and 0R01 would be 0.01Ω.
The EIA-96 System
Higher precision resistors, combined with the decreasing sizes of resistors, have created the need to have a new, more compact marking for SMD resistors. Therefore the EIA-96 marking system has been created. It is based on the E96-series, thus aimed at resistors with 1% tolerance.
In this system, the marking exists out of three digits: 2 numbers to indicate the resistor value and 1 letter for the multiplier. The two first numbers represent a code that indicates a resistance value with three significant digits. In the table below the values for each code is given, which are basically the values from the E96 series. For example, the code 04 means 107 ohms, and 60 means 412 ohms. The multiplying factor gives the final value of the resistor, for example:
- 01A = 100 Ω ±1%
- 38C = 24300 Ω ±1%
- 92Z = 0.887 Ω ±1%
The usage of a letter prevents the confusion with other marking systems. However, pay attention because the letter R is used in both systems. For resistors with tolerances other than 1%, different letter tables exist.
As with package codes, these resistance value codes are common, but a manufacturer may use a variation on these or even something completely different. It is therefore always important to verify the manufacturer’s marking system.