Resistor SMD Code

Chapter 2 - Resistor Standards and Codes

What are SMD Resistors?

 

SMD resistors on a printed circuit boardSMD resistors on a printed circuit board. Image used courtesy of TE Connectivity

 

SMD stands for Surface Mounted Device. An SMD is any electronic component that is made for use with SMT, or Surface Mount Technology. SMT was developed to meet the ongoing desire for printed circuit board (PCB) manufacturers to use smaller components and to be faster, more efficient, and cheaper when assembling components to the PCBs.

SMDs are smaller than their traditional through-hole counterparts. They are often square, rectangular or oval in shape, with very low profiles. Instead of wire leads that go through the PCB, SMDs 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 thereby allows both sides of the board to be more fully populated with components.

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 pick-and-place machine. The components are fed to the machine in long tubes, on rolls of tape, or in trays. These pick-and-place machines can pick SMD device and place onto PCBs at rates of thousands of components per hour (cph); one manufacturer advertises a rate as high as 60,000 cph.

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 Packages

The term package refers to the size, shape and/or lead configuration of an electronic component. 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 millimeters. It is therefore important to check the manufacturer's documentation. The shape and size of surface mount resistors are standardized with most manufacturers use the JEDEC standards. The size of SMD resistors is indicated by a numerical code, such as 0603. This code describes the length and width of the package. More information is available in the discussion of Resistor Sizes and Packages.

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 Systems

In these systems, 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.

Three digit system

  • 450 = 45 Ω x 100 is 45 Ω
  • 273 = 27 Ω x 103 is 27,000 Ω (27 kΩ)

Four digit system

  • 7992 = 799 Ω x 102 is 79,900 Ω (79.9 kΩ)
  • 1733 = 173 Ω x 103 is 173,000 Ω (173 kΩ)

The letter "R" is used to indicate the position of a decimal point for resistance values lower than 10 Ω. Thus, 0R5 would be 0.5 Ω and 0R01 would be 0.01 Ω.

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 first table below, the resistance values for each code are given, which are basically the values from the E96 series. For example, the code 04 means 107 Ω, and 60 means 412 Ω. The second table below provides the multiplying factor for each letter code gives the final value of the resistor

Examples of converting EIA-96 markings to a resistance value:

  • 01A ⇒ Code 01 = 100 with Multiplier A = 1 ⇒ 100 x 1 = 100 Ω ±1%
  • 38C ⇒ Code 38 = 243 with Multiplier C = 100 ⇒ 243 x 100 = 24,300 Ω ±1%
  • 92Z ⇒ Code 92 = 887 with Multiplier Z = 0.001 ⇒ 887 x 0.001 = 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.

SMD Resistance Value Code Table
Code Value Code Value Code Value Code Value Code Value Code Value
01 100 17 147 33 215 49 316 65 464 81 681
02 102 18 150 34 221 50 324 66 475 82 698
03 105 19 154 35 226 51 332 67 487 83 715
04 107 20 158 36 232 52 340 68 499 84 732
05 110 21 162 37 237 53 348 69 511 85 750
06 113 22 165 38 243 54 357 70 523 86 768
07 115 23 169 39 249 55 365 71 536 87 787
08 118 24 174 40 255 56 374 72 549 88 806
09 121 25 178 41 261 57 383 73 562 89 825
10 124 26 182 42 267 58 392 74 576 90 845
11 127 27 187 43 274 59 402 75 590 91 866
12 130 28 191 44 280 60 412 76 604 92 887
13 133 28 196 45 287 61 422 77 619 93 909
14 137 30 200 46 294 62 432 78 634 94 931
15 140 31 205 47 301 63 422 79 649 95 953
16 143 32 210 48 309 64 453 80 665 96 976

 

SMD Resistor Multiplication Factors
Code Multiplication Factor
Z 0.001
Y / R 0.01
X / S 0.1
A 1
B / H 10
C 100
D 1000
E 10,000
F 100,000
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