SMD Resistors: Surface Mount Resistor

Surface mount resistors, SMD resistors use surface mount technology, SMT to provide considerable advantages in terms of space saving and automated manufacture of printed circuit boards.

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Surface mount resistors are used in truly vast quantities. As they are the component style of preference for mechanised manufacture of electronic circuits, surface mount resistors, SMD resistors, surface mount device resistors or SMT resistors, surface mount technology resistors are used virtually exclusively in these environments.

Surface mount technology

SMD resistors are just one form of component that uses surface mount technology. This form of component technology has now become commonplace for manufacturing electronic equipment as it enables much faster and more reliable construction of electronic printed circuit boards.

Note on the Surface Mount Technology:

Surface mount technology offers significant advantages for the mass production of electronic equipment. Traditionally components possessed leads at either end and these were attached to either terminals or later they were mounted through holes in a printed circuit board. Surface mount technology does away with the leads and replaces them with contacts that can be mounted directly onto the board enablong easy soldering.

Read more about Surface Mount Technology, SMT.

SMD resistor construction

SMT resistors or SMD resistors are rectangular in shape and as a result they are often known as chip resistors.

They have metallised areas at either end of the main ceramic body, and in this way they can be set onto a printed circuit board that has pads onto which the two ends are set to provide the connection.

Overall construction of a surface mount resistor showing the external view
Surface mount resistor features

The resistor is made by taking a ceramic substrate and then depositing a thin film of resistive material - typically metal oxide or a metal film. The length, thickness and material used all determine the resistance of the component.

The resistor itself consists of a ceramic substrate and onto this is deposited a metal oxide film. The thickness, and the length of the actual film determines the resistance. In view of the fact that the SMD resistors are manufactured using metal oxide, means that they are quite stable and usually have a good tolerance.

Cross section through a surface mount resistor showing the resistive element and other areas of the SMD
Surface mount resistor cross section

The terminations at either end of the SMD resistor are key to the overall performance of the resistor. The internal connection between the resistor element and the terminations typically uses a nickel based layer and then the outer layer of the connection uses a tin based layer to provide good solderability a key requirement for these components.

SMD resistor packages

The SMD resistor packages generally conform to the standard SMD outlines for passive SMD components. Needless to say, occasionally other less standard packages may be used.

There is a growing trend in new designs to migrate towards some of the very small packages where power dissipation allows. This saves board space and allows further miniaturisation of the equipment, or to pack more functionality into the same space.

Common Surface Mount Resistor Package Details
Package style Size (mm) Size (inches)
2512 6.30 x 3.10 0.25 x 0.12
2010 5.00 x 2.60 0.20 x 0.10
1812 4.6 x 3.0 0.18 x 0.12
1210 3.20 x 2.60 0.12 x 0.10
1206 3.0 x 1.5 0.12 x 0.06
0805 2.0 x 1.3 0.08 x 0.05
0603 1.5 x 0.08 0.06 x 0.03
0402 1 x 0.5 0.04 x 0.02
0201 0.6 x 0.3 0.02 x 0.01

SMD resistor specifications

SMD resistors are manufactured by a number of different companies. Accordingly the specifications vary from one manufacturer to the next. It is therefore necessary to look at the manufacturers rating for a specific SMD resistor before deciding upon exactly what is required. However it is possible to make some generalisations about the ratings that might be anticipated.

  • Power rating:  The power rating needs careful consideration in any design. For designs using surface mount resistors, the levels of power that can be dissipated are smaller than those for circuits using wire ended components. As a guide typical power ratings for some of the more popular SMD resistor sizes are given below. These can only be taken as a guide because they may vary according to the manufacturer and exact type.

    The power rating of a resistor is very dependent upon its size. Accordingly it is possible to generalise on the power ratings for SMD resistors of different sizes.

    Typical SMD Resistor Power Ratings
    Package style Typical Power Rating (W)
    2512 0.50 (1/2)
    2010 0.25 (1/4)
    1210 0.25 (1/4)
    1206 0.125 (1/8)
    0805 0.1 (1/10)
    0603 0.0625 (1/16)
    0402 0.0625 - 0.031 (1/16 - 1/32)
    0201 0.05
    Some manufacturers will quote higher power levels than these. The figures given here are typical.
  • Temperature coefficient:     Again the use of metal oxide film enables these SMD resistors to provide a good temperature coefficient. Values of 25, 50 and 100 ppm / C are available.
  • Tolerance:  In view of the fact that SMD resistors are manufactured using metal oxide film they available in relative close tolerance values. Normally 5%, 2%, and 1% are widely available. For specialist applications 0.5% and 0.1% values may be obtained.

Surface mount resistors are manufactured in their billions and used in every electronic circuit using SMDs. The SMD resistors are simple to manufacture now and can be obtained for a very low cost, especially when used in quantity. SMD resistors are now the most widely used form of resistor technology.

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