SMD Resistor

- an overview of surface mount technology (SMT) resistors, their packages and properties.

Surface mount device , SMD, resistors are the most widely used electronic component. Every day many millions are used to produce the electronic equipment from cell phones to televisions and MP3 players, and commercial communications equipment to high technology research equipment.

Basic Construction
SMD resistors are rectangular in shape. They have metallised areas at either end of the body of the SMD resistor and this enables them to make contact with the printed circuit board through the solder.

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.

Packages
SMD resistors come in a variety of packages. As the technology has moved forward so the size of the resistor packages has fallen. The main packages with their sizes are summarised below:

• 1812 - 4.6 mm x 3.0 mm (0.18" × 0.12")
• 1206 - 3.0 mm x 1.5 mm (0.12" × 0.06")
• 0805 - 2.0 mm x 1.3 mm (0.08" × 0.05")
• 0603 - 1.5 mm x 0.8 mm (0.06" × 0.03")
• 0402 - 1.0 mm x 0.5 mm (0.04" × 0.02")
• 0201 - 0.6 mm x 0.3 mm (0.02" × 0.01")

It can be seen from the dimensions in Imperial measurements, that the package names correspond to the dimensions in hundredths of an inch. This an SMD resistor with an 0805 package measures 0.08 by 0.05 inches.

Other sizes including 1210, 2010, 2512, 3616, and 4022 are available for some high power or specialist SMD resistor applications.

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 SMDs 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.

• 1206:       1/8 watt
• 0805:       1/10 watt
• 0603:       1/16 watt

Some manufacturers will quote higher power levels than these. The figures given are conservative.

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.

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.

Markings
Most SMD resistors do not have any markings on them to indicate their value. For these devices, once they are loose and out of their packaging it is very difficult to tell their value. There are a few of the larger style SMD resistors that are marked with their value. Three figures are used. The first two indicate the significant figures, and the third is a multiplier. This is the same as the coloured rings used for wired resistors, except that actual numbers are used. Therefore an SMD resistor with the figures 333 would have a resistance of 33 x 10³ ohms, or 33 kohms. However beware of resistors marked with figures such as 100. This is not 100 ohms, but it follows the scheme exactly and it is 10 x 10⁰ or 10 x 1 = 10 ohms. Sometimes these resistors are marked with two figures, i.e. 10 to prevent any misunderstandings.

Applications
SMD resistors are used in many designs. Their size not only means that they are suitable for compact circuit boards, and for automatic assembly techniques, but it also ahs the advantage that they perform well at radio frequencies. Their size means that they have little spurious inductance and capacitance. Nevertheless care has to be taken when calculating their power dissipation as they can only dissipate small levels of power.