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.
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