- an overview of surface mount technology SMT resistors, or SMD resistors, their packages and properties.
Surface mount device , SMD, resistors are the most widely used electronic component.
Every day many millions of these chip resistors 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 SMD resistor construction
SMD resistors are rectangular in shape. Chip resistors have metallised areas at either end of the body and this enables them to make contact with the printed circuit board through the solder.
SMD resistor diagram
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.
SMD resistor cross section
Typically the ceramic substrate consists of a high alumina ceramic element. This gives a very stable insulating based onto which the resistive metal oxide element is set down.
The SMD resistor terminations are also important. They need to make a good reliable contact with the resistive element of the chip resistor, while also being able to provide high levels of solder-ability. This is often achieved by making the internal connection using a nickel based layer and then the outer layer of the connection uses a tin based layer to provide good solderability.
SMD resistor packages
Surface mount resistors come in a variety of packages. As the technology has moved forward so the size of the chip resistor packages has fallen. The main packages with their sizes are summarised below:
|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|
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.
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.
|Typical SMD Resistor Power Ratings|
|Package style||Typical Power Rating (W)|
|0402||0.0625 - 0.031 (1/16 - 1/32)|
Some manufacturers will quote higher power levels than these. The figures given here are typical.
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.
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. ..............
By Ian Poole
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