Understanding SMD Resistors (Surface Mount Resistors)

The SMD resistors used surface mount technology SMT to offer considerable advantages in terms of space-saving and automated manufacture of the printed circuit boards. Surface-mount resistors are used in large quantities, most professionals and industrial electronics now employ this technology. These types of resistors are designed to be surface mounted and they are typically much smaller than the traditional resistors. This is why they take less space in a circuit board.

Today you’ll get to know the definition, applications, code, construction, packages, diagram, specifications, and markings of SMD resistors. You’ll also learn the surface mount technology and the advantages and disadvantages of SMD resistors.

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What is an SMD resistor?

SMD is an abbreviation of the words Surface Mounted Device. It is an electronic component that can be mounted directly to a PCB circuit board using Surface Mount Technology (SMT). One must come across these resistors in professionally manufactured PCB’s and for most homemade circuits, you will make use of the more classic through-hole technology resistors. This is because through-hole resistors can be easily mounted and do not require any specialized equipment as the SMD.

Surface mount technology was invented to reduce the size of components and to significantly reduce the time taken to manufacture a circuit. The SMT improves manufacturing, enabling a high level of automation and improving reliability. Also, it enables greater levels of functionality to be achieved within a reasonable size and reduces cost significantly.

The surface mount resistors are the style of preference for virtually all electronic equipment in terms of quantities used. Also, surface mount resistors provide the same functionality as the more traditional axially leaded resistors with a lower dissipation capability. They often lower stray inductance and capacitance. The device is available in all popular values, E3 to E192, and is available in a variety of sizes, some are too small that they cannot be handled manually.

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Surface mount technology

Apart from the SMD resistors, other components employ surface mount technology. This technology has now become the technique for manufacturing electronic equipment which is much faster and more reliable construction of electronic printed circuit boards. Surface mount technology offers great benefits for mass production. Conventionally, components possessed at either end and were attached to either terminal or through holes in a printed circuit board. Surface mount technology replaces them with contacts that can be mounted directly onto the board and easy soldering can be done. It does away with the leads.

SMD Resistor Code

The SMD resistors are not designed with a color band system like the normal resistors. This is because they are too small that the color band code can be printed on them. Thus, they come with three code systems that specify them. Two of these codes are defined under the IEC 60062:2016 standard, a four-digit system, and a three-digit system. The third system is a numbering system called “EIA-96”, specified by the Electronic Industries Alliance who ceased operation in 2o11.

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Construction

The SMT or SMD resistors have rectangular shapes which are why they are often called chip resistors. They have metalized areas at either end of the main ceramic body which is why they can be set onto a printed circuit board that has pads. The pads allow the two ends to be set and provide the connection. In the construction of SMD resistors, alumina or ceramic substrate are used and the end connection electrode bases are placed to it. It is then fired to ensure they are perfectly held in place.

A thin film of resistive material is then deposited to it – this is either a metal oxide or a metal film, the resistor is fired again. The reactance of resistors can be determined by the length, thickness, and material used. The component is covered with successive layers of a protective coat which are all allowed to prevent mechanical damage and avoid ingress of moisture and other contaminants. The final stage of carryout in the construction of SMD resistors is applying a marking, that is, if the resistor is sufficiently large for it.

In addition, SMD resistors are manufactured using metal oxide or metal film and are protected using a robust coating. This is to say, they are stable and have a good temperature and time tolerance. The terminations at either end of the SMD resistor are key to the overall performance of the resistor. A nickel-based layer is used for the internal connection between the resistor element and the terminations. However, the outer layer of the connection employs a tin-based layer so that good solderability can be achieved.

SMD resistors diagram:

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SMD resistor packages generally conform to the standard SMD outlines for passive SMD components. Although, less standard packages are available for use only that they are not widely used. In the new design of very small packages that allows power dissipation, thus, board space is saved and further miniaturization of the equipment is allowed. This also allows more functionality to be packed into the same space.

The table below shows the surface mount resistor package:

With the above table, you can see that the package size descriptor is taken from the measurement of the resistor package measured in inches. This is to say, an 0603 SMT resistor package measures 0.06 x 0.03 inches.

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Specifications

Because there are different manufacturers of SMD resistors out there, their specifications vary. So, it is important to look at the manufacturer’s rating for a specific SMD resistor before choosing. However, there are some general specifications such as power rating, temperature coefficient, tolerance, etc. that must be known to give a clear understanding of the SMD resistors.

Power rating:

The power rating of any resistor is very important, for surface mount resistors, the levels of power that can be dissipated are smaller than those of the normal types. The table below will serve as a guide for a typical power rating of some of the popular SMD resistor sizes. Note, this is just a guide, It may vary according to the manufacturer.

Although most manufacturers claim their SMD resistors have higher power levels than the above list ones. So, it is always better to de-rate the components and not run them close to their maximum ratings. Derating below 0.5 or 0.6 will further improve the reliability.

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Temperature coefficient:

The metal oxide film enables SMD resistors to provide a good temperature coefficient. They are available in different values such as 25, 50, and 100 ppm / ⁰c. the technology used for SMT resistors is way better than some older tech employs for the conventional types. With this new tech, the circuits experience much better temperature stability. Finally,

Tolerance:

Manufacturing SMD resistors with metal oxide film have to offer them relatively close tolerance values. 5%, 2%, and 1% are the ones available, although some specialist applications 0.5% and 0.1% values are available. Even though close tolerance resistors are not often required, their use will help ensure better repeatability from one circuit or module to the next. This reduces the number of wide tolerance components used within the circuit. The 2% resistors are more expensive than the 5% and are widely used and the use of 0.5% and 0.1% tolerance SMT resistors are not usually needed except for highly demanding requirements. They are likely to cost much than the 2% electronic components.

Advantages and disadvantages of SMD resistors

Advantages:

Below are the benefits of SMD resistors in their various applications:

• Surface-mount resistors are naturally much smaller in size than conventional resistors.
• The size and construction of these types of resistors reduce inductance, that is, they have much lower levels of stray inductance and capacitance.
• This is why they are used for much higher frequency operations.
• The SMT resistors can be manufactured to high tolerances.
• They possess a good temperature coefficient of resistance and long-term resistance stability.

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Disadvantages:

Despite the good advantages of SMT resistors, some limitations still occur. Below are the disadvantages of SMT resistors in their various applications:

• The power rating of these surface mount resistors is smaller than the conventional resistors.
• They are used in applications where their circuit levels are lower.
• Serious care and concentration are required to ensure power ratings are not exceeded.
• Rework of surface mount resistors is often required.
• Separate technology is required to fix them
• The component is very tiny.

Conclusion

SMD resistors used surface mount technology SMT to offer considerable advantages in terms of space-saving and automated manufacture of the printed circuit boards. SMD is an abbreviation of the words Surface Mounted Device. It is an electronic component that can be mounted directly to a PCB circuit board using Surface Mount Technology (SMT). That is all for this article, where the definition, applications, code, construction, packages, diagram, specifications, and markings of SMD resistors are being discussed. You also learned about the surface mount technology and the advantages and disadvantages of SMD resistors.

I hope you get a lot from the article, if so, kindly share with other students. Thanks for reading, see you around!