SMD Resistor: All You Need to Know

SMD resistors have been accepted as the standard resistor type across the electronics industry. They offer high efficiency, low power dissipation and compact size. What are these resistors? How do they operate, and how is it used in manufacturing PCBs? 

After reading this article, you will know all about SMD Resistors, their function, applications and uses. You will have a clear understanding of which type of SMD Resistor suits your business the best.

What is a SMD Resistor?

An SMD resistor is a small electronic component often of a rectangular shape, which acts as a passive source of resistance. SMD resistors can be mounted on the surface of the PCB, thereby removing the need for holes. Therefore, PCBs can be reduced in size without any compromise to their functionality.

Various types of SMD resistors

These are classified on the basis of their construction, performance, size and material. 

• Thick film resistors are cost-effective and used in several general-purpose applications. 
• Thin film resistors are used in applications where precision is key. They have a low temperature coefficient, which makes them reliable for use in varying environments. 
• Metal strip resistors are used for current sensing and have great stability. 
• Wire wound resistors feature molded construction and can handle high power circuits.

These resistor types are widely used in various electronic devices. This includes lighting systems like CFL and LEDs, home electronics, automotive electronics and medical devices.

What are the Functions of SMD Resistors?

SMD resistors provide a variety of functions in electronic devices. Some of these are:

1. One of the most important functions of SMD resistors is current regulation. By controlling the amount of current flowing into each branch, they protect other devices. SMD resistors are also used to set the bias point of an electronic component, supplying it with a steady amount of current.
2. Another vital function of SMD resistors in electronic circuits is voltage division. By setting the value of divided voltage across other components connected in series, we can ensure that each device is operating at its rated value.
3. In circuits with a high operating frequency, SMD resistors provide high quality undistorted signals. SMD resistors possess low parasitic inductance and capacitance. Due to their small size, they remove unwanted impedance from the circuit, which is the desired property for circuits designed for alternating currents.
4. SMD resistors act as signal attenuators, where excessive signal strength can lead to noise.
5. By impedance matching between two circuits, SMD resistors are able to effectively transmit power with minimal losses.

SMD Resistors: Advantages and Disadvantages

In this section, we will discuss the advantages and disadvantages of SMD Resistors.

Advantages

1. Due to impedance matching and power conservation, SMD resistors are ideal for high frequency circuits.
2. Due to their small size and precision, SMD resistors are compatible with automotive devices.
3. SMD resistors allow for a greater component density on the PCB than their through-hole counterparts. They also support more connections per component.
4. SMD resistors are manufactured in automated assembly lines. It is faster as compared to other manufacturing processes. Modern assemblies can produce hundreds of thousands of components in an hour.
5. SMD components are cheaper than through-hole components due to their automated manufacturing process.
6. Since SMD resistors do not require holes to attach, they can be placed on both sides of a PCB, thus saving space and decreasing circuit size.
7. SMD resistors and SMT components have better performance under mechanical stress and shock, making them suitable for portable electronic equipment.
8. SMD resistors offer low impedance, making them suitable for circuits designed for alternating current.
9. Due to their heat dissipation properties, SMD resistors are excellent in thermal management.

Disadvantages

Despite becoming the industry standard for PCB production, SMDs have certain disadvantages as well.

1. Due to their small size and complexity, SMD resistors are not suitable for manual assembly and prototyping. This means that any modification to the circuit board after manufacturing needs to be done by professionals with appropriate tools. It is difficult to perform DIY tasks with SMDs.
2. The solder connection that is used to mount an SMD to the circuit board may be damaged by potting compounds during tests such as thermal cycling.
3. SMD resistors cannot be modified or replaced easily in case of damage. While all through hole components can be installed into sockets, SMDs do not support this operation.
4. SMD resistors cannot be used on breadboards. This makes them unsuitable for manual prototyping and testing. For testing SMDs, a breakout board may be used, which is an inexpensive form of PCB. Manual testing with through-hole resistors is significantly easier, however.
5. The small size of SMDs place a restriction upon the heat they can dissipate. Therefore, they are not suitable in circuits where heat management is essential.
6. SMDs are not suitable as connectors for external components, which are exposed to a lot of external friction. For this purpose, through-hole resistors might be better suited.
7. SMD resistors have substantially lower surface area as compared to their through-hole counterparts. This means that their resistance value has to be recorded in code rather than color strips. They often require magnification to be studied, and are disadvantageous for repair work, reverse engineering and DIY applications.

Applications of SMD Resistors

SMD Resistors have replaced through-hole resistors to a great extent in industry applications. Currently they are used in a wide variety of electronic devices spanning through different segments. In this section we will discuss some applications of SMD Resistors.

Home Electronics

The most widespread use of SMD resistors can be seen in home electronics. With each passing generation, the size of electronics has decreased while their computational power has increased. A simple smart phone can perform more tasks than the best computers of the 1980s. This is possible due to the decrease in the chip size and circuits.

SMD resistors have a huge role to play in this. From smartphones and tablets to televisions and monitors, SMD resistors are used in all devices to keep the PCB size small. SMD resistors are also used in gaming consoles and accessories, which often require high power efficiency and portability.

Automotive Industry

Modern vehicles rely on electronic systems for a number of essential operations. The industry of automotive electronics has seen tremendous growth in the past century, with more and more car manufacturers depending upon electronic devices for functions such as engine management, ignition, entertainment, heating and radio.

Advanced Driver Assistance Systems rely on PCB design which incorporates SMD resistors. Lighting systems are becoming increasingly complex, and rely upon SMTs for dimming control and headlight beam shaping.

Automation Industry

In the automation industry, various electronic equipment and machinery is used to reduce human involvement. Control systems are an integral part of automating the processes in factories, boilers, ships, aircrafts and vehicles. In PLCs, SMD resistors are used for signal termination, power filtering and input/output interfacing.

Power Supplies and Converters require efficient voltage regulation, which is provided by SMD resistors. In Robotics and Machinery, there are several moving parts and motors, in which SMDs are highly suitable. Finally, SMD resistors are used in sensors and actuators for temperature sensing and signal amplification.

Medical Equipment and Devices

In the medical industry there is a need for highly precise equipment with robust construction. Diagnostic equipment such as ECG machines require signal conditioning and voltage regulation. Portable medical instruments are used for imaging and feature several LED indicators.

SMD resistors are used in these devices for their efficient power management and stress handling. Monitoring systems are another critical component of healthcare that require high performance for long hours. SMD resistors are ideal for these as they are used for sensor interfacing, signal filtering and power distribution among the monitoring units.

Lighting Systems

LED Lighting boards and panels often have thousands of LED bulbs installed over a few inches of space. LED line arrays connected in series require efficient current regulation in order to avoid failure. SMD resistors are also used in dimming control in LED lights. Smart Lighting Systems are another domain in which SMD resistors are used to great benefit.

Smart Lighting Controls are often portable handheld devices that need to connect with several small sensors. SMD resistors are used for signal conditioning and impedance matching in these communication circuits.

 What limitations do small size resistors bring?

While the small size of SMD resistors has made it possible for them to be used in portable, high speed and complex electronic products, there are some limitations on resistor size that it is important to discuss. In this section, we will go over these limitations in detail.

1. The greatest drawback of SMD resistors are its thermal dissipation properties. Due to small size, there is less surface area, and therefore the resistor is unable to dissipate large amounts of heat. This makes SMD resistors prone to overheating.
2. Due to smaller size and less surface area, SMD resistors are not able to operate in high-power circuits. Larger resistors are better suited for power dissipation in such cases.
3. Manufacturing of SMD resistors is a complex process and cannot be carried out manually. This is due to the small size of its parts. As size decreases, the placement and handling of the soldering equipment becomes increasingly difficult, hence the need for machines.
4. Prototyping of SMD resistors cannot be performed manually. The small size of these resistors means that they can easily be damaged by soldering guns or other equipment. Therefore, they require robotic handling. This is often expensive.
5. SMD resistors are small in size, therefore, it is not possible to denote their resistance value through color coding like in through-hole resistors. Therefore, different codes like three- and four-digit systems and the EIA-96 systems have been developed to define their resistance.

Impact of SMD Resistor Size on PCB Design

SMD resistors are manufactured in different sizes according to certain specifications, also known as packages. Each package has certain characteristics like size, shape, lead type etc. The choice of SMD resistor may impact PCB design to a great extent.Knowing these in advance can avoid problems that may arise when we make PCBs.

1. Smaller SMD resistors like 0201 and 0402 enable electronic components to be packed closely together. This increases the component density, reducing the material cost. These compact PCB designs are useful for small devices like smart watches. However, it is not possible for these small components to be mounted manually, therefore, robotic help is required in construction.
2. PCB routing is greatly affected by SMD resistor size, smaller resistors offer greater options for routing design. They vacate enough space to allow other bulkier components to be placed.
3. In power-intensive electrical circuits, it is preferred to use larger SMD resistors like 1206 series. This is because larger resistors are better at power and thermal dissipation than smaller ones. Smaller resistors may suffer from overheating or failure in a power-hungry circuit.
4. In terms of physical robustness and durability, large SMD resistors are better performing than small SMD resistors. Small SMD resistors are susceptible to damage from environmental factors such as mechanical stress, temperature changes or humidity.
5. The cost of a PCB depends on many factors such as components, density, board size, layers, copper thickness etc. Some small SMD resistors may be more expensive due to their manufacturing process, however, they may save cost by reducing circuit size.

Conclusion

SMD resistors are an important electronic component which are widely used in modern electronic devices. They offer several advantages over through-hole resistors like smaller size, automated manufacturing, flexible PCB design and lower cost.

However, they are not without limitations and it is important to understand what factors affect your electronic design the most in order to determine which kind of resistor is right for your business.