In the early 1980s, surface mount technology emerged as a new technique for mounting electronic components that directly mount the electronic components on the surface of printed circuit boards. Since its invention, surface mount technology has revolutionized the electronic manufacturing industry. This is because it is more reliable, and has lower cost, weight, and volume. In contrast, components are inserted into the printed circuit board in Through Hole Technology (THT). This conventional method is costly, less effective, and has more weight and dense.

The requirements of modern electronic designs are shifting towards smaller, lighter, and higher computational capabilities designs. Surface mount technology has become the pioneer in meeting these requirements with its capabilities to accommodate more components in a given area, higher efficiency, automated assembly process, and cost-effectiveness. In this article, we will explore Surface Mount Technology (SMT) and its differences from through-hole technology, assembly process, and soldering techniques.

What is SMT?

SMT stands for Surface Mount Technology. In SMT, components are directly mounted on the surface of a printed circuit board. The components that are used for directly mounting on the surface of the PCB are known as Surface Mount Devices (SMD). Conversely, in Through Hole Technology (THT), components are inserted into the printed circuit board. This technology is time-consuming and more prone to errors due to the human involvement. The SMT, on the other hand, is a completely automated process that reduces cost and time and increases quality and efficiency. However, both surface mount technology and through-hole technology can be used on the same electronic board. This is due to the reason that some electronic components are not suitable for SMT such as high-power components like DC-DC converters and transformers.

Surface Mount Technology VS. Through Hole Technology  

Surface Mount Technology (SMT) and Through Hole Technology (THT) have become the industry standards in printed circuit board manufacturing. Both these technologies are primarily used for assembling electronic components on the printed circuit board. However, these technologies have distinct features, uses, processes, advantages, and applications.

Table I: Surface Mount Technology VS Through Hole Technology

When To Use Surface Mount Technology

Modern electronic devices are getting smart and lightweight with each passing year such as mobile phones, laptops, and other electronic gadgets. This transition has emerged as a new challenge for scientists to make more complex designs in less electronic board areas. Surface Mount Technology has emerged as the best solution to incorporate these challenges due to its ability to fit maximum surface mount devices (SMDs) in less area, resulting in lightweight and smart electronic devices. It is due to this reason that surface mount technology is extensively being used in almost every electronic industry such as medical devices, aviation, defense, and consumer electronics.

Surface Mount Technology uses an automated machine known as a pick and place machine to produce high volume production, making it ideal for many large-scale industries. SMT's extensive automated process enables it to achieve high efficiency and cost-effectiveness. Components used in surface mount technology known as Surface Mount Devices are very small in size which not only helps to fit more components in a given printed circuit board but is also beneficial for high-frequency circuits. Due to these reasons, large-scale electronic manufacturing industries mostly use SMT technology to manufacture compact, lightweight, smart, and high-quality electronic devices.

Advantages of Surface Mount Technology

Surface Mount Technology offers numerous advantages over through-hole technology, resulting in a major shift in the printed circuit board manufacturing industry to choose surface mount technology. Some of the advantages are：

·       In SMT, components are directly mounted on the surface of the electronic board unlike in THT components are drilled into the printed circuit board. This feature of SMT enables to assembly of components on both sides of a printed circuit board, making the more complex designs fit in a less given area. 

·       Unlike through-hole components, surface mount devices are very small in size, resulting in a maximum number of components to fit on an electronic board.

·       The assembly process of surface mount devices is expeditious with the use of a fully automated pick-and-place machine, resulting in high-volume production. The automated assembly process of SMT also enables fast production, high efficiency, and the least margin of error.

·       Signal integrity is one of the critical challenges in today’s modern electronic designs. In SMT, the components are smaller and mounted on the surface of the electronic board, resulting in less distance for signals to travel. This improves the problems of cross-talk between signals and reduces inductance and capacitance. Therefore, surface mount devices are ideal for high-frequency circuits.

·       The automated pick-and-place machines enable high-volume production, resulting in reduced overall manufacturing costs.

Disadvantages of Surface Mount Technology

Although, there are a lot of advantages of surface mount technology. However, there are certain disadvantages of SMT as well.

·       Surface mount devices are more prone to physical damage. This is due to the reason that SMDs are mounted on the surface of printed circuit boards, unlike in THT, where components are drilled to form greater mechanical bonds.

·       It is generally not suitable for high-power components such as DC-DC converters and transformers except for some low power regulators (LDOs). It is poor in heat dissipation.

·       The automated machines used in surface mount technology, such as pick-and-place machines, printing screens, and reflow ovens, are costly. Therefore, the initial capital cost of installing the necessary equipment for industry setup is quite high.

·       Due to the very small size of components, it is difficult to do any repair and reworks. It is also tough to identify the potential faults in the printed circuit board due to the complex design and small size of components. Special tools such as X-ray machines are required to identify potential faults.

·        It is difficult to inspect surface mount technology printed circuit boards because the components are of tiny sizes. 

SMT Assembly Process

The process that mounts the surface mount devices onto the surface of the printed circuit board is known as the SMT assembly process. There are four main steps in the SMT assembly process. Each step is explained as follows：

Solder paste is a type of mixture that is made up of solder powder and flux. This mixture is used to connect surface mount devices like resistors, transistors, inductors, and ICs to the electronic board assembly. In the first step, solder paste is applied to the pads available on the PCB. This is done by a thin sheet with pre-cut holes known as a stencil.

In the 2^nd step, surface mount devices such as resistors, capacitors, and ICs are picked and placed onto the pads of the PCB using an automatic pick-and-place machine. 

At this point, components are exactly placed onto the surface of the PCB using a pick-and-place machine. In 3^rd step, the Printed Circuit Board is now passed through a heating oven known as a reflow oven. This makes the solder paste melt and solder to form an electrical connection between different components. In the reflow oven, temperature is maintained such that solder paste is accurately soldered to the components. The temperature is programmed as per the reflow profile.

In this step, visual inspection is carried out using the X-ray machine to identify any soldering faults and any other defects.

In the final step, PCBs undergo a rigorous cleaning process to eliminate any flux residues left after the reflow soldering process.

Common Surface Mount Device Packages

The components used in surface mount technology for mounting on the surface of printed circuit boards are known as surface mount devices (SMD). These surface mount devices include several components such as resistors, capacitors, and Integrated Circuits (ICs). All these SMDs come in various sizes and lengths known as packages.

There are different packages of resistors, capacitors, inductors, diodes, and ICs available in the market. Some of the most commonly used packages are:

Table Ⅱ: Common Surface Mount Device Packages

Soldering Techniques Employed in SMT

In the printed circuit board manufacturing industry, mostly two techniques are employed known as wave soldering and reflow soldering.

1.     Wave Soldering

Wave soldering is not as common as reflow soldering. This technique is mainly used for Through Hole Technology but it can be used for SMT.

This technique is less efficient and more prone to errors as compared to reflow soldering.

2.     Reflow Soldering

Reflow soldering is one of the most widely used techniques. It is used for high-volume production and has the least margin of error. This technique is mostly used for surface mount technology. The reflow soldering process has the following steps.

Conclusion

To sum up, surface mount technology is the game changer in the printed board circuit manufacturing industry. Surface Mount Technology has emerged as the best solution to incorporate maximum surface mount devices (SMDs) in less area, resulting in lightweight and smart electronic devices. SMT enables the industries to produce high volume production, making it less costly and increasing efficiency.