Surface Mount Technology (SMT), SMT Components, and SMT Equipment are all terms for this technology. Almost all commercially produced equipment in the electronics assembly industry today uses Surface Mount Technology (SMT), mainly because of its advantages in the PCB manufacturing process and the fact that a large number of electronic components can fit into a given space due to SMT components. This article explains everything about SMT PCB assembly and its uses, parts, and considerations.

What is Surface Mount SMT PCB Assembly? How does it work?

In the 1970s and 1980s, PCB assembly for various devices became increasingly automated. It was not easy to assemble PCBs using standard components with leads. Resistors and capacitors required pre-formed information, and even the leads of integrated circuits had to be adjusted to the exact right spacing in order to insert them into the holes.

Due to tight tolerances, it was necessary to make the leads fit perfectly through the holes. Due to the lack of proper component fit, the machine often stopped and required operator intervention. If the SMT PCBA process was slowed down, it would lead to a significant increase in costs.

Devices with SMT Connections

Surface mount components are unique compared to leaded components. SMT components are designed to be placed on a circuit board and soldered, rather than wired between two points.

Traditional leaded components usually have their leads run through holes in the circuit board; but that is not the case here. Components can be packaged in a variety of ways, each with its own design aesthetic. Passive components, transistors and diodes, and integrated circuits can all be classified as SMT components, and the table below shows three types of SMT components.

Passive SMDs:
Passive SMD packages come in a wide variety of styles. On the other hand, most passive SMDs are either SMT resistors or SMT capacitors, both of which have very uniform package sizes. Many different components require their own packages due to the unique requirements of various components.

Transistors and Diodes
SMT transistors and SMT diodes are usually housed in a small plastic box called an SMT package. The leads in the package are bent to touch the circuit board, making the connection. These kits usually come with three tips. This makes it clear which direction the device should be turned.

Integrated Circuits
Integrated circuits come in a variety of package options. Depending on the level of interconnect required, the package is chosen. Unlike a VLSI processor and its companion chip, which may require up to 200 or more pins, a simple logic chip may only require 14 or 16 pins. Because of the wide range of requirements, there are a variety of packages to choose from.

Due to the widespread use of SMT PCBAs, there is a wide variety of components to choose from. Surface mount packages offer a wider variety of functions than standard leaded packages. This is the case due to the high demand.

However, the most common basic components (such as transistors and logic) and analog ICs (such as op amps) are usually available in both leaded and surface mount forms. Typical transistors (such as the BC109) and many op amps and virtual logic devices are available in both forms.

Surface Mount Component Design Technology

The main motivation for the move to surface mount technology was to greatly improve the speed, reliability, and cost-effectiveness of the PCB assembly process. Although this was the most obvious impact on technology adoption, it also created new electronic circuits and devices. Fortunately, the benefits of this shift outweighed the drawbacks in terms of development and circuit performance.

SMT PCB assembly has several advantages for development engineers, but there are a few things to note:

High-quality SMT resistors are closer to ideal resistors than leaded resistors due to their smaller size and lower stray capacitance and inductance.

Low parasitic capacitance and inductance: Similar to SMT capacitors, SMT capacitors have significantly less parasitic inductance. Faster and higher frequencies can be achieved by using standard SMT components instead of leaded components.

The lower the power rating, the better the performance of surface mount components

Surface mount resistors are a good example of such devices. Traditional leaded resistors will dissipate at least 0.25 watts of power. Surface mount resistors also dissipate less because they are smaller. If you are unsure, check the data provided by the manufacturer.

The electronics industry pursues smaller and more compact package sizes to achieve better performance, and SMT PCBA technology is a key enabler of miniaturization. Traditional leaded components cannot be mounted as tightly on a printed circuit board as ceramic components due to size and weight constraints. This shows that the increased functionality of integrated circuits has made the responsibilities of development engineers possible.

There are some additional considerations to be aware of with SMT PCB assembly technology; most designs remain the same, although designs tend to be more complex and feature more functionality than before. In this way, surface mount technology has facilitated the development of electronic products, allowing for a higher degree of complexity and enabling more functionality.

Assembling Circuit Boards Using Surface Mount Technology

Today, PCB assembly and manufacturing relies almost entirely on SMT. SMT allows more electronics to be packed into a smaller space. Due to their smaller size and generally higher performance, surface mount components can be used in automated pick and place machines, reducing or even eliminating the need for manual assembly.

Positioning of wired components is difficult, as the wires must be preformed to fit throughout the pitch, and even then they are prone to placement issues.

In recent years, automated placement of most PCB components has become commonplace. The need for manual intervention is decreasing over time. In the past, the placement of some connectors and some other components required manual assistance, but this has decreased over time.

In modern times, printed circuit board designs often limit this to a minimum, or even modify the design to include components that can be inserted automatically. Component manufacturers have produced several other surface mount versions of parts, allowing for almost complete automation of assembly of most boards.

Some components have problems with withstanding high temperatures. Some technologies have problems with soldering due to the high temperatures required for the soldering process. To combat this, you have the following options:

Integrated Circuits
Surface Mount Resistors
Several Types of Surface Mount Capacitors

For this reason, surface mount electrolytic capacitors were not initially adopted. Later, electrolytic capacitors that could withstand the heat of soldering were designed instead to use surface mount tantalum capacitors.

Board Expansion and Flexing

Surface mount boards can have problems due to temperature changes and board flexing. This is not a big problem as long as the components are leaded, as the wires on the components absorb the movement and relieve any stress.

This may not be the case with surface mount components. The printed circuit board is held in place by soldering. Unlike surface mount transistors and integrated circuits, which have leads extending from the body of the device, transistors and capacitors mounted on the surface of the board have no method of allowing movement.

Surface mount ceramic MLCC capacitors are most susceptible to board strain. They are more likely to break when subjected to tensile stress. It is no doubt a huge problem from a reliability perspective.

How to avoid warping and temperature expansion?

There are several protective measures you can take during SMT PCBA design and assembly to avoid warping and temperature expansion issues, as follows:

PCB power and ground planes must be placed evenly to avoid board warping during soldering, which can be severe in some oversized PCBs as the board is processed through the soldering process.
As many ground and power planes as possible should be laid out on the board to alleviate this problem. In some cases, this may cause the printed circuit board to warp.
It is recommended to use short and wide components instead of long and thin components. When the components are short and wide, expansion and bending are less noticeable.
Conclusion

While some SMT components can be used in house construction, care must be taken when soldering. Even ICs with many pins can be difficult to solder.

Without the right tools, it is impossible to solder more than 50 pins. Mass production is the only use of these tools. Care must be taken even after the board has been made.

However, PCB assembly manufacturers can save a lot of money by using SMT PCB assembly components, which is why they are popular. Since they can still be hand-soldered, classic leaded components are still easily accessible to anyone building their own, and they offer a better alternative.

However, in some cases, SMT components can be used in DIY projects because the leads and connections of SMT components are not too small to be handled with more typical soldering irons and tools.

KKPCB offers Design for Manufacture and Assembly (DfMA) services. We conduct phased process design reviews, evolving from localized to comprehensive reviews. In production, we effectively identify manufacturing risks and process mismatches, allowing us to proactively address these issues.