In electronics assembly, you typically deal with two main methods of component assembly: surface mount technology (SMT) assembly and through-hole assembly (THT).

SMT assembly is a technology that places components directly onto a printed circuit board (PCB). This efficient method produces a densely packed but lighter weight end product.

In contrast, through-hole assembly has been around for some time. Here, components are inserted into pre-drilled holes in the PCB and then wave soldered from the bottom to fill the holes and create the necessary interconnections. The difference, while seemingly small, changes a lot of things, such as the board design, the materials and processes used, heat dissipation and the associated labour and setup costs.

That's where this guide comes in. It is designed to help you understand the differences between SMT and THT so you can choose the best approach for your next project. In addition, it provides some useful tips for designing SMT board designs to ensure they are easy to assemble.

Understanding the Basics of SMT PCB Assembly

SMT assembly is the process of assembling electronic products by mounting components directly to the surface of a printed circuit board through a batch reflow process.

The process begins with the application of solder paste to the PCB. Subsequently, the components are strategically placed onto the board. The entire assembly is then heated in a controlled manner through a reflow oven. The heat causes the solder paste to melt or ‘reflow’, creating a connection between the component and the PCB. Upon cooling, the solder solidifies and forms a strong bond between the component and the PCB.

SMT assembly is highly automated. This can significantly reduce labour costs for high volume production. However, for smaller operations, this method may be less efficient. In addition, skilled workers are required to manage complex equipment and perform any required manual rework.

The components used in SMT are called surface mount devices (SMD) or surface mount components (SMC). Unlike traditional components, SMDs have a unique package. Instead of having wired leads on the underside, they have small metal or ceramic contacts that allow them to be soldered directly to pads on the PCB. This unique package gives SMDs certain characteristics:

They are smaller and lighter than conventional components.

They have shorter lead lengths, which reduces inductance and capacitance and improves electrical performance.

SMDs are more susceptible to moisture-related cracking.

There are three main types of SMT assembly:

Type I: This type uses only SMDs. components can be mounted on one side (single-sided) or both sides (double-sided) of the PCB.

Type II: This type uses a mix of SMD and through-hole components. It is often used when certain components are not available in SMD form. This assembly type is the most complex to manufacture because it has many process steps.

Type III: This type replaces only discrete components (such as diodes or resistors) with surface mount components. Typically, these discrete SMT components are fixed on the lower side of the PCB, while the THT components are placed on the upper side.

SMT assembly offers several design and manufacturing advantages, such as:

High Component Density: SMT assembly allows more components per square inch of PCB space because SMD components are small and can be placed on both sides of the board. This facilitates more complex circuit designs without the need to expand the PCBA.

Weight Reduction: SMT components are up to ten times lighter than traditional components. This weight reduction is especially important in aerospace, where weight reduction is key.

Improved Electrical Performance: Surface mount packages have lower parasitic effects (unwanted inductive and capacitive components). This reduces propagation delays and lowers noise, and as clock speeds increase, this improved electrical performance becomes more important and critical.

Easier Automation: SMT is highly compatible with automated assembly processes. Automated pick-and-place machines can place components faster and more accurately than humans can, greatly speeding up the assembly process. This helps reduce the time required for prototyping and increases scalability.

Cost Efficiency: SMT assembly reduces board costs due to fewer drilled holes. Fewer drilled holes also facilitate the routing of the board's alignment.

Disadvantages of SMT Assembly
While SMT assembly has many advantages, there are several challenges to keep in mind:

Quality control challenges: Due to the high-speed, automated nature of the surface mount assembly process, problems in the assembly line can lead to a large number of defective assemblies before a problem is detected.

Repair or Replacement Difficulties: The small size of surface mount components and the high density of SMT boards make replacement or repair difficult. In addition, manual visual inspection is challenging and requires automated inspection equipment, which can increase overall production costs.

Reliance on solder joints: SMT relies more on solder joints for mechanical and electrical connections than through-hole technology. Therefore, any defects in these joints can compromise the reliability of the component.

Difficulty in part identification: The small size of surface mount components often makes clear labelling challenging, especially on passive components. This can lead to problems if devices are confused. They need to be positively identified or discarded.

Increased heat and moisture sensitivity: Surface mount packages encounter higher temperatures during soldering than through-hole components, which can lead to thermal stress if the components are not adequately acclimatised to these temperatures. In addition, some types of SMT components are also susceptible to absorbing moisture, which can lead to internal damage and a phenomenon known as ‘popcorn’ when heated up quickly during the soldering process.

Strict Cleanliness Requirements: Surface mount components are placed very close to the surface of the substrate with less clearance than through-hole packages. This requires strict process controls to ensure cleanliness and prevent contamination that can disrupt assembly and performance.

Determining the suitability of SMT for different applications

Surface mount technology is not intended to be a one-size-fits-all solution for all situations. As with any technology, it excels in specific applications, but may not be the best choice for others. So when is it appropriate to consider SMT for your project?

High-speed, high-density applications: SMT is particularly beneficial in situations where a high density of electronic components is required. This type of equipment significantly benefits from SMT's ability to mount more components onto a printed circuit board, on either side if desired.

Lightweight, compact devices: The use of SMT components reduces the weight and size of the entire device. This is beneficial for applications where weight and form factor are important, such as aerospace applications, drones and portable devices.

High-volume, low-cost production: From an electronics manufacturing perspective, SMT assembly lines offer speed and precision. The automation involved can significantly reduce production time, enabling efficient manufacturing of high volume finished boards. SMT offers significant advantages for mass production equipment where cost-effectiveness is critical.

Reliability and performance-driven applications: Smaller components and shorter signal paths in SMT assemblies can improve speed and signal quality performance. In addition, reducing the need for drilling enhances the structural integrity of the finished component, thereby improving its reliability.

However, SMT may not always be the best choice; SMT relies heavily on automated machinery and is therefore not well suited to manual assembly scenarios or solo prototyping. For hobbyists or in a prototyping environment, through-hole technology may be more convenient because it is compatible with manual assembly techniques.