Assembly Challenges: Navigating the Differences Between SMT and THT

The debate between Surface-Mount Technology (SMT) and Through-Hole Technology (THT) has been going on for years. Both technologies have their advantages and challenges to face, and choosing one or the other can sometimes impact production timelines and product performance. 

For instance, just in May 2024 the volume of printed circuit boards (PCBs) in Thailand was around 30 million units. And those PCBs can be both SMT or THT designed. Still, deciding for one or another is important as that can decide the nature of your project. 

What is Surface-Mount Technology?

SMT is a method where components are mounted directly onto the surface of PCBs. It’s a main form of assembly in modern electronics because of its versatility, efficiency, and space-saving advantages. Manufacturers don’t need to use leads that go through the PCB because SMT components are soldered onto pads on the surface of the board.

SMT components are normally smaller and lighter, which makes them ideal for compact designs like laptops, smartphones, and other wearables. The process is also largely automated, with robots handling the precise placement of these small components, so the production is faster. 

So far, SMT’s been largely popular, but it can still have some challenges, especially when it comes to fine or complex assemblies.

What is Through-Hole Technology?

Unlike SMT, THT is the older method for assembly. Here components have leads inserted through holes drilled into the PCB. These leads are soldered onto pads on the opposite side of the board. 

THT components are larger and bulkier and they provide a stronger mechanical bond. All of this makes them more suited to applications that require durability and longevity. 

Through-hole components are usually used in industrial equipment, aerospace, and other applications where durability under harsh conditions is critical. So, this is the main difference between SMT vs THT in PCB assembly–SMT is better for lightweight, compact designs, while THT excels in heavy-duty applications where vibration, physical stress, or heats is a problem. Somewhat, it’s like the difference between a ballerina and a Hulk. 

As you might expect, drilling through PCBs and manually inserting components can be time-consuming, leading to several assembly challenges that must be overcome. 

Key differences between SMT and THT

We already mentioned some differences, but let’s give it a thorough go at it. 

SMT components are typically smaller and lighter, while THTs are bulkier and designed to handle more robust applications.

When it comes to placing components, SMT relies on automated machines, but THT needs manual labor or at least semi-automated processes to insert components through the board.

If the cost is what interests you, SMT can be more cost-effective for mass production because of automation, whereas THT is more labor-oriented and thus more expensive. 

THT can last longer. It offers greater mechanical strength, making it more suitable for applications subject to stress or harsh conditions (think cars, submarines, airplanes, and space explorations). 

Do you know who uses these PCBs? Well, SMT is the go-to for customer electronics, like laptops, smartphones, and such. THT is more meant for industries like aerospace, automotive, and industrial machinery where durability is the key. So, if you were wondering which PCBs you have more chances of seeing, there goes your answer. 

Assembly challenges of SMT 

One of the main challenges of SMT assembly is dealing with the incredibly small size of the components. Capacitors, resistors, and integrated circuits in SMT form are often just fractions of a millimeter in size. This demands expreme precision in placement to make sure that components are correctly aligned with the PCB pads. 

To overcome this challenge, manufacturers are using automated pick-and-place machines. These machines are fast nas accurate but they also need to be regularly calibrated to maintain precision. The trouble is, any misalignment can lead to defects, such as tombstoning (where one side of a component lifts off the pad during soldering) or poor solder joints. All this can result in failed connections or circuit malfunctions. 

The soldering process is another critical stage. The most common method is reflow soldering, where the entire board is passed through an oven to melt solder paste and secure the components in place. What can go wrong? Temperature control in the reflow process for one, because overheating or underheating can bring soldering defects.

For instance, too much heat can lead to component damage or the formation of voids (air pockets) in the solder, and that will reduce its strength. On the other hand, insufficient heat can make cold solder joints, where the solder doesn’t properly bond to the pads, and this can lead to electrical connectivity issues. 

And what’s the story with thermal stress? It’s the main challenge when it comes to multi-layered PCBs. Boards are exposed to high temperatures during reflow soldering, and the materials used in the PCB can contract and expand, which might lead to stress on both the solder joints and the components. If thermal stress is not kept under control, it can result in delamination (separation of PCB layers) or cracked solder joints.  

To get a hold of this, manufacturers need to carefully choose materials that can withstand temperature fluctuations, and to optimize their reflow to ensure gradual heating and cooling of the board. 

Since those components are so small, can we actually inspect them? Well, visual inspection alone is often insufficient. Manufacturers use automated optical inspection (AOI) systems to detect defects like misaligned components, insufficient solder, or bridge formations (unintended solder connections between pads). 

In some cases, X-ray inspection is used, especially for complex or densely populated boards. X-rays can reveal hidden defects beneath components, such as Ball Grid Array (BGA) packages. Sadly, these inspection systems require a lot of money and highly trained personnel. 

Assembly challenges of THT 

One of the biggest challenges with THT is the time and labor required for assembly. Unlike SMT where machines place components with envious speed, THT needs a manual touch. Even with semi-automated systems, this process is considerably slower and prone to human error. 

THT assembly is in danger of human error, like misaligned components, missed solder joints, or components inserted in the wrong direction. 

On top of this, THT needs holes to be drilled through the PCB for the component leads. This extra step in the manufacturing process can slow down the production even more, not to mention the cost increase. With the drilling process, you can have mistakes like misaligned holes or excessive wear on drill bits, and that can lead to inconsistencies in the final product.

For multi-layered PCBs, the drilling process becomes even more complex, as the holes need to be precisely aligned to avoid damaging internal layers of the board. To avoid these mistakes, manufacturers use highly accurate drilling machines and regular maintenance.

Soldering is here mostly done via wave soldering or manual hand soldering. Wave soldering is the process of passing PCB over a wave of molten solder, which coats the exposed leads and makes the necessary electrical connections. But wave soldering can be prone to defects, like making solder bridges.

Manual soldering offers greater control, but on the other hand it’s even more labor-intensive and brings greater risk of human error. 

As mentioned before, THT components tend to be larger than their SMT counterparts, so it’s harder to blend them in lightweight electronics. In applications where space is limited (khm smartphones khm), THT components may simply be too large to fit into the design. 

Also, heavier components can put extra strain on the solder joist, especially if the device is going to suffer some vibration or movement. In these cases, the mechanical strength of THT components is an advantage.

Can SMT and THT combine? 

Well yes, they can. Many electronics use a combination of both SMT and THT components. The hybrid approach lets manufacturers take advantage of the strengths of each technology.

For example, SMT is used for most of the compact, lightweight components, while THT is reserved for connectors, large capacitors, or other components that need greater mechanical strength. 

Hybrid assemblies also have their set of challenges. For one, manufacturers must balance the different soldering processes (reflow for SMT and wave for THT). This can complicate production lines, since both processes require different equipment and conditions. 

Also, can you imagine the placement of SMT and THT components on the same boards? Managing that needs careful design and layout planning. For instance, SMT components must be placed in areas that won’t interfere with the through-holes used for THT components.

Overcoming challenges of assembly 

Working with differences between SMT and THT is a number one priority for any serious electronics manufacturer. Both are important–SMT has speed, compactness, and automation, and THT has durability and strength for more demanding applications. 

Manufacturers need to know about all these challenges in order to choose proper manufacturing design for PCB. Over time, more and more quality products would be available. 

Petra Rapaić is a B2B SaaS Content Writer. Her work appeared in the likes of Cm-alliance.com, Fundz.net, and Gfxmaker.com. On her free days she likes to write and read fantasy.

James Ewen

James is the head of marketing at Tamoco