Through-Hole PCB Assembly: When THT Is Still the Best Choice for Reliable Electronics

Surface-mount technology dominates modern electronics production. It allows manufacturers to place tiny components at high speed and pack far more circuitry onto a board than older assembly methods could manage.

Yet through-hole technology has never disappeared.

Look inside industrial controls, power supplies, test equipment, automotive electronics, or other hardware expected to survive years of physical use, and through-hole components are still easy to find. They are often used alongside surface-mount devices on the same PCB.

There is a practical reason for that. Some components simply benefit from being anchored through the board rather than soldered only to pads on its surface.

What Is Through-Hole PCB Assembly?

Through-hole technology, commonly shortened to THT, uses components with leads that pass through drilled holes in the printed circuit board. The leads are soldered on the opposite side, creating both an electrical connection and a mechanical attachment.

This differs from surface-mount technology, or SMT, where components are soldered directly onto pads on the surface of the PCB.

THT was once the standard method for assembling electronic boards. SMT eventually took over most high-volume production because smaller components, automated placement, and higher component density made it better suited to compact electronics.

But “newer” does not always mean “better” for every part of a board.

Where Through-Hole Still Has an Advantage

The most obvious strength of through-hole assembly is mechanical support.

A component lead that passes through a plated hole has more physical anchoring than a component attached only to a surface pad. That can matter when the PCB will experience repeated plugging and unplugging, vibration, movement, or other mechanical loads.

Consider a large connector mounted at the edge of an industrial control board. Every time a cable is inserted or removed, force is transferred into the connector and PCB.

For that type of component, through-hole mounting can make more sense than relying entirely on surface solder joints.

The same reasoning applies to many switches, terminal blocks, transformers, relays, large capacitors, and other components that may be heavy or physically stressed during normal use.

This is one reason through-hole PCB assembly remains relevant even in production lines where most other components are placed using SMT.

High-Power Components Are Another Common Use

Not every component can or should be reduced to a tiny surface-mount package.

Power electronics often use physically larger parts because they need to carry more current, handle higher voltages, or dissipate heat.

Large capacitors, power resistors, transformers, inductors, connectors, and certain semiconductor packages may therefore still use through-hole mounting.

Board layout and thermal design still determine whether THT is appropriate, but the technology gives designers more options when components are too large or mechanically demanding for ordinary surface mounting.

This is common in power supplies, industrial equipment, motor controllers, charging hardware, and other products where size is less important than durability and electrical performance.

THT and SMT Often Work Together

Choosing through-hole technology does not mean choosing an entirely through-hole PCB.

Most modern boards that use THT are mixed-technology assemblies.

Small resistors, capacitors, processors, memory devices, and other compact components can be placed using SMT. Larger connectors, switches, relays, or power components can then be installed through the board.

This lets designers use each assembly method where it makes sense.

A PCB might contain hundreds of surface-mount components and only six through-hole parts. Those six parts may still be the right choice if they carry external cables, switch significant power, or receive repeated mechanical stress.

The decision is therefore less about THT versus SMT and more about choosing the right mounting method for each component.

Reliability Depends on More Than the Component Type

Through-hole mounting can provide strong mechanical connections, but using THT does not automatically make a board reliable.

Hole dimensions, lead size, solder quality, component placement, thermal conditions, and manufacturing workmanship all affect the finished joint.

Industry standards are useful here. IPC’s J-STD-001 and IPC-A-610 standards cover soldering process requirements and acceptance criteria for electronic assemblies, including requirements relevant to through-hole solder connections.

NASA also maintains detailed workmanship standards for electronic assemblies used in demanding applications. Its guidance emphasizes control of design, materials, assembly processes, inspection, and workmanship rather than treating reliability as the result of one manufacturing technique alone.

In practice, good THT assembly depends on controlling the whole soldering process.

Manual Soldering or Wave Soldering?

Through-hole parts can be soldered in several ways.

For prototypes and low-volume builds, manual soldering is often practical. An operator installs the components and solders each lead individually.

At higher volumes, wave soldering may be used. The bottom of the PCB passes over a controlled wave of molten solder, allowing multiple through-hole joints to be soldered in one process.

Selective soldering is another option, particularly on mixed SMT/THT boards where only certain areas should contact solder.

The manufacturing method depends on board design, component mix, quantity, temperature sensitivity, and cost.

This is worth discussing with the assembler before the PCB layout is finalized. Component spacing and placement can make a board much easier—or unnecessarily difficult—to manufacture.

When SMT Is the Better Choice

Through-hole technology has strengths, but there are good reasons SMT replaced it for most electronics.

SMT supports much smaller components and much higher board density. Components can also be mounted on both sides of the PCB, which helps when board space is limited.

Automated pick-and-place equipment can populate surface-mount boards very quickly. That makes SMT particularly attractive for high-volume products such as smartphones, computing hardware, consumer electronics, and compact IoT devices.

Drilling through-holes also consumes PCB space that could otherwise be used for routing.

If a component does not require the additional mechanical support or physical package offered by THT, surface mounting will often be the more practical choice.

Choosing the Right Assembly Approach

The best manufacturing method starts with the product rather than with a preference for one technology.

A compact wearable and an industrial motor controller have very different requirements. One may need the smallest possible components. The other may prioritize connector strength, service life, heat handling, and resistance to vibration.

Prototype requirements matter too. BigNewsNetwork has previously covered factors involved in choosing a PCB prototype manufacturing service, and assembly capability should be part of that evaluation when a board uses both SMT and through-hole components.

When comparing PCB assembly services, engineers should check whether the manufacturer can handle the actual mix of technologies required by the design. That may include SMT placement, manual through-hole work, wave or selective soldering, inspection, testing, and rework.

A supplier built mainly around SMT volume production may not be the best match for a board containing many unusual through-hole parts.

Why THT Hasn’t Gone Away

Electronics manufacturing keeps moving toward smaller components and denser boards, but physical products still have connectors, switches, transformers, power components, and parts that people push, pull, plug in, and remove.

Those mechanical realities are why through-hole assembly remains useful.

THT no longer needs to compete with SMT for every position on the PCB. In many designs, the two technologies complement each other.

Use surface mounting where size, density, and automation matter. Use through-hole mounting where the component benefits from stronger physical attachment or its package naturally demands it.

A modern PCB does not have to choose one side.

It only has to use the right connection in the right place.