Through-hole technology (THT) is an assembly method in which component leads are inserted through holes in the PCB and soldered on the underside. Although SMT has dominated mass production, THT remains indispensable wherever mechanical strength, high power and resistance to extreme operating conditions matter.
This article explains how through-hole assembly works, which components require this technology and when a project should combine THT with SMT.
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What is through-hole technology (THT) and how does it differ from SMT?
THT (Through-Hole Technology) is a component assembly method in which the leads (pins) of a component are inserted through specially prepared holes in the PCB and then soldered on the underside — manually, with a selective soldering machine or by wave soldering.
Unlike surface mount technology (SMT), where the component sits on the board surface, a THT component is physically anchored in the board. This mechanical connection — not just soldered pads on the surface — is key to strength.
When to use through-hole technology (THT)?
Although SMT has dominated the market, there are many applications where THT remains the only right choice or is clearly preferred:
Connectors and mechanical connectors
USB, audio, power connectors, DB9/DB25, Phoenix — any connector exposed to mechanical forces in the process of mating and unmating should be mounted through the hole. SMD pads will not withstand repeated connections.
High-power components
Power transistors, thyristors, large rectifier diodes, high-capacity electrolytic capacitors — components that generate large amounts of heat or carry large currents. Through-hole leads dissipate heat better and withstand thermal loads.
Transformers and chokes
Magnetic components with high mass and size — through-hole mounting provides mechanical stability impossible to achieve on SMD pads.
Devices exposed to vibration and shock
Electronics for vehicles (automotive), industrial machines, military equipment and railway systems — wherever the board is exposed to constant vibration or sudden mechanical overload.
Prototyping and servicing devices
When a project requires repeated component replacement during development — through-hole components can be desoldered and replaced manually, without a BGA station.
Components with non-standard leads
Some specialist components — especially older or niche ones — are only available in THT packages.
The through-hole (THT) assembly process — stages
Component preparation
Verification of components against the BOM. Components with long leads are trimmed to the correct length. Polarized components (diodes, electrolytic capacitors) are checked for correct orientation before placement.
Placing components in the board
Manual or automatic placement of components into the PCB holes. For larger series and projects with a repeatable structure we use assembly templates that speed up and standardize the process.
Soldering
Selective soldering — preferred for mixed assembly (SMT+THT) or projects with high component density. The soldering head precisely solders the indicated pads without exposing other components on the board to heat.
Manual soldering — for small series, prototypes and components requiring precise operator supervision. Used as a standard for temperature-sensitive components, non-standard connectors and repairs.
Wave soldering — for large series with a dominant share of THT — the board passes over a wave of molten solder. Cost-effective for simpler projects without sensitive SMD components on the same board side.
Quality control
Visual inspection of every solder joint as per IPC-A-610. Verification of assembly completeness, solder quality, absence of cold joints and bridges. Functional tests according to the client's specification.
When does a project require mixed SMT and THT assembly?
The vast majority of modern electronics projects use mixed assembly: SMT as the main technology for logic and passive components, THT for connectors, high-power components and large capacitors.
Mixed assembly requires the production line to handle both technologies in the correct order:
- SMT assembly of side A (paste, pick&place, reflow)
- SMT assembly of side B if double-sided (paste, pick&place, reflow)
- Placing THT components
- Selective or wave soldering of THT components
- AOI inspection and final control
Order and temperature matter: SMT components mounted on the side that will go through the wave must be protected in advance, or the design must account for this stage at the DFM stage.
At Siltegro we carry out mixed SMT+THT assembly as a standard — the DFM analysis before production covers verification of the process order and the thermal profile.
Through-hole (THT) assembly at Siltegro
Siltegro, with many years of experience in the electronics industry since 2009, is a certified partner in comprehensive electronics assembly. Our THT services, combined with advanced SMT surface mount assembly and our own design department, allow us to offer full support for your project:
Soldering methods — selective soldering for mixed SMT+THT projects, manual soldering for small series and prototypes, wave soldering for selected projects.
Quality standard — visual inspection as per IPC-A-610 — the entire operator team holds certification. Every solder joint is verified before shipment.
Component range — connectors, transformers, chokes, electrolytic capacitors, power transistors, IC sockets and other THT components according to the client's documentation. PCB assembly covers both THT components and mixed SMD + THT assembly.
Traceability — every THT batch is traceable just like SMT — a unique batch ID, documentation archived for up to 10 years.
Prototypes and series — electronic prototypes and series electronics production with THT assembly from 1 piece. Contract electronics production for Polish and European manufacturers.
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Series SMT and THT production →
SMT or THT — what to choose for your project?
In most cases the answer is: both. Purely THT projects are rare today — it is usually about which components to mount through the hole and which on the surface.
| THT | SMT | |
|---|---|---|
| Component mounting | Through holes, soldered from below | On the surface, solder pads |
| Mechanical strength | Very high | Standard |
| Miniaturization | Limited | High |
| Automation | Partial (selective) | Full |
| High-power components | Yes | Limited |
| Servicing and replacement | Easy | Difficult |
| Cost – large series | Higher than SMT | Lower |
A simple rule:
- SMT — logic circuits, passive components, ICs, communication modules, everything that fits in an SMD package
- THT — external connectors, high-power components, large electrolytic capacitors, transformers, components exposed to mechanical forces
If you are not sure how to split your project between SMT and THT — a DFM analysis before the first prototype will indicate the optimal distribution.
→ Contract electronics manufacturing — more about the EMS process
