SMT assembly and THT assembly are the two core electronics assembly technologies, but they rarely compete head to head — most projects combine both. See how they differ in packing density, mechanical strength and cost, and how to work out what your project actually needs.
SMT (Surface Mount Technology) and THT (Through-Hole Technology) are the two fundamental electronics assembly technologies, distinguished by how components are attached to the PCB. In SMT assembly, components are soldered directly onto the surface of the board. In THT assembly, component leads pass all the way through drilled holes and are soldered on the reverse side. In practice, most electronics projects don't choose one technology over the other — they combine both in mixed assembly. The real question isn't "SMT or THT", it's which components in a given project actually require which technology.
What separates SMT assembly from THT assembly
The core difference lies in how a component is physically attached to the board. SMT components have flat contacts or short leads soldered directly onto the laminate surface — no through-holes required. THT components have longer leads (wire-form or pin-type) that pass through drilled holes and are soldered on the underside of the board, creating a mechanical bond that runs through the full thickness of the laminate.
This structural difference has several practical consequences:
Packing density. SMT components are significantly smaller and can be placed on both sides of the board, closer together, fitting more functionality into less space. THT components take up more room due to their size and the need for drilled holes.
Mechanical strength of the joint. A THT joint, running through the entire laminate, is mechanically stronger and better withstands stress, vibration and repeated plugging/unplugging — think connectors and sockets. An SMT joint, relying solely on a surface layer of solder, is more prone to cracking under heavy mechanical load.
Automation and unit cost. SMT assembly is fully compatible with automated pick&place assembly and mass production — hence the lower unit cost at higher volumes. THT assembly more often requires manual or semi-automated work, which raises labour cost per unit, but remains the only practical option for large or high-power components.
When to use SMT assembly, and when THT assembly
In most modern projects, both technologies appear on the same board — the question isn't "SMT or THT", it's which specific components need which one.
Typical applications of SMT assembly
Surface mount technology dominates wherever miniaturization and high function density matter: integrated circuits, chip resistors and capacitors, logic elements, microcontrollers, communication modules. If a project has limited board space or a high count of small passive components, SMT assembly is the natural choice — and it accounts for most of the surface area on a typical modern PCB.
Typical applications of THT assembly
Through-hole technology remains the standard wherever mechanical strength of the joint matters: edge connectors, power sockets, relays, large electrolytic capacitors, transformers, components exposed to vibration or frequent plugging. In industrial and automotive applications, where the board operates under shock or temperature swings, THT assembly is sometimes required even for components that are technically available in an SMT version.
Mixed assembly — how it works in practice
Mixed assembly, combining SMT and THT on a single board, is the standard rather than the exception in most projects of medium to high complexity. The typical process sequence runs as follows: SMT components are placed first (by machine, using pick&place, with reflow soldering), and only after that stage are THT components added, hand-soldered or wave-soldered selectively. This order follows from process temperatures — through-hole elements often wouldn't survive the thermal profile of a reflow oven, so they have to be added afterwards.
For a company outsourcing electronics assembly, this means the design documentation (BOM, Gerber files) must clearly distinguish which components are SMT and which are THT — a misclassification at the design stage causes problems as early as production planning, not just at the point of assembly.
How to assess what your project needs
When evaluating your own project, it's worth answering three questions:
Will the component be exposed to mechanical stress, vibration, or frequent plugging? If so, consider THT assembly, even where an SMT version of the same component is available.
Is miniaturization and packing density the priority? If board space is limited and the function count is high, surface mount will be the only realistic option in most cases.
What's the planned production volume? At higher volumes, SMT assembly reduces unit cost through full automation. For single units or very small batches, the cost difference between technologies matters less than component availability in a given package type.
In practice, the answer is rarely "we choose one assembly technology" — identifying which elements of a project require SMT assembly and which require THT assembly, and sequencing mixed assembly so it doesn't add unnecessary cost or quality risk, is the manufacturer's job at the documentation review stage. Examples of projects combining both technologies in a single device can be seen among our R&D projects.
SMT vs THT Assembly — frequently asked questions
Can a THT component be swapped for its SMT equivalent without changing the design?
It depends on the component. Many passive elements and some integrated circuits have equivalents in both technologies, but the swap affects the mechanical strength of the joint and may require redesigning the board's trace layout. This is a decision best made during DFM review, not after assembly has started.
Is mixed assembly, combining SMT and THT, more expensive than single-technology assembly?
Yes, because it requires two separate stages of the production process — machine-based SMT assembly plus THT assembly and soldering. In practice, though, most medium-complexity projects need both technologies for functional reasons anyway, so the comparison to a "pure" single-technology assembly rarely applies.
Which assembly technology is more resistant to vibration?
THT assembly, thanks to a mechanical joint that runs through the full thickness of the board. That's why in industrial and automotive applications, and anywhere a device operates under shock, mechanically critical elements — connectors, relays, large capacitors — most often stay in THT even when the rest of the design is SMT-based.
Can THT component assembly be automated?
Partially. Machines for automated through-hole insertion do exist (axial/radial insertion), but they're less common than SMT lines and cost-effective mainly at very high volumes. In practice, THT assembly is more often done manually or using selective soldering, especially for smaller and medium production runs.
How should project documentation be prepared to avoid confusion between SMT and THT?
The key is clearly marking in the BOM and Gerber files which components are SMT and which are THT, and accounting for that split as early as the board layout stage — for instance, through-hole placement. The manufacturer verifies this classification during documentation review before production and flags any ambiguity before assembly starts, not during it.
Summary
SMT assembly and THT assembly aren't competing technologies — they're complementary tools, chosen per component based on the specific requirement. SMT wins where miniaturization and cost at higher volumes matter; THT wins where mechanical joint strength is critical. Most medium and high-complexity projects need both at once, in the form of mixed assembly with a properly planned process sequence.
Send us your BOM — we'll flag which components need SMT assembly, THT assembly, or both.
