SMT or THT? How to choose a component assembly technology

Once the schematic is drawn and the board is routed, the question arises of how the components will actually end up on the board. The choice between SMT (Surface Mount Technology) and THT (Through-Hole Technology) is not just a technical question; it determines the assembly cost, component availability, the size of the device and even how you will test and repair the board. In practice most modern boards combine both technologies – what matters is doing it deliberately.

What SMT and THT are

With SMT, components in packages such as 0402, 0603, SOT-23, QFN, QFP and BGA are placed directly onto pads on the surface of the board and soldered with solder paste in an oven (reflow). The components need no holes, so they can sit on both sides of the board.

With THT, components have wire leads that pass through plated holes and are soldered on the opposite side – by hand, by wave soldering or by selective soldering.

A practical comparison

When to choose SMT

SMT is today the default technology for almost all electronics. The reasons:

• Modern semiconductors exist only in SMT packages. Microcontrollers, wireless chips, voltage regulators and sensors rarely come in DIP packages any more.
• A smaller board means a lower PCB price and a smaller enclosure.
• Automation. A pick-and-place machine places thousands of components per hour with a precision that cannot be achieved by hand.
• Better electrical properties. Shorter connections and lower parasitic inductance are essential for high-speed signals and switching regulators.

A typical SMT process for a small series:

1. Solder paste printing through a laser-cut stainless-steel stencil (0.10 to 0.15 mm thick).
2. Component placement with a pick-and-place machine from reels, tubes or trays. In small series some components are sometimes placed by hand under a microscope.
3. Reflow soldering in an oven with a profile matched to the paste (for lead-free SAC305 paste, a peak of 240 to 250 °C).
4. Optical inspection (AOI or manual) and rework where needed.

When THT is still the right choice

THT is not an outdated technology – it is a specialised one. Use it for:

• Connectors exposed to mechanical forces: USB, power jacks, terminal blocks, pin headers, DC sockets. A plated hole withstands pulling and plugging far better than an SMT pad.
• Large capacitors, inductors and transformers, where the component's mass exceeds what an SMT joint can carry or where an SMT version simply does not exist.
• Power components in TO-220 and TO-247 packages that are screwed to a heat sink.
• Relays, fuses and switches that the user presses or replaces.
• Educational and hobby projects, where anyone with a soldering iron should be able to build and repair the board.

Mixed assembly: standard practice in most devices

Almost every industrial board we see is mixed: SMT for the logic and passive components, THT for the connectors and a few power parts. The order and method of soldering matter:

1. The SMT components are placed and reflow-soldered first (if they are on both sides, the bottom side goes first and must stay in place during the second reflow; glue helps with heavier components).
2. The THT components are then inserted and soldered by hand, selectively or by wave.

With wave soldering, the SMT components on the bottom side must be glued and oriented so that the wave does not create shadows; for small series it is therefore often cheaper to solder the THT part by hand or selectively.

Pin-in-paste (intrusive reflow) is a technique in which paste is also applied into the holes of the THT components, which are then soldered in the same reflow cycle as the SMT parts. It works for connectors that withstand reflow temperatures and have short leads, and it saves an entire separate step.

Assembly costs for small series

For prototypes and series of 5 to 500 boards the cost structure is different from mass production:

• Fixed SMT costs: the stencil (a few tens of euros), machine programming and feeder setup. These costs are the same for 5 or 500 boards.
• Variable SMT costs: machine time per board is low; with 100 components per board it is a matter of seconds.
• THT costs: practically entirely variable; every component means manual insertion and soldering.

The consequence: for very small quantities (1 to 3 boards) hand-placing SMT components can be cheaper than setting up the machine, but from about 10 boards machine assembly usually pays off. A board with 20 THT components will have higher assembly costs at 200 boards than the same board with 100 SMT components.

How the design affects the assembly price

A few layout decisions that directly lower the cost:

• All SMT components on one side. Double-sided assembly doubles the setups and reflow cycles.
• Uniform passive sizes (e.g. 0603 only) reduce the number of feeders.
• Avoid exotic packages and components available only in tubes or as loose parts.
• Polarity and orientation should be clearly marked on the silkscreen; an unclear marking means a question, a delay or an error.
• Fiducial marks let the machine align precisely; without them, placing fine-pitch packages is not reliable.
• THT components on the same side, with enough clearance for a soldering iron or a selective soldering nozzle.
• A complete BOM with manufacturer part numbers and board designators saves hours of back-and-forth.

Hand soldering: when and how

Hand soldering SMT components is perfectly realistic for prototypes: 0603 and SOT-23 with a soldering iron, QFN and fine-pitch QFP with paste and hot air. BGA packages, however, require a reflow oven or a temperature-controlled rework station and X-ray inspection of the joints, which in practice almost always means leaving the assembly to a specialised workshop.

For THT components, hand soldering is the standard for small series. We recommend a temperature-controlled station, flux-cored solder and cleaning off the residues after soldering, especially on sensitive analogue circuits.

Recommendations by project type

• Prototype, 1 to 5 boards: SMT placed by hand or by machine, THT by hand. Speed is what matters.
• Pilot series, 10 to 100 boards: machine SMT assembly with a stencil, THT by hand or selectively. Production documentation already pays off here.
• Series of 100 to 1000 boards: machine SMT, selective or wave THT, AOI inspection, a test fixture.
• Over 1000 boards: minimise THT, consider pin-in-paste, standardise components.

Need help?

At Selektronik we assemble SMT and THT components in small and medium series, with a pick-and-place machine, a reflow oven and experienced manual assembly for special cases. When we develop a board, we make sure in the layout that assembly will be as cheap as possible. Check the price estimate for your boards in our PCB configurator, and for an assembly quote send us your Gerbers and BOM at info@selektronik.si.