SMT vs Through-Hole (THT): Choosing the Right Assembly Method
Surface mount is the default, but through-hole is still the right call for some parts. A practical guide to choosing between them, and why most real boards use both.
Ask most engineers today and they will say surface mount technology (SMT) has won. On density and cost at volume, that is true. But open almost any industrial controller, power supply or EV charger and you will still find through-hole parts on the board. The real question is not which technology is better, but which parts belong in which.
How the two methods differ
Through-hole technology (THT) uses components with leads that pass through drilled, plated holes and are soldered on the opposite side. It was the standard until the 1980s.
Surface mount technology (SMT) places components directly onto pads on the board surface. They are held in position by solder paste and soldered in a reflow oven. Packages range from tiny 0201 passives to large BGAs.
Side-by-side comparison
SMT | Through-hole | |
|---|---|---|
Component density | Very high, both sides of the board | Lower, holes use space on every layer |
Assembly | Fully automated placement and reflow | Insertion plus wave, selective or hand soldering |
Cost at volume | Lower | Higher per part |
Mechanical strength | Good for most parts | Excellent: leads anchor through the board |
High current and heat | Limited by pad size | Handles heavy currents and large parts well |
Prototyping by hand | Harder for fine pitch | Easy |
Electrical performance at high frequency | Better: shorter leads, less inductance | Leads add inductance |
When SMT is the right choice
Use SMT for most of the board: logic, microcontrollers, memory, passives, small power devices and RF parts. It gives you smaller boards, two-sided placement and better high-frequency behaviour, and it is the most economical route once volumes go beyond a few hundred pieces.
When through-hole still wins
Through-hole makes sense where a part takes mechanical stress or carries real power:
- connectors that are plugged and unplugged many times;
- terminal blocks and screw terminals;
- transformers, chokes and large electrolytic capacitors;
- relays and high-current parts;
- parts on boards that see heavy vibration, as in vehicles and heavy machinery.
Mixed technology: what most real boards use
Most industrial and automotive boards are mixed-technology assemblies. The SMT parts go on first. The through-hole parts are then inserted and soldered by one of three methods:
- Wave soldering: fast for boards with many through-hole joints, but the whole underside meets the wave, which limits SMT parts on that side.
- Selective soldering: a small programmable solder nozzle visits each joint, so SMT parts close by are left alone. It is slower than wave soldering but gives more control.
- Hand soldering: practical for a few parts or low volumes.
We go deeper into these in reflow, wave, selective and vapor phase soldering.
Design tips for mixed boards
Keep SMT parts on the solder side away from through-hole pins if the board will be wave soldered, or plan for selective soldering instead. Group through-hole parts where you can. And think about the order of assembly early, because it affects stencil design, panelization and test access.
Building SMT, THT and mixed boards in Chennai
Polyfit's EMS division builds surface-mount, through-hole and mixed-technology assemblies on a fully automated SMT line, with SPI and AOI, in an ISO 7 cleanroom. Share your design and we will come back with a quote.
Send us a part number, a drawing or just the application. Our engineers reply within one working day.
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