Quick answer
Compare SMT and through-hole assembly on cost, size, strength, inspection and mixed-technology use cases before choosing a process.
How the Two Processes Actually Differ
The difference is not just component shape — it changes the entire production line, the inspection method, and the cost structure.
Review the dedicated SMT assembly and through-hole assembly process pages before selecting the production route.
- SMT — solder paste is stencil-printed onto surface pads, a pick-and-place machine positions components, then the whole board passes through a reflow oven. No holes required for the component itself, and both sides of the board can be populated.
- THT — component leads are inserted through plated holes, then soldered from the other side by wave, selective, or hand soldering. The mechanical anchor through the board is what gives THT its strength.
THT was not replaced by SMT — it was displaced from the parts of the board where it added no value. The two coexist on almost every professional design.
SMT vs THT: Side by Side
| SMT | Through-Hole (THT) | |
|---|---|---|
| Component size floor | Fine-pitch packages; confirm the assigned line and package | Limited by lead pitch, typically ≥1.27 mm |
| Finest pitch | Fine-pitch BGA/QFN; confirm the assigned line and package | Not applicable |
| Placement accuracy | Confirm accuracy for the assigned line and package | Manual or semi-automatic insertion |
| Board sides usable | Both | One (leads occupy the other side) |
| Automation | Fully automated | Partially — insertion often manual |
| Mechanical strength | Adequate for most circuitry | Significantly stronger — leads anchor through the board |
| Soldering method | Reflow (air or nitrogen) | Wave, selective, or hand soldering |
| Primary inspection | SPI, 3D AOI, X-ray (for BGA) | Visual, electrical test, selective X-ray |
| Throughput | Very high — automated lines | Lower — insertion is the bottleneck |
| Best for | ICs, passives, dense digital circuitry | Connectors, transformers, high-current parts |
The size numbers are where the practical gap shows. Fine-pitch SMT packages can be much smaller than typical through-hole parts; confirm the package range, placement accuracy and process window for the assigned line rather than treating a nominal package size as a universal capability.
The Cost Picture (and Where It Flips)
Buyers often assume SMT is always cheaper. It is cheaper at volume — but SMT carries fixed setup costs that THT does not, so the two swap places at low quantities.
- SMT fixed costs — a stencil must be made and the pick-and-place programme written and verified. These are one-time per design, then amortised across the run.
- SMT variable cost — extremely low. Once running, a line places components far faster than any human can insert them.
- THT fixed costs — near zero. No stencil, minimal programming.
- THT variable cost — high and roughly linear, because insertion labour scales with quantity.
For a handful of prototype boards with few components, THT can genuinely come out cheaper. At higher quantities or when the design needs fine-pitch parts, SMT may have a cost advantage; compare setup, placement, soldering, inspection and quantity assumptions for the project.
Component cost usually dominates either way. On a typical assembly the parts cost more than the board and the labour combined, which is why BOM optimisation often saves more than switching assembly method.
Inspection: An Underrated Difference
This is the dimension most comparisons skip, and it directly affects your defect escape rate.
SMT has a mature, fully automated inspection chain. Solder paste is measured before placement, every joint is optically inspected after reflow, and hidden joints under BGAs are X-rayed. Nothing equivalent exists for through-hole joints — they are largely inspected visually and verified electrically.
| Method | What it checks | Applies to |
|---|---|---|
| 3D SPI | Solder paste volume and alignment before placement; line-specific accuracy is confirmed during engineering review | SMT only |
| 3D AOI | Placement, polarity and solder fillets; false-call performance is measured against the project acceptance criteria | SMT primarily |
| X-Ray (AXI) | Hidden joints under BGA/QFN; voiding is evaluated against the applicable class and project criteria | SMT, BGA especially |
| ICT / Flying probe | Electrical continuity; netlist coverage is defined by the ICT or flying-probe test program | Both |
| FCT | Application-level function with a custom fixture | Both |
A board that is heavily through-hole is harder to inspect automatically. If reliability is critical, budget for electrical and functional testing rather than relying on visual inspection alone.
When Through-Hole Is Still the Right Answer
THT is not legacy technology — there are cases where substituting an SMT part would be a design mistake.
- Connectors and anything a human touches — repeated plugging and unplugging applies force that will eventually tear an SMT pad off the board.
- High-current paths — a through-hole lead carries more current and dissipates heat better than a surface pad.
- Large or heavy components — transformers, big electrolytic capacitors, relays. Mass plus vibration is exactly what SMT joints are worst at.
- High-vibration or high-shock environments — automotive, industrial machinery, aerospace. The mechanical anchor matters more than the space saving.
- Parts only available in through-hole packages — still common in power and RF components.
Mixed Technology: What Most Boards Actually Are
The realistic answer to "SMT or THT" is usually both. A typical industrial controller runs SMT for the microcontroller and all signal circuitry, then through-hole for the power connector, terminal blocks and bulk capacitors.
- 01SMT first
Paste printing, placement and reflow are completed while the board is still flat and free of tall components.
- 02Through-hole insertion
THT parts are normally inserted after reflow when the process route requires it; fitting them earlier can obstruct SMT placement or expose them to a reflow profile they were not selected for.
- 03Selective or wave soldering
Wave soldering for boards with many THT joints; selective soldering when nearby SMT parts must be protected from the solder wave.
- 04Inspection and test
AOI covers the SMT side, then electrical and functional testing verifies the assembly as a whole.
Mixed-technology boards need both an SMT line and a soldering process, so they carry more setup than a pure-SMT board. Telling your manufacturer early lets them plan the routing and quote accurately.
FAQ
Questions engineers ask before release
Is SMT always cheaper than through-hole?+
No. SMT has one-time setup costs — a stencil and a pick-and-place programme — that through-hole does not. For very small prototype quantities with simple, low-count boards, through-hole can be cheaper. Past a few dozen units, or as soon as fine-pitch components are involved, SMT is clearly more economical and the advantage grows with volume.
Can I mix SMT and through-hole on the same board?+
Yes, and most production boards do. SMT is processed first through reflow, then through-hole components are inserted and wave- or selective-soldered. Mixed technology is standard practice, not an exception.
What is the smallest component you can place with SMT?+
We place down to 01005 (0.4 × 0.2 mm) passives, with minimum BGA pitch of 0.25 mm and QFN down to 0.3 mm width. Placement accuracy is ±0.025 mm on X/Y and ±0.2° on rotation.
Is through-hole more reliable than SMT?+
Mechanically, yes — the lead passes through the board and is soldered on the far side, which resists pull-out and vibration far better than a surface pad. Electrically and thermally, a well-executed SMT joint is entirely reliable. The distinction matters for connectors, heavy parts and high-vibration environments, not for general circuitry.
Does through-hole take longer to assemble?+
Generally yes, because insertion is not fully automated the way SMT placement is. On a mixed board the through-hole stage is usually the throughput bottleneck, which is why designers minimise THT count where it is not mechanically necessary.
Which should I choose for a prototype?+
Choose whichever matches your production intent. Prototyping in through-hole and then moving to SMT for production means revalidating a different board — you gain little and risk finding new problems late. If the product will ship as SMT, prototype it as SMT.
Reference points
Sources and verification starting points
External standards and industry references help frame the decision. Confirm current supplier evidence and project-specific requirements before release.