Short answer: Use surface-mount technology (SMT) for everything you can: it is automated, denser, cheaper per placement and more repeatable. Keep through-hole (THT) only where a part needs mechanical strength or high current: connectors that are plugged repeatedly, large electrolytic capacitors, transformers, power terminals and some sensors. A typical industrial board today is 90–98 % SMT with a handful of THT parts soldered by selective or wave soldering.
What is the practical difference?
| SMT | THT | |
|---|---|---|
| Placement | Pick-and-place machine, thousands per hour | Manual or automated insertion, tens per hour |
| Soldering | Reflow oven, both sides possible | Wave or selective soldering, one side |
| Cost per placement | Lowest | 5–20× higher than SMT |
| Mechanical strength | Adequate for most parts | Best: leads through the board |
| Board density | High, both sides used | Low, holes consume all layers |
| Rework | Needs hot-air station and skill | Easier with basic tools |
When THT is still the right choice
- Connectors that are mated many times or carry cable weight (screw terminals, D-sub, RJ45 with latch).
- High-current or high-voltage parts: power terminals, large inductors, relays.
- Heavy components: large electrolytic capacitors, transformers.
- Parts that must survive vibration and shock without underfill, e.g. in marine and mobile machinery.
- Legacy designs where requalification would cost more than the THT process.
How mixed-technology boards are built
At Comtec Labs a mixed board follows one flow: SMT paste printing, placement and reflow first, then THT parts inserted and soldered by selective soldering, then automated optical inspection (AOI) of both. Selective soldering keeps THT joints repeatable without exposing the SMT side to a wave, which matters for IPC class 3 products.
Key facts
- Typical industrial board today: 90–98 % SMT placements.
- THT placement costs roughly 5–20× an SMT placement.
- Selective soldering handles THT on a mostly SMT board without a second reflow cycle.
- Moving a THT connector to a press-fit or SMT-with-pegs variant often removes the last manual step.
Design decisions that reduce THT cost
Group THT parts on one edge so the selective-solder nozzle path is short, keep them on the same side, choose SMT variants with mechanical pegs where the connector allows it, and specify the THT hole-to-lead clearance per IPC-2221 so wave or selective soldering fills the barrel reliably.
Frequently asked questions
Is SMT more reliable than THT?
For most components, yes: fewer manual steps and controlled reflow profiles give more repeatable joints. THT remains stronger for parts under mechanical load.
Can one board mix SMT and THT?
Yes, this is the normal case. SMT is reflowed first and THT is added by selective or wave soldering.
Is THT being phased out?
Not for connectors and power parts. New components are almost always SMT, so the THT share keeps shrinking.
Does Comtec Labs do THT assembly?
Yes, both selective soldering and hand soldering for prototypes and series, on the same lines as SMT.
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