From hand-soldered through-hole to machine-assembled SMD
Many legacy boards were designed for through-hole components and hand or wave soldering. Rebuilding them is a chance to modernise: surface-mount parts are easier to source and allow automated assembly. But not every part should change.
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THT and SMD – the difference
With through-hole technology (THT) the component leads are inserted through drilled holes and soldered on the opposite side. With surface-mount technology (SMT, the parts are called SMD – surface-mount devices) the components sit directly on pads on the surface and are soldered in a reflow oven.
| Through-hole (THT) | Surface mount (SMD) | |
|---|---|---|
| Assembly | Manual insertion or axial/radial insertion machines | Pick-and-place machine |
| Soldering | Hand, wave or selective soldering | Reflow soldering with solder paste |
| Mechanical strength | Very high – good for connectors and heavy parts | Lower, depends on pad size and adhesive |
| Space | Large, leads go through the whole board | Compact, both sides usable |
| Availability of new parts | Decreasing for many semiconductors | Widest range, longest availability |
Soldering processes in brief
- Hand soldering: flexible and fine for prototypes and small series, but slow and dependent on the operator.
- Wave soldering: the whole underside passes over a wave of molten solder – the classic process for THT series production.
- Selective soldering: a small solder fountain solders only selected THT joints – used for mixed boards.
- Reflow soldering: solder paste is printed through a stencil, components are placed by machine, the board passes through an oven. Standard for SMD.
When the conversion pays off
- The original THT semiconductors are obsolete, but SMD versions of the same function are available.
- You need more than a handful of boards and want repeatable, automated assembly.
- Hand soldering is a quality risk or too expensive in your volumes.
- The board is being rebuilt anyway, so the layout has to be recaptured in any case.
What should usually stay through-hole
- Connectors, terminals and switches that take mechanical force when plugged or operated
- Heavy components: transformers, large electrolytic capacitors, relays
- Power components that are mounted on a heat sink
- Parts exposed to strong vibration – relevant in vehicles, rail and aviation ground equipment
The result is often a mixed-technology board: SMD for the logic and small passives, THT for the mechanically and thermally stressed parts. That keeps the board robust and still allows most of it to be assembled automatically.
What has to be checked
- Electrical equivalence of every replaced part – value, tolerance, voltage, power, temperature range.
- Footprints according to datasheet or IPC-7351 land patterns.
- Thermal behaviour: SMD parts dissipate heat through copper, not through their leads.
- Mechanical fit: the board outline, mounting holes and connector positions stay identical so the board still fits into the housing.
- Testing: a prototype is built and checked against the original before series production.
Questions
Does converting to SMD change the function of the board?
It should not. The goal is form, fit and function compatibility. Every replaced component is checked against the original and documented in the review report.
Is SMD less reliable than through-hole?
Not in general – most modern electronics, including in aviation, is SMD. For mechanically stressed parts THT remains the better choice, which is why many rebuilds use mixed technology.
Can you keep the board hand-solderable?
Yes. If you assemble small quantities yourself, we can choose larger SMD packages or keep THT where it makes sense.