Keep THT to a minimum
Modern electronics manufacturing is SMD-driven: components sit as surface-mount devices (SMD) on the board and are soldered in a single reflow pass. Yet almost every assembly still needs a few through-hole (THT) components — connectors, relays, larger electrolytic caps, transformers. These parts are pushed through the board and soldered on the back side.
The first and most important design rule is therefore: keep the number of THT components to an absolute minimum. Every THT part requires an additional soldering process after reflow and thus drives cost and lead time. Wherever an SMD variant exists, it is almost always the more economical choice.
Where THT is unavoidable, the chosen soldering process determines cost and layout rules. There are three automated processes: wave soldering, selective soldering and robot soldering. Each has its own profile of strengths, limits and clearance rules that must be respected in the layout.
Wave soldering
In wave soldering the populated board passes over a standing wave of molten solder that wets all THT leads on the underside at once. This makes it the fastest and most cost-effective THT soldering process in series production — at high, consistent quality.
Drawback: a solder adapter (pallet) is usually required to mask SMD areas and let the wave reach only the intended spots. This adapter causes one-time setup cost and pays off mainly at higher volumes.
For wave soldering to work cleanly, the layout must respect clear rules:
- Clearance SMD pad to THT annular ring ≥ 7 mm. The wave must not reach sensitive SMD pads.
- No tall SMD components next to THT solder joints — they would disturb or shadow the wave.
- Maximum SMD component height 5 mm in the wave area.
- THT components on one side if possible. Spread across both sides, the assembly is not ideal for wave soldering.
Selective soldering
In selective soldering a small, programmable solder nozzle addresses each THT joint individually. The process is fast and cost-effective — already from prototypes, because it works without an adapter and the solder program is created quickly. Another advantage: double-sided SMT and THT population is possible.
Drawbacks: in high-volume series the longer process time matters, since each joint is addressed one by one. In addition, a grid pitch below 2.00 mm is prone to short circuits, because the nozzle can bridge adjacent leads.
Layout rules for selective soldering:
- Clearance SMD pad to THT annular ring ≥ 3 mm.
- No tall SMD components next to THT components, so the nozzle can approach freely.
- Grid pitch at least 2.00 mm, ideally 2.54 mm.
For prototypes and small to medium series, selective soldering is often the best compromise: no expensive adapter, flexibly programmable, usable on both sides.
Robot soldering
In robot soldering a robot arm guides a soldering tip with wire feed precisely to each joint — almost like hand soldering, but automated and reproducible. The process is especially suited for soldering fine structures and small clearances where the wave and the selective nozzle reach their limits. And because heat is applied locally and in a metered way, heat-sensitive components are soldered gently.
Drawbacks: as with selective soldering, the process time matters, since each spot is addressed individually. In addition, an assembly fixture is required to hold the board during soldering.
Robot soldering is therefore the specialist process for demanding geometries and thermally critical components — not the tool of choice for fast high-volume series, but indispensable where precision and heat protection count.
| Process | Advantages | Drawbacks | Layout clearance |
|---|---|---|---|
| Wave soldering | Fastest, cheapest THT soldering in series; high, consistent quality | Solder adapter usually required; THT on one side if possible; max. SMD height 5 mm | SMD pad → THT ring ≥ 7 mm |
| Selective soldering | Fast & cheap from prototypes; no adapter; double-sided SMT/THT population | Process time in high volumes; grid pitch < 2.00 mm prone to shorts | SMD pad → THT ring ≥ 3 mm |
| Robot soldering | Fine structures & small clearances; gentle on heat-sensitive parts | Process time; assembly fixture required | Fine structures / fixture needed |
// Source: SMTEC Design Guidelines — THT soldering processes compared
Designing the THT layout right
Regardless of the process, a few basic rules are decided in the layout:
- THT components on one side if possible — this keeps the soldering process simple and is a prerequisite for wave soldering.
- Grid pitch for selective soldering at least 2.00 mm, ideally 2.54 mm.
- Thermal reliefs on large copper areas are mandatory. Without them the soldering heat drains into the copper plane, the joint does not get hot enough and cold solder joints occur.
Planning these points early avoids later layout loops and keeps all three soldering processes open to you.
Choosing the process
There is no universally "best" soldering process — the choice depends on volume, components and layout. As a rough guide: wave soldering for high volumes with single-sided THT, selective soldering for prototypes and small to medium series as well as double-sided assemblies, robot soldering for fine structures and heat-sensitive components.
SMTEC manufactures SMT and THT on the same series lines. The right soldering process is chosen project-specifically after consultation — so you do not have to decide it yourself. In the Speedplace configurator, THT population is selectable (single- or double-sided, +2 working days each).
Quote your THT assembly?
In the online configurator you select THT population with one click — single- or double-sided. SMTEC picks the right soldering process for your project.
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