Wave and Selective Soldering

Where Wave Soldering Still Fits

Wave soldering remains the efficient way to solder through hole components at volume. A board passes over a standing wave of molten solder, and every joint on the underside is made in one pass, which is far faster than touching each pin with an iron. The process is well suited to a board with a moderate number of through hole parts, particularly connectors, transformers and switches that cannot be placed by a pick and place machine. Where the through hole content is small or concentrated in one area, the process becomes wasteful, since the whole board is heated to solder a few joints.

Flux, Preheat and the Thermal Path

The sequence is flux, preheat and wave contact. The flux removes the oxide and prepares the surface, and how it is applied, by spray or by foam, decides how uniform the coverage is and how much residue remains. The preheat brings the assembly up to a temperature that activates the flux and reduces the thermal shock of the wave, and its setting depends on the board’s mass, layer count and copper distribution. A board that is too cold on contact finds the solder solidifying before it has wet the barrel; a board that is too hot burns the flux and oxidises the joint. The wave contact itself is measured by the dwell time and the depth of contact, which the conveyor speed and the wave height control.

Thermal Profiling for Through Hole

Preheat and wave contact are measured on the board, on the topside and the underside, using the same discipline as a reflow profile. The topside temperature at the wave is what decides whether the flux is still active when the solder arrives, and the difference between the leading edge and the trailing edge of a large board is often larger than expected. Where the board carries both a wave soldered side and a reflowed side, the thermal history of the first pass has to be compatible with the second, and the sequence of processes has to be planned rather than discovered. A board that has been reflowed twice and waved once has been through three thermal cycles before it is functional.

Defects the Process Produces

The faults of wave soldering are characteristic. Solder bridges between closely spaced pins appear when the flux is inadequate, the dwell is too long or the lead length leaves a large thermal mass. Icicles and webbing form where the solder does not drain cleanly, often at the trailing edge of a connector. Insufficient fill in a plated barrel is usually a preheat or a flux problem rather than a solder problem. Solder on the topside, sometimes called blowthrough, comes from trapped moisture or volatiles escaping through the barrel, which is why moisture sensitive parts and unsealed boards are a risk in this process. Skips on a thermal pad point to a cold joint that the solder never reached.

Selective Soldering

Selective soldering solders individual joints with a small nozzle of flowing solder or with a focused laser, guided by the same program as the placement. It gives a much smaller thermal load than a wave, which suits a board with surface mount parts on the underside, a mixed assembly or a design with a few through hole components in an awkward position. It is slower per joint than a wave, so it is economical where the number of joints is small or where the alternative would be hand soldering. Its limits are the nozzle geometry, which decides the access, and the cycle time, which falls as the joint count rises.

Choosing Between Them

The decision usually comes down to the number of through hole joints, their location and the sensitivity of the rest of the board. A high count of through hole parts on a board with no underside surface mount suits a wave. A few connectors on a board that is otherwise surface mount fits selective. Hand soldering remains a legitimate option for a handful of joints, but it depends on operator skill and it is hard to control, so it should be a deliberate choice with a written standard rather than the default for anything awkward.

Process Control

The controls that keep these processes stable are the flux type and quantity, the preheat temperature measured on the board, the wave or nozzle temperature, the dwell and the conveyor speed, and the solder pot chemistry, which changes with the metals that dissolve into it. The pot should be analysed periodically, since copper and other elements accumulate and change the alloy’s behaviour. Records of the settings and the measured thermal profile are what allow a defect to be traced, and they are the first thing to check when a run starts producing a defect that it did not produce yesterday.

Board Design for Wave Soldering

The layout decides how well the process works. A board that will be waved should keep the through hole pads on one side, provide enough spacing between adjacent pins to avoid bridging, and use thermal reliefs on a plane connection so that a joint on a heavy plane can reach temperature. Solder thieves, small pads placed to draw excess solder away from a dense area, reduce bridging on a connector. Where the board carries underside surface mount parts, they have to be bonded with an adhesive before the wave, because the wave will otherwise wash them off. These are design decisions, and a wave process started on a layout that ignored them will always be a struggle.

FAQ

When is wave soldering appropriate? When a board has a significant number of through hole joints concentrated on the underside and few sensitive parts elsewhere.

What does preheat do? It activates the flux and reduces thermal shock, and its setting depends on the board’s mass and copper.

Why do bridges form? Usually from inadequate flux, an excessive dwell or a lead and pad geometry that holds too much solder.

What is selective soldering for? Soldering individual joints without the thermal load of a wave, useful for mixed assemblies and few through hole parts.

What causes solder on the topside? Trapped moisture or volatiles escaping through the barrel, which is why board preparation matters.

Conclusion

Wave and selective soldering each solve a different problem, so choose by joint count, location and the sensitivity of the rest of the board, then control the flux, the heat and the dwell. Through hole processes belong to PCB assembly, the surface mount work alongside them sits in SMT PCB assembly, and the acceptance criteria are part of quality management. Process selection for a new design is settled during prototype PCB assembly in 2026.