What is the wave soldering process in PCB board making?

When a printed circuit board leaves the design stage, the real work begins on the production floor. Most people picture surface-mount machines when they think of PCB assembly, but a large share of the boards used in automotive, industrial, medical, and power electronics still rely on through-hole components. Soldering those components reliably and at volume is exactly what the wave soldering process is for. This article explains what wave soldering is, how it works step by step, and what to look for when choosing a wave soldering pcb assembly service.

What is wave soldering?

Wave soldering is a bulk soldering technique used to join through-hole (DIP) component leads to the plated holes of a PCB. The assembled board travels on a conveyor over a reservoir of molten solder. A pump inside the machine pushes the solder upward into a standing wave, and the underside of the board sweeps through that wave so that every exposed lead and pad is wetted at the same time. Instead of soldering one pin at a time by hand, a single pass can complete hundreds of joints in a few seconds.

The process was developed in the 1950s as electronics manufacturers looked for a way to speed up through-hole assembly, and it remains one of the most cost-effective methods for boards that contain connectors, transformers, relays, and other components too large or too robust for surface-mount placement.

Where wave soldering fits in the PCB board making process

Wave soldering is one stage in a longer pcb board making process. After the bare board is manufactured, components are placed and soldered, and the finished assembly is tested. On a mixed-technology board, surface-mount parts are usually soldered first with reflow, and the through-hole parts are then soldered in a wave soldering machine. A complete DIP line typically runs from component forming and insertion, through wave soldering, to lead cutting, repair welding, board washing, and functional testing.

The wave soldering process step by step

A modern wave soldering machine combines several stages in one continuous line. The main steps are flux application, preheating, wave contact, cooling, and cleaning.

Step 1: Flux application

Flux is sprayed or foamed onto the underside of the board before it reaches the solder. Its job is to remove oxides from the pads and leads and to promote good wetting so the solder flows evenly into the plated holes. Too little flux leaves joints poorly wetted, while too much can cause bridging and leave residues behind.

Step 2: Preheating

The board then passes through a preheat zone, typically raising it to around 90 to 110°C. Preheating dries the flux solvent and activates the flux chemicals, and it gradually warms the board so it does not suffer thermal shock when it meets the molten solder. Boards that skip proper preheating are far more likely to warp or produce solder balls.

Step 3: Wave contact

The heated board travels over the solder pot, where a pump maintains a stable wave. The underside of the board touches the wave for roughly two to five seconds, and the molten solder wicks up into the plated holes by capillary action to form the joint. Many machines use a two-wave system: a turbulent first wave breaks through oxide and flux residues, followed by a smooth laminar wave that produces clean, even joints. Conveyor speed and wave height are controlled carefully so excess solder drains back into the pot instead of leaving bridges.

Step 4: Cooling

After leaving the wave, the board is cooled with forced air so the solder solidifies quickly into a strong, uniform joint. Controlled cooling also helps prevent damage to heat-sensitive components.

Step 5: Cleaning

Depending on the flux used, the board may pass through a washing stage to remove flux residue. In many production lines this is followed by lead cutting and repair welding to tidy up any imperfect joints before inspection.

Key equipment in a wave soldering line

A wave soldering line is more than just the soldering machine. A well-equipped DIP department includes the flux applicator, preheat section, solder pot with its wave pump, conveyor system, and a board washing machine. Around the machine, the line needs inspection stations, repair workstations, and test equipment. For example, a professional facility such as Farway Electronic runs two DIP plug-in lines with two wave-soldering machines, 24 rear-welding stations, and a board-washing machine, supported by plug-in AOI inspection and trained, certified operators.

Wave soldering vs. reflow soldering

Wave soldering and reflow soldering are complementary rather than competing processes. The table below summarises the main differences.

Most modern assemblies use both: reflow for the surface-mount side and wave soldering for the through-hole side, often on a single mixed-technology line.

Advantages and limitations of wave soldering

Advantages:

• High throughput: hundreds of joints are soldered in a single pass, which keeps unit cost low for volume production.
• Consistent quality: automated control of wave height, conveyor speed, and temperature gives repeatable results that hand soldering cannot match.
• Strong joints: through-hole solder joints are mechanically robust, which matters for connectors and components subject to vibration.
• RoHS compatibility: lead-free solder alloys are handled routinely in modern machines.

Limitations:

• The whole board is exposed to heat, so very heat-sensitive components need protection or alternative processes.
• Standard wave soldering is not ideal for fine-pitch surface-mount parts because of shadowing and bridging risks.
• The board layout must follow design rules such as proper pad and hole sizing and thermal relief on large copper areas.

Design considerations for wave soldering

A little planning at the design stage prevents most wave soldering defects. Keep these rules in mind:

• Orient polarized components such as diodes in the same direction, at a right angle to the solder flow, to reduce shadowing.
• Space leads properly so solder does not bridge between adjacent pins.
• Size the plated hole about 0.3 to 0.4 mm larger than the lead diameter to allow good capillary flow.
• Add thermal relief pads on large ground planes and copper pours to avoid cold joints caused by heat sinking.
• Panelize small boards with break-away tabs that are strong enough to survive the conveyor and the wave.

Quality control after wave soldering

Soldering is only half the story. A reliable wave soldering service follows up with inspection and testing to catch defects before boards ship. Typical checks include plug-in visual inspection, AOI optical inspection, X-ray inspection for hidden joints, ICT circuit testing, and FCT functional testing. Some boards also go through thermal imaging and high- and low-temperature reliability testing. Farway Electronic, for example, combines these checks with IPQC and QA sampling on its DIP lines, and its quality systems are certified to ISO 9001, ISO 13485, and IATF 16949, with assembly work following the IPC-A-610 standard.

Choosing a wave soldering partner

When selecting a contract manufacturer for through-hole assembly, look beyond the price per board. Check the equipment on the line, the inspection and testing available, the certifications held, and how the company handles mixed SMT and DIP boards. A one-stop partner that can manage component sourcing, SMT, DIP, conformal coating, testing, and final assembly saves you from coordinating several suppliers. Farway Electronic in Shenzhen offers this full chain, serving customers in transportation, new energy, security, medical, and communication electronics, and it has worked with more than 100 industry customers across over 20 countries and regions.

Frequently asked questions

What is wave soldering used for?

Wave soldering is used mainly for high-volume soldering of through-hole components such as connectors, relays, transformers, and electrolytic capacitors, and for mixed-technology boards where surface-mount parts sit on the top side.

What is the difference between wave soldering and reflow soldering?

Wave soldering uses a molten solder wave to solder through-hole components, while reflow soldering melts pre-printed solder paste in an oven to attach surface-mount components. Most boards use both processes.

Can surface-mount components be wave soldered?

Some surface-mount parts can be wave soldered if they are secured with adhesive and the layout follows special design rules, but the process is best suited to through-hole components. Fine-pitch SMDs are normally handled with reflow instead.

What equipment is used in wave soldering?

A wave soldering machine includes a flux applicator, preheat section, solder pot with a wave pump, conveyor system, and cooling section, usually followed by a board washing machine and inspection stations.

Why does preheating matter in wave soldering?

Preheating activates the flux, dries its solvent so solder balls do not form, and warms the board gradually to prevent thermal shock and warpage when it meets the molten solder.