What is wave soldering?
Wave soldering is a bulk soldering process in which an assembled PCB is passed over a standing wave of molten solder, soldering all exposed through-hole joints on the underside in one pass. It is mainly used for through-hole components and can also solder certain glued SMT components on the bottom side of the board.
Wave soldering explained
In a wave soldering machine, the board travels on a conveyor through three main stages. First, flux is applied to the underside, usually by spraying, to clean the surfaces and help the solder wet. Next, the board is preheated to activate the flux, evaporate solvents and reduce thermal shock. Finally, the underside passes over a wave of molten solder pumped up from a solder pot. The solder flows into the plated through-holes and around the component leads, forming joints as the board leaves the wave.
Many machines use two waves: a turbulent wave that reaches into tight areas and around obstacles, followed by a smooth laminar wave that removes excess solder and reduces bridging. Some machines use a nitrogen atmosphere to reduce oxidation of the solder.
Through-hole components are inserted from the top side, either by hand or by machine. On mixed-technology boards that also have SMT components on the bottom side, those parts must either be glued in place and suitable for wave soldering, or protected using a wave solder pallet that masks the areas that must not touch the solder.
Board design affects the result. Component orientation relative to the direction of travel, lead spacing, pad size, the position of tall parts and the use of solder thieves all influence bridging and shadowing. Wave soldering is efficient for boards with many through-hole joints, but it heats the whole underside of the board, which can be a problem for sensitive components.
When wave soldering matters
Wave soldering matters on designs with many through-hole components, such as power supplies, industrial controls and boards with large connectors, terminals and transformers. It solders all these joints at once, which is efficient for volume production.
It matters especially on mixed-technology boards. If there are SMT components on the bottom side, they must be compatible with passing through the wave or be shielded by a pallet, and the layout must leave room for pallet walls. If that is not possible, selective soldering is usually used instead.
Typical defects include solder bridges between closely spaced leads, insufficient hole fill, solder balls and skips, shadowing behind tall components, and icicles. Flux residues may need cleaning, depending on the flux type and the product's requirements. Thermal stress from the wave can damage heat-sensitive parts or cause board warpage. Designing through-hole parts with suitable lead spacing and orientation reduces these risks.
Wave vs selective vs reflow soldering
Wave soldering is fastest for boards dominated by through-hole parts. Selective soldering is preferred when through-hole parts sit near bottom-side SMT parts. Reflow is the standard for SMT, and many boards use reflow followed by wave or selective soldering.
Wave soldering in your quote request
In your quote request, mark which components are through-hole in the BOM and note whether there are SMT parts on the bottom side, since this affects whether wave or selective soldering is used. Include an assembly drawing showing component orientation and any keep-out areas for pallets, and state the required IPC-A-610 class for through-hole fill. Our free DFM check flags through-hole pad and spacing issues that can cause bridging.
Need boards assembled? Send your Gerber files and BOM for a free turnkey quote from the right assembly partner.
PCB assembly →FAQ about wave soldering
What is wave soldering used for?
Wave soldering is used to solder through-hole components to a PCB in a single pass. The board passes over a wave of molten solder that fills the plated holes and forms joints around the leads. It is efficient for boards with many through-hole parts, such as power supplies and industrial electronics, and can also solder glued SMT parts.
What is the difference between reflow and wave soldering?
Reflow soldering melts solder paste that has been printed on the pads, and is used for surface-mount components. Wave soldering passes the board over molten solder, and is used mainly for through-hole components. Many boards use both: reflow for the SMT parts first, followed by wave or selective soldering for the through-hole parts.
What are the downsides of wave soldering?
Wave soldering heats the whole underside of the board, which can stress sensitive components and cause warpage. Bottom-side SMT parts must be glued and wave-compatible or protected with pallets. It is prone to bridging on fine-pitch leads and to shadowing behind tall parts, and it often uses more flux, which may require cleaning.
Related terms
- THT
THT (through-hole technology) is a PCB assembly method in which component leads are inserted through plated holes in the board and soldered, usually on the opposite side. It creates strong mechanical joints and is used today mainly for connectors, large capacitors, transformers and other parts that face mechanical stress or carry high current.
- Selective soldering
Selective soldering is an automated through-hole soldering process that solders specific joints one at a time using a precision nozzle of molten solder. Unlike wave soldering, it requires no fixtures and only applies flux to targeted joints. Ideal for mixed-technology boards with SMT on both sides and few THT components.
- Reflow soldering
Reflow soldering is the process used to solder surface-mount components to a PCB. Solder paste is printed onto the pads, components are placed on the paste, and the board passes through an oven that follows a controlled temperature profile. The paste melts, wets the pads and leads, and forms solder joints as it cools.
- IPC-A-610
IPC-A-610 is the IPC standard that defines visual acceptance criteria for electronic assemblies. It describes what acceptable and defective solder joints, component placement, cleanliness and damage look like on a populated PCB, using photographs and illustrations, across three product classes. It is the most widely used inspection standard in PCB assembly.
- PCBA
PCBA (Printed Circuit Board Assembly) is the process of populating a bare PCB with electronic components — resistors, capacitors, ICs, connectors, and other parts — to produce a functional electronic assembly ready for testing or integration into a finished product.