Wave soldering

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Wave soldering is a bulk soldering method used to attach components to printed circuit boards. A PCB moves over a pot of molten solder, where a pump creates a solder wave. As the board passes through the wave, joints are formed. This method works for through-hole components and for some surface-mount parts (SMT parts are usually glued on before they go through the solder wave). While reflow soldering is common for many SMT boards, wave soldering remains in use for large power devices, high-pin connectors, or boards that still rely on through-hole technology.

How it works and the main equipment
- The process uses a conveyor to move the PCB through several zones, a pan of solder, a pump that makes the wave, a flux sprayer, and a preheating stage.
- The solder is typically a metal alloy. Traditional leaded solder is around 50% tin, 49.5% lead, and 0.5% antimony.
- Because of RoHS rules, lead-free alloys are common today, such as tin-silver-copper (SAC) or tin-copper-nickel, with SN100C (high tin, copper, nickel, and trace germanium) being one well-known example.

Flux and cleaning
- Flux cleans oxide from parts before soldering. There are corrosive and noncorrosive fluxes. Noncorrosive fluxes require more pre-cleaning; corrosive fluxes work quickly but can be more aggressive.
- Some fluxes are “no-clean,” leaving residues that are benign and may not require cleaning, though process conditions still matter. Other fluxes require cleaning with solvents or deionized water after soldering.

Preheating and quality factors
- Preheating helps soldering occur smoothly and reduces thermal shock to the board.
- Quality depends on proper temperatures and clean surface conditions. Common issues include insufficient flux, inadequate preheating, poor conductivity, too-fast conveyor speed, too-thin copper, or unwanted solder bridging.

Alloy choices and cooling
- Alloy choice depends on RoHS compliance and long-term reliability. Leaded Sn63Pb37 is a common, fast-melting eutectic, while lead-free SAC alloys are popular for RoHS compliance. Higher tin content gives better corrosion resistance but raises the melting point.
- Boards should cool at a controlled rate. Cooling too fast can warp the PCB or weaken joints; cooling too slowly can make boards brittle or damage parts. Cooling is usually done with a fine water spray or with air.

Thermal profiling and process control
- Thermal profiling measures temperatures at several points on the board to ensure consistent heating and cooling.
- Statistical process control (SPC) tools help keep the process within specification. Special fixtures and software can measure temperature profiles, contact times, wave height, and wave alignment.
- The height of the solder wave is important. Typical contact time is 2–4 seconds and is controlled by conveyor speed and wave height (adjusted by pump speed). Some machines offer a smooth laminar wave, while others use a higher-pressure “dancer” wave for different results.