Wave Soldering is a large-scale soldering process by which through-hole components are soldered to an electronic assembly. The name is derived from the fact that the process uses a tank to hold a quanitity of molten solder; the components are placed on the board and the component side (back side) of the board is run across the surface of the pool of solder. The solder sticks to the areas of the board not protected with solder mask via surface tension effects, creating a reliable mechanical and electrical connection. The process is much faster and can create a higher quality product compared to manual soldering of components.
As pin-through-hole components have been largely replaced by surface mount components, wave soldering has been supplanted by reflow soldering methods in large-scale electronics applications.
Wave Solder Process
There are many types of wave solder machines, however the basic components and principles of these machines are the same. A standard wave solder machine consists of three zones: the fluxing zone, the preheating zone and the soldering zone. An additional fourth zone, cleaning, is used depending on the type of flux applied.
Fixtures
The printed circuit board with through-hole components on top of the board and/or surface mount components glued on the bottom side are placed on a fixture. The fixture is a fiberglass or titanium frame with openings exposing the components to be soldered.
The fixture is then placed on a conveyor which will carry the PCB through the machine. The conveyor consists of titanium fingers. Titanium is used because solder will not bond to this metal.
Fluxing
The PCB will then enter the fluxing zone, which applies flux to the underside of the board. Two types of fluxers are used: the spray fluxer and foam fluxer.
Spray fluxer
The spray fluxer consists of a robotic arm which travels from side to side while spraying a fine mist of flux onto the bottom side of the board. Some systems will then use compressed air to remove excess flux or to completely remove flux from some areas.
Foam fluxer
The foam fluxer consists of a tank of water soluble flux into which a plastic cylinder with tiny holes are immersed. The plastic cylinder is covered with a metal chimney. Air is passed through this cylinder which causes flux foam to rise up the chimney. As the PCB passes over the foam head flux is applied to the PCB.
For either flux application method, precise control of flux quantities are required. Too little flux will cause poor joints, while too much flux may cause cosmetic or other problems.
Preheating
The PCB will then enter the preheating zone. The preheating zone consists of convection heaters which blow hot air onto the PCB increase its temperature. For thicker or densely populated PCBs, an upper preheater might be used. The upper preheater is usually an infrared heater.
Preheating is necessary to activate the flux. Preheating is also necessary to prevent thermal shock. Thermal shock occurs when a PCB is suddenly exposed to the high temperature of the molten solder wave from the ambient room temperature.
Soldering
The tank of molten solder has a pattern of standing waves (or, in some cases, intermittent waves) on its surface. When the PCB is moved over this tank, the solder waves contact the bottom of the board, and stick to the solder pads and component leads via surface tension. Precise control of wave height is required to make sure solder is applied to all areas but does not splash to the top of the board or other undesired areas. This process is often performed in an inert gas atmosphere to increase the quality of the joints.
Cleaning
Some types of flux, called "no-clean" fluxes, do not require cleaning, and are completely removed during the soldering step. Others, however, require a cleaning stage, in which the PCB is washed with solvents and/or deionized water to remove flux residue.
Process monitoring
Due to the precise requirements needed for wave soldering, the soldering equipment must be closely monitored. Common tests include visually inspecting boards for signs of problems with the soldering process, a resistance test to make sure no flux or other deposits are causing conduction between traces, halide content tests to check for proper flux activation, and others. In modern equipment, virtually all control of the process is computerized, and little human interaction is needed to monitor and adjust the equipment.
