Wave soldering is a technique used for large scale PCB assembly for fast soldering of boards that use either or both SMDs and leaded components.
Wave soldering is far less widely used in PCB assembly than it was in previous years. Despite this, wave soldering remains as a process which can be used effectively in a number of areas, especially when the PCB assembly uses a mix of leaded and SMT components.
What is wave soldering
The wave soldering process gains its name from the fact that the process passes the printed circuit boards to be soldered over a wave of solder.
In this way a complete board can be soldered in a matter of seconds producing joints that are reliable mechanically and electrically. Apart from being much faster than manual soldering, wave soldering is also produces joints with a much higher degree of reliability and this makes it ideal for large scale PCB assembly.
Wave soldering can be used in PCB assembly for both conventional through hole mounted components as well as surface mount components. However other methods such as infra-red reflow soldering are more applicable to the fine features being used today on printed circuit boards for surface mount components. As a result wave soldering is less widely used for PCB assembly than it was many years ago.
Wave soldering machine
The wave soldering machine consists of a heated tank of solder. This is maintained at the required temperature for the soldering process. Within the tank, a wave of solder is set up and the printed circuit boards are passed over this so that the underside of the board just contacts the solder wave. Care must be taken in adjusting the height of the wave so that it does not flow over the top side of the board where it would cause solder to enter places where it is not required.
The boards are held firmly in place on a conveyor using metal fingers. These are typically made of titanium because it is able to withstand the temperatures and it is not affected by the solder.
In order that a electronics printed circuit board may be successfully processing using a wave soldering machine, it is necessary that it is designed and manufactured in the correct manner.
- Solder resist layer: The first is standard practice when designing boards these days. A solder resist or solder mask layer is included in the PCB design, and this adds a layer of "varnish" like material to the board to which the solder will not adhere. Only those areas where the solder is required are left exposed. This solder resist is most commonly green on colour.
- Pad spacing: The second main precaution is to ensure that there is sufficient spacing between the solder pads requiring soldering. If they are too close then there is the possibility that the solder may bridge the two pads causing a short circuit.
In view of the way that wave soldering works, where the solder wave is caused by the solder flowing out of the reservoir tank, and the board passes over it, the spacing requirements are dependent upon the direction of the board relative to the solder flow. Pads that are spaced apart in the direction of the solder flow should have a greater spacing than those that are spaced at right angles to the solder flow. This is because it is much easier for solder bridges to occur in the direction of the solder flow.
To ensure that the areas to be soldered are clean and free from oxidation, etc, flux is required. Flux is applied to the side of the board to be soldered, i.e. the underside. Careful control of the quantities of flux are needed. Too little flux and there is a high risk of poor joints, and too much flux and there will be residual flux on the board. While this does not look good cosmetically, there is also the risk of long term degradation because of the acidic nature of the flux.. There are two main methods of applying the flux::
- Spray flux; A fine mist of flux is sprayed onto the underside of the board that is to be soldered. Some systems may even use a compressed air jet to remove the excess flux.
- Foam flux; The electronic printed circuit board is passed over a cascading head of flux foam. This is generated using a tank of flux into which a plastic cylinder with tiny holes is immersed. The plastic cylinder is covered with a metal chimney and air is forced through the cylinder. This causes flux foam to rise up the chimney.
The wave soldering process exposes the electronic printed circuit boards to considerable levels of heat, far greater than those it would be subjected to if it were to be manually soldered. This thermal shock would give rise to a considerably increased level of failure if it were not addressed. To overcome this the board is preheated so that it can be steadily brought up to the required temperature steadily so that any thermal shock is minimised.
The preheating area normally uses hot air heaters that blow hot air onto the boards as they pass towards the wave soldering machine. On some occasions, particularly if the board is densely populated, infra-red heaters may be used as well. This ensures that all the board is evenly heated and no shadow areas are present.
While the pre-heating is required to prevent the thermal shock that the wave soldering machine would generate, the heating is also necessary to activate the flux. This flux is required to ensure that the areas to be soldered are clean and will take the solder.
Applications of wave soldering in PCB assembly
Wave soldering is not as widely used for PCB assembly as it was at one time. It is not suited to the very fine pitches required by many of the boards in manufacture today. However it is ideal for the many boards still manufactured with conventional leaded components and some surface mount boards that use larger components. These boards are often those which are used in lower volume and possibly more specialised products.