Unless the surfaces are unusually clean, flux always has to be applied in order to encourage wetting. When flux is heated, first its low-boiling constituents evaporate, and then it starts to decompose, generating smoke. It is easy to tin a component by dipping first in flux and then in solder, because the vapour and fumes can escape easily. However, just placing a flat fluxed board in contact with hot solder will trap solvent vapour between the two surfaces, preventing even contact between solder and joint, and resulting in solder skips
Within a short time of exposure to air, the surface of molten solder grows a layer of oxide. Not only is this oxide unsolderable, inhibiting wetting, but the layer impedes free flow of the solder
Unless the operation is very carefully carried out, it is difficult to avoid leaving surplus solder or solder spikes, even when the joint is fully molten when the source of solder is removed
Some degree of movement of solder relative to the surfaces to be joined helps accelerate the wetting process, and is needed to make sure that solder reaches areas of the joint that are difficult to access.

The relative motion scrubbed the solder across the board and allowed flux volatiles to escape, and solder peel-back from the joint was enhanced by arranging for the board to exit smoothly at a slight angle to the pot surface. Automatic machines fluxed the board before solder immersion, and could incorporate pre-drying of the assembly to reduce the quantity of flux volatiles. A cover layer of oil was generally used to reduce oxidation, although this meant that cleaning was almost unavoidable.
 Relative motion between board and solder is enhanced by the movement of the solder wave, and the surface kept free of oxide by drawing up fresh solder from underneath. By way of analogy, think how you might be able to take (relatively) clean water from the body of a pond whose surface is covered with algae!

This process has to be carried out in a controlled and reproducible manner to ensure a high yield of good quality joints at the lowest possible cost. As a result, wave soldering machines have become increasingly sophisticated, in an attempt to control the many variables.

The temperature experience of the wave soldered joint, typically shows a steady temperature rise to over 100°C, then a rapid rise to a peak of 240–250°C at the time of solder immersion. On immersion, the area of the board in contact with the wave rapidly attains thermal equilibrium with the molten solder, so that all joints reach the same temperature. The fact that a large quantity of liquid metal is present to transfer heat is a key difference between wave soldering and reflow soldering, and one that explains the lack of a stabilisation plateau region.

As with reflow, there is a critical contact time for soldering to take place – that is a minimum time a joint must be in contact with the solder to ensure a good joint. This depends on the type of joint, the solder pot temperature, and the board type – constructions differ in their thermal characteristics.

There is a corresponding optimum contact time for an assembly, which is just long enough to ensure that all joints become fully wetted. This time will depend on what joint types are in the assembly and must be as long as the longest individual critical contact time. Generally contact times between 3–4s are suitable for most applications, but 1–2s is used for boards with sensitive components. Working backwards, contact time determines the required conveyor speed and wave dimensions.

Through-hole components have to be held in position to prevent movement during handling and soldering, and especially to prevent them being pushed out of the hole during soldering. The upward force on the leads is a combination of their buoyancy and the pressure of the solder wave. This ‘component lifting’ problem is most commonly seen with parts such as connectors, with multiple terminations and often little interference between the leads and the holes in the board. Mechanical retention may also be needed where the leads are either to be left long or to be sheared very short before or after soldering.

The many ways of keeping components in place include:

'Clinching’ component leads after insertion, that is bending over the end of the lead that projects from the board, so that the component is pulled and held against the board
Preforming the leads, using the residual spring in the lead to ‘interfere’ with the hole
With dual-in-line packages, the leads on opposite sides are set at an angle and have to be compressed to the vertical for insertion. Whilst they interfere with the holes, and are thus loosely retained, the leads on opposite corners are usually clinched for extra security
Fitting retention clips to the handling jigs
Applying temporary weights. These can be as simple as a bean bag, or as sophisticated as a shaped and weighted magnetic cover that is automatically loaded and removed
‘Shrink wrapping’ with a plastic film heat-shrunk over the entire assembly. Problems arise when the components are not of a uniform height: tall parts can become twisted, whilst nearby components are left loose. Also the adhering plastic film may not allow flux fumes and expanding air to escape, causing blow-holes and insufficient solder rise
‘Spray webbing’: applying a top surface spray that glues the components in place and comes off during cleaning. This can be applied as a hot melt system or as an evaporating spray
Applying a molten low temperature material to the bottom of the board. Various wax-like materials are used that become detached and float to the top of the solder bath when passing through the wave.

The methods most frequently seen are the first four in the list above, but the choice will depend very much on the design requirement and equipment available. For example, although requirements for low joint profile are now more normally met by SM solutions, the heat shrunk plastic film method is still used for assemblies where the leads have to be cut short prior to soldering.

Whatever the method, careful attention must be paid to static protection for sensitive assemblies. Also, the component leads must project below the board sufficiently both to ensure contact with the solder and to create joints where good wetting allows the underlying termination to be seen. This so-called ‘pin witness’ forms part of the specification requirement for all through-hole joints: if solder is just ‘plastered’ over the surface to cover the lead, as can happen if the solder temperature is too low, there is no guarantee that a proper joint has been formed underneath.

Conveyor vertical control

Because the depth of immersion of every part of the board surface in the crest of the wave affects the final soldering result, geometrical precision is required where the board travels over the crest, or crests of the wave. The position of every board in the vertical axis must be defined, in reference to both its longitudinal edges, to ±0.3 mm. Any sideways tilt of the board relative to the wave crest must be held within these limits. It is advisable that they should not be exceeded because some unsteadiness of the wave, and warping and bowing of the board itself, must also be accommodated.

Conveyor speed

The speed of the conveyor is a critical parameter in the wave soldering process. The main considerations are:

The heat received by the board is inversely proportional to the speed at which it travels through the preheat stage at a given setting of the heater panels
The maximum practicable soldering speed of a wave machine is governed not only by the ability of the solder wave to get the necessary amount of heat into the board within the time available, but also by the complexity of its pattern and the density of its population of components
Multilayer boards with a high heat capacity must travel more slowly than simple single-layer boards
Boards with closely packed devices and fine-pitch multi-leaded devices must travel over the wave more slowly to give the solder the chance to flow into the narrow gaps between adjacent devices, and to drain away from the fine pattern of leads.
Board support

Boards entering the wave must always be kept flat, otherwise some areas may not be properly in contact with the solder. Also, long components, such as connectors, at right angles to the conveyor, may be mounted flush with the board at the ends, but with unacceptable clearance in the middle. This may result in unsatisfactory joints, will add to stresses, and will ‘freeze’ the board in its non-flat state. Even worse, the board leading edge may dip under the wave front, allowing solder to come over the top of the assembly. Such ‘flooding’ is very difficult to rework.

Boards may warp when heated. But, even without such warping occurring, heavy unsupported boards may flex under their own weight, and thin boards may be too flexible. Where some possibility of board sag is anticipated, and finger conveyors are fitted to the machine to be used, there are three ways in which this can be prevented:

by applying stiffeners across the board width – these would usually be titanium clips mounted on the more vulnerable leading edge of the board
by using pallets, preferably with clamps to hold the board flat
by running a central support cable along the wave solder machine, as with reflow ovens.
Support cables are usually thin multi-stranded stainless steel wire, and move at the same speed as the conveyor. They are adjustable across the width of the conveyor, so that their position can be arranged to coincide with unused areas of the board, such as the fret between circuits on a multi-circuit panel.

Hot air knives

Although not a cure-all for badly designed assemblies, hot air knives are often recommended for problem circuits. These provide directed streams of hot air to separate bridges before they solidify, using the fact that solder bridges between pads are less stable than those bridges that form the joints between pads/holes and leads. Of course, this can only be done when the solder is still liquid, so the gas flow has to be applied to the board as close as possible to the point where the board exits the wave.

Mixed results have been reported with older designs of air knives, which were fixed and operated across the whole width of the board: it was an art to get them set up correctly, as a result of which they were often not used.

More recently, methods of selective de-bridging have been developed, responding to user pressure for higher yields even with designs where lack of space prevents ideal layouts. These use carefully controlled streams of warm air, whose velocity and angle of attack is ‘fine tuned’ to avoid disturbing the desirable solder bridges that form the solder joints. Just enough gas is applied to disturb the capillary/cohesive forces that maintain the unwanted bridge, without reducing the amount of solder available to make a good joint. The excess solder is forced back towards the wave, and falls back into the pot by gravity.

A good impression of the way that the solder is blown away from bridges is given in this Seho video clip of a hot air debridging tool in operation. The gas flow is directed only to the parts of the board that have been identified as potential sites for bridging, leaving the remaining areas untouched. As de Klein and Schouten comment: “most solder bridging can be more or less predicted, unless the bridges are caused by a lack of flux activity . . . (when) . . . solder bridging can be found randomly across the board and there is no cure other than improving the fluxing process.”

Inert atmospheres
In a normal atmosphere, molten solder quickly acquires a tough surface film of mixed tin and lead oxides. As soon as the solder is moved or disturbed, the oxide skin breaks and mixes with the solder underneath. The resultant mix of oxides and clean solder is called ‘dross’. Because the process of wave soldering involves moving solder around and letting it fall back into a bath of molten solder from a height, the formation of dross is unavoidable unless measures are taken to protect the surface from oxygen in the atmosphere.
Implementation
One way of protecting the surface is to remove oxygen from the surrounding atmospher.

The advantages of soldering in this inert atmosphere are:
a substantial reduction in dross
decrease in solder consumption
reduced machine maintenance
more consistent process yield.
In addition inert soldering gives improved solderability by improving wettability even with the best of modern fluxes and can minimise or even eliminate the need for post-solder cleaning.
Users report that:
Reduction in solder consumption has resulted in projected annual savings nearly equal to the cost of retrofitting the wave solder machine with a nitrogen-atmosphere system
By eliminating dross, the maintenance required to clean solder pot and nozzle has been reduced from once every 40 operating hours to less than once every 1400 hours
Solder wetting and through-hole wicking have been substantially improved, with the virtual elimination of skips and bridging.
Although nitrogen is not a truly inert gas, it remains by far the most popular option because of its ready availability and low price compared to other inert gases. The best performance comes from machines that are inerted throughout their length and have entrance and exit air-locks. Such machines can easily provide an environment containing <50 ppm of oxygen.

However, machines with this special construction are relatively expensive both to purchase and run. For economic reasons, probably a majority of users chose one of the solutions in which nitrogen is provided only at the wave surface. Not only is such a ‘nitrogen wave’ generally available as an retrofit add-on to older machines, but it gives most of the benefits of the fully inerted machine:

The rate of oxidation increases with temperature, and the preheat areas are relatively cool, so that the majority of the oxide is formed in the vicinity of the wave
Substantial improvements in the rate of dross formation can be achieved with quite modest levels of nitrogen injection, and most of the problem can be eliminated by reducing the oxygen content only to <1,000 ppm.
Designs vary considerably in the ways in which they both inject nitrogen and define the inerted volume. Typically they use nitrogen sprays or diffuse nitrogen through porous stones, and take advantage of the fact that the board creates a seal over the wave, retaining the inert atmosphere.

Maintaining performance

If you have seen a number of wave soldering machines in different companies, you will almost certainly have come across some machines that are in less than pristine condition! Partly this reflects the nature of the process, and the difficulty of removing dross and dealing with flux maintenance in older machines. However, with newer equipment, your observation may relate more to the low level of expectation of operators and management, used to machines carrying out less exacting work. A review of equipment and maintenance practice is an enlightening part of any supplier audit.

Fluxer

Sealed spray fluxer systems need little more maintenance than checking the free operation of any mechanical movements and keeping the feed tubes and nozzles unblocked. Problems in those areas result in inconsistent flux coverage, a fault that is soon apparent and easily diagnosed.

The same cannot be said of wave and foam fluxers, where the two mechanisms that can affect the solder joint take place over time:

the density of the flux increases as solvents evaporate, producing a more viscous flux and a thicker flux deposit
the flux gradually degrades through exposure to air, especially if the environment is hot and/or humid.
The first of these is normally addressed by monitoring the density (also referred to as ‘specific gravity’) of the flux. This relies on the difference in density between the flux and the thinners used. The second problem area can only be dealt with by regular (fortnightly or monthly) cleaning of the flux tank and replacement of the flux.

Solder

Here there are three topics to consider:

having enough solder to ensure that the wave height is correct
making sure that the joints are not adversely affected by the build-up of oxide on the solder surface
ensuring that the solder in the pot has not become contaminated.
The solder level in the bath must be maintained, and the pot replenished, which means that the level of solder in the pot must be monitored regularly, either automatically or by the operator.

Dross is formed when solder is moved or disturbed, and the oxide skin formed by reaction with the oxygen in the atmosphere breaks up and mixes with the solder underneath. Because the basic principle of wave soldering involves moving solder around and letting it fall back into a bath of molten material, the formation of dross is unavoidable unless an inert atmosphere is used. Allowed to build up, dross has a number of negative effects, so it needs to be removed regularly: depending on the application, some intervention may be needed every hour!

For hand and reflow soldering, purity of the starting material is not an issue since fresh solder is used. However, with wave soldering, solder in the bath is continually reused, and may gradually pick up contamination dissolved from the product being processed. Silver (from passive component terminations), copper (from boards without a nickel barrier layer) and gold (from boards with nickel-gold finishes) are the materials most frequently found. It is normal practice to use a laboratory to carry out an analysis of the bath on a regular basis (perhaps three-monthly) and then make the necessary additions of tin, to replace tin oxide lost in the dross, or even replace the whole of the solder in the bath if it is contaminated.