In this part of the unit we aim to help you understand:
This part is divided into four main sections: after a preliminary reminder about the process sequence and the materials used, the second section describes the three main stages of the wave solder process. The third section gives some information about practical implementation, machine parameters and set-up, but is deliberately less detailed than a process engineer would need – wave soldering is a complex process, and there are many options and trade-offs. The final section majors on identifying, curing and preventing faults, including aspects of design.
Although we do not explicitly refer to other processes that use liquid solder, such as lead tinning, wire stripping and Hot Air Solder Levelling for PCBs, much of the information on materials, equipment and control contained here will also be found relevant in those contexts.
This booklet is supplemented by the units on Solder materials and Solder joints. You should refer to these if you find difficulty with any aspects relating to the materials used.
The advent of the printed wiring board made it much easier, quicker and cheaper to assemble electronic equipment. The time saving benefit when making multiple solder joints was found first with hand soldering. However, bringing all the joints into a single plane, with the board as a barrier between solder and components, also created a structure in which soldering could be automated by solder dipping.
Before reading further, think in some detail about your response to the question ‘What are the potential problems in simply taking an assembly and dipping it in liquid solder?’.
In considering this, you should recollect any experience you may have had in hand soldering (for example, reworking) or in tinning components by solder dipping. To jog your memory:
Our ideas on this can be found in the remainder of this section.
Probably you will have thought of most of the points below, which highlight the challenges in developing any method of machine soldering:
Many of the first in-line machines used the ‘drag soldering’ principle, where the board was moved across a static pot. 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.
Wave soldering, also known as ‘flow soldering’, was patented by Fry’s Metals in 1956 and by the mid 1960s had become commonly used as a way of enhancing productivity and yield. 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!
Wave soldering is an in-line process, during which the underside of the board is successively fluxed, preheated, immersed in liquid solder, and then cooled (Figure 1).
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, the ‘profile’ (Figure 2), 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.
Measuring the temperature profile has been particularly important in wave soldering to reduce the damage to surface mount devices: thermal damage is less of a problem with through-hole components, because the board acts as a heat shield.
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.
Wave soldering has numerous applications including component lead tinning, component manufacture, hybrid circuit assembly, and continuous wire tinning, but its main application is for soldering circuit board assemblies. The process for a through-hole component starts with selecting the part, cropping and forming it where necessary, inserting it into the board, and then applying molten solder to form the bond between the circuit board and component termination.
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 (leads are less dense than solder) 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:
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.
SM components were originally conceived in the late 1960s for ‘hybrid microcircuits’, using ceramic substrates. Parts were either hand-soldered or reflowed using a hotplate. This second process presented no problem, as circuits were usually of single-sided construction and the ceramic was stable, with good heat conductivity.
However, soldering problems began when SM components started to be wave soldered to polymer circuit boards. Whilst chip components presented no problems, active component formats were not very ‘soldering-friendly’, small outline integrated circuit packages (SOICs) and plastic leaded chip carriers (PLCCs) being especially difficult to wave solder.
This is because the ends of the leads are too close to the relatively high body mouldings. The solder wave finds it difficult to access these corners, because of the high surface tension of the molten solder. Until wetting takes place, the solder surface in contact with a component is like a balloon pressing against the walls of a room – in a tight corner, at best it will only make contact at the periphery.
This ‘angle of aspect’, formed between the upper edge of the component body and the end of the solderable lead, is about 60° for SOICs and can reach 90° for standard PLCCs.
A similar sort of situation exists where SM parts are closely spaced, making it difficult for the solder to access the joint. There is no single solution: as you will be able to deduce from your later studies, this problem is both addressed during board design and tackled during manufacture by using waves with high turbulence and an appropriate angle of attack.
The trend towards thinner packages might be expected to make gull-wing formats easier to solder, because the angle of aspect is reduced. However, the reverse is the case: usually the lead pitch also becomes finer, and the reduced separation increases the incidence of bridging. Table 1 indicates the range of SM components for which wave soldering is suitable.
|Lead pitch||Applicability of wave soldering|
|1.27 mm (0.05 in.)||straightforward|
|0.75 mm (0.03 in.)||more difficult, requiring special layout provision|
|0.5 mm (0.02 in.)||usually need to be soldered in an inert atmosphere|
|<0.5 mm (0.02 in.)||wave soldering not recommended|
|Table 1: Range of application of wave soldering|
The solder joint faults introduced by the wave soldering process are normally ‘major’ defects, that is they require rectification. Typical of these are bridges, spikes and solder skips, excess solder defects being the most common. When problems happen, they tend to affect wide areas of the board, and most test systems have difficulty in dealing with a number of short-circuits on a single assembly. It is therefore common practice to introduce a rework stage immediately after wave soldering. If a cleaning process has been specified, this rework is usually carried out after cleaning, but may sometimes be done beforehand, provided that the board is not excessively contaminated by flux and that care is taken to avoid the flux residues hardening and becoming more difficult to remove.
In what is graphically referred to as ‘view and touch-up’, an operator both inspects and reworks the board. This is usually done under low magnification using a magnifier. A benefit is that there is direct feedback from inspector to rework operator on the location and type of defect! However, there are dangers in assigning dedicated operators to this task:
Mixed technology (‘Type 3’) assemblies are often made by combining reflow and wave soldering. A typical process is to apply solder paste, place surface mount (SM) components on the top of the board, dry the paste, and then reflow the paste to create the joint. Next leaded components are inserted, the board is inverted, adhesive is applied, SM components are placed, and the adhesive is cured. After inverting the board once more, wave soldering completes the process.
Note that the SM parts are totally immersed in the solder wave. To avoid being washed away, they must be firmly secured to the board by applying glue before placement and curing the adhesive afterwards. Typical glues used are epoxies that are heat cured, the process taking place in a belt oven similar to that used for reflow.
Unfortunately, with wave soldering it is not easy to incorporate complex components on the underside, which limits the freedom of the layout designer. It also introduces a second type of soldering process, one that is (comparatively) low yield and difficult to control. The alternative of using solder paste and reflowing all the components is attractive, and cost reduction and process simplification are major driving forces behind what is called ‘intrusive reflow’ or (more descriptively) ‘pin-in-paste’. We will be exploring that theme in Alternative techniques and off-board assembly.
Fluxes used for wave soldering (also referred to as ‘preparation fluids’) are usually low-viscosity liquids, and consist of a flux base combined with:
There are four main categories of flux used in wave soldering:
The trend to no-clean and water-soluble fluxes resulted from the CFC ban following the Montreal Protocol agreement of 1987. There have since been environmentally driven moves both to control the discharge of water used for washing and to reduce the emission of Volatile Organic Compounds (VOCs). The result has been a trend towards using water-based no-clean fluxes. Because water has a higher boiling point and is relatively slow to evaporate, these fluxes typically have a higher (10–20%) solids content, and generally require modification to the preheat part of the process. This aspect is discussed at greater length in Design for eXcellence.
Flux changes in composition when in contact with the atmosphere. These changes take two forms:
In order to get consistent fluxing results, either the condition of the flux has to be maintained scrupulously (with foam, wave or brush methods) or virgin flux has to be used once only (with spray fluxing). Low solids and no-clean fluxes tend to be most difficult to maintain in consistent condition, which in turn means that the industry is moving towards sealed spray fluxing systems.
Although 60:40 tin : lead used to be favoured, the material of choice for all current wave-soldering applications is 63:37 tin : lead, that is, Pb37A (in the ANSI/J-STD-006 standard) or Sn63 (in QQ-S-571E). [The moves to lead-free materials as a result of environmental pressures are considered in Design for eXcellence]
As you will have learned, there are benefits of adding small amounts of silver to tin-lead eutectic solder, and the Pb36A alloy with 2% silver is very commonly used for solder paste. However, for wave soldering the benefits are normally outweighed by the greatly increased cost.
Fortunately, using the weaker alloy is not a major problem: a wave soldered joint tends to be stronger by design than a reflowed joint, being generally applied to through-hole components and surface mount parts on coarse lead pitch, with correspondingly large fillets. Restricting the use of Pb36A to solder pastes, where the demands on the joint are more stringent, and the metal cost is only a small percentage of the total, makes good economic and engineering sense.
Note that there will always be a substantial differential between the quoted trading prices of commodity metals and the cost of solders. Only a portion of this represents profit, as there are real costs associated with the additional refining and casting processes involved. Whilst specifications and manufacturing processes vary, electronic solders are very much purer than their counterparts in other industries, and have a lower oxide content.
Solder resist, or ‘solder mask’, is an organic coating applied selectively to the surface of the board to restrict the contact area with the molten solder. As a result only non-masked areas will be wetted. Solder resist is usually a permanent coating and is left on the final product. It offers the benefits of increased electrical insulation, fillet control, and greater simplification of circuit design. Solder mask is essential for all surface mounting applications because of the fine tracks and small pad areas.
The material selected for a resist depends on the final substrate (rigid or flexible), the cost of the assembly, and the final product operating environment. Solder resists are generally made of epoxies and polyurethanes, but other materials such as acrylics and polyimides have been used.
It is extremely important that the chosen material forms a flat, smooth, hard surface of consistent thickness onto the board, otherwise solder webbing will occur. In addition, the materials must withstand soldering temperatures, as well as exposure to flux and any cleaning solvents.
The main advantages of solder resists are that they:
This last item is particularly important from the reliability point of view – the increase in surface insulation resistance (SIR) is around two orders of magnitude.
The only downside for solder resist is that some resists may interact with solder to produce unexpected effects: an example of this is solder balling which is reported to take place during wave soldering under nitrogen.
Solder masks are mainly applied in one of three ways:
The latter two are necessary for fine-line applications because of their better pattern definition, and liquid solder mask is the most commonly used for high-density SM work, because of its thin coating and inherent consistency of thickness. More details are given in Solder mask.
Make as complete a list as possible of the ways in which aspects of the materials used might impact on the wave soldering process. Include in your list:
Can you think of any aspects relating to the materials used that have not been considered so far in this section? [If you have experience of wave soldering, you might think about any materials-related defects you have seen]
The correct quantity of flux has to be applied evenly to the entire surface to be soldered: unfluxed areas will not be soldered well, and over-thick deposits will lead to voids and solder balling. It is also desirable to flux the inside of any through holes, to aid wetting and solder pull-through. The in-line application of flux can be achieved by a number of different methods, of which the main current techniques of foam and spray fluxing are described below.
Until the mid-1990s, the foam fluxer was the type most commonly fitted in automatic soldering lines. With this method (Figure 4) a flux ‘foam’ is generated by passing low pressure air (<1bar) through aerator tubes immersed in a tank of liquid flux. Despite their common name, most of these ‘stones’ are in fact made of polypropylene! [Having said this, ceramic tubes are reported to be more reliable and easier to keep clean] The tubes, often fitted in pairs, are designed to break up the stream into tiny bubbles, and are covered by an open chimney that channels the foam upwards. The assembled board travels across the crest of this wave of foam, and a thin coating of flux is left on the board as the bubbles burst.
The flux used needs to be specifically formulated, with additives to aid the creation of a stable foam with small bubbles. Typically, the flux is of low viscosity, achieved by having a relatively low solids content. The thinner may be based on solvent or water, the latter becoming increasingly favoured because of environmental issues associated with VOCs.
The normal foam fluxer without any special support is often labelled a free foam head, and can reach a height of around 15 mm. If greater heights are needed, for example, where long through-hole leads are used, a brush support can be introduced on both sides of the foam chimney and the total depth can be extended to above 25 mm.
The maximum head of foam for a given type of flux depends on the nozzle type, the air pressure and the amount of flux above the aerator. The latter affects the stability of each flux bubble and therefore the foam head support and height. Figure 5 shows different types of nozzle used in a foam fluxing unit, and illustrates both how contact width can be increased for a given flux type and how the height of the foam head increases as the flux level is raised (3 > 2 > 1).
When setting up a fluxer, the flux level typically starts 10–15 mm above the aerator, and is increased gradually (up to about 35–40 mm) until no further increase in foam head height is observed. At the same time the bubbles become smaller, because the foam head is better supported.
The amount of flux deposited by a foam fluxer cannot be controlled directly by the operating parameters of the fluxer itself, but depends primarily on the speed of the board set by the conveyer and the viscosity of the flux. Note that the latter parameter depends on the solvent content of the flux and foam fluxers need constant addition of thinner to replace that which evaporates.
Since the introduction of SM technology, spray fluxing has gained in popularity because the fine droplets can be propelled into the narrow gaps between closely-packed SM components, whereas foam bubbles may burst before they reach the joints at the base of the component body.
Most spray systems use a pressurised canister of flux feeding a spray head at which the required amount of material is atomised by air pressure or ultrasonic energy. A major benefit of a sealed system is that the flux is applied as received, so that there is no chance of contamination, and complicated flux controls are unnecessary. There are other advantages that outweigh the increased cost of the equipment compared to foam fluxers, and have made sealed spray systems popular, even as a retrofit option:
Many of the spray fluxers in the market place have easily replaceable, air-powered nozzles, often fitted in pairs. Heads are fixed to a moving bar that provides a reciprocating action similar to that used in paint sprayers, in order to give complete even coverage of the board (Figure 6).
Combined with a board sensor, the spray fluxer can be set to spray only the required area. Depending on whether the head movement is pneumatic or motorised, the limits of head travel may be set mechanically or electronically. The latter option makes it possible to control all fluxing functions by computer.
The main problems with reciprocating air-powered nozzles are that:
As a result, there are a number of competing spray systems that vary in the way that they address these problems and achieve wide, even coverage. Many of these have an ultrasonic spray mechanism, in which the energy in high frequency sound waves atomises the flux.
The amount of flux deposited varies directly with conveyor speed, but can also be controlled by adjusting the fluxer parameters. How this is done depends on the equipment being used. Generally, the more sophisticated the equipment, the more flexibility there is over the amount of flux deposited.
Common to all spray fluxers is the need for an efficient extraction system, which gathers any over-spray and the aerosol of small flux droplets that unavoidably forms wherever flux is atomised. This airborne flux must not be allowed to settle on the top surface of boards or on the soldering machine at large.
Flux application is rarely totally even, and a fluxed board will usually drip surplus flux onto the preheater, reducing its efficiency and creating a fire hazard. There are two ways of removing excess flux, both of which are usually integrated into the fluxing unit:
On current machines, the air knife is the option normally fitted, as this avoids physical contact with the board. A brush head is only suitable for assemblies that are not sensitive to such contact. An additional benefit is that the air knife will drive flux up into plated holes to enhance top side wetting.
What are the main types of fluxer unit? Think about a range of different board types and list the tasks for which each is suitable. Are there any problems or limitations?
Justify your choice of the method you believe to be best for a high-density surface mount board?
A freshly fluxed board cannot be wave soldered successfully unless its underside has been heated to a temperature of about 100°C, the exact temperature depending on the flux being used. The reasons for preheating a board are:
Should excess flux solvent be left on a board through insufficient preheat application, a vapour blanket can form between the board and the solder wave. Needless to say this not only slows down the heat transfer between the molten solder and the board, but can also cause the solder to spit. This is a major cause of small globules adhering to the underside of a board.
Where the flux thinners are solvents, they are easy to evaporate. However, water-based fluxes are more difficult to handle since they can retain enough moisture to spatter during the soldering process.
Removing volatiles requires removing saturated air from below the board. With an inclined conveyor, there is usually sufficient movement caused by natural convection above the preheater. However, with horizontal or near horizontal conveyors, additional artificial air movement may need to be generated.
Even after preheating, a certain amount of ‘sizzling and spattering’ can usually be anticipated when the board enters the wave. This is directly related to the amount of volatiles still left in the flux. However, over-drying is not recommended, as this makes the flux on the surface relatively immobile, interfering with solder wetting because it is not easily displaced by the wave.
Note that preheating alone is not enough to remove volatiles (such as moisture) that may have been absorbed into the PCB structure. These need to be removed by pre-baking 1.
1 On soldering, any volatiles in the printed wiring laminate will give off gaseous materials and cause blow-holes and entrapped gas pockets in the solder joint. This gas evolution may also create sufficient force to rupture the plated via barrel or cause delamination. These volatiles can be contaminants deposited during storage, handling, and assembly operations, trapped processing solutions, organic volatiles from the materials used in board fabrication, or natural moisture. However, pre-baking is a costly and time-consuming operation, and one which may impair solderability. Users should only need to bake a board before wave soldering either when packages have been left open for an extended period or when blow-holes are discovered.
Some fluxes like rosin, depend on heat (70–80°C) to become active at all. The level of activity of other types of flux also increases with increasing temperature.
The thermal gradient between room ambient and soldering temperature is enough to cause serious damage to many materials, especially non-metals, and SM components should be specified to be compatible with wave soldering. This can be done simply by immersing the parts directly in a solder bath. Suitable tests are referenced in IPC standard 9501 PCB Assembly Process Simulation for Evaluation of Electronic Components.
For boards, the main concern is the distortion that occurs, often referred to as ‘warpage’ or ‘warping’. This is mainly caused by the difference in thermal expansion between the underside of the hot board, which is exposed to solder, and the cooler top. The result is that the centre depresses and pushes itself further into the wave, whilst the sides curl up and may not come into contact with the molten solder. The warpage is made worse by the random location of holes drilled in the board, internal copper layers, and by the uneven distribution of component weight.
Preheating the assembly reduces both the thermal gradient between the top and the bottom of the board, so reducing the potential for warping, and the thermal shock to which any SM components on the underside are subjected.
When making joints with liquid solder, you need to have sufficient thermal energy available to ensure that the interface remains liquid. If you dip a totally cold surface into solder, you will first freeze a film of solidified solder that masks the surface but is not in intimate contact with it. This has the result that heat transfer is impaired, so that the joint area takes longer to reach a temperature high enough for the solder to wet. In all soldering processes, the parts to be assembled must be hot enough before solder is applied to avoid this happening.
In wave soldering, the heat required to raise the surfaces to the wetting temperature comes from both preheating and contact with the solder wave. These work together to supply the heat necessary for the joining process. The higher the preheat temperature, the less heat is required from the wave. This can be translated into a shorter time in the solder wave, or higher production speeds.
Without an efficient preheating stage, high conveyor speeds would not be possible, nor (during the brief time available) could the molten solder be persuaded to rise through the plated holes in a multilayer board to form a meniscus on the top surface.
Most systems provide the majority of their heat from underneath using infrared radiation. For the first drying stage of preheat, panels are common. These are hot plates or rods emitting long/medium wave infrared. For the more intense heating required later, quartz lamps emitting short wave infrared are frequently preferred for functional and maintenance reasons:
Convection panels are becoming increasingly used, primarily to provide the air flow that is necessary to dry water-based fluxes.
When preheating is applied only from below, the rate of temperature rise of the assembly can be insufficient for heavy or densely populated boards, because too much energy is conducted away from the underside. For that reason, many machines have additional pre-heaters mounted above the work, especially in the section immediately before the wave.
The preheat requirements of products vary greatly, depending on the thermal demand of the assembly and the drying properties of the flux specified. For this reason, many wave soldering machines have modular preheat systems, which can be reconfigured on site for a specific application.
Whatever the set-up of preheaters in a machine it is most important that the whole board receives the same amount of thermal energy, because uneven preheating is a dangerous source of soldering faults. However most modern soldering lines give a warning if a heater in the preheating section should fail; some even prevent further boards from entering the machine in case of failure. Boards still in the machine must of course continue to travel forward, if they are not to be cooked or get stuck over the solder wave.
Before reading further, think about what happens during the preheating stage, and make a list of the features you would look for in selecting a preheater design for a specific application.
The solder wave provides direct contact between the solder and the component joints on the PCB. It can be divided into two distinct physical events:
As indicated, both parts of the process depend on wave dynamics or, more plainly, on the shape of the solder being pumped, its fluidity, flow rate and turbulence. There are a large number of wave designs but most are variations on the same technology (Figure 7).
Solder waves are produced by forcing molten solder upwards, from an area where there is no dross, through a vertical conduit that ends in what is commonly called the wave nozzle. This nozzle will contain baffles to ensure uniformity of flow to the top. Originally, the wave nozzle had the form of a narrow slot, set at right-angles to the direction of board travel, with the emerging solder forming a hump of molten metal and falling in a symmetrical wave over both sides back into the main solder bath.
The symmetrical wave was soon replaced by the asymmetrical wave shown above, which gives neater joints, reduces solder bridging and permits higher soldering speeds. The term ‘Lambda wave’, though an Electrovert trademark, is often applied to this waveform because of its similarity in shape to the Greek capital letter G.
At the front of the wave, the solder flows in the opposite direction to the board, providing a scrubbing action, and assisting wetting. At the back of the wave, the solder flows in the same direction as the board, so that it peels away smoothly, and as much solder as possible drains back into the pot, reducing the incidence of shorts and bridges.
The reason for this effective action lies in fluid dynamics. The object is to drain the solder excess from the pin, leaving only what can be retained by the meniscus around the board/pin interface. Pins should therefore be moved vertically away from the solder, since any other angle will expose a larger surface area of the pin, and a greater force would be required to separate pin and solder. This is why Electrovert chose to make the flow of the wave equal to the conveyor speed, as this gives a vertical downward vector to the separation, and hence the best drainage.
Depending on the wave design, the presence of the board may modify the flow characteristics. On some machines solder does not move over the back plate until the board comes into contact with the front of the wave, pushing the solder over the back-plate, to leave a clean soldering surface and complete the G shape.
With most types of wave solder machine, an impeller pump, driven by a variable speed motor, propels the solder downward into a pressure chamber, from which it flows up through a vertical conduit towards the wave nozzle. This arrangement keeps the movement of the solder towards the weirs at both sides of the nozzle as free as possible from any turbulence. Before the introduction of SMDs, this waveform, with the board skimming through the top of the solder wave, was the best way of achieving a clean, solder-bridge free assembly at conveyor speeds of over two metres per minute.
To minimise dross, solder waves are generally only pumped when needed. Sensors determine the imminent presence of a board, and the pump is then activated, but allowing enough time for the wave to stabilise before contact between board and wave occurs.
Sources: Electrovert (LH); Iemme (RH)
The concept of the double wave is shown in Figure 8:
Most double-wave machines have two solder pumps, one for each wave, but both take their solder from the same reservoir.
The chip wave leaves the joints somewhat untidy, with some bridging. However, most of these imperfections are tidied up in the second wave, with its smooth exit conditions.
A dual wave system may be used for both leaded and SM components, but on most machines the pumps can be operated independently, allowing the chip wave to be turned off for applications that have only leaded components.
There is an inevitable gap between the two waves, and this has been criticised for several reasons:
This has led to the development of a wave form that combines the functions of both chip and main waves in a single asymmetrical wave. This is done by generating a zone of high multi-directional kinetic energy within the wave at the point where the board enters, typically using an electromagnetic transducer to vibrate a movable element within the wave. The position of the element defines the vibration zone, and its amplitude of movement can be varied to optimise the action and eliminate defects. The exit side of the wave follows the normal asymmetrical pattern with a smooth, laminar overflow. The process is claimed to be reliable, repeatable and efficient.
In some machines, the vibrating wave is preceded by a conventional high-energy chip wave – belt and braces! In defence of the manufacturers, it must be said that all these wave functions can be selected, so that the machine can be configured very flexibly for a wide variety of applications.
The interaction of a conventional laminar wave and the assembled board can be divided into three zones, as shown in Figure 9:
The point of entry is at the most dynamic part of the wave since the directions of board travel and solder flow are in direct opposition to one another. At this point the solder flows rapidly down the wave, while the board moves in the opposite direction.
This differential motion creates a washing action that removes the flux from the board, and will also flush away organic layers such as any surface contamination.
Flux removal is total on the metallic pads where wetting occurs. However, some of the more viscous fluxing materials may cling to the laminate between conductors. The extent of flux retention depends on the physical layout of the lands and components, but inevitably some flux is left on the bottom of the board between lands.
If only the bottom of the PCB had to be wet, the wave solder operation would be complete shortly after the point of entry. However, component leads with a substantial heat content also need to be soldered, and plated through holes must be wetted and filled with solder.
This filling is primarily produced by wetting taking place between solder and metallisation and the resulting surface tension forces: capillary action ensures that solder will rise up the holes. There is some contribution from wave pressure, although, as the board approaches the point of exit, the upward push due to fluid dynamics decreases in importance.
In addition to the heat supplied by the wave, heat is absorbed by the assembly during the preheat stage. This additional heat is critical in good fillet formation. A board that is already warm can pass more rapidly through the hot wave. Contact time is thus reduced, which decreases thermal damage. Note that the greatest damage to components and board happens during exposure to the wave. This is because the damage mechanisms are accelerated at higher temperatures, whereas the assembly can tolerate the lower temperatures of preheat for an extended period with few ill effects.
Because of the importance of heat transfer, the part of the wave between the point of solder to metal contact and the point of wave exit is often referred to as the heat transfer zone.
In order to achieve the best wave solder results, a high degree of uniformity in fillet configuration has to be obtained. This ultimately eases inspection and dramatically reduces unnecessary operator touch-up. To obtain such uniformity the forces shaping the underside of the fillet at the wave exit must be controlled. These forces fall into two categories:
Surface energies. These forces are predictable and can be subdivided into:
Hydraulic forces. These are often random and depend on the following factors:
The best point of exit corresponds to where these hydraulic forces can be neutralised. This is achieved by withdrawing the fillet from the wave at a static location, which is found where the board travel speed and direction are similar to that of the solder flow.
Describe the way in which a wave acts to create solder joints, in particular what happens in each part of the wave.
Which waveform do you believe would be best for a low density mixed technology (Type II) assembly? Justify your choice!
There are two methods of holding PCB assemblies in the conveyor:
Source: Blundell (RH)
Sources: Seho (inset)
On larger machines, pallets usually ride on chains, pulled along by their own prongs: on smaller machines, a synthetic rubber belt provides friction drive to the pallet. Some advantages are that pallets:
Pallets are, however, not very cost effective for medium to large volumes, because a high labour element is required to load and unload the pallets. This is especially true if finger conveyors are fitted, since the loading and unloading operations must be carried out carefully to avoid damage to the fingers. Pallets also interfere with the final cleaning process and must be unloaded before degreasing or washing. However, the pallets themselves need to be cleaned occasionally in order to avoid contaminating the fluxer. Also, when pallets are recycled whilst still hot, they tend to depress the foam in the fluxer, resulting in skips or uneven coverage.
Fixtures and pallets can be made in many different materials, but the service conditions are harsh! Pallet material must:
Anodised aluminium, Teflon-coated steel, and titanium are all used, the last being preferred because it has no surface coating and is less subject to damage. However, although easier to clean, metal jigs lose out to a range of non-metallic materials similar to FR-4 laminate, except for light items such as clips. Fixtures made from composites are easier to handle in production because of their lighter weight and lower thermal conductivity, and have less effect on the process because they absorb and retain less heat.
Durostone® is one such composite. Made of epoxy reinforced with 65–80% of glass fibre, it has a similar thermal coefficient of expansion to the board it supports. It is robust and easy to machine, but will wear in contact with metallic surfaces. For this reason pallets are often fitted with metal glides, usually made of titanium.
Depending on the production volume, pallets may be customised for the circuit (in which case they can incorporate anti-flood protection, solder masks for selective soldering and clamps for component retention) or general purpose styles that can be adjusted to fit differing sizes of board. Typically, the clamps on these flexible types will integrate solder dams to prevent solder washing round the side of the board.
Note that pallets have to be cleaned regularly, because they always pick up flux, which decomposes as it passes through the machine. This even happens when spray fluxers are adjusted correctly, because inevitably there is some over-spray. Not only does this make the jigs difficult to handle, but it will impact on both the yield and the cleanliness of the final product. No-clean fluxes produce less of a problem but, after many passes, almost any material will build up a film and then solder sticks to the film, rather than the metal.
Finger conveyors are permanently attached to the machine. The fingers, which come in contact with the solder, are always made from titanium, which is untinnable with any type of flux. Most of the fingers fitted have a vee-groove, as this restrains the board vertically, and prevents it being pushed up whilst in contact with the wave. However, unless very carefully maintained and adjusted, fingers with this design are prone to dropping boards. It is common, therefore, to replace a proportion of the fingers (say 1 in 3) by fingers with an L-shape, to provide a more secure platform for the board. This design of finger is also easier to use in conjunction with pallets, where the upward solder pressure can be overcome by the combined weight of assembly and pallet.
Finger conveyors have the following advantages:
Finger conveyors are very cost effective from small to large volumes and fit more easily into in-line conveyerised manufacturing systems. However, they can only handle boards with parallel edges and cannot deal with mixed width batches. They are less suitable than carriers for use with boards that are less than fully self-supporting – flexible boards may be dropped, or dip into the wave crest, leading to flooding.
For problem-free operation, the fingers on the conveyor must be properly adjusted and kept free of tacky residues. Cleaning the fingers helps hold the board correctly and stops contamination of the flux station. Unlike pallet cleaning, an automated mechanism for doing this can be fitted to the machine. In adjusting the conveyor, attention must be paid to:
The conveyor system is a key element in the construction of a wave soldering machine. On most British and American machines, boards travel on one single continuous conveyor from one end of the machine to the other. The slope of the conveyor is used to make a bridge between the low level of the hand assembly operation and a higher level, from which the cooling section (frequently separate) returns the product to standard machine conveyor height.
By contrast, other European machines frequently operate with the sections at different angles. One constraint that this design introduces is great difficulty in incorporating a finger conveyor, and most such machines use ‘rubber bands’ at the edge, driving pallets holding the product. However, there is an advantage that the speeds at fluxing, preheat and over the wave can be set independently. For example, with high-mass products, the pre-heat section can run at a slower speed than is used for the soldering operation.
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.
The speed of the conveyor is a critical parameter in the wave soldering process. The main considerations are:
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:
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.
Summarise the benefits and drawbacks of the options for transporting an assembly through a wave soldering machine. Which of these would you recommend for:
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.
One way of protecting the surface is to remove oxygen from the surrounding atmosphere, and since 1985 much work has been done on finding practical ways of providing a nitrogen environment for wave solder machines.
The advantages of soldering in this inert atmosphere are:
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:
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 (Figure 12). 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:
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.
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.
We have had our current wave solder machine for about six years and it still looks new and operates very well with almost no down-time. The secret is maintenance and operator pride.
I used to do consulting on wave solder machines and often saw them stuffed in little sheds and dirty corners because they are stinky and dirty. We have ours right in front and make it an important part of customer tours.
Another important thing we do is having an annual maintenance done by the factory. This gives a new set of eyes to look at the machine for little things that we may overlook day to day. Also the technician gets to see all kinds of problems with poorly maintained machines in the field and can help you dodge a bullet.
When the factory tech comes to perform maintenance on our machine they are always amazed at the condition of our machine and tell us some horror stories of what they see elsewhere.
Kenny Bloomquist on TechNet, 10 Mar 1999
There is no more to say!
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 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.
Here there are three topics to consider:
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.
The main function of the solder is to make electrical interconnection, but there is a mechanical aspect: even where parts have been clinched or glued in position, the solder also serves to strengthen the joint. As a reminder of what was said in How joints are made, it is generally agreed that:
In wave soldering, the form of the joint is determined by mechanical and process conditions, and is not limited by the amount of solder available, which is essentially infinite. Solder should flow evenly over the surfaces to be soldered and run out thinly towards the edges of the joint, with a contact angle <30°, unless the solder fillet is small and the contact angle constrained by the closeness of the edge of the solder land, as may be the case with small SM components.
For through-hole pins, there is usually also a requirement that the solder fillet to the lead should be visible on the top surface, with the solder having been pulled upwards by capillary attraction. This is not so much for reasons of joint strength or connectivity, but to ensure that there are no defects in the plating (such as cracks) which indicate potential unreliability.
Our list of typical wave solder defects can be divided into four main categories:
Apart from the mechanical problems, many of these defects are related to the quality of the wetting that is achieved, and the way in which the solder flows away from the joint during the peel-back that happens in Zone 3. For wave soldering, vital requirements are:
The following sections discuss two specific problem areas: the first of these concerns solder balling, which is more complex than other problems, depends on a number of factors, and may occur intermittently even in the best regulated processes; the second is a group of design-related issues.
Solder balling occurs both with wave soldering and reflow soldering, and its mechanism can be extremely complex, with the root cause lying in a number of areas. There is also considerable interaction, and factors that do not produce balling on their own may do so in combination. For example, soldering in nitrogen, which changes the surface tension of the molten solder, has often been reported as leading to an increased incidence of very small solder balls.
Figure 13 tries to indicate the most likely causes for solder balling in wave soldering. Note that clues to the origins of the problem can be gleaned from observing the nature and distribution of the problem. For example, solder balling associated with particular components can be a design issue, whereas balls embedded in the solder mask, that leave discoloured marks on the board when removed, indicate solder mask incompatibility.
In the case of wave soldering, a rough surface is preferred, especially with low solids fluxes: trapped flux is able to reduce the surface tension of the solder as it peels away from the board, so rough finishes on solder masks generally display fewer solder ball problems than smooth.
Cases have also been reported of increased solder balling caused by insufficient cure of the solder mask. The effect here is probably related to producing boards with different surface tension characteristics.
Recommended land patterns and other design information can be found in the IPC-SM-782A Surface Mount Design & Land Pattern Standard. This includes pad layouts for all types of passive and active components, with guidelines for both wave and reflow soldering. This IPC standard and its predecessors have formed the basis of design practice throughout the world, being designed to ensure acceptable solder joints under a wide range of conditions. However, it must be said that they sometimes represent a relatively conservative view: as with all standards, you should be prepared to discover differences between in-house practice and the published information.
For through-hole components, the pad configuration is a compromise between several competing requirements. Board density and insulation resistance demand the use of the smallest outside diameter possible, whilst joint strength and reliability are best served if the pad is as large as possible to increase the copper peel strength. Easy soldering, however, requires a pad that is wide enough for repetitive wetting and not too wide for solder to drain away from the lead wire.
The following guidelines should be followed:
For SM components, as in through-hole designs, layout density favours the smallest area, while joint stability indicates the largest pad possible. Manufacturing tolerances, including adhesive application and component placement, also dictate the need for large pads. Pad dimensions for wave soldering of surface mounted assemblies should also ensure as large a contact area as possible, to reduce any problems of lack of consistent wetting. Because of the geometry of the components glued to the surface, there are areas where gaseous material can be trapped, causing skipping or misses on small pads: the larger the pad, the less likely this problem is to occur.
As well as being used to connect to components, contact areas may also be dedicated as test pads. The majority of automatic testing is carried out by placing the board on an array of spring-loaded-probes (bed of nails). These probes make contact with the test points which should preferably be separate pads, away from the component joint in order to prevent component damage.
Two specific problems associated with design for wave soldering are skipped joints and solder bridges. Skipped joints are frequently associated with surface mount assemblies. The potential for their occurrence can be reduced by extending the footprint, or exposing a short length of track not covered by the solder mask. These can help lead the solder to a joint close to a high component body (Figure 14).
Footprints for MELFs and chips should extend far enough to provide an aspect angle of about 60°. This allows for slight misalignment of the component, so that in no circumstances does the angle get steeper than 45°.
The majority of defects on wave-soldered boards are however solder bridges, formed because contact with the solder wave is lost before a sufficient amount of solder has drained from the joints. Often these bridges are linked to particular designs and components, such as some styles of connector behind chip components, and on trailing leads of flat-packs.
With a row of pads, it has proven easier to avoid bridges when they emerge from the wave in single file, rather than all of them together in a broad front. In examining the impact on solder bridging of pad design and of IC orientation, Comerford concluded that “Bridges occurred three to ten times as frequently on ICs oriented perpendicular to the line of travel”. The board should therefore be laid out with all multi-lead packages oriented perpendicular to the wave (Figure 15). With SM components, this is also the optimum orientation to avoid shadowing, where solder fails to reach certain joints because the component body impedes solder flow.
(a) favourable alignment
(b) alignment resulting in formation of bridges
An observation first made with through hole components was that, as a row of leads or footprints leaves the wave in a single file, the peelback seems to jump from lead to lead, until the last two emerge, when a bridge tends to form between them. Placing a somewhat larger dummy footprint, called a ‘solder thief’ or ‘robber pad’ (Figure 16), at the end of the row draws the bridge to a place where it does no harm.
Avoiding bridges is obviously more difficult when components have leads on all four sides! Another way of reducing bridges (it improves drainage by increasing the effective lead spacing by 41%) and of helping solder reach the joints on what would be the shadow side is shown in Figure 17, where components such as PGAs and PLCCs are rotated at 45° to the direction of travel.
Unfortunately, not every design can be tackled in this way, nor is every designer willing to produce a 45° design! Alternatives are
Avoiding bridges is also more difficult with fine pitch packages. One suggested design approach, which is feasible down to 0.8 mm pitch, is ‘long and thin’ – thin to avoid shorting, and long to provide enough solder to wet the pad and wick up the lead:
Since the conveyor is a vital part of the process, the board has to be held properly in the fingers or pallet. To enable this:
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.
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.”
Imagine that one of your colleagues is designing a board for wave soldering. How might you summarise the advice given in this unit to help him/her?
From the perspective of someone who had designed a board that gave problems during wave soldering, what kind of improvements might you expect if you were told that your new assembler’s wave-soldering machine was inerted and had a selective debridging capability?