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Wave soldering
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Wave soldering is a large-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The name is derived from the use of waves of molten solder to attach metal components to the PCB. The process uses a tank to hold a quantity of molten solder; the components are inserted into or placed on the PCB and the loaded PCB is passed across a pumped wave or waterfall of solder.
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Encyclopedia
Wave soldering is a large-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The name is derived from the use of waves of molten solder to attach metal components to the PCB. The process uses a tank to hold a quantity of molten solder; the components are inserted into or placed on the PCB and the loaded PCB is passed across a pumped wave or waterfall of solder. The solder wets the exposed metallic areas of the board (those not protected with solder mask, a protective coating that prevents the solder from bridging between connections), creating a reliable mechanical and electrical connection. The process is much faster and can create a higher quality product than manual soldering of components.
Wave soldering is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued by the placement equipment onto the printed circuit board surface before being run through the molten solder wave.
As through-hole components have been largely replaced by surface mount components, wave soldering has been supplanted by reflow soldering methods in many large-scale electronics applications. However, there is still significant wave soldering where SMT is not suitable (e.g. large power devices and high pin count connectors), or where simple through-hole technology prevails (certain major appliances).
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 preheating zone, the fluxing zone, and the soldering zone. An additional fourth zone, cleaning, is used depending on the type of flux applied. The basic equipment used during the process is a conveyor that moves the PCB through the different zones, a pan of solder used in the soldering process, a pump that produces the actual wave, the sprayer for the flux and the preheating pad. The solder is usually a mixture of metals. A typical solder has the chemical makeup of 50% tin, 49.5% lead, and 0.5% antimony. There are three types of waves: normal wave, a medium speed, long leads used for horizontal soldering; cascade wave, high speed, short leads, used for inclined soldering; and flat wave with extenders; medium to high speeds, long leads that is used for horizontal soldering.
Process Characteristics
The following are characteristics of the wave soldering process:
- The solder connection is very reliable and also a clean connection
- The process is automated
- The process reuses the flux and solder that is left over
- It does require inspection, some touch ups, and also testing
- The productivity and efficiency is increased.
Preheating
The PCB will then enter the preheating zone. The preheating zone consists of convection heaters which blow hot air onto the PCB to 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, and to remove any flux carrier solvents. 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.
Fluxing
When the PCB enters the fluxing zone, a fluxer applies flux to the underside of the board. Two types of fluxers are used: the spray fluxer and foam fluxer.
Spray fluxer
Some spray fluxers consist of a robotic arm which travels from side to side while spraying a fine mist of flux onto the bottom side of the board. Other spray fluxers consist of a stationary bar with a series of nozzles that spray a fine mist. There are also additional ones that can consist of a single stationary ultrasonic head and/or an oscillating ultrasonic head. 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 flux into which a plastic cylinder with tiny holes is immersed; this is sometimes called a "stone". The plastic cylinder is covered with a metal chimney. Air is passed through this cylinder which causes flux foam to rise up the chimney, forming a cascading head of foam. 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. Also the types of flux affect the end result. There are two types of flux, corrosive and noncorrosive. Noncorrosive flux requires precleaning and is used when low acidity is required. Corrosive flux is quick and requires little precleaning, but has a higher acidity. (Todd p.396
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 ensure solder is applied to all areas but does not splash to the top of the board or other undesired areas. This process is sometimes performed in an inert gas nitrogen (N2) atmosphere to increase the quality of the joints. The presence of N2 also reduces oxidization known as solder dross.
Solder dross, its reduction and elimination, is a growing industry concern as lead soldering is being replaced by lead-free alternatives at significantly higher cost. Dross eliminators are entering the market and may hold some solutions for this concern.
Finish and Quality
Quality depends on proper temperatures when heating and on properly treated surfaces.
| Defect | Possible causes | Effects |
|---|
| Cracks | Mechanical Stress | Loss of Conductivity | | Cavities | Contaminated surface
Lack of flux
Insufficient preheating | Reduction in strength
Poor conductivity | | Wrong solder thickness | Wrong solder temperature
Wrong conveyor speed | Susceptible to stress
Too thin for current load
Undesired bridging between paths | | Poor Conductor | Contaminated solder | Product Failures |
Cleaning
Some types of flux, called "no-clean" fluxes, do not require cleaning; their residues are benign after the soldering process. Others, however, require a cleaning stage, in which the PCB is washed with solvents and/or deionized water to remove flux residue.
Geometrical Possibilities
The typical range of the wire diameter used in wave soldering is about 0.01 in, but the feasible range is from 0.001 in to 0.1 in. THe PC board is usually anywhere from about 2 in to 10 in. But it also has a much larger feasible range of about 0.5 in to 600 in.
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 such as: bridging- if the wave is too high or too low then the board will not make proper contact with the wave and it can lead to undesirable bridging, causing short circuits. Ventilation does not affect the boards' quality, but if the ventilation in the machine is poor, the fumes from the flux as it goes through the pre-heat can lead to a fire, in which case a foam or powder extinguisher should be used. Due to the extreme temperatures of the wave when it comes into contact with the water, the water flashes off so rapidly it can cause splashes or solder jumping into the air with heights up to 1 meter or even more. Smearing- another flux related problem, is caused by not enough flux being applied during the fluxing stage. Solder sticks to the board and causes streaking. Quality insurance often includes a resistance test, to make sure no flux or other deposits cause 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, with little human interaction needed to monitor and adjust the equipment.
Some defects may occur during the soldering process. Cracks occur due to mechanical stress and can cause loss of conductivity. Cavities, also known as porosity, is caused from either contaminated surfaces, lack of flux, or insufficient heating. Decrease in strength and conductivity is the result. If the wrong solder thickness is produced, the product may be susceptible to stress, or too thin for the load placed upon it. This can be caused by the wrong temperature being used or wrong conveyor speed.
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 sometimes placed on a fixture. The fixture is a fiberglass composite or titanium frame with openings exposing the components to be soldered. Wave solder fixtures are required for PCBs where bottom side components cannot be exposed to the solder, or the board is either too small or has a non rectangular shape and cannot be 'picked up' by the titanium fingers. The fixtures can also be used to incorporate clamping and alignment devices designed to hold components in place and prevent 'lifting' of the components as the leads hit the wave, whilst ensuring correct vertical alignment. In some cases fixtures are also used to reduce changeover time by having fixtures of the same width for different assemblies. This way no changes are needed for the conveyor of the wave solder.
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 adhere to this metal.
The introduction of RoHS and the subsequent development of lead free solder (which has a much higher melting point than conventional solder), has caused problems for the manufacturers of material used in fixtures. The higher temperatures causes resin burn out which, in turn, compromises the structural integrity of the fixture. This has created a significant challenge to the manufacturers of both the material and fixtures.
Solder types
Although there are many different types of solder that can be used in wave soldering, tin/lead-based solders are the most common. However, these types of solder are quickly being replaced by lead-free solder. Tin/lead-based solder is highly toxic, and government regulations regarding all lead products are becoming increasingly strict. Workers dealing with tin/lead-based solder can be contaminated at work or carry it home on their clothes, where it can cause contamination. Lead-free solder provides a solution to this problem, but it also has its own set of problems that must be taken into consideration.
Solder adheres best to copper. The next best metals to use are nickel, brass, aluminum, tungsten, and lastly steel.
Lead-free solder problems
In addition to higher material costs, lead-free solder has a higher melting point, slower flow rates, and can sometimes cause leeching in the iron components. Higher melting points and decreased flow rates require a longer contact period between the solder and the connections in order for the solder to completely fill the holes. Higher melting points also require a melting pot made of more stable material. The melting pot and ducts also need to be made from material that will prevent dissolution. Replacing those parts in a typical wave soldering machine can cost up to $25,000.
Common solder compositions
Different combinations of tin, lead and other metals are used to create solder. The combinations used depend on the desired properties. The most popular combination is 60% tin 39%, lead, and 1% alloys. This combination is strong, has a low melting range, and melts and sets quickly. Higher tin compositions gives the solder higher corrosion resistances, but raises the melting point. Another common composition is 11% tin, 37% lead, 42% bismuth, and 10% cadmium. This combination has a low melting point and is useful for soldering components that are sensitive to heat. (Todd p. 395)
Effects of wave soldering
Defects
Cracks in the solder can occur and are usually caused by stress on the solder and can cause a loss of conductivity. Cavities, pores in the solder, can be caused by contaminated surfaces, insufficient flux, and insufficient preheating. Cavities can reduce strength and conductivity. Improper solder temperature or conveyor speed can cause an undesired thickness. If the solder is too thin, then it is susceptible to stress and may not support the required load. If it is too thick, then unwanted shorts and bridging can occur. Poor conduction in the solder can be caused by contaminated solder and can cause the product to be dysfunctional.
Excess heat
Excess heat can cause the circuit board to delaminate, become brittle, and become warped. Excess heat can also damage components that are sensitive to heat.
Effects of cooling rate
It is important that the PCBs be allowed to cool at a reasonable rate. If they are cooled too fast, then the PCB can become warped and the solder can be compromised. On the other hand if the PCB is allowed to cool too slowly, then the PCB can become brittle and some components may be damaged by heat. The PCB should be cooled by either a fine water spray or air cooled to decrease the amount of damage to the board.
Feed rate and production time calculation
The conveyors can feed PCBs at a rate of 120 in./min to 180 in./min. The higher the melting point of the soldering material the faster the feed rate. Each board takes about 3 seconds to be soldered. The production time can be calculated by this formula.
Total Time = (L/V x N) + Setup Time
V = Conveyor velocity (ft/min)
L = Conveyor length (ft)
N = Production size
Production costs and safety precautions
Things that should be taken into consideration when estimating cost of production are:
- Initial heatup time
- Load/unload time
- Soldering time
- Cooling time
- Conductor and flux costs
- Direct labor rates
- Overhead rates
- Amortization of equipment and tooling costs
Some of the risks that one should be aware of is that the equipment will get hot and can cause serious burns. Also corrosive chemicals and fumes can be dangerous to workers and the environment.
Thermal profiling
In the electronics manufacturing industry, a statistical measure, known as the Process Window Index (PWI) is used to quantify the robustness of a thermal process. PWI helps measure how well a process "fits" into a user-defined process limit known as the Specification Limit.
Each thermal profile is ranked on how it "fits" in a process window (the specification or tolerance limit). The center of the process window is defined as zero, and the extreme edge of the process window as 99%. A PWI greater than or equal to 100% indicates that the profile does not process the product within specification. A PWI of 99% indicates that the profile processes the product within specification, but runs at the edge of the process window. A PWI of 60% indicates a profile uses 60% of the process specification. By using PWI values, manufacturers can determine how much of the process window a particular thermal profile uses. A lower PWI value indicates a more robust profile.
For maximum efficiency, separate PWI values are computed for peak, slope, reflow, and soak processes of a thermal profile. To avoid the possibility of thermal shock affecting the output, the steepest slope in the thermal profile must be determined and leveled. Manufacturers use custom-built software to accurately determine and decrease the steepness of the slope. In addition, the software also automatically recalibrates the PWI values for the peak, slope, reflow, and soak processes. By setting PWI values, engineers can ensure that the reflow soldering work does not overheat or cool too quickly.
Bibliography
- Seeling, Karl (1995). A study of lead-free alloys. AIM, 1, Retrieved April 18, 2008, from http://www.aimsolder.com/techarticles/A%20Study%20of%20Lead-Free%20Solder%20Alloys.pdf
- Biocca, Peter (2005, April 5). Lead-free wave soldering. Retrieved April 18, 2008, from EMSnow Web site: http://www.emsnow.com/npps/story.cfm?ID=10669
- Todd, Robert H.; Allen, Dell K.(1994). Manufacturing Processes Reference Guide. New York: Industrial Press Inc.
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