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What is Wave Soldering Machine?


Wave Soldering Machine - for through hole PCB assembly

One of the techniques that has been used in PCB assembly is wave soldering as it provides swift soldering for boards using either or both SMT and DIP devices.

Wave soldering is a bulk soldering process used in the manufacture of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the circuit board makes contact with this wave, the components become soldered to the board. Wave soldering machine is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued onto the surface of a printed circuit board (PCB) by placement equipment, before being run through the molten solder wave. Wave soldering is mainly used in soldering of through hole components.


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 surface-mount technology (SMT) is not suitable (e.g., large power devices and high pin count connectors), or where simple through-hole technology prevails 

Wave soldering Machine used for large scale PCB assembly for fast soldering of boards that use both SMDs and DIP components.

Wave soldering machine is far less widely used in PCB assembly than it was in previous years. Despite this, wave soldering machine can be used effectively in a number of areas, especially when the PCB assembly uses a mix of DIP and SMT components.

What is wave soldering Machine

The wave soldering process gains its name from the fact that the process passes the printed circuit boards to be soldered over a wave of solder, Melting the Solder bar in the solder pot, use a pump send the melted solder in a certain height, when PCB pass this solder wave, the solder pass the surface of the PCB, solder on soldering area sticked to the component pins and solder pad, other area is anti solder stick part, solder will go away by it's weight reason. after temperature cooling down, the component pins will soldered to the solder pad.

In this way a complete board can be soldered in a matter of seconds producing joints that are reliable mechanically and electrically. Apart from being much faster than manual soldering, wave soldering is also produces joints with a much higher degree of reliability and this makes it ideal for large scale PCB assembly.

Wave soldering Machine can be used in PCB assembly for both conventional through hole mounted components as well as surface mount components. However other methods such as infra-red reflow soldering are more applicable to the fine features being used today on printed circuit boards for surface mount components. As a result wave soldering is less widely used for PCB assembly than it was many years ago.

Wave soldering machine

The wave soldering machine consists of a heated tank of solder. This is maintained at the required temperature for the soldering process. Within the tank, a wave of solder is set up and the printed circuit boards are passed over this so that the underside of the board just contacts the solder wave. Care must be taken in adjusting the height of the wave so that it does not flow over the top side of the board where it would cause solder to enter places where it is not required.

The boards are held firmly in place on a conveyor using metal fingers. These are typically made of titanium because it is able to withstand the temperatures and it is not affected by the solder.

Preparation

In order that a electronics printed circuit board may be successfully processing using a wave soldering machine, it is necessary that it is designed and manufactured in the correct manner.

· Solder resist layer:   The first is standard practice when designing boards these days. A solder resist or solder mask layer is included in the PCB design, and this adds a layer of "varnish" like material to the board to which the solder will not adhere. Only those areas where the solder is required are left exposed. This solder resist is most commonly green on colour.

· Pad spacing:   The second main precaution is to ensure that there is sufficient spacing between the solder pads requiring soldering. If they are too close then there is the possibility that the solder may bridge the two pads causing a short circuit.
In view of the way that wave soldering works, where the solder wave is caused by the solder flowing out of the reservoir tank, and the board passes over it, the spacing requirements are dependent upon the direction of the board relative to the solder flow. Pads that are spaced apart in the direction of the solder flow should have a greater spacing than those that are spaced at right angles to the solder flow. This is because it is much easier for solder bridges to occur in the direction of the solder flow.

Fluxing

To ensure that the areas to be soldered are clean and free from oxidation, etc, flux is required. Flux is applied to the side of the board to be soldered, i.e. the underside. Careful control of the quantities of flux are needed. Too little flux and there is a high risk of poor joints, and too much flux and there will be residual flux on the board. While this does not look good cosmetically, there is also the risk of long term degradation because of the acidic nature of the flux.. There are two main methods of applying the flux::

· Spray flux;   A fine mist of flux is sprayed onto the underside of the board that is to be soldered. Some systems may even use a compressed air jet to remove the excess flux.

· Foam flux;   The electronic printed circuit board is passed over a cascading head of flux foam. This is generated using a tank of flux into which a plastic cylinder with tiny holes is immersed. The plastic cylinder is covered with a metal chimney and air is forced through the cylinder. This causes flux foam to rise up the chimney.

Preheat

The wave soldering process exposes the electronic printed circuit boards to considerable levels of heat, far greater than those it would be subjected to if it were to be manually soldered. This thermal shock would give rise to a considerably increased level of failure if it were not addressed. To overcome this the board is preheated so that it can be steadily brought up to the required temperature steadily so that any thermal shock is minimised.

The preheating area normally uses hot air heaters that blow hot air onto the boards as they pass towards the wave soldering machine. On some occasions, particularly if the board is densely populated, infra-red heaters may be used as well. This ensures that all the board is evenly heated and no shadow areas are present.

While the pre-heating is required to prevent the thermal shock that the wave soldering machine would generate, the heating is also necessary to activate the flux. This flux is required to ensure that the areas to be soldered are clean and will take the solder.

Cleaning
Some types of flux, called "no-clean" fluxes, do not require cleaning; their residues are benign after the soldering process. Typically no-clean fluxes are especially sensitive to process conditions, which may make them undesirable in some applications. Other kinds of flux, however, require a cleaning stage, in which the PCB is washed with solvents and/or deionized water to remove flux residue.
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

Solder types
Different combinations of tin, lead and other metals are used to create solder. The combinations used depend on the desired properties. The most popular combinations are SAC (Tin(Sn)/Silver(Ag)/Copper(Cu)) alloys and Sn63Pb37 (Sn63A) which is 63% tin, 37% lead. The latter combination is strong, has a low melting range, and melts and sets quickly. Higher tin compositions give the solder higher corrosion resistances, but raise 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. Environmental and performance requirements also factor into alloy selection. Common restrictions include restrictions on lead (Pb) when RoHS compliance is required and restrictions on pure tin (Sn) when long term reliability is a concern.
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.
Thermal profiling
Thermal profiling is the act of measuring several points on a circuit board to determine the thermal excursion it takes through the soldering process. In the electronics manufacturing industry, SPC (Statistical Process Control) helps determine if the process is in control, measured against the reflow parameters defined by the soldering technologies and component requirements.[10] Products like the Solderstar WaveShuttle and the Optiminer have been developed special fixtures which are passed through the process and can measure the temperature profile, along with contact times, wave parallelism and wave heights. These fixture combined with analysis software allows the production engineer to establish and then control the wave solder process.
Solder Wave Height
The height of the solder wave is a key parameter that needs to be evaluated when setting up the wave solder process. The contact time between the solder wave and assembly being soldered is typically set to between 2 and 4 seconds. This contact time is controlled by two parameters on the machine, conveyor speed and wave height, changes to either of these parameters will result in a change in contact time. The wave height is typically controlled by increasing or decreasing the pump speed on the machine. Changes can be evaluated and checked using a tempered glass plate, if more detailed recording are required fixture are available which digitally record the contact times, height and speed.

Applications of wave soldering in PCB assembly

Wave soldering Machine is not as widely used for PCB assembly as it was at one time. It is not suited to the very fine pitches required by many of the boards in manufacture today. However it is ideal for the many boards still manufactured with conventional leaded components and some surface mount boards that use larger components. These boards are often those which are used in lower volume and possibly more specialised products.


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