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Wave Soldering

Author:GLICHN Date:3/25/2012 8:33:37 PM

1 . wave soldering description

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 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).

 

 

 2 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.[citation needed] 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.

 

3 Flux on wave solder machine

1)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.

2)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, cleanliness, or other problems. An air knife, can be used ensure flux is evenly applied to the bottom of the circuit board and remove excess flux. 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.

 

4 Preheating

 

Once fluxed, the PCB enters 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.

 

5 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 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.

 

6 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.

 

7  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

 

8 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.

Lead 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 (4 – 6 seconds)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 63% tin, 37% lead. 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.