Wave soldering

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

A simple wave soldering machine.

There are many types of wave solder machines; however, the basic components and principles of these machines are the same. 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.

Fluxing

Flux in the wave soldering process has a primary and a secondary objective. The primary objective is to clean the components that are to be soldered, principally any oxide layers that may have formed.^[1] 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.^[2]

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.

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.

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.

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 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. (Todd p. 395)

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.^[3]

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. ^[4] 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.

References

↑ http://www.ipctraining.org/dvd/47c/script.pdf
↑ Todd p. 396
↑ Todd, Robert H.; Allen, Dell K.(1994). Manufacturing Processes Reference Guide. New York: Industrial Press Inc.
↑ http://www.ipc.org/TOC/IPC-7530.pdf