D2. water washing - There are a number of waterbased cleaning methods that may be used individually or in sequence. Which one is used depends on the nature of the soils to be removed, the application to which the circuit board is to be put and the tolerance of that application for small amounts of contaminants, the board design, and the environment it will face. All the water-washing methods have the advantage of avoiding the environmental problems and costs involved in the use of chlorinated solvents. Waste water is economical to process and dispose of. Water without additives is sometimes used as a cleaning agent when the only contaminations to be removed are soluble in water. Organic fluxes and water-soluble oils are two examples. Sometimes, boards are repeatedly flushed with the same water, with the last flush of any particular board being made with the freshest water, and the first flush being made with used water. Thus, repeated flushes are made with successively cleaner, fresher water. This system leaves the board cleanest at the end of the sequence and is referred to as countercurrent rinsing . Fig. 13D2 illustrates countercurrent rinsing schematically. The water may be heated, since this aids in the dissolution of materials to be removed. Sometimes water with neutralizing agents is used. These agents react with flux acid and other acidic dirts, neutralizing them and making them easier to remove. Neutralizing agents also aid in the removal of metallic salts. Water with surfactants (detergents) is effective in removing non-polar contaminants, including oils, waxes, rosins and grease. Water with saponifiers (alkaline materials that react with rosin and oils to produce a soap-like material that is washable) are also effective in removing rosin and nonpolar contaminants. Sometime a foam suppressor is added to the solution. Water rinses, often those that are multi-stage and countercurrent, follow cleaning steps that use additives with the water.  

The water and water-augmented liquid used in these approaches is sprayed, flushed, and agitated, usually at a temperature in the range 120 to 180 ° F (49 to 82 ° C) 4 . Pressure spraying can loosen soils that are in protected spaces. Spraying at an angle of 45 to 60 degrees has been found to be more effective than vertical spraying. Ultrasonic cavitation (See 8A2b.) can be utilized when the board is immersed, to help loosen adhering soils (though ultrasonics are used less with fine pitch assemblies where there is concern that the cavitation could damage fine-wire leads). Water washing can be a batch or continuous process, depending on the production volume. Batch washers resemble kitchen dishwashers in their arrangement and operation. 2 For higher-level production, these operations can be performed on a conveyorized, continuous basis. Air knives are often used to remove water from the boards after the washing cycle and sometimes also as an in-process step to more thoroughly remove soils from the board when the wash water contains soils. Radiant heat in a final station may be used to enhance the drying of the rinse water. A simple water-washing operation sequence is shown schematically in Fig. 13D2-1.  

Fig. 13D2 Countercurrent rinsing shown schematically. Circuit boards on the conveyor are subjected to four rinses. The first rinse, with the board at its dirtiest, is with water that has been through three rinse cycles. The second rinse is with water that has been through two rinse cycles. The last rinse, with the board at its cleanest, is with the cleanest rinse water.

Fig. 13D2-1 Typical water washing operation sequences for circuit boards after soldering: Boards with water soluble fluxes are given a pre-wash which may be with a neutralized water or the least fresh water from the first countercurrent rinse station. The wash water may contain surfactants and/or saponifiers. Rinsing is often done in several stages with a countercurrent sequence. The air knife removes rinse water retained on the boards and the last station provides heating to dry the boards. Any or all of the wash or rinse stations may use heated water.

D3. semi-aqueous cleaning - uses solvents that do not have the disadvantage of a significant ozone-depletion property, or other serious environmental drawbacks. However, they are flammable. The most common solvents used are terpene (commonly extracted from orange peel) or an alcohol. Terpene has a flash point of 160 ° F (70 ° C). Explosion-proof equipment with fire prevention properties is used. The semi-aqueous cleaning process usually has two stages. The first stage involves washing the workpiece with the organic solvent (terpene or alcohol), to remove soils that are solvent-soluble, including rosin flux and non-polar materials. An inert atmosphere may be used during this step for fire prevention. The second stage is a water wash that removes traces of the solvent. A surfactant is added to the water. This stage also removes any water-soluble soils that were not removed by the organic solvent. A nitrogen knife, like an air knife, may be used after the solvent wash, and an air knife after the water wash. If a nitrogen atmosphere is not used in the solvent wash stage, the flushing with the solvent is done by immersion or another method that avoids creating a solvent mist that, in air, could be explosive. Terpene drained from the solvent-wash stage and that recovered from the wash water is treated and reused. The solvent is separated from the water by differences in specific gravity. The cleaning system may also have a final water rinsing stage followed by a drying stage. Fig. 13D3 schematically illustrates a semiaqueous cleaning system for circuit boards.  

Fig. 13D3 Semi-aqueous cleaning of flux from printed circuit boards using terpenesolvent.