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9 Cleaning
1. Requirement of cleaning
2. Cleaning options
2.1 Aqueous cleaning
2.2 Solvent cleaning
2.3 No-clean flux
3. Cleanliness test methods
3.1 Visual examination
3.2 Solvent Extraction
4. Conformal coating
Cleaning of printed boards is required to remove flux residues and other contaminants which are left behind after soldering . Cleaning or washing of the boards prevent potential electrical failures due to electromigration. Cleaning operations does the removal of the following contaminants:
i) Ionic contaminants
ii) Non-ionic contaminants
iii) Particulate contaminants
Fluxes that are water soluble generally produce ionic(also called polar) contaminants that require aqueous cleaning. Non-ionic(also called non-polar) contaminants produced by rosin fluxes require non-ionic solvents such as trichloroethane.
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Note: Rosin and Rosin Mildly Activated (RMA) fluxes need not be cleaned. However, for high reliability applications and for aesthetic reasons, it may be necessary to clean these boards.
Water soluble fluxes need to be cleaned thoroughly due to corrosive elements that are present in the flux residues, aqueous cleaning is ideal. Because rosin is not soluble in water, when aqueous cleaning is used for rosin fluxes, alkaline chemicals called saponifiers are added to the water. The efficiency of cleaning treatment is significantly increased if the cleaning is assisted by ultrasonic vibration. However, the ultrasonic vibrations are not confined to the cleaning fluid and the surfaces to be cleaned, but are also transferred into the electronic components, which may be damaged . Bonding wires, inside active components between the die and the bonding pads may break if the bonding wires are free and have not been tightly encapsulated in plastic or any other packaging material. High-power, high frequency vibration induce fracture in external leads also. The following set of parameters is generally accepted:
- maximum frequency of 40 kHz
- maximum time of ultrasonic load 1 to 5 minutes
- maximum power of 10 W/ litre
- boards in racks, so that they cannot touch each other.
In all cases, the time interval between soldering and cleaning be reduced to a minimum (less than an hour) to obtain good cleaning results.
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