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Aqueous cleaning is used for cleaning of ionic contaminants left by use
of organic acid fluxes. The cleaning process can be done by either of the
following methods.
1. Batch aqueous cleaning system
2. In-line aqueous cleaning system
Batch aqueous cleaning is good for low volume production. A batch aqueous
system is very much like a house-hold dishwasher, the assemblies are loaded
vertically like dishes, detergents are added, and cleaning and drying are
accomplished in different timer-controlled cycles.
In-line systems are very much like batch systems except that the board
passes different modules rather
than different cleaning cycles as in batch systems. In-line systems are used for
high volume production. A typical
sequence for in-line cleaning system is given below:
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The prerinse module is used
for the removal of gross contaminants. It is kept separate from the wash stage
to prevent contamination. The wash stage needs saponifiers for rosin fluxes.
Water is heated to 60 to 70 0 C. Generally additives are not required
for Organic acid fluxes. There may be additional recirculating wash stages if
detergents are used, or if higher cleanliness is necessary. In recirculating
stages, water is directed towards the conveyor carrying the cards at pressures
ranging from 10 psi to 30 psi.
The assembly goes through the final rinse to remove primarily the
contaminated water that got accumulated during recirculating wash stage.
The clean water is blown away with air knives and dried. It may be noted
that the boards must be completely dry before it comes out of the cleaner to
prevent dust accumulation.
Hydrocarbon solvent or water with saponifiers is needed for cleaning of
boards with rosin fluxes. Water is not a natural solvent for rosin, therefore,
when aqueous cleaning is used for rosin fluxes, alkaline chemicals called
saponifiers added to the water. The cleaning solvents for rosin-containing
fluxes that are typically used are chlorinated and fluorinated hydrocarbons,
such as 1.1.1 trichloro-ethane or trichlorotrifluro-ethane mixed with ethanol,
isopropanol and methylene chloride (called azeotrope mixtures). Because halogenated hydrocarbons are found to be harmful to
the environment, the use is being restricted. Given below are typical process
steps involved in solvent cleaning of rosin based flux residues.
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In the solvent cleaning process the boards are placed in a basket with
their solder side facing downwards and the basket is placed in the vapour phase
for about a minute. During this phase, the vapour condensates on the boards and
dissolves the contaminants. The basket is then transferred to second stage
(rinse) and immersed in the solvent. Dwell time is 1 to 2 minutes. The rinse
stage is sometimes given ultrasonic vibrations for better results. In some
cases, the solvent is forced onto the boards
through nozzles. The above
process can be repeated until desired cleanliness is achieved. The process is
operator dependent because the entire process may have to be repeated a number
of times for desired cleanliness.
The cards can be dried with compressed air or be allowed to dry by
natural means. It is important that the cleaning operation is performed soon
after the soldering process. With longer time lapse, the rosin becomes hard and
it would be very difficult to remove the residues. A maximum time limit of one
hour may be allowed if required.
Adopting a no-clean soldering
process is preferred for not only due to the rapid phase out of cleaning with
CFCs but also because no-clean can be very cost-effective process. No-clean
fluxes contain less than 5% of solids after soldering process is completed which
is negligible for consumer and industrial electronics products. However, if
active fluxes are to be used due to any reason, then the option of using
no-clean flux does not arise.
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