Korntal, Germany T he ever-increasing demand for reliability, continuing miniaturization, and the growing number of faults in electronic components manufactured in no-clean pro- cesses all combine to put the focus back on cleaning in
electronics manufacturing. The industry offers a variety of solu-
tions to finding the optimal cleaning process.
The development of no-clean fluxes and soldering pastes
has done much to turn attention away from the need to clean
components in electronics manufacturing. For many components that are only used in non-critical atmospheric environments, this mostly poses no problem. However, where they are
utilized in adverse environments (such as humid or fluctuating
temperatures), the protective layer applied in the no-clean
process can be gradually eroded, releasing ionizing substances
that promote electro-migration and dendritic growth. This
occurs chiefly in narrow spaces beneath components and
between their connections or other contact surfaces.
Increased requirement for surface cleanliness
In addition, protective coatings (conformal coatings), progressive miniaturization, wire bonding, and the increased use high
voltage components all call for a high level of surface cleanli-
Cleaning with water-based media is generally done in dipping systems with multiple cleaning and rinsing baths. (Photo: Amsonic)
ness. A further aspect is the use of lead-free solders, containing
a higher proportion of fluxes and more aggressive activators
that can cause problems. Cleaning of electronic components
also involves removing potentially hazardous impurities such
as fluxes, residues of soldering agents and adhesives, and such
contaminants as dust and residue from previous manufacturing stages.
Choosing the right cleansing agent
A key factor in achieving economy and efficiency in the cleaning
process is the selection of a suitable cleansing agent. Selection
criteria include the nature and quantity of the impurities to be
removed, and the subject material. Cleaning agents currently
used in electronics manufacturing include solvents, water-based media containing alkaline surfactants, and water-based
tenside-free cleaning agents.
The electronics industry mainly uses solvents containing
non-halogenated hydrocarbons, modified alcohols, or hydro-fluorethers (HFEs). HFEs were developed as an alternative to
the previously preferred chlorofluorocarbons (CFCs), after
CFC manufacture was stopped about 20 years ago due to their
high potential for breaking down ozone. Non-inflammable
HFEs have similar properties to CFCs, but pose no danger
to ozone, do not persist in the atmosphere, and have low
greenhouse gas potential. At the same time, they offer physical
properties that are in demand for the cleaning of electronics,
such as relatively high density, low viscosity, and low surface
tension. These solvents are used in monosolvent, cosolvent,
and bisolvent systems.
A monosolvent system usually uses a pure HFE or an
azeotrope—a mixture of two or more components that vaporizes without changing its chemical composition. It is used
to remove slight impurities such as light oils, halogen compounds, residue of easy clean solvent, particles, and dust.
The cosolvent system consists of an HFE combined with
a low-volatility organic solvent as a solubility promoter. The
solubility promoter removes impurities from the surface of
the workpiece and the HFE rinses away the solvent and the
impurities from the components. Cleaning with a cosolvent
procedure is extremely versatile and also gives good results
with the most stubborn impurities such as heavy oils, grease,
waxes, NC-flux residues, adhesives, and hot-melt glues. Choice
of a low-volatility organic solvent allows material compatibility to be tested.
Cosolvent and bisolvent systems differ mainly in that, for the
cosolvent system, the solvent and the rinsing agent are mixed
together, while in the bisolvent process they are kept separate.