of fluorocarbon solvents
Rosin flux residues require cleaning, especially when activated fluxes have
been used, and cleaning was very commonly carried out, especially throughout
Europe and the USA, driven by a perception held by military customers that
all flux residues are potentially harmful. The original approach used a variety
of solvents but these gave problems:
- Organic solvents often have low flash points and present fire hazards
- Chlorinated solvents can be too aggressive.
Some solvents present health hazards and/or have low TLV limits.
Fluorocarbons had been developed during the 1930s as refrigerants, and in
the late 1960s were introduced for the cleaning of electronic assemblies.
The particular compound most commonly used was 'CFC-113', 1,1,2-trichloro-1,2,2-trifluoroethane1.
- is inherently non-toxic (TLV 1,000 ppm)
- is non-flammable
- can be mixed with other solvents (such as isopropanol) to give a range
of cleaning properties – even with a small percentage of additive,
cleaning is effective
- allows cleaning to be carried out at low temperature (typically less than
- when formulated with other suitable solvents, can be used for cleaning
by immersion, by vapour phase, or by hand after repair or rework
For these reasons, fluorocarbons rapidly became the preferred means of cleaning,
despite their expense. The solvents were marketed under a variety of trade-names,
of which Arklone and Freon were the best-known in the UK.
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Following scientific evidence of the damaging effect on the stratospheric
ozone layer, which protects us from the harmful effect of the sun's radiation,
over 100 countries agreed, in the Montreal Protocol of September 1987 (amended
in London, June 1990 and at Copenhagen, November 1992), to phase out man-made
ozone-depleting chemicals as quickly as possible. Not only was CFC-113 affected,
but another of the common chlorinated solvents (1,1,1-trichloroethane, or
methyl chloroform, and sold as Genklene or Chlorothene) was also covered by
the resulting European-wide regulations, where phase-out was scheduled by
the mid-1990s. The Protocol and its associated EC regulations imposed control
on the supply to the market of the substances rather than their use as such,
but the effect is the same, to force electronic companies to seek different
ways of approaching the problem, and these materials have now essentially
The Ozone Depletion Potential (ODP) is the ratio of the impact on ozone of
a chemical compared to the impact of a similar mass of CFC-11, and indicates
the relative ability of substances to damage the Earth's ozone layer. There
are two groups of chemicals, classified according to ODP:
- Class I substances2 have an ozone-depletion
potential of 0.2 or higher. They include chlorofluorocarbons (CFCs), halons,
carbon tetrachloride, and methyl chloroform:
- CFCs were commonly used as refrigerants, solvents, and foam blowing
- Halons were used as fire extinguishing agents, both in built-in systems
and in handheld portable fire extinguishers.
- Carbon tetrachloride was widely used as a raw material in many industrial
uses, including the production of CFCs, and as a solvent. Solvent use
ended when it was discovered to be carcinogenic.
- Methyl chloroform was used as an industrial solvent.
- Class II substances have an ozone-depletion potential
of less than 0.2.
- Hydrochlorofluorocarbons (HCFCs) contain chlorine and thus deplete
stratospheric ozone, but to a much lesser extent than CFCs, with ODP
values ranging from 0.01 to 0.1.
- Hydrofluorocarbons (HFCs) contain only hydrogen, fluorine, and carbon,
and do not deplete the ozone layer. However, some HFCs have high Global
Warming Potential (see later)
Class I materials, those with significant ODP, have already been phased out.
HCFCs and other halogenated materials with some ODP are subject to increasing
restrictions and will also be phased out, those with the highest ODP values
first. For example, the commonly used HCFC-141b ceased production on 1 January
If you are interested in researching this in more detail, a good starting
point in the US Environmental Protection Agency website at http://www.epa.gov/docs/ozone/.
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The list of environmental issues involved with cleaning materials is in fact
much more extensive than mere ozone-depletion, although it is that aspect
which has been nearest the headlines. A fuller list includes:
- Global Warming Potential (GWP): Carbon dioxide induces
atmospheric warming as a result of the 'greenhouse effect', by absorbing
the infrared radiation emitted from the Earth's surface. How much materials
contribute depends on both the emission level and their 'GWP', the ratio
of the warming caused by a substance to the warming caused by a similar
mass of carbon dioxide. The seriousness also depends on the length of their
persistence in the upper atmosphere.
All solvents3 have substantially greater global warming impact
per molecule than does carbon dioxide. HCFCs and HFCs have GWP values ranging
from 93 to 12,100, the value being generally lower than with CFCs (CFC-12
has a GWP of 8,500; CFC-11 has a GWP of 5,000). Their GWP also degrades
more quickly with time, whereas CFCs are effective for over 500 years. Water,
a substitute in numerous end-uses, has a GWP of 0.
- Photochemical Ozone Creation Potential (POCP): Many solvents
are able, in the presence of light, to assist in the formation of ozone
at ground level. This 'photo-chemical smog' is a damaging pollutant at high
concentrations, which can adversely affect human health, plant growth and
building materials. Whilst there are no agreed regulatory values, many of
the alcohols and substitutes have significant POCP.
- Ground water contamination: Many industrial chemicals
are not degraded on disposal, and survive land-fill operations to pollute
the water table. The philosophy now embodied in legislation is, in essence,
that the person who creates the problem has to clear it up! This has led
to the emergence of a whole new industry of licensed disposal operations,
with the effect that the cost of disposing of all kinds of waste has increased.
- Waste water: This has a number of problem areas, including
its acidity or alkalinity (pH), the heavy metal content (lead, copper, nickel,
tin) and biodegradability. The significance is that waste water needs to
be both neutralised and treated before discharging to remove heavy metals,
and be assessed in terms of the impact it makes on the dissolved oxygen
in water within the environment - an essential for aquatic life.
The Environmental Protection Act 1990 controls emission from large cleaning
operations (the trigger levels for which are constantly reducing) and, through
Water Authority consents, is also affecting the ability of a company to discharge
even treated effluent. Legislation in the EPA and elsewhere, regulates how
many materials, including spent solvents, may be carried, transferred, stored,
treated and disposed of. These regulations are likely to get tighter and involve
higher costs in the future.
Probably the most unfriendly material from the GWP
perspective is sulphur hexafluoride. Used as a cover gas in magnesium
production and casting, as a dielectric gas and insulator in electric
power equipment, as a fire suppression discharge agent in military systems,
and formerly as an aerosol propellant, this has a GWP of 22,200 and
a life of 3,200 years!
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Hazards in cleaning
Almost all cleaning options have some potential to harm operators if not
properly controlled to minimise risk. Precautions taken have to be reasonably
practical but also to take into account technical, cost and environmental
issues. Three aspects of Health and Safety hazard are generally considered:
- Toxicity - the main risk is inhalation of vapours, and
the more volatile the solvent, and/or the higher the temperature, the higher
the risk. Exposure must be kept within operational exposure limits. Some
solvents are suspected of being carcinogens, for which no safe level of
exposure is allowed.
- Skin and eye hazards - solvents designed to dissolve
oil, grease and dirt will probably be very good at removing natural oils
and greases from the skin, and most cleaning chemicals have the potential
to cause skin irritation, sensitivity or dermatitis. Direct skin contact
should be avoided as far as it is practicable. Many of the cleaning substances
may also be harmful to the eyes if splashed, and suitable eye protection
may be required.
- Flammability - the ozone-depleting solvents that are
being phased out are non-flammable, but most of the alternative hydrocarbon
solvents will burn. The flash-point is the lowest temperature at which a
solvent will emit sufficient vapour to form a flammable vapour-air mixture
which will flash when ignited momentarily by application of a flame. Its
value only indicates the hazard, which is increased if the solvent is sprayed,
atomised or strongly agitated, for example, using ultrasonics. Normally
one recommends a margin of safety of at least 15°C between the operating
temperature and the material's flash-point, otherwise stringent precautions
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Alternatives to CFCs
As with the replacement for tin-lead solder, there is no immediate drop-in
replacement for the CFCs used in cleaning. The nearest to such a material,
n-propyl bromide, was developed too late in the day, and its use has been
somewhat controversial. Although it has an ODP rating of zero, is relatively
cheap, non-flammable and fits a range of applications, it is somewhat toxic,
with an OEL of 50 ppm, and some concern that this will reduce. In consequence,
there has been no change to the four basic approaches to the withdrawal of
CFCs that were proposed at the outset:
- Use a volatile organic solvent in a process similar to CFC vapour cleaning
using either non-flammable HCFC solvents or flammable solvents with a lower
global warming potential.
- Use an organic solvent with a high boiling point, either evaporating the
solvent at elevated temperature or choosing a water-miscible material which
can be rinsed with water and dried once the solvent has completed its task
of removing the 'soil' - this is referred to as a 'semi-aqueous' process.
- Use water as a solvent, either in combination with detergent to remove
rosin flux residues or in conjunction with water soluble fluxes. Some components
are not compatible with water or are difficult to clean because of fluid
entrapment in small crevices. Water-soluble fluxes are more active than
rosin fluxes, providing a larger process window, but even traces need to
be removed because they are acid.
- Use a no-clean process.
A somewhat unexpected problem with a number of the processes, especially
those based on water/saponifier combinations, is that the lead from solder
will actually dissolve in the solvent, whereas this was not a problem
with CFCs. This points out the need to consider every aspect of the process
when evaluating a proposed change!
With no 'drop-in replacement' for CFCs available, much attention has inevitably
been focussed on removing the need for cleaning. Figure 1 gives an example
of a review carried out by a user of the cost and difficulty of alternative
cleaning approaches, from which the no-clean option can clearly be seen to
be desirable, if less easy than using CFCs.
Figure 1: Difficulty : cost ratio for different cleaning methods, Source:
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No-clean processes are not the same as not cleaning! In the latter
case there will be flux residues, and they may or may not be deleterious to
the long term reliability of the end product. A no-clean process involves
selecting flux materials which will be effective in use but whose residues
- Electrically insulating.
- 'Pin testable' (in other words, the pins on test fixtures are able easily
to penetrate the coating on test lands and solderable areas).
- Chemically inert, and will not react with any of the materials on the
assembly during extended life (which may include elevated temperature and
- Cosmetically acceptable. This is particularly important where a non-technical
end-user gets sight of a board which is less than critically clean. Would
you want to eat with dirty cutlery, even if the deposits were shown to be
Of these, the major issues in recent years have been associated with pin
testability and cosmetic acceptability, and work continues on solder ball
elimination and maximising the process window.
No-clean systems have been formulated for both wave-soldering and reflow
soldering. The basis for both is a flux which has a higher proportion of either
solvent (for wave soldering) or rheology modifiers (for reflow soldering)
and a lower concentration of both flux base and activator. Moreover both flux
base and activators are chosen so that their decomposition products are not
The ultimate of no clean is 'no residue', where the flux is expended entirely
during the process of producing the solder joint. This also presents the ultimate
challenge for the user! The issues with all no-clean/no residue processes
relate to process control, so that solderability is achieved and, the joint
satisfactorily wetted, before the protective qualities of the flux are dissipated.
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