Note: Descriptions are shown in the official language in which they were submitted.
CA 0221~438 1997-11-12
"Non-CFCs Solder Flux Composition (I)"
Background of the Invention
1. Field of the Invention
The present invention relates generally to environmentally safe flux removing
compositions specifically formulated for cleaning electronic circuit assemblies, such as printed
circuit boards, during the fabrication thereof. Hydrofluoric acid, formic acid, and fluoroaliphatic
ammonium salts in a solvent, such as isopropanol, are used to remove solder flux, oils, waxes and
greasy substances, as well as adhesive and other residues from various substrates.
2. Description of the Prior Art
Soldering with a soft solder, such as tin-lead based alloy melting at between 200~C to
260~C, is widely employed in the electrical and electronics industries. In order to produce a
s~ti~f~ctory soldered joint, it is necessary to use a flux with the soft solder in order to remove any
residual surface oxide films and hence provide a clean surface and in order to reduce the surface
tension of the molten solder and hence promote good wetting of the surface by the solder. It is
important that the molten solder fully wet the surfaces to be joined during the tinning step, as
unwet portions result in voids at the surfaces after soldering is complete, and the voids can cause
later failure of the bond.
Therefore, the surface to be fluxed must be cleaned of a portion of the oxide and
co~ "~ l coating prior to fluxing. Equally important, the soldered array must be carefully
cleaned after soldering is complete, in a postcleaning process, to remove any unreacted flux and
CA 0221~438 1997-11-12
reaction products of the flux treatment. Any rem~ining flux or reaction products can continue
reacting with the bonded pieces and corrode them during service. Such corrosion caused by
fluxes is a major concern for many types of solder bonding, as the bonded pieces are sometimes
used in environments having conditions such as high humidity and high temperature that
5 accelerate corrosion reactions that might otherwise proceed slowly. Tiny amounts of unreacted
flux or reaction products that remain on the surface following soldering and postcleaning can
therefore have disasL.o~ls consequences. Because of the consequences of solder failures and the
significance of solder joints, the soldering process has been studied extensively.
Fluxes now in use include mixtures of inorganic acids in inorganic vehicles or solvents
o such as water, inorganic acids in petrolatum pastes, salts in water, petrolatum paste or organic
solvent, organic acids in water, organic halogens in water, amines and amides in water, and
natural or modified rosins. All of these fluxes require postcleaning of the soldered joint. Fluxing
can also be carried out in a reducing environment, as by accomplishing the soldering process in a
hydrogen gas atmosphere. Postcleaning is not required, but m~int~n~nce and use of a pure
5 hydrogen atmosphere in large scale production operations is difficult, and can lead to related
hydrogen embrittlement. In sum, there is known no effective, generally applicable approach to
fluxing which avoids the need for postcleaning of the soldered parts.
There exists a need for an improved approach to soldering, and particularly to the fluxing
operation, which increases the reliability of the joining process by promoting effective wetting of
20 the solder to the piece during the tinning step, and also reduces the incidence of post-soldering
failures due to corrosion or related causes. Work continues in an effort to develop suitable fluxes
that permit making a solder joint faster and better, in a manner that is both environmentally
CA 0221~438 1997-11-12
acceptable and also does not require special precautions to protect the health and productivity of
production line workers.
Summaly of the Invention
s The present invention provides a solder flux, and related method for fluxing a surface and for
joining two pieces together using the flux. The flux promotes even, smooth, regular wetting of
the surface of the piece by solder, with a virtual absence of unwetted portions. Use of the flux
does not require cleaning of the surface prior to fluxing or postcleaning of the bonded portions of
the pieces after soldering is complete. The flux evaporates entirely during the fluxing process,
o leaving no residue or reaction products to interfere with the use of the bonded components, or
cause corrosion or other harmful post-bonding effects. At ambient temperature, the flux is a
liquid having a viscosity comparable with that of water, and can be readily provided in open
contail-el~ at the point of use. The vapors from many embodiments of the flux are not odorous
and offensive to persons who use the flux, and contact of the flux to the body does not cause
5 injury. The flux can therefore be used in assembly operations involving hand labor without special
precautions such as required with some other fluxes.
In the past, strong inorganic acids such as hydrochloric and hydrobromic acids have been
incorporated into certain synthetic organic acids, which are then processed into esters for use in
fluxes. However, hydrofluoric acid has been generally considered too strong an acid for
2() widespread use in fluxes, because tiny amounts of residues could cause serious corrosion damage,
and because the hydrofluoric acid attacks glass and ceramics used in many electronic components.
Hydrofluoric acid has occasionally been used in water and petrolatum mixtures, but such use has
CA 0221~438 1997-11-12
been restricted due to the activity of the acid. It has now been found that hydrofluoric acid effects
the fluxing operation when mixed with an evaporative organic solvent. The flux of the invention
is applied to the surface to be fluxed by briefly hl"nel~ g the surface into the flux, spraying the
flux onto the surface, or by other convenient means. The surface is then tinned by dipping the
5 surface into liquid solder, brushing the liquid solder onto the surface, or other convenient means.
When the flux of the invention is used, the surface to be fluxed and bonded need not be
specially cleaned prior to application of the flux. The surface should not have grease, thick dirt,
or other impenetrable barrier that prevents contact of the liquid flux to the surface. In the other
aspect of the invention, an evaporative organic acid is mixed with an evaporative organic solvent
o to form a flux. The evaporative organic acid must be present in an effective amount, which is
typically in the range of from about 1 part of acid to 50 parts of solvent. Full fluxing effectiveness
is normally reached with about I part of the acid for 50 parts of the solvent. However, the
general level of effectiveness reached using evaporative organic acids is less than that reached
using hydrofluoric acid, and therefore the flux mixture of hydrofluoric acid and evaporative
5 organic solvent is plerel,ed. Acceptable evaporative organic solvent would be isopropallol and
ethyl alcohol. Acceptable evaporative organic acids are those that evaporate fully at the fluxing
temperature when mixed with the solvent. Formic acid are especially plerelled in this
embodiment. Significant amount of fluoro-containing surface active agent, such as fluoroaliphatic
ammonium salts, would improve the surfaces to be joined during the tinning step. Inorganic acids
20 such as hydrofluoric acid, hydrochloric acid, hydrobromic acid and hydroiodic acid can be added,
either singly or in conlbil~alion.
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A wide variety of types of pieces can be fluxed and joined by the processes of the
invention. Conventional electronic components such as resistors, diodes, capacitors, transistors,
integrated circuits, and the like are included. Subassemblies can be formed or joined. Wires,
connectors, printed wiring boards, and the like can be joined together or with active components.
s The processes are also operable in conjunction with non-electronic soldering operations, such as
general metal joining, fabrication, and the like. There are no known limitations on such uses of
the flux.
It will be applecialed that the present invention presents an important advance in the art of
soldering and fluxes. The flux of the invention is highly ef~ective in promoting wetting of the
portions of components to be joined by solder, producing an evenly tinned surface. The flux and
its reaction products evaporate completely by the conclusion of the fluxing operation, so that
postcleaning is not required even though the flux contains a strong acid. The flux is easy to use in
fluxing the surfaces and in bonding components. It is not dangerous to contact to the skin, and
does not have an objectionable odor, so that the flux can be used in mass production soldering
s operations.
Further, the purpose of the foregoing Abstract is to enable the Canadian Intellectual
Property Office (CIPO) and the public generally, and especially the scientists, engineers and
practitioners in the art who ate not familiar with patent or legal terms of phraseology, to
determine quickly from a cursory inspection the nature and essence of the technical disclosure of
2() the application. Accordingly, the Abstract is neither intended to deffne the invention or the
application, which only is measured by the claims, nor is it intended to be limiting as to the scope
of the invention in any way.
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It is therefore an object of the present invention to provide a new and improved
composition for cleaning the surface of various soldered articles and which has all of the
advantages of the prior art and none of the disadvantages.
It is another object of the present invention to provide a new and improved composition
s for cleaning the surface of various soldered articles which may be efficiently m~nllf~ctured and
marketed.
An even further object of the present invention is to provide a new and improved
composition for cleaning the surface of various soldered articles which is susceptible of a low cost
of manufacture with regard to both materials and labor, and which accordingly is then susceptible
o of low prices of sale to the con~llming public, thereby making such composition available to the
buying public.
Still yet a further object of the present invention is to provide a new and improved
composition for cleaning the surface of various soldered articles and which is adapted to be
topically applied to such articles in a colourless thin liquid, easily sprayable form.
These together with still other objects of the invention, along with the various features of
novelty which characterize the invention, are pointed out with particularity in the claims forming a
part of this disclosure. For a better understanding of the invention, its opelaling advantages and
the specific objects attained by its uses, reference should be made to the following detailed
description of the pl ere-led embodiments of the invention.
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De~c. ;I~lion of the preferred Embodiment
The present invention is embodied in a flux used in conjunction with soldering. The flux
preferably is a mixture of hydrofluoric acid, formic acid, fluoroaliphatic ammonium salts and the
e~,~po~ e organic solvents inclu(~ing isopropanol, with the ratio of acid and solvent being from
s about 1 to about 10 parts of hydrofluoric acid for each 46 parts of evaporative organic solvent.
While greater proportions of acid are operable, completely effective results are obtained with less
than about 10 parts of acid, and there is less danger from acid burns. Preferably, the solvent is
isopropanol. Both the hydrofluoric acid and isopropanol should be without impurities that would
not evaporate during fluxing.
10 A flux having reagent grade hydrofluoric acid provided as an aqueous solution of 49 percent
hydrogen fluoride, and reagent grade formic acid provided as an aqueous solution of 85 percent
formic acid, and reagent grade isopropanol, mixed in the ratio of 2 parts by volume of
hydrofluoric acid to 46 parts by volume of isopropanol, is the presently most pl ere. I ed
embodiment of the invention. This flux mixture is prepared by adding the required volume of
hydrofluoric acid slowly to the isopropanol, taking care not to splatter the acid or overheat the
solution. Concenll~led hydtofluoric acid is potenially dangerous, as it is a very strong acid that
rapidly attacks human tissue. Consequently, the preparation of the flux mixture should be
accomplished by a person skilled in making acid mixtures, using good ventilation and applopliate
safety equipment including at least goggles, a rubber apron, and rubber gloves.
Once, the flux mixture is prepared, the concentration of hydrofluoric acid is sufficiently
low that there is a greatly reduced risk of injury. The flux mixture can be contacted to the skin for
brief periods without burns or injury, although it is recommended that any flux touching the skin
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be washed away as soon as possible. Nevertheless, the flux mixture is much safer than many
other commercial liquid fluxes, which quickly burn the skin. The dol-linalll odor of the flux is that
of the solvent, in the plt;relled case the isopropanol. While prolonged exposure to this odor is not
recomrnended, exposure to a weak odor or for brief periods is not immediately harrnful. Thus,
5 the flux is safe to handle in ordinaly conditions of good ventilation and a reasonably safe working
enviloll.l~ ll. It is seen that the flux and approach of the present invention yield important
advantages in soldering. Good tinning and soldering are achieved, without the need for cleaning
and postcleaning. The flux is nontoxic, and can be used in production operations where hand
.
WlrlnglS done.
0 While the present invention has been fully described above with particularity and detail in
connection with what is presently deemed to be the most practical and plerelled examples of the
invention, it will be appare"l to those of ordinary skill in the art that many modifications thereof
may be made without depa~ g from the principles and concepts set forth herein. Hence, the
proper scope ofthe present invention should be determined only by the broadest inte.l~lelalion of
1 5 the appended claims so as to encompass all such modifications.
2()
