Note: Descriptions are shown in the official language in which they were submitted.
CA 02221086 1998-01-27
"Organic Acids Activated No Clean Solder Flu~ Composition"
Background of the Invention
1. Field of the Invention
The present invention is directed to a liquid solder flux, and, more particularly, to an
organic acid activated liquid solder flux specifically form~ ted for cleaning electronic circuit
assemblies, such as printed circuit boards, during the fabrication thereo~ Adipic acid, citric acid,
and formic acid in a solvent, such as isopropanol and ethyl acetate, are used to remove solder
flux, oils, waxes and greasy substances, as well as adhesive and other residues from various
o 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
15 s~ti~f~ctory soldered joint, it is necessaly 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
20 later failure of the bond.
Thelerole, the surface to be fluxed must be cleaned of a portion of the oxide and
cont~lni~ coating prior to fluxing. Equally in~po~lanl, the soldered array must be carefully
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cleaned after soldering is complete, in a postcleaning process, to remove any unreacted flux and
reaction products of the flux treatment. Any l~"~ ing flux or reaction products can continue
reacting with the bonded pieces and corrode them dunng service. Such corrosion caused by
fluxes is a major concern for many types of solder bonding, as the bonded pieces are somelillles
s used in environments having conditions such as high humidity and high te,l,pelal~lre that
accelerate corrosion reactions that might otherwise proceed slowly. Tiny amounts of unreacted
flux or reaction products that remain on the surface following soldering and postcleanirlg can
therefore have disastrous consequences. Because of the consequences of solder failures and the
significance of solder joints, the soldering process has been studied extensively.
o Pluxes now in use include mixtures of inorganic acids in inorganic vehicles or solvents
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 en~ on,llelll, as by accomplishing the soldering process in a
15 hydrogen gas atmosphere. Postcleaning is not required, but m~int.on~nce and use of a pure
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
20 operation, which increases the reliability of the joining process by promoting effective wetting of
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
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that permit making a solder joint faster and better, in a manner that is both en~/hur~nel.lally
acceptable and also does not require special precautions to protect the health and productivity of
production line workers.
Summary of the Invention
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
o the pieces afler soldering is complete. The ingredients of an organic acids activated solder flux
increase the reaction to metal oxides and make wetting by the solder easier and faster. Any
residue of the activated agent boils off with the heat of soldering. The flux evaporates entirely
during the fluxing process, 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
tell.pelal~lre, the flux is à liquid having a viscosity co~.p~able with that of water, and can be
readily provided in open conlail~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 injury. The flux can thel erol e be used in assembly operations involving hand
labor without special precautions such as required with some other fluxes.
In accordance with the invention, organic acids activated liquid solder flux comprising a
flux base and the organic acids dissolved in sufficient solvent to form a liquid solder flux is
provided. The organic acids consist essentially of carboxylic acid derivatives of a short to
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medium-chain hydrocarbon. The amount of the organic acid derivatives range from an amount
effective to provide improved fluxing action to an amount sufficient to form a saturated solution.
The flux of the invention is applied to the surface to be fluxed by briefly immersing the
surface into the flux, spraying the flux onto the surface, or by other convenient means. The
s surface is then tinned by dipping the 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
o aspect of the invention, the organic acids are mixed with an evaporative organic solvent to form a
flux. The organic acids must be present in an effective amount, which is typically in the range of
from about I part of acid mixture to 30 parts of solvent. Full fluxing effectiveness is normally
reached with about 1 part of the acid mixture for 20 parts of the solvent. Acceptable evaporative
organic solvent would be isopl.,panol and ethyl acetate. Acceptable organic acids are those that
5 sublime fully at the fluxing temperature when mixed with the solvent. Adipic acid and citric acid
are especially plt;r~lled in this embodiment. Significant amount of evaporative organic acid, such
as formic acid, would improve the surfaces to be joined during the tinning step.
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,
20 integrated circuits, and the like are included. Subass~ ,blies can be formed or joined. Wires,
connectors, printed wiring boards, and the like can be joined together or with active components.
The processes are also operable in conjunction with non-electronic soldering operations, such as
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general metal joining, fabrication, and the like. There are no known limitations on such uses of
the flux.
It will be appreciated that the present invention presents an important advance in the art of
soldering and fluxes. The flux of the invention is highly effective 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. 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 operations.
0 Further, the purpose of the foregoing Abstract is to enable the C~n~ n Intellectual
Property Office (CIPO) and the public generally, and especially the scienlis~s, engineers and
practitioners in the art who are 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
the application. Accordingly, the Abstract is neither intended to define 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.
It is thelero~e an object of the present invention to provide a new and improvedcomposition for .,leaning 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
for cleaning the surface of various soldered articles which may be efficiently m~nllf~ red and
marketed.
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An even further object of the present invention is to provide a new and improved
co~l.pos;lion for cle~ning the surface of various soldered articles which is susceptible of a low cost
of m~nllf~chlre with regard to both materials and labor, and which accordh~gly is then susceptible
of low prices of sale to the con~uming public, thereby making such composition available to the
s buying public.
Still yet a further object of the present invention is to provide a new and improved
composition for cle~ning 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
lo novelty which characterize the invention, are pointed out with particularity in the claims fo~ ing a
part of this disclosure. For a better underst~nding of the invention, its opel~lh-g advantages and
the specific objects attained by its uses, lerelence should be made to the following detailed
description of the plerel led embodiments of the invention.
15 Description of the preferred Embodiment
The present invention is embodied in a flux used in conjunction with soldering. The flux
prefelably is a mixture of adipic acid, citric acid, formic acid and the e~apo~ e organic solvent
system inclu(~ing isopropanol and ethyl acetate, with the ratio of acid and solvent being from
about 1 to about 10 parts of acid mixture for each 20 parts of ev~pol ali.~e organic solvent system.
20 While greater propo. lions of acid are operable, completely effective results are oblained with less
than about 8 parts of acid mixture. Preferably, the solvent system is isoplupanol and ethyl
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acetate. Both the acid mixture and solvent system should be without impurities that would not
evaporate during fluxing.
A flux having reagent grade adipic acid, reagent grade citric acid, reagent grade formic
acid, reagent grade isoplopallol and ethyl acetate, mixed in the ratio of 1 part by volume of adipic
5 acid to 3 parts by volume of citric acid to 2 parts by volume of formic acid to 90 parts by volume
of isopropanol to 10 parts by volume of ethyl acetate, is the presently most plere--ed embodiment
of the invention. This flux mixture is prepared by adding the required volume of adipic acid, citric
acid, formic acid slowly to the isopropanol and ethyl acetate, taking care not to splatter the acid
or overheat the solution. Consequently, the plepa-~Lion of the flux mixture should be
lo accomplished by a person skilled in making acid mixtures, using good ventilation and approp.;ate
safety e luipll~ l inclu~ling at least goggles, a rubber apron, and rubber gloves.
The flux mixture can be contacted to the skin without burns or injury, although it is
recommended that any flux toucl~ing the skin be washed away as soon as possible. Nevertheless,
the flux mixture is much safer than many other col..n-t;--;ial liquid fluxes, which quickly burn the
5 skin. The dol.--nalll odor of the flux is that of the solvent, in the plerel-ed case the isoplupanol
and ethyl acetate. While prolonged exposure to this odor is not It;con~ll.ellded, exposure to a
weak odor or for brief periods is not immediately harmful. Thus, the flux is safe to handle in
oldin~y conditions of good ventilation and a reasonably safe working envi-ur~--elll. It is seen that
the flux and approach of the present invention yield important advantages in soldering. Good
20 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 wiring is done.
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While the present invention has been fully described above with particularity and detail in
comleclion with what is of ordillaly skill in the art that many modifications thereof may be made
without dep~ ling from the principles and concepts set forth herein. Hence, the proper scope of
the present invention should be dete-lllined only by the broadest intell,lelalion of the appended
5 claims so as to encompass all such modifications.
