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2068357
WATER-SOLUBLE FLUX FOR CORED SOLDER
TECHNICAL FIELD
The present invention relates to a novel water-
soluble, environmentally-safe flux for cored solders,
primarily useful in electronic industry soldering
processes.
BACKGROUND ART
Most fluxes and defluxing chemicals used by industry
to manufacture electronic circuits, such as military hard-
ware, are either contributing to ozone depletion of the
atmosphere or are considered by local environmental
agencies, such as the Air Quality Management District in
the Los Angeles basin, as environmental pollutants or
health hazards. For example, it has been reported that if
CFCs (chlorofluorohydrocarbons), which are used in vapor
degreasing to clean soldered parts where a rosin flux is
employed, are released to the atmosphere, they will remain
there as an ozone depletant for nearly 100 years. other
chemicals such as rosin fluxes, alcohols, and the like pose
health risks and disposal problems for industry.
Water-soluble fluxes for cored solders are
formulations that combine a soldering flux with a
thickening agent maintained within a hollow tube of solder.
What is desired is a simple, non-toxic, non-corrosive,
environmentally-safe solder flux for cored solders that
produces solder joints of high metallic luster and
excellent quality.
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DISCLOSURE OF INVENTION
In accordance with the invention, a water-soluble flux
for cored solder is provided, comprising citric acid. The
citric acid is co-extruded with a hollow cylinder of solder
to form the composite cored solder.
The flux of the invention eliminates harmful
environmental emissions normally associated with rosin-
based fluxes, flux thinner (such as iso-~ropanol), and
associated defluxing solvents (such as CFCs The flux of the
invention is environmentally safe, non-toxic, and easy to
use. Soldered assemblies can be cleaned in deionized
water, thus eliminating the high capital costs associated
with vapor degreasing equipment. The resulting soldered
joint exhibits high metallic luster and excellent
electrical quality.
Various aspects of the invention are defined as
follows:
A composite cored solder comprising a hollow cylinder
of a solder, said hollow filled with flux, said flux
comprising citric acid.
A method of soldering a metal comprising applying to
a surface thereof of a cored solder containing a flux
therein and heating said metal to a desired soldering
temperature, characterized in that said soldering flux
comprises citric acid.
BEST MODES FOR CARRYING OUT THE INVENTION
The flux of the invention comprises citric acid.
Citric acid has a melting point of about 149 C. The
inventors have found that in the liquidous state, citric
acid can be co-extruded with any of the common tin-based
solders.
Citric acid granules or crystals are heated to a
liquid state (approximately 149 in a stainless steel
container. The heated liquid citric acid is then poured
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into a pressurized heated extruding vat. As the solid
solder wire is extruded into a hollow cored wire, the
liquid citric acid is fed into the extruding chamber and
then into the core. Air pressure inside the vat is
preferably adjusted to assure void-free extrusion inside
the core. The extruded, flux-filled solder wire is then
wound on spools, labeled, and packaged for use.
Without subscribing to any particular theory, it
appears that the citric acid chelates the oxide of the
basis metal, but not the metal itself. As a result, there
is little or no corrosion of the basis metal by the flux of
the invention.
The grade or class of the citric acid is not critical
in the practice of the invention. However, for extremely
sensitive electrical circuits, it may be desirable to em-
ploy a fairly high grade of citric acid.
The flux may contain other components, such as impuri-
ties, whether accidental or deliberate, so long as such ad-
ditives have no adverse effect on the soldering results.
Any of the common tin-based soldering alloys may be
employed, including 60/40 Sn/Pb, 62/2/36 Sn/Ag/Pb, 63/37
Sn/Pb, and 96/4 Sn/Ag. Typically, 60/40 and 63/37 tin-lead
solders are used.
The flux/cored solder is most advantageously employed
in surface mounted and through hole mounted component sol-
2068357
dering processes. The flux contains no volatile organic
compounds (VOCs) or other evaporative solvents, such as
iso-propyl alcohol. The water-soluble solder flux of the
invention avoids the use of rosin-cored solders, which ne-
cessitate the use of chlorofluorocarbon-type defluxing
agents; rather, the flux of the invention is easily de-
fluxed using deionized water.
The flux of the invention provides the following ad-
vantages:
1. It eliminates harmful environmental emissions
normally associated with rosin-cored fluxes, flux thinner
(such as iso-propyl alcohol), and associated defluxing
agents (such as 1,1,1-trichloroethane). Indeed, cleaning
of the flux is simply done in warm or hot water. Thus, the
new flux is environmentally safe.
2. It provides soldering personnel with a non-
toxic, extremely safe, and highly effective flux/cored sol-
der to use.
3. Its use provides solder joints that evidence
a more lustrous connection with excellent wetting and an
estimated 80% reduction in solder defects, compared with
rosin-cored solders.
INDUSTRIAL APPLICABILITY
The flux/cored solder of the invention is expected to
find use in commercial soldering operations, particularly
in soldering electrical components in circuit boards.
EXAMPLE
Five electronic assembly personnel conducted a hand
soldering operation comparing solder wire cored using Mil-
Spec rosin flux to that employing citric acid. A total of
280 solder joints were made. The solder joints (connec-
tions) involved the insertion of the leads of dual in-line
packages (DIPs) into plated-through holes on printed wiring
~8~5~
boards. Following component (DIP) installation, each as-
sembler performed the soldering/flux comparison using both
types of flux cored solder. Following the evaluation, the
five assemblers jointly concluded that the solder wire com-
prising citric acid produced solder ~oints superior inquality and luster to those of rosin cored solder. Both
types of solder wire were of the same solder composition,
Sn 63/Pb 37.
Thus, there has been disclosed a non-toxic, non-cor-
rosive, environmentally-safe, water-soluble flux for cored
solders, suitable for use in soldering electrical compo-
nents. It will be clear to those skilled in the art that
various changes and modifications of an obvious nature may
be made, and all such changes and modifications are consid-
ered to fall within the scope of the invention, as defined
by the appended claims.
