Note: Descriptions are shown in the official language in which they were submitted.
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PATENT
PD-90466
WATER-SOLUBLE FLUX FOR CORED SOLDER
CROSS-REFERENCE TO RELATED APPLICATIONS
~he present application is a continuation-in-part ap~
plication of Serial No. 07/ , , filed [PD-
89640A], which in turn is a continuation-in-part applica-
tion of Serial No. 07/523,765, filed May 15, 1990, both ap-
plications filed in the name of Ray L. Turner and assigned
to the same assignee as the present application. The ap-
plications disclose and claim a method of soldering using
a water-soluble flux which co~tains a water-soluble organic
acid having at least two carboxylic acid groups. A pre-
ferred example of such an organic acid is citric acid.
The present application is also related to Serial No.
07/ ~ PD-90465] filed the same date as the present
application and assigned ko the same assignee as the pres-
ent application. That application discloses and claims a
composition of matter useful as a solder paste and a method
for using the same.
TEcHNIcAll FIE~D
The present invention relates to a novel water-solu-
ble, environmentally-safe ~lux for cored solders, useful in
electronic industry soldering processes.
BACKGROUND AR~
Most fluxe~ and defluxing chemicals used by industry
to manufacture electronic circuits, such as military hard-
ware, are either contributing to ozone depletion of the at-
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mosphere or are considered by local environmental agencies,
such as the Air Quality Management District in the Los An~
geles basin, as environmental pollutants or health ha~ards.
For example, it has been reported that if CFCs (chlorofluo-
rohydrocarbons), which are used in vapor degreasing to
clean soldered parts where a rosin flux is employed, are
releasad 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 formula-
tions 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 solclers that
produces solder joints of high metallic luster and excel-
lent quality.
DISCLOSURE OF INVENTION
In accordance with the invention, a water-soluble flux
for cored solder is provided, comprising (a) botanical oil,
(b) emulsifier, and (c~ a water-soluble organic acid having
at least two carboxylic acid groups.
The flux of the invention eliminates harmful environ-
mental emissions normally associated with rosin-~ased flux-
es, flux thinner (such as iso-propanol), and associated de~
fluxing solvents (such as CFCs)~ The flux of the invention
is environmentally safe, non-toxic, and easy to use. Sol-
dering time is cut in half, and defluxing (cleaning) time
is reduced from ten to fifteen minutes (required for rosin-
based fluxes) to about three minutes. Soldered assemblies
can be cleaned in deionized water, thus eliminating the
high capital costs associated with vapor degreasing e~uip-
ment. The resulting soldered joint exhibits high metallic
luster and excellent electrical quality.
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BEST MODES FOR CARRYING OUT THE IMVENTION
The flux of the invention comprises (a) botanical oil,
(b) emulsifier, and (c) a waker-soluble organic acid having
at least two carboxylic acid groups. Examples of suitable
organic acids include citric acid, malic acid, tartaric
acid, glutamic acid, phthali~ acid, succinic acid, and the
like.
However, the best results have been obtained with cit-
ric acid, and while the class of organic acids indicatedabove is useful, the remainder of the description which
follows is directed to the use of citric acid as a flux in
cored solders used in soldering components.
Without subscribing to any particular theory, it ap-
pears that the citric acid chelates the oxide of the basismetal, but not the metal itself. As a result, there is
little or no corrosion of the basis metal by the flux of
the invention. Thus, to the extent that members of the
above class of organic acids behaves similarly to citric
acid, then such members are also included within the ambit
of the invention.
The botanical oil serves to level the solder, minimize
splattering during the soldering process, and prevent dry-
ing of the paste. The oil is used to assist in produ~ing
a smooth solder surface. Examples of suitable botanical
oils include ~ojoba oil, palm oil, and peanut oil. Animal
fat oils do not clean up easily, and are to be avoided in
the practice of the invention.
Any of the well-known water-soluble emulsifiers and
surfactants of the cationic, anionic, and non-ionic type
may be used in the practice of the invention to assure a
reduction in surface tension, enhance solder spread, and
aid in flux removal, and as used herein, the term "emulsi-
fier" is intended to also include surfactants. One or more
such emulsifiers and surfactants may be employed. Examples
of suitable emulsifiers and surfactants include ethylene
oxide and propylene oxide and adducts thereto, adducts of
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glycerol esters, polyoxyethylene sorbitan monopalmitate,
polyoxyethylene sorbitan monolaurate, alkyl aryl polyether
alcohol, polyoxyethylene sorbitan monooleate, polyoxyethyl-
ene sorbitan monostearate, and ethoxylated linear alcohols.
5The emulsifier keeps the oil and water in suspension
and can act as a thickening agent. The -Elux must be thin
enough to be co-extruded with the solder, but thick enough
so as not to exude. The viscosity range of fluxes employed
in cored solders is well-known, and does not form a part of
10this invention.
An aqueous solution of citric acid is used. Tha cit-
ric acid concentration may range from about 3 wt% to satu-
ration of the total flux composition. ~he amount of water
should be minimized to avoid splatter, and accordingly, a
15saturated solution of citric acid in water is preferably
employed. In this connection, an amount of water suffi-
cient to dissolve citric acid (powder, crystals, or gran-
ules) is employed. The grade or class of the citric acid
is not critical in the practice of the invention, nor is
20the type or purity of the water used. However, for ex-
tremely sensitive electrical circuits, it may be desirable
to employ distilled or deionized water and a fairly high
grade oP citric acid.
Alternatively, a non-toxic medium which ties up free
25water may be used for lower concentrations of citric acld.
An ethoxylated linear alcohol may be used for this purpose.
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.
30The ratio of emulsifier to citric acid solution is
preferably about 1:1; the emulsifier is used to suspend the
oil in the citric acid.
An especially preferred flux composition for cored
solder consists essentially of the following:
35(1) about 1 to 5 wt% botanical oil;
(2) about 47.5 to 49.5 wt% emulsifier, and
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~3) about 47.5 to 49.5 wt% citric acid solution
(3% to saturated).
Any of the common 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 solders are used.
As is well-known, a hollow cylinder of the solder is co-
extruded with the flux to form the composit~ cored solder.
Tha flux/cored solder is most advantageously employed
in surface mount component soldering processes. The flux
contains no volatile organic compounds (VOCs) or other
evaporative solvents, such as iso-propyl alcohol. The wa-
tar-soluble solder flux of the invention avoids tha use oE
rosin-cored solders, which necessitate the use of chloro-
fluorocarbon-type defluxing agents; rather, the flux of the
invention is ~asily defluxed 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
ag~nts (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 cuts the actual soldering time by ap-
proximately 70%. rrhis enables soldering to proceed at a
considerably higher rate than permitted by rosin-cored sol-
ders, thereby subjecting electronic components to the heat
of the solder for a far shorter period of time.
4. 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.
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INDUSTRIAL APPLICABILIrrY
The flux/cored solder of the invention is expected to
find use in commercial soldering operations, particularly
in soldering electrical components in circuit boards.
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 thatvarious 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.
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