Foaming flux for automatic soldering process

Abstract: A non-toxic, non-corrosive foaming soldering flux ( 14 ) comprises citric acid and water, together with at least one foaming agent. The foam is fast-breaking, dissipating almost immediately. Such a fast-breaking foam is novel in an aqueous-based flux. The unique flux of the invention produces solder joints of high metallic luster and excellent quality. Disposal presents no health hazards, and clean-up of flux residues is accomplished using only water and, optionally, a surfactant.

Description: TECHNICAL FIELD The present invention relates to a method for soldering a metal and to a foaming flux for automatic soldering process. BACKGROUND ART Most fluxes and defluxing chemicals used by industry to manufacture electronic circuits, such as military hardware, 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 could be the simple solution for this major problem. However, most water-soluble fluxes are formulated with harsh activators, such as hydrochloric acid and complex glycols, which create major cleaning and residue problems for the printed circuit board and the electrical circuitry thereon and which tend to severely corrode the metal being soldered or contaminate the dielectric, causing a tendency for electromigration. Other water-soluble fluxes are formulated with iso -propanol and/or plasticizers, which create disposal and health problems. What is desired is a simple, non-toxic, non-corrosive foaming soldering flux that produces solder joints of high metallic luster and excellent quality when used in automatic soldering processes. From EP-A-0 458 161 A1 there is a known a water-soluble soldering flux comprising an organic acid and a non-toxic carrier, such as water. Furthermore, said flux may comprise an odorant as well as surfactants such as ethoxylated alcohol. DISCLOSURE OF INVENTION In accordance with the invention, a foaming soldering flux is provided, which consists essentially of at least one water-soluble organic acid having at least two carboxylic groups, water, and one foaming agent in form of sodium alkyl sulfonate. The flux of the invention produces, through aeration, a continuous head of foam which permits automatic soldering using a water-based flux in lieu of glycol/alcohol type liquid fluxes normally found in commercial fluxes. As a result of the unique formulation, automatic soldering can be performed, utilizing an environmentally safe, water-based flux. Further, this flux negates the need for costly and environmentally-harmful defluxing solvents, comprising chlorofluorocarbons and/or volatile organic compounds (VOCs). The flux is readily removed from soldered circuit card assemblies, using water and a surfactant. The resulting soldered joint exhibits high metallic luster and excellent electrical quality. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view, partially exploded, showing a conventional foam fluxing system; FIG. 2 is a cross-sectional view of the foam fluxing system of FIG. 1; and FIG. 3 is a side-elevational view of a conventional wave soldering system incorporating the foam fluxing system of FIGS. 1 and 2. BEST MODES FOR CARRYING OUT THE INVENTION A conventional foam fluxing system 10 is illustrated in FIG. 1. An outer housing, or flux tank, 12 contains liquid flux 14 (visible in FIG. 2). An aerating stone 16 is situated in a flux chimney 18, which in turn is arranged in the flux tank 12 so as to permit the liquid flux in the flux tank to constantly flow into the flux chimney as required. An air connect 20, connected to a source of air (not shown) and pressurized to a controlled pressure by control means (not shown), provides aeration of the liquid flux, as seen more clearly in FIG. 2, to provide a foam head 22. It is desirable for the height of the foam head 22 to extend above the top 18a of the flux chimney 18. Desirably, when used in conjunction with the wave soldering system depicted in FIG. 3, the height of the foam head 22 is about 0.5 inch (1,27 cm), shown at "A". However, for other soldering systems, that height may differ. The primary factors governing the height of the foam head 22 include the composition of the foaming agent(s) used in the foaming flux, the concentration of the foaming agent(s), the air pressure applied to the aerating stone, and the physical structure of the stone itself. The flux of the invention consists essentially of (a) at least one water-soluble organic acid having at least two carboxylic acid groups, such as citric acid, (b) water, and (c) a foaming agent in form of sodium alykl sulfonate surfactants. Examples of suitable organic acids include citric acid, malic acid, tartaric acid, glutamic acid and phthalic acid. However, the best results have been obtained with citric acid, and while the class of organic acids indicated above is useful, the remainder of the description which follows is directed to the use of an aqueous solution of citric acid as a solder flux in foam soldering. Without subscribing to any particular theory, it appears that the citric acid chelates the oxide, but not the basis metal. 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 concentration range of citric acid in the solder flux may range from 0.5 to 99.5 percent (by weight). A concentration of at least about 5 wt% provides noticeably improved soldering results, while concentrations above 50 wt% provide no additional improvement, and accordingly, this range is preferred. A concentration of at least 20 wt% ensures consistently improved soldering results, even for less-than-ideal soldering surfaces, and accordingly, the range of 20 to 50 wt% is most preferred. The quality of water used and the grade of citric acid employed are not material in the practice of the invention, although for extremely sensitive electrical circuits, it may be desirable to employ distilled or deionized water and a fairly high grade of citric acid. The higher the grade of water and citric acid, the better the results. The source of the citric acid is also immaterial, consistent with the foregoing considerations relating to sensitive electrical circuits, and may comprise, for example, commercially-produced powder or crystal or even fruit juice or fruit juice concentrate, such as juice from oranges, lemons, limes, grapefruits, pineapples, tomatoes, and the like. Finally, the flux may contain other components, such as impurities, whether accidental or deliberate, so long as such additives have no adverse effect on the soldering results. The concentration of the foaming agent ranges from 0.0001 to 5 wt%, depending on the particular foaming agent or combination of foaming agents selected. Preferably, the concentration ranges from 0.0002 to 0.04 wt%. The composition and concentration of the foaming agent must be selected so as to provide a foam head height of about 0.5 inch (1.27 cm). If the concentration is too high, there will be too much foam, and the foam height will be too high; if the concentration is too low, there will not be enough foam to form the foam head 22. The foaming agent used in the present invention is a sodium alkyl sulfonate, which is preferably used in a range from 0.0315 to 0.0385 wt%, and more preferably about 0.035 wt%. Additives may be added to the flux for specific purposes. For example, an odorant, such as wintergreen oil, spearmint oil, or peppermint oil, may be added to the flux to provide a pleasant odor for the user. Such an odorant is typically added in the amount of about 1 to 30 ppm. A colorant may be added for visual purposes; this enables the operator to see the flux, which is otherwise colorless. About 1 to 30 ppm of at least one coloring agent (or acid-stable, non-photoreactive dye) may be added in this regard. The flux is most advantageously employed in the soldering of electrical connections of electronic components to printed circuit boards (PCBs). Such printed circuit boards typically comprise copper-plated lines surrounding via openings, which are copper-plated followed by a tin-lead coating over the copper, in the PCB through which the leads of the components extend. The solder employed during component soldering is typically a tin-lead solder, and the flux of the invention has been successfully used with 60-40, 63-37, and 96-4 tin-lead solders. In most applications, 63-37 tin-lead solder is employed. FIG. 3 depicts apparatus 24 commonly used to solder printed circuit boards, employing wave soldering. The apparatus is seen to include a transporter rail 26 which carries the printed circuit boards (not shown) at the desired height above the foam fluxing unit 10 and the solder wave unit 28 . After passing over the foam fluxing unit 10 , the PCB is exposed to a flux air knife 30 to blow off excess flux from the PCB assembly, an optional auxiliary heater 32 , and a preheater 34 to heat the board to condition the flux and reduce thermal shock to the board and components thereon, before contact with the solder wave 36 . Various aspects of the operation are controlled from a control panel 38 . The temperature of the solder in the solder pot 28 is also controlled by temperature controller 40 . While the flux of the invention is specifically intended for use in foaming operations, such as used in automated soldering processes described above, the flux may also be applied by a variety of techniques, such as wave, spray, or dipping techniques. The flux of this invention was formulated to produce a rapid movement of flux foam that could rise up through the fluxing chimney 18 with sufficient force to produce a head of flux foam 22 0.5 inch (1.27 cm) or more of free height (rapid movement of foam through a narrow rectangular opening rising above the top chimney surface 18a ), and as the foam descends, it must result in a timely collapse of the foam bubbles sufficient to prevent the flux foam from collecting and flowing outside the flux holding pan 12 (which houses the flux 14 , fluxing chimney 18 , and aeration stone 16 ). Such timely collapse is substantially immediately, and is referred to as "fast-breaking". Surprisingly, while fast-breaking foam is known in solvent-based fluxes, such foam is not generally known in aqueous-based fluxes. The foaming flux is easily removed using an aqueous cleaning system, which may contain surfactants to enhance cleaning, if desired. Non-ionic wetting agents, such as ethoxylated alcohol, available under the tradename Triton X-100 from Rohm & Haas (Los Angeles, CA), may be added up to about 0.1 wt% of the cleaning solution. The degree of cleanness and the need for a surfactant is somewhat dependent upon user cleanness requirements. Using an ethoxylated alcohol, such as Triton X-100, in an amount of about 0.007 wt% in water provides a cleanness of between about 7.5 and 20 megohm/cm, as measured by an Omega meter, model 200 (available from Kenco Industries, Inc., Atlanta, GA). These values are not per se critical, but are guidelines to meet military specifications for cleanness requirements for this particular testing system. This water-based soldering flux produces a good head of fluxing foam without any noticeable decomposition following weeks of use. The flux of this invention avoids the use of such components as glycols, alcohols, or other volatile organic compounds (VOCs) normally found in commercially available liquid fluxes. The flux of the invention provides the following advantages: 1. It eliminates harmful environmental emissions normally associated with rosin-based 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 and a surfactant (if needed). Thus, the new flux is environmentally safe. 2. It provides soldering personnel with a non-toxic, extremely safe, and highly effective flux to use. 3. Its use cuts the actual soldering time by approximately 50%. This enables automated soldering, using foam flux soldering, to proceed at a rate normally twice as fast as with rosin-based fluxes, thereby subjecting electronic components to the heat of the solder for a far shorter period of time. 4. Wave soldered assemblies can be cleaned in deionized water, thus eliminating the high capital cost of vapor degreasing equipment. The recurring costs of defluxing solvents and the costs associated with re-distillation are also eliminated. 5. Since use of the flux of the invention eliminates degreasing, the total cleaning time is reduced by about ten to fifteen minutes. 6. Use of the flux of the invention eliminates the need for costly control of solvent usage and equipment permits required by local environmental agencies (such as AQMD in the Los Angeles basin). 7. Use of the flux significantly reduces waste disposal and waste management costs; the flux is biodegradable and water soluble. 8. Use of the flux of the invention provides solder joints that evidence high metallic luster and excellent electrical quality. INDUSTRIAL APPLICABILITY The solder flux of the invention is expected to find use in commercial and military foaming automatic soldering operations, particularly in soldering electrical components in circuit boards. EXAMPLES Example 1 (invention): A base flux solution was prepared consisting essentially of 46 wt% citric acid granules in water. To this solution was added the following foaming agent: 0.035 wt% sodium alkyl sulfonate. Example 2 (comparison): The base flux solution was prepared as in Example 1. To this solution was added the following foaming agent: 0.0036 wt% octyl phenoxy polyethoxy ethanol. Example 3 (comparison): The base flux solution was prepared as in Example 1. To this solution was added the following foaming agent: 0.0030 wt% ethoxylated alcohol. Example 4 : The fluxes prepared in Examples 1-3 were evaluated in terms of foam height, foam bubble breaking status, longevity, and cleanness level. The results are tabulated in Table I below. Foam Flux Evaluation Results. Example Foam Height Bubble Breaking Status Longevity (days) Clearmess Acceptance Performance 1 ≥0.5" (≥ 1.27 cm) small, fast-breaking bubbles ≥5 passed 2 2 ≥0.5" (≥ 1.27 cm) medium, slow-breaking bubbles ≤2 failed 5 3 ≥0.5" (≥1.27 cm) large, slow-breaking bubbles ≤2 passed 4 Example 5 : A solder/flux spread test was performed to compare the soldering flux formulae with and without the foaming agent(s). The comparison was made to determine if the spread of solder was affected by the foaming additive(s). The flux formulations included the three formulations set forth in Examples 1-3 plus 46 wt% citric acid granules in water. The test involved the placement of 0.060 inch (1.5 mm) diameter 60/40 solid solder wire rings (3/8 inch, 9.5 mm inside diameter) onto a copper panel (approx. 2 x 2.5 x 0.060 inches, 5.08 x 6.35 x 0.15 cm). Three drops of flux were placed in the center of each ring. The copper specimens were then heated on a hot plate above the melt temperature of the solder. The specimens were removed from heat at the point of solder liquefaction, cleaned, and visually examined. The solder spread of each of the foam flux formulations of Examples 1-3 was comparable with the citric acid flux without foam additives. Thus, there has been disclosed a non-toxic, non-corrosive foam soldering flux, suitable for use in soldering electrical components.

Claims: A method for soldering a metal comprising applying to a surface a foaming soldering flux, heating said metal to a desired soldering temperature, and applying solder to said surface, said soldering flux consisting essentially of (a) a water-soluble organic acid having at least two carboxylic acid groups, (b) water, and (c) 0.0001-5.0 wt% of sodium alkyl sulfonate surfactants, acting as a foaming agent. The method of claim 1, wherein said foaming agent has a concentration from 0.0315-0.0385 wt%. The method of claim 1 or 2, wherein said surface is copper or tinned copper and said solder is a tin-lead solder. The method of claim 1, wherein said organic acid is selected from the group consisting of citric acid, malic acid, tartaric acid, glutamic acid, and phthalic acid. The method of claim 1, wherein said organic acid consists of citric acid. The method of claim 4 or 5, wherein the concentration of said citric acid in the soldering flux ranges from 0.5 to 99.5 wt %. The method of claim 6, wherein said concentration of citric acid ranges from 20 to 50 wt%. A non-corrosive, non-toxic, rosin-free foaming solder flux composition, characterized in that said soldering flux composition consists of (a) a water-soluble organic acid having at least two carboxylic acid groups, (b) water, and (c) 0.0001-5.0 wt% of sodium alkyl sulfonate surfactants, acting as a foaming agent. The flux composition of claim 8 wherein the sodium alkyl sulfonate concentration is 0.0315-0.0385 wt%. The flux composition of claim 8, wherein said organic acid is selected from the group consisting of citric acid, malic acid, tartaric acid, glutamic acid, and phthalic acid. The flux composition of claim 8, wherein said organic acid consists of citric acid. The flux composition of claim 11, wherein the concentration of said citric acid in the soldering flux ranges from 0.5 to 99.5 wt%. The flux composition of claim 12, wherein said citric acid is present in an amount ranging from 20 to 50 wt %.

Inventors: TURNER RAYMOND L, TURNER, RAYMOND L.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1992
Main classification: B23K35
IPC codes: B23K35