The present invention relates to environmentally safe aqueous cleaning compositions comprising alkaline salts, organic adjuvants and a hydrotrope comprised of alkali metal salts of carboxylic acids having a chain length of 7-13 carbon atoms so as to maintain the organic adjuvant in solution.
The present invention is directed to improvements in aqueous cleaning compositions which are useful in a wide variety of household and industrial cleaning products and which utilize high concentrations of alkaline salts and include at least one organic adjuvant such as a surfactant to improve the cleaning efficiency of the product. A particularly preferred usage of the cleaning compositions of this invention is for cleaning electronic circuit assemblies. It has been discovered that aqueous cleaners such as disclosed in the aforementioned commonly assigned application are effective in removing soldering fluxes from the electronic circuit assemblies such as printed circuit or wiring boards without the corrosive effects of previous highly alkaline circuit board cleaners and are substantially safer for the environment exhibiting lower biological oxygen demands and chemical oxygen demands than formulations currently available. The addition of surfactants and/or anti-foam agents to these lower alkaline aqueous cleaners substantially improves cleaning performance. However, it has become problematic to maintain the organic adjuvants in solution, in particular, when such solution is an aqueous concentrate.
The cleanliness of electronic circuit assemblies (ECA), such as printed circuit boards (PCB) or printed wiring boards (PWB), is generally regarded as being critical to their functional reliability. Ionic and nonionic contamination on circuit assemblies is believed to contribute to premature failures of the circuit assemblies by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic contamination can accumulate after one or more steps of the process. Circuit assembly materials are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes and finally soldered.
In the fabrication of electronic circuit assemblies, e.g., printed circuit boards, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin and non-rosin, or water soluble, fluxes. The rosin fluxes, which are generally only moderately corrosive and have a much longer history of use, are still widely used throughout the electronics industry. The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. Because water soluble fluxes contain strong acids and/or amine hydrohalides, such fluxes are very corrosive. Unfortunately, residues of any flux can cause circuit failure if residual traces of the material are not carefully removed following soldering and thus remain on an electronic circuit assembly.
While water soluble fluxes can be easily removed with warm, soapy water, the removal of rosin flux from printed circuit boards is more difficult and has therefore traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane, trichloroethylene, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or mixtures or azeotropes of these and/or other solvents. These solvents are undesirable, however, because they are toxic and when released into the environment deplete the ozone layer and/or contribute to the greenhouse global warming effect. Thus, use of such solvents is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA), and stringent containment equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
Alkaline cleaning compounds known as the alkanolamines, usually in the form of monoethanolamine, have been used for rosin flux removal as an alternative to the toxic chlorinated hydrocarbon solvents. These high pH compounds (e.g., about 12 pH), chemically react with rosin flux to form a rosin soap through the process of saponification. Other organic substances such as surfactants or alcohol derivatives may be added to these alkaline cleaning compounds to facilitate the removal of such rosin soap. Unfortunately, these compounds, as well as the water soluble soldering fluxes, have a tendency to cause corrosion on the surfaces and interfaces of printed wiring boards if such compounds and fluxes are not completely and rapidly removed during the fabrication process.
In other approaches, Daley et al., U.S. Pat. No. 4,635,666 utilize a highly caustic solution having a pH of 13 in a batch cleaning process. This method severely oxidizes the solder applied to the circuit board. In Hayes et al., U.S. Pat. Nos. 4,640,719 and 4,740,247 rosin soldering flux and other residues are removed from electronic assemblies by means of terpene compounds in combination with terpene emulsifying surfactants by rinsing in water.
The complete removal of adhesive and other residues also poses a problem. During the manufacture of electronic circuit assemblies the components are mounted on the upper surface of the board with leads protruding downwardly through holes in the board and are secured to the bottom surface of the board by means of an adhesive. Further, it is sometimes necessary to temporarily protect certain portions of the board from processing steps such as the process of creating corrosion resistant gold connecting tabs at the board edges. This transient protection of portions of the circuit board can be achieved by the application of special adhesive tape to susceptible areas. Once such protection is no longer needed, the adhesive tape must be removed. In both instances, a residue of adhesive generally remains which, if not thoroughly removed, can cause premature board failure. Removal of this adhesive residue has traditionally been carried out by the use of chlorinated solvents which, as already described, are toxic and environmentally undesirable.
Thus, the residual contaminants which are likely to be found on electronic circuit assemblies and which can be removed by the compositions and method of the present invention include, but are not limited to, for example, rosin flux, photoresist, solder masks, adhesives, machine oils, greases, silicones, lanolin, mold release, polyglycols and plasticizers.
The above-identified commonly assigned U.S. Ser. No. 731,512 is directed to improvements in electronic circuit assembly cleaners and discloses a cleaning composition comprising an aqueous solution of alkaline metal salts, in particular, alkaline metal carbonate, bicarbonate and mixtures of such salts. The addition of at least one organic adjuvant such as a surfactant or anti-foam agent to such compositions has been found to effectively increase the cleaning proficiency of such aqueous alkaline salt cleaners. Unfortunately, at high concentrations of the alkaline salts, such as in the aqueous concentrate form in which the cleaning compositions are typically supplied to the end user, the organic adjuvants are not very soluble and tend to precipitate from the solution. It is desirable to supply the cleaning concentrates as uniform solutions so that no mixing is necessary before addition of the concentrates to the wash bath. Moreover, solubility of the organic adjuvants in the aqueous concentrate ensures solubility in the dilute wash bath. Thus, there is a need to supply a means of maintaining uniformity of the concentrate.
It is known, in general, to utilize hydrotropes to maintain the solubility of organic materials such as surfactants in aqueous solution. In some cleaning compositions and, in particular, cleaning compositions utilized to remove solder fluxes from electronic circuit assemblies it is important that the composition be low foaming. Accordingly, it is also important that the addition of a hydrotrope not increase the foaming tendency of the aqueous cleaning composition.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an aqueous cleaning composition which is formed of a high concentration of cleaning salts and a cleaning enhancing amount of at least one organic adjuvant and which further contains a low foaming hydrotrope to maintain the organic adjuvant in solution.
It is another object of the present invention to provide an aqueous composition and method for the safe and effective removal of soldering fluxes from electronic circuit assemblies without otherwise adversely affecting the boards. It is a further objective of this invention to provide a cleaning composition and method for removing soldering fluxes from electronic circuit assemblies which comprises a high concentration of cleaning salts, at least one organic adjuvant such as a surfactant or anti-foaming agent and a low foaming hydrotrope which maintains the organic adjuvant in the solution and prevents separation of the organic adjuvant from an aqueous concentrate of the composition.
It has now been found that cleaning compositions for use in aqueous solution and comprising at least about 60 weight percent of cleaning salts and up to about 25 weight percent of an organic adjuvant have enhanced cleaning proficiency, are low foaming and are stabilized such that the organic adjuvant does not precipitate from aqueous solution including aqueous concentrates of the composition if up to 15 weight percent of a hydrotrope comprising a monocarboxylic fatty acid salt having a carbon chain of between C.sub.7 -C.sub.13 is added to the cleaning composition. The cleaning compositions of this invention are particularly effective for cleaning electronic circuit assemblies such as printed circuit or wiring boards if the composition comprises at least about 60 weight percent of alkaline metal salts, up to about 25 weight percent of an organic adjuvant such surfactants and/or anti-foam agents and up to about 15 weight percent of a hydrotrope comprising an alkali metal fatty acid salt having a carbon chain length of from about C.sub.7 -C.sub.13. Any balance of the composition may comprise corrosion inhibitors and other known adjuvants. The cleaning compositions of this invention are particularly useful in the form of aqueous concentrates containing 5-45 wt. % of the cleaning composition. Such concentrates are stable, uniform and do not contain flocculated organics.
This invention, in particular, provides cleaning compositions and methods for the removal of rosin solder fluxes and other residues during the fabrication of printed circuit or wiring boards. As a result, the possibility of premature circuit failure that might occur in the absence of such cleaning is eliminated or greatly reduced. The cleaning efficacy of the compositions of the invention is such that printed wiring boards thus treated meet stringent U.S. Department of Defense specifications.
The compositions of the invention are characterized by non-corrosiveness and low environmental impact, unlike the chlorinated hydrocarbon solvents and highly alkaline cleaners that have heretofore been employed for printed wiring board and printed circuit board cleaning. Advantageously, the flux removing compositions, as used herein, exhibit lower biological oxygen demands (BOD) and chemical oxygen demands (COD) than formulations currently available. For example, BODs and CODs below 20,000 ppm in the wash water and considerably lower, e.g. less than 300 ppm in the rinse water result upon using the cleaning compositions of this invention. Accordingly, the rinse water can be sewered without further treatment and minimal, if any, treatment is needed to remove the organics from the wash water before sewering, thus eliminating the need for costly water treatment.
The present invention provides aqueous printed circuit/wiring board cleaning compositions comprising alkaline salts so combined that they have, when used in concentrations of about 0.1 to 1.5 percent by weight, a pH of at least about 10 to 13, preferably to 12 or less. The compositions, when used in the above-mentioned concentrations, should have an adequate reserve of titratable alkalinity, for example, at least equivalent to from about 0.2 to 4.5 percent caustic potash (potassium hydroxide), when titrated to the colorless phenolphthalein end point, which is about pH 8.4. The cleaning compositions of this invention are further substantially improved, in particular, when provided in the form of an aqueous concentrate, by the addition of at least one organic adjuvant such as a surfactant and a hydrotrope as above described for maintaining the adjuvant in solution. Antifoam agents, corrosion inhibitors, wetting agents, and emulsifiers can also be included with the salts per se or in any solution thereof no matter what the concentration of salts therein to improve cleaning performance or provide stabilization of the cleaning solutions or concentrates. When used according to the above, the compositions do not leave an undesirable residual film and are effective in removing the fluxes and other residue from electronic circuit boards.
BRIEF DESCRIPTION OF THE DRAWINGS
The efficacy of this invention will be better understood by reference to FIGS. 1-7 herein wherein the test results of certain embodiments of the cleaners of this invention are illustrated.
FIGS. 1, 2, 4, and 6 represent typical curves showing the cleaning efficiencies of various concentrations of cleaning solutions resulting from visual testing as described herein.
FIGS. 3, 5, and 7 represent typical curves showing the cleaning efficiencies of various concentrations of cleaning solutions resulting from equilibrium resistivity measurements as described herein.
DETAILED DESCRIPTION OF THE INVENTION
The objects and advantages mentioned above as well as other objects and advantages may be achieved by the compositions and methods hereinafter described.
Essentially, the flux removing compositions of the invention comprise mixtures of alkali metal salts. Accordingly, the term "flux removing compositions" as used herein is intended to define the mixture of essentially active ingredients comprised of the alkali metal salts, and additional performance enhancers such as surfactants, corrosion inhibitors, antifoam agents, hydrotropes and any other adjuvants as hereinafter described.
As hereinlater set forth, the flux removing compositions are preferably formulated into concentrated solutions. The terms "flux removing concentrated solutions" or "concentrates" as used herein define aqueous mixtures containing from about 5 to 45 or more percent by weight of the flux removing compositions with the balance being essentially water.
As used herein the terms "flux removing solutions" or "flux removing solutions in use" is meant to define aqueous mixtures containing from about 0.1 to 1.5 percent by weight of the flux removing composition with the balance comprised essentially of water and which are the solutions employed in the cleaning methods of the invention. Also, as used herein, "flux removing composition" and "cleaning composition" have the same meaning since as stated previously, the electronic circuit assemblies including printed circuit boards and printed wiring boards often contain residues other than fluxes which the compositions of this invention are able to remove and thus "flux removing composition" is intended as an all-purpose cleaner.
In accordance with the invention, additives, adjuvants, or the like, may be included with the flux removing compositions, flux removing concentrates, or the flux removing solutions in use.
The flux removing compositions of the present invention contain mixtures of alkaline salts. Suitable alkaline salts or mixtures thereof for the invention are those capable of providing the desired pH. Most suitable are the salts of potassium and sodium with potassium being preferred. Especially preferred are the carbonates and bicarbonates which are economical, safe and environmentally friendly. The carbonate salts include potassium carbonate, potassium carbonate dihydrate, and potassium carbonate trihydrate, sodium carbonate, sodium carbonate decahydrate, sodium carbonate heptahydrate, sodium carbonate monohydrate, sodium sesquicarbonate and the double salts and mixtures thereof. The bicarbonate salts include potassium bicarbonate and sodium bicarbonate and mixtures thereof.
Also suitable alkaline salts include the alkali metal ortho or complex phosphates. The complex phosphates are especially effective because of their ability to chelate water hardness and heavy metal ions. The complex phosphates include, for example, sodium or potassium pyrophosphate, tripolyphosphate and hexametaphosphates. Additional suitable alkaline salts useful in the cleaning compositions of this invention include the alkali metal borates, acetates, citrates, tartrates, succinates, silicates, and phosphonates, edates, etc.
Generally, the flux removing compositions of the invention will contain the alkaline salts in amounts of from about 60-98 wt. %, preferably from about 70-98 wt. %. The alkaline salts are utilized in combination and in concentrations such that the resultant solutions have a pH of from about 10, or somewhat less, to 13, preferably from about 10 to less than 12 and, more preferably from 10.5 to 10.9. The desired pH of the cleaning solution may depend on the type of flux being removed. Thus, the lower pH range is desirable and effective for removing the more easily removed fluxes. However, a pH of above 11.5 is preferred when removing the more difficult to remove solder paste fluxes. As stated above, it is most desirable that the alkaline salts utilized in combination at the dilution of the wash bath and at the desired pH also have an adequate reserve of titratable alkalinity, at least equivalent to from about 0.2 to 4.5 percent caustic potash (potassium hydroxide), when titrated to the colorless phenolphthalein end point, which is at about pH 8.4 to maintain enhanced performance.
The cleaning compositions of the present invention also contain one or more surfactants in order to enhance the wetting and emulsifying ability of the flux remover and permit maximum penetration thereof within regions of the circuit boards most difficult to clean. The surfactant should be low foaming and may be used also to control foaming, and, thus, double as an anti-foam agent. Suitable surfactants include anionic, nonionic, cationic surfactants or amphoteric surfactants or combinations thereof. A combination of surfactants may be employed. The term "surfactant", as used herein, may include other forms of dispersing agents or aids.
It has been found especially effective to use alkoxylated alcohols which are sold under the tradename of "Polytergent SL-Series" surfactants by Olin Corporation. Also, the polycarboxylated ethylene oxide condensates of fatty alcohols manufactured by Olin under the tradename of "Polytergent CS-1" have also been found effective, especially in combination with the above Polytergent SL-Series surfactants. An effective surfactant which also provides antifoam properties is "Polytergent SLF-18" also manufactured by Olin. A combination of this surfactant together with the above two surfactants has been found to provide excellent cleaning with low foam.
Examples of other suitable surfactants are the block copolymers of ethylene oxide and propylene oxide such as those supplied by the BASF Corporation as Pluronics.
Ethoxylated alcohols with 8 to 20 carbons, such as those containing from 3 to 30 moles of ethylene oxide per mole of alcohol could be used as surfactants in this invention. The monocarboxylated derivatives of these surfactants could also be used.
Sodium or potassium salts of sulfonated benzene or naphthalene derivatives such as alkyl benzene sulfonate, or alkyl naphthalene sulfonate or disulfonate could be used. However, caution would have to be employed since these surfactants might tend to impart excessive uncontrollable foam to the wash water.
The amounts of surfactant utilized is usually small, typically no more than 15 weight percent of the cleaning composition. Preferably, the surfactant comprises from about 0.5-8 weight percent of the composition. The amount of surfactant is not overly critical and as such the amount present in the composition will vary depending on the conditions and the contamination encountered. Thus, higher surfactant levels may be employed if so desired. The surfactant is typically added to the aqueous concentrate and will tend to precipitate therefrom in the absence of additional stabilization.
If the surfactant utilized does not also act as a defoamer, it may be useful to include at least one antifoam agent in any of the flux removing compositions of this invention. The antifoam agent is utilized to prevent the formation of excessive foam caused by the rosin flux/flux removing combination. The presence of foam interferes with the mechanical action of the cleaning equipment used to wash the circuit boards. It is important, if not critical, that the antifoam agent used herein does not act by replacing the flux film with another residual surface film which could affect the performance of the electronic circuit board in use. The antifoam agent could be an agent which solely acts to inhibit foam or it could be the surfactant which helps clean the boards and emulsify soils.
Preferred examples of antifoam agents include compounds formed by condensing ethylene oxide with a hydrophobic base formed by the condensation of propylene oxide with propylene glycol. The hydrophobic portion of the molecule which exhibits water insolubility has a molecular weight of from about 1,500 to 1,800. The addition of polyoxyethylene radicals to this hydrophobic portion tends to increase the water solubility of the molecule as a whole and the liquid character of the product is retained up to the point where polyoxyethylene content is about 50 percent of the total weight of the condensation product. Examples of such compositions are the "Pluronics" sold by BASF--Wyandotte. These compounds also enhance flux removal.
Other suitable antifoam agents that also enhance flux removal include: the polyethylene oxide/polypropylene oxide condensates of alkyl phenols, e.g., the condensation products of alkyl phenols having an alkyl group containing from about 6 to 12 carbon atoms in either a straight chain or branched chain configuration, with ethylene oxide/propylene oxide, the ethylene oxide being present in amounts equal to 1 to 25 moles of ethylene oxide per mole of alkyl phenol and the propylene oxide being present in amounts equal to 1 to 25 moles of propylene oxide per mole of alkyl phenol. The alkyl substituent in such compounds may be derived from polymerized propylene, diisobutylene, octene, or nonene, for example.
Also suitable are those derived from the condensation of ethylene oxide with the product resulting from the reaction of propylene oxide and ethylene-diamine or from the product of the reaction of a fatty acid with sugar, starch or cellulose. For example, compounds containing from about 40 percent to about 80 percent polyoxyethylene by weight and having a molecular weight of from about 5,000 to about 11,000 resulting from the reaction of ethylene oxide groups with a hydrophobic base constituted of the reaction product of ethylene diamine and excess propylene oxide, and hydrophobic bases having a molecular weight of the order of 2,500 to 3,000 are satisfactory.
In addition, the condensation product of aliphatic alcohols having from 8 to 18 carbon atoms, in either straight chain or branched chain configuration, with ethylene oxide and propylene oxide, e.g., a coconut alcohol-ethylene oxide propylene oxide condensate having from 1 to 30 moles of ethylene oxide per mole of coconut alcohol, and 1 to 30 moles of propylene oxide per mole of coconut alcohol, the coconut alcohol fraction having from 10 to 14 carbon atoms, may also be employed.
The antifoam agents of the present invention are preferably employed in the flux removing compositions at about 0.01 to about 10 wt. %. Like the surfactant, the antifoam agents are preferably included in the aqueous concentrates.
To maintain the enhancement of cleaning proficiency achieved by the addition of organic adjuvants such as surfactants, anti-foam agents, etc., and to maintain such adjuvants in aqueous solution in the ultimate cleaning solution during use or, more particularly, in the aqueous concentrate, it is necessary to add a hydrotrope. The term "hydrotrope" herein refers to a substance which has a property of increasing the aqueous solubility of a variety of otherwise insoluble or only slightly water-soluble organic chemicals. For many cleaning applications, in particular for cleaning of circuit and wiring boards as described above it is also important that the cleaning composition be characterized as low foaming inasmuch as foam can inhibit the detersive action of the composition. It is also highly desirable to utilize a hydrotrope which is effective at low concentrations to reduce BOD and COD loadings on water effluents.
In accordance with the present invention, a low foaming hydrotrope which has use in the aqueous cleaning compositions of the present invention comprises alkali metal salts of intermediate chain length monocarboxylic fatty acids. The monocarboxylic fatty acid salts within the scope of the present invention are those with carbon chain lengths between C.sub.7 -C.sub.13. Preferred are the sodium and potassium salts of such fatty acids and more preferred are the sodium and potassium salts of octanoic and nonanoic acids. For maximum biodegradability, the straight chain fatty acids are preferred.
The hydrotrope of the present invention is added in amounts effective to maintain the organic adjuvants as solubilized in aqueous solution, whether such aqueous solution represents the dilution of the wash bath or, more particularly, in the aqueous concentrated form which is generally how the cleaning solutions are supplied to the end user. Thus, the hydrotrope is present in the cleaning compositions in amounts ranging from about 0.2-15 weight percent of the composition, more preferably from about 0.5-6 weight percent. To minimize COD and BOD in the used wash water, it is desirable to use the minimum quantity of hydrotrope needed to solubilize the organic adjuvant and, thus, such minimum amount constitutes at least the "effective amount" of hydrotrope added.
The flux removing compositions of the invention should also contain one or more corrosion inhibitors to prevent corrosion or pitting of the connecting tabs or solder joints, metals or other materials present on the circuit boards being cleaned. Preferably, the corrosion inhibitor is an alkali metal silicate salt with the sodium and potassium silicate salts being most preferred. The alkali metal silicates which are used can be in a variety of forms which can be encompassed generally by the formula [Alk].sub.2 O:SiO.sub.2 wherein [Alk] represents the alkali metal and in which the ratio of the two oxides can vary. Most useful alkali metal silicates will have an [Alk].sub.2 O to SiO.sub.2 mole ratio of between 1:0.5 to 1:4.5. Most preferably, the [Alk].sub.2 O to SiO.sub.2 ratio is between 1:1.6 and 1:4.0. Such silicates also provide additional alkalinity to the wash water to help cleaning. Surprisingly, it has been found that the addition of silicate actually promotes the brightness and shininess of the solder joints. However, other corrosion inhibitors could be used. For sufficient corrosion protection, it is useful to add 0.1 to 10 wt. % of the corrosion inhibitor based on the amount of cleaning composition (without water).
The corrosion inhibitor may comprise part of the composition, be added separately during use to the flux removing solution or may be included in the concentrate. When using the preferred alkali metal silicates as the corrosion inhibitor in the concentrates of this invention, a silicate stabilizer may be required to prevent the silicate from precipitating. A suitable silicate stabilizing system is described in a commonly assigned, copending application D-15792.
The flux removing compositions of the present invention which are comprised of alkaline salts, surfactants, hydrotropes and, preferably, corrosion inhibitors, antifoam agents and other optional adjuvants as set forth above are preferably prepared as aqueous concentrates. Such aqueous flux removing concentrates may contain from about 5 up to about 45, or more, percent by weight of the components which form the cleaning compositions depending on the solubility thereof in water. Preferably, the concentrates contain about 10 to 20 percent, for example, about 15 percent by weight of the flux removing composition.
The balance of the concentrate formulation is water. The dilutions of these concentrates are determined by manufacturing, packaging, shipping, storage, and other factors. It should be understood that the amount of solute in these concentrates is not especially critical.
The flux removing solutions which are employed in the cleaning procedures described herein usually contain from about 0.1 to 15, or more, percent, preferably, from about 0.6 to 15 percent and, more preferably, from about 1 to 3 percent by weight of the flux removing compositions of this invention with the balance being essentially water. The upper limit of concentration of the flux removing composition is not critical and is determined by fabrication conditions, the amount of residues and the difficulty of removing same from the circuit assemblies, etc.
The compositions of this invention are characterized by low environmental impact, unlike the chlorinated hydrocarbon solvents and other materials that had been used prior to this invention for printed circuit board cleaning. For example, the alkali metal carbonate and bicarbonate salts are naturally occurring and environmentally benign. The flux removing compositions of the invention have biological oxygen demand (BOD) and chemical oxygen demand (COD) values (as determined by methods hereinafter described more fully) which are much lower than alternative compositions currently available. Thus, low concentrations of surfactants, anti-foam agents and other organic adjuvants are preferably employed. The hydrotropes of this invention are also characterized by their effectiveness at low concentration. As described in the Examples herein, the flux removing compositions result in very low BODs and CODs in the rinse water allowing the rinse water to be sewered without further treatment. In comparison, terpenes, e.g., limonene result in rinse water having BODs and CODs which may require removal before sewering.
The applicability of the compositions of the invention to various aspects of the printed circuit/wiring board fabrication process can best be understood by a description of a representative assembly process.
The assembly manufacturing process involves the placement of components such as integrated circuits, resistors, capacitors, diodes, etc. on the surface of the board or their insertion through pre-drilled holes. The components are then secured by soldering by mechanical or automatic means. Interspersed with the soldering operations are cleaning procedures and inspections to ensure that tape and solder flux residues than could lead to premature circuit failure do not remain.
For the removal of rosin soldering flux deposits and other residues during printed circuit/wiring board fabrication, the compositions of the invention may be applied to the boards by immersion in dip tanks or by hand or mechanical brushing. Alternatively, they may be applied by any of the commercially available printed wiring board cleaning equipment. Dishwasher size units may be employed, or much larger cleaning systems such as the "Poly-Clean+" and the various "Hydro-Station" models produced by Hollis Automation, Inc. of Nashua, N.H.
Depending upon their design, these washers may apply the flux removing compositions of the invention by spraying with mechanical nozzles or by rolling contact with wetted roller surfaces. The temperature at which the compositions may be applied can range from room, or ambient, temperature (about 70.degree. F.) to about 180.degree. F., preferably, about 140.degree. to 170.degree. F. The flux removing compositions or concentrates are diluted with water to from as low as about 0.1 percent by weight (or volume) concentration and up to about 15 percent by weight.
Once solder flux has been loosened and removed during a period of contact which typically ranges from about 1 to about 5 minutes, but may be longer up to about 10 minutes, the boards are taken from the flux removing solution. Another advantage of the instant invention is that the flux removing solutions need not be flushed with solvents as with the processes of the prior art. Herein, the boards may simply be flushed with water for a period of up to about 2 minutes. Deionized water is preferred. The optimal rinsing time varies according to the kinds of surfactants and the concentrations of the flux removing solutions used and can easily be determined by routine experimentation.
The cleaned boards are then dried, preferably with forced air. Drying is expedited if the air is warmed, preferably to above about 100.degree. F.
The efficacy of rosin soldering flux removal from printed wiring boards is such that the boards meet stringent military specifications for low resistivity after cleaning. For example, the boards meet the Mil-P-28809A standard for low resistivity of the solvent extracts resulting when the contamination has been removed from a circuit board cleaned according to Mil-P-55110C. The resistivity of such solvent extracts after the cleaning of the boards is complete is most easily determined with an Omega Meter. Omega Meter is the registered trademark of Kenco Industries, Inc., Atlanta, Ga., for a microprocessor-controlled contamination test system that rapidly measures changes in resistivity due to contaminating ions.
The results of Omega Meter measurements are expressed in equivalent units of ug NaCl/in.sup.2 or its metric equivalent. According to MIL-P-28809A, the acceptable resistivity value for a cleaned board is equivalent to 2.2. ug NaCl/cm.sup.2 or 14 ug NaCl/in.sup.2, but far better results are routinely obtained after solder flux has been removed with the flux removing solutions of the present invention. A value of about 0.31 ug NaCl/cm.sup.2, or 2.0 ug NaCl/in.sup.2, or even less, is typical.
The flux removing solutions of this invention are also effective in removing other undesirable and deleterious substances and residues. One particularly troublesome substance is the residue left by adhesive tape used during fabrication of the electronic circuit assemblies.
During the process of gold plating connecting tabs to improve corrosion resistance, tin-lead residues must first be removed from the unplated tabs. Removal of these residues is carried out through the use of etching chemicals that can damage other unprotected printed circuit/wiring board components. To protect vulnerable components from the etching chemicals, boards are wrapped on both sides with an adhesive plating tape which forms a shield or splash guard for all but the exposed tab area. The etching chemicals then remove the tin-lead residues on the tabs, a nickel plate is applied as a base for the gold, and gold plating of the tabs is finally carried out. The adhesive plating tape which is maintained in place through all of these etching and plating steps, is then removed. When the tape is removed following the nickel and gold plate step, it is at this point that the cleaning compositions of the invention may most advantageously be used.
Thus, following removal of the tape, a silicone-based and/or rubber-based adhesive residue may remain on the board. This residue may easily be removed by employing the compositions of the invention under the same conditions described above for solder flux removal. The exact operational parameters will be determined by the nature of the adhesive residue and the tenacity with which it adheres to the board, but the conditions described above are generally effective. As in the case of solder flux removal, treatment of the board with the flux removing solutions of the invention is generally followed by water flushing and air drying.
The efficiency of removal of adhesive residues from printed circuit/wiring boards by the compositions of the invention is such that no residues are visible after cleaning. A simple 5-10.times. stereomicroscope can facilitate visual inspection for tape residues following cleaning.
The following non-limiting Examples are provided to further illustrate the present invention. All percentages, unless otherwise noted, are by weight. However, due to the near equivalence of the weight and volume of the materials utilized, volume percent is essentially the same.
EXAMPLES I-IV
To illustrate the cleaning ability of the flux removing compositions of the invention, a series of demonstration printed wiring boards were cleaned in a mechanical cleaning system.
The cleaning composition contained, by weight, 75 percent potassium carbonate, 12.5 percent sodium bicarbonate, and 12.5 percent sodium carbonate monohydrate. Cleaning solutions having various concentrations were prepared.
The cleaning system was a "Poly-Clean+" machine which is manufactured by Hollis Automation, Inc. of Nashua, N.H.
The cleaning sequence comprised the operations of loading, washing, drying, first rinsing, final rinsing and high speed drying carried out in succession. The washing operation utilizing cleaning solutions of the invention was done in two stages, i.e., a first regular wash at spray nozzle manifold which directed a regular wash spray at 40 psig followed by a "hurricane" spray at 80 psig. The cleaning solutions were maintained at 160.degree. F. The rinses were also two stage operations; the first at 40 psig regular rinse followed by an 80 psig "hurricane" rinse with the rinse water having a temperature of 160.degree. F. A final rinse was effected under substantially the same conditions. The circuit boards were subjected to Alpha air knife drying after the washing and final rinse stages. In air knife drying, turbine propelled air shears fluids from the boards' surfaces.
Cleaned and dried boards were evaluated for cleaning efficiency both visually and by an Alpha 600 5MD Omega Meter resistivity measurements.
The visual test method uses a dyed flux and carrier base injected between glass components and a glass board. This provides excellent access for visual inspection. The analysis is further quantified by placing the board and components against a grid. Each block of the grid is then read as being completely clean or containing residue.
The test method utilizes straight flux and carrier from a rosin mildly activated (RMA) flux or paste. It is essentially the solder paste minus the solder. "Carrier" refers to both the flux paste and all other additives included in solder paste, except the solder. This carrier is then injected with red dye so that visual examination can be made more rapidly. The dye does not affect the carrier density or melting properties. The dyed carrier is then injected under the glass components on specially made test boards. RMA solder paste is not considered an aqueous-compatible flux. The test boards are constructed of glass. A 1".times.1" square coupon that simulates the component is mounted onto a glass substrate. The coupon is glued in place by first laying shim stock of the desired standoff height on the glass. Next, the glue is applied and the coupon set in place until it dries. When dry, the shim stock is removed. Six coupons are mounted on a single board at 1/2" spacing. The interior coupons are further shielded from any nozzles by the first coupons in the placement array.
The flux carrier stock is injected under each coupon to entirely fill the inch-square area. Flux is also added to the area surrounding each coupon. The board is IR-reflowed at a typical dwell time of five minutes at reflow temperature. All boards are then stored for 24 hours at ambient temperature prior to cleaning. Reflowing and storing increases cleaning difficulty by allowing the board to cool and the flux carrier to set up.
Prior to reflow, the entire area under the coupon is filled with the dyed flux carrier. During reflow, a small percentage of the area under the coupon develops voids due to expansion and escape of flux volatiles. The area under the coupons filled with baked-on residue is measured prior to cleaning. The application method causes most of the flux to be bridged across the component standoff height. These regions entirely filled with flux are the most difficult to clean. They are also much less likely to occur in actual manufacturing processes since much less flux is applied. For the purposes of this test, however, no special measurement qualification is given to this category. By regarding all areas with flux trapped under them as the same, the test method is made more rigorous. This method is directed toward the measurement of cleaning effectiveness, which is defined as the percentage of residue removed. This aqueous cleaning test method is described more fully in a publication by Janet R. Sterritt, "Aqueous Cleaning Power," Printed Circuit Assembly, Sep. 1989, pp. 26-29.
The results from the cleaning experiments are measured in terms of cleaning effectiveness as follows. The area of reflowed flux carrier is measured prior to cleaning. The test board is then cleaned and the amount of flux residue is visually measured with the aid of a grid pattern. The cleaning effectiveness rating is established by dividing the area still containing flux after cleaning by the total area containing flux prior to cleaning. The measurement technique shows that a completely clean board would result in a cleaning effectiveness rating of 100%. A test board on which three quarters of the initial residue is removed would show a 75% reading.
To make the resistivity measurements, cleaned and dried boards were loaded into a test cell of the instrument and then extracted with a circulating solution of isopropanol: water (25:75), v/v) as specified by MIL-P-55110C and MIL-P-28809A. The resistivity of the solution was measured at a rate of 24 times per minute over a period of about 5-15 minutes until equilibrium was reached, indicating that extraction of board surface contamination was essentially complete. Equilibrium was defined as the point at which the change in measured resistivity of the solution was less than or equal to 5% of any value measured in the previous two minutes.
EXAMPLE I
In this example, demonstration glass printed wiring boards (as developed by Hollis Automation to evaluate cleaning solution and as hereinbefore described more fully) which were reflowed with Alpha flux paste as disclosed above were subjected to the sequence of cleaning operations also disclosed above. Five different concentrations of cleaning solutions, i.e., concentrations of 1.0, 1.7, 2.6, 3.5 and 6.0 percent were employed. Three different standoff distances were employed, viz., 2 mils, 6 mils, and 10 mils, respectively.
The results are shown in FIG. 1 in terms of cleaning effectiveness. These results clearly demonstrate the efficacy of the cleaning solutions of the present invention, especially at concentrations of 2.0 percent and above. The efficacy of the cleaning solutions at standoffs as low as 2 mils is especially noteworthy because of the difficulty in accessing the flux.
EXAMPLE II
Demonstration (H-40) circuit boards (circuit boards produced by Hollis Automation and which ware provided with drilled holes for the passage of leads therethrough) were immersed in flux and wave soldered with Kester 185 and evaluated both visually and on the Alpha 600 5MD Omega Meter for ionic contamination. FIGS. 2 and 3 illustrate the results of such evaluations. The visual evaluation in FIG. 2 again illustrates the effectiveness of the cleaning solutions of this invention. FIG. 3 confirms the visual results by extremely low ionic contamination results. FIG. 3 shows a concentration of about 2 percent results in an acceptable resistivity value of about 14 ug NaCl/in.sup.2 according to MIL-P-28809A. The equilibrium resistivity measurements for the cleaning tests of Example II are also shown in Table I.
TABLE I
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RESISTIVITY MEASUREMENTS OF CLEANED ROSIN
FLUX SOLDERED PRINTED WIRING BOARDS
Cleaner Equivalent NaCl
Test Concentration
Contamination
Number (weight %) (ug/in.sup.2)
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1 1.0 29.6
2 1.7 16.0
3 2.6 12.0
4 3.5 4.0
5 6.0 1.6
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As shown in Table I, the flux removing solutions examined were effective at concentrations below 2.0 percent in producing levels of residual board surface contamination that were far below the MIL-P-28809A requirement of 14 ug NaCl/in.sup.2 equivalent. This is especially noteworthy in view of the configuration of the boards subjected to testing.
EXAMPLE III
Demonstration (H-50) circuit boards (produced by Hollis Automation and having fewer joints, etc. as the H-40 boards) were wave soldered with Kester 185.
Both FIGS. 4 and 5 support the surprising effectiveness of the cleaning solutions of the present invention by both visual testing (FIG. 4) and by Omega Meter testing for ionic contamination (FIG. 5).
Table II also shows the equilibrium resistivity measurements for the cleaning tests of Example III.
TABLE II
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RESISTIVITY MEASUREMENTS OF CLEANED ROSIN
FLUX SOLDERED PRINTED WIRING BOARDS
Cleaner Equivalent NaCl
Test Concentration
Contamination
Number (weight %) (ug/in.sup.2)
______________________________________
1 1.7 5.0
2 2.6 3.6
3 3.5 2.4
4 6.0 0.8
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As shown in Table II, the concentrations examined were all effective in producing levels of residual board surface contamination that were far below the MIL-P-28809A requirement of 14 ug NaCl/in.sup.2 equivalent.
EXAMPLE IV
In this example, demonstration circuit boards similar to those of Example III were evaluated again both visually and for ionic contamination on the Omega Meter. These demo boards were reflowed with Kester R-229RMA paste. The results are shown in FIG. 6 which represents the results from visual testing and FIG. 7 which represents the ionic contamination results.
The equilibrium resistivity measurements for the cleaning tests effected in Example IV are also shown in Table III.
TABLE III
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RESISTIVITY MEASUREMENTS OF CLEANED ROSIN
FLUX SOLDERED PRINTED WIRING BOARDS
Cleaner Equivalent NaCl
Test Concentration
Contamination
Number (weight %) (ug/in.sup.2)
______________________________________
1 1.7 3.0
2 2.6 0
3 3.5 0
4 6.0 0
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As shown in Table III, again, all the concentrations examined were effective in producing levels of residual board surface contamination that were far below the MIL-P-28809A requirement of 14 ug NaCl/in.sup.2.
EXAMPLE V
For this Example separate tests were performed for effluent chemistry and aluminum abrasion in order to exemplify the environmental efficacy of the cleaning compositions of this invention. The effluent tests measured pH, BOD and COD. The methods employed are described in the following: (a) for chemical oxygen demand analysis see "Method for the Chemical Analysis of Water & Wastes, USEPA 600/4 79 020, Method 410.1 and (b) for biological oxygen demand analysis see "Method for the Chemical Analysis of Water & Wastes, USEPA 600/4 79 020, Method 405.1.
Two samples were drawn from the wash tank utilized in the previous Examples I-IV. Sample No. 1 was at a concentration of 2.6 percent by weight and Sample No. 2 was at a concentration of 3.5 percent by weight. No additional dilution was made, e.g., by mixing with the rinse water utilized. Each Sample was also measured for its pH. The results of these tests and measurements are presented in Table IV.
TABLE IV
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Sample Number
1 2
______________________________________
Concentration (wt %)
2.6 3.5
BOD (ppm) 35 33
COD (ppm) 71 71
pH 10.51 10.56
______________________________________
The results from the aluminum abrasion test showed no discoloration on the heat sinks. The evidences no corrosion or other chemical attack of aluminum surfaces.
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