Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
1999-11-24
2003-01-28
Gulakowski, Randy (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S002000, C134S007000, C134S040000, C134S042000
Reexamination Certificate
active
06511546
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an aqueous cleaning composition and a process for the use of such composition in removing organic material from a substrate. This invention will be described initially in connection with cleaning electronic parts whose surfaces are soiled with an organic residue, for example, raw solder pastes and uncured epoxy-based surface-mount adhesives that are present on the surfaces of circuit boards and stencils as a result of the assembly process. It should be understood, however, that the invention has broader applicability, as explained below.
In electronics assembly, soldering is a process for forming a metallurgical bond between components and a printed circuit board to make an electronic circuit. This is done by contacting the board and components with molten solder. The solder connection is formed when the solder solidifies upon cooling. Solder flux is used to remove oxides from the surfaces so that the solder may wet effectively. Solder paste is a viscous combination of flux paste and dry solder particles which are approximately spherical in shape. The solder paste is a substantially stable composition that can be dispensed through an orifice or printed through a stencil. Solder paste is used for soldering surface-mount components, that is, those that are soldered directly onto the surface of a printed circuit board. Surface-mount adhesives are used frequently to hold surface-mount components in place prior to soldering. These adhesives are based usually on silicone, epoxy, urethane, or acrylic chemistry, with epoxy being the most common. Epoxies used for electronics assembly are typically curable compositions comprising a bisphenol A epoxy resin, an aliphatic polyol epoxy resin, a modified amine, fillers, and additives.
Solder paste and/or epoxy surface mount-adhesive is applied to circuit boards by stenciling or dispensing the paste or the adhesive through a small nozzle prior to placement of electronic components onto the boards. Periodically, there will be a “misprint,” that is, a board onto which the paste or adhesive was not dispensed in the proper location. In such cases, it is more economical to clean the board and reuse it rather than dispose of it. It is also necessary to clean the stencils after several printings to keep the apertures free from soil to avoid future aperture clogging, misprints, and consequent reliability problems with the completed circuit assembly.
Desirable cleaning compositions, in addition to being environmentally benign, should be safe to the user. Accordingly, they should be nonflammable under the conditions of use and be non-toxic or have low toxicity, among other attributes well known in the art. Desirable cleaning compositions should be effective also in removing contaminants from the substrates without damaging the substrates or the materials of construction of the equipment in which the composition is being used.
The present invention relates to a cleaning composition which is effective in removing organic residues from substrates which are soiled therewith, particularly surfaces of electronic parts which are soiled with organic-based adhesives and solder paste.
REPORTED DEVELOPMENTS
Prior-art cleaning compositions to clean solder paste and surface-mount adhesive residues often comprise ozone-depleting solvents and high levels of volatile organic compounds (VOCs) and are either flammable or combustible.
Presently in the electronics industry, there is a movement toward the use of cleaning compositions with favorable environmental properties, specifically those with low concentrations of volatile organic compounds (VOCs). Traditionally, the industry has relied heavily on the use of cleaning compositions which are now considered as having unfavorable environmental properties such as high ozone-depletion potentials or high global-warming potentials. Examples of such compositions are 1,1,2-trichlorotrifluoroethane (CFC-113) and 1,1,1-trichloroethane.
Because of their undesirable properties, the aforementioned compositions have been replaced with high vapor-pressure solvents. Examples of such solvents are isopropyl alcohol and aliphatic hydrocarbons, such as, for example, mineral spirits. Such solvents are now considered unfavorable because they have high concentrations of VOCs, which contribute to ground-level ozone formation. At present, the standards established by the State of California are generally regarded as the most stringent in the United States and, therefore, are considered a guide for industry. Under California law, the present standard requires compositions to have a VOC content of less than 50 g/l.
Aqueous cleaning compositions for removing solder paste from stencils are presently known in the industry. For cleaning raw solder pastes, a solvent or water-based detergent or inorganic saponifier, that is, a product with alkaline components and a pH greater than 10, are cleaners of choice. For example, U.S. Pat. No. 5,593,504 to Cala et al. discloses an aqueous alkaline cleaner comprising alkaline salts such as alkali metal carbonates, a surfactant formulation which contains at least one nonionic surfactant, an alkali metal silicate corrosion inhibitor, and stabilizers to maintain the components in aqueous solution.
A commercially available cleaning composition marketed by Smart Sonic Corporation under the name 440-R SMT Detergent is a proprietary highly alkaline cleaner which has a pH of over 12 at its recommended concentrations for cleaning solder pastes and surface mount device (SMD) adhesives.
These and similar cleaners have a variety of shortcomings. First, because they are alkaline and have a pH greater than about 8, the spent material must be neutralized with acid prior to disposal. Furthermore, such alkaline cleaners can be used to clean solder pastes, but not surface-mount adhesives because the water will initiate prematurely the epoxy curing mechanism, as discussed below. Accordingly, it would be necessary for electronics manufacturers who are required to remove not only solder paste, but also surface-mount adhesives, to use multiple cleaning chemistries. Alkaline cleaners may also attack certain metals on circuit boards, components, or stencils.
For cleaning uncured epoxies, water-based products are not used usually because exposure to water tends to cause the epoxy to cure prematurely. Non-hygroscopic solvents such as propylene glycol ethers, aliphatic hydrocarbons, or d-limonene are preferred cleaning solvents. If the epoxy cures on the stencil or misprinted board, it is extremely difficult to remove and would require an aggressive cleaner like that disclosed in U.S. Pat. No. 4,729,797 to Linde et al. The composition described in this patent contains pyridine or substituted pyridines either alone or combined with dimethylsulfoxide (DMSO). Cured epoxy is removed by placing the soiled substrate in a boiling solution (refluxing in a distillation apparatus) of the aforementioned composition. Pyridine is a dangerous chemical with a very low 8-hour exposure threshold limit value of 5 parts per million established by the American Conference of Governmental Industrial Hygienists (ACGIH).
The present invention relates to a cleaning composition which overcomes the disadvantages associated with the use of prior art cleaning compositions, for example, as described above.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a process for cleaning a substrate having thereon a residue which includes a curable organic material. The process comprises contacting the substrate with an aqueous cleaning composition which removes the curable organic material from the substrate while inhibiting the material from curing prior to its removal. The process is particularly suitable for removing uncured surface-mount adhesives, including, for example, epoxy-based materials, and solder paste from electronic components and manufacturing apparatus.
A preferred cleaning composition for use in the present invention is one comprising water, preferably deionized water, and a
Bivins Elizabeth A.
Hayes Michael E.
Prieto Nelson E.
Starkweather Byron A.
Gulakowski Randy
Petroferm Inc.
Synnestvedt & Lechner LLP
Winter Gentle E.
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