Cleaning compositions for solid surfaces – auxiliary compositions – Cleaning compositions or processes of preparing – For cleaning a specific substrate or removing a specific...
Reexamination Certificate
1995-06-06
2001-02-13
Del Cotto, Gregory R. (Department: 1751)
Cleaning compositions for solid surfaces, auxiliary compositions
Cleaning compositions or processes of preparing
For cleaning a specific substrate or removing a specific...
C510S170000, C510S175000, C510S177000, C510S178000, C510S407000, C510S408000, C510S409000, C510S411000, C510S412000, C134S002000, C134S026000, C134S030000, C134S031000, C134S038000, C134S040000, C134S042000
Reexamination Certificate
active
06187729
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processes for removing adherent soils from substrates. More specifically, the invention relates to effective cleaning processes which utilize non-azeotropic mixtures of solvating agents and rinsing agents.
The present invention provides cleaning methods having characteristics and features which are highly desirable in numerous and varied commercial applications. For example, most metallic components are treated with an oil or other processing agent during the fabrication process, and this oil must be removed before the component is installed in the finished product. It is also frequently required that excess rosin flux must be removed from printed circuit boards before the boards are acceptable for use. The present processes are adaptable for use in these and many other applications. Thus, although the present invention is described initially herein in connection with its applicability to the cleaning of printed circuit boards, it will be appreciated from a reading of the entire application that the invention has wider application.
Printed circuit boards typically consist of a rigid or flexible sheet of fiberglass-reinforced dielectric plastic having electrical contacts and conductors on one or both sides thereof. Electrical components are electrically connected to these connectors and/or contacts using any one of a number of soldering techniques. Most of the soldering techniques currently used in commercial manufacturing processes include the step of coating the entire circuit side of the printed circuit board, or at least a portion thereof, with a solder flux prior to carrying out the actual soldering step. Rosin flux is commonly used alone or in combination with activating amine-based additives, such as amine hydrochloride, to clean the conductive metal parts and to promote strong mechanical and electrical bond with the solder.
After the soldering process is complete, the presence of residual flux on the printed circuit board is detrimental to the operability of the electrical circuitry and components contained on the board. Accordingly, any residual flux present on the board must be removed.
REPORTED DEVELOPMENTS
The techniques used to remove adherent residues from printed circuit boards are numerous and varied, ranging from simple brushing of the board with solvent to relatively sophisticated emulsion cleaning. See Leonida,
Handbook of Printed Circuit Design, Manufacture, Components and Assembly,
Chapter 9, pp. 464-489 (1981). One of the most widely used cleaning techniques is known as vapor degreasing or vapor-liquid-vapor cleaning. According to this process, the printed circuit board is contacted in succession by: (1) relatively hot solvent-containing vapors; (2) by relatively cool solvent-containing liquid; and (3) finally by relatively hot solvent-containing vapor.
Equipment typically used in connection with vapor degreasing consists of a two-section tank. The first section of the tank contains boiling solvent and the second section of the tank contains relatively cool, non-boiling solvent. Refrigerated coils are provided in the vapor space above the boiling solvent, and solvent vapor condenses onto the coils and is transferred to the cold solvent in the second section of the tank. A certain portion of the cold liquid solvent is returned to the first section of the tank to maintain a sufficient quantity of boiling liquid solvent. The cleaning process typically proceeds by first introducing a relatively cold, soil-containing circuit board into the vapor space above the boiling solvent. Due to the temperature difference between the circuit board and the solvent vapor, solvent condenses on the printed circuit board and achieves a solvating action on the residual rosin fluxes. The condensed vapors and the rosin fluxes solvated thereby are allowed to return to the first section of the tank. After the desired amount of cleaning action has occurred, the board is then moved to the second section of the tank and immersed in the cold solvent, thereby cooling the board and effecting any final cleaning which may be desired. The relatively cool board is then introduced once again into the relatively hot vapor space, where condensing vapors perform a final rinse on the board. Such vapor-liquid-vapor cleaning processes are described on pages 475-477 of Leonida. According to the teachings of Leonida, vapor degreasing processes require solvents having a boiling point of below about 75° C. (167° F.).
The types of materials that have heretofore been used for the removal of residual fluxes from printed circuit boards are also numerous and varied. For example, chlorinated hydrocarbons, aliphatic hydrocarbons, alcohols, and terpenes are known rosin flux solvents. Because of their high vapor pressures and good solvating ability in the vapor state, chlorinated hydrocarbons, including chlorofluorocarbons, have been widely used in vapor degreasing type processes. However, chlorinated hydrocarbons are generally relatively poor solvents for any ionic residues which may be present on the printed circuit board or other substrate to be cleaned. See Leonida, page 466. For this and other reasons, chlorinated hydrocarbons in general, and chlorofluorocarbons in particular, have sometimes been used in combination with other low-boiling solvents.
Mixtures comprising chlorinated hydrocarbons and other low-boiling solvents have been suggested for use in vapor degreasing type processes. In general, however, the prior art has failed to suggest, and in some cases has even discouraged, the use of mixtures having components with widely different vapor pressures in vapor degreasing processes. This has been especially true for prior art directed to the use of solvent mixtures which include chlorinated hydrocarbons, as illustrated, for example, in U.S. Pat. No. 3,640,884—Scholfield et al:
Although mixtures of solvents have been used for [removing rosin flux from printed circuit boards] they have the disadvantage that they boil over a range of temperatures and consequently undergo fractionation in vapor degreasing or ultrasonic applications which are open to the atmosphere.
(Col. 1, lines 47-51). Accordingly, Schofield requires the use of tetrachlordifluoroethane in the form of binary and ternary azeotropic mixtures for use as solvents in vapor degreasing processes. Other patents which disclose the use of azeotropic mixtures containing chlorinated hydrocarbons as solvents in vapor degreasing type cleaning processes are U.S. Pat. Nos. 3,960,746—Gorski; U.S. Pat. No. 3,733,218—Begun; and U.S. Pat. No. 4,062,795—Hutchinson.
While chlorinated hydrocarbons and mixtures containing chlorinated hydrocarbons have been used widely and with advantage as solvents in printed circuit board cleaning processes, the use of such materials has recently been strongly discouraged for environmental reasons. In particular, the use of chlorinated hydrocarbons, including chlorofluorocarbons, has been severely criticized by ecologists because the dispersal of such materials into the atmosphere has been found to damage the ozone layer. For this reason, the use of chlorinated hydrocarbons has been greatly restricted and, in some situations, prohibited entirely. Thus, despite their attractive solvency characteristics, chlorinated hydrocarbons are no longer the solvent of choice in vapor degreasing processes. Thus, a need now exists for an alternative to the typical prior art degreasing processes, which rely heavily on chlorinated hydrocarbons for effective cleaning.
SUMMARY OF THE INVENTION
Applicants have discovered cleaning processes in which adherent soils are effectively removed from substrates. In particular, the present processes generally comprise (a) contacting the substrate with a liquid solvating agent and, preferably simultaneously and/or subsequently, (b) contacting the substrate with a rinsing agent, the ratio of the vapor pressure of said rinsing agent to the vapor pressure of said solvating agent being relatively high, preferably no less than about 20.
Accordin
Hayes Michael E.
Hosman Donald P.
Hrebenar Kevin R.
Sell Robert D.
Del Cotto Gregory R.
Petroferm Inc.
Synnestvedt & Lechner LLP
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