Cleaning process using a non-azeotrope forming contaminated cleaning mixture

Abstract

Printed circuits are cleaned by passing them through a heated non-azeotropic solvent mixture comprising a halogenated hydrocarbon solvent preferably fluorochlorohydrocarbons containing two or three carbon atoms and an auxiliary solvent such as n-butanol, alkoxy alcohols, or cyclic ethers contained in a sump compartment and then rinsing them in a similar body of a solvent mixture but containing a lower proportion of the auxiliary solvent in a rinsing compartment.

Description

This invention relates to a method of cleaning contaminated articles and apparatus therefor.

Contaminated articles, especially printed circuits, on which may be mounted components containing polymeric insulations and markings and having rosin-based soldering fluxes attached thereto do require cleaning. Such articles may be treated with halogenated hydrocarbon solvents especially fluorochlorohydrocarbon solvents such as for instance 1,1,2-trichloro-1,2,2-trifluoroethane in association with auxiliary solvents. In particular the articles may be treated with azeotropic mixtures of the solvents or with mixtures capable of forming an azeotrope. It is usual to employ azeotropic mixtures at the boil. Azeotropic mixtures of the primary solvent with an auxiliary solvent are useful for some purposes but such mixtures may not contain sufficient of the auxiliary solvent to remove the fluxes effectively. Azeotrope-forming mixtures comprising a mixture of the primary solvent with a high concentration of the auxiliary solvent well above that of the azeotrope or mixtures of solvents not containing such an azeotrope might be expected to solve the problem and these are utilised at a relatively cold temperature, that is, near to ambient temperature, e.g., 20.degree.C or less. If desired such treatment may be followed by then bringing the article into contact with a clean cold mixture of the primary solvent and the auxiliary solvent in a separate tank, or by then bringing the article into contact with a liquid produced from a body of the liquid mixture by evaporation and condensation of the vapour the article being within a vapour zone and allowing the liquid to drain off the article. In such methods there may be quite serious loss of solvent due to an adhering layer of cold solvent on articles withdrawn from the plant and/or in that in certain applications a sufficiently high degree of removal of flux from the contaminated article may still not be effected.

Cleaning plants for treating contaminated articles may comprise a liquor tank in which the articles are immersed, the liquor tank being provided with an overflow to a sump tank. This latter tank contains a relatively small amount of liquor which is held at the boil and the purpose of the sump is not to bring the articles into contact with the liquor but to collect dirty solvent. Vapour above the level of the liquor is condensed and returned to the liquor tank thereby ensuring the presence of clean solvent in the liquor tank while impure solvent overflows to the sump. The articles are withdrawn from the liquor tank then through a vapour zone and finally out of the plant. Use of hot or boiling solvent mixtures containing high concentrations of auxiliary solvents of higher boiling point than the primary solvent in both tanks of such plants would not be effective inasmuch as there is depletion of the auxiliary solvent from the liquor tank and increase of auxiliary solvent in the sump. The system is therefore not stable and will not do the cleaning job required of it.

We now provide a method of cleaning contaminated articles which is quite distinct from what has been previously used, which permits effective cleaning of contaminated articles at elevated temperatures and which is stable.

According to the present invention we provide a method of cleaning a contaminated article in a system which comprises using a solvent mixture comprising a halogenated hydrocarbon solvent and an auxiliary solvent which does not form an azeotrope with said halogenated hydrocarbon solvent wherein the article is contacted with a first heated liquid mixture comprising the halogenated hydrocarbon solvent and a proportion of the auxiliary solvent which is at least 2.5 parts by weight per hundred parts by weight of the halogenated hydrocarbon solvent, and the article is subsequently rinsed with a second liquid mixture containing a lower proportion of the auxiliary solvent than is contained in the first liquid mixture.

Preferably there is introduced into the cleaning system, continuously or intermittently from an external source, a supply of the halogenated hydrocarbon solvent and/or the auxiliary solvent in quantities required to maintain the composition of the said first and second liquid mixtures substantially constant.

Preferably in any method of carrying out the process of the invention the second liquid mixture is allso heated.

It is preferred that at least part and preferably all of the condensate from the vapour of the first and second liquid mixtures be returned to a vessel (the rinsing compartment) containing the second liquid mixture, the latter overflowing to a vessel containing the first liquid mixture.

According to a preferred method of carrying out the process of the invention we provide a method of cleaning a contaminated article which comprises immersing the article in said first heated liquid mixture, said mixture being contained in a sump compartment of a cleaning system, and subsequently passing the article into said second liquid mixture in a rinsing compartment of said system, the first and second liquid mixtures having a common vapour zone. Preferably the second liquid mixture is also heated. This second liquid mixture is in equilibrium with the vapour in the vapour zone above the sump and rinsing compartments and with the said mixture in the sump compartment. A plurality of rinsing compartments may be employed if desired.

Suitably the appropriate proportion of the halogenated hydrocarbon solvent and the auxiliary solvent is introduced continuously or intermittently into the sump and/or rinsing compartments of the cleaning system.

In said preferred form of carrying out the present process at least part, and preferably all, of the liquid obtained by condensation of vapours from the common vapour zone is returned to the rinsing compartment. Most suitably it is also arranged that liquid mixture overflows from the rinsing compartment and passes to the sump compartment.

Preferably the first headed liquid mixture contains at least five parts by weight of the auxiliary solvent per hundred parts by weight of the total mixture.

Of the halogenated hydrocarbons which may be used in the present process there may be mentioned fluorochlorohydrocarbons especially those containing two or three carbon atoms, for example 1,1,2,2-tetrachloro-1,2-difluoroethane. 1,1,2-trichloro-1,2,2-trifluoroethane gives especially good results.

Examples of auxiliary solvents which can be employed in association with the halogenated hydrocarbon include alcohols (which do not form azeotropes with the halogenated hydrocarbon solvent), for example, n-butanol; alkoxy alcohols for example 2-methoxy ethanol, 2-ethoxy ethanol and 2-butoxy ethanol; and cyclic ethers, for example 1,4-dioxane. The non-azeotropic solvent mixtures used in the present process are those which do not have the capability of forming an azeotrope as for example they do not include mixtures of 1,1,2-trichloro-1,2,2-trifluoroethane having a high concentration of isopropyl alcohol which on fractionation give an azeotrope of said trichlorotrifluoroethane and a small proportion of isopropyl alcohol.

In the sump compartment when using for example 1,1,2-trichloro-1,2,2-trifluoroethane the concentration of the auxiliary solvent is suitably not greater than 90% by weight of the liquid mixture therein. It is preferred that said concentration of auxiliary solvent is not greater than 70% and it is particularly preferred that this concentration is not greater than 40% by weight of the liquid mixture in the sump compartment.

The concentration of the auxiliary solvent in the rinsing tank is different from that of the liquid mixture in the sump compartment. The liquid mixture in the rinsing compartment, once stable conditions are established, has a concentration of the auxiliary solvent in the primary halogenated hydrocarbon solvent which generally corresponds to 0.5 to 10% by weight of the said liquid mixture. In operating the present process, with supply from an external source of said primary and auxiliary solvent, this second liquid mixture is in equilibrium with the first liquid mixture in the sump compartment and with the vapour in the common vapour zone, that is, the composition of said second liquid mixture is substantially constant while that in the sump compartment and the vapour in the common vapour zone remain substantially constant.

The concentration of the auxiliary solvent in the halogenated hydrocarbon solvent in the sump and rinsing compartments will also differ according to the particular auxiliary solvent used. Thus when using 1,1,2-trichloro1,2,2-trifluoroethane as the primary solvent useful results can be obtained, for example, when using a concentration of 15 to 25% by weight of n-butanol (auxiliary solvent) with reference to the total liquid mixture in the sump compartment and 1 to 1.5% by weight n-butanol in the rinse compartment, both liquids being at the boil. In a similar way there may be usefully employed a concentration of 8 to 12% by weight 1,4-dioxane in the sump compartment and 2 to 4% by weight 1,4-dioxane in the rinse comparting, both liquid being employed at the boil.

The external source of halogenated hydrocarbon solvent and auxiliary solvent supplied to the system will vary according to the particular solvents employed. The two solvents may be introduced separately into the sump and/or rinsing compartments or a preformed mixture of the solvents can be employed. The amount of said solvents introduced into the system is preferably that required to maintain the level constant in the sump compartment.

The solvent mixtures in the sump and rinsing compartments are held at a temperature greater than ambient temperature (the latter in some climates being 20.degree.C). Usually the mixtures are held at a temperature of at least 30.degree.C and preferably at least 40.degree.C. More preferably the solvent mixtures are held at the boiling point which in the case of mixtures using 1,1,2-trichloro1,2,2-trifluoroethane as the primary solvent are often in the range 45.degree.C to 50.degree.C or higher.

The procedure of immersing the contaminated article in said heated liquid mixture (which does not form an azeotrope) in the sump compartment of a cleaning system alone or in association with the further immersion in said heated liquid mixture in the rinsing compartment is inter alia an unusual feature of the present process.

Other solvents or additives can be added to the solvent compositions used in the present process if it is desired to modify their cleaning or solvent power. Suitable additives include cationic, anionic, and non-ionic detergents. Water may also be added in some circumstances, particularly when the compositions include a detergent but this is not essential.

It is usually unnecessary to use stabilisers in the solvent mixtures. However it is possible that stabilisers may be desirable under corrosive conditions, for example those conditions in which the solvent mixtures come into contact with light metals for example zinc and aluminium.

The present process is useful in a wide range of applications for cleaning contaminated articles including removal of tenacious soldering fluxes from electrical equipment. In particular it is useful for removing fluxes from equipment carried on a plastic or resin substrate while not damaging the boards or components thereon.

In another method of carrying out the process of the invention a contaminated article is contacted with said first heated liquid mixture in a cleaning system having one compartment (sump compartment), the article is withdrawn to the vapour zone above said compartment and therein is rinsed with said second liquid mixture which is first obtained by condensation of vapours from said vapour zone. In a modification of that process the contaminated article after immersion in said first heated liquid mixture is withdrawn through and out of said vapour zone, allowed to cool above the vapour zone but within the cleaning vessel and is then returned to said vapour zone where it is rinsed by said second liquid mixture derived by direct condensation of vapours from said vapour zone on to said cooled article.

In another variant of the present process the first and second liquid mixtures are in compartments separate from each other and not possessing a common vapour zone, condensed liquid from vapour above said first heated mixture being returned to the second heated mixture.

One suitable type of apparatus which can be used in the present process is shown in the accompanying drawing (FIG. 1), which represents a schematic view in vertical cross section and are not to scale. In the drawing, a vessel 1 is divided into a sump compartment 2 and a rinse compartment 4 separated by wall 3. Sump compartment 2 contains a body of the first mixture (which does not form an azeotrope) of sufficient depth for the contaminated articles to be immersed therein, and is provided with heater 5. Rinse compartment 4 adjacent to the sump compartment 2 is provided with heater 6 and contains a volume of the second liquid mixture as hereinbefore described, of composition different from that in the sump compartment 2. Vapour zone 7 communicates with both sump and rinsing compartments 2 and 4. Cooling coil 8 is fitted in the vapour zone 7 to condense vapour, together with trough 9. The latter is for collection of condensed liquor and pipe 10 is for return of condensate to rinsing compartment 4. A cold line 11 is attached to the outside of the vessel, through which a cooling medium can be passed to provide extra cooling. An inlet 12 may be provided for feeding a supply from an external source of a mixture of primary and auxiliary solvents as hereinbefore described; it is shown in the drawing as leading into the sump compartment 2, but it can be placed so as to feed into the system at any convenient point. Means (not detailed) are also provided to transport the contaminated articles through the apparatus, the track of the articles being represented by the line beginning at the entry point 13 and leading through sump compartment 2, vapour zone 7 and rinsing compartment 4 and ending at exit point 14.

In operation, the solvent mixtures in both compartments 2 and 4 are heated and the vapours from the mix in the vapour zone 7, condense on the cooling coil 8 and flow back as liquid into rinsing compartment 4, from which excess liquid flows over wall 3 into the sump compartment 2. This continuing flow of liquid and vapour maintains a clean rinsing liquid in compartment 4 and ensures that the dirt and contamination accumulate in compartment 2, from which it may be removed in any convenient manner, for example by periodically removing all or part of the dirty liquid. The levels of liquid are maintained by adding fresh solvent mixture, as required, to make up for any amounts thus removed.

The present invention includes with its scope an apparatus which comprises a vessel having a sump compartment provided with a heating device and a volume of a first non-azeotrope forming liquid mixture as hereinbefore described of depth sufficient to immerse the contaminated articles, a rinsing compartment or compartments adjacent to the sump compartment provided with a heating device or devices and containing a volume of liquid as hereinbefore described and being different from that in the sump compartment, said rinsing compartment being adapted to permit overflow of liquid into the sump compartment, a vapour zone communicating with both sump and rinsing compartments, cooling means adapted to permit condensation of vapour above the sump and rinsing compartments and means adapted to return condensed liquor to the rinsing compartment or compartments.

The following Examples illustrate but do not limit the invention.

EXAMPLE 1

The material to be cleaned consisted of printed circuit boards consisting of resin-bonded paper boards on which adhered a deposit of flux. The flux coating had been produced by applying one brush stroke of flux to the board, followed by 2 minutes drying at 70.degree.C, dip-soldering for 5 seconds at 250.degree.C, followed by a 15-minute wait. The flux was an activated rosin-based fluxes available commercially under the Trade Mark Zeva C4.

The method of cleaning was as previously described and involved dipping the contaminated articles in the sump compartment of a cleaning plant for a period of one minute followed by immersion in an adjoining rinsing compartment for ten seconds. The solvent mixture in the sump consisted of 1,1,2-trichloro-1,2,2-trifluoroethane containing 14.8% by weight of n-butanol. The mixture in the rinsing tank consisted of 98.8% by weight of said trichlorotrifluoroethane and 1.2% by weight of n-butanol. Both liquids were maintained at the boil. Vapour above the liquid levels was condensed by cooling coils and condensate was returned to the rinsing tank. A mixture of said trichlorotrifluoroethane containing 8.4% by weight n-butanol was required to maintain the composition and level of the liquids in the sump and rinsing compartments.

All traces of flux residues were removed by this treatment without damaging the boards.

Example 2

In this Example the solvent mixtures for treating printed circuit boards were as follows:

i. Sump: 1,1,2-trichloro-1,2,2-trifluoroethane containing 9.3% by weight of 1,4-dioxane,

ii. Rinse: 97.3% by weight of said trichlorofluoroethane and 2.7% by weight 1,4-dioxane,

iii. the mixture required to maintain the composition and level of liquids in the sump and rinsing compartment consisted of said trichlorotrifluoroethane containing 9.8% 1,4-dioxane.

All traces of flux residues (as described in Example 1) were removed by this treatment without damaging the boards.

Claims

What we claim is:

1. A method of cleaning a contaminated article which comprises contacting the article with a first contaminated liquid mixture which is heated to at least 30.degree.C., removing contamination from the article and accumulating the removed contamination in the first liquid mixture, said first liquid mixture containing a fluorinated hydrocarbon solvent and an organic auxiliary solvent which together form a non-azeotrope-forming mixture and wherein the concentration of the auxiliary solvent in the mixture is at least 2.5 parts by weight per hundred parts of the fluorinated hydrocarbon solvent, and subsequently rinsing the article with a second liquid non-azeotrope-forming mixture containing the fluorinated hydrocarbon solvent and a proportion of the auxiliary solvent which is lower than that contained in the first liquid mixture, but at least equal to 0.5% by weight of said second liquid mixture.

2. A method as claimed in claim 1 in which there is introduced, continuously or intermittently from an external source a supply of the fluorinated hydrocarbon solvent or the auxiliary solvent in quantities required to maintain the composition of the said first and second liquid mixtures substantially constant.

3. A method as claimed in claim 2 in which the external source of the fluorinated hydrocarbon solvent and the auxiliary solvent are introduced into a sump compartment.

4. A method as claimed in claim 1 in which the second liquid mixture is also heated.

5. A method as claimed in claim 1 which comprises immersing the article in said first heated liquid mixture, said mixture being contained in a sump compartment and subsequently passing the article into said second liquid mixture in a rinsing compartment, the first and second liquid mixtures having a common vapour zone.

6. A method as claimed in claim 5 in which at least part of the condensate from the vapour of the first and second liquid mixtures is returned to the rinsing compartment containing the second liquid mixture.

7. A method as claimed in claim 5 in which liquid mixture overflows from said rinsing compartment to said sump compartment.

8. A method as claimed in claim 5 in which there is employed a plurality of rinsing compartments.

9. A method according to claim 5 in which the fluorinated hydrocarbon is 1,1,2-trichloro-1,2,2-trifluoroethane.

10. A method as claimed in claim 5 in which the auxiliary solvent is n-butanol.

11. A method as claimed in claim 5 in which the auxiliary solvent is an alkoxy alcohol.

12. A method as claimed in claim 11 in which the alkoxy alcohol is 2-ethoxy ethanol.

13. A method as claimed in claim 1 in which the first liquid mixture contains at least five parts by weight of the auxiliary solvent per hundred parts by weight of the total mixture.

14. A method as claimed in claim 13 in which the fluorinated hydrocarbon solvent is a fluorochlorohydrocarbon.

15. A method as claimed in claim 14 in which the fluorochlorohydrocarbon contains two or three carbon atoms.

16. A method as claimed in claim 15 in which the fluorochlorohydrocarbon is 1,1,2-trichloro-1,2,2-trifluoroethane.

17. A method as claimed in claim 1 is which the auxiliary solvent is n-butanol.

18. A method as claimed in claim 1 in which the auxiliary solvent is an alkoxy alcohol.

19. A method as claimed in claim 18 in which the alkoxy alcohol is 2-methoxy ethanol.

20. A method as claimed in claim 18 in which the alkoxy alcohol is 2-ethoxy ethanol.

21. A method as claimed in claim 18 in which the alkoxy alcohol is 2-butoxy ethanol.

22. A method as claimed in claim 1 in which the auxiliary solvent is a cyclic ether.

23. A method as claimed in claim 22 in which the cyclic ether is 1,4-dioxane.

24. A method as claimed in claim 3 in which the concentration of the auxiliary solvent in the sump compartment is not greater than 90% by weight of the liquid mixture therein.

25. A method as claimed in claim 24 in which the concentration of auxiliary solvent in the sump compartment is not greater than 70% of the liquid mixture therein.

26. A method as claimed in claim 25 in which the concentration of auxiliary solvent in the sump compartment is not greater than 40% by weight of the liquid mixture therein.

27. A method as claimed in claim 1 in which the concentration of auxiliary solvent in the second liquid mixture is in the range of 0.5 to 10% by weight of the second liquid mixture.

28. A method as claimed in claim 1 in which the auxiliary solvent is n-butanol and the fluorinated hydrocarbon solvent is 1,1,2-trichloro-1,2,2-trifluoroethane and the concentration of n-butanol in 1,1,2-trichloro-1,2,2-trifluoroethane employed in the first liquid mixture in the sump compartment is 15to 25% by weight with reference to the total liquid mixture.

29. A method as claimed in claim 28 in which the concentration of n-butanol in said trichlorotrifluoroethane employed in the second liquid mixture in the rinsing compartment is 1 to 1.5% by weight with reference to the total liquid mixture.

30. A method as claimed in claim 1 in which the auxiliary solvent is 1,4-dioxane and the fluorinated hydrocarbon solvent is 1,1,2-trichloro-1,2,2-trifluoroethane and the concentration of 1,4-dioxane in said trichlorotrifluoroethane employed in the first liquid mixture in the sump compartment is 8 to 12% by weight with reference to the total liquid mixture.

31. A method as claimed in claim 30 in which the concentration of 1,4-dioxane in said trichlorotrifluoroethane employed in the second liquid mixture in the rinsing compartment is 2 to 4% by weight with reference to the total liquid mixture.

32. A method as claimed in claim 1 wherein the temperature of the first and second liquid mixtures is at least 30.degree.C and up to the boiling point of the mixtures.

33. A method as claimed in claim 32 wherein the temperature of said mixtures is at least 40.degree.C and up to the boiling point of the mixtures.

34. A method as claimed in claim 33 wherein the liquid mixtures have 1,1,2-trichloro-1,2,2-trifluoroethane as the fluorinated hydrocarbon solvent and the temperature of such mixtures is in the range 45.degree.C to 50.degree.C.

35. A method as claimed in claim 1 in which the contaminated article is contacted with said first liquid mixture, withdrawn to a vapour zone above said first liquid mixture and rinsed with said second liquid mixture which is obtained by condensation of vapors from the vapor zone.

36. A method as claimed in claim 35 in which the article is immersed in said first liquid mixture, withdrawn through and out of said vapor zone, allowed to cool above the vapour zone but within a cleaning vessel containing the liquid mixtures and then returned to said vapour zone where it is rinsed by said second liquid mixture derived by condensation of vapours from said vapour zone on to said cooled article.

37. A method as claimed in claim 1 wherein said first and second liquid mixtures are in compartments separate from each other and not possessing a common vapor zone, and vapours from above said first liquid mixture are condensed and returned to the second liquid mixture.