GB2274286A

GB2274286A - Cleaning circuit boards with hydrogen

Info

Publication number: GB2274286A
Application number: GB9300587A
Authority: GB
Inventors: Yue Sern Kho; Kai Hwa Chew; Lock Kee Yip
Original assignee: Singapore Asahi Chemical and Solder Industries Pte Ltd
Current assignee: Singapore Asahi Chemical and Solder Industries Pte Ltd
Priority date: 1993-01-13
Filing date: 1993-01-13
Publication date: 1994-07-20
Anticipated expiration: 2013-01-13
Also published as: WO1994016545A1; GB2274286B; GB9300587D0; AU5821294A

Abstract

A method of cleaning the metal surfaces of an electric circuit board, such as a printed circuit board (PCB), by removing metal oxides therefrom comprises directing diatomic hydrogen onto the surface of the PCB to be cleaned. The hydrogen reacts with metal oxide on the metal surfaces of the PCB to form metal, thereby effectively removing the oxide layer and cleaning the metal surfaces. The cleaned PCB is then introduced to a flow or wave soldering machine. The cleaned PCB may be coated with a mild acidic flux to prevent re-oxidisation of the metal surface prior to soldering. An apparatus for carrying out this method comprises an elongate chamber 1 comprising a hydrogen generator 3, a gas storage tank 4, a sprayer mechanism and a conveyor system 6, positioned adjacent a conventional soldering machine 2. The sprayer mechanism is formed with a plurality of outlets for the hydrogen distributed along its length and directed upwards. This mechanism is mounted on a carriage which moves back and forth along the length of the chamber 1. The conveyor system 6 comprises two elongate rails 17 which extend along sides of the top surface of 1.A jig 18, on which the PCB 19 is mounted, has free ends which are received by rails 17. <IMAGE>

Description

"METHOD OF AND APPARATUS FOR CLEANING METAL SURFACES" This invention relates to the cleaning of metal surfaces and, more particularly, to the cleaning of the metal surfaces of an electric circuit board such as a printed circuit board (PCB).

Conventionally, prior to flow/wave soldering, an electric circuit board such as a PCB is pre-cleaned using either a rosin-based flux or an organic flux. These fluxes introduce contaminants such as chlorides, sulphates, bromides and organic acids which cause corrosion of the PCB. The reliability of the PCB will be affected unless such contaminants are removed completely. Therefore, postcleaning, i.e. cleaning of the PCB after soldering, is a necessary and crucial step in a conventional flow/wave soldering process.

The three types of flux remover which are commonly used in post-cleaning are: a hydrocarbon/solvent wash; an aqueous wash; and a semi-aqueous wash. Each of these flux removers has disadvantages.

The hydrocarbon/solvent wash contains high amounts of CFC compounds which are linked to the depletion of the ozone layer. It is envisaged that in time to come, such compounds will be banned from use. While there may be alternative chemicals which are CFC-free, these are not cost effective.

The aqueous wash does not always assure the reliability of the PCB. Even in the current state of technology so called "water soluble" fluxes still exhibit problems such as the formation of white residues which give an unsightly appearance to the PCB. Additionally, the water used for post-cleaning must be free from contaminants and particles and, after being used in the post-cleaning process, the water must be treated prior to disposal.

Considerable costs can be incurred in providing both an uncontaminated water supply of sufficient quality for use both during and after the aqueous wash and a water treatment process.

The semi-aqueous wash can be used for both rosinbased fluxes and organic fluxes but the cost of using this method is high. The waste produced by this method has to be treated before disposal and the equipment used in this treatment process is also expensive.

Recent technological improvements have enabled the use of a non-clean flux, i.e. a flux which does not require post-cleaning of the PCB, to counter CFC-related solvent problems. However, at present, the non-clean fluxes require a 2% solid content for spraying and dipping applications and a 3% to 5% solid content for foaming applications. These formulations also have their disadvantages.

The use of the 2% solid content flux requires a sprayer to be incorporated in the flow/wave soldering machine which incurs significant costs. Additionally, although the residues that are left behind after soldering do not need to be cleaned, it is not yet known whether these residues will pose any problems since these residues invariably contain carboxylic acids which may cause corrosion of the PCB over time.

In view of the above, this invention seeks to eliminate the post-cleaning process by eliminating the use of highly acidic and corrosive pre-cleaning fluxes through an alternative but effective method of pre-cleaning the PCB.

Accordingly, the present invention provides a method of cleaning an electric circuit board by removing substantially all the metal oxides from the metal surface of the electric circuit board, comprising the steps of: providing a supply of diatomic hydrogen; and directing hydrogen from the supply onto the surface of the electric circuit board to clean the electric circuit board.

In a further aspect, the present invention provides an apparatus for cleaning a electric circuit board including: a supply of diatomic hydrogen; and means for directing the hydrogen from the supply onto the surface of the electric circuit board to remove substantially all the metal oxides from the metal surface of the electric circuit board, thereby cleaning the electric circuit board.

In one embodiment of the present invention, there is provided a method in which diatomic hydrogen gas (which may be generated by a variety of methods described herein) is sprayed onto the PCB surface at a pre-determined pressure and for a pre-determined length of time thereby removing the oxide layer of the metal surface. When this hydrogen cleaning process is completed, a very mild acidic flux (having an acid level less than 0.1% by weight) is applied to the PCB to prevent re-oxidation of the metal surface prior to soldering. As an alternative to providing a mild acidic flux to the cleaned PCB, the hydrogen cleaning process and the flow/wave soldering process may be located in an inert environment to likewise prevent reoxidation of the cleaned surface.

Since the mild acidic flux used is not highly acidic or corrosive, a post-cleaning process is not necessary as no residues of a corrosive nature are left behind.

The use of diatomic hydrogen either in conjunction with a mildly acidic flux or in an inert atmosphere provides an effective pre-cleaning of metal surfaces prior to soldering and is thereby a viable alternative to the conventional approach of using highly acidic and corrosive pre-cleaning fluxes which, by introducing contaminants onto the metal surface, necessitate the need for the postcleaning process, which in many cases uses chemicals containing CFC compounds and/or is costly to perform.

In order that the present invention may be more readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates a front view of a cleaning apparatus embodying the present invention; Figure 2 is a plan view of the cleaning apparatus of Figure 1; and Figure 3 is a side view of the cleaning apparatus of Figures 1 and 2.

Diatomic hydrogen H2 may be generated by a variety of known processes, the most common of which is by electrolysis.

For the purposes of simplicity and cost effectiveness, one embodiment of the present invention employs the process of electrolysis in generating hydrogen.

A specially built electrolysis tank or a commercially available hydrogen generator may be used. Bottled hydrogen in cylinders may also be used although this is an expensive option.

Referring now to Figure 1, a cleaning apparatus embodying one aspect of the present invention includes an elongate cabinet 1 which is positioned adjacent a conventional flow/wave soldering machine 2. The cabinet 1 houses or supports the following four main components: a hydrogen generator 3 consisting of a custom built electrolysis tank 3 in the present embodiment; a gas storage tank 4; a sprayer mechanism 5; and a conveyor system 6.

The cabinet 1 and the components of the cabinet 1 which come into contact with the hydrogen should be made of either a non-metallic material (such as polypropylene) or a metallic material coated with a non-metallic substance.

The hydrogen generator 3 and the gas storage tank 4 are housed within a lower compartment 7 of the cabinet 1 away from the sprayer mechanism 5 and the conveyor system 6. A gas detector 8 is provided within this compartment 7 to detect any leakage of hydrogen gas within the compartment and to operate an alarm in the event of such leakage.

The hydrogen generator 3 delivers hydrogen with a purity of at least 99.99% at a flow rate of at least 250 cc/min at standard temperature and pressure (STP). The hydrogen generator 3 may also include a means for drying the generated hydrogen such as by the use of desiccants.

The hydrogen gas generated by the hydrogen generator 3 is fed via a first tube 10 to the gas storage tank 4 which serves as a buffer reservoir to ensure that a constant quantity of hydrogen is available for supply to a sprayer cartridge 5 of the sprayer mechanism 5. An outlet 11 of the gas storage tank 4 is connected via a second tube 12 to the sprayer cartridge 5.

The capacity of the storage tank 4 is about 0.5 cubic metres. A gauge (not shown) measures the pressure of hydrogen within the storage tank 4 such that, when the pressure in the tank 4 falls below a pre-determined level, the outlet 9 of the hydrogen generator 3 is opened to release more hydrogen into the storage tank 4. Similarly, the input of hydrogen is stopped by closing the outlet 9 when a pre-determined upper pressure level is reached.

A regulating valve (not shown) is provided at the outlet 11 of the storage tank 4 to control the pressure of the hydrogen delivered to the sprayer cartridge 5. The delivery pressure of the hydrogen should be at least 10 psi.

As mentioned above, the outlet 11 of the storage tank 4 is connected via the second tube 12 to the sprayer cartridge 5. The cartridge 5 has a length which spans substantially across the width of the cabinet 1 and is formed with a plurality of outlets 13 (or spray ports) distributed along the length of the cartridge and directed substantially upward. The cartridge 5 is mounted on a carriage 14 which is secured to a plunger 15 of a pneumatic cylinder 16. When operated, the stroke of the pneumatic cylinder 16 moves the carriage 14 back and forth along a portion of the length of the cabinet 1. The length of the stroke of the pneumatic cylinder 16 is controllable and is adjustable in dependence upon the length of the PCB. The time period of the stroke(s) may also be controlled.

Similarly, the number and location of the outlets 13 along the length of the cartridge 5 which are opened is dependent upon the width of the PCB to be cleaned. A PCB as wide as the cabinet 1 will require all the outlets 13 to open whilst a narrow PCB will only require a few outlets 13 to open. When open and supplied with hydrogen from the tank 4, the outlets 13 spray hydrogen upwards towards the PCB.

The above factors are, in turn, determined by both the length of the PCB as well as by the extent of cleaning required.

Referring now to Figures 2 and 3, the conveyor system 6 comprises two elongate guide rails 17 which extend along respective longitudinal sides of the top surface of the cabinet 1. A PCB jig 18, on which the PCB 19 is mounted, has opposite free ends which are adapted to be received in the guide rails 17.

The conveyor system 6 has a drive means which engages the opposite free ends of the PCB jig to convey the PCB jig 18 across the cabinet 1, over the sprayer cartridge 5 and to release the PCB jig 18 onto a receiving conveyor system of the adjacent flow/wave soldering machine 2.

The PCB jig 18 is so formed as to expose substantially all of the downward facing surface of the PCB to the top surface of the cabinet 1 and hence to the cartridge 5.

In operation, the PCB jig 18 is loaded onto the conveyor system 6 and conveyed along the conveyor system 6 up to a pre-determined location substantially opposite the sprayer cartridge 5. A sensor (not shown) indicates the presence of the PCB jig 18 at this predetermined position, stops the conveyor system 6 and simultaneously opens the outlet 11 of the storage tank 4 as well as initiating the operation of the sprayer cartridge 5 and the pneumatic cylinder 16.

Hydrogen gas is then sprayed onto the exposed bottom surface of the PCB for a predetermined length of time at the predetermined pressure.

The hydrogen gas interacts with metal oxide on the metal surface of the PCB to form metal, thus effectively removing the oxide layer. The process involved is as follows M"+ + nH2 --- M + nH+ where M is metal compound.

eg. Cu2+ + H2 --- Cu + 2H+ The stroke length of the pneumatic cylinder 16 and the number of outlets 13 which are open along the length of the sprayer cartridge 5 may either be predetermined or controlled by sensors in accordance with the dimensions of the PCB to be cleaned and the extent of cleaning required.

Having completed the cleaning operation, the sprayer cartridge 5, the pneumatic cylinder 16 and the storage tank outlet 11 are all de-activated. The drive means of the conveyor system 6 is re-activated to move the PCB jig 18 onwards to be passed (either manually or automatically) onto the conveyer system of the adjacent flow/wave soldering machine 2.

Upon entering the flow/wave soldering machine, the PCB is immediately subjected to a fluxing process. The flux used in this fluxing process need only be a mildly acidic flux containing less than 0.1% acid by weight. This fluxing process serves the following functions: to coat a thin protective layer on the PCB to prevent re-oxidation; to improve wettability during a following soldering step; and to clean out any other contaminants, finger prints or oils left on the PCB during handling and which may not have been removed during hydrogen cleaning.

As an alternative to the fluxing process, both the cabinet and the adjacent flow/wave soldering machine may be located in an inert environment. Thus, the PCB, after being cleaned by the hydrogen, can remain clean without the danger of re-oxidation and can therefore proceed directly onto a soldering process.

The mild acidic flux (as mentioned above) is of a very low acid content of less than 0.1% by weight and does not cause corrosion. Thus, the use of highly acidic/corrosive fluxes prior to soldering has been eliminated.

The alternative of conducting the process in an inert environment also avoids the problem of corrosion caused by highly acidic corrosive fluxes and does not require their use.

Since no contaminants are introduced into the solder pot due to either the use of an inert environment or the low acid content of the flux, the formation of dross is reduced.

By eliminating the post-cleaning process, the problems normally associated with using post-cleaning solvents or chemicals are also eliminated, particularly those relating to the use of CFC-related solvents which are ozone depleting substances.

The present invention can be implemented inexpensively. The major cost component, the hydrogen generator or electrolysis tank, can be sourced inexpensively. The pre-cleaning flux, being made from very dilute or mild chemicals, is also inexpensive. On the other hand, the elimination of post-cleaning will achieve substantial cost-savings.

Claims

CLAIMS:

1. A method of cleaning an electric circuit board by removing substantially all the metal oxides from the metal surface of the electric circuit board, comprising the steps of: providing a supply of diatomic hydrogen; and directing hydrogen from the supply onto the surface of the electric circuit board to clean the electric circuit board.

2. A method according to Claim 1, wherein the electric circuit board is a printed circuit board (PCB).

3. A method according to Claim 2, wherein the cleaning of the PCB is performed as a pre-cleaning step prior to introducing the cleaned PCB to a flow or wave soldering machine.

4. A method according to Claim 3, wherein the cleaned PCB is coated with a mild acidic flux to prevent reoxidisation of the metal surface prior to soldering.

5. A method according to Claim 4, wherein the mild acidic flux has an acid level of less than 0.1% by weight.

6. A method according to Claim 3, wherein the cleaning process is carried out in an inert atmosphere to prevent re-oxidisation.

7. A method according to any one of Claims 2 to 6, wherein the hydrogen is dried before being directed onto the surface of the PCB.

8. A method according to Claim 7, wherein the hydrogen is dried by the use of desiccants.

9. A method according to any one of Claims 2 to 8, wherein the hydrogen is directed onto the surface of the PCB by means of spraying.

10. A method according to Claim 9, wherein the hydrogen is sprayed upwardly towards the PCB.

11. A method according to Claim 9 or 10, wherein the hydrogen is-sprayed at a pressure greater than or equal to 10 psi (68,950 N/m2).

12. A method according to any one of Claims 9 to 11, wherein the duration for which the hydrogen is sprayed onto the PCB is controlled.

13. A method according to any one of Claims 9 to 12, including controlling the area covered by the spraying.

14. A method according to Claim 13, wherein the area covered by the spraying is controlled by selectively operating one or more spraying outlets.

15. A method according to Claim 14, including conveying the PCB into a position in proximity to one or more of the spraying outlets and holding the PCB in a jig.

16. A method according to any preceding claim, wherein the hydrogen is stored in a tank prior to being directed to the surface of the electric circuit board.

17. A method according to Claim 16 including sensing the pressure of the hydrogen within the tank and supplying more hydrogen to the tank when the pressure drops below a pre-determined level.

18. A method according to any preceding claim, comprising detecting the presence of gas within a compartment housing the supply of hydrogen and providing an alarm signal if gas is detected.

19. An apparatus for cleaning an electric circuit board including: a supply of diatomic hydrogen; and means for directing the hydrogen from the supply onto the surface of the electric circuit board to remove substantially all the metal oxides from the metal surface of the electric circuit board, thereby cleaning the electric circuit board.

20. An apparatus according to Claim 19 in combination with a wave or flow solder machine.

21. An apparatus according to Claim 19 or 20, wherein the electric circuit board is a printed circuit board (PCB).

22. An apparatus according to any one of Claims 19 to 21, wherein the hydrogen supply comprises an electrolytic hydrogen generator.

23. An apparatus according to any one of Claims 19 to 21, wherein the hydrogen supply comprises a cylinder containing bottled hydrogen.

24. An apparatus according to any one of Claims 19 to 21, wherein the hydrogen supply provides hydrogen of a purity of at least 99.99% at a flow rate of at least 250cc per minute at standard temperature and pressure.

25. An apparatus according to any one of Claims 19 to 24, wherein any component of the apparatus which comes into contact with hydrogen is either non-metallic or coated with a non-metallic substance.

26. An apparatus according to any one of Claims 19 to 25, wherein the means for directing the hydrogen comprises one or more outlets which are mounted along the length of a spray cartridge.

27. An apparatus according to Claim 26, wherein the spray cartridge is reciprocated by actuator means.

28. An apparatus according to Claim 27, wherein the actuator means is a piston and cylinder device, the stroke of which is controlled and dependent upon the size of the electric circuit board being cleaned.

29. An apparatus according to any one of Claims 26 to 28, wherein the spray cartridge has an adjustable length which can be controlled in dependence upon the width of the electric circuit board being cleaned by opening or closing the one or more outlets.

30. An apparatus according to any one of Claims 26 to 29, wherein the electric circuit board is positioned in proximity to the one or more outlets of the hydrogen supply by a conveyor system, the electric circuit board being supported by two opposite free edges on a rail or rails of the conveyer system.

31. An apparatus according to any one of Claims 26 to 30, wherein the one or more outlets are operated to open or close by a sensor detecting the position and size of an electric circuit board to be cleaned.

32. An apparatus according to any one of Claims 19 to 31, wherein the hydrogen from the hydrogen supply is stored in a tank prior to being directed onto the electric circuit board.

33. An apparatus according to claim 32, wherein the tank has a volume of at least 0.5 cubic metres.

34. An apparatus according to Claim 32 or 33, wherein the apparatus is housed in a cabinet and the supply of hydrogen and the tank are housed in a separate compartment of the cabinet.

35. An apparatus according to Claim 34, wherein the compartment is remote from the means for directing the hydrogen onto the electric circuit board.

36. A method of cleaning an electric circuit board by removing substantially all the metal oxides from the metal surface of the electric circuit board, comprising the steps of: providing a supply of diatomic hydrogen; and bringing the diatomic hydrogen and the surface of the electric circuit board to be cleaned into contact with one another to clean the electric circuit board.

37. Any novel feature or combination of features disclosed herein.

37. A method of cleaning an electric circuit board substantially as hereinbefore described with reference to the accompanying drawings.

38. An apparatus for cleaning an electric circuit board substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

39. Any novel feature or combination of features disclosed herein.

Amendments to the claims have been filed as follows 1. A method of cleaning an electric circuit board by removing substantially all the metal oxides from the metal surface of the electric circuit board, comprising the steps of: providing a supply of diatomic hydrogen; directing hydrogen from the supply onto the surface of the electric circuit board to clean the electric circuit board; and introducing the cleaned PCB to a flow or wave soldering machine.

3. A method according to Claim 1 or 2, wherein the cleaned PCB is coated with a mild acidic flux to prevent re-oxidisation of the metal surface prior to soldering.

4. A method according to Claim 3, wherein the mild acidic flux has an acid level of less than 0.1% by weight.

5. A method according to Claim 1 or 2, wherein the cleaning process is carried out in an inert atmosphere to prevent re-oxidisation.

6. A method according to any preceding claim, wherein the hydrogen is dried before being directed onto the surface of the electric circuit board.

7. A method according to Claim 6, wherein the hydrogen is dried by the use of desiccants.

8. A method according to any preceding claim wherein the hydrogen is directed onto the surface of the electronic circuit board by means of spraying.

9. A method according to Claim 8, wherein the hydrogen is sprayed upwardly towards the electronic circuit board.

10. A method according to Claim 8 or 9, wherein the hydrogen is sprayed at a pressure greater than or equal to 10 psi (68,950 N/m2).

11. A method according to any one of Claims 8 to 10, wherein the duration for which the hydrogen is sprayed onto the electronic circuit board is controlled.

12. A method according to any one of Claims 8 to 11, including controlling the area covered by the spraying.

13. A method according to Claim 12, wherein the area covered by the spraying is controlled by selectively operating one or more spraying outlets.

14. A method according to Claim 13, including conveying the PCB into a position in proximity to one or more of the spraying outlets and holding the PCB in a jig.

15. A method according to any preceding claim, wherein the hydrogen is stored in a tank prior to being directed to the surface of the electronic circuit board.

16. A method according to Claim 15 including sensing the pressure of the hydrogen within the tank and supplying more hydrogen to the tank when the pressure drops below a pre-determined level.

17. A method according to any preceding claim, comprising detecting the presence of gas within a compartment housing the supply of hydrogen and providing an alarm signal if gas is detected.

18. An apparatus for cleaning an electronic circuit board including: a supply of diatomic hydrogen; means for directing the hydrogen from the supply onto the surface of the electronic circuit board to remove substantially all the metal oxides from the metal surface of the electric circuit board, thereby cleaning the electric circuit board; and means to subsequently introduce the cleaned electric circuit board to a wave or flow solder machine.

19. An apparatus according to Claim 18, wherein the electric circuit board is a printed circuit board (PCB).

20. An apparatus according to any one of Claims 18 or 19, wherein the hydrogen supply comprises an electrolytic hydrogen generator.

21. An apparatus according to any one of Claims 18 to 20, wherein the hydrogen supply comprises a cylinder containing bottled hydrogen.

22. An apparatus according to any one of Claims 18 to 21, wherein the hydrogen supply provides hydrogen of a purity of at least 99.99% at a flow rate of at least 250cc per minute at standard temperature and pressure.

23. An apparatus according to any one of Claims 18 to 22, wherein any component of the apparatus which comes into contact with hydrogen is either non-metallic or coated with a non-metallic substance.

24. An apparatus according to any one of Claims 18 to 23, wherein the means for directing the hydrogen comprises one or more outlets which are mounted along the length of a spray cartridge.

25. An apparatus according to Claim 24, wherein the spray cartridge is reciprocated by actuator means.

26. An apparatus according to Claim 25, wherein the actuator means is a piston and cylinder device, the stroke of which is controlled and dependent upon the size of the electric circuit board being cleaned.

27. An apparatus according to any one of Claims 24 to 26, wherein the spray cartridge has an adjustable length which can be controlled in dependence upon the width of the electric circuit board being cleaned by opening or closing the one or more outlets.

28. An apparatus according to any one of Claims 24 to 27, wherein the electric circuit board is positioned in proximity to the one or more outlets of the hydrogen supply by a conveyor system, the electric circuit board being supported by two opposite free edges on a rail or rails of the conveyer system.

29. An apparatus according to any one of Claims 24 to 28, wherein the one or more outlets are operated to open or close by a sensor detecting the position and size of an electric circuit board to be cleaned.

30. An apparatus according to any one of Claims 18 to 29, wherein the hydrogen from the hydrogen supply is stored in a tank prior to being directed onto the electric circuit board.

31. An apparatus according to Claim 30, wherein the tank has a volume of at least 0.5 cubic metres.

32. An apparatus according to Claim 30 or 31, wherein the apparatus is housed in a cabinet and the supply of hydrogen and the tank are housed in a separate compartment of the cabinet.

33. An apparatus according to claim 32, wherein the compartment is remote from the means for directing the hydrogen onto the electric circuit board.

34. A method of cleaning an electric circuit board by removing substantially all the metal oxides from the metal surface of the electric circuit board, comprising the steps of: providing a supply of diatomic hydrogen; and bringing the diatomic hydrogen and the surface of the electric circuit board to be cleaned into contact with one another to clean the electric circuit board.

35. A method of cleaning an electric circuit board substantially as hereinbefore described with reference to the accompanying drawings.

36. An apparatus for cleaning an electric circuit board substantially as hereinbefore described with reference to and as shown in the accompanying drawings.