GB2063414A

GB2063414A - Cleaning of Printed Circuit Boards

Info

Publication number: GB2063414A
Application number: GB8024255A
Authority: GB
Original assignee: Electrovert Ltd
Current assignee: Electrovert Ltd
Priority date: 1979-07-27
Filing date: 1980-07-24
Publication date: 1981-06-03

Abstract

The invention relates to the cleaning of a workpiece 1 comprising a printed circuit board in which, in addition to impacting the upper and lower surfaces of the workpiece 1 with high velocity jets of cleaning liquid from nozzles (6, 7 Fig. 1, not shown) disposed both above and below a conveyor 3 the workpiece is also subjected to the action of jets of cleaning fluid directed generally horizontally towards the workpiece from further nozzles 9 alongside of the conveyor 3 to clean and flush any blind holes, crevices and recesses which are not cleaned by the top and bottom jets. The further nozzles 9 operate at lower pressure and velocity and may be adjustable or oscillatory. The jets may all be fan or conical and/or as defined in GB 1456729A. <IMAGE>

Description

SPECIFICATION Cleaning of Printed Circuit Boards This invention relates to the cleaning of socalled "printed circuit boards". A typical printed circuit board or PCB is a plastics plate, board or card having copper circuitry imprinted or otherwise applied on one or both surfaces thereof and formed with apertures therethrough for mounting electronic components and/or component sockets and to receive the leads for connection thereof to the printed circuit board.

In this specification references to the "upper surface" and "under surface" of a printed circuit board mean respectively the upper and under surfaces of the board when the latter is supported with the plane of the board substantially horizontal, the upper surface having electronic components and/or component sockets mounted thereon.

After positioning of the components and/or sockets on the printed circuit board with their leads inserted through the apertures therein, the leads are soldered to the printed copper circuitry in a series of steps in which the board is advanced, with the plane of the board substantially horizontal, through the soldering apparatus. In the first step, the under surface of the printed circuit board is fluxed by, for example, passing the printed circuit board through a standing wave of liquid flux. Following fluxing of the under surface of the printed circuit board, the under surface is preheated to evaporate the solvent of the flux and to avoid heat shock during soldering. The preheated circuit board is then brought into contact with molten solder, as by being passed through a standing wave of molten solder.The hot molten solder further heats the printed circuitry and the leads, with the flux being driven up into the holes in the board, followed by the solder, and finally the leads are soldered to the printed circuitry. When the soldering operation is finished, both the under and upper surfaces of the printed circuit board are covered with flux residue, which later can produce corrosion or current creepage problems.

Therefore, it is necessary that this flux residue be thoroughly and completely cleaned off both surfaces of the printed circuit board.

The specification of our British Patent No.

1456729 describes a method and apparatus for cleaning printed circuit boards in which multiplicities of high velocity coherent jets (as therein defined) of cleaning liquid are impacted perpendicularly on both the upper surface and the under surface of a board while it is resting substantially horizontally on a screen conveyor.

The high velocity coherent jets effect a very thorough cleaning of the upper and lower surfaces of the board, including those parts of the upper surface beneath the components thereon, the board being maintained against the screen conveyor during cleaning by the force of the upper multiplicity of jets without the necessity of mechanical holding means.

It has been found that where the workpiece (which term includes any components and/or component sockets thereon) has blind holes, crevises or recesses, such blind holes, crevices or recesses may not be reached or adequately flushed by coherent jets or fan-type spray jets which attack the workpiece from the top and the bottom only, and the object of the invention is to ensure the cleaning of such areas of the surfaces of the workpiece.

To this end, according to this invention, in addition to impinging jets of cleaning liquid (which may be of the coherent and/or spray type) from both above and below on to the upper and lower surfaces of the workpiece as it is advanced on a conveyor, the workpiece is also subjected to the action of further jets (which may be of coherent and/or spray type) of cleaning liquid directed sideways and/or obliquely towards the workpiece being advanced therepast such that said further jets clean and flush any blind holes, crevices and recesses which are not cleaned by the top and bottom jets.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawing, in which~ Fig. 1 is a diagrammatic cross-section of a known cleaner for printed circuit boards, and Fig. 2 is a similar cross-section of a cleaner incorporating additional jets of cleaning liquid in accordance with this invention.

Referring to the drawing, the workpiece 1 to be cleaned is advanced through a chamber 2 on a screen conveyor 3 between upper and lower manifolds 4, 5 respectively for a multiplicity of upper and lower nozzles 6, 7 respectively. The manifolds may comprise tubes which may run lengthwise or transversely of the conveyor, or may comprise tanks in which the nozzles are arranged in the desired array. The nozzles 6, 7 may produce a fanlike, generally conical, spray as shown, or may produce coherent jets of cleaning liquid as described in British Specification No.

1456729.

A typical crevice on the workpiece which is not satisfactorily cleaned with an arrangement as depicted in Fig. 1 is indicated at 8.

In order to clean such difficult areas, the cleaner according to the invention is provided with additional nozzles directed in such a way that the workpiece to be cleaned is exposed to jets (coherent and/or spray type) of cleaning liquid which clean and flush it from all directions, not only from the top and bottom.

As shown in Fig. 2, additional nozzles 9 are provided at or along each side of the coneyor 3 and directed generally horizontally towards the workpieces being advanced thereby. These nozzles 9 may have their own separate manifolds 10, which may be of the tubular or tank type, or may connect with a manifold of rectangular, oval or other shape which is also common to the nozzles 6 and/or 7. The additional nozzles 9 may discharge the cleaning liquid in the form of jets or sprays, and may discharge in fixed directions.

Preferably, however, the directions in which the further nozzles discharge the cleaning fluid are adjustable, either individually or in groups to meet the requirements of the workpieces to be cleaned.

In another embodiment the further nozzles may oscillate or otherwise move automatically to change the direction of discharge during operation of the apparatus.

Means may be provided for regulating the pressure and velocity of the liquid jets produced by said upper and lower nozzles and said further nozzles.

In the case of blind holes on the upper surface of a workpiece, while downward jets may have little effect on the cleaning liquid which has collected therein, a jet directed obliquely to the surface thereof can have a greater effect on displacing liquid from a blind hole with resulting change of liquid in the hole and improved cleaning.

The pressure and velocity of the liquid discharged from the additional nozzles may be appreciably less than that of the liquid discharged from the top and bottom nozzles so that the sideways force of the additional nozzles has no substantial effect on the workpiece being maintained on the conveyor screen by the top jets without the necessity of mechanical holding means. The fact that often the workpiece has wiring ends or pins projecting from its under surface which engage in the mesh of the screen conveyor, also resists movement of the workpieces on the conveyor under the effect of the side jets.

In the following claims, each reference to "jets" of cleaning liquid is intended to include jets of the coherent type (as defined in our aforesaid Patent No. 1456729) and/or jets of the fan or conical spray type.

Claims

1. A method of cleaning a workpiece comprising a printed circuit board which has been fluxed and soldered and has leads of components and/or component sockets connected to the printed circuitry thereon by fluxing and soldering, by moving the workpiece while resting substantially horizontally on a screen conveyor between generally vertically projected jets of cleaning liquid which impinge on the workpiece from both above and below, characterized in that in addition to projecting said jets of cleaning liquid from both above and below on to the upper and lower surfaces of the workpiece as it is advanced on the conveyor, the workpiece is also subjected to the action of further jets of cleaning liquid directed sideways and/or obliquely towards the workpiece being advanced therepast such that said further jets clean and flush any blind holds, crevices and recesses which are not cleaned by the vertically projected jets.

2. A method according to claim 1, characterized in that the pressure and velocity of said further jets of cleaning fluid is less than that of the vertically projected jets.

3. Apparatus for cleaning a workpiece comprising a printed circuit board which has been fluxed and soldered and has leads of components and/or component sockets connected to the printed circuitry thereof by fluxing and soldering, in which the workpiece is advanced by an endless screen conveyor having a substantially horizontal run, on which a workpiece rests, between a plurality of upper nozzles through which jets of cleaning liquid are directed downwardly on to the upper surface of a workpiece moving therepast and a plurality of lower nozzles through which jets of cleaning liquid are directed upwardly on to the lower surface of a workpiece moving therepast, characterized in that, in addition to said upper and lower nozzles, the apparatus also includes further nozzles through which jets of cleaning liquid are directed sideways and/or obliquely towards a workpiece moving therepast.

4. Apparatus according to claim 3, characterized in that said further nozzles are adjustable.

5. Apparatus according to claim 3, characterized in that means are provided for automatically moving the direction in which cleaning liquid is discharged from said further nozzles during operation thereof.

6. Apparatus according to claim 3, 4 or 5, characterized by means for regulating the pressure and velocity of the liquid jets produced by said upper and lower nozzles and said further nozzles.

7. Apparatus according to claim 3, 4, 5 or 6, characterized by said further nozzles being located at opposite sides of said conveyor run.

8. Apparatus for cleaning a workpiece substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.