The present invention relates to a so-called fluid knife, that is to say a device having one or more nozzles for producing a jet of fluid which is wide and thin. The fluid knife is for use in the treatment of boards, particularly though not exclusively printed circuit boards.
Fluid knives find particular application in a solder leveller, that is to say a machine through which printed circuit boards are passed for application and leveling of molten solder, the leveling being carried out by causing the fluid jet to impinge on the board and smooth the solder to a uniform thickness. Both liquid and gas can be passed through the nozzle(s) of a fluid knife to form the jet(s). Fluid knives have other applications in circuit board treatment, for instance in treating, washing and drying them. They also have application in other fields.
In a solder leveller, control of the thickness of the leveled solder is of importance. For this, control of the position of the board with respect to the air knivesxe2x80x94as they usually arexe2x80x94is important. Nevertheless there are a limited number parameters which can be adjusted to control the position of the board.
In a conventional air knife, although the jets are directed against the direction of travel of the board through the air knife, a substantial quantity of the air leaves the air knife in the downstream direction of travel of the board. With the air comes a substantial quantity of oil and solder droplets. Not only is this undesirable in terms of lost materials and contamination of other equipment; but also the biggest problem is that it can lead to contamination of the tinned boards by unwanted solder deposition.
We have surprisingly found that with an asymmetric air knife apparently having a freer air passage downstream, we can direct more oil and solder upstream of the board travel than with a symmetric air knife
The object of the present invention is to utilise this effect to provide an improved fluid knife.
According to one aspect of the invention there is provided a fluid knife for producing a jet of fluid which is wide and thin for use in the treatment of a board, the fluid knife comprising:
a body having a nozzle side with a nozzle surface,
one or more nozzles in the nozzle side of the body for producing the wide, thin jet of fluid,
the nozzle(s) defining by their orientation a mean, jet plane and
the nozzle(s) intersecting the nozzle surface of the body at orifice(s),
the mean jet plane dividing the nozzle surface of the body into:
a wing face and a lip face,
the wing face having a dimension transverse to the jet plane which is greater than the lip face dimension transverse to the jet plane and
the wing face sloping shallowly and the lip face sloping steeply when the body is arranged with the jet plane substantially vertical,
the arrangement being such that in use with a board closely spaced from the orifice(s), the board is drawn towards the fluid knife and the bulk of the fluid passes over the wing and not the lip.
Whilst either of the wing face and the lip face may have non-planar features, they will normally be planar. In this case, the bisector plane of the two planar facesxe2x80x94in its extent through the bodyxe2x80x94is on the wing face side of the jet plane.
In use, in the absence of a board, the fluid jet leaves the nozzle(s) in the jet plane. With a board closely spaced from the orifice(s), the fluid exiting from the nozzles is directed to either side of the plane. We have noted two surprising effects:
1. The air passing between the wing and the board establishes a partial vacuum, or at least a local pressure depression, such that the board is drawn towards the fluid knife despite the jet being directed against the board; and
2. The bulk of the fluid passes over the wing and not the lip despite the lip being narrower.
These effects can be utilised in a solder leveling fluid knife to:
i. Control the separation of the board from the orifices; and
ii. Direct the bulk of the leveling fluid back upstream of the board travel and with it the oil and solder droplets.
Preferably, the fluid knife includes a datum surface defining knife inclination in use with respect to a board path in a board treatment machine, the jet plane being angled to make an acute angle with respect to the board path on the lip face side of the jet plane.
Again, the fluid knife preferably includes a narrow land on either or both of the wing and the lip adjacent the nozzle(s), the narrow land(s) preferably being parallel a board path in use of the fluid knife in a board treatment machine.
Various configurations are possible. In a first, the body of the fluid knife comprises two tips, the tips being attached together and providing respectively the lip and the wing, and the nozzle comprises a single slot between the tips.
Alternatively, the body of the fluid knife comprises a solid block, and the nozzle comprises a series of closely spaced bores extending in the solid block from its nozzle surface to a back surface.
In another alternative:
the body of the fluid knife comprises a single block;
the nozzle comprises a single slot extending only part way through the block from the nozzle surface, and
a plurality of bores are provided in a back surface of the block to communicate with the slot.
The plurality of bores can be arranged in the jet plane defined by the slot; or the plurality of bores can be arranged in two planes angled to either side of the jet plane defined by the slot.