The invention concerns a method for cleaning printed circuit boards, with any related components, by which, while being submerged in a cleaning fluid, the board is washed by spraying a cleaning fluid at high pressure on to one or both sides of the board, and where after washing, the board is removed from the bath for rinsing and drying.

In the production of printed circuit boards, i.e. printed boards on to which are mounted electronic components, a number of soldering operations are carried out, where a variety of fluxes are used which have to be cleaned of the boards before they are put to use. This cleaning process has traditionally been performed by means of chlorofluorocarbon cleaning agents (CFC cleaning agents) , and various types of washing devices have been used, including the use of brushes where the boards are conveyed through a fully automatic process.

In recent years the dangers of using this kind of solvent have become increasingly apparent, leading to a development aimed at finding alternative cleaning methods and cleaning agents.

Thus alcohol-based solvents on an isopropanol-ethanol basis have been found to be well-suited for use as alternative cleaning agents and such alcohol-based solvents have also been used together with the known CFC solvents in order to reinforce the cleaning power. A drawback with the alcohol-based agents, however, is that they have a reduced cleaning efficiency, so that substantially larger quantities of cleaning agents have to be used in order to achieve the same cleaning effect. Moreover, the alcohols have the disadvantage of being highly inflam able.

The object of the present invention has been to provide a new cleaning method with related apparatus which enables alcohol- based cleaning agents to be used in more limited quantities than is usual for this type of solvent, the cleaning method

being so efficient that a satisfactory cleaning effect is achieved in each case. A further object of the invention is to provide a cleaning method for printed circuit boards, which ensures that those areas of the board which are situated under the mounted components can also be safely cleaned, resulting in a complete cleaning of the boards.

These objects are achieved by a method together with an apparatus which are characterized by the features in the claims presented.

From DE-OS a printed circuit board cleaner is known where a slotted nozzle is used for cleaning printed circuit boards. However, this differs significantly from the invention. Firstly, it concerns only the cleaning of holes in printed circuit boards. Thus there are no problems with projecting components where a washing effect has to be achieved under the components. The German machine cannot be used for printed circuit boards. Moreover, a slotted nozzle is used, which will mean that the cleaning will be performed at low pressure. If high pressure has to be used through nozzles of this kind it will involve an enormous consumption of fluid which will render the equipment unusable. Thus the apparatus can only be used for printed circuit boards and does not give the desired cleaning effect.

As a cleaning fluid in the invention alcohols, mainly of the ethanol and isopropanol type, are used. The method is based on the principle of high pressure cleaning under the surface of the fluid in the cleaning bath. By means of high pressure spraying the spraying pressure will be maintained even when the stream of fluid strikes the board surface. Thus the kinetic energy is not utilized exclusively at the point of impact of the jet, but remains effective as the liquid is forced in under the components since it follows the surface of the board, thus also cleaning.the space between the components and the board surface where a capillary effect will normally prevent an effective flushing. The special feature of the invention is

therefore that nozzles are used arranged in groups or rows which are arranged in order to achieve a highly effective jet effect along the surface of the board even in otherwise inaccessible places. The distance between the nozzle and the board surface is adjusted optimally, it being possible to design the nozzle panel to be movable and adaptable to various board dimensions, and the nozzle panel can be adapted to suit the purpose of the operation with regard to the distance between the nozzles.

In the following section the invention will be described in more detail by means of an embodiment which is illustrated in the drawings:

Fig. 1 is a schematic sketch illustrating the principles for the design of an apparatus for washing printed-circuit-boards in which the invention is incorporated,

fig. 2 and fig. 3 illustrate the jet from a nozzle according to the invention, with impact against the board and with the flow path for the fluid movement viewed face on to the plate and at right angles to the plate respectively.

The apparatus according to the invention as illustrated schematically in fig. 1 comprises a washing zone A, a rinsing zone B, a cooling zone C, a drying zone D together with various valves V and pumps P which are arranged in the manner illu¬ strated. A printed-circuit-board (PC-board) which is to be cleaned is conveyed vertically along a path 1 and down into the apparatus, then conveyed through the drying zone, the cooling zone and the rinsing zone B down into the actual washing zone A in which the invention is incorporated. The washing zone A is composed of a closed vessel which is filled with cleaning fluid or washing fluid to the desired level which covers a set of nozzles which in the illustrated embodiment are arranged on both sides of. the PC-board insertion zone. The nozzle plates are indicated by 2. In these nozzle plates are formed or fitted a desired number of nozzles from which jets of cleaning fluid

can be sprayed on to a PC-board which is indicated in the drawing by 3 and which is placed symmetrically between the plates 2. The jets which also indicate the position of the nozzles are indicated by 4. The board is thus lowered into the fluid bath in that it is secured in a special way and held between the nozzle plates 2 from which cleaning fluid is sprayed in a jet 4 against the surface of the PC-board 3. When the jets 4 strike the plate, since this jet spraying is performed below the level of the fluid, they will be deflected parallel with the surface of the plate 3 and thus clean it in its entirety, since the number and positioning of the nozzles and jets are arranged to suit this purpose and to ensure a complete coverage of the whole surface. Thus a highly effective cleaning of the surface is achieved with a stream of cleaning fluid along the board which ensures the removal of undesirable flux. The spraying is performed at a pressure of approximately 60 bar. The temperature of the cleaning fluid is adapted to suit the type of fluid and the cleaning purpose. With a machine of this kind in which the boards are lowered, cleaned and raised it will be possible to clean between 50 and 100 boards per hour in one chamber.

After cleaning in washing zone A the board is raised, rinsed in zone B, cooled in zone C, since the board has become hot in the fluid bath, dried in zone D, in that means are arranged for retention of alcohol by means of an airstream specially for this purpose. These are elements which do not constitute any part of the invention and thus are not described in more detail here. The board is then withdrawn. The cleaning fluid in the main cleaning bath is circulated through a coarse filter before being conveyed to a high pressure pump and through a fine filter before it reaches the cleaning nozzle. The addition of new fluid for dilution/maintenance of the cleaning fluid is carried out from the rinsing nozzles in zone B. The new fluid for rinsing is preheated by passing it through the heat exchanger submerged in the main cleaning bath and in this way rinsing is performed at the same temperature as the temperature of the main cleaning process.

New cleaning fluid is added to the machine's supply by means of the rinsing pump and the machine can be supplied with automatic refilling from an external source and also adapted to allow it to be supplied directly from an external source, so that the machine's reservoir is used when the supply vessel is replaced. This cleaning and cleaning fluid handling arrangement is illustrated in the left hand part of the figure, but it is not described in more detail, since it does not constitute any part of the invention.

In the invention the jet from the nozzle should be of a coherent, solid nature, striking the PC-board at one spot. When submerged below the fluid level, such a jet will have a dispersion in the order of 10 degrees, and for this reason the distance to the contact face will be critical in combination with the jet pressure. The impact area for the jet under the fluid level on the PC-board is vital for the ability of the fluid to "escape" from the defined zones without the cleaning effect of the movement of the fluid along the board surface being diminished. This is illustrated in figs. 2 and 3. If the relationship between the shape/angle of dispersion of the jet and the distance to the board surface is incorrect, counter- pressure could be created under the components with a fluid stream from the opposite direction if the area where the fluid escapes is not defined. If the distance between the fluid jets is too great, this can lead to uncovered areas on the board surface where the fluid pressure may be too low to overcome the capillary forces under the components. Fig. 2 illustrates a view of a PC-board in the direction of the nozzle jets 4, and the arrows indicate how the jet spreads in a circular pattern along the surface of the plate. The arrows also illustrate how the fluid from the jet 4 from two nozzles will meet and be deflected to the side and could deviate in the area between the circular areas, i.e. in the hatched areas. Fig. 3 illustrates the board and. the nozzles 4 viewed from the side and also shows how the jet has a solid, coherent path. The arrows indicate how the fluid flows.

The example shown illustrates the use of two nozzle plates, one on each side of the PC-board. This constellation can, of course, be varied in many different ways. If the board is only equipped with components and solderings on one side, only one nozzle plate on one side may be sufficient. Moreover, in the case of larger PC-boards it may be advantageous to arrange only one small nozzle group or only one nozzle at the top of the fluid bath, but still below the fluid surface, so that the board is moved past such a row of nozzles operating under high pressure and thereby is continuously cleaned as it passes. If necessary the board can be passed back and forward several times in front of a row of this kind or a continuous process can be provided where the boards are conveyed one after the other down such a path, until the side at the bottom of the fluid bath and up again is pushed aside. It is also conceivable that the conveyer system could be arranged horizontally with nozzle plates or nozzle rows arranged above and below the plate, if a continuous passage is required. In the drawing the embodiments for a nozzle plate have not been illustrated more accurately, since it will be easy for the specialist to vary the technical design on the basis of the indications given in the present case. It has been discovered that an effective cleaning is achieved if the nozzles are arranged at a distance of between 1 mm and 250 mm from the surface of the PC-board, depending on the cleaning effect required and the pressure at which the spraying is performed. In practice a distance of 50 mm or less has proved to be particularly well suited. With regard to the points of impact of the jets against the surface of the PC-board, it is important in order to achieve an even distribution that this distance too is kept within defined limits. These limits have been found to be 1 mm and 100 mm, while it is preferred to keep the distance between the points of impact for the jet to under 50 mm. These limits will give an even flushing or cleaning of the entire surface. In this connection it.is also vital that the angle of incidence of the jets against the surface of the board should be kept within specially defined limits. A perpendicular angle of incidence is

best suited, but oblique angles down to 45 degrees have also proved to be useful, especially in cases where access is required to a large group of components arranged on special parts of the board.

Thus many modifications are possible within the scope of the invention.