Method and apparatus for washing circuit boards

Abstract: The present invention concerns a method for washing off flux residues subsequent to soldering from a circuit board or equivalent component. The circuit board washing procedures of prior art have either been detrimental to the environment or poor in their washing result. These problems have in the present invention been solved by including at least one step in the procedure in which steam is sprayed on the circuit board or equivalent. The steam spraying step is advantageously combined with preceding water washing and rinsing steps. A circuit board or equivalent component rinsed with hot steam is recovered in a cleaner condition than before, and moreover, entirely dry.

Description: The present invention relates to a method for washing off flux residues after soldering from a circuit board or an equivalent component on which said soldering has been made. The invention also relates to apparatus for carrying out said washing procedure. Such method and apparatus are disclosed in GB-A- 2084 865 and DE-A- 3201 880. When high-active or active and smearing fluxes are used in soldering circuit boards, the circuit boards need a careful washing after being soldered. This requirement is supported not only by cosmetic aspects but also the requirements of the test results implying the properties of the circuit board. The problem is greatest when flux remains on high-impedance boards which in use are subjected to continuous thermal variations causing corrosion. No needle bed testing can be done in the production process without washing. Flux residues have traditionally been washed off from the circuit board with freon-based solvents which are chlorine fluorine hydrocarbons possessing an extremely great solving capacity. For these solvent washing systems, which are expensive and detrimental both for man and environment, e.g. causing ozone losses, endeavours have been made for a long time to find some replacement. An important alternative is washing with water in which water washable fluxes are either directly used or original solvent-washable fluxes are used, these being first treated with certain alkaline agents with which the fluxes are 'saponified'. An advantage of the water washing has been thought to be better soldering results, which are due to the higher activity of the fluxes, and a higher purity which may be even manifold in comparison with the solvent cleaning systems. The purity is due to the easy dissolution of water washable fluxes in water, and on the other hand, water, which is inexpensive, can freely be consumed for rinsing in great quantities. Water washing, which in other respects is very useful, is, however, subject to two problems: moisture remains inside the components after washing, anf from the narrow spaces between the components, flux residues often remain unremoved. Said drawbacks are due to the fact that water has a greater surface tension than the conventional solvents. Endeavours have been made to solve the problem by reducing the surface tension of water by means of expensive distillation and ion exchange systems. This has not, however, turned out to be economical. The present invention provides a method of washing off flux residues after soldering from a circuit board or equivalent component, characterized in that it comprises at least one step in which steam is sprayed on the circuit board or equivalent component. Thus, it is taught that the washing result is considerably better than before but not more expensive, when the washing contains at least one step in which steam is sprayed on the circuit board or equivalent component. The steam jet step can be used as such for washing off flux residues from the circuit board or an equivalent component. It can also be combined with any known washing process of circuit boards or equivalent as an extra or optional process step. According to a preferable embodiment, the method can also include at least one of the following steps: the circuit board or an equivalent component is washed with water, advantageously in a closed space either after pretreatment with a detergent or in the presence of a detergent, the circuit board or an equivalent component is rinsed by showering water thereon, this being advantageously heated. The above steps in general precede said steam jet step, and finally the circuit board or equivalent component is allowed to dry and cool. This type of water-and-steam-washing method is advantageously a so-called in-line process in which the circuit board or equivalent component to be washed is transported e.g. with a belt through the zones performing the above, in series connected steps. The steam jet step of the cleaning process including water washing steps is preferably accomplished with water vapour which is at such a pressure and temperature that it also eliminates substantially all moisture from the circuit board or equivalent component in addition to the impurities. A slightly pressurized vapour is completely gaseous and will efficiently turn the water drops remaining on the circuit board or equivalent into gaseous form. The steam jet step is in general carried out by vaporizing water, by conducting the steam thus produced into nozzles positioned in a steam chamber from which it is from many directions sprayed towards the circuit board or equivalent located in the steam chamber. By maintaining the steam chamber at a small overpressure the steam can be let out of the chamber directly into the air or, if desired, through a condenser into a sewer. The apparatus the invention comprises, in order to wash off the flux residues subsequent to the soldering from a circuit or an equivalent component, at least a zone in which steam is sprayed on the circuit board or equivalent. The zone consists advantageously of a steam producing means, feed members for feeding steam into jet ejecting nozzles designed for said purpose, jet ejecting nozzles being directed towards the circuit board from different directions, and of steam chamber walls therearound with apertures for feeding the circuit board or equivalent into the steam chamber and out of it. The steam chamber may also be provided with an outlet aperture for discharging steam, and is on the outside of said chamber optionally provided with a condenser for condensing the steam discharged from the chamber. The means comprises also preferably a closed washing space with requisite transport and washing members and washing water feed and outlet, a rinsing zone with rinsing liquid supply, spraying members and discharge. The washing space and the rinsing zone are placed before said steam spraying zone. After the steam spraying zone is preferably disposed a drying and cooling zone with dry air feed and discharge. Said zones, that is, the closed washing space, the rinsing space, the steam spraying zone and the drying and cooling zone have advantageously been disposed one after the other and surrounded with an intact outer wall but separated with inner walls from one another to produce an in-line-process apparatus through which a circuit board or equivalent component to be washed is transported e.g. on a belt conveyor. An example of the invention is illustrated in the following with the aid of the figure presenting a perspective image in principle of the in-line-washing means of a circuit board or equivalent components. A circuit board or equivalent is fed into a washing means 1 through a loading projection 2 located on the left in the figure, wherefrom a conveyor belt carries the piece to a first washing zone 3. The washing zone 3 is advantageously so closed that detergent can be added in the washing step for providing a better washing result, and on the other hand, water can be saved to reduce environmental loading. After the first water washing step the conveyor belt conveys the circuit board through a hole in the partition to the next zone 4 of the washing apparatus, in which a shower rinsing is so carried out on the circuit board that hot water is sprayed towards the circuit board. An outlet pipe 5 for discharging water vapour is also shown in the figure. The last washing step which is accomplished in the steam chamber 6 is totally deviating from the conventional washing steps. With a small steam producing means 7, such as is used in industry, water vapour is produced, the heat content of which, that is, temperature and pressure, is keeping it in steam form during the entire period, and it will in no phase turn into water drops. In the steam chamber 6 steam is sprayed on the circuit board from all sides so that the reaction forces are not able to move the circuit board progressing on the conveyor belt. The conveyor belt is advantageously a metal mesh belt. In the steam chamber 6 prevails a slight overpressure which then is discharged out of an outlet pipe 8 positioned above the unit, e.g. out through a condenser (not shown) and into a sewer. The last step is a drying and cooling unit 9 operating merely by suction principle. No separate driers are needed for the hot aquaeous gas has removed all moisture from the circuit board. The drying air exits through the aperture 10 and the washed circuit board from the discharge projection 11. Since the end washing is always done with hot steam, the water used in earlier washing steps need not be distilled or purified with ion exchange. The advantages of the new procedure and the new means of the invention are as follows. They are less expensive than the earlier ones, the means is smaller and more practical than earlier, the operating costs are lower, a totally dry washing result is obtained therewith, steam penetrates more efficiently into the narrow spaces of the circuit board or equivalent component and gives a better washing result, the kinetic energy removes small particles, the steam used in the procedure has a better dissolving ability, the water used at the initial step need not be purified by distilling or ion exchange, the procedure is environmentally acceptable; it generates no freons affecting the ozone layer, nor great quantities of flux-polluted water, they are faster than before, etc.

Claims: A method of washing off flux residues after soldering from a circuit board or equivalent component, characterized in that it comprises at least one step in which steam is sprayed on the circuit board or equivalent component. A method according to claim 1, characterized in that said steam jet step is accomplished with water vapour prevailing under such pressure and at such temperature that it removes, in addition to the impurities, all moisture from the circuit board or equivalent component. A method according to claim 1 or 2, characterized in that the steam jet step is accomplished by vaporizing water, by directing the produced steam into nozzles positioned in a chamber, where it from several directions is sprayed against the circuit board or equivalent component, and by maintaining the steam chamber under small overpressure for letting steam out of the chamber directly into the air or through a condenser into a sewer. A method according to any one of claims 1,2 or 3, characterized in that it comprises, in addition to the steam jet step, at least one of the following steps: the circuit board or equivalent component is washed with water, advantageously in a closed space in the presence of a detergent; the circuit board or equivalent component is rinsed by spraying water thereon, this advantageously being heated, which steps precede said steam jet step; and finally the circuit board or equivalent component is dried and cooled. A method according to any one of the preceding claims, characterized in that it is an in-line-process in which the circuit board or equivalent is conveyed through zones accomplishing the above-mentioned steps, said zones being arranged in series. Apparatus for washing off flux residues subsequent to the soldering from a circuit board or an equivalent component, characterized in that it comprises at least a zone (6,7,8) for spraying steam towards the circuit board or equivalent component. Apparatus according to claim 6, characterized in that the steam jet zone consists of a steam producing means (7), supply members for supplying steam into its jet ejecting nozzles, jet ejecting nozzles directed towards the circuit board from different sides, and of a steam chamber (6) therearound, provided with holes for introducing a circuit board or equivalent component into the steam chamber (6) and out of it, and of a discharge aperture (8) on the wall of the steam chamber for discharging the steam out of the chamber, and possibly of a condenser for condensing the steam exiting the chamber. Apparatus according to claim 6 or 7, characterized in that the apparatus also comprises a closed washing space (3) with requisite transport and washing members, and with a supply and an outlet for washing water; a rinsing zone (4,5) with rinsing liquid supply, spraying members and outlets (5), whereby the washing space (3) and the rinsing zone (4,5) are placed before the steam spraying zone (6,7,8); and subsequent to the steam spraying zone (6,7,8) a drying and cooling zone (9) with drying air supply and outlet. Apparatus according to any one of the preceding claims 6-8, characterized in that said closed washing space (3), said rinsing zone (4,5), said steam spraying zone (6,7,8), and said drying and cooling zone (9) have been disposed in succession and surrounded with compact outer walls but separated with inner walls from each other, and that through the zones a conveyance of the circuit board or equivalent component has been arranged with including loading (2) and discharging (11) projections.

Inventors: TUOMINEN AULIS, TUOMINEN, AULIS
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1990
Main classification: H05K3
IPC codes: H05K3, B08B3, H05K13