NO881611L - CLEANING MACHINE, SPECIFICALLY FOR ELECTRONIC COMPONENTS. - Google Patents

Abstract

Ransaaaskin, in particular for electronic components, comprising a substantially closed row (10) 'ed entrance windows (12) and exit windows (14), through which passes a conveyor belt (16) for transporting the components, and cleaning devices (20, 18, 30 ) for solvent placed along the path of the conveyor belt, which devices at least comprise: - a pre-washing step (2) with showering of solvent on the components, - a washing step by immersing the components in a reading salt bath contained in a immersion tank (18), (30) with showering of solvent on the components. The heating means (26) are connected to the immersion tank. (18) to keep the solvent bath in permanent boiling, the heating means () are connected to the rinsing step (30) to bring the solvent permanently close to its boiling temperature. , and the space (10) is covered by a condenser unit (64) for condensing solvent vapors, the condensates of the latter being collected in a trip rvoar for pure solvent (36) for feeding the rinsing step (30), the solvent coming from the rinsing step (30) is collected at one end of the immersion tank (18) and evacuated at the other end, to keep a flow of solvent in the tank (18) with a direction soa is the opposite of the direction of movement of the components.

Description

The present invention relates to a cleaning machine, particularly for electronic components.

In the electronics industry, the components on boards with printed circuits are grouped according to higher and higher densities, either on the side of the board opposite the circuit, or on the same side (a technology called components mounted on the surface: CMS or also assembly in FLAT PACK), or both sides at the same time.

When the soldering is secured in the wave soldering machines, it is appropriate to clean the cards after passing through the soldering machine to eliminate traces of flux and small drops of solder that can stick during soldering and which can lead to the risk of short circuits.

When the soldering is done by remelting with hair solder cream, which solder cream consists of eutectic microspheres in a binder that forms a flux, and deposited by screen printing on the selected sides of the board, the components are then placed on the board and this is exposed to heat to cause remelting of the solder cream and form the solder. In this case, a certain number of microspheres in the periphery of the boards escape from the solder cream by coalescence and migrate on the board and can cause short circuits.

The cleaning machines in question often use fluorocarbon solvents and combine one or more of the following methods:

- spraying the solvent on the cards

- immersion of the cards in a bath of solvent with or without the use of ultrasound.

The solvent loaded with contaminants and products in solution is distilled in a still that is combined with the cleaning machine to return the clean solvent to the machine.

This recycling of solvent is made necessary not only because of the price of the solvent, but especially because it is avoided as much as possible to emit vapors of these fluorocarbon solvents into the atmosphere, which are currently held responsible for the erosion of the protective ozone layer that surrounds the Earth's atmosphere.

In continuous-function cleaning machines, the cards pass through the machine from an entry window to an exit window, which of course may remain open to allow the cards and the conveyor belt that carries them through the machine to pass.

First, these windows provide exits through which solvent vapors can escape to the atmosphere.

Second, despite the precautions taken to drain the solvent from the cards and from the conveyor belt after the final rinse, droplets of solvent will remain attached to the cards and to the conveyor belt and be carried outside the machine through the exit window and their eventual evaporation leads to the disadvantages already mentioned.

In certain machines, a forced evaporation is carried out by ventilation, which presupposes the arrangement of an additional step to achieve this, which includes suitable arrangements for condensation of the thus evaporated solvent.

In addition, the stills include an air duct to lead them to the atmosphere, which likewise constitutes an outlet for releasing the solvent vapor to the atmosphere.

Finally, the machines in question in their composition lack the efficiency of cleaning the boards with printed circuits that have the components mounted on the surface to the extent that the space (of the order of 25 pm) between these components and the board is poorly rinsed of the solvent.

With the aim of remedying these various disadvantages, the present invention proposes a cleaning machine for components, especially electronic ones, comprising an essentially closed space through which there is a movement path for these components and devices for cleaning with solvent placed along this path, which devices at least comprises: a pre-washing step with spraying solvent on the components, a washing step with immersing the components in a solvent bath and - a rinsing step with spraying solvent on the components,

characterized in that it comprises heating means which are connected to the solvent bath to keep this latter in permanent boiling, and heating means connected to the cleaning step which is to keep the solvent permanently in its vicinity

boiling temperature, and that the room is surrounded by a condenser unit for the recondensation of solvent vapors, the condensates from the latter being collected in a pure solvent reservoir for feeding the purification stage, the solvent coming from the purification stage being collected at one end of the immersion bath and evacuated to the other end, to maintain a flow of solvent in the bath in a direction opposite to the direction of movement of the components.

Other details and advantages of the invention will become clear when reading the description that follows, with reference to the accompanying drawings in which: Figure 1 schematically shows the cleaning machine according to the invention seen from the side,

Figure 2 schematically shows this machine in a plan and

Figure 3 shows a detail of one of the windows in the machine.

The machine shown in Figure 1 comprises a closed room 10 with an entrance window 12 and an exit window 14 through which room there is a conveyor belt 16 which will carry the cards with printed circuits along a path which exhibits a downward sloping section 16a from the entrance window 12, followed by a horizontal piece 16b passing through a tank 18 and a rising piece 16c passing through the exit window 14.

Between the entrance window 12 and the tank 18, along the downward section, shower lamps for pre-washing 20i, 2Os are placed above and below the conveyor belt, by conveying a high-pressure pump 22 which leads the solvent to a receiving reservoir 24 which collects the solvent from the following the cleaning steps as will be seen the tendons,, and the solvent is near its boiling point. The lamps are designed so that they deliver a "continuous" beam, i.e. in the form of a narrow strip and with an inclination that is chosen in relation to the cards so that the amount of movement of liquid hitting the surface of the cards is largely transferred to a movement amount that is directed tangentially to the board, enabling the liquid solvent to effectively penetrate the small gaps between the board and the surface-mounted components. The number and orientation of these jets is determined, on the other hand, to avoid a "clogging" of liquid on the surface of the card, which could damage the desired conversion of momentum of liquid necessary for penetration into the intervals between the card and the components. Finally, an inclined direction of the rays in relation to the card makes it possible to avoid that the presence of the components constitutes an "umbrella" effect or expressed in another way, that the rays striking the components should be deflected in the lateral direction and not reach the zones of the card that located in the immediate vicinity of the components. The inclination of the jets is advantageously achieved by an assembly for orientation of the ramps that carry the nozzles for the showering.

As an example, the pump emits a quantity of 4 m<3>/hour under a pressure of 10 bar and eliminates 80-90% of impurities and polluting products on the card.

Along the horizontal section 16b, the conveyor belt and the cards it carries are immersed in a solvent bath of small thickness, which is contained in the tank 18 and flows in the opposite direction to the direction of travel of the cards (countercurrent flow) thanks to an influx of permanent renewal which comes from the final rinsing stage, as will be seen later, and overflow of waste water over the edge of the tank located on the side of the entrance. Heating elements 26 are arranged in the tank 18 which keep the solvent slightly above its boiling point, so that the cards pass through a layer of solvent which is kept in a permanent mixture of vapor bubbles from the solvent which are formed by boiling. In addition, on a certain part of the length of the tank, a certain number of ultrasonic generators 28 are placed, which provide a vibrating beam energy to the interior of the liquid, which promotes the penetration of solvent into the smallest cracks in the printed circuit board and the removal of the polluting elements that present in these small cracks.

To the extent that the solvent contained in the tank 8 is at its boiling point, it would normally be expected that this addition of ultrasonic generators 28 would be completely redundant, since the usual phenomena of micro-cavitation caused by the formation of ultrasonic waves in the liquid no longer can be formed, because these micro-cavities would then be immediately occupied by the steam. However, it has been established that the vibrating energy that the ultrasound generators provide (in the form of resonant wave trains in the boiling liquid has the effect of displacing solid particles) and the mixing energy due to the boiling provide a synergism, the result of which is a better penetration of solvent and a better elimination of polluting products.

The liquid's dissolving power is of course at its highest level because this dissolving power increases with temperature.

Finally, the printed circuit boards and the conveyor belt passing through the tank are kept at a temperature close to the boiling point of the solvent, and the benefits of this temperature rise will become apparent later.

The conveyor belt and the cards leave the tank over the rising piece 16c in the direction towards the exit window 14 and go through a final cleaning step 30 which ensures a shower of clean solvent above and below the cards.

It should be noted that this final cleaning must comply with a number of requirements: - it is the last opportunity to remove the impurities, the microspheres from the solder or polluting products, - the amount of disposable, clean solvent is determined by the capacity of the distiller,

- the cleaning must not reduce the card's temperature.

The retained solution is subjected to an atomization at very high pressure (adjustable for example to 20-60 bar and under the supervision of a servo control) in a small quantity of for example 400 litres/hour and at a temperature which is practically equal to the boiling temperature of the solvent.

It should be noted that thanks to the high pressure emitted by the pump 32, it is possible to arrange an ultrafiltration station 34 (less than 5 pm) just downstream of the pump.

However, it is not advisable to boil the solvent in the reservoir 36 for supply to the pump as there may be a risk of cavitation and that the pump will operate with poor yield.

The supply is thus made "at ambient temperature", i.e. the temperature of the pure solvent when it leaves the still and is brought to the buffer reservoir 36.

The heating is therefore carried out on the high-pressure line 38 downstream of the pump and upstream of the atomizing heads 40, thus in a zone where there is no danger of boiling phenomena appearing due to the high pressure that prevails in this line.

As an example, the boiling temperature of a solvent is preferably 39<*>C at atmospheric pressure, and this temperature rises to 70"C at 20 bar. A significant amount of heat can thus be supplied very quickly without the risk of reaching a level due to appearance of the first boiling cores along the heating elements 42.

The regulation of the heating is controlled by means of a temperature probe 43 which is placed in the jets of solvent immediately downstream of the atomization heads 40, thus after the intervention of phenomena such as expansion and evaporation which occur at

the outlet of the heads.

This temperature can thus be kept as close as possible to the boiling temperature (39°C-0.5<0>C) and completely constant.

The heating device 42 likewise has a very small inertia thanks to the small amount of solvent present in the line 38, which enables regulation with a very short time response. The jets formed by the atomizing heads are preferably in the form of lamellae and oriented according to an angle of incidence chosen in relation to the printed circuit boards, in order to favor the penetration of liquid into the smaller cracks between the board and the printed circuits and the components that they carries.

The thus atomized liquid is drained towards the tank 18 and maintains a circulation of solvent in this in the opposite direction to the movement of the cards with printed circuits.

Finally, it should be noted that the cards leave the final cleaning step 30 practically at the evaporation temperature of the solvent, so that the few drops of solvent that may remain "attached" to the cards and/or on the conveyor belt evaporate in the zone between the final cleaning step and the exit window 14.

The few drops of solvent which are not evaporated at this stage and which are carried outside the machine by the cards and/or the conveyor belt thus represent a minimal percentage.

The machine also ensures recondensation and distillation of solvent in situ, thanks to the dispositions listed below: a) Protection of the input window 12 and the output window 14 with "plug" capacitors.

These windows are surrounded by tube spirals 44, 46 through which a cooling liquid passes and on which the solvent vapors will condense, in the picture you can see the distillers' condensation tube spirals.

However, improvements have been made to the function and yield of these "plug" capacitors in the following manner (see details in Figure 3).

In addition to the outer wall 48 which surrounds the tube spirals, there is provided an inner wall 50 which is formed by two wall pieces joining the outer wall, an upper piece 50s and a lower piece 50i, which two pieces are separated by an interval 52, thanks being which there is a connection between the inner zone 54 of the window situated in the interior of the inner wall, and an annular zone 56 which is delimited between the inner and outer walls and in which the condenser coils are placed.

The vapors present in the annular zone 56 are thus quickly condensed on the tube spirals and this phenomenon creates a negative pressure in the annular zone which positively "sucks" all the vapors that may try to escape through the window, as soon as these vapors reach the level for the interval 52 for communication with the annular zone.

In order to avoid drops of condensed solvent from falling on the cards with the circuits above the interval 52' on the side where the inner wall 50'i, 50<7>s overhangs the conveyor belt (with references "first"), this is produced side of the inner wall in one piece, the interval 52' being defined by holes, or "punctures", in this wall.

It should be noted that the inlet and outlet windows are the only zones in the machine where solvent vapor is in contact with the atmospheric air with the risk of charging the atmospheric air. To the extent that these windows are of small size, such entrainment of water is relatively small and the drying apparatus 58 which is integrated in the machine (Figure 1, hygroscopic salts or "siliporite") to capture the water molecules present in the condensates of steam from the input and output windows via lines 57 and 59 can be reduced in size with a possible reduction in the frequency of regeneration in the dryer. Downstream of the drying device 58, the clean solvent is led towards the reservoir 36 by means of a line 61.

Another advantage lies in the possibility of allowing the machine to operate discontinuously, i.e. stopping the diffusion pumps 22, 32 when they are not required.

When the pumps are started there is a slight jolt which in a classical machine tends to cause the steam in the inlet and outlet windows to rise to a level just at the point of flooding outside the machine.

Thanks to dispositions as mentioned above, an immediate absorption of the vapors takes place which, in the event of a jerk, tends to pass the interval level of communication to the

annular zone.

b) Use of the internal volume of the machine as a distillation chamber.

It is shown that the tank 18 in which the printed circuit boards are immersed during their washing is heated to the boiling point of the solvent. With this tank's large surface area, the amount of steam produced per time unit large and spreads throughout the internal volume 60 of the machine above the tank.

The solvent that flows successively in the various stages of the machine is also finally collected in a final receiving tank 62, which is located on the side of the immersion tank 18, but so that the zones located above the final tank 62 and the immersion tank 18 form the one and the same zone 60 which is covered by a capacitor unit 64 as described below.

It should be understood that in the schematic representation in Figure 1, the final receiving tank 62 is not located below the immersion tank 18, but is located at a distance in the lateral direction, as shown schematically in Figure 2, on one or the other side of the machine.

In the final tank 62, the heating elements 66 bring the solvent to a boil and, with this tank's large surface area, the amount of steam produced per time unit large and it distributes throughout the entire internal volume 60 of the machine while mixing with the vapors coming from the immersion tank.

The entire volume in the machine is completely isobar, so that annoying convection movements of steam in the whole machine are avoided.

The accumulated evaporation surfaces in the immersion tank 18 and the final tank 62 are also very significant and the heating capacity can be regulated in the two tanks so that a "gentle" boiling is achieved, i.e. that the bubbling caused by steam bubbles that come up to the surface is moderated and entrainment is thus avoided ( mechanically) with these vapors fractions of volatile products which undesirably have close boiling points.

The machine's evaporation capacity is e.g. 400 litres/hour, which can be compared with the usual capacities of distillers for these types of machines (180 litres/hour).

The condenser unit 64 forms a jacket that covers the entire width of the machine, i.e. the width of the actual cleaning zone plus the width of the final receiving tank. This jacket is closed at its upper end and equipped with a large number of condenser tubes 68 through which coolant passes.

The vapors thus condense on the pipes 68, after which the condensed pure solvent is recovered in the drain pipes 70 which open into the buffer reservoir 36 for pure solvent.

The machine comprises a heat pump (not shown) whose evaporator consists of the condenser tubes 68 of the condenser unit 64, as well as of the tube coils 44 of the "plug" condensers which are connected to the inlet and outlet windows 12, 14 and whose condenser consists of the heating elements 26 of the immersion tank 18 and the heating elements 66 in the final receiving tank 62, as well as a classic air condenser. A suitable regulation ensures distribution of the hot compressed gases between the heating elements and the air condenser.

To enable the evacuation of air originally present in the volume of the jacket 64, this is connected at its top to one of the input or output capacitors, by means of an air duct 72.

This device also has the advantage that the entire volume of the jacket 64 is filled with saturated steam and that all the condenser tubes 68 work at maximum power. If variations occur in the steam level, these variations are actually automatically transferred to the 11 propp11 capacitors in the input or output.

In a classic condenser for distillation, by comparison, the upper part is connected to the atmosphere by means of an air channel which is a source of unwanted water vapor as shown earlier. Moreover, only the condenser tubes located at the bottom are immersed in the unsaturated steam and operate with reduced power. These latter tubes are known in classical devices under the name of the "protection" tubes.

It will be understood that these different features provide significant savings in two respects: First, it is not necessary to connect the machine to a separate distillation unit, which reduces the price and to some extent the dimensions.

Second, the heat energy supplied to the solvent to bring it near its boiling point to improve purification need not be re-supplied for its distillation.

Finally, the machine includes various other features that should reduce the entrainment of solvent outside the machine, as well as improve the quality of the cleaning, such as in particular the use of a mesh cloth conveyor belt.

In the structure for chain conveyors which are usually used in known machines, there is actually for each link a hollow axis, and these axes likewise consist of reservoirs which are filled with solvent during the passage through the immersion bath and it is practically impossible to evacuate this solvent before the exit through the exit window, so that a non-negligible amount of solvent eventually escapes to the atmosphere.

In the structure with mesh cloth conveyor belts, on the other hand, the only places where the solvent droplets can still be collected at the outlet of the machine are the knots in the joints of the mesh, which represent a much smaller amount.

In the machine described here, the mesh cloth is additionally split into two divided cloths of small width (50 mm) which support the printed circuit boards only by means of their side edges. Thanks to this, the number of joints is reduced even more and, as a result, the entrainment of solvent droplets.

The undersides of the printed circuit boards are also not covered by the mesh and can be cleaned effectively.

Although the present description only mentions the cleaning of printed circuit boards, it goes without saying that the cleaning machine can find many applications for cleaning:

- electrical components,

- parts and constructions for precision mechanics,

- optical parts and constructions,

as this list is not limiting.

The machine is advantageously controlled by a microprocessor which integrates all functional data (speeds, quantities, temperatures, pressures) and controls the various equipment according to selected instruction values and/or predetermined function programs.

Finally, it should be noted that the transducers 18 which will produce the ultrasound are located in a zone of free air and that it is not necessary to take special precautions to protect them from an atmosphere saturated with solvent vapors.

Moreover, the lower part of the machine is located below the immersion tank and is completely free and can allow the passage of the return part of a conveyor belt when this cleaning machine is integrated into a complete production line (assembly station, soldering machine, cleaning machine) that includes such a general conveyor belt.

Claims (11)

1. Cleaning machine, especially for electronic components, comprising a substantially closed room (10) with entrance windows (12) and exit windows (14), through which a conveyor belt (16) passes that transports the components, and cleaning devices (20, 18, 30) with solvent placed along the path of the conveyor belt, which devices include at least: - a pre-washing step (20) with showering of solvent on the components, - a washing step with immersion of the components in a solvent bath contained in an immersion tank (18), - a rinsing step (30) with showering of solvent on the components, characterized in that it comprises heating means (26) connected to the immersion tank (18) and which must keep the solvent bath in permanent boiling, and heating means (42) connected to the rinsing step (30) which must keep the solvent permanently close to its boiling temperature, and that the room ( 10) is covered by a condenser unit (64) for recondensation of solvent vapours, the condensates of the latter being collected in a reservoir for pure solvent (36) for feeding the rinsing step (30), the solvent coming from the rinsing step (30) being collected in a end of the immersion tank (18) and is evacuated at the other end, to maintain a solvent flow in the tank (18) in the opposite direction to the movement of the components. 2. Machine according to claim 1, characterized in that it comprises ultrasound generators (28) connected to the immersion tank (18). 3. Machine according to any one of claims 1 and 2, characterized in that it comprises a final receiving tank (62) for solvent for use in the pre-washing step, and a heating device (66) which is connected to the final receiving tank (62) which should heat this solvent to boiling. 4. Machine according to claim 3, characterized in that the final receiving tank (62) is placed laterally in relation to the immersion tank (18), the condenser unit (64) simultaneously covering the immersion tank (18) and the final receiving tank (62). 5. Machine according to any one of the preceding claims, characterized in that the input windows (12) and the output windows (14) in the space (10) are equipped with "plug" capacitors. 6. Machine according to claim 5, characterized in that the "plug" capacitors consist of condenser tube spirals (44, 46) which extend in an annular zone (56) which lies between an outer wall (48) surrounding these tube spirals and an inner wall (50) formed of two pieces, respectively an upper piece (50s) and a lower piece (50i) separated by an interval (52). 7. Machine according to claim 6, characterized in that the capacitor unit (64) is closed at its upper end and communicates by means of an air channel (72) with one of the "plug" capacitors. 8. Machine according to any one of claims 5 to 7, characterized in that the "plug" capacitors are connected to the reservoir for pure solvent (36) by means of lines (57, 59, 61) in which a dry - device (58). 9. Machine according to any one of claims 1 to 8, characterized in that the reheating device (42) which is connected to the final rinsing step with showering (30) is placed on an outlet line (38) downstream of a high-pressure pump (32) and upstream of the shower heads (40). 10. Machine according to any one of claims 1 to 9, characterized in that the conveyor belt (16) for the components is a mesh cloth conveyor belt. 11. Machine according to claim 10, characterized in that the mesh cloth conveyor belt (16) is designed as two partial cloths which support the components laterally.