Closed-Loop Dual-Cycle Printed Circuit Board Cleaning Apparatus and Method

This invention relates to a method and apparatus for cleaning printed circuit boards (PCBs) and other electronic assemblies. More particularly, the invention relates to a PCB cleaning system that recycles cleaning fluids employed.

During assembly of components on printed circuit boards, soldering fluxes are applied to the board material to facilitate soldering. After soldering, remaining flux residue must be removed. The flux chemistry determines what fluid is suitable for removing the flux. Unlike some fluxes, rosin-based fluxes are not removable with a water wash, and require more aggressive solvents that have environmental disadvantages. Such solvents generate flammable and environmentally undesirable vapors that must be safely contained, and require costly disposal methods because they cannot be drained into conventional sewer systems.

A batch cleaning system using such combustible solvents is disclosed in U.S. Pat. No. 5,027,841 to Breunsbach et al. This system is a closed loop system using only solvent to wash the PCBs in a sealed chamber that is purged with inert gas before it is opened. In the closed loop system, solvent is reused and recycled until its useful properties are exhausted. Solvent is sprayed onto the boards to mechanically dislodge and dissolve contaminants. After the PCBs are cleaned, they must be removed from the chamber and inserted in a separate apparatus for rinsing the remaining solvent off the boards. Although the inert gas purge is sufficient to eliminate any fire or explosion hazard, some solvent vapor escapes to the environment as the PCBs are transferred to the rinse apparatus.

A printed circuit board cleaning apparatus that uses a first wash fluid and a second rinse fluid in a single chamber is shown in U.S. patent application Ser. No. 07/715,441, filed Jun. 14, 1991, by Breunsbach et al. In this system, a saponifier solution wash is sprayed onto the PCBs and then drained from the chamber. A similar water rinse follows. Finally, a water and alcohol solution is sprayed onto the boards to be subsequently tested to determine cleaning effectiveness. This system does not, however, recycle or reuse the saponifier wash fluid or the rinse fluid. Only the cleanliness test solution is recycled for reuse.

The primary objects of the invention are to provide:

1. A printed circuit board cleaning apparatus having at least two separate cleaning cycles using cleaning fluids from separate sources, and including an ultrasonic generator for agitating the fluid.

2. A printed circuit board cleaning apparatus having an ultrasonic generator for agitating cleaning fluid, and which apparatus recycles the cleaning fluid.

3. A printed circuit board cleaning apparatus having at least two separate cleaning cycles using cleaning fluids from separate sources, and which recycles the cleaning fluid to avoid discharging a substantial quantity thereof.

4. A printed circuit board cleaning apparatus having a single receptacle for holding the boards for a solvent fluid wash and a separate water rinse.

According to the present invention, the primary objects are achieved by providing a printed circuit board cleaning apparatus having two separate wash and rinse fluid circuits, each separate and in communication with a tank for immersing boards to be cleaned. Each fluid circuit collects and filters drained fluid for reuse. The rinse fluid is also further purified. An array of ultrasonic transducers connected to the tank facilitates thorough and rapid cleaning. An inert gas purge system eliminates flammable vapors and oxygen from areas of the apparatus where there are potential electrical ignition sources.

The foregoing additional features and advantages of the present invention will be more readily apparent from the following detailed description which proceeds with reference to the accompanying drawings.

FIG. 1 is a perspective view of a printed circuit board cleaning apparatus according to the present invention.

FIG. 2 is a schematic diagram of the apparatus of FIG. 1, showing the functional components thereof.

FIG. 1 shows an electronic assembly cleaning apparatus 10 with an upper portion defining a forwardly open chamber 12 that is closable by a hinged door 14. The door is hinged at its lower edge to open perpendicularly away from the front of the apparatus 10. A wheeled carriage 20 resides in the chamber 12 and may be rolled out of the chamber 12 onto the open door 14. A perforated fume ventilation plenum 22 is attached to the chamber 12 at the upper perimeter of the opening to draw in fume vapors that would otherwise be released when the door is opened.

As shown in FIG. 2, the carriage 20 includes an inner cleaning tank 26 and an outer vessel 28. The upper rim edges of the tank 26 and outer vessel 28 are sealed together with an upper vessel panel 30 to define an enclosed jacket 34 surrounding the tank 26 on all its sides and bottom, and contained within the outer vessel 28.

An array of ultrasonic transducers 36 is positioned within the jacket 3 and attached to the exterior of the tank 26. In a preferred embodiment, a conventional transducer such as that provided by Ney Company of Hartford, Conn., is suitable for providing 10 to 100 watts per gallon.

The tank carriage 20 further includes a sump fill level sensor 38 for indicating the fluid level in the tank. A sump sensor 42 resides in the tank to be in contact with fluid contained therein to determine when the assemblies are cleaned. A sparge 44 is a vessel connected to a source of gas such as nitrogen and includes a plurality of apertures for expelling the gas onto the inner surface of the tank 26 to blow remaining fluid droplets off the walls of the tank to facilitate drainage. A sparge may alternatively be provided in the bottom of the tank to laterally direct fluid across the floor of the tank 26.

The lower portion of the carriage 20 defines a solvent drain 48 and a rinse drain 50 on opposite sides of the tank 26 at the rear portion thereof. The drains pass downward out of the tank 26 and through the jacket 34 and outer vessel 28 to provide fluid communication between the interior of the tank and the area below the carriage 20. The drains are each sealed from the jacket area 34 to maintain an environmentally sealed jacket.

A solvent drain plug assembly 52 and a rinse drain plug assembly 54 are attached to the carriage 20 to provide selectable opening and closing of the respective solvent drain 48 and rinse drain 50. Each drain plug assembly includes a plug 58 suspended by a rod 60 above the respective drain by a lifting mechanism 62 controlled by a gas piston 64.

The carriage 20 further includes a water overflow drain 66 in communication at an upper end with the interior of the tank 26 at an upper portion thereof, and open at a lower end substantially adjacent the outlet of the rinse drain 50. The carriage 20 rests on a set of rollers 68 to permit it to be rolled forwardly out of the chamber 12 when the door 14 is open, as shown in FIG. 1. The plenum 22 is positioned above the door, and includes an exhaust sensor 69. The plenum is connectable to any exhaust fan (not shown).

A solvent fluid circuit 70 is included in the cleaning apparatus 10 to collect fluid from the solvent drain 48, to store, test and condition the fluid for recycling. A solvent collection trough 72 is an elongated receptacle positioned beneath the solvent drain 48 for collecting fluid drained from the tank 26. Because of the rearward positioning of the solvent drain 48 on the carriage 20, the trough 72 effectively collects drained fluid regardless of whether the carriage 20 is fully installed in the chamber 12, fully rolled out onto the door 14, or any position in between. The trough 72 drains through a conduit 74 into a solvent holding tank 76 having a volume comparable to the cleaning tank 26. The holding tank 76 includes a level sensor 80 and a temperature sensor 82. A heater coil 88 within the tank circulates heated fluid selectively provided by a heater 90 in response to a signal from the temperature sensor 82 to provide a thermostatically controlled solvent temperature.

To fill the cleaning tank 26 with solvent, a solvent fill pump 92 draws fluid from the solvent holding tank and sends it through a particle filter 96, and thereafter through a conduit 98 that terminates in a solvent outlet 100 positioned in the chamber 12 directly above the upwardly open tank 26. The solvent fill pump 92 is driven by an external source of pressurized air. This avoids placing the ignition sources found in conventional electric motors in proximity with potentially flammable solvent. The pump 92 may alternatively be driven by an external source of pressurized inert gas such as nitrogen to further avoid introduction of oxygen.

A rinse fluid circuit 106 circulates rinse fluid, typically water, through a system that is similar to, but entirely independent of the solvent fluid circuit, except that both circuits communicate with the cleaning tank 26. A rinse trough 108 similar to the solvent trough 72 is positioned below the rinse drain 50 and overflow drain 56 to catch fluid emitted therefrom. A conduit 110 empties from the trough 108 into a rinse holding tank 114. A tap water source 116 is connected to the conduit 110 to add additional water to the holding tank 114 as needed when the water is depleted due to evaporation and dragout losses as wet boards are removed. A heater coil 118 passes through the holding tank 114 to heat the fluid contained therein by circulating heated fluid provided by the heater 90. A rinse fill pump 122 similar to the solvent fill pump 92 pumps the rinse fluid through a particle filter 124, a carbon purifier 126 and a deionizer 128. The rinse fluid is thereafter transmitted through a conduit 130 that terminates in a rinse fluid outlet 134 positioned above the tank 26. A rinse fluid conductivity tester 136 is positioned in communication with the conduit 130 after the filters and purifiers to indicate when the rinse filters are sufficiently contaminated to require replacement. The rinse holding tank 114 may optionally include a solvent outlet valve 138 for decanting any solvent that remains floating on the surface of the rinse fluid in the holding tank. The outlet may include a floating intake (not shown) and a receptacle (not shown) for collecting any decanted solvent.

An inert gas supply system 142 provides inert gas such as nitrogen to the various system components requiring a gas supply. A nitrogen tank 144 is connected through a regulator 145 to a manifold 146 with a plurality of outlets. A purge line 148 including a purge flow rate meter 150 communicates with the carriage jacket 34 to evacuate any oxygen or solvent vapors from the jacket before the electrically-powered ultrasonic transducers 36 are activated. A door lock 152 connected to the purge line prevents opening of the door while a purge is in process. The manifold further includes connections to the sparge 44, the solvent and rinse drain lifting mechanisms 64, and the solvent and rinse fill pumps 92, 122. Alternatively, the solvent and rinse fill pumps may be powered by compressed air to conserve the nitrogen supply, as discussed above.

The ultrasonic transducers 36 are electrically powered by an ultrasonic power source 154 that includes a safety interlock purge sensor 156 mounted inside the jacket 34 to prevent operation of the ultrasonic system when the jacket is inadequately purged. The ultrasonic power source 154 may be controlled by a Ney-proSONIK model process control ultrasonic generator 158 mounted in the cabinet of the cleaning apparatus 10. Alternatively, the ultrasonic system may be controlled by an external microprocessor controller (not shown). If the electrical process controller 158 is mounted in the cleaning apparatus 10, a purge line (not shown) is connected to provide a selective flow of nitrogen gas through the process controller 158 to evacuate oxygen and any accumulated solvent vapors to avoid ignition by the electrical contacts and switches contained therein.

Having illustrated and described the principles of my invention by what is presently a preferred embodiment, it should be apparent to those skilled in the art that the illustrated embodiment may be modified without departing from such principles. For instance, a conventional agitation system may be substituted for the ultrasonic agitation, and alternative fluids may be substituted for the semi-aqueous solvent and water rinse. I claim as my invention not only the illustrated embodiments but all such modifications, variations and equivalents thereof which come within the true spirit and scope of the following claims.