CO.sub.2 Cleaning System and Method

The present invention relates to an apparatus and method for creating abrasive CO.sub.2 snow at supersonic speeds and for directing the snow onto an area of contaminants to be removed from a workpiece.

The use of liquid carbon dioxide for producing CO.sub.2 snow and subsequently accelerating it to high speeds for cleaning particles from a substrate is taught by Layden in U.S. Pat. No. 4,962,891. A saturated CO.sub.2 liquid having an entropy below 135 BTU per pound is passed though a nozzle for creating, through adiabatic expansion, a mix of gas and CO.sub.2 snow. A series of chambers and plates are used to enhance the formation of larger droplets of liquid CO.sub.2 that are then converted through adiabatic expansion into solid CO.sub.2 snow. The walls of the ejection nozzle are suitably tapered at an angle less than 15 degrees so that the intensity or focus of the stream of the solid/gas CO.sub.2 will not be reduced below that which is necessary to clean the workpiece. The nozzle, which may be manufactured of fused silica or quartz, does not utilize any precooling.

Lloyd, in U.S. Pat. No. 5,018,667 at columns 5 and 7, teaches the use of multiple nozzles and tapered concentric orifices for controlling the flow of the CO.sub.2 and snow mixture. These references seek to disperse the snow rather than to focus it after exiting the exhaust nozzle. Lloyd teaches that a small portion of the liquid CO.sub.2 is routed through a pilot orifice and then into an expansion cavity for allowing the liquid CO.sub.2 to flash from the liquid to the solid state, which in turn causes a significant drop in temperature. This cooled mixture of solid, liquid and gas cools the inside surface of the nozzle, which then cools the remainder of the nozzle through conduction. This cooling acts as a constant temperature heat sink that precools the liquid CO.sub.2 as it enters the primary orifices in the body, which in turn enhances the conversion of the main flow of the liquid CO.sub.2 flowing through the primary orifices of the nozzle. No precooling gasses are used in the vicinity of the nozzle to improve the flashing conversion of the liquid into the solid phase.

Hayashi, in U.S. Pat. Nos. 4,631,250 and 4,747,421, discloses the use of liquified nitrogen (N.sub.2) for cooling a jacket-type peripheral wall defining a sealed cavity in which a flow of CO.sub.2 gas is introduced under pressure. The cooling produced by the cooled peripheral walls causes the CO.sub.2 to change into snow within the chamber. N.sub.2 gas is introduced into the chamber at high pressure in order to agitate and carry the CO.sub.2 snow from the chamber at high velocity though a jetting nozzle. While liquid N.sub.2 is used for cooling the peripheral walls, the ambient N.sub.2 is used only for agitating and transporting the CO.sub.2 snow from the cooled cavity.

In contrast to these prior art teachings, the present invention utilizes inexpensive components and readily available low pressure shop air for improving the efficiency of creating CO.sub.2 snow and for improving the coagulation of the CO.sub.2 snow into larger CO.sub.2 snow particles. It is therefore an object of the present invention to utilize shop air which is introduced into an elongated expansion area adjacent to the direct conversion liquid CO.sub.2 snow nozzle, and produce moderate size snow particles suitable for agglomeration into larger CO.sub.2 particles by controlling the pressure and temperature of the shop air. The shop air may be precooled by the injection of relatively small volumes of liquid N.sub.2 to precool low pressure shop air that then is introduced into the expansion area adjacent the nozzle in order to improve the efficiency of the flash conversion of liquid CO.sub.2 into snow. The shop air cooled by the injection of the liquid N.sub.2 also flows across and cools the nozzle for improving the efficiency of the direct flash conversion of the CO.sub.2 from liquid to solid.

In an apparatus for cleaning a workpiece with abrasive CO.sub.2 snow, a nozzle is provided for receiving and expelling liquid CO.sub.2 through an orifice sized for converting the liquid into CO.sub.2 snow. A body, defining a cavity therein, is coupled to the nozzle such that the snow is ejected into the cavity. An exhaust nozzle is coupled to the body and the cavity therein for directing the pressurized CO.sub.2 snow toward the workpiece. A mixing device is coupled to the nozzle for receiving and mixing pressurized shop air and liquid nitrogen, and then directing the cooled shop air into the cavity for cooling the area adjacent to the nozzle. In this manner, the precooled shop air enhances the efficiency of the conversion of liquid CO.sub.2 into CO.sub.2 snow particles by cooling and pressurizing the area adjacent to the orifices in the nozzle within the cavity.

Other objects, features and advantages of the present invention will be apparent from a study of the written descriptions and the drawings in which:

FIG. 1 is a pictorial diagram of the CO.sub.2 cleaning system in accordance with the present invention as it operates on a printed circuit board workpiece.

FIG. 2 is a cross-section view of the first preferred embodiment of the CO.sub.2 generator nozzle in accordance with the present invention that is contained within the circled portion of FIG. 1 identified with the reference numeral 2.

FIG. 3 is a perspective view of a first preferred embodiment of the exhaust nozzle in accordance with the present invention. Hidden lines and cutaway sections reveal the shapes of the interior dimensions of nozzle.

FIG. 4 is an enthalpy diagram showing the transition or flashing of the liquid CO.sub.2 into snow in accordance with the operation of the method of the present invention.

A CO.sub.2 cleaning system in accordance with the present invention is illustrated generally in FIG. 1. A CO.sub.2 snow generator 10 is connected to a reservoir 20 of liquid CO.sub.2, a source of compressed shop air 30 and a source of liquid nitrogen N.sub.2 40. The solid CO.sub.2 snow which is exhausted from the exhaust nozzle of the CO.sub.2 generator 10 is focused on the workpiece 90 shown generally as a printed circuit board of the type having electronic components mounted thereon. The size of the workpiece is enlarged for purposes of clarity and does not necessarily represent the size of the CO.sub.2 footprint on the PC board or other workpiece to be cleaned

The reservoir 20 of liquid CO.sub.2 is stored at approximately 0.degree. F. and is pumped under a pressure of approximately 300-400 psi through line 24, and through a control valve 22 and then into the CO.sub.2 snow generator 10. The control valve 22 regulates the pressure and the flow rate under which the liquid CO.sub.2 is fed into the CO.sub.2 snow generator 10, which in turn regulates the amount of snow in the output.

The source of "shop air" 30 generally comprises an air compressor and reservoir of the type normally found in a manufacturing or production environment. The air compressor is capable of pumping a large volume of air, typically 200 cfm at room temperature, through a feedline 34. A control valve 32 is interposed along the feedline 34 for regulating the pressure and flow rate of the air from the shop air reservoir 30. The use of existing shop air in the pressure range of 50 psi to 100 psi significantly reduces the initial capital cost of the present system.

A reservoir 40 of liquid nitrogen (N.sub.2) is coupled through a supply line 44 into a mixer 50 that allows the liquid nitrogen to be injected into the flow of shop air as required for proper performance of the system. A control valve 42 is inserted into the liquid nitrogen line 44 for controlling the pressure and volume of the liquid nitrogen that mixes with and therefore cools the shop air in the mixer 50. As illustrated generally in FIG. 2, the mixer 50 can be constructed by merely inserting the line 44 carrying the liquid nitrogen into the line 34 transporting the shop air from the reservoir 30 into the CO.sub.2 snow generator nozzle, illustrated generally as 60.

With continuing reference to FIG. 2, the CO.sub.2 snow generator nozzle 60 includes a body 62 having a generally cylindrical shape and defining therein a body cavity 64 having a diameter of approximately 1 to 4 inches, with 4 inches being used in the preferred embodiment, in which the CO.sub.2 snow is generated. The cavity 64 is at least 10 to 15 diameters long, which provides a sufficiently restricted volume in which the CO.sub.2 snow particles can coagulate to form larger CO.sub.2 particles.

The line 24 carrying the liquid CO.sub.2 from the reservoir 20 is coupled through the closed end of the body 62 and extends into the body cavity 64 by approximately 4 inches. The body 62 is sealed with the line 24 to allow pressure to accumulate within the body cavity 64. An injector nozzle 70 is coupled to the distended end of the line 24 carrying the liquid CO.sub.2. A plurality of orifices 72 are arranged generally around the circumference and on the end of the injector nozzle 70. Whereas the inside diameter of the injector nozzle 70 is approximately 1/2 inch, the orifices 72 are only 0.04 inches in diameter. The orifices generally comprise bores or channels into the nozzle 70 that are angled with respect to the longitudinal axis of the nozzle 70 and the cavity 64 so that when the liquid CO.sub.2 is expelled through the orifices 72, the snow will have some forward velocity toward the elongated section of the cavity 64. The exact angle at which the CO.sub.2 snow is expelled through the orifices 72 will vary by design, but in the preferred embodiment is between approximately 30 degrees and 60 degrees with respect to the longitudinal axis.

With continuing reference to FIG. 2, the shop air line 34 from the mixer 50 is coupled into the body 62 of the CO.sub.2 snow generator nozzle 60 at a point generally between the closed end of the body and the orifices 72 in the injector nozzle 70. The angle at which the line 34 is coupled into the body 62 not only provides a forward momentum for the shop air as it is introduced under pressure into the cavity 64, but the location and angle of the line 34 with respect to the body 62 also cause the shop air to be directed toward the injector nozzle 70. The inside diameter of the shop air line 34 is approximately 1.25 inches, which in the preferred embodiment is appropriate to provide the volume of shop air to propel the CO.sub.2 snow from the system with the appropriate velocity.

The method of operation of the CO.sub.2 snow generator 10 will now be explained with continuing reference to FIG. 2. The liquid CO.sub.2 is pumped from the reservoir 20 through the feedline 24 under a pressure controlled by the control valve 22. The liquid CO.sub.2 is forced under pressure through the orifices 72 in the injector nozzle 70 and thereby "flashes" from the liquid state into a state that includes a solid form of CO.sub.2, which herein is referred to generally as CO.sub.2 snow. The CO.sub.2 snow will be mixed with either liquid CO.sub.2 or CO.sub.2 in the gaseous form depending on the combination of temperature and pressure as illustrated in the enthalpy diagram of FIG. 4. In the preferred mode of operation, the liquid CO.sub.2 will have a temperature of approximately 0.degree. F. and will be pumped through the orifices 72 in the injector nozzle 70 under a pressure of approximately 300 psi. This combination of characteristics is illustrated as point 1 in the enthalpy diagram of FIG. 4. As the liquid CO.sub.2 exits the orifices 72, it will move to point 2A on the enthalpy diagram. It will be understood by one skilled in the art that point 2A may be transferred into the area in which the exiting CO.sub.2 is in the solid and gaseous phase by increasing the pressure of the gas within the cavity 64 to point 2C, where the exiting CO.sub.2 is in the solid and liquid phases, by decreasing the temperature of the gas within the cavity 64.

Both of these objectives may be accomplished by either controlling the pressure of the shop air flowing through line 34, or by injecting a controlled volume of liquid nitrogen through the mixer 50 into the shop air to carefully control the resulting temperature of the mixture of gases, or by doing both. Assuming that liquid nitrogen at a temperature of -450.degree. F. is injected into the mixer 50 in a ratio of 15 parts of gaseous nitrogen to 85 parts of air, the shop air at a pressure of 80 psi can be precooled to a temperature in the range of -40.degree. F. to -120.degree. F. As this precooled mixture of shop air and nitrogen is directed toward the nozzle 70, point 2A on the enthalpy diagram in FIG. 4 moves to point 2C which produces more snow and less liquid CO.sub.2.

The precooled air and nitrogen mixture flowing through the line 34 from the mixer 50 will also cool the injector nozzle 70 to remove latent heat generated as the liquid CO.sub.2 flashes through the orifices 72 in the injector nozzle. This cooling effect also will improve the efficiency of the conversion of the liquid CO.sub.2 to snow. The conversion of part of the liquid CO.sub.2 injected into the cavity 64 from the liquid state to the gaseous state also adds additional pressure to the shop air in the body cavity 64. This compensates for system pressure losses and increases the pressure at the inlet to the exhaust nozzle 100 by up to approximately 20 percent. This increases nozzle exit velocities, thereby improving the cleaning efficiency of the process.

With reference to FIG. 2, the mixture of CO.sub.2 snow and gas from the orifices 72 within the injector nozzle 70 are exhausted toward the elongated end 66 of the body cavity 64 within the exhaust gas nozzle 100. The exhaust nozzle 100 expands the stream isentropically to the ambient pressure. Further conversion of any remaining liquid or gaseous CO.sub.2 into CO.sub.2 snow will occur during this process. As illustrated in FIG. 3, the exhaust nozzle 100 includes a generally cylindrical section 110 that is sized for coupling with the distended section of the body 62 of the CO.sub.2 snow generator nozzle 60. This coupling may be accomplished either directly or by the use of a hose 95 of sufficient diameter and length. The cylindrical section 110 is approximately 0.9 inches in inside diameter, and tapers over a length of approximately 6 inches to a throat section 120 that has a generally rectangular cross section approximately 0.9 inches by 0.1 inches. This compound tapering shape between the cylindrical section 110 and the throat section 120 causes a decrease in the pressure of the CO.sub.2 snow and gases flowing therethrough. The throat section 120 expands and opens into an enlarged exit nozzle section 130 that defines a generally rectangular exhaust aperture 132 through which the solid CO.sub.2 snow and gases flow as they are directed toward the workpiece. The generally cylindrical section 110 of the exhaust nozzle 100 is manufactured of aluminum and is designed to contain and channel a subsonic flow rate of the CO.sub.2 gas and snow flowing therethrough. The enlarged exit nozzle 130 is designed to direct a supersonic flow of the CO.sub.2 gas and snow from the exhaust aperture 132.

The contour or curvature of the inside surface of the subsonic section 110 of the nozzle 100 is designed according to the matched-cubic design procedure described by Thomas Morel in "Design of 2-D Wind Tunnel Contractions", Journal of Fluids Engineering, 1977, vol. 99. According to this design the gaseous mixture of air and CO.sub.2 flows at subsonic speeds of approximately 40 feet per second at the inlet to 1000 feet per second at temperatures of from -60.degree. F. to -120.degree. F. as it converges at the throat section 120.

The contour or curvature of the inside surfaces of tile supersonic section 130 are designed according to a computer program employing the Method of Characteristics as explained by J. C. Sivells in tile article "A Computer Program for the Aerodynamic Design of Axisymmetric and Planar Nozzles for Supersonic and Hypersonic Wind Tunnels", AEDC-JR-78-63, that can be obtained from the U.S. Air Force.

The exact contour of the enlarged exit nozzle section 130 is more particularly defined with reference to the table of dimensions as follows:

In the preferred embodiment of the present invention, the air and carbon dioxide gas, exiting from the exhaust aperture 132 of the exhaust nozzle 100 has a temperature of approximately -150.degree. F. and a velocity of approximately 1700 feet per second. The output mixture is approximately 10% by mass of solid CO.sub.2 snow which has a mean particle size of approximately 100 micrometers. The exhaust nozzle 100 was designed for an inlet pressure of approximately 100 psi and produces and exit flow Mach number of approximately 1.92. The CO.sub.2 snow exits at a velocity of approximately 600 feet per second with a generally uniform distribution. The exhaust aperture 132 is designed to be approximately 2 to 6 inches from the workpiece 90. The exhaust gases and snow exiting from the exhaust aperture 132 are generally parallel to the longitudinal axis of the nozzle 100 and do not substantially diverge. While the particle size of the CO.sub.2 snow exiting the nozzle 70 is only about 0.0005 to 0.001 inches, as a result of the coagulation and agglomeration process within the elongated cavity 64 the size of the CO.sub.2 particles exiting the exhaust nozzle 100 is approximately 0.004 to 0.006 inches. The angle of attack of the snow against the workpiece 90 can be varied from 0.degree. to 90.degree., with an angle of attack of approximately 30.degree. to 60.degree. being the best for most operations.

The method of operation of the CO.sub.2 cleaning system will now be explained. Assuming a shop air pressure of approximately 85 psi and an ambient temperature of approximately 75.degree. F., the effect of controlling the pressure and temperature of the gaseous mixture of air and liquid N.sub.2 into the mixer 50 can be illustrated with reference to FIG. 4. Point 1 on FIG. 4 represents the state of the saturated liquid CO.sub.2 within the nozzle 70 which is controlled by the controller 22 at a pressure of 300 psi and a temperature of approximately 0.degree. F. Point 2A represents a pressure of 100 psi and indicates the state of the CO.sub.2 after flashing through the orifices 72 in the injector nozzle 70. The CO.sub.2 exiting the nozzle 70 comprises CO.sub.2 in both the liquid and gaseous phase having a temperature of approximately -40.degree. F. If the pressure of the shop air in the cavity 64 is adjusted to approximately 60 psi instead of 100 psi at point 2B, then the resulting CO.sub.2 exiting from the nozzle 70 will be a combination of solid and vapor, and the temperature of the resulting combination will be approximately -80.degree. F. Therefore, the relative levels of liquid and gaseous CO.sub.2 produced in conjunction with the CO.sub.2 snow can be controlled by adjusting the pressure of the air in the cavity 64. If the air and Nitrogen mixture exiting from the mixer 50 is maintained at a temperature of approximately -50.degree. F., this would cool the CO.sub.2 mixture exiting the injector nozzle 70 so that the resulting mixture would be represented by point 2C on FIG. 4, which corresponds to a mixture of solid and liquid phase CO.sub.2. Thus, the composition of the CO.sub.2 mixture within the cavity 64 can be controlled by adjusting the pressure or the temperature of the air within the cavity 64, or both. The elongated shape of the cavity 64 allows sufficient length for the coagulation of the CO.sub.2 snow into larger particles before it enters the exhaust nozzle 100.

During the injection of the liquid CO.sub.2 through the injector nozzle into the cavity 64, a boost of up to 15 psi in the pressure within the cavity is obtained because of the partial conversion of the liquid CO.sub.2 into vapor. This increase in pressure results in an increase in the particle speeds exiting the nozzle 100 by about 10 percent, which further improves the efficiency of the cleaning process.

The inlet pressure at the cylindrical section 110 of the exhaust nozzle 100 can be varied from 40 to 300 psi, although in the preferred embodiment the pressure is designed to be from 60 to 100 psi with a temperature of between -40 to -100.degree. F. The pressure at the exhaust aperture 132 of the exhaust nozzle 130 is designed to be at atmospheric pressure, while the exit temperature is estimated to be approximately -200.degree. F. The percentage of solid to gaseous CO.sub.2 entering the exhaust nozzle 100 is estimated to be about 10-40%.

The CO.sub.2 snow produced by the first preferred embodiment of the present invention was directed at a Koki rosin baked pallet (8" by 14") of the type used in wave-soldering applications. The pallet had a coating of baked Koki rosin flux of approximately 0.005 inches in thickness, and had been through numerous wave-soldering cycles in a manufacturing environment. At a shop air pressure of 85 psi, the Koki rosin flux was completely cleaned from the pallet in about 30 seconds, whereas commercially available CO.sub.2 cleaning systems were not able to remove the accumulated flux in a similar period of time. In a similar manner, a 3 inch by 3 inch face of an FR4 printed circuit board of the type used in a speedometer assembly was coated with a combination of fluxes (including Koki) to a depth of approximately 0.003 inches and then was cleaned in approximately 5-10 seconds using the present invention. Finally, an 8 inch by 10 inch glue-plate application fixture of the type used in an electronic manufacturing assembly process was coated with approximately 0.05 inches of rosin glue and then was cleaned in approximately 120 seconds using the present invention. This performance is at least comparable to, if not better than, common available systems utilizing compacted CO.sub.2 pellets.

If the pressure of the shop air is increased from 85 psi to approximately 250 psi, then the present invention could be operated in approximately the same manner, except that direct conversion liquid CO.sub.2 snow nozzle, CO.sub.2 conversion efficiencies may be somewhat reduced. However, the system would be more expensive because the lower cost shop air equipment could not be used.

While the present invention has been particularly described in terms of specific embodiments thereof, it will be understood that numerous variations of the invention are within the skill of the art and yet are within the teachings of the technology and the invention herein. Accordingly, the present invention is to be broadly construed and limited only by the scope and spirit of the following claims.