Abstract:
An ultrasonic cleaning tank for use in cleaning electronic parts having a top portion and a bottom portion operably divided by a perforated dispersion plate. The cleaning tank is assembled to avoid internal projections or obstructions within the top portion to create a piston-like, laminar flow region. The dispersion plate is constructed to provide a backpressure within the bottom portion so as to promote even flow of a cleaning fluid through the perforations. The cleaning fluid flows upward past an electronic part. At the same time, an ultrasonic transducer supplies ultrasonic energy within the cleaning fluid creating cavitation such that any particulate matter is scrubbed from the electronic parts. The particulates are subsequently carried upward by the laminar flow and over a tank lip. The cleaning tank can be used in either a batch or recirculating mode.

Description:
PRIORITY CLAIM 
   The present invention claims priority to U.S. Provisional Application Ser. No. 60/444,752 entitled, “ULTRASONIC CLEANING TANK”, filed Feb. 4, 2003, and hereby incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to an ultrasonic system for precision cleaning of parts. In particular, the invention relates to an ultrasonic cleaning system that includes a cleaning tank with an internal dispersion plate adapted to promote upward laminar flow within the cleaning tank for improved part cleaning. 
   BACKGROUND OF THE INVENTION 
   Precision cleaning and drying systems typically utilize a wide variety of cleaning solutions including various solvents, detergents, or other aqueous mixtures. These systems operate to clean and dry various devices or parts such as medical devices, optical instruments, wafers, PC boards, hybrid circuits, disk drive components, precision mechanical or electromechanical components, or the like. In the precision cleaning industry in particular, there exists a need for an efficient cleaning system generally having a high tank turnover rate. 
   Ultrasonic systems for processing and cleaning parts within a tank are generally known. In a typical prior art ultrasonic system, the tank contains a cleaning solution and the parts to be cleaned are introduced therein. Ultrasonic energy is applied to the tank, and the ultrasonic vibrations generate pressure gradients within the cleaning solution, forming minute cavitation bubbles. These cavitations implode against a surface of the part to be cleaned releasing tremendous energy thereby dislodging contaminants. 
   In prior art systems, the ultrasonic energy is turned off while the solution within the tank is refreshed. For example, new or filtered solution is pumped into bottom of the tank, while the solution within the tank containing the contaminants overflows one or more sides out of the tank, to be filtered and reused or discarded. It is necessary to apply ultrasonic energy separately from refreshing the tank in these systems because the turbulence associated with a high rate of tank refreshing flow disrupts the ultrasonic wave pattern that produces the ultrasonic cavitations. In prior art ultrasonic systems, mixing of contaminants within the tank with the refreshed solution still occurs such that the contaminants are eliminated slowly in a logarithmic manner over time. Logarithmic elimination of all contaminants theoretically takes an infinite amount of time, greatly reducing the overall turnover clean up rate. 
   One prior art ultrasonic system, described in U.S. Pat. No. 6,181,052, attempted to create laminar flow within the tank by including at least two baffles at the bottom of the tank. The purpose of the baffles was to reduce the velocity of the incoming cleaning solution, equalize the pressure of the clean solution, and introduce the solution in the bottom of the tank with equal spatial distribution. However, these baffles as described have two serious shortcomings to achieve the desired results. First the upper baffle was welded into place within the tank, or mounted within the tank such that the mounting bracket interferes with uniform flow up along the sidewalls of the tank, which introduces a counter-current within the tank causing turbulent mixing which again slows down the elimination of contaminants from the tank and the overall turnover rate. Secondly, the large open area of this baffle plate, a minimum of 45% open, prevents uniform upward flow from developing by failing to develop uniform pressure behind the second baffle. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to create laminar flow characteristics within an ultrasonic cleaning tank by providing a diffusion plate having a predetermined number of perforations of a calculated size. This method allows for uniform flow without interference at the sidewalls and provides a high turnover at a given flow rate to achieve efficient cleaning. By providing an external flange-mounted diffusion plate that is removable, an appropriate diffusion plate can be provided to accommodate different flow and turnover rate requirements of the ultrasonic cleaning system. The external flange design allows the construction of a cleaning tank with no obstructions to induce turbulence within the cleaning fluid. Further, the external flange design provides a simple means for removing the plate to make modifications if required. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a cleaning tank of the present invention. 
       FIG. 2  is a perspective view of the cleaning tank of  FIG. 1 . 
       FIG. 3  is a top view of a lower tank assembly. 
       FIG. 4  is a top view of a dispersion plate. 
       FIG. 4A  is a top view of an alternative embodiment of a dispersion plate. 
       FIG. 5  is a top view of a plurality of perforations on the dispersion plate of  FIG. 4 . 
       FIG. 5A  is a top view of a plurality of perforations on the dispersion plate of  FIG. 4A . 
       FIG. 6  is a flow diagram of an embodiment of a recirculating ultrasonic cleaning system of the present invention. 
       FIG. 7  is a flow diagram of the cleaning tank used in the recirculating ultrasonic cleaning system of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  illustrate a cleaning tank  100  of the present invention. Cleaning tank  100  typically has a welded construction using stainless steel. Alternatively, cleaning tank  100  can be constructed of other materials when the use of stainless steel is not recommended. Alternative materials could include tantalum, titanium, quarts or plastics such as PEEK. As depicted, cleaning tank  100  has a rectangular cross-section though other geometrical configurations, such as cylindrical can be used without departing from the scope of the present invention. 
   As shown in  FIGS. 1 and 2 , Cleaning tank  100  comprises an upper tank assembly  102 , a lower tank assembly  104 , a dispersion plate  106  and a pair of flange gaskets  108   a ,  108   b . Flange gaskets  108   a ,  108   b  are comprised of a suitable gasket material that is both chemically inert and non-leaching. For example, flange gaskets  108   a ,  108   b  can comprise polymers such as Teflon, PVDF, EPDM, Viton or perflourinated elastomer. Upper tank assembly  102  includes a top lip  110  and an upper perimeter flange member  112 . Lower tank assembly  104  includes a floor  116 , an inlet port  118  and a bottom perimeter flange member  120 . Floor  116  as shown in  FIG. 3  can further include an inlet plate  122  mounted above the inlet port  118 . Upper perimeter flange member  112  and bottom perimeter flange member  120  are substantially identically shaped and sized. 
   Preferably, dispersion plate  106  comprises the same material of construction as cleaning tank  100 , for example stainless steel. Dispersion plate  106  is constructed so as have essentially the same size and shape as defined by the upper perimeter flange member  112  and the bottom perimeter flange member  120 . As illustrated in  FIG. 4 , dispersion plate  106  includes a plurality of spaced apart perforations  124 . Perforations  124  are preferably uniform and can be formed by processes including laser cutting, mechanical punching, drilling or other suitable mechanical operations. In a preferred embodiment, perforations  124  are arranged in a close hex pattern  126  on the dispersion plate  106  as shown in  FIG. 5 . Perforations  124  are preferably circular but can be can be fabricated in other geometric configurations, for example squares, circles, ovals, rectangles or other suitable shapes. Perforations  124  are configured to have a perforation diameter  128  as small as possible for the specific cleaning application, for example, between 0.001 inches to 0.250 inches. When manufactured, a total perforation area  129  representing the sum of all the perforations  124  represents an amount slightly less than, equal to or greater than an inlet area  130  of the inlet port  118 . In all embodiments, the total perforation area  129  represents less than 45% percent of the total area of the dispersion plate  106 . 
   In assembling the cleaning tank  100 , the dispersion plate  106  is placed over the bottom perimeter flange member  120  such that flange gasket  108   a  resides between them. Flange gasket  108   b  is placed on top of the dispersion plate  106 . Finally, upper tank assembly  102  is positioned such that the upper perimeter flange member  112  resides on top of the flange gasket  108   b.  The lower tank assembly  102  and upper tank assembly  104  can then be operably coupled with a plurality of fasteners  132 , for example nuts and bolts that project through aligned bores in the bottom perimeter flange member  120 , the dispersion plate  106  and upper perimeter flange member  112 . Fasteners  132  can be exterior to or pass through the flange gaskets  108   a,    108   b.  In an alternative embodiment, fasteners  132  can take the form of external clamps, for example c-clamps. By assembling the cleaning tank  100  in such a manner, it is possible to removably exchange alternative configurations of the dispersion plate  106 , i.e., a second dispersion plate  107  having differing perforation  124  geometries, sizes and/or quantities. By varying the perforations  124 , dispersion plate  106  and second dispersion plate  107  can be tailored for specific cleaning rates, part geometries and/or part loading arrangements. 
   Cleaning tank  100  can be used as part of a single-pass or recirculating ultrasonic cleaning system. A recirculating ultrasonic cleaning system  150  is shown schematically in  FIG. 6 . Generally, the recirculating ultrasonic cleaning system  150  comprises the cleaning tank  100 , a pump  152 , an in-line filter  154  and a weir assembly  156 . In a preferred embodiment, pump  152  has a pumping capacity providing for at least one tank volume per minute or more. Pump  152  preferably has an adjustable pump speed for varying flow rates based upon a variety of cleaning variables. In-line filter  154  comprises a commercially available in-line filter including a filter media, for example polyether sulfone, Teflon, PVDF, polyester, or polypropylene, capable of removing particulates down to 0.03 microns in size. As shown in  FIG. 7 , cleaning tank  100  includes a plurality of exterior bonded, ultrasonic transducers  158 . In a preferred embodiment, ultrasonic transducer  158  is a Crest Ultrasonic Corp. ceramic enhanced transducer supplying ultrasonic energy at a suitable frequency of between 28 KHz and 2.5 MHz. Ultrasonic transducers  158  are bonded directly to the exterior of the upper tank assembly  102  with an adhesive such as epoxy. Recirculating ultrasonic cleaning system  150  can further comprise an inline heat exchanger  160 . In addition, recirculating ultrasonic cleaning system  150  can include a degasification unit  162  for removing dissolved gases, which can have adverse effects on the delivery of ultrasonic energy. While not depicted, it will be understood that recirculating ultrasonic cleaning system  150  can include suitable valve and or sensors for use during operation and draining. 
   To use recirculating ultrasonic cleaning system  150 , a electronic, medical or optical part is placed within the cleaning tank  100 , typically using a basket, a rack or a cleaning fixture, adapted for insertion into the cleaning tank  100 . Prior to placing the loaded within the cleaning tank  100 , the cleaning tank  100  is filled with a cleaning solution  166 . Cleaning solution  166  can be suitable aqueous, semi-aqueous or solvent based solutions comprising any combination of deionized water, detergents, or any number of suitable organic solvents alone or in mixtures. When cleaning solution  166  is an aqueous or semi-aqueous solution, inline heat exchanger  160  selectively heats or cools to maintains the temperature of the cleaning solution  166  in the recirculating loop between ambient and two hundred degrees F. 
   Once cleaning tank  100  is filled with the cleaning solution  166  and the loaded basket, a process logic controller (PLC) can be used to start the pump  152  to recirculate the cleaning solution  166  through the in-line filter  154  and into the cleaning tank  100  through the inlet port  118 . The flow within the cleaning tank  100  is shown in  FIG. 7 . At inlet port  118 , incoming cleaning solution  166  is distributed to the sides of cleaning tank  100  with inlet plate  122 . The combination of inlet plate  122  and the backpressure applied by dispersion plate  106  results in a turbulent flow pattern  168  within the lower tank assembly  104 . The backpressure applied by dispersion plate  106  causes the cleaning solution  166  to distribute and flow upward evenly through the perforations  124  and into the upper tank assembly  102 . The even flow of the cleaning solution  166  through the perforation  124  results in a substantially parallel, laminar flow pattern  170  within the upper tank assembly  102 . The laminar flow pattern  170  is maintained as cleaning solution  166  approaches the top lip  110  as there are no internal projections or obstructions along the sides of upper tank assembly  102  to disrupt the substantially parallel, upward flow of the cleaning solution  166 . 
   As the cleaning solution  166  flows upward through the upper tank assembly  102 , the ultrasonic transducer  158  supplies ultrasonic energy within the cleaning solution  166 . The ultrasonic energy causes alternating patterns of low and high pressure phases within the cleaning solution  166 . In the low pressure phase, bubbles or vacuum cavities are formed. In the high pressure phase, the bubbles implode violently. This process of creating and imploding bubbles is commonly referred to as cavitation. Cavitation results in an intense scrubbing process along the surface of the parts causing any particulate to be removed from the parts. The bubbles created during cavitation are minute and as such are able to penetrate microscopic crevices to provide enhanced cleaning as compared to simple immersion or agitation cleaning processes. 
   When particulates are removed from the part, the laminar flow pattern  170  carries the particulate upward and over the top lip  110 . Once cleaning solution  166  overflows the upper tank assembly  102 , the cleaning solution  166  and any removed particulate flows into the overflow weir  156 . Overflow weir includes a drain whereby the cleaning solution  166  and any particulates are returned to an inlet side of the pump  152 . Pump  152  circulates the cleaning solution  166  and particulates through the in-line filter  154  whereby the particulate is retained and the cleaning solution  166  is again directed into the cleaning tank  100  through the inlet port  118 . 
   In a preferred embodiment, the recirculating ultrasonic cleaning system  150  is fully contained within a cabinet to present a pleasing, aesthetic appearance. In such a cabinetized system, a user need only supply the cleaning solution  166 , a dispersion plate  106  including the desired perforation configuration, the parts and an electrical power source to power the recirculating ultrasonic cleaning system  150 . 
   It is understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only.