Abstract:
A method and apparatus are disclosed for improving drying of a module, tooling, and solder stencils via the introduction of acoustic pressure waves and/or vibrational energy to the module, tooling, or solder stencils. The acoustic pressure waves may be created by a transducer where the waves are transferred to the module through air or a vibrational interface medium. The acoustic pressure waves impinge on the water droplets to atomize the droplets on the surface of the module and in the cracks, crevices and hard to reach areas of connectors and other components, without undesirable heat. The acoustic energy may further be used to assist in cleaning solder stencils within an automated screen printer.

Description:
This patent application claims priority to Provisional Patent Application No. 60/145,524 filed Jul. 24, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to the field of drying systems, and in particular to a method and apparatus for acoustic and vibrational energy assisted cleaning and drying of electronic modules and solder stencils. 
     BACKGROUND OF THE INVENTION 
     Hot air drying systems are an established method of drying bare Printed Circuit Boards (PCB&#39;s), various components on a (PCB), and tooling which may require cleaning such as stencils, board supports, and the like. There are a wide variety of equipment and processes available to manufacture, solder, clean and dry PCBs, and to clean and dry stencils and other tooling, however, the general principles of the process remain the same, as explained below. 
     After components have been assembled to the PCB, the assembly is often referred to as a module. Flux residue and other contaminants may remain on the module after assembly, necessitating the cleaning and drying process. These residues may be similar to those found on stencils and other tooling required for the assembly process. The module, stencil or other tooling may be cleaned in an aqueous cleaning system used to remove flux residue or other contaminates such as solder balls associated with the component or module manufacturing process. Once cleaned, it is important to remove all of the moisture from the interior of open components on the module and the exterior of the module. It is also necessary to remove moisture from stencils or other tooling required in the process. 
     The limitations of this process continue to be challenged with the inclusion of smaller openings within connectors, smaller gaps under components, and the like which can entrap moisture. Any excess water or moisture will cause corrosion over time. This is especially a problem when power is applied to a module which is not dry, causing a galvanic reaction and, therefore, corrosion. 
     Tooling, such as solder stencils and wave solder pallets, require cleaning after becoming contaminated with either solder paste or flux residue. Solder stencils need to have any remaining solder paste removed prior to storage. If the solder paste dries within the apertures of the stencil, the dried solder paste will interfere with the release of the solder paste during the next assembly process and cause defects. Build up of flux residue on wave solder pallets will hinder the application of the flux onto the assembly and cause defects. 
     The solder stencil printing process sometimes includes an under wiping process. The under wiping process may further apply a solvent to either the under wiping paper or the underside of the stencil. The under wiping process can further include a vacuum system which removes the loose solder particles located inside the apertures of the stencil and any solvent in the direct flow of the vacuum. The under wiping process may not sufficiently remove excess solvent remaining on the top-side of the stencil. The process may not remove all of the solder residue within the apertures of the stencil. 
     After completion of the reflow and/or wave soldering processes, the assemblies are cleaned to remove the remaining residue or contaminants. The cleaning process applies some form of liquid, generally water. Chemicals with relatively low flash points were used in the past, but those chemicals are expensive and some were found to be harmful to the environment. One of the more desirable chemicals used to clean assemblies is water. Water, or other cleaning solutions with similar flash points, are difficult to dry in a short time period. The desirable outcome of the drying process is for components and the module to be sufficiently dried to preclude corrosion. Various processes and devices are available to dry electronic modules. 
     In one case, hot air is blown over and across the module with sufficient velocity, volume and thermal content to evaporate some of the moisture and urge some of the remaining moisture off the module. The limitations of this are that the dryers require a great deal of thermal energy and large capacity air blowers to provide sufficient drying. Additionally, these dryers are generally loud and require sound dampening. Drying depends on convection of hot gases past the module. The rate of drying decreases after a portion of moisture has been removed. The last few points of moisture removal take the longest and increase the cost of drying. If one attempts to increase the temperature of the drying gas, there is a risk of thermally damaging the electronic components on the module. The efficiency of drying is proportional to the temperature of the drying gas. Thermally damaging the module sets a practical upper limit for the gas temperature. Additionally, this process continues to be limited when moisture is trapped in components such as connectors. 
     In another case, infrared energy is applied to the module in an attempt to evaporate excess moisture. This process is somewhat limited by the time required for drying excess moisture. Because of this limitation, infrared dryers are often used in conjunction with hot air dryers. Infrared energy transfers heat to the exposed surfaces; where the infrared energy would have a difficult time to evaporate entrapped moisture from within pockets of components such as connectors or under components designed to have a space between them and the surface of the PCB, such as ball grid array packages. 
     In another case, reference is made to U.S. Pat. No. 4,334,366 which teaches a method of drying objects in a perforated drum. Hot gas and sonic energy are used to dry the food objects which are tumbled in the perforated drum, and upon sufficient drying, the objects are removed from the drum. The limitations of this patent are that electronic modules cannot be tumbled in a drum and are most often processed on a conveyor to preclude damage to the module. 
     In yet another case, reference is made to U.S. Pat. No. 3,592,395 filed Sep. 16, 1968, to Lockwood et. al. This dryer uses pulsating hot gas and sonic energy to dry a stirred slurry. This dryer readily handles slurries or other fine powdery materials. This type of dryer would not work with electronic modules as any stirring of electronic modules would cause mechanical damage to the modules. 
     In yet another case, reference is made to U.S. Pat. No. 5,113,882 filed Aug. 28, 1990 to Gileta. A dryer system for a liquid cleaning apparatus has a dehumidifier to remove vapors, droplets of liquid cleaning agent and recirculate dry gas onto workpieces moving on a conveyor. Gileta teaches lowering the relative humidity within the atmosphere to increase the efficiency of the drying of printed circuit assemblies. 
     Ultrasonic transducers are used in wave soldering technology to atomize liquid flux into a fine mist and transferring the flux in mist form from the source reservoir to the bottom side and into the plated through holes of the module. This is commonly referred to as a spray fluxer. This clearly shows that moisture can be atomized when near or contacting a source of vibrational energy. 
     It can be recognized that improvements made to the drying process of modules, can also be utilized in the drying processes applied to tooling as well as stencils within solder printers and stencils in stencil cleaners. 
     While each of these improvements has contributed to the art, the limitations of these processes continue to be challenged. 
     Thus, what is desirable, is a means to reliably clean and dry electronic modules and tooling utilizing a minimal amount of energy and time and precluding any mechanical or thermal damage to the module. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the deficiencies in the art by applying acoustic pressure waves and vibrational energy proximate to the surface of the module or tooling such that the energy aids in the improvement of drying of electronic modules. The contacting and/or non-contacting pulsating energy increases the drying efficiency over the prior art solutions by atomizing moisture droplets into a fine mist, as well as allowing the combined use of prior art solutions such as hot air blowers and infrared energy. The deficiencies within the present art may further be addressed by including the same vibrational energy in conjunction with the cleaning fluid during the cleaning process to further aid in cleaning the object. 
     One aspect of the current invention is to provide a means to apply acoustic pressure waves to the desirable side of a module causing the atomizing of excess moisture. 
     A second aspect of the present invention is the use of ultrasonic sources for the acoustic pressure waves. 
     A third aspect of the present invention is to provide a means to use acoustic pressure waves to atomize moisture in small openings, crevices and hard to reach places. 
     A fourth, aspect of the invention is to combine at least one of acoustic pressure waves and vibrational energy to prior art solutions such as hot air knives allowing improved efficiency and lower time and energy costs. 
     A fifth aspect of the present invention is the ability to dry tooling such as wave solder pallets or solder stencils. 
     A sixth aspect of the present invention is the inclusion of a vibrational energy source within an under stencil cleaner within an automated or semi-automated solder paste printing apparatus. 
     A seventh aspect of the present invention is the inclusion of a vibrational energy source in conjunction with a fluid to aid in the cleaning process. 
     An eighth aspect of the present invention is the inclusion of a vibrational energy source in conjunction with the cleaning process of an under stencil wiper. 
     A ninth aspect of the present invention is the inclusion of a vibrational energy source to atomize residual moisture on stencils. 
     A tenth aspect of the present invention is the inclusion of a vibrational energy source to atomize residual moisture on stencils, used in conjunction with an under stencil cleaner. 
     An eleventh aspect of the present invention is the combination of using the vibrational energy source for both cleaning and drying. 
     A twelfth aspect of the present invention is the inclusion of a vibrational transfer medium placed between the vibrational energy source and the object. 
     A thirteenth aspect of the present invention is the use of air to transfer the vibrational energy to the object. 
     A fourteenth aspect of the present invention is the use of at least one of ultrasonic transducer, speaker, tuning fork, horn, and sonar transducer to generate the vibrational energy. 
     A fifteenth aspect of the present invention is the application of the disclosed technology within an automated in-line cleaner. 
     A sixteenth aspect of the present invention is the application of the disclosed technology within an automated stencil cleaner. 
     A seventeenth aspect of the present invention is the application of the disclosed technology within an automated solder printer. 
     An eighteenth aspect of the present invention is a means for changing the relational proximity of the vibrational energy source and the at least one of electronic assemblies and tooling related to manufacture of electronic assemblies to one another. One example would be a conveyor. A second would be a means for moving the vibrational energy source. 
     The present invention comprises an apparatus used to apply acoustic pressure waves and resulting vibrational energy to a module to atomize moisture during the drying phase of assembly of the module. The apparatus preferably includes equipment known in the art: a conveyor to move the module through an in-line cleaner wash, rinse, and drying sections, which can include hot air blowers and infrared heaters. Alternatively, the present invention may be included within other equipment known in the art such as stencil cleaners and batch cleaners or solder printers. 
     The present invention discloses the application of acoustic pressure waves and resulting vibrational energy to atomize excess moisture and entrapped moisture in hard to reach cracks and crevices, thus overcoming the surface tensional forces and allowing increased efficiency of the hot air dryers and the infrared heaters. The acoustic pressure waves, generated by a transducer and transferred to the module through the air, will impinge the module at the angle proscribed and not be substantially affected by the volume or velocity of the hot air flow caused by the hot air dryers. 
     The present inventions further discloses the application of vibrational energy through close proximity to dry preferably planar tooling such as stencils where heat is not desirable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional side elevation illustrating a module with water droplets on the top surface and bottom surface. 
     FIG. 2 is a sectional side elevation illustrating a module where the water droplets have been removed. 
     FIG. 3 is a flow diagram which illustrates the general assembly and cleaning process in conjunction with the present invention. 
     FIG. 4 is a sectional side elevation of a module being dried utilizing acoustic pressure waves and hot gas. 
     FIG. 5 is a sectional side view of a stencil cleaner in the drying cycle in conjunction with the use of ultrasonic transducers. 
     FIG. 6 is a sectional side view of a solder stencil cleaning apparatus using vibrational energy for cleaning and drying. 
     FIG. 7 is a sectional side view of a solder stencil in conjunction with an under wiping system with the use of an ultrasonic transducer for drying. 
     FIG. 8 is a sectional side view of a solder stencil in conjunction with an under wiping system with the use of an ultrasonic transducer for cleaning and drying. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a sectional side elevation of a module  13 . The module  13  consists of a Printed Circuit Board (PCB)  10  which has two 2 sides; a solder side  11  and a component side  12 . Electromechanically attached to the illustrated PCB  10  are surface mount components  14 , a leaded component  15 , and a connector  18 . An assembly with a PCB  10 , surface mount components  14 , a leaded component  15 , and a connector  18  is collectively referred to as a module  13 . After the module  13  is washed, moisture droplets  16  may be found on the solder side  11  and the component side  12  of the module  13 . Surface tension may cause the moisture droplets  16  to adhere to the module  13  increasing the difficulty of drying. Additionally, moisture droplets  16  may become entrapped in the cavities  19  within the connector  18 , or under components such as Quad Flat Packs (QFPS&#39;s), Thin Small Outline Packages (TSOP&#39;s), Ball Grid Arrays (BGA&#39;s), Micro Ball Grid Arrays (uBGA&#39;s) and the like, further increasing the difficulty of drying, as forced air and infrared heating can not accelerate the evaporation of moisture within the cavities  19 . 
     FIG. 2 illustrates a module  13  in the desirable state, which has been through the drying process. All of the moisture droplets  16  which were previously shown in FIG. 1 have been removed by the drying process. Additionally, moisture droplets  16  entrapped in the connector  18  and under components have also been removed. 
     FIG. 3 is a flowchart which illustrates the assembly and drying process. The process begins with the first step  30  where surface mount components  14  and leaded components  15  are assembled onto a PCB  10 . This can be accomplished using any of the known technology. During the second step  32 , the module  13  is placed on a conveyor (not shown) of an automated cleaning apparatus (not shown). During the third step  34 , the module  13  is washed. During the fourth step  36  the module  13  is rinsed. During the fifth step  38 , vibrational energy  50  in the form of pressure waves (shown in FIG. 4) is directed towards to the module  13 . During the sixth step  40 , hot gas  54  (shown in FIG. 4) can be applied to the module  13 . During the seventh step,  42  infrared energy (not shown) can applied to the module  13 . It can be recognized that the sixth step  40  and the seventh step  42  can be used independently, or any other technology currently known to assist in drying a module  13 . It can also be recognized that the use of vibrational energy  50  can be applied in simultaneously with any other known drying apparatuses. During the final step  44 , the module  13  is removed from the conveyor of the automated cleaning apparatus (not shown). 
     FIG. 4 illustrates a module  13  traveling on a path  46  from left to right, the path representative of a conveyor. The module  13  has moisture droplets  16  on the component side  12 , the solder side  11  and entrapped in the connector  18 . At least transducer  48  emits vibrational energy  50  in the form of pressure wave which impinge the module  13  and the moisture droplets  16  at a predetermined angle. The vibrational energy  50  atomizes the moisture droplets  16  on the component side  12 , on the solder side  11 , in the cavities  19  of the connector  18 , and under components  14 . By atomizing the moisture droplets  16 , the process reduces the gathered volume per droplet and increases the surface area per droplet. These two changes increase the rate of evaporation. A second drying means can be used to further dry the module  13 . The figure illustrates a hot gas  54 , preferably hot air, can be emitted from a hot air nozzle  52  which is directed at the module  13  in a predetermined angle. The hot air  54  evaporates the atomized moisture droplets  16  and removes them from the module  13 . The module  13  may then be treated with infrared energy to complete the drying process. Other methods can include forced air using turbines, infra-red heating, convection heating, and the like. 
     FIG. 5 illustrates an embodiment utilized to assist in the drying process used during the cleaning of tooling such as stencils and wave solder pallets. A stencil  60  is mounted within a stencil frame  61 . The stencil  60  includes apertures  62  which are used to transfer a pattern of solder paste to the PCB during the assembly process. Upon completion, the solder paste must be removed from the stencil and from within the apertures  62 . The optimal process to remove the solder paste from the apertures  62  of the stencil is to utilize a stencil cleaner  68 . A cleaning fluid is applied to the stencil  60  to remove the remaining solder paste. The cleaning fluid leaves moisture droplets  16  on the surfaces of the stencil  60  and within the apertures  62 . At least one transducer  48  emits vibrational energy  50  shown in the form of pressure waves which impinge the stencil  60  and the moisture droplets  16  at a predetermined angle. The acoustic and vibrational energy  50  turn the moisture droplets  16  on the stencil  60  and in the apertures  62  into atomized moisture droplets  66  on the stencil and suspended in the air. By atomizing the moisture droplets  16 , the process reduces the gathered volume per droplet  16  and increases the surface area per droplet  16 . These two changes increase the rate of evaporation. Other drying methods can be employed as described for FIG.  4 . One can recognize this apparatus may be used for drying other objects such as bare PCB&#39;s, batch cleaning of assembled modules  13 , or tooling such as wave solder pallets and squeegee blades. It can be recognized that the hot air nozzle  52  and the at least one transducer  48  may be on a varying angle or on a moving carriage (not shown) to increase the drying efficiency. 
     FIG. 6 illustrates a cross sectional view of a stencil cleaner  68  using vibrational energy  50  in close proximity to or contacting the stencil. Illustrated is an transducer  48  such as an ultrasonic horn located proximate the stencil  60 , whereby the transducer  48  passes across  70  the stencil  60 . A vibrational interface medium  126  can be coupled to the transducer  48  to place a thermal barrier between the transducer  48  and stencil  60  to reduce heat transfer, while mechanically transferring vibrational energy  50 . The vibrational energy  50  can be used in conjunction with fluids to aid in removing solder paste residue (not shown) from the stencil  60  and apertures  62 . The same vibrational source  48  can be used for both cleaning assistance and drying. 
     FIG. 7 illustrates a cross sectional view of a solder stencil  60  and under wiping system  100  in conjunction with a preferred embodiment of the present invention. The system described would normally be found within a semi-automated or automated solder screen printer (not shown), but it can be recognized that it may be applied to other system devices. The solder screen printer includes a stencil  60  and a squeegee  108 , where the squeegee  108  passes a material such as solder paste  100  across a plurality of stencil apertures  62 . The stencil  60  would separate from the object, such as a printed circuit board (PCB) (not shown) depositing most of the material, such as solder paste  110  onto the object. Solder paste residue  112  sometimes remains within the stencil apertures  62  or along the bottom (contact) side of the stencil  125 . Under wipe systems such as the one described with solvent  107 , wiper paper  102 , and vacuum  105  are already known. The solvent dispenser  106  applies a solvent  107  from a solvent reservoir  128  generally to the under wiping paper  102 . The under wiping paper  102  is transferred between the two under wiper paper handling rollers  116  and pressed against the bottom (contact) side  125  of the stencil  60  by an under wiper paper support  114 . The solvent  107  soaked under wiping paper  102  passes across the stencil  60  removing the undesirable residual solder paste  112  from the bottom (contact) side  125  of the stencil  60 . The wiping process leaves resident moisture (solvent)  64  inside the stencil apertures  62  and on the top (squeegee) side  124  of the stencil  60 . A vacuum system  118  is introduced to remove solder paste residue  112  and resident moisture  64  from inside the stencil apertures  62 . The vacuum system  118  includes a vacuum fan  120  which provides a vacuum force  105  to a vacuum nozzle  104 . The vacuum nozzle  104  and vacuum force  105  collects the solder paste residue  112  and resident moisture  64  from inside the stencil apertures  62  and transfers it to a collection filter  122 . The vacuum system  118  may not remove resident moisture  64  from the top (squeegee) side  124  of the stencil  60 . An ultrasonic transducer  48  applies acoustic and vibrational energy  50  to the stencil  60 , preferably without contacting the stencil  60 . This may be accomplished by providing an air gap or a vibrational interface medium  126  between the transducer  48  and the stencil  60 . The vibrational energy  50  causes the resident moisture  64  to atomize into atomized moisture  66  and lift off the top (squeegee) side  124  of the stencil  60 . The preferred embodiment would be to incorporate all features into one apparatus. It may also be recognized that the transducer  48  may be incorporated within the vacuum nozzle  104 . It can be recognized that other under wiping and/or under wiping vacuum systems exist or may be developed which should not limit the spirit or intent of the present invention. 
     FIG. 8 illustrates a stencil cleaning apparatus  100  similar to FIG. 7 with the additional utility of using vibrational energy  50  in conjunction with fluid  107  applied to the stencil  60  to aid in the cleaning process to further assist in loosening flux and particles  112  from the stencil  60  and apertures  62 . Fluid can be retained within the apertures by the wiping paper  102  or other mean, including surface tension. The vibrational energy  50  would further assist in drying the stencil  60  by atomizing  66  residual moisture droplets  64 . A vacuum system  104 ,  118  as described by FIG. 7 can further be included to remove loosened particles  62  and remaining moisture droplets  64 . The system can be incorporated within any variation of stencil cleaning system based upon the principles described herein. 
     Various changes may be made to the embodiment shown herein without departing from the spirit and scope of the present invention.