Abstract:
When dross is removed from a molten-solder reservoir, a substantial amount of molten solder is often removed with the dross. This molten solder can be separated from the dross and returned to reservoir via a conduit for reuse. Additionally, a skimmer for removing the dross from the reservoir includes a skimming plate pivotally attached to a displaceable structure; a stop is provided to restrict the degree to which the skimming plate can pivot so that the skimming plate will not pivot more than 90° from vertical to enable the skimming plate to dig into the dross and collect dross when the displaceable structure is displaced toward on outlet of the reservoir. Further still, the displaceable structure of the skimmer can be controlled via a computer control system storing software code instructions for a motor to extend and retract the displaceable structure such that the skimming plate extends to a position further from the outlet port of the reservoir with each displacement cycle.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S.S.N. 09/759,506, filed Jan. 12, 2001, the entire disclosure of which is incorporated herein by reference. This application is also a continuation-in-part of U.S.S.N. 10/051,407, filed Jan. 18, 2002, the entire disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    When many molten metals contact air, compounds of those metals, primarily oxides but also nitrides and other non-metallic impurities, can be formed. When the molten metal is moving, the metal and the metal oxide combine to form a material known as dross. The dross often forms a sponge-like network and has a lumpy, granular appearance.  
           [0003]    In a wave soldering device, such as that described in U.S. Pat. No. 6,431,431 B2, the disclosure of which is incorporated by reference herein in its entirety, the dross floats on the surface of a molten solder bath. The molten solder is pumped through a nozzle to form a wave. Substrates, such as printed wiring boards, are then passed over and into contact with the wave to deposit solder on the substrate surface. If the dross is allowed to build up, the dross can become entrained in the solder wave and adversely affect the quality of soldering  
           [0004]    Accordingly, the dross is periodically removed from the solder reservoir to prevent contamination of the solder. Existing methods of dross removal also often remove a substantial amount of usable molten solder along with the dross. It is estimated that dross removed from the surface of a molten solder bath can included 30-90% usable solder that can be reclaimed for reuse.  
           [0005]    Where molten solder has been separated from the dross after removal from the solder reservoir, the molten solder is cooled to form a brick. The brick is then returned to the solder reservoir, where the heat of the molten-solder reservoir re-melts the brick so that the solder is again usable for application to a printed wiring board.  
         SUMMARY  
         [0006]    Disclosed herein are apparatus and methods for (a) removing dross from a molten-solder reservoir, wherein an amount of molten solder is removed with the dross; (b) separating the molten solder from the removed dross; and (c) returning the separated molten solder to the molten-solder reservoir via a conduit. The reservoir, separation device and return conduit can all be positioned adjacent one another. These apparatus and methods provide a highly efficient means of recycling the molten solder that was trapped with the dross without solidifying the solder at any point during this process.  
           [0007]    Also disclosed is a skimmer for removing the dross from the reservoir. The skimmer includes a skimming plate pivotally attached to a displaceable structure, such as a trolley mounted in a track. A stop is provided to restrict the degree to which the skimming plate can pivot so that the skimming plate will not pivot more than 90° from vertical. Accordingly, the stop positions the skimming plate so that it will dig into and collect dross when the displaceable structure is displaced toward on outlet of the reservoir. This configuration enables frequent and repeated reciprocation of the skimmer while preventing the skimming plate from floating horizontally atop the dross when the skimmer is displaced toward the outlet of the reservoir.  
           [0008]    Further still, the displaceable structure of the skimmer can be controlled via a computer control system storing software code instructions for a motor to reciprocally displace the displaceable structure such that the skimming plate extends to a position progressively further from the outlet port of the reservoir with each displacement cycle. This approach offers a manageable, incremental procedure for removing dross without overloading the skimmer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    In the accompanying drawings, described below, like reference characters refer to the same or similar parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating particular principles of the methods and apparatus characterized in the Detailed Description.  
         [0010]    [0010]FIG. 1 is a cross sectional side view of one embodiment of an automatic dross removal apparatus in accordance with the invention.  
         [0011]    [0011]FIG. 2 is a front isometric view of a dross reclamation device used in conjunction with the automatic dross removal apparatus of FIG. 1.  
         [0012]    [0012]FIG. 3 is a cross sectional side view of another dross reclamation device used in conjunction with the automatic dross removal apparatus of FIG. 1.  
         [0013]    [0013]FIG. 4 is a perspective view of another embodiment of an automatic dross removal apparatus in accordance with the invention.  
         [0014]    [0014]FIG. 5 illustrates the drive mechanism for displacing the skimmer in FIG. 4.  
         [0015]    [0015]FIG. 6 is a top view of the automatic dross removal apparatus of FIG. 4.  
         [0016]    [0016]FIG. 7 is a sectioned side view of the dross removal apparatus illustrated in FIGS.  4  and  
         [0017]    [0017]FIG. 8 is a cross sectional side view of yet another embodiment of an automatic dross removal apparatus in accordance with the invention.  
         [0018]    [0018]FIG. 9 is a front isometric view of the conveyor of the automatic dross removal apparatus of FIG. 8.  
         [0019]    [0019]FIG. 10 is an illustration, partially sectioned, of a solder reclamation apparatus.  
         [0020]    [0020]FIG. 11 is a cross-sectional illustration of the details of the impeller housing in the apparatus shown in FIG. 10.  
         [0021]    [0021]FIG. 12 is a flow chart illustrating the processing of signals from sensors for measuring the height of dross in the solder reclamation apparatus of FIG. 11 and the decision-making based thereon.  
         [0022]    [0022]FIG. 13 is a view of a skimmer mounted on a reservoir filled with molten solder.  
         [0023]    [0023]FIG. 14 is a magnified side view of the skimming plate and carriage of the skimmer illustrated in FIG. 13.  
         [0024]    [0024]FIGS. 15A, 15B and  15 C illustrate sequential passes of the skimmer of FIGS. 13 and 14 over the molten solder bath, the skimming plate extending incrementally further across the bath with each pass. 
     
    
     DETAILED DESCRIPTION  
       [0025]    [0025]FIG. 1 shows an embodiment of the automatic dross removal apparatus  10 . The apparatus  10  includes a solder reservoir  12  and a conveyor  14  that is driven by a motor  16 . The reservoir  12  contains liquid solder  18 , which has dross  20  forming on its top surface. A dross removal mechanism, such as a conveyor  14 , has one end protruding into the solder reservoir  12  to skim the dross  20  from the surface of the solder  18  and carry it out of the reservoir  12 . The conveyor  14  is driven by a motor  16  that drives a sprocket  30  that engages and drives the endless belt or chain of the conveyor  14  for removing the dross  20  from the reservoir  12 . The conveyor  14  can have one or more brackets  15 , such as an angle bracket and can be adjustably positioned (e.g., raised or lowered) to allow the brackets  15  to sweep across the top surface or beneath the surface of the liquid solder  18 . When power is supplied to the motor  16 , the conveyor  14  draws the angled bracket  14  forward, dragging the dross  20  on the conveyor  14  and over the edge of the solder reservoir  12 . The conveyor motor  16  is preferably driven by a computer  32  such that the dross removal apparatus  10  can operate automatically with or without operator intervention. Alternatively, the conveyor motor  16  can be controlled by a switch (not shown). Preferably, the conveyor motor  16  is computer controlled through timers with conventional overload protection for the device.  
         [0026]    The conveyor  14  or other dross removal mechanism removes the dross from the solder reservoir, sends it over chute  17  and deposits it into a container or a dross separation device  34  for further processing. One embodiment of the dross separation device  34 , which is shown in FIGS. 1 and 2, includes a first portion  36  and a second portion  38 . The first and second portions  36  and  38  are movable in relation to one another such that the dross  20  recovered from the solder reservoir  12  can be compressed to remove any usable molten solder. For example, the first portion  36  of device  34  can include a pivot  40  or hinge and be connected to a piston  42  or cylinder that is driven by a motor  43 . The second portion  38  can include holes  44  of an appropriate size such that molten solder drops  45  flow through the holes and drop into a receptacle  46 .  
         [0027]    As shown in FIG. 2, the dross separation device  34  can include an ejection device  48  that can include, for example, a plate  50  for ejecting the solder depleted dross, and a piston  51  or cylinder that is driven by a motor  52 . The ejection device  48  can eject dross by moving the plate  50  outwardly against the dross separation device  34  after the first portion  36  is opened such that the first portion  36  is pivoted outwardly from the second portion  38 . A conveyor chute  19  for depositing the dross into the dross separation device  34  can also be provided.  
         [0028]    [0028]FIG. 3 shows an embodiment of the automatic dross removal apparatus  60  that uses a dross separation device  62 , which separates the dross into receptacle  64  and reusable solder into receptacle  46 . The dross separation device  62  can be a device such as the dross muncher manufactured by the company, Solet, located in the United Kingdom. FIGS.  4 - 7  show a second embodiment of the automatic dross removal apparatus  70 , wherein the reservoir  72  is partially cut-away and the molten solder is made transparent to show elements of the pumping apparatus. The apparatus  70  has a solder reservoir  72  and a dross skimmer  74  having a skimming plate  77  protruding into the reservoir  72  for moving or pushing the dross within its pathway. The apparatus  70  also has means for moving the dross within the reservoir towards the skimmer assembly  94 , such as one or more pumps  76  and pressure nozzles  78  (see FIG. 7), wherein the pumps  76  pump solder and dross through channels  83  and then out through the nozzles  78  (see FIG. 7) above the bath surface. The solder flows out of the nozzles  78  in one direction within channels  81  flushing any dross with it, toward the skimmer apparatus  94 , which removes the dross. A gas, such as nitrogen, that is inert with solder can be used to pressurize the nozzle to remove and/or limit oxygen to further reduce dross formation.  
         [0029]    Like the dross removal apparatus  10  of FIG. 1, the dross removal apparatus  70  of FIG. 4 can be driven by a motor  82  (in this case, a rotary motor) for automatically moving the skimmer  74  to place the dross in a receptacle  75 . The motor  82  is preferably driven by a computer  84  such that the dross removal apparatus  70  can operate automatically with or without operator intervention. Alternatively, the motor  82  can be controlled by a switch. Preferably, the motor  82  is computer controlled via timers with overload protection for the device.  
         [0030]    As FIG. 5 illustrates, the rotary motor  82  reciprocally drives a cable  79  back and forth via a pulley  85  over rollers  87 . The skimmer  74 , which has a pivotally mounted skimming plate  77 , is mounted to the cable  79  to reciprocate back and forth as the cable  79  is reciprocally displaced. As the skimmer  74  is drawn toward the motor  82 , the skimmer  74  is pulled across the molten solder at, near, or into the top surface of the molten solder and up a ramp  119  along a track  165  that positions the skimming plate  77  in contact with the ramp  119  (see FIG. 4) to or beyond the edge of the reservoir  72  so as to expel the dross that is captured by the skimmer  74  from the reservoir  72 .  
         [0031]    [0031]FIGS. 8 and 9 show a second embodiment of an automatic dross removal apparatus  110 . The dross removal apparatus  110  is generally similar to the dross removal apparatus  10  of FIG. 1, with the exception that the conveyor  140  moves in a direction opposite to that of the conveyor  14  in a direction shown by arrow  115 . As shown in FIG. 8 and best in FIG. 9, the conveyor  140  has two endless chains  134  that support a plurality of L-shaped brackets  131 . The brackets  131  preferably have one or more slots  136  to allow liquid solder  118  to pass therethrough. The brackets  131  typically penetrate into molten solder  118 , though the brackets  131  can still be used to remove dross  120  if they do not extend down to or into the molten solder  118 , as illustrated in FIG. 8. The brackets  131  contact the ramp  119  as the brackets  131  are drawn up the ramp  119 .  
         [0032]    The dross removal apparatus  110  includes a solder reservoir  112  and a conveyor  114  that is driven by a motor  116 . The reservoir  112  contains liquid solder  118  which has dross  120  forming on its top surface. A conveyor  122  has one end protruding into the solder reservoir  112  to skim the dross  120  from the surface of the solder  118 , push it up the surface of ramp  119  and dump it into receptacle  175 . Alternatively, the dross could be placed in a dross separation device, as shown in either FIGS.  2  or  3 . The conveyor  114  is driven by a motor  116  which drives a sprocket  130  that engages and drives the endless belt or chain of the conveyor. The conveyor can have one or more brackets  131 , such as an angle bracket. When power is supplied to the motor  116 , the conveyor  122  draws the angled bracket in the direction of arrow  115 , and drags the dross  120  on the ramp  119  and over the edge of the solder reservoir  112 . The conveyor motor  116  is preferably driven by a computer  132  (which is the same as or similar to computers  32 ,  84  and  158  in other FIGS.) such that the dross removal apparatus  110  can operate automatically with or without operator intervention. Alternatively, the conveyor motor  116  can be controlled by a switch. Preferably, the conveyor motor  116  is computer controlled through timers with overload protection for the device.  
         [0033]    A solder reclamation receptacle  46  is illustrated in FIG. 10. The receptacle  46  contains solder  18  that has been separated from dross  20  by a dross separation device  34  (see FIG. 1) after the dross  20  is skimmed from a molten solder reservoir  12  (see FIG. 1). Other suitable embodiments of a dross separation device (also referred to as a solder-dross mixture separation apparatus) are described in U.S.S.N 10/051,407, which is incorporated herein by reference in its entirety.  
         [0034]    A solder return pump  139  driven by a motor  140  is placed in the solder reclamation receptacle  46 . The pump  134  operates much like a water sump pump common in many residential basements; within the solder reclamation receptacle  46 , the pump  139  draws molten solder from near the bottom of the receptacle  46 . The pump motor  140  transmits a rotary force through a shaft  142  to an impeller  144  (illustrated in greater detail in FIG. 11), which draws the molten solder through a pump intake  146 , through the pump housing  147 , and into a solder return conduit  148 , which channels the molten solder back to the molten solder reservoir. The solder return conduit  148  is heated by a heating element  150  to prevent the solder from cooling and solidifying within the conduit  148 .  
         [0035]    The solder level within the solder reclamation receptacle  46  gradually rises as more dross is skimmed from the molten solder reservoir and separated into dross and reclaimed solder components. The solder level in the receptacle  46  can be dropped by activating the pump  139  to reclaim molten solder  18  through the solder return conduit  148 .  
         [0036]    The solder return pump  139  includes a float  152  positioned about the pump shaft  142 . The float  152  moves up and down with the level of the solder (or dross) in the solder reclamation receptacle  46 . When the float  152  reaches a predetermined height, an upper-limit sensor  154  is activated. When activated, the upper-limit sensor  154  activates the pump motor  140 , thereby beginning the transfer of solder  18  from the solder reclamation receptacle  46  to the main solder reservoir and lowering the level of the solder  18  in the solder reclamation receptacle  46 .  
         [0037]    A lower-limit sensor  156  senses the float  152  position at a lower level and sends an electronic signal turning off the pump motor  140  when the lower-limit sensor  156  detects the height of the float  152  to have dropped to a lower limit so as to prevent dross  20  at the surface from being drawn into the pump intake  146  along with the molten solder  18 . Sensing can be performed by a pair of on/off-type sensors  154 ,  156  or by a continuously reading analog-type sensor through software control. The sensors  154 ,  156  can be fiber-optic sensors, and the float  152  can include a flag positioned to break the beam of the sensors  154 ,  156  when the float  152  is at the limit heights. In one embodiment, the sensors  154 ,  156  are Model 0BT 200L-18GM70 E5-V1 fiber-optic sensors from Pepperl+Fuchs, Inc. (Twinsburg, Ohio, USA), and the sensors  154 ,  156  are coupled with the computer  158  via Model LMR 18-2, 3-0, 5-K 11 cables from Pepperl+Fuchs, Inc.  
         [0038]    The software control is provided via a computer  158  coupled with the sensors  154 ,  156  and with the motor  140 . The computer  158  is thereby able to receive measurements from the sensors  154 ,  156 , process those measurements utilizing software code stored on a computer-readable storage medium, and generate instructions, which are communicated to the motor  140  to commence or stop pumping based on the measurements received from the sensors  154 ,  156 . The computer is a standard personal computer (e.g., a PENTIUM-based desktop computer running a WINDOWS operating system).  
         [0039]    If for any reason the upper-limit sensor  154  does not sense the float  152  when the float  152  is in its up position, the solder  18  would eventually overflow from the solder reclamation receptacle  46  absent another fail-safe mechanism. Such a mechanism is provided in the form of an additional independent overflow sensor  160  used to provide a high-level shut off in the solder reclamation receptacle  46 . The overflow sensor  160  is a metallic strip that is normally electrically isolated from the reclamation receptacle  46 , but that has a DC potential between the sensor  160  and the molten solder  18  to ground and is mounted such that it will touch the molten solder  18  just prior to the solder  18  overflowing the reclamation receptacle  46 , thereby creating an electrical connection between the overflow sensor  160  and the ground  162 . A computer system  158  in communication with the overflow sensor  160  senses the voltage connection and causes the entire system to be stopped, thereby preventing the solder  18  from spilling from the solder reclamation receptacle  46 .  
         [0040]    Within the solder reclamation receptacle  46 , a thin layer of dross  20  will typically form on the surface of the solder  18  due to interaction between the solder and oxygen in the air. Additionally, some of the dross  20  from the dross separation device  34  (see FIG. 1) will find its way into the solder reclamation receptacle  46 . This dross  20  tends to collect in the upper part of the receptacle  46 . Accordingly, when pumping reclaimed solder, particular embodiments, such as that illustrated in FIG. 11, position the pump intake  146  located as low as is practical in the solder reclamation receptacle  46  so as to reduce the likelihood of entraining dross particles in the reclaimed solder that is being returned to the main solder reservoir, which is coupled with the nozzle that generates a wave of molten solder in a wave soldering device.  
         [0041]    The melting points of solders typically range from 183° C. (361 ° F.) for 63/37 Sn/Pb electronic solder to close to 260° C. (500° F.) for a variety of lead-free solders that are growing in popularity. To ensure the flowability of the solder in the conduit  148 , the solder  18  should be maintained at a temperature above its melting point throughout the system. The temperature of the solder  18  can be maintained by heating the conduit  148  along its entire path with a heat-tape-type heating element  150  wrapped around the conduit.  
         [0042]    In one embodiment, the heating element  150  is in the form of a heat tape and is designed (dimensions, resistivity, etc.) to maintain the solder return conduit  148  at a constant temperature. Constant temperature can be maintained via a closed-loop thermocouple control system, where a thermocouple monitors the temperature of the conduit and communicates with the computer  158  to raise or lower the wattage of the heating element  150  to maintain the desired temperature. Alternatively, heat tape can be sized to the proper watt density, thereby eliminating the costs of a thermocouple closed loop control system.  
         [0043]    An additional embodiment of a skimmer assembly  94  is illustrated in FIG. 13, which can likewise be used in the apparatus of FIG. 4. The skimmer assembly  94  is mounted to a lip of the molten solder reservoir  12 , and it includes a reciprocating structure including a skimmer cartridge  166  and skimming plate  168 . In this embodiment, the skimmer cartridge  166  takes the form of a trolley mounted in a track  165  that extends across the reservoir  12 . The skimming plate  168  skimming plate is made of a heat-resistant material that will not melt or deform when heated to the temperature of the molten solder and is pivotally mounted to the skimmer cartridge  166 . The skimmer cartridge  166  can be mounted to displacement mechanism including a motor  82  and pulley  85 , a cable  79  drive train, and a retractable arm  164 , wherein the cable  79  is secured to the motor-driven pulley  85  at one end and to a remote end of the arm  164  at or proximate the skimmer cartridge  166  at the other end; the motor rotates the pulley  85  in either direction to collect or release the cable  79 , thereby extending or retracting the arm  164 .  
         [0044]    Operation of the skimmer is further illustrated in FIGS. 14 and 15A-C. As can be seen in FIG. 14, the skimming plate  168  is pivotally mounted to the skimmer cartridge  166  via a hinge  170  that permits the skimming plate  168  to extend orthogonally to or into the molten solder  18  in the reservoir  12 . At the other extreme, the skimming plate  168  can be collapsed to a position near, but not quite, parallel with the surface of the molten solder bath  18 , and the layer of dross formed thereon.  
         [0045]    As shown in FIG. 13, when the arm  164  is retracted from a remote position, which may or may not be fully extended, the skimming plate  168  digs into the dross  20  and pivots about the hinge  170  to achieve an orientation normal to the top surface of the molten solder bath  18 . The skimming plate  168  is mechanically constricted from further pivoting so as to prevent formation of a substantially acute angle between the skimming plate  168  and bath surface that would allow dross  20  to be dragged under the skimming plate  168 , into the molten solder  18  only to reemerge on the opposite side.  
         [0046]    As the skimmer cartridge  166  is retracted toward an outlet ramp  119 , the skimming plate  168  collects dross  20  floating on the molten solder  18  between the skimming plate  168  and ramp  119 . The skimming plate  168  scoops the collected dross  20  up the ramp  119  (as shown in FIG.  15 A) and out of the reservoir  12  to a dross separation device. As the skimmer cartridge  166  is again extended to a position remote from the outlet, the skimming plate  168  pivots about the hinge  170  in the opposite direction so that the skimming plate  168  skims over the top surface of the dross  20  without substantially displacing the dross  20 . A stop  174  extends from the skimmer cartridge  166  to provide a physical barrier against which the skimmer plate  168  collapses to prevent the skimming plate  168  from collapsing to a position parallel to the surface of the molten solder bath  18 . If the skimming plate  168  were to collapse to this parallel position, the skimming plate  168  may not be able to dig into the dross  20  and rotate to its downward extended position to collect the dross  20  when the skimmer cartridge  166  is again retracted toward the outlet. The stop  170  can be in the form of a screw.  
         [0047]    If a substantial quantity of dross  20  accumulates on the molten solder bath, as shown in FIGS.  15 A-C, the skimmer cartridge  166  can be controlled via a computer  84  and a motor  82  to extend the skimmer cartridge  166  and plate  168  across the reservoir to progressively greater distances from the outlet with each reciprocal pass. Accordingly, the skimmer cartridge  166  may be only minimally extended across the reservoir on a first pass, as shown in FIG. 15A, to collect only a small section of the dross  20  proximate the outlet ramp  119  so that the skimmer is not overloaded. With each progressive pass of the arm  164  across the molten solder bath (see FIGS. 15B and 15C), the cartridge  166  and plate  168  extend to a further reach from the ramp  119  to reach a section of the dross  20  that was not skimmed on the previous pass. The cartridge  166  can be displaceably mounted in a track  165 , as illustrated in FIG. 13, which guides the positioning of the cartridge  166  along the path.  
         [0048]    The computer  84  includes a processor in communication with a computer-readable storage medium storing software code for communicating with the motor  82  coupled with the arm  164  to displace the arm  164  along its longitudinal axis, as described above. The software code includes a sequence of instructions to control the motor  82  and extend and retract the arm  164  (including the skimming plate  168 ) by a distance, for example, x, on a first pass and then to extend and retract the arm  164  by a distance, x+y, on a second pass. For a third pass, the software instructions can cause the motor to extend and retract the arm  164  by a distance, for example, x+2y, and so on until the pass extends the arm  164  substantially across the width of the bath.  
         [0049]    The various elements described and illustrated in various embodiments, described above, can readily be interchanged. For example the trolley and track in the embodiment of FIG. 13 can readily be used in place of the arm in FIG. 15; moreover, other illustrated skimmers and conveyors can readily be interchanged in different embodiments.  
         [0050]    While this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various changes in form and details may be made therein without departing from the scope of the invention, which is limited only by the following claims.