Abstract:
The invention features soldering parts (e.g., printed circuit boards) by indexing the parts sequentially and continuously through a series of processing stations. Flux may be applied to precise areas of the parts at a flux station, and the flux station may include a rough part locator that roughly locates a part indexed into the flux station and a precise part locator that precisely locates the part. The flux station may further include a guide rail for supporting the part and a mechanism for raising and lowering the guide rail. The flux station may include a flux sprayer for applying flux to the part, where the flux sprayer includes an air valve, a flux valve, and a controller coupled to activate and deactivate separately the air valve and the flux valve. The controller may activate the air valve before activating the flux valve and deactivate the flux valve before deactivating the air valve.

Description:
This application is a divisional of application Ser. No. 08/931,913, filed Feb. 24, 1997, now issued as U.S. Pat. No. 5,941,444, which is a divisional of application Ser. No. 08/640,018, filed Apr. 30, 1996, now issued as U.S. Pat. No. 5,642,850 on Jul. 1, 1997, which is a divisional of application Ser. No. 08/420,553, filed Apr. 11, 1995, now issued as U.S. Pat. No. 5,560,537 on Oct. 1, 1996. 
    
    
     BACKGROUND 
     This invention relates to soldering. 
     In one method of soldering, printed circuit boards (PCBs) which are populated with components pass, one at a time, through a three step process: flux is applied to electrical connection points on both the PCB and the components; the PCB and the components are preheated; and the electrical connection points are brought in contact with molten solder. 
     SUMMARY 
     In general, in one aspect, the invention features soldering parts (e.g., printed circuit boards) by indexing the parts sequentially and continuously through a series of processing stations. Each of the series of processing stations includes at least one processing position, and the indexer indexes the parts from processing position to processing position. At different processing stations, the parts indexed into those processing stations are simultaneously processed. 
     Implementations of the invention may include one or more of the following features. The indexer may have a chain and flights connected to the chain and separated along the chain by an indexing distance. Solder may be applied to the parts at a soldering station. The soldering station may include a solder fountain. The solder fountain may have a solder manifold with a first aperture, a solder well plate with second apertures, where the solder well plate is mounted above the solder manifold with the second apertures above the first aperture, modular solder well plates mounted above the second apertures, and a solder chimney mounted to each of the modular solder well plates. The solder chimneys may provide passageways from the solder manifold to a top of the solder chimneys where the parts are soldered, the parts may be of different part types, and each of the modular solder well plates may correspond to one of the different part types. The solder fountain may also include another solder chimney mounted to each of the modular solder well plates. The soldering station may further include a rough part locator that roughly locates a part indexed into the soldering station and a precise part locator that precisely locates the part. The soldering station may also include a guide rail for supporting the part, and a mechanism for raising and lowering the guide rail. Additionally, flux may be applied to precise areas of the parts at a flux station, and the flux station may include a rough part locator that roughly locates a part indexed into the flux station and a precise part locator that precisely locates the part. The flux station may further include a guide rail for supporting the part and a mechanism for raising and lowering the guide rail. The flux station may include a flux sprayer for applying flux to the part, where the flux sprayer includes an air valve, a flux valve, and a controller coupled to activate and deactivate separately the air valve and the flux valve. The controller may activate the air valve before activating the flux valve and deactivate the flux valve before deactivating the air valve. Furthermore, the parts may be preheated and glue used to mount components to the parts may be cured in an oven. The oven may include sparging tubes for blowing heated gas on the parts as the parts are indexed to an exit end of the oven. The oven may also have a guide rail for supporting the parts as they are indexed through the oven and a support rail for supporting the guide rail, where the support rail includes thermal expansion slots for allowing the support and guide rails to thermally expand longitudinally and where the support rail is held in a fixed position at one end and is free to expand away from the fixed position. Moreover, the processing stations may include an identification station having a sensor for determining a type of a part indexed into the station, and the soldering system may include a controller for controlling subsequent processing stations in response to the sensor. The controller may control the amount of time flux is sprayed at each of the parts according to the identified part type, and the controller may control the pump speed of a solder pump according to the identified part type. The parts may be printed circuit boards, and the parts may include pallets having an aperture for holding one of the printed circuit boards. The pallets may include additional apertures for holding additional printed circuit boards. 
     In another aspect, the invention features a solder fountain. The solder fountain includes a solder manifold for containing solder, and the solder manifold has a first aperture. The solder fountain also includes a solder well plate having second apertures, where the solder well plate is mounted above the solder manifold with the second apertures above the first aperture. Further, the solder fountain includes modular solder well plates mounted above the second apertures, and a solder chimney mounted to each of the modular solder well plates. The solder chimneys provide passageways from the solder manifold to a top of the solder chimneys where solder is applied to the parts. The parts are of different part types, and each of the modular solder well plates corresponds to one of the different part types. 
     Implementations of the invention may include one or more of the following features. The chimney passageway may be an unrestricted passageway. The solder fountain may include a rough part locator having a stopper arm with an end that provides a datum point and a pusher arm for pushing the part against the stopper arm and a precise part locator having a datum bushing and a slotted bushing on the part and a datum pin and an expansion pin for respectively engaging the datum bushing and the slotted bushing. 
     In another aspect, the invention features a flux unit for applying flux to precise areas of printed circuit boards. The flux unit includes a rough part locator that roughly locates a printed circuit board within the flux unit and a precise part locator that precisely locates the printed circuit board. 
     Implementations of the invention may include one or more of the following features. The rough part locator may include a stopper arm having an end that provides a datum point and a pusher arm for pushing the part against the stopper arm. The precise part locator may include a datum bushing and a slotted bushing on the printed circuit board and a datum pin and an expansion pin for respectively engaging the datum bushing and the slotted bushing. 
     In another aspect, the invention features a flux unit for applying flux to precise areas of printed circuit boards. The flux unit includes a flux sprayer for applying flux to the printed circuit boards, where the flux sprayer includes an air valve, a flux valve, and a controller coupled to activate and deactivate separately the air valve and the flux valve. 
     In another aspect, the invention features a convection oven for preheating printed circuit boards. The oven includes multiple sequential indexed positions, a nitrogen input mechanism for providing a nitrogen environment, and a pair of parallel guide rails for supporting the printed circuit boards within the oven. The oven also includes sparging tubes for blowing heated nitrogen on a printed circuit board at an oven exit and a pair of parallel support rails for supporting the pair of guide rails. The support rails include thermal expansion slots for allowing the support and guide rails to thermally expand longitudinally and a fastening mechanism extending through the slots for maintaining a set width between the guide rails. 
     In another aspect, the invention features a method for use in connection with soldering printed circuit boards of different types moving along an automated production line. The method includes identifying a type of each of the printed circuit boards that approaches a flux station on the production line and applying flux to a specific area of the printed circuit board for an amount of time corresponding to the type of the board. 
     In another aspect, the invention features a method for use in connection with soldering printed circuit boards of different types moving along an automated production line. The method includes identifying a type of each of the printed circuit boards that approaches a solder station on the production line and setting a solder pump speed appropriate to providing a stable top surface of a column of solder of a predetermined height in a solder fountain used to apply solder to a specific area of the printed circuit board. 
     Advantages of the invention may include one or more of the following. The amount of handling required to solder components to printed circuit boards (PCBs) is reduced and the number of PCBs soldered in a given period of time is increased. The PCBs are uniformly heated and not subject to damaging temperatures. The top surface of solder columns are stable which permits precise soldering of multiple PCB areas while preventing solder from spreading to other closely spaced PCB areas. 
     Other advantages and features will become apparent from the following description and from the claims. 
    
    
     DESCRIPTION 
     FIGS. 1 a  and  1   b  are side and top views, respectively, of a continuous soldering system. 
     FIG. 2 a  is a cut-away view at  2   a — 2   a  in FIG. 1 a.    
     FIGS. 2 b  and  2   c  are cross-sectional views at  2   b — 2   b  and  2   c — 2   c , respectively, in FIG. 2 a.    
     FIG. 2 d  is an enlarged view of a chain tensioner in FIG. 2 a.    
     FIG. 3 is a perspective view of a pallet loaded with printed circuit boards (PCBs). 
     FIGS. 4 a  and  4   b  are cross-sectional views of an identification station at  4   a — 4   a  and  4   b — 4   b , respectively, in FIG. 1 a.    
     FIG. 5 is a block diagram of a control system for a continuous soldering system. 
     FIGS. 6 a  and  6   b  are cross-sectional views of a flux unit at  6   a — 6   a  in FIG. 1 b  and  6   b — 6   b  in FIG. 1 a.    
     FIG. 6 c  is a top view of a pallet not loaded with PCBs over a flux mask. 
     FIG. 7 a  is a block diagram of a flux sprayer. 
     FIG. 7 b  is a block diagram of a nozzle. 
     FIG. 8 a  is a cross-sectional view of an oven at  8   a — 8   a  in FIG. 1 a.    
     FIGS. 8 b  and  8   d  are cross-sectional views at  8   b — 8   b  and  8   d — 8   d , respectively, in FIG. 8 a.    
     FIG. 8 c  is a perspective view at  8   c — 8   c  in FIG. 8 a.    
     FIG. 9 is a perspective view of a pallet and a slot and a shoulder screw in a support rail. 
     FIGS. 10 a  and  10   b  are cross-sectional views of a solder station at  10   a — 10   a  in FIG. 1 a  and  10   b — 10   b  in FIG. 1 b , respectively. 
     FIG. 10 c  is a top view of a pallet not loaded with PCBs over a solder fountain. 
     FIG. 10 d  is a perspective view of a solder fountain. 
     FIG. 11 a  is a perspective view of a solder fountain with top surfaces of molten solder columns extending above solder chimneys. 
     FIG. 11 b  is a perspective view of a pallet loaded with PCBs over the solder fountain of FIG. 11 a.    
     FIG. 11 c  is a cross-sectional view of a PCB in contact with top surfaces of solder columns extended above two solder chimneys. 
    
    
     Referring to FIGS. 1 a  and  1   b , a system  10  for continuously soldering printed circuit boards (PCBs) includes a flux station  12 , an oven  14 , and a solder station  16 . System  10  also includes a controller  18 , for example, a programmable logic controller, of the type manufactured by Allen-Bradley Corp., Highland Heights, Ohio, USA, that monitors and controls the operation of system  10  and a solder feed unit  20  coupled to solder station  16  which, under control of controller  18 , feeds solder to solder station  16 . 
     Referring to FIGS. 2 a - 2   c , flux station  12 , oven  14 , and solder station  16  are interconnected by two parallel chains  22 . Chains  22  are supported on chain guides  24  which are connected to mounting rails  26 , and at predetermined distances, e.g., 8 inches, along each chain, a flight  25  is mechanically connected to the chain. The chains  22  are indexed forward (arrow  21 ) periodically, e.g., every 10 seconds, by the predetermined distance, e.g., 8 inches, between the chain flights to locate each flight and its neighbor on either end of an indexed chain position, e.g.,  14   a , within system  10 . When system  10  is powered up, controller  18  advances chains  22  until home sensor  23  (e.g., a proximity sensor of the kind manufactured by Turck, Inc.) detects a leading edge of a flight. Chains  22 , chain guides  24 , mounting rails  26 , and flights  25  are stainless steel. 
     As chains  22  pass through oven  14  they are heated and, although they are stainless steel, which has a low thermal expansion coefficient, they expand with the heat. Chains  22  also stretch with age. A weighted chain tensioner  27  (FIGS. 2 a  and  2   d ) is mechanically coupled to chains  22 , through sprockets  29   a ,  29   b ,  29   c , and  29   d , and together with a weight  31 , the chain tensioner pushes down on the chains to maintain a constant chain tension (arrows  33   a  and  33   b ) regardless of temperature or age. 
     Referring to FIG. 3, PCBs are carried through the system on pallets. A pallet  28  includes three apertures  30 ,  32 , and  34 . Each PCB  36 ,  38 , and  40  (shown unpopulated, i.e., without mounted components, for clarity, but which normally have mounted components) has a locating pin hole  39   a  and a locating slot  39   b , and for each aperture  30 ,  32 , and  34 , pallet  28  includes a tooling pin  39   c  and an extension pin  39   d . Each PCB  36 ,  38 , and  40  is precisely located within apertures  30 ,  32 , and  34 , respectively, when tooling pin  39   c  engages locating pin hole  39   a  and extension pin  39   d  engages locating slot  39   b . Further, each PCB  36 ,  38 , and  40  is supported within apertures  30 ,  32 , and  34 , respectively, by locating clips  42 . Each clip applies force to a side of a PCB in a direction from that side of the aperture toward an opposite side of the aperture. 
     The PCBs may be of a single type or, as shown, may be of three different types with each aperture  30 ,  32 ,  34  corresponding to a particular type of PCB. When the PCBs are three different types, each pallet  23  is generally loaded with only one of the three types, which allows the system to be configured to tailor PCB processing in accordance with the type of PCB loaded in each pallet. 
     Pallets  28  are loaded onto stainless steel guide rails  44  (FIGS. 2 a ,  2   c ) within system  10  either manually or automatically, e.g., by a Bosch conveyor belt  46  which is indexed synchronously with chains  22 . Pallets  28  are centered on conveyor belt  46 . Apertures  30 ,  32 , and  34  (FIG. 3) are at least a distance, d, e.g., about 0.70 inches, from edges  35  and  37  of pallets  28  to allow edges  35  and  37  to rest on guide rails  44 . To allow the pallets to smoothly pass from conveyor belt  46  to guide rails  44 , the top surface of conveyor belt  46  is approximately 0.005-0.010 inches above the top surface of guide rails  44  and guide rails  44  have a lead-in chamfer, e.g., of about 0.030 inches. Flights  25  engage a back edge  48  of each pallet  28  and push the pallet through system  10  as chains  22  are indexed. System  10  includes a sequence of processing stations and, at each indexed chain position, each processing station may execute a certain procedure, such as applying flux, solder, or heat to PCBs loaded in pallets. The index period is limited, therefore, by the slowest procedures, for example, by the application of solder at solder station  16 , and by the time required to index chain  22 , for example, 1.5 seconds. As an example, where the distance between the flights is about 8 inches, the width W 1  of the pallets  28  is about 6.30 inches which provides a clearance of about 1.70 inches between pallets  28 . After being indexed through solder station  16 , pallets  28  are pushed by flights  25  from guide rails  44  to a second indexed conveyor belt  47 . The top surface of conveyor belt  47  is approximately 0.005-0.010 inches below the top surface of guide rails  44  to allow the pallets to smoothly pass from guide rails  44  to belt  47 . 
     Referring to FIGS. 4 a  and  4   b , within flux station  12 , a first indexed chain position locates a pallet  28  at an identification (ID) station  50 . ID station  50  includes four retroreflective sensors  52 ,  54 ,  56 , and  58  (e.g., a fiber photoelectric sensor having a transmitter, for example, a light emitting diode (LED), and a receiver). Sensor  52  detects the presence of the pallet by receiving light reflected from the pallet. Sensors  54 ,  56 , and  58  detect the presence of the particular PCBs  36 ,  38 , and  40 , respectively, by receiving light reflected from those PCBs. 
     Referring to FIG. 5, the sensors within ID station  50  send electrical signals to controller  18  to notify controller  18  that a pallet  28  is present and to notify controller  18  which pallet aperture(s)  30 ,  32 , or  40  contains a PCB (i.e., which PCB type(s) is present within that pallet). If sensor  52  detects a pallet, but none of the sensors  54 ,  56 , and  58  detect a PCB, then controller  18  indicates an error condition on control screen  60  to notify an operator. The controller may automatically prevent chain  22  from indexing until the pallet is removed or the operator, using control screen  60  and controller  18 , may manually prevent chain  22  from indexing until the pallet is removed. Alternatively, the controller can automatically or the operator cn manually allow the pallet to proceed through system  10  and prevent the flux unit  62  (FIG. 6 a ) and solder station  16  from operating when that pallet is indexed into those stations. 
     Referring to FIGS. 6 a  and  6   b , a second indexed chain position locates the pallet  28  on flux rails  61  of a flux unit  62  within flux station  12 . Because chains  22  can stretch as they age and as they are heated, two pneumatic actuator arms, a stopper arm  64  and a pusher arm  66 , are used to roughly locate the pallet. When the pallet is indexed onto flux rails  61 , stopper arm  64  is activated and rotates (arrow  65 ) a predetermined amount to position an end  68  at a datum point beyond a front edge  70  of the pallet, and when activated, pusher arm  66  also rotates (arrow  67 ) a predetermined amount to position an end  72  against and push (arrow  73 ) back edge  48  of the pallet such that front edge  70  contacts stopper arm  64 . This action positions the front edge  70  of the pallet at the datum point (i.e., against end  68  of stopper arm  64 ). Pusher arm  66  includes a spring loaded lever  74  that compresses (arrow  75 ) slightly when front edge  70  of the pallet contacts stopper arm  64 . spring loaded lever  74  compensates for the pallet width tolerance and the rotation tolerances of stopper arm  64  and pusher arm  66  and prevents the arms from applying significant pressure to the front and back edges of the pallet. 
     Once the pallet is roughly located, controller  18  determines whether the pallet is properly seated on rails  61  with seating sensors  80  and  82 , e.g., retroreflective sensors, located at two diagonal corners of the pallet. The amount of reflected light which should be received by sensors  80 ,  82  when the pallet is properly seated on rails  61  is predetermined during a set up procedure, and if the amount of reflection indicates, for example, that one or both corners f the pallet are a predetermined distance, e.g., 0.015 inches, or more above or below rails  61 , then controller  18  indicates an error on control screen  60  (FIG. 5) to notify the operator of the error. Controller  18  may then automatically prevent chains  22  from indexing until the sensors indicate that the error has been fixed or the operator, through control screen  60  and controller  18 , may manually prevent the chain from indexing until he or she fixes the error. 
     Rails  61  are mechanically coupled to an axis or a rail table  63  having a central aperture  63   a . Rail table  63  is mechanically coupled to a lead screw (not shown) which is driven by a stepper motor  84 . If the pallet is properly seated on rails  61 , controller  18  sends electrical signals to stepper motor  84  to cause stepper motor  84  to turn the lead screw to lower (arrow  86 ) rail table  63  and rails  61  to a predetermined position above, approximately 1.0-1.5 inches, flux mask  88 . With a lead screw having a repeatability of about +/−0.00039 inches and a position accuracy of about +/−0.0003 inches and a motor  84  such as an SX57-102 motor manufactured by Compumotor, Inc. a rail height position accuracy of about +/−0.001 inches is possible. 
     The pallet includes a datum bushing  90  and a slotted bushing  92  (FIGS. 3,  6   c ) both having radiused lead-in edges. As the pallet is lowered, a datum pin  94  and an expansion pin  96 , which are press fitted to flux mask  88  and extend above flux mask  88 , are inserted within datum bushing  90  and slotted bushing  92 , respectively. The shanks (shank  94   a  of datum pin  94  is shown in FIG. 6 a ) are straight and round and have a tapered top ( 94   b ). The tapered shanks engage the radiused bushings to precisely align the pallet. As an example, the shank diameter of both pins  94 ,  96  is approximately −0.187 inches and the diameter of the top of both pins is about 0.157 inches. The datum bushing has a diameter of about 0.191 inches with a {fraction (1/32)} inch radiused lead-in, while the slotted bushing has a length of about 0.234 inches, a width of about 0.191 inches, and a {fraction (1/64)} inch radiused lead-in. 
     The combination of the datum bushing  90  and slotted bushing  92  ensures that the pins  94 ,  96  will properly mate with the bushings despite thermal expansion or contraction of the pallet. As the pallet passes through system  10 , temperature variations may cause the pallet to expand and contract. Thermal expansion is greatest toward the areas of least resistance, typically, edges  35 ,  37 ,  48 , and  70 . Because datum bushing  90  is substantially centered with respect to the length L and width W 1  of the pallet, the potential thermal expansion of the pallet from the datum bushing toward back edge  48  and front edge  70  of the pallet is substantially equal and the potential thermal expansion of the pallet from the datum bushing toward edges  35  and  37  is substantially equal. As a result, the position of datum bushing  90  remains substantially centered with respect to the edges of the pallet as the pallet thermally expands and contracts and, therefore, very little clearance, approximately 0.004 inches, between datum pin shank  94  and datum bushing  90  is required. Similarly, because slotted bushing  92  is substantially centered with respect to the width W 1  of the pallet, the potential thermal expansion of the pallet from the slotted bushing toward back edge  48  and front edge  70  is substantially equal and very little clearance, approximately 0.004 inches, is required between the sides  92   a ,  92   b  of slotted bushing  92  and the shank of expansion pin  96 . On the other hand, slotted bushing  92  is not centered with respect to the length L of the pallet and the potential for thermal expansion of the pallet from the slotted bushing toward edge  35  is far greater than the thermal expansion of the pallet from the slotted bushing toward edge  37 . As a result, the length of slotted bushing  92  provides a large length-wise clearance, approximately 0.047 inches, between expansion pin  96  and the sides  92   c ,  92   d  of slotted bushing  92  to compensate for unequal potential thermal expansion and contraction of the pallet between slotted bushing  92  and edges  35  and  37 . 
     Similarly, the combination of locating pin hole  39   a  (FIG. 3) and locating slot  39   b  is used to compensate for potentially unequal thermal expansion and contraction of the pallet and a PCB located within an aperture of the pallet by tooling pin  39   c  and extension pin  39   d.    
     Once the pallet is lowered to the predetermined distance above flux mask  88  and the pallet is precisely located by pins  94 ,  96 , controller  18  checks, using seating sensors  80 ,  82 , whether the pallet is properly seated on rails  61 . If the pallet is not properly seated, controller  18  notifies the operator of the error through control screen  50  (FIG. 5) and either controller  18  automatically prevents chains  22  from indexing until the sensors indicate the pallet is properly seated, or the operator, through control screen  60  and controller  18 , prevents the chains from indexing until the pallet is properly seated, or the process is continued and the PCB(s) in that particular pallet is considered a reject. The controller may automatically or the operator, through control screen  60  and controller  18 , may manually raise and lower rails  61  again to try to properly seat the pallet. 
     Once the pallet is properly seated, controller  18  sends electrical signals to a flux sprayer  104  to cause flux to be sprayed upward (arrows  106 , FIG. 6 a ) toward flux mask  88  but only in the area(s) of a loaded PCB(s). Flux mask  88  can be machined from many materials, including plastic or plated steel. 
     Referring to FIG. 6 c , flux mask  88  includes apertures  108 ,  110 ,  112  which correspond to particular areas of PCBs  36 ,  38 ,  40 , respectively (not shown, for clarity) where electrical connection points between components mounted on the PCBs and the PCBs are to be soldered in solder station  16  (FIG. 1 a ). When flux sprayer  104  sprays flux, the flux passes through apertures  108 ,  110 , and  112  and provides a thin coat on the particular PCB areas. Portions  114 ,  116 ,  118  of the flux mask prevent flux from being applied to other areas of the PCBs. 
     Flux mask portions  114 ,  116 ,  118  can be divided into two or more sections of differing heights. For example, flux mask portions  114   a ,  116   a , and  118   a  are lower (i.e., closer to flux sprayer  104 ) than flux mask portions  114   b ,  116   b , and  118   b . Lower flux mask portions  114   a ,  116   a , and  118   a  can accommodate components (e.g., single turn transformer windings, not shown) that extend from a bottom surface of the PCBs and allow connection points located on the PCBs next to the upper flux mask portions  114   b ,  116   b , and  118   b  to be brought as close as possible to flux sprayer  104 . 
     Referring to FIG. 7 a , flux sprayer  104  includes a set of two valves, a pneumatic air valve  120  and a flux valve  122 , for each aperture  30 ,  32 , and  34  (FIG. 3) of pallet  28 . Both valves are controlled by controller  18 . Flux sprayer  104  also includes a set of one or more nozzles  124  directed at each aperture  30 ,  32 , and  34  to provide an even application of flux to PCBs loaded with the apertures. For example, three nozzles are directed at large PCB  36 , two nozzles are directed at medium PCB  38 , and one nozzle is directed at small PCB  40 . Referring to FIG. 7 b , each nozzle  124  includes one flux jet  126  and multiple (four are shown, but there may be more) air jets  128  directed at flux mask  88 . Air expelled by the air Jets atomizes flux expelled by the flux jet. The controller selectively activates only those sets of air and flux valves associated with apertures within which PCBs are loaded. 
     In typical flux stations, the air valve may be used to activate the flux valve: turning on the air valve turns on the flux valve; and turning off the air valve turns off the flux valve. However, because the flux jets may expel flux for a short period of time after the air jets have stopped expelling air, the air jets can become clogged with flux. 
     In system  10 , controller  18  sends electrical signals to each set of air and flux valves  120 ,  122 , respectively, to separately control when each valve is turned on and off and to control which nozzles  124  receive air and flux. When rails  61  are lowered and hold a properly seated pallet, controller  18  turns on air valve  120  and after a first predetermined amount of time, approximately 1.0 seconds, turns on flux valve  122 . Flux is sprayed for a second predetermined amount of time through selected nozzles to apply the thin layer of flux to a PCB loaded in the pallet. Flux station  62  includes an exhaust pipe  130  which allows venting of flux gasses. The second predetermined amount of time may vary depending upon the type of PCB loaded in the pallet, for example, the second predetermined amount of time is about 1.5-2.0 seconds for PCB  36  (FIG.  3 ), about 1.0-1.5 seconds for PCB  38 , and about 0.5-1.0 seconds for PCB  40 . After the second predetermined amount of time passes, controller  18  turns off flux valve  122  and waits a third predetermined amount of time for the flux jets to finish expelling flux and for flux in the surrounding air to settle, before turning off air valve  122 . As a result, air is continuously expelled from the air jets both before and after flux is expelled from the flux jets to substantially prevent the air jets from becoming clogged with flux. The third predetermined amount of time may also vary with the type of PCB loaded in the pallet, for example, where three nozzles are directed at large PCB  36  and only one nozzle is directed at small PCB  40 , it may take less time, for example, about 0.6 seconds, for the three nozzles directed at large PCB  36  to finish expelling flux than the time, for example, about 1.4 seconds, for the one nozzle directed at small PCB  40  to finish expelling flux. Periodically and without a pallet on rails  61 , controller  18  causes the air jets to expel short quick bursts of air to purge or clean the air jets which provides additional protection against clogged air jets. 
     After applying flux, controller  18  sends electrical signals to stepper motor  84  to cause motor  84  to turn the lead screws and raise rails  61  and pallet  28  such that rails  61  are level with guide rails  44 . A rail home sensor (not shown) and a rail bottom sensor (not shown) can be used by the controller to determine if rails  61  are in a home or top position or in a bottom or flux position, respectively. The steps of roughly locating the pallet, lowering the rails, spraying flux, and raising the rails are accomplished within the index period. 
     The next indexed position following the flux station indexed position locates the pallet in convection oven  14 . Oven  14  has multiple indexed positions, e.g., thirteen (for clarity, only four  14   a ,  14   b ,  14   c , and  14   d  are shown in FIG. 2 a ), and runs at a temperature of about 140° C. to slowly preheat a PCB(s) loaded in the pallet to about 105-110° C., before the pallet is indexed into solder station  16 . Preheating the PCB enhances the solder wettability of the connection points and also activates the flux just applied to the connection points. Slowly preheating the pallet and PCB to a desired temperature provides a uniform temperature across the PCg and reduces the risk of damage to the PCB if controller  18  prevents chain  22  from indexing for a period of time when an error is detected in system  10 . Past systems have used high temperature, e.g., greater than 280° C., infra-red (IR) or convection panels to quickly preheat individual PCBs to about 105-110° C. However, a PCB left between such high temperature panels beyond a short threshold time may be damaged. 
     Pallets  28  (FIG. 3) can be made from many materials, including hard coated teflon and aluminum. Aluminum is a preferred pallet material because aluminum pallets typically cost less to manufacture and are more easily detected by metal detecting proximity detectors located in other stages of the overall PCB manufacturing system (not shown). Additionally, aluminum is heavier than teflon and may assist in seating the pallets on the datum and expansion pins. 
     Aside from preheating PCBs loaded in pallets, oven  14  can also cure the glue used to mount some components to the PCBs. This may eliminate a prior glue curing stage (not shown) in the overall PCB manufacturing system (not shown). If the glue curing temperature is higher, e.g., 125° C., than the PCB soldering preheat temperature, e.g., 105-110° C., then oven  14  may need to be run at a higher temperature, e.g., 160° C., in order to cure the glue. Care should be taken that in raising the temperature the PCBs are not damaged and the application of solder to the PCBs in solder station  16  is not adversely affected. 
     Nitrogen is continuously added, through a nitrogen input mechanism  83  (FIG. 5) activated by controller  18 , to oven  14  at a rate of, e.g., 20 cubic feet per minute (cfm), which pushes contaminants out of the oven and provides a nitrogen environment. An oxygen analyzer  85  (FIG. 5) is used by controller  18  to monitor the oven environment, and controller  18  flags an error to the operator if the parts per million (ppm) of oxygen within oven  14  exceeds a predetermined threshold, for example, 100 ppm. The nitrogen environment reduces the possibility of oxides forming on PCB connection points as pallets are indexed through the oven. Additionally, because solder station  16  also has a nitrogen environment, directly connecting the oven to the solder station will not degrade the nitrogen environment of solder station  16 . 
     At the entrance  132  (FIG. 1 a ) of oven  14 , sparging tubes (tubes with multiple holes; not shown in the Figure) above and below entering pallets are used to blow nitrogen at the pallets. A nitrogen containment curtain (not shown) can also be draped across the entrance to prevent contaminants from entering oven  14 . 
     Referring to FIGS. 8 a  and  8   b , several, e.g., ten, heated aluminum plates are distributed within and heat oven  14 . Half, e.g., five, of the aluminum plates are supported within oven  14  below (for clarity, only three aluminum plates  138  are shown in dashed outline in FIG. 8 a ) guide rails  44  (FIG. 2 c ), which support pallets being indexed through the oven, and half of the aluminum plates (not shown) are supported within oven  14  above guide rails  44 . As the pallets are indexed through the thirteen indexed chain positions within oven  14 , the pallets pass between the top and bottom heated aluminum plates  138 . 
     On a side of each aluminum plate  138  that is opposite to the side facing the pallets, a nitrogen input tube  133  expels (arrow  135   a ) nitrogen into oven  14  on one side of a fan  134 , and fan  134  pushes (arrows  135   b ) the expelled nitrogen and environmental (i.e., already within the oven) nitrogen (arrows  135   c ) against aluminum plate  138 . Aluminum plate  138  has many small vertical holes  136  through which the nitrogen passes and becomes heated. Holes  136  in aluminum plate  138  provide a steady, even flow of warm nitrogen between guide rails  44  and, hence, against pallets on those rails. 
     Additionally, underneath the two aluminum plates  138  closest to an oven exit chamber  148 , fan  134  pushes nitrogen past tubes  144 ,  146 , filled with nitrogen, to heat the nitrogen in the tubes. Referring also to FIGS. 8 c  and  8   d , tubes  144 ,  146  are then passed to oven exit chamber  148  where tube  144  is separated into two sparging tubes  144   a ,  144   b  (tubes with multiple holes) which are extended across a top of exit chamber  148  and where tube  146  becomes a sparging tube and is extended across a bottom of exit chamber  148 . Sparging tubes  144   a ,  144   b , and  146  direct (lines  151 ) the heated nitrogen across a top and a bottom, respectively, of exiting pallets. A nitrogen containment curtain (not shown) may also be draped across exit  150  of exit chamber  148 . 
     In system  10 , exit chamber  148  is only one indexed position wide and heated nitrogen is blown across PCBs loaded in pallets to substantially prevent the PCBs from cooling. 
     Support rails  45  (FIG. 2 c ) in oven  14  are fixed to flux station  12  within ID station  50  (FIG. 1 a ) to prevent longitudinal thermal expansion in a direction toward flux station  12 , while guide rails  44  in oven  14  are not fixed at oven exit  150  to allow for longitudinal thermal expansion in a direction toward solder station  16 . Referring to FIGS. 2 a  and  9 , guide rails  44  in oven  14  are mounted on support rails  45  which include slots  152  through which shoulder screws  154  are anchored to the oven frame. Slots  152  allow support rails  45  and guide rails  44  to thermally expand and contract longitudinally, for example, by as much as about 0.312 inches. Shoulder screws  154  keep the rails on each side of the oven parallel and separated by a set width W 2  (FIG. 2 c ) which insures that guide rails  44  always support pallet edges  35  and  37  and maintain the pallets on a straight path through oven  14 . 
     Referring to FIGS. 10 a  and  10   b , from exit chamber  148 , the pallet  28  is indexed onto solder rails  155  of solder station  16 . A stopper arm  156  and a pusher arm  158  operate to roughly locate the pallet, as discussed above with respect to stopper arm  64  (FIG. 6 a ) and pusher arm  66  of flux unit  62 , and seating sensors  160 ,  162  are used by controller  18  to determine the proper seating of the pallet on solder rails  155 , similar to the use of seating sensors  80 ,  82  (FIG. 6 b ). Solder rails  155  are mechanically coupled to a rail table  163  having an aperture  163   a . Rail table  163  is mechanically coupled to a lead screw (not shown) which is driven by a stepper motor  164 , similar to stepper motor  84  (FIG. 6 b ). Controller  18  uses motor  164  to lower solder rails  155 , and, hence, the pallet, toward a stainless steel solder fountain  166  and to raise solder rails  155  away from solder fountain  166 . A datum pin  170  and an expansion pin  172  are press fit into and extend above solder fountain  166  and are used to precisely locate the pallet, and are similar to datum pin  94  and expansion pin  96  (FIG. 6 b ). A nitrogen input mechanism  161  (FIG.  5 ), controlled by controller  18 , provides nitrogen to two sparging tubes  157 ,  159  (for clarity, shown only in FIG. 10 a ) which extend across and above the solder fountain and expel nitrogen at a rate of, e.g., about 30 cfm, toward solder fountain  166 . 
     When rail table  163  and, hence, solder rails  155  are lowered to a predetermined distance above solder fountain  166 , if seating sensors  160 ,  162  indicate an improper seating of the pallet, controller  18  automatically causes stepper motor  164  to raise solder rails  155  to prevent a PCB(s) in the pallet from being damaged. Controller  18  also flags an error, through control panel  60  (FIG.  5 ), to the operator. Controller  18  may automatically prevent chains  22  from indexing and re-lower solder rails  155  or the operator, through control panel  60  and controller  18 , may manually prevent chains  22  from indexing and re-lower solder rails  155 . Alternatively, the PCB loaded in the pallet can be considered a reject. 
     Referring to FIGS. 10 c  and  10   d  (for clarity, pins  170 ,  172  are not shown in FIG. 10 d ), solder fountain  166  includes a solder manifold  174  (i.e., main solder chimney) filled with solder (not shown) and a solder well plate  176 . Solder well plate  176  includes modular solder wells  178 ,  180 , and  182  which correspond to PCBs  36 ,  38 ,  40  (FIG.  3 ). The modular solder wells are mounted over apertures (not shown) in solder well plate  176 . These mounted modular solder wells can be replaced with redesigned modular solder wells. For instance, if a PCB is redesigned and the locations of connection points are changed, a redesigned modular solder well corresponding to the redesigned PCB can mounted to the solder well plate in place of the existing modular solder well. 
     Each modular solder well  178 ,  180 ,  182  includes multiple solder chimneys  184  which correspond precisely to PCB areas having connection points to be soldered. A heater  185  (FIG. 5) heats the solder within solder manifold  174  such that the solder flows freely. Each solder chimney  184  provides an open, unrestricted passageway (P, FIG. 11 c ) from solder manifold  174  to a top of the solder chimneys  184  to allow for the flow of solder. 
     Pumping nitrogen into the solder fountain reduces the amount of contaminants, e.g., oxygen, and, therefore, prevents the flowing solder from oxidizing. An oxygen analyzer  187  (FIG. 5) is used by controller  18  to monitor the parts per million (ppm) of oxygen in solder station  16 . Controller  18  flags an error to the operator through control screen  60  if the level of oxygen exceeds a predetermined threshold, e.g., 100 ppm. 
     Referring to FIGS. 11 a - 11   c  (pins  170 ,  172  are not shown in FIG. 11 a  and PCBs  36 ,  38 , and  40  are shown without mounted components in FIG. 11 b ), when a pallet  28  is lowered on solder rails  155  (not shown), controller  18  sends electrical signals to pump  183  (FIG. 10 b ) to cause pump  183  to pump solder from solder manifold  174  through chimneys  184  at a soldering pump speed suitable to form stable surfaces  190  on the tops of molten solder columns passing through solder chimneys  184 . The pump speed depends upon the size and characteristics of the PCB being soldered, for example, the soldering pump speed for PCB  36  is, for example, approximately 80% of pump capacity, while the soldering pump speed for PCB  40  is, for example, approximately 78% of pump capacity. The areas of a PCB having connection points are then brought in contact, for a predetermined amount of time, e.g., about 5.5-5.7 seconds, with top surface  190 . The solder wets to the fluxed and pre-tinned (i.e., pre-treated) connection points between the PCB and components (not shown) mounted on the PCB. 
     Referring to FIG. 11 c , when areas of PCB  36  are brought in contact with top surfaces  190 , the solder wets to pre-treated connection points  191   a - 191   g . Those connection points which lie directly over a chimney are soldered whereas those which do not lie above a chimney are not: thus, connection point  191   a  is precisely soldered while an adjacent, extended component, e.g., a single turn transformer winding  196 , is not. The connection points can lie flush with a bottom side  36   a  of PCB  36 , such as  191   a ,  191   c ,  191   e , and  191   g , or the connection points can lie on a top surface  36   b  of PCB  36  and adjacent to through-holes  194  in PCB  36 . For instance, a surface mount component  195 , e.g., a capacitor, has a connection point  191   b  on top surface  36   b  and adjacent to through-hole  194 . When the bottom surface of PCB  36  is brought in contact with top surface  190 , the solder wets to through-hole  194  and to connection point  191   b , as described in U.S. patent application Ser. No. 08/225,263, filed Apr. 8, 1994, and assigned to the same assignee as this application and U. S. patent application Ser. No. 08/337,245, filed Nov. 10, 1994, and also assigned to the same assignee as this application. Additionally, connection points  191   d  and  191   f  of components  197   a  and  197   b  are leads which extend through adjacent through-holes  194 , and when bottom surface of PCB  36  is brought in contact with top surface  190 , the solder wets to connection points  191   d  and  191   f  and through-holes  194 . The leads can extend partially into through-hole  194 , completely through through-hole  194  and a small distance, e.g., 0.010 inches, beyond bottom surface  36   a , e.g., connection point  191   d , or completely through through-hole  194  and significantly, e.g., 0.10 inches, beyond bottom surface  36   a , e.g., connection point  191   f.    
     One type of solder fountain provides small inlets at a bottom of solder chimneys between the solder manifold and the chimneys. The flow of solder through these inlets may disrupt the top surface of the molten column of solder passing through the chimneys, and, as a result, vibrating or unstable top surfaces may apply solder beyond the PCB areas containing connection points when the PCB is brought in contact with the top surfaces. 
     Stable top surfaces  190  allow connection points in precise locations to be soldered. The unrestricted passageways, P, between solder manifold  174  and the tops of chimneys  184  may avoid the disruption that may be caused by the flow of solder through inlets in the bottom of the chimneys. Additionally, solder fountain  166  is heavy, e.g., approximately 160-200 lbs, which tends to dampen vibrations in system  10  and substantially prevent the vibrations from passing through solder fountain  166  and disrupting the stable top surfaces  190  of the molten solder columns passing through the chimneys. Solder fountain  166  is also leveled, to approximately 0.005 inches or better, with a precision grade level to insure that the stable top surfaces  190  are also level. 
     Stepper motor  164  (FIG. 10 b ), like stepper motor  84  (FIG. 6 b ), brings solder rails  155  to a predetermined height with an accuracy of approximately +/−0.001 inches. Using pump speed, solder temperature, and solder level, controller  18  can estimate the height of top surfaces  190  above chimneys  184 . The solder pump preferably includes a closed loop rotation per minute (rpm) controlled motor system which precisely maintains a selected motor speed. Through control panel  60  (FIG.  5 ), the operator selects a pump speed and, in response, controller  18  sends an electrical signal to a motor controller (not shown) which corresponds to the selected pump speed. The motor controller uses the electrical signal to set the motor rpm to the desired level and uses an rpm sensor (not shown) to detect the actual speed of the motor. The motor uses the detected actual motor speed to maintain the motor speed at the desired level. 
     Instead of estimating the height of top surfaces  190 , a method for accurately determining the height to which solder rails  155  should be lowered begins by loading a high temperature glass plate, e.g., neoceran, into one or more of the apertures in a pallet. The pallet is then placed on solder rails  155  and rails  155  are lowered until the operator visually detects that the glass plate has come in contact with top surfaces  190  and a consistent displacement of solder is detected across the glass plate at the location of each chimney. A teach button on control panel  60  (FIG. 5) can then be activated by the operator to set the height to which controller  18  will cause stepper motor  164  to bring rails  155 . 
     The height to which rails  155  are brought and the pump speed of the pump may vary with the type of PCB loaded in the pallet. For example, small PCBs, e.g., PCB  40 , often use modular wells, e.g, modular well  182 , with thicker walled, smaller chimneys  184 . The small amount of solder passing through thick walled, small chimneys may heat the chimneys less than the larger amount of solder passing through thinner walled, larger chimneys, e.g., modular well  178 . Consequently, the pump speed may have to be increased to pass the solder through these smaller chimneys and achieve the same top surface  190  height. The operator can manually or controller  18  can automatically set three different pump speeds depending upon which PCB is loaded in the pallet. 
     After a PCB in a pallet has been soldered and solder rails  155  raise the pallet, or when solder rails  155  do not hold a pallet, controller  18  (FIG. 5) sends electrical signals to a solder pump  183  (FIGS. 5 and 10 b ) to cause solder pump  183  to pump solder from solder manifold  174  through solder chimneys  184  at an overflow rate, e.g., 85% of pump capacity, or to clean out the solder chimneys, controller  18  periodically sends electrical signals to solder pump  183  causing solder pump  183  to pump solder from solder manifold  174  through solder chimneys  184  at a purge rate, e.g., 90-95% of pump capacity. The solder which overflows the chimneys is caught by weirs  186  and recirculated to solder manifold  174 . 
     The flowing, heated solder heats solder well plate  176  and chimneys  184  and also heats the environmental nitrogen. The solder contained within solder manifold  174  provides a certain heat mass, and the larger this mass, the less likely it will be that heat transferred to the solder well plate, chimneys, and environmental nitrogen will affect the overall temperature of the solder. To direct the overflow of solder and prevent undirected overflow paths from applying solder to undesired areas of the PCB, overflow indentations  188  (FIGS. 10 d ,  11   c , and  11   c ) can be machined into one or more of the chimneys (for clarity, only one is shown). 
     After a certain number of pallets have passed through system  10 , controller  18  detects the level of solder in manifold  174  with, e.g., a retroreflective solder level sensor  192  (FIG. 10 a ) or a floating solder level sensor (not shown). When the level of solder is below a threshold level (determined by the placement of solder level sensor  192 ) controller  18  sends electrical signals to solder feed unit  20  (FIG. 1 b ) to cause solder feed unit to add solder to solder station  16 . 
     In a partially loaded system  10  (i.e., only three pallets are shown within system  10  in FIG. 2 a ), controller  18  causes the procedures at flux unit  62  and solder station  16  to be executed only when a pallet with a loaded PCB is located at that station. When system  10  is fully loaded (i.e., one pallet with a loaded PCB is located between each pair of flights), controller  18  causes the procedures at flux unit  62  and solder station  16  to be executed during each index period. 
     Other embodiments are within the scope of the following claims. 
     For example, although controller  18  was described as providing different flux spray times and different solder pump speeds for the three different types of PCBs which may be loaded in a pallet, a single flux spray time and a single solder pump speed may be sufficient for all three PCB types. If so, two or all three types of PCBs may be loaded at one time in a pallet as the pallet is passed through system  10 . Similarly, the pallet can be designed to hold one PCB or one or more of the same type of PCB. 
     Controller  18  may include a central controller electrically connected to processing station sub-controllers, for instance, identification station  50  (FIG. 1 b ), flux unit  62 , oven  14 , and solder station  16 , may each include a sub-controller specifically designed to control the operation of the station. The central controller monitors the operation of the entire system and coordinates the operation of the sub-controllers. 
     A motor  198  (FIG. 10 b ) which drives chains  22  can include a slip clutch  199 , e.g., a torque limiter clutch. With a slip clutch, if chains  22  are prevented from indexing, for instance, by a jammed pallet, the clutch slips and, as a backup, motor electronics limit the current to the motor to prevent the motor from forcing chains  22  to index which may damage chains  22 , system  10 , or a jammed pallet. Controller  18  can monitor chain movement by determining whether a flight  25  periodically passes home sensors  23  (FIG. 2 a ). If flights are not detected within a predetermined amount of time, then controller  18  flags an error to the operator. 
     The walls of chimneys  184  can be machined to change the shape of the chimneys and the wall thickness. This may be required for minor specification changes or if solder is not being properly applied to desired PCB areas. For more significant specification changes, a modular well  178 ,  180 , or  182  (FIG. 10 d ) can be removed from well plate  176  and replaced with a new modular well reflecting the specification changes. Additionally, a modular well corresponding to a different PCB type altogether may replace one of the existing modular wells attached to well plate  176 . Similarly, well plate  176  can be replaced with a new well plate having different sized apertures, possibly in different locations, for receiving different sized modular wells. of course, such changes may required similar changes to pallet  28 . 
     Identification station  50  could include a bar code scanner (not shown) for reading a bar code  200  (FIG. 3) on pallet  28  and for notifying controller  18  of the results of the bar code scan. Controller  18  could then use the results of the bar code scan to determine using, for example, a table look up, which PCB or PCBs are loaded in pallet  28 . Bar codes allow controller  18  to keep track of individual pallets. 
     Many sensors can be placed throughout system  10  and monitored by controller  18  to detect error conditions. For example, a solder temperature sensor  202  (FIG. 10 a ) can be monitored by controller  18  to prevent controller  18  from activating the solder pump, and possibly damaging the solder pump, when the solder is below a first predetermined temperature, e.g., 230° C. Similarly, solder temperature sensor  202  can be monitored by controller  18  to prevent controller  18  from activating the solder pump, and possibly damaging a PCB loaded in a pallet on lowered solder rails  155 , when the solder temperature is above a second predetermined temperature, e.g., 270° C. 
     Upon the detection of an error condition, controller  18  can execute automatic procedures or wait for instructions from the operator. When controller  18  or an operator prevents chains  22  from indexing, controller  18  can automatically or the operator can manually reduce the temperature at which oven  14  is running to further reduce the possibility that PCBs being indexed through oven  14  will be heat damaged. If controller  18  is connected to conveyor belt  46  (FIG. 1 b ), and controller  18  detects an error, controller  18  can automatically stop conveyor belt  46  and prevent new pallets from entering system  10 . Aside from notifying the operator of errors through control panel  60  (FIG.  5 ), controller  18  could notify the operator through alarm lights or bells.