Abstract:
A wave soldering apparatus includes a solder reservoir adapted to contain molten solder, and a solder nozzle disposed in the solder reservoir and extending up above the molten solder. The nozzle provides a substantially turbulent free solder wave under a printed circuit board while the board is moved in a predetermined path. A tray is pivotably mounted to the nozzle and angularly moved to vary the flow rate of the molten solder. A shroud is mounted adjacent to and associated with the tray to define a contained space into which an inert gas is supplied to provide an inert gas atmosphere. The shroud includes a canopy extending over a portion of the tray. The canopy extends substantially parallel to the predetermined path and is adjustably positioned in response to angular position of the tray to ensure that the board exits from the solder wave within the inert gas atmosphere.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an apparatus and method for automatically wave soldering workpieces such as printed circuit boards under an insert gas atmosphere.  
           [0002]    A conventional automatic wave soldering apparatus typically includes a flux applicator, a preheater, a solder station and a cooling station subsequently arranged to process printed circuit boards. While the printed circuit boards are transported by a conveyor with their side edges supported by gripping fingers, flux is first applied by contacting each circuit board with a foam of flux. Alternatively, flux may be applied to the circuit board by spraying. The fluxed board is preheated by the preheater in order to evaporate excess flux solvent and prevent sudden extreme temperature changes or “heat shock” to the board. The circuit board is then contacted with multiple waves of molten solder. Typically, the solder station includes a relatively narrow nozzle to produce a turbulent wave. This turbulent wave enables the molten solder to fill the gap between leads of electronic components and through holes in the circuit board. However, solder bridges, icicles or excess solder deposits are apt to remain on the underside of the circuit board. To remove such undesirable bridges, a relatively wide nozzle is provided downstream of the narrow nozzle to produce a smooth turbulent free solder wave through which the circuit board passes.  
           [0003]    The electronic components are heated to a high temperature when current flows therethrough. On the other hand, the electronic components are cooled to a room temperature in an inoperative state. This thermal cycling causes expansion and contraction of the board as metallic solder and the plastic printed circuit board have different coefficients of thermal expansion. The resulting mechanical stress causes the solder to fracture or peel.  
           [0004]    To avoid such solder fracture or fatigue, attempts have been made to deposit a relatively large amount of solder on selected regions of a board to be soldered, as disclosed by Japanese laid-open patent application No. 9-283912. A nozzle is arranged in a solder bath and has a fixed front guide and an adjustable rear guide. A baffle plate is attached to the nozzle near the rear guide. The rear guide is inclined downwards in the direction of movement of a board to thereby form a step between the baffle plate and the rear guide. The molten solder flows at a relatively fast rate after it flows over the baffle plate. This enables a relatively large amount of molten solder to be deposited on selected regions of the circuit board to be wave soldered. However, application of such a large amount of molten solder is apt to form solder bridges, particularly in case that electronic components are packaged with high density. It is, therefore, necessary to adjust the flow rate of molten solder depending on the density of packaged components. This adjustment is made manually, or automatically as disclosed by Japanese laid-open patent application No. 9-293959.  
           [0005]    A tin-lead solder has superior wetting characteristics and is conventionally used with RA flux or RMA flux. The use of RA flux minimizes or eliminates the occurrence of bridging regardless of whether solder is applied to high density circuit boards or a large amount of solder is applied. However, RA flux residues are corrosive or hydrolyze to corrosive constituents in the presence of moisture. Those flux residues must therefore be rinsed with chlorine solvent, fluorine solvent, hydrocarbon solvent, terpene solvent or other solvents. All of those solvents are considered to be environmental pollutants. The RMA flux is less corrosive and requires no cleaning after soldering. However, the RMA flux is less active than the RA flux and tends to form solder bridges. The use of a lead-free solder also forms solder bridges since the lead-free solder exhibits a high degree of surface tension and a low degree of wettability.  
           [0006]    It has been found that formation of such solder bridges can be avoided if soldering occurs in a substantially oxygen-free atmosphere. Typically, an inert gas is directed to the point, known as “peel back” region, at which a printed circuit board exits from a solder wave. To reduce oxygen content in that region, an inert gas nozzle is placed as close to the peel back region as possible. However, the single use of the inert gas nozzle is not satisfactory since the concentration of oxygen in the peel back region are apt to fluctuate.  
           [0007]    Accordingly, it is an object of the present invention to provide an automatic wave soldering apparatus and method which can provide a steady supply of a reduced oxygen atmosphere near the point at which circuit boards exit from a solder wave.  
         SUMMARY OF THE INVENTION  
         [0008]    According to one aspect of the present invention, there is provided an apparatus for wave soldering a workpiece such as a printed circuit board, which comprises a solder reservoir adapted to contain molten solder, and a solder nozzle extending up above the molten solder and arranged to provide a substantially turbulent free solder wave under the workpiece. A tray is pivotably mounted to the downstream side of the solder nozzle. The tray is angularly moved to vary or adjust the rate of flow of the molten solder after the molten solder is pumped up through the solder nozzle. A shroud or enclosure is mounted adjacent to and associated with the tray to define a contained space into which an inert or non-oxidizing gas is supplied to provide an inert or non-oxidizing gas atmosphere. The shroud includes an adjustable canopy extending over a portion of the tray and selectively moved toward and away from the solder nozzle in a direction substantially parallel to the path in which the workpiece is moved. The canopy is adjustably positioned in response to angular position of the tray to ensure that the workpiece exits from the solder wave within the inert gas atmosphere.  
           [0009]    The point at which the workpiece exits from the solder wave is referred to in the art as a “peel back region”. This peel back region is displaced depending upon the rate of flow of the molten solder. The peel back region should be blanketed with an inert gas atmosphere in all cases; otherwise, solder bridges or icicles are likely to occur. According to the present invention, the tray is angularly moved downwards to increase the rate of flow of the molten solder when a relatively large amount of solder deposits are required. At this time, a peel back region is defined at near the point where the tray is attached to the solder nozzle. It is therefore necessary to position the canopy as close to that point as possible so as to ensure that the peel back region is blanketed with an inert gas atmosphere. As stated above, the canopy extends substantially parallel to and below the path in which the workpiece is moved. This arrangement allows the canopy to position as close to the peel back region as possible. Such close position aids in reducing oxygen content near the peel back region. When a relatively small amount of solder deposits are required, the tray is angularly moved to a horizontal position or slightly upwardly inclined position to decrease the rate of flow of the molten solder. At this time, a peel back region is displaced away from the point at which the tray is attached to the nozzle. To this end, the canopy is moved in a direction away from the nozzle and positioned near the peel back region. This movement of the canopy will not interfere with the position of the tray as the canopy is movable along the path in which the workpiece is moved.  
           [0010]    In a preferred embodiment, a baffle may be located at the downstream end of the solder nozzle and associated with the tray to form a step when the tray is oriented in a downwardly inclined position. By this arrangement, the molten solder is caused to drop when it flows over the baffle. This enables the molten solder to flow at a relatively fast rate. To allow angular movement of the tray, a bracket may be secured to the solder nozzle, and a crank may be disposed between the bracket and the tray. The crank may be connected to a motor and driven to cause the tray to rotate in a vertical plane.  
           [0011]    In a preferred embodiment, the shroud may include a mount secured to the solder reservoir and shaped to slidably support the canopy thereon. A second solder nozzle may be located upstream of the solder nozzle to provide a turbulent wave under the workpiece. A weir member may be mounted adjacent to the tray and cooperate with the tray to form an opening through which the molten solder flows back to the solder reservoir. The weir member may be selectively rotated toward and away from the tray to vary the degree of opening of the opening.  
           [0012]    According to another aspect of the present invention, there is provided a method of wave soldering a workpiece, which comprises the steps of providing a wave soldering apparatus including a solder reservoir within which molten solder is contained, a solder nozzle disposed in the solder reservoir, a tray pivotably mounted to the solder nozzle, and a shroud including an adjustable canopy and associated with the tray to form a contained space, forcing the molten solder through the solder nozzle to generate a substantially turbulent free solder wave under the workpiece while the workpiece is moved in a predetermined path, angularly moving the tray to vary the rate of flow of the molten solder, providing an inert gas atmosphere within the contained space, and adjusting position of the canopy in response to angular position of the tray to ensure that the workpiece exits from the solder wave within the inert gas atmosphere.  
           [0013]    Depending on angular position of the tray, the canopy is moved toward and away from the solder nozzle in a direction substantially parallel to the path in which the workpiece is moved. The tray is oriented in a downwardly inclined position so as to increase the flow rate of the molten solder when a relatively large amount of solder deposits are required. When a relatively small amount of solder deposits are required, the tray is oriented in a horizontal position or slightly upwardly inclined position to decrease the flow rate of the molten solder. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The above and other objects, features and advantages of the present invention will become more apparent from a reading of the following description of preferred embodiments when taken in conjunction with the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1 is a sectional view of one embodiment of an automatic wave soldering apparatus with a tray placed in an downwardly inclined position;  
         [0016]    [0016]FIG. 2 is an enlarged perspective view of a shroud; and  
         [0017]    [0017]FIG. 3 is a view similar to that of FIG. 1, but with the tray placed in a substantially horizontal position. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Referring first to FIG. 1, there is shown an automatic wave soldering apparatus according to one embodiment of the present invention and generally designated at  10 . The wave soldering apparatus  10  includes a solder reservoir  12  for holding a supply of molten solder  14 , first and second solder nozzles  16 ,  18  for providing solder waves of molten solder, and an shroud  20  for providing an inert gas atmosphere. A printed circuit board  22  is passed along a path  24 , shown by broken line in FIG. 1, above the solder reservoir  12 . The path  24  is sloped upwards at an angle, for example, of 3 to 5 degrees to the horizontal.  
         [0019]    The first solder nozzle  16  includes a pair of upstream and downstream sides or inwardly sloping walls  25 ,  26 , and vertical side walls (not shown) secured to opposite sides of the inwardly sloping walls  25 ,  26 . The first nozzle  16  extends up above the solder level and has a relatively narrow opening  28  through which the molten solder is pumped up to form a solder wave. In the illustrated embodiment, provision is made to generate a turbulent wave. Specifically, a transverse rod or shaft  30  extends below the narrow opening  28  of the first solder nozzle  16 . Opposite ends of the rod  30  are supported by springs (not shown). When the molten solder is pumped up through the nozzle opening  28 , it flows against the rod  30 . This causes the molten solder to flow in a vortex above the rod  30 . The rod  30  is first bent upward and then, moved back under the influence of the springs. This up and down motion of the rod  30  creates a turbulent wave. One example of such turbulent means is disclosed in Japanese patent publication No. 1-59073, the content of which is incorporated herein by reference. Illustratively, the turbulent wave is arranged to only contact the underside of the board  22 . The turbulent wave promotes the penetration of the molten solder into narrow spaces between electronic components, the filling of through holes in the circuit board, the filling of crevices and corners adjacent solder masks.  
         [0020]    The second solder nozzle  18  is provided downstream of the first nozzle  16  and has a nozzle opening  32  wider than that of the first nozzle  16 . The second nozzle  18  also extends up above the solder level and is adapted to provide a smooth turbulent free solder wave. As best shown in FIG. 2, the second nozzle  18  includes a pair of upstream and downstream sides or front and rear inwardly sloping walls  34 ,  36 , and a pair of opposite side walls (only one is shown)  38  secured to the front and rear walls  34 ,  36 . These side walls  38  are also secured to the shroud  20  as will later be described. A front guide  40  extends forwardly and downwardly from the upper end of the front wall  34 . An adjustable second guide or substantially flat tray  42  is mounted to the upper end of the rear wall  36  and can be rotated about a horizontal pivot pin  44 . A horizontal bracket  46  is secured to the rear side of the rear wall  36  below the tray  42 . A crank  48  is disposed between the tray  42  and the bracket  46 . The crank  48  has a shaft  50 . The shaft  50  has a L-shaped end  52  contacted with the tray  42 . The other end of the shaft  50  is connected to a reversible motor  54  which is, in turn, mounted outside the solder reservoir  12 . The crank  48  is rotated in directions, as shown by the double-headed arrow in FIG. 1, so as to adjust angular position of the tray  42 .  
         [0021]    A baffle plate  56  is fixedly attached to the front side of the rear wall  36  and extends up above the pivot pin  44 . The baffle plate  56  forms a step when the tray  42  is oriented in a downwardly inclined position as shown in FIG. 1. This step causes the molten solder to drop when it flows over the baffle plate  56 . An adjustable weir member  58  is mounted adjacent to the free end of the tray  42  to form an opening  60  through which the molten solder can flow back to the solder reservoir  12 . The weir member  58  is connected to a motor (not shown), whereby the weir member  58  is rotated to allow the molten solder to flow at variable flow rates. When the weir member  58  is rotated upwards to block the flow of the molten solder, the molten solder flows at a relatively slow rate. On the other hand, when the weir member  58  is rotated downwards to decrease the size of the opening  60 , a significant volumetric portion of the molten solder flows over the weir member  58  at a relatively fast rate. The-weir member  58  thus serves as an additional means to adjust the rate of flow of the molten solder.  
         [0022]    The shroud  20  is arranged adjacent to and associated with the tray  42  to define a contained space into which an insert gas is supplied to provide an insert gas environment, as will later be described in detail. The shroud  20  generally includes a T-shaped mount  60 , an adjustable canopy or shield plate  62 , and a skirt  64 . T-shaped mount  60  has a vertical plate  66  secured to the rear wall of the solder reservoir  12 , and a downwardly sloping plate  68  secured to the top end of the vertical plate  66  and extending substantially parallel to the path  24 . The skirt  64  is attached to the front side of the vertical plate  66  and extends down below the level of the molten solder in the reservoir  12 . The vertical plate  66  has a vertical slot  70  through which a bolt  72  extends. A nut  74  is threadably engaged with the bolt  72  so as to secure the mount  60  in place. This arrangement allows vertical adjustment of the mount  60  relative to the reservoir  12 . The canopy plate  62  extends substantially parallel to and below the path  24  and has a longitudinal slot  76  through which a bolt  78  extends. The bolt  78  is secured to the downwardly sloping plate  68 . A nut  80  is threadably engaged with the bolt  78  to secure the canopy plate  62  in place. This arrangement allows longitudinal adjustment of the canopy plate  62  relative to the downwardly sloping plate  68  and thus, the tray  42 , as shown by the double-headed arrow in FIG. 2. The side walls  38  are secured to opposite sides of the shroud  20  to provide a seal. A gas inlet  82  is defined in at least one of the side walls  38  and is communicated with a gas pipe  84  (FIG. 2) which is, in turn, connected to a source of inert gas  86 . The canopy plate  62  extends over a portion of the tray  42 . The shroud  20  and the tray  40  cooperate to form a contained space into which an inert gas is supplied to blanket the solder wave. The gas inlet  82  is located between the canopy plate  62  and the skirt  64  so as to effectively direct an insert gas toward the contained space.  
         [0023]    [0023]FIG. 1 shows the manner in which a large amount of solder is applied to the underside of the circuit board  22 . Specifically, the motor  54  is energized to rotate the crank  48  so as to incline the tray  42  downwards in the direction of movement of the circuit board as shown by the arrow in FIG. 1. A step is thereby formed between the baffle plate  56  and the tray  42 . The molten solder is caused to drop when it flows over the baffle plate  56 . This enables the molten solder to flow at a relatively fast rate. In this case, a peel back region P 1  is defined at a point substantially above the baffle plate  56 . The canopy plate  62  is moved as close to the peel back region P 1  as possible. At this time, the height of the canopy plate  62  must be so adjusted as to clear the tips of leads or the lowest extremities of exposed metallic surfaces of the circuit board to be wave soldered. An inert gas is introduced into the shroud  20  through the gas inlet  82 . The inert gas then flows between the canopy plate  62  and the tray  42  and is directed toward the peel back region P 1 . The circuit board  22 , the tray  42  and the canopy plate  62  cooperate to ensure that a minimum of inert gas escapes from the shroud  20 . A substantially oxygen-free atmosphere is thus maintained in the peel back region P 1 .  
         [0024]    [0024]FIG. 3 shows the arrangement suitable for applying a relatively small amount of molten solder to printed circuit boards with high density packaging of electronic components. If a large amount of molten solder is applied to such high density circuit boards, bridging of solder between adjacent conductors or metallic surfaces results. To this end, the tray  42  is so positioned as to reduce the rate of flow of molten solder. Specifically, the motor  54  is energized to rotate the crank  48  so as to orient the tray  42  in a substantially horizontal position or slightly upwardly inclined position in the direction of movement of the circuit board  22 . Now that the top of the baffle plate  56  is in substantially the same level as the tray  42 , the molten solder smoothly flow through the space between the circuit board  22  and the baffle plate  56 . This enables the molten solder to flow at a relatively slow rate. In other words, a relatively small amount of molten solder can be applied to the underside of the circuit board. This prevents the occurrence of undesirable bridging. In this case, a peel back region P 2  is defined downstream of the peel back region P 1 . The canopy plate  62  is moved in a direction away from the solder nozzle  18  so that the peel back region P 2  is blanketed with an inert gas atmosphere. This also prevents interference of the canopy plate with the tray  42 .  
         [0025]    Tests were conducted with circuit boards with high density packaging of electronic components as well as circuit boards in which a large amount of solder needed be deposited. The angle of inclination of the tray was varied depending on applications. The canopy plate was longitudinally adjusted accordingly. Flux used was of RMA type. Solder comprised of 63% by weight tin and 37% by weight lead. The results showed no bridging. The concentration of oxygen was less than 500 ppm. Another test was carried out with a lead-free solder and RA flux. The lead-free solder comprised 95.75% by weight tin, 3.5% by weight silver, and 0.75% by weight copper. The test result also showed no formation of bridges or icicles.  
         [0026]    An additional test was conducted without shroud. An insert gas was directed to a peel back region by the use of an inert gas nozzle. Bridging resulted in all cases. Also, fillets were formed only at the lower end of leads when the lead-free solder was used. In some instances, molten solder did not sufficiently fill through holes. The concentration of oxygen was over 1,000 ppm.  
         [0027]    Although the present invention has been described with respect to its preferred embodiments, it is to be understood that various changes and modifications may be made without departing from the scope of the invention as defined by the appended claims.