Patent Publication Number: US-2002007782-A1

Title: Heater in a conveyor system

Description:
BACKGROUND OF THE INVENTION  
       [0001] This invention relates to a heater for an oven with a conveyor, and more particularly to a heating system for use with a liquid dispensing system.  
       [0002] In the manufacture of printed circuit boards, liquid dispensing systems can be used to dispense liquids on the circuit board. Different types of dispensers can be used to dispense different types of liquids for different purposes. One application of a liquid dispenser is for dispensing glue for mounting components on the board. Another application for a liquid dispenser arises after a packaged chip has been mounted to a circuit board. The liquid dispenser dispenses a bead of liquid around the perimeter of the chip. The liquid wicks under the chip to underfill it, thereby thermally conducting heat away from the packaged chip. In another application, a die can be mounted to a circuit board, and then the liquid dispenser dispenses a viscous liquid over the die. The liquid hardens to encapsulate the die in place on the board. In such liquid dispensing systems, a conveyor transports a circuit board along a direction of travel until it reaches a desired position where the conveyor stops and the dispensing is performed.  
       [0003] The underfill and encapsulation processes are typically performed in a heated environment. A radiant heater heats the circuit board from below while the liquid dispenser dispenses liquid from above. One type of heater has a number of quartz tubes mounted in parallel, transverse to the direction of travel. The quartz tubes enclose coils that quickly heat to a high temperature with an electric current. These tubes, however, tend to create relatively hotter and relative cooler areas on a circuit board.  
       [0004] In ovens used for reflow soldering, convection can be added to the radiant heating by providing a fan to blow air across the radiant heat source. A plate with a series of holes is positioned between the fan and the radiant heat source, or the radiant heat source can itself be a panel with holes. Because of the circular air flow from the fan, however, the temperature of the air that flows through the openings is inconsistent from one hole or group of holes to others. Another drawback with such a system is that the fan pushes a large volume of hot air that provides great heat to the outside environment.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0005] It is an object of the present invention to provide convection heating in a conveyor system, and particularly for use with a liquid dispensing system in which liquid is dispensed on a circuit board in a heated environment.  
       [0006] It is another object of the present invention to provide convection to a heating system without producing excessive airflow.  
       [0007] According to the present invention, a system for operating on a workpiece, such as a circuit board, has a conveyor for transporting the circuit board along a direction of travel to a working position. A heater is mounted on one side of the circuit board when the circuit board is in the working position, and a hollow tube is mounted so that the heater is between the circuit board and the hollow tube. The tube has holes oriented to direct a gas, such as air, toward the heater and the circuit board. Preferably a number of tubes are arranged in parallel, with each made to facilitate heating of the air inside the tubes.  
       [0008] The present invention provides a conveyor system with a simple and easily manufactured convection system that provides a low volume of air in a consistent manner to provide even and consistent convection heating to a workpiece, such as a circuit board, when it is being operated on in a working position. Other features and advantages will become apparent from the following detailed description, drawings, and claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is a perspective view of a conveyor system according to the present invention.  
     [0010]FIG. 2 is a part elevational side view, part cross-sectional view of the system of FIG. 1 shown with a liquid dispenser.  
     [0011]FIG. 3 is a part schematic, part plan view illustrating heating coils and tubes according to the present invention.  
    
    
     DETAILED DESCRIPTION  
     [0012] Referring to FIGS. 1 and 2, a conveyor system  10  has two elongated rails  12 ,  14  spaced apart and parallel to each other. Each rail  12 ,  14  supports a conveyor belt  16 ,  18  driven by a controllable motor  20 ,  22  for transporting workpieces, such as printed circuit boards  24 , along a direction of travel  26 . Circuit boards are transported by conveyor belts  16 ,  18  one at a time to a working position as detected by sensors (not shown). The rails are mounted in a track transverse to the direction of travel such that the distance between rails  12 ,  14  can be adjusted for different size circuit boards.  
     [0013] Referring particularly to FIG. 2, in a liquid dispensing system, when the circuit board is in the working position, motors  20 ,  22  stop conveyor belts  16 ,  18  and a liquid dispensing valve  28  is lowered to dispense liquid onto the circuit board  24 . The liquid for dispensing is held in a syringe  30  coupled to valve  28 . Valve  28  and syringe  30  are mounted for movement along three mutually orthogonal axes. As shown generally in FIG. 2, lead screws  32  and  34  are used to move the valve along the x-axis and the z-axis (y-axis is not shown but could include a rail that supports lead screws  32 ,  34 ). A computer system (not shown) controls the movement of valve  28  and its operation to dispense liquid at desired locations. Exemplary liquid dispensing systems are manufactured and sold by Camelot Systems, Inc. of Haverhill, Mass. under the name CAM/ALOT ®. These dispensing systems use a rotary positive displacement pump approach with liquid held under low continuous pressure. Other types of liquid dispensing valves could be used.  
     [0014] A number of different liquids can be dispensed to accomplish one of a number of different objectives. For example, the liquid can be a glue for bonding components to the circuit board, an underfill material deposited around a mounted packaged chip to underfill the chip, or an encapsulating liquid that hardens over a die to encapsulate it on the circuit board.  
     [0015] As shown in FIGS.  1 - 2  and particularly in FIG. 3, a number of elongated quartz heating tubes  50  are oriented in parallel and transverse to direction of travel  26 . Each heating tube  50  encloses a coil  56  connected at each end to a conductive end cap  58 . End caps  58  snap in and out of spring clamps  59 , which are electrically coupled to an electric power source  52 . Source  52  energizes coils  56 , causing heating tubes  50  to heat very quickly and provide radiant heat to the circuit board. Underneath the heating tubes is a highly reflective floor  54 . While tubes  50  can be replaced, they are stationary during operation of the system.  
     [0016] While three heating tubes  50  are shown in FIGS. 1 and 3 for illustrative purposes, there would typically be many more such tubes, e.g., one every 1.5inches (3.75 cm) over a length of 6 feet (180 cm). Furthermore, groups of heating tubes can be controlled separately from other groups of heating tubes to define multiple heating zones, such as a pre-heat zone, a dispensing zone, and a post-heat zone.  
     [0017] The discussion to this point relates to components that are known or have been used in heated dispensing systems.  
     [0018] According to the present invention, a number of elongated hollow tubes  60   a - 60   c,  coupled at each end to an air supply  62 , are mounted transverse to heating tubes  50 , i.e., parallel to the direction of travel  26 . In an exemplary embodiment, these tubes have a 0.25 inch (6.35mm) outer diameter and a {fraction (1/32)} inch (0.8 mm) wall for a cross-sectional area of 0.04in 2  (24mm 2). Tubes 60   a - 60   c are made of black anodized aluminum to facilitate the absorption of radiant energy from heating tubes  50  and thus to effectively preheat the air in tubes  60   a - 60   c.    
     [0019] Tubes  60   a - 60   c  have holes  64  oriented to provide air from air supply  62  upwardly toward heating tubes  50  and hence toward the circuit board. In the exemplary embodiment, holes  64  are each about 0.010 inches (0.25 mm) in diameter and are at locations  66  about midway between heating tubes  50 . At each location  66 , there can be one or more holes  64  for providing air. As shown in FIG. 3, the holes are preferably located halfway between pairs of adjacent heating tubes  50 , or outside heating tubes at the ends of rows.  
     [0020] Referring to FIGS. 1 and 2, in an exemplary embodiment, the system has two outer tubes  60   a ,  60   c  that are parallel and adjacent rails  12 ,  14 . Outer tubes,  60   a,    60   c  have one hole per location  66 , with these holes directed slightly at an angle so that air flow is upwardly and inwardly as shown by arrows  70 ,  72  in FIG. 2. Positioning outer tubes  60   a,    60   c  near rails  12 ,  14  is useful because the rails tend to function as heat sinks. Tubes  60   a,    60   c  are mounted with mounting blocks  76 ,  78  that are coupled to rails  12 ,  14  for lateral movement with the rails.  
     [0021] A third tube  60   b  is mounted parallel and between tubes  60   a,    60   c  with a mounting block  80 . Block  80  is laterally movable in a channel  82  so that tube  60   b  can be repositioned midway between rails  12 ,  14  if the distance between the rails is adjusted. Tube  60   b  has three holes at each location  66 . One hole is oriented to direct air straight upwardly, and two holes on each side are directed upwardly and outwardly as shown by arrows  86  in FIG. 2.  
     [0022] The amount of air provided into each end of each tube is preferably about one pound, or 25 lb/in 2  (8.8 kg/cm 2 ), to produce a gentle turbulence from holes  64 . While fan-based convection produces a high volume of air flow with uneven heat, the convection according to the present invention provides a more gentle and even flow of warm air underneath the circuit board to reduce variations in heating from heating tubes  50 .  
     [0023] Having described a preferred embodiment of the present invention, it should be apparent that other modifications can be made without departing from the scope of the present invention as defined by the appended claims. For example, the hollow tubes can be made of a different material and have different dimensions from those described; moreover, more or fewer air tubes could be used as desired, and depending on the distance between the rails and the size of each circuit board. While the invention has been described for use with a conveyor, other systems, such as robotic systems, could be used for moving a workpiece, such as a circuit board, while other supports, such as stationary shelves or clamps could be used to support the workpiece while it is worked on.