Abstract:
A convection heat attachment and removal system for surface mounted assemblies is disclosed which uses a flow of heated gas to heat and melt solder contacts between surface mounted components and printed circuit boards. The system protects components adjacent to the workpiece by confining the flow of hot gas within a nozzle conduit which encompasses the workpiece. The system also protects the workpiece from overheating and thermal shock by shielding the workpiece from direct incidence of hot gas and by cooling the top surface of the workpiece with cool air. Relative positioning of the workpiece, the printed circuit board and the source of convection heat is accomplished by an arm mechanism as well as an X-Y table. The positioning means in this system are designed to accommodate a method of precise positioning disclosed herein. Convenient physical and visual access is provided to the workpiece and its location on the printed circuit board at any time during positioning.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a method and an apparatus for soldering surface mounted electronic components to and desoldering them from metallic contacts on substrates such as printed circuit boards. 
     2. Summary of the Invention and Description of the Prior Art 
     A convection heat system for metallic solder attachment and detachment of components surface-mounted on printed circuit boards is disclosed. For soldering, the system conveniently, rapidly and precisely positions the workpiece component, the printed circuit board and a source of convection heat relatively to each other through either moving the printed circuit board using X-Y table or through moving the vacuum-held workpiece with the source of convection heat to a desired relation with the printed circuit board using an arm system that moves in X, Y and Z directions independently of the X-Y table. The source of convection heat used for melting the solder is hot gas which is forced to flow down the terminus of the arm inside a nozzle conduit that encompasses the component work area where the hot gas deflecting inwardly from the nozzle conduit walls moves rapidly past the area to be heated and then outward from the printed circuit board working area. At the same time that the hot gas flow is melting solder, cooling air is forced down a tube inside the nozzle conduit onto the top central portion of the workpiece component surface. The tube used for passage of the cooling air may advantageously be used also for the vacuum holding of the workpiece since the two operations take place at different times. A thermal sensor in the immediate vicinity of the solder joints effectively measures the temperature of the solder so that the heating process can be controlled or terminated. 
     Hot gas has been used in a number of existing systems to melt solder contacts between surface mounted components and printed circuit boards so as to release or secure these contacts. One such system was disclosed in the U.S. Pat. No. 4,426,571, where the main problem is that the hot air flow is not restricted to the immediate vicinity of the workpiece, and overheating of components adjacent to the workpiece occurs. Large amount of hot air required in this case often causes damage to the printed circuit board. In the present invention hot gas is restricted to the immediate vicinity of the workpiece by means of a nozzle conduit. 
     Another problem encountered in practical applications of the method disclosed in the U.S. Pat. No. 4,426,571 is damage of circuitry printed on the printed circuit board. This damage may easily occur when the workpiece is picked up by hand or a hand held tool while the solder contacts connecting the workpiece to the printed circuitry are not completely melted. The present invention provides vacuum pick-up so that no excessive force may be applied to lift the workpiece off the printed circuit board. 
     The idea of restricting hot gas to the immediate vicinity of the workpiece has been previously disclosed in the U.S. Pat. No. 4,552,300, where hot gas fills the cavity formed between the nozzle walls and the printed circuit board, and the workpiece is located in the cavity. This method proved to be inefficient in transferring heat from hot gas to solder contacts. In the present invention the heat exchnge occurs between hot gas rapidly moving directly past the solder contacts into the area outside the nozzle conduit. 
     Another problem that often occurs when soldering or desoldering contacts between the workpiece and the printed circuit board is overheating and/or thermal shock damage of the workpiece itself. The present invention provides means for cooling the workpiece and shielding it from direct incidence of hot gas to avoid overheating and thermal shock damage of the workpiece. 
     In systems such as CRAFT-100 by PACE Inc. and systems manufactured by AIRVAC little or no visual and physical access is provided to the workpiece during positioning of the workpiece on the printed circuit board. In the present invention convenient visual and physical access is provided to the workpiece at any time during positioning by allowing the nozzle conduit to move away from the workpiece during positioning and completely enclose the workpiece during soldering. 
     In existing systems, relative positioning of the workpiece and the printed circuit board is accomplished by moving the printed circuit board so that the area of interest is precisely underneath the component and the source of heat. Moving the printed circuit board, which is often many times larger than the component, in precise manner is often inconvenient. The present invention provides means for moving the workpiece held by vacuum at the end of an arm mechanism together with the source of heat as well as means for moving the printed circuit board to position the workpiece, the printed circuit board and the source of heat relatively to each other. 
     It is, thus, a principle object of the present invention to provide an apparatus for soldering and desoldering of contacts between surface mounted components and printed circuit boards that is more convenient and efficient than existing apparatuses. 
     It is an important object of this invention to provide an apparatus as described in which an efficient transfer of heat from hot gas to solder contacts is achieved by creating a turbulent rapid flow of hot gas directly past the solder contacts. 
     It is a further object of this invention to provide an apparatus as described where the printed circuit board and the components adjacent to the workpiece are protected from overheating by restricting the application of hot gas to the immediate vicinity of the workpiece. 
     Another important object is to provide an apparatus as described in which the workpiece itself is protected from overheating and thermal shock by providing means of cooling the workpiece and shielding it from direct incidence of hot gas during the process of soldering and desoldering. 
     It is a further object of this invention to provide an apparatus as described in which the hot gas is delivered to the solder contacts at high flow velocity without significant impact, eliminating splattering of the solder, by distributing uniformly the hot gas flow along and deflecting the flow from the walls of the nozzle conduit used to restrict hot gas application. 
     It is an important object of this invention to provide a method and an apparatus to position precisely and conveniently the surface mounted components, the printed circuit board and the hot gas flow relatively to each other. 
     It is yet another object to provide an apparatus as described above in which there is an easy and convenient visual and physical access to the workpiece and its location on the printed circuit board by allowing the nozzle conduit to move relatively to the vacuum-held workpiece. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view in cross-section of the preferred embodiment of a thermal device for soldering-desoldering; 
     FIG. 2 is an isometric diagram of relative motions of the parts of the present invention; 
     FIG. 3 is a diagram illustrating a soldering-desoldering process performed therewith. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Thermal device for soldering and desoldering is shown in FIG. 1 to be downwardly dependent from the upper terminus 34 of pivoting arm 1 where venturi device 2 is connected though venturi inlet chamber 35 to the suction cup 3 by vacuum conduit 4 to provide vacuum to the suction cup 3 for holding the workpiece during positioning. This vacuum is created in the venturi inlet chamber 35 when air entering venturi inlet 36 is allowed to escape though venturi outlet 39. When venturi shutter 37 is closed, air is not allowed to escape though venturi outlet 39 and instead flows through vacuum conduit 4 to the suction cup 3 where such air flow may be used to cool the workpiece during soldering-desoldering process. 
     Soldering and desoldering is accomplished by hot gas which is obtained from the cool gas passed via flexible conduit 5 through connection box 6 into preheat chamber 7 of the heater assembly and thereafter into heater chamber 8 which is heated by an electric cartridge heater 9 and which is filled by stainless steel cloth 10 to facilitate heating of the gas. Flexible conduit 5 also contains metallic wires to conduct electric power through connection box 6 and via metallic wires 11 to the electric cartridge heater 9. Flexible conduit 5 also contains sensor wires which conduit electric signal from thermocouple sensor 12 through thermocouple wires 13 and through connection box 6 to electronic controls for automatic control of the temperature in the proximity of the solder contacts. 
     Hot gas leaving the heater chamber 8 enters manifold chamber 15 which is tightly connected to the heater chamber 8 and thereafter is forced to flow rapidly through jet needles 17 from where it moves rapidly in a multitude of jets along walls of the nozzle conduit 18 which is held to planar member 19 by means of brackets 20, toward the lower opening of the nozzle conduit 18. Vacuum conduit 4 is pressed into supporting block 21 which is held together with cover block 22 by screws 23. Bushing 24 surrounding vacuum conduit 4 is pressed into the the upper terminus 34 of the pivoting arm 1 directly underneath the venturi inlet chamber 35. When cover block 22 is rotated around its axis, vacuum conduit 4 with attached to it suction cup 3 is rotated inside bushing 24 to achieve angular alignment of the workpiece held by suction cup 3 and the printed circuit board. Manifold chamber 15 is formed between the inner tube 25 and an outer tube 26 placed so that the axis of both tubes coincide with the vacuum conduit 4 axis and the inner tube 25 extending outside the length of the outer tube 26. The inner tube 25 is permanently joined with the outer tube 26 by a ring 33 and the planar member 19 which enclose manifold chamber 15 from the top and the bottom, respectively, and the hot gas is allowed to escape the manifold chamber 15 only through jet needles 17 inserted into the planar member 19 between the inner tube 25 and the outer tube 26. The inner tube 25 has two linear bearings 27 pressed into it at opposite ends of the tube so that the bearings&#39; axis coincide with the tube axis which facilitates the sliding motion of the inner tube 25 along the vacuum conduit 4. Gear holder 28 is permanently attached to the inner tube 25 above the outer tube 26 and holds a gear 29 which is engaged through a slot in the inner tube 25 with a rack gear 31 cut on a portion of the vacuum conduit 4 surface. When gear 29 is rotated, its axis moves up and down the vacuum conduit 4, and with it moves the gear holder 28, inner tube 25, outer tube 26 and all parts attached to them. Guiding pin 32 is pressed into the supporting block 21 and slides within the gear holder 28 during the motion of the manifold tubes 25 and 26 along the vacuum conduit 4 preventing the manifold tubes 25 and 26 and all parts attached to them from rotating around the vacuum conduit 4 axis. 
     Installation of the printed circuit board is explained with the help of Figure 2. The printed circuit board is held between clamps 100 with the board edges fitting into V-shaped grooves 101 cut along the width of the clamps 100 in Y-direction. The V-shaped grooves 101 are capable of accommodating boards of different thicknesses. The clamps 100 are capable of moving independently along clamp guides 102 and can be stopped in any position by rotating clamp stops 103 which, pushing against the clamp guides 102, prevent the clamps 100 from moving. Underneath the printed circuit board, fixed to the base 108, is a heater assembly 104 for heating the passing through it cool air which is then used to preheat the printed circuit board to avoid thermal shock damage to the board. The heater assembly 104 for board preheating is constructed identically to the heater assembly used for heating cool gas for soldering and desoldering. Having been installed between the clamps 100, the printed circuit board may be moved in Y-direction by sliding the board within the V-shaped grooves 101. X-direction motion of the board is accomplished by moving a frame 105 which is joined with the clamp guides 102 by a yoke 106. The frame 105 moves within Thomson bearings housed within bearing housings 107. By moving the printed circuit board in X- and Y-directions independently any area of interest on the printed circuit board may be placed over the heater assembly 104. 
     In FIG. 2 the thermal device described before in FIG. 1 is located underneath the upper terminus 34 of pivoting arm 1 and the workpiece 30 is shown to be held for positioning on the printed circuit board by the suction cup 3 attached to the vacuum conduit 4. The function of raising and lowering and locating in azimuth and radially the suction cup 3 to place or remove the workpiece 30 is performed by the pivoting arm 1. When the workpiece 30 is clear of the printed circuit board, the pivoting arm 1 can quickly be swung with the workpiece 30 directly to any desired location with the required degree of precision by means of pivoting mechanism to be described. In a convenient operation, simultaneously as the workpiece 30 is being located over the portion of interest on the printed circuit board, the workpiece is also rotated to a desired angular alignment by rotating the vacuum conduit 4 and lowered into soldering contact. 
     The pivoting arm 1 is generally a &#34;J&#34;-shaped tubular member and contains air conduits for air to be used by venturi device to create vacuum or air flow through the suction cup 3, gas conduits for gas to be heated and used for melting solder contacts, electric wires to provide power to heater and conduct signals from the thermocouple sensor. Lower terminus 111 is an integral terminal portion of the pivoting arm 1 which fits between the jaws of vice 112. The vice 112 is placed on a vertical stub axle 113 so that top portion of the stub axle 113 is in between the jaws of the vice 112, and the vice 112 rests on a flat surface of vertical adjustment knob 114. The stub axle 113 has a threaded portion with which the vertical adjustment knob 114 is engaged so that when the knob 114 is rotated it is raised or lowered raising or lowering the vice 112. Horizontal pivot axle 115 is pressed into one jaw of the vice 112 and goes through guideway 118 in the lower terminus 111 of the pivoting arm 1 and through a hole in the other jaw of the vice 112. Azimuthal motion of the pivoting arm 1 is accomplished through rotation of the vice 112 around the vertical stub axle 113. Radial motion of the arm 1 is accomplished through sliding motion of the lower terminus 111 of the arm 1 between the jaws of the vice 112 within the limits determined by the length of the guideway 118. Vertical motion of the arm is accomplished through rotation of the lower terminus 111 of the arm 1 around the horizontal pivot axle 115. All these motions can be prevented by rotating fixator handle 116 attached to a nut 117 which, in turn, is engaged with a threaded portion of the horizontal pivot axle 115. Turning the fixator handle 116 tightens the nut 117 clamping the jaws of the vice 112 together and preventing rotational motion of the vice as well as any motion of the pivoting arm 1. 
     FIG. 3 illustrates, in a diagrammatic way, the process of melting solder contacts 120 between the workpiece 30 and the printed circuit board 121 and the essential devices involved in this process. The workpiece 30 is generally a rectangular small thickness object with the top surface being flat, and solder contacts 120 are located on the sides of the workpiece around the perimeter. The nozzle conduit 18 is a conduit shaped as a truncated pyramid with a rectangular cross-section, and the bottom opening of the nozzle conduit encloses the workpiece allowing only a small clearance between the nozzle conduit walls and the corresponding sides of the workpiece. 
     Gas, having been previously heated, enters manifold chamber 15 and escapes through the jet needles 17 which are directed downward and at the nozzle conduit walls 18. Each jet of hot gas is then deflected by the nozzle wall 18 toward the solder contacts 120 and loses most of its impact energy spreading in width along the nozzle wall 18. The multitude of such jets create a flow of hot gas distributed along the perimeter of the nozzle conduit 18 toward the solder contacts 120. When such flow reaches the bottom opening of the nozzle conduit 18 turbulence is created because the workpiece 30 and the printed circuit board 121 constrict this opening. Such turbulence facilitates heat exchange between the solder contacts 120 and the hot gas. The hot gas, having given away its heat to the solder contacts escapes from the inside of the nozzle conduit 18 into surroundings clearing the way for the arrival of new hotter gas into the nozzle conduit 18. 
     The suction cup 3 is made of silicone rubber 122 formed in the shape of a rectangular object of small thickness so that when installed on top of the workpiece 30 its bottom surface could be made to cover the top surface of the workpiece in such a way that the two surfaces coincide exactly. When the suction cup 3 is positioned in this manner with respect to the workpiece it shields the workpiece from direct incidence of the hot gas during soldering or desoldering. In addition, installing the workpiece in such a way that its entire top surface is covered by the suction cup 3, centers the workpiece with respect to the vacuum conduit 4 axis so that, when the bottom opening of the nozzle conduit 18 is lowered to enclose the workpiece, equal small clearances will exist between the sides of the workpiece and the walls of the nozzle conduit 18. The suction cup 3 as well as the nozzle conduit 18 may be changed in correspondence with different sizes and rectangular shapes of the workpiece 30. 
     The silicone rubber 122 of the suction cup 3 is formed around the bottom part of a tubular metal member 123. The inside diameter of the tubular member 123 is such that it can be installed over the outside surface of the vacuum conduit 4 and held there by friction. Flat bottom portion 124 of the tubular member 123 is an integral part of this tubular member and lies in the plane perpendicular to the vacuum conduit axis, and the purpose of the bottom portion 124 is to support and reinforce the silicone rubber. The formed silicone rubber 122 has a hole extending through its thickness and connecting with the inside of the tubular metal member 123. Through this hole vacuum or air flow can be delivered to the top surface of the workpiece 30. 
     During positioning of the workpiece 30 over the corresponding board contacts, the workpiece is held by vacuum created by the venturi device and delivered to the suction cup 3 though the vacuum conduit 4. During the process of melting solder contacts, venturi device may be used to provide air flow, as discussed previously, through the vacuum conduit 4 and the suction cup 3 to the top surface of the workpiece 30. This air flows between the suction cup 3 and the top surface of the workpiece 30 and then, mixing with the hot gas, leaves the inside of the nozzle 18. Such air flow is used to cool the component during soldering-desoldering process, and does not interfere with heating of the solder contacts by hot gas since the amount of this air flow is much smaller than the amount of hot gas flow. 
     The thermocouple sensor 12 is lowered through a hole in the planar member 19 into the hot gas flow in the close proximity to the workpiece 30. Thermocouple signal is used to control the timing of soldering-desoldering process.