Abstract:
Reflow soldering apparatus comprising a vapour chamber in communication with a reservoir of heat transfer fluid such that a volume of vaporised heat transfer fluid is created and held in the vapour chamber by heating of the heat transfer fluid. A heating chamber is provided for receiving a board and a transportation mechanism is provided to transport vapour, and condensate formed from the vapour, from the volume of vaporised heat transfer fluid to the heating chamber. The transported vapour and condensate applies heat to the heating chamber for the reflow soldering process.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and apparatus for reflow soldering. 
       BACKGROUND TO THE INVENTION 
       [0002]    Reflow soldering is a commonly used process for connecting components to a circuit board. The components are placed on solder paste located on the circuit board and the board is heated in an oven to melt the solder and thereby connect the components. 
         [0003]    The ovens used are arranged to provide some control over the temperature throughout the process. Producing the correct temperature profile for the process is extremely important to producing good connections between the components and the board. A common process uses an initial preheat zone in which the temperature is ramped up, a thermal soak zone in which temperature is maintained relatively constant and a reflow zone in which the temperature is ramped up again to melt the solder. The rate of increase of temperature in the preheat and reflow zones is particularly important in obtaining good connections. A final cooling zone is also used to cool down the board and components. Again, the rate of cooling is important to the connection quality. 
         [0004]    A number of different ovens for this purpose are available which use different methods of heating the board. Convection and infrared radiation are two such methods which have been employed either separately or in combination. Ovens using these heating techniques may utilise a method of moving the board through various zones in the oven in order to provide the different temperatures required at the various stages of the process. 
         [0005]    A further method involves the use of vapour phase heating. In such an oven, a liquid with a predefined boiling point is heated until a vapour of the liquid is formed. The liquid used is one such that the vapour produced is heavier than air and therefore forms a vapour layer which sits above the liquid in a vessel. The board is then lowered into the vapour layer to apply heating. In order to control the temperature of the board throughout the process, a means of moving the board in and out of the formed vapour layer may be employed. 
         [0006]    Vapour phase ovens are known to have poor control of the temperature profile causing problems such as thermal shock to parts and increased tombstoning (being parts standing up during the reflow process) compared to convection ovens. 
         [0007]    Infrared and convection ovens result in higher peak temperatures and variations in peak temperatures and temperature profiles for parts across the PCBs and this leads to higher failure rates. 
         [0008]    The present invention relates to an improved method and apparatus for applying heat to a circuit board for the purpose of reflow soldering. 
       SUMMARY OF THE INVENTION 
       [0009]    According to one aspect of the present invention there is provided reflow soldering apparatus comprising: 
         [0000]    a vapour chamber in communication with a reservoir of heat transfer fluid such that a volume of vaporised heat transfer fluid is created and held in the vapour chamber by heating of the heat transfer fluid;
 
a heating chamber for receiving a board; and
 
a transportation mechanism arranged to transport vapour, and/or condensate formed from the vapour, from the volume of vaporised heat transfer fluid to the heating chamber;
 
wherein transported vapour applies heat to the heating chamber for the reflow soldering process.
 
         [0010]    Preferably the transportation mechanism comprises a pressurising device to create a pressure differential between the vapour chamber and the heating chamber such that vapour and/or condensate are transported by the pressure differential. 
         [0011]    Preferably the heating chamber is of low thermal mass and/or lined with material having low thermal conductivity so temperature profiles remain constant throughout the chamber during temperature cycling. The sides and any observation windows may be lined with reflective material to reduce temperature variations caused by radiant heat loss. 
         [0012]    Preferably the pressurising device comprises a first fan and a first conduit is provided connecting the vapour chamber and the heating chamber such that operation of the first fan causes vapour from the vapour layer to be transported to the heating chamber via the first conduit. A second conduit is preferably provided connecting an upper end of the vapour chamber with the heating chamber, the second conduit including the first fan which is operated to pressurise the air above the vapour layer such that vapour passes via the first conduit into the heating chamber. 
         [0013]    Preferably operation of the first fan is varied to control the rate of vapour delivered to the heating chamber, thereby regulating the heat applied to the board. 
         [0014]    In one embodiment, there is provided a cooling system conduit including a cooling system fan, the cooling system conduit including a cooling means to cool air passing through the cooling system conduit. 
         [0015]    In a preferred embodiment, the heating chamber is provided with a circulation fan to circulate the vapour around the heating chamber. Preferably the operation of the circulation fan is varied to regulate the heat applied to the board. 
         [0016]    In one embodiment, the circulation fan is provided within the heating chamber. The first conduit preferably connects a lower end of the heating chamber with the vapour chamber such that condensate falling to the lower end of the heating chamber will return to the vapour chamber via the first conduit. The first conduit may be provided with a flow diverter adjacent the upper end such that when the circulation fan is circulating air and condensate within the heating chamber in a first direction, the diverter limits air and condensate passing into the return conduit. Preferably the diverter is arranged such that when the circulation fan is reversed, the flow is directed down the first conduit. 
         [0017]    A cooling chamber is preferably provided in the cooling system conduit and the cooling system fan draws air from the heating chamber into the cooling chamber from which it is returned via a cooling return conduit. The cooling chamber may be provided with an expansion bladder or vent to allow for changes in volume in the heating and cooling process 
         [0018]    In a further embodiment, the heating chamber is provided with a first internal chamber at a first end thereof and a second internal chamber at a second end thereof, each of the first and second internal chambers including apertures into the heating chamber, wherein the circulation fan is provided in a circulation conduit connecting the first and second internal chambers such that operation of the circulation fan causes air flow from the first end of the heating chamber to the second end. 
         [0019]    A condensation trap may also be provided adjacent the upper end of the vapour chamber and a condensate return line having a second fan may be provided extending from the heating chamber to the vapour chamber below the condensation trap such that slowing operation of the first fan relative to the second fan causes air to flow up through the condensation trap and into the second internal chamber via the second conduit. 
         [0020]    Preferably the cooling system conduit connects from the second internal chamber to the first internal chamber. 
         [0021]    In one embodiment, a third fan is provided within the first conduit and the third fan and the second fan are run to draw vapour back into the vapour chamber. 
         [0022]    A temperature sensor may be provided in the heating chamber such that temperature information provided by the temperature sensor is used to control operation of the first and cooling system fans and thereby the temperature around the board in the heating chamber. 
         [0023]    According to a further aspect of the present invention, there is provided a method of reflow soldering comprising the steps of:
       heating of a heat transfer fluid to create and hold a volume of vaporised heat transfer fluid in a vapour chamber;   placing a board to be soldered in a separate heating chamber; and   transporting vapour, and/or condensate formed from the vapour, from the vapour chamber to the heating chamber;   wherein heat transfer fluid applies heat to the board for the reflow soldering process.       
 
         [0028]    Preferably a pressure differential is created between the vapour chamber and the heating chamber such that vapour and/or condensate are transported by the pressure differential. 
         [0029]    Preferably a first conduit is provided connecting the vapour chamber and the heating chamber and a first fan is provided such that operation of the first fan creates the pressure differential to transport vapour to the heating chamber via the first conduit. 
         [0030]    The vapour is preferably circulated in the heating chamber by a circulation fan to distribute heat evenly to the board. Preferably operation of the first fan is varied to control the volume of vapour delivered to the heating chamber, thereby regulating the heat applied to the board. 
         [0031]    Further, the operation of the circulation fan is preferably varied to regulate the heat applied to the board. 
         [0032]    There is also preferably provided a cooling system conduit including a cooling system fan, the cooling system conduit including a cooling means to cool air passing through the cooling system conduit and the cooling system fan is operated to cool the heating chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The invention will now be described, by way of example, with reference to the following drawings: 
           [0034]      FIG. 1  is a diagrammatic view of apparatus for reflow soldering in accordance with the present invention; 
           [0035]      FIG. 2   a  is a top cross sectional view of a heating chamber of a second embodiment of the apparatus for reflow soldering; and 
           [0036]      FIG. 2   b  is a side cross sectional view of the vapour chamber of the second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    Referring to  FIG. 1  there is shown apparatus  10  for reflow soldering according to the method of the present invention. 
         [0038]    The reflow soldering apparatus comprises a vapour chamber  12  and a heating chamber  14 . The vapour chamber  12  is located below the heating chamber  14  and is used to produce a volume of vapour in a manner similar to that used in known vapour phase reflow soldering. That is, a reservoir  16  is provided adjacent a lower end of the vapour chamber  12  which is filled with a suitable heat transfer fluid  18 . The heat transfer fluid may be of a known type such as that sold under the name Galden LS230. 
         [0039]    The reservoir  16  includes a heating element  20  which applies heat to the heat transfer fluid  18 . Once the fluid reaches its boiling point, which in the case of Galden LS230 is 230 degrees Celsius, a layer of vapour  22  is formed above the fluid  18 . The vapour chamber  12  therefore is used to create and hold a volume of vapour for use in the reflow soldering process. 
         [0040]    The heating chamber  14  receives a printed circuit board  24  which includes components and solder paste to undergo a reflow soldering process. The heating chamber  14  may include an upper door  26  through which the board  24  is placed into the heating chamber  14 . The door  26  may include a window to allow the board  24  to be viewed during the soldering process. 
         [0041]    The heating chamber  14  may be constructed from segments of stainless shim material, thinner than 0.3 mm. The segments would be joined by expansion joints. Also, the surfaces of the heating chamber  14  may be formed from curved surfaces to provide strength to reduce deformation that may be caused by the heating and cooling cycles. 
         [0042]    The apparatus  10  includes a first conduit  34  connecting between a position adjacent the lower end of the vapour chamber  12  and a lower end of the heating chamber  14 . The first conduit  34  is provided for delivering vapour created in the vapour chamber  12  to the heating chamber  14 . Also provided is a second conduit  28  connecting an upper end of the vapour chamber  12  and an upper end of the heating chamber  14 . 
         [0043]    The apparatus  10  includes one or more pressurising devices provided to create a pressure differential between the heating chamber  14  and the vapour chamber  12  such that vapour from the vapour layer  22  passes through the first conduit  34  into the heating chamber  14 . 
         [0044]    In the embodiment shown, the pressurising device comprises a first fan  30  provided in the second conduit  28 . The first fan  30  is operated to pressurise the air above the vapour layer  22  such that vapour passes up through the first conduit  34 , the lower end of which is located below the top of the vapour layer  22 , into the heating chamber  14 . 
         [0045]    In an alternative embodiment, the first fan  30  may be provided in the first conduit  34  to draw vapour up into the heating chamber  14 . 
         [0046]    The vapour  22  is delivered through the first conduit  34  to the heating chamber  14  in order to provide the heat to the board  24  for the reflow soldering process. As the vapour  22  enters the heating chamber  14 , the vapour  22  will condense forming a suspension of condensed heat transfer fluid around the board  24 . The heating chamber  14  is provided also with a circulation fan  32  in order to provide consistent temperature throughout the heating chamber  14  and to circulate the suspended condensate evenly around the heating chamber  14 . The suspended condensate will come into contact with the board  24  rapidly delivering heat to the board  24 . 
         [0047]    The amount of vapour delivered to the heating chamber  14  is controlled by operation of the first fan  30 , thereby allowing the amount of heat delivered to the board  24  to be controlled throughout the process. The operation of the circulation fan  32  is also controlled to further regulate the heat delivered to the board  24 . The control of operation of the fans  30  and  32  thereby allows the temperature of the board to be varied according to the desired heating profile. 
         [0048]    To reduce tombstoning, the board  24  is first heated according to the heating profile to just below the eutectic of the solder paste then the first fan  32  is stopped so the mixing action creating condensation ceases. The speed of the first fan  30  is increased so the heating chamber  14  is rapidly filled with the vapour to melt the solder paste evenly and quickly. 
         [0049]    A temperature sensor  36  in the heating chamber  14  is also used to provide temperature information used in controlling operation of the first fan  30  and the circulation fan  32 . 
         [0050]    The condensed vapour which forms larger droplets throughout the process will fall to the lower end of the heating chamber  14  and can return to the vapour chamber  12  and the reservoir  16  via the first conduit  34 . The first conduit  34  may be provided with a flow diverter  38  adjacent the upper end such that when the circulation fan  32  is circulating air and condensate within the heating chamber  14  in a first direction, the diverter  38  limits air and condensate passing into the return conduit  34 . The diverter  38  is arranged such that when the circulation fan  32  is reversed, the flow is directed down the return conduit  34 . Reversing of the direction of the circulation fan  32  therefore allows vapour to be directed back towards the vapour chamber  12 . The bottom of the heating chamber  14  may also be sloped toward the first conduit  34  so that condensed vapour and fluid return to the vapour chamber  12 . 
         [0051]    The apparatus  10  is also provided with a cooling tank  40  connected to the heating chamber  14  by a cooling system conduit  42 . The cooling system conduit  42  includes a cooling system fan  44  arranged such that operation of the cooling system fan  44  draws air from the heating chamber  14  into the cooling tank  40 . A cooling return line  46  is also provided between the cooling tank  40  and the heating chamber  14 . Operation of the cooling system fan  44  thereby draws air from the heating chamber  14  into the cooling chamber  40 . Returned cooler air is delivered back to the heating chamber  14  to cool the interior of the heating chamber  14 . Operation of the cooling system fan  44  can therefore be used to control the cooling phase of the reflow soldering process. 
         [0052]    A return conduit  50  is provided between the cooling chamber  40  and the reservoir  16 . The return conduit  50  is provided with a U moisture trap  52  and isolation valve  54  so that condensed vapour in the cooling chamber  40  returns to the vapour chamber  12 . 
         [0053]    The apparatus  10  may be provided with an outer casing (not shown) provided around both the heating chamber  14  and the vapour chamber  12 . The cooling chamber  40  may be formed by the outer casing. To reduce heat loss and improve thermal control in the heating chamber  14 , a layer of insulating material such as foam glass and/or reflective material may line the vapour and heating chambers  12  and  14 . An air gap layer could be provided between the outer casing and the insulating material. Vapour from the heating chamber  14  could then be directed to flow through the air gap layer between the insulating material and the outer casing of the apparatus for the purposes of cooling and condensation. A filter material having a large surface area (such as a course stainless gauze) could be placed in the path of the cooling air for condensation to collect onto. Alternatively cooled air could be directed to flow through a fine pore material such as synthetic chamois. 
         [0054]    The second conduit  28  and the cooling system conduit  42  may include a common entry to the heating chamber  14 . This arrangement may be utilised to pump vapour formed in the vapour chamber  12  through the heating chamber  14  to the cooling chamber  40  for the purpose of removing impurities from the heat transfer fluid. In such an operation, the lid  26  is opened and the isolation valve  54  opened. Then heater  16  heats the heat transfer fluid  18  to above the volatile temperature of common impurities but below the boiling point of the heat transfer fluid and the first fan  30  and the cooling system fan  44  are operated to remove unwanted vapours from the system. When venting is complete the lid  26  is closed, isolation valve  54  closed and heat transfer fluid is heated until it all forms vapour and is transferred to the cooling tank  40  where it condenses and is prevented from returning. The apparatus is cooled and the reservoir  16  may be removed for cleaning. The reservoir  16  may then be replaced, the isolation valve  54  opened again and heat transfer fluid returned to the reservoir  16 . 
         [0055]    The cooling chamber  40  may be provided with an expansion bladder or vent (not shown) to allow for changes in volume during the cooling process. A vent may contain cooled material with a large surface area on which vapour can condense. The condensed vapour forms fluid and is returned to the reservoir  16 . 
         [0056]    A further fan (not shown) is provided between the system  10  and the expansion bladder or vent such that air is blown out of the system during the heating process. This ensures the system operates with a slight negative pressure during the reflow process, thereby reducing loss of heat transfer fluid  18 . 
         [0057]      FIGS. 2   a  and  2   b  show a second embodiment of apparatus for reflow soldering in accordance with the present invention. The apparatus is similar in function to that of  FIG. 1  and like reference numerals are used to denote like parts. 
         [0058]      FIG. 2   a  shows a top view of the heating chamber  14 . The heating chamber is provided with a first internal chamber  50  at a first end thereof and a second internal chamber  52  at a second end opposite end thereof. Each of the first and second internal chambers  50  and  52  is defined by an associated internal wall  51  and  53  extending across the heating chamber  14 . Each of the internal walls  51  and  53  includes a plurality of apertures such that air can flow through the apertures. 
         [0059]    The circulation fan  32  in the second embodiment is provided externally of the heating chamber  14  and is connected to the first and second internal chambers  50  and  52  via a circulation conduit  33 . Operation of the circulation fan  32  is such that a negative pressure is created in the second internal chamber  52  and a positive pressure in the first internal chamber  50 . Air is thereby drawn through the apertures in the first wall  51  and flows along the heating chamber  14  into the apertures in the second wall  53 . That is, air flows from the first end of the heating chamber  14  to the second end of the heating chamber  14  and is returned through the circulation conduit  33 . 
         [0060]    The first fan  30  is provided in the second conduit  28  which is connected to the second internal chamber  52 . The first conduit  34  connects the first internal chamber  50  to the lower end of the vapour chamber  12 . The first fan  30  operates to draw air from the second internal chamber  52  into the upper end of the vapour chamber  12  when it is required to transport vapour into the heating chamber  14 . The vapour passes through the first conduit  34  into the first internal chamber  50  and then into the heating chamber  14  around the board. 
         [0061]    The cooling system conduit  42  connects the first and second internal chambers  50  and  52  and includes the cooling system fan  44  inline to draw air from the second internal chamber  52  and direct it to the first internal chamber  50 . The cooling system conduit  42  passes through a cooling device  54  to remove heat from the air drawn through the cooling system conduit  42 . The cooling device  54  may comprise an outer casing provided around both the heating chamber  14  and the vapour chamber  12 . The cooling system conduit  42  may be thermally connected to the outer casing or may connect to a chamber provided within the outer casing. Alternatively, a dedicated heat exchanger with associated fan may be provided as the cooling device. 
         [0062]    The apparatus of  FIG. 2  also includes a condensation trap  56  adjacent the upper end of the vapour chamber  12 . The condensation trap  56  is provided to allow control over the level of condensation throughout the heating process as will be described below. As it is expected that the heat transferred to the board  24  will be related to the amount of condensation in the heating chamber  14 , such control over the condensation level allows increased control over the heating profile. Also provided for this purpose is a condensate return line  61  extending from the heating chamber  14  to the vapour chamber  12  below the condensation trap  56 . The condensate return line  61  includes a second fan  59  therein. 
         [0063]    A probe tube  39  is provided extending from a lower end of the vapour chamber  12  to an upper end of the vapour chamber  12 . The probe tube  39  is external to the vapour chamber  12  so that air above the vapour will cool relative to the vapour. A plurality of temperature probes  37  are provided along the length of the probe tube  39  wherein the temperature readings along the tube are used to determine the level of the vapour within the vapour chamber  12 . 
         [0064]    In use, the heat transfer fluid  18  is heated until the vapour layer  22  is formed in the vapour chamber  12 . That is, a volume of vapour is created and held in the vapour chamber  12  prior to commencement of the reflow soldering process. When the vapour is at the correct level to commence the heating process, power to the heating element  20  is controlled to maintain an appropriate vapour level. 
         [0065]    The circulation fan  32  is then started to create the air flow through the heating chamber  14  from the first end to the second end. The cooling system fan  44  is then run at a speed sufficient to stop reverse airflow by pressurising the cooling system conduit  42 . Alternatively, the cooling system conduit  42  may extend between sides of the heating chamber  14  such that the air flow caused by air passing through the cooling system conduit  42  is generally perpendicular to that flowing from the first internal chamber  50  to the second internal chamber  52 . In this way, it will not be necessary to pressurise the cooling system conduit  42 . The first fan  30  and second fan  59  are then run to pressurise the vapour chamber  12  via conduits  28  and  61 , transporting vapour into the heating chamber  14  via the first conduit  34 . The first fan  30  and the second fan  59  are slowed to either reduce the amount of vapour entering or stop further vapour entering the heating chamber  14 . 
         [0066]    The amount of vapour in the heating chamber  14  can be controlled during the process by operation of the first and second fans  30  and  59 . Further, the amount of condensation can be reduced if required without significant reduction in temperature by reducing the speed of the first fan  30  relative to the second fan  59 . When the first fan  30  runs at a speed slower than second fan  59  condensate flows through condensate return line  61  into the vapour chamber  12  where the large area permits droplets to fall to the bottom of the chamber  12 . As the condensate return line  61  extends from the first internal chamber  50  to the vapour chamber  12  at a position just below the condensation trap  56 , by increasing the pressure of second fan  59  relative to the first fan  30 , air will flow through second conduit  61 , up through the condensation trap  56  and into the first conduit  28 . As air passes up through the condensation trap  56 , the air above the condensation trap  56  will be relatively dry heated air which will flow back into the second internal chamber  52  for circulation by the circulation fan  32 . To speed the removal of condensate further, a third fan  60  provided within the first conduit  34  can be run at a speed suitable to counter increased pressure from second fan  59 . 
         [0067]    At the end of the heating phase vapour is removed from the heating chamber  14 . First fan  30  is turned off and the third fan  60  provided in the first conduit  34  may be run, along with second fan  59  to draw vapour back into the vapour chamber  12 . Cooling is effected by operation of the cooling system fan  44  to pass air from the heating chamber  14  through the cooling device  54 . Removing vapour and condensate by the abovementioned method prior to the cooling process speeds the cooling process and reduces the amount of condensate generated in the cooling means  54 . 
         [0068]    As in the embodiment of  FIG. 1 , an insulation layer  58  is provided around the vapour chamber  12 . As the vapour chamber  12  is provided as a separate insulated chamber it is expected that the heat in the vapour chamber  12  will be retained well between uses of the apparatus in comparison to existing vapour phase ovens. During standby, the heater  20  could be controlled using little energy, maintaining a suitable standby vapour level. Re-heating of the heat transfer fluid to operating temperature is therefore not required, providing a benefit when compared to existing vapour phase ovens. 
         [0069]    A means to vibrate the PCB during the reflow process is included in the preferred embodiment. The vibration improves the wetting of parts by the solder and reduces solder dags. A waterproof audio device  62  is provided in the cooling system conduit  42  adjacent the first end of the heating chamber  14 . The audio device operates with a high sound pressure level, suitable to vibrate the PCB. The sound is not a constant frequency as this can cause resonances and movement of parts on the PCB. This may be in the form of a swept frequency or noise. To provide best outcomes, the sound is applied at a low level when the solder starts to melt and increases in level until fully melted. The sound is stopped before the cooling process to ensure dry joints are not created. This movement could be applied to other oven types by using high temperature speakers or other directly connected mechanical vibration means such as mass drivers. 
         [0070]    It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention.