Reflow oven gas management system and method

[0002] This application claims priority of Provisional Application No. 60/383,165, entitled REFLOW OVEN GAS MANAGEMENT SYSTEM AND METHOD filed on May 24, 2002, and is incorporated herein by reference.

[0003] This invention relates generally to the field of reflow soldering ovens, and, more particularly to a method and an apparatus for removing flux vapors and other volatile contaminants from the atmosphere of such ovens.

[0004] Printed circuit boards are commonly fabricated using the reflow solder technique. A paste containing solder particles mixed with flux, adhesives, binders, and other components is applied to selected areas of a printed circuit board. Electronic devices are pressed into the applied solder paste. Adhesives in the paste hold the devices to the printed circuit board. A conveyor within a reflow oven tunnel carries the printed circuit board and devices through regions of varying temperatures within the oven, each region performing a specific step in the reflow process. Generally an initial (preheat) region will bring the circuit board and devices to a predetermined temperature for flux reaction. A second (soak) region will maintain this reaction temperature for a predetermined time. Flux in the solder paste reacts with the contacts to remove oxides and to enhance wetting. A third (reflow) region will heat the circuit boards and devices to a temperature sufficient to cause the solder particles in the paste to melt. Molten solder wets metal contacts on the devices and printed circuit board. The conveyor moves the heated printed circuit board to a fourth (cooling) region of the oven where the molten solder solidifies forming a completed electronic circuit.

[0005] The reaction of the flux with the contacts liberates vapors. Further, heat within the oven vaporizes flux as well as adhesives, binders, and other components of the solder paste, devices, and circuit boards. The vapors from these materials accumulate within the oven leading to a number of problems. If the vapors migrate to a cooler region they will condense on the circuit boards, contaminating the boards and making subsequent cleaning steps necessary. The vapors will also condense on cooler surfaces within the oven, clogging gas orifices, gumming up moving parts, and creating a fire hazard. This condensate may also drip onto subsequent circuit boards destroying them, or making subsequent cleaning steps necessary. In addition, condensed vapors may contain corrosive and toxic chemicals, which can damage equipment and create a hazard to personnel.

[0006] The flux vapors and other volatile compounds generated by the reflow operation collectively are referred to in this application as “volatile contaminants.” It is understood that this term is intended to encompass all reaction products released when the printed circuit board is heated, including flux vapors, vapors from all other components of the solder paste, as well as all vapors out-gassed from the printed circuit board and the electronic devices.

[0007] Volatile contaminants can be flushed from the oven by providing a continuous supply of clean gas. This is commonly done in machine operating with air as the process gas by drawing fresh air in both ends of the oven and exhausting the air along with volatile contaminants from the inner regions of the oven as described in U.S. Pat. No. 5,345,061 by Chanasyk et al. This is not an ideal solution for the cases where the oven must be filled with a substantially inert gas, for example nitrogen. Generating additional inert gas in volumes adequate to flush these ovens (2000 cfh or more) is expensive.

[0008] The approaches of volatile contaminant removal that do not involve condensing tend to lack feasibility due to cost, scientific complexity, physical impracticality or a combination of all of these. Incineration, for instance, requires impracticably high temperatures. UV decomposition utilized very expensive equipment. Filtration in the gaseous state requires a filter far to fine to consider at the volumes needed. Nuclear disintegration has obvious problems related to radioactive materials.

[0009] Generally, the prior art involves removal, filtration, and reintroduction of inert gas to the oven, however, they fail to address the complex gas flow patterns necessary to maintain a clean, substantially inert atmosphere while also maintaining the precise temperature profile required by the reflow process.

[0010] These complex flow patterns in the tunnel are both natural and created. They may either help or hinder attempts to optimize various process parameters. It is necessary to consider each of these various parameters separately before designing a flow scheme that is the best compromise for the process as a whole.

[0011] Flows for substantially inert gas optimization need to purge the entire tunnel of air and pressurize the center such that flows are out both ends impeding any flow of outside environment in. Flows optimized for substantially inert gas containment are not compatible with control of volatile contaminant condensation as all volatile contaminants pass through the coolest regions of the oven where they condense before they reach the exhaust, and also may not be optimized for zone temperature definition.

[0012] It is therefore an object of this invention to provide an apparatus and a method for the control of the complex gas flow patterns in a reflow soldering oven to maintain a clean inert atmosphere while maintaining the precise temperature profile.

[0013] It is another object of this invention to provide an apparatus and a method to reduce operating expenses by conserving substantially inert gas.

[0014] It is yet another objective of this invention to provide an apparatus and a method to minimize the sensitivity of oven external pressure and/or temperature variations on the oven internal environment.

[0015] The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described herein below.

[0016] The present invention introduces an substantially inert gas, for example nitrogen, in a reflow oven tunnel at predetermined points to influence the oven tunnel gas flow and to dilute and expel excess oxygen from the system. Tunnel gas is ducted from the tunnel at selected points to a flux separation system to be cleaned, for example remove flux vapors, and returned to the tunnel for recirculation, thereby conserving the substantially inert gas and maintaining a low oxygen environment. Additionally, the substantially inert gas is introduced in amounts larger than required to create and maintain tunnel gas flow. The system maintains sufficient pressure to supply end baffle boxes with clean gas to create a gas barrier to effectively seal off the tunnel ends from air infiltration or migration into the tunnel making the system insensitive to external pressure and/or temperature variations.

[0017] The substantially inert gas, such as nitrogen, is cold relative to the tunnel gas at the point of introduction. The cold gas will mix with the hotter gas thereby absorbing excess heat for better zone temperature control. The cold source gas mixing with the hotter gas causes an expansion of the tunnel gas. The tunnel gas is drawn in both directions by the tunnel gas outlets of the recirculation system.

[0018] Preferably, substantially inert gas is introduced into the pre-heat and/or soak zones of the tunnel mixing with the tunnel gas. The mixed gas primarily flows toward the reflow zone to prevent condensation and for zone temperature definition. Some mixed gas splits and flows toward select outlets in the pre-heat and soak zones to be ducted to the flux separation system for cleaning prior to recirculation. Additional inert gas is introduced into the cooling zone of the tunnel mixing with the tunnel gas and flowing back toward the reflow zone. The counter flowing gases converge at the boundary of the high temperature region and a cooling region to mix and exit the tunnel toward the flux separation system for cleaning and recirculation. The amount of substantially inert gas can be varied at each introduction inlet to account for gas expansion differentials and flow direction.

[0019] After cleaning, some of the clean gas is ducted back to recirculation ports for introduction back into the tunnel and the remaining clean gas is ducted to the end baffle boxes located at either or both ends of the oven tunnel to create a gas barrier preventing outside air from entering the tunnel. Recirculated gas can be reintroduced at points in the tunnel to cause flow generally from colder areas into hotter areas, with maximum flow out of the cool zones and flow out of a zone where flux build up problems are the worst. Recirculated flows are introduced into the baffle boxes for pressure control, baffling, and exhaust. The system exhausts through the baffle boxes more gas than input to account for expansion of the substantially inert gas due to heating.

[0020] The exhaust system, defined by the baffle boxes, recirculated gas and exhaust hoods, generally are not be used to remove contamination from the system, but only to remove the excess expansion gas, which has been cleaned, recirculated, used to pressurize the baffle box, and allowed to draft out of the baffle space to the exhaust hood, thereby inhibiting the in-flow of oxygen contained in the outside air. The exhaust hoods are located above the tunnel ends outside of the baffle boxes. The exhaust hoods can be ducted to the customers facility exhaust. The flow from the recirculation system into the baffle boxes are preferably set at a flow equal to the substantially inert gas input plus expansion to keep the pressure inside the tunnel constant, with no-flow through the baffle area and into the oven tunnel.

[0021] Baffle boxes are, preferably, incorporated at both ends of the oven to form isolation zones between the constantly changing pressure and drafts of the factory and the expanding and contracting gas flows within the tunnel. Since the baffle box preferably is pressurized by clean recirculated gas at a rate slightly higher than the substantially inert gas, such as nitrogen, input rate plus an expansion factor, the recirculated gas is metered through a series of small orifices onto a flat angled surface to create a series of low-level laminar flows. These flows come from both top and bottom of the reflow oven. The outer most flows are, preferably, allowed to draft out of the oven to be picked up by a low level flow exhaust hoods at the ends of the tunnel constituting the entire exhaust volume. The innermost flows will cause a pressurized space to keep expansion gas from the tunnel from flowing through the baffle space. The fact that the baffle boxes are supplied with recirculated gas is significant in several ways. It is preferred that no inert gas is dumped to the factory essentially unused and un-expanded; thus decreasing inert gas consumption. There is also no tunnel gas migrating through the baffle area disturbing the desired flow. This action also de-couples the flow of the baffle boxes from the flow in the tunnel in order to reduce the buoyancy driven mixing at the tunnel ends. Also since this recirculated gas is heated and ducted to top and bottom of both tunnel ends, the buoyancy effect is much reduced.

[0022] In alternative embodiments, low oxygen content in the tunnel gas is desired to improve system performance, for example filtration of contaminants. In such cases, low flow valves are employed to bleed low volumes of air into the tunnel at one or more pre-selected locations, thereby controlling the parts per million of oxygen mixed with the tunnel gas.

[0023] In air only machines, the possibility exists to exhaust the cleaned gas up a stack rather than returning it to the oven. This may result in an extremely clean air machine with simple gas flow, with resultant possible energy conservation.

[0024] For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawing and detailed description of preferred embodiments of the invention.

[0025] FIG. 1 is a schematic diagram of an reflow oven according to a preferred embodiment of the present invention;

[0026] FIG. 2 is a partial cross-section view showing a temperature cell used in the reflow oven of FIG. 1;

[0027] FIG. 3 is a pictorial representation of an air valve used to bleed low volumes of air into the oven;

[0028] FIG. 4 is a schematic cross-section view of a preferred embodiment of a condensation and filtration system usable in the FIG. 1 oven and other embodiments of the present invention;

[0029] FIG. 5A is a pictorial representation of a tunnel end baffle usable in the FIG. 1 oven and other embodiments of the present invention;

[0030] FIG. 5B is a preferred embodiment of a schematic diagram of a tunnel end baffle usable in the FIG. 1 oven and other embodiments of the present invention;

[0031] FIG. 6 is a schematic diagram of an oven with attached apparatus according to an alternative preferred embodiment of the present invention; and

[0032] FIG. 7 is a diagram showing a temperature cell used in the oven of FIG. 6.

[0033] Prior to providing the detailed description of preferred embodiments of the apparatus and method of this invention, it is first preferable to provide the following information.

[0034] Flows optimized for volatile contaminant control are designed to move gas from cooler toward hotter regions of the tunnel to eliminate any condensation, and with a high exhaust throughput to expel contaminants. The most important flow in this scheme is the flow through the cool region toward the hotter region to eliminate any counter flow of volatile contaminants.

[0035] For zone temperature definition optimization, flows are desired which move from cooler zone toward hotter zones. Gas input has been used both to accomplish flow direction and to add cooling especially next to high heat areas. Flows optimized for zone definition are entirely dependent upon the profile selected.

[0036] Once flow patterns are understood, it is also important to consider that the gas in tunnel reflow ovens is used for many different purposes. Gas flow separation between zones in the tunnel allows temperature separation between those zones. Controlling the direction of tunnel flows from cooler to hotter can not only allow higher zone differentials, but can also inhibit condensation in the cooler areas. Nitrogen or other gases are used to inert the tunnel by diluting and displacing oxygen. For example, nitrogen input and expansion in the tunnel pressurizes and isolates the tunnel from outside pressure fluctuations. Nitrogen is used to create flows and pressure in the baffle areas to inhibit the reentry of oxygen. Hot gas is the heat transfer medium of the convection oven. Gas recirculation in a zone carries the particular heat energy of that zone to the printed circuit board. Hot gas is the transport media for the volatile contaminants from the tunnel to an exhausting or filtering mechanism. Therefore, inert ovens generally have several features within them to provide and maintain the inert atmosphere. These include: Process Gas Input and Distribution system, Oven Gas Circulation system, Condense and Filter system, Gas Reintroduction system, Tunnel End Baffle system, and the Exhaust system. Gas introduced to inert the oven will pass through and impact the effectiveness all of these systems. Therefore, to be most effective, these systems must be combined into one comprehensive gas management system.

[0037] The preferred embodiment of the present invention is shown in FIG. 1. Reflow oven 2 includes two vertically opposed rows of temperature cells 4 forming the process tunnel 42, which is sectioned off into a high temperature region 22 and a cooling region 24. The high temperature region 22 can be divided into several zones, for example, pre-heat zones 58, soak zone 62, and reflow zones 66, as illustrated in FIG. 1. A conveying mechanism 44 for carrying objects to be heated, for example printed circuit boards, runs through the process tunnel 42. A process gas reservoir 46 for storing a substantially inert gas, for example nitrogen, can be provided to supply the process tunnel 42 with a particular process gas. Selected inlet ports 6 are piped from the process gas reservoir 46 through flow meters 56. Substantially inert gas enters the process tunnel 42 through the temperature cells 4 (described in detail below with respect to FIG. 2). The volume of substantially inert gas through each of these flow meters 56 is set at a rate to produce the desired pressure and flow direction in that portion of the process tunnel 42. Pressure is caused not only by the introduction of this process gas, but also by the expansion of the cold gas due to heat, as described in FIG. 2, in the process tunnel 42. The location of these selected inlet ports 6 are such that the flows caused in the process tunnel 42 are generally from the cool region 24 to the high temperature region 22, and from cooler to hotter areas within the high temperature region, to impede the flow of volatile laden gas from migrating to the cooler regions where it will condense. This also concentrates the volatile laden gas in regions where the outlets 8 and barrier vent 32 connect to the condensation and filtration chamber 34 (described in greater detail in FIG. 3), thereby removing the volatile laden gas from the process tunnel 42 to be cleaned and recirculated. The barrier vent 32 is located between the high temperature region 22 and the cooling region 24 of the oven 2. The barrier vent 32 allows gas from the high temperature region 22 that would otherwise migrate into the cooling region 24 to be drawn out of the process tunnel 42 into the recirculation ducting 30. Selected outlet ports 8 are interconnected through recirculation ducting 30 to a condensation and filtration chamber 34 (described in greater detail in FIG. 3). A recirculation blower 36 located inside chamber 34 powers and controls the flow of gases within the recirculation ducting 30 the process tunnel 42. Clean gas is ducted from the condensation and filtration chamber 34 to the process tunnel 42 by two separate routes, both powered by recirculation blower 36. The first route is into several selected inlet ports 6 with a volume bias toward the cooling region 24 such that a flow is created from the cooling region 24 to the high temperature region 22 to inhibit volatiles from the high temperature region 22 from migrating into the cooling region 24. This prevents condensation of these volatiles on cool surfaces in the cooling region 24. The positioning of inlet ports 6 and outlets 8 are derived from specific operating parameters.

[0038] A preferred embodiment of a temperature cell 4 of the present invention is illustrated in FIG. 2, shown as a top and bottom pair, as described in U.S. Pat. No. 5,345,061, entitled CONVECTION/INFRARED SOLDER REFLOW APPARATUS UTILIZING CONTROLLED GAS FLOW, issued Sep. 6, 1994, and incorporated by reference herein. An inlet port 6 and an outlet port 8 are provided on each cell 4. The inlet port 6 is connected with a return channel 16. The return channel 16 is connected with a centrifugal fan 12 driven by a motor 10. The centrifugal fan 12 is contained within a plenum 26. A diffuser 14 forms the bottom surface of the plenum 26. Imbedded in the diffuser 14 is a heating element 18. Holes 20 are provided through the diffuser 14 and heating element 18 so that gas driven by the fan 12 exits the plenum 26 into the process tunnel 42. The return channel 16 has inlets 28 opening into the process tunnel 42 allowing the gas exiting the plenum 26 to return to the fan 12 and back out through the diffusers 14 and heater 18. The outlet port 8 is connected with the plenum 26. The pressure drop of the gas drawn along the return channel 16 by the fan 12 causes suction through the inlet port 6. Positive pressure within the plenum 26 forces gas out of the outlet port 8. The gas forced by the fan 12 is heated by the heating element 18 and heats objects, for example printed circuit boards, in the process tunnel 42 by convection.

[0039] In an alternative temperature cell embodiment of the present invention for use with low oxygen content in the tunnel gas desired to improve the system performance, for example filtration of contaminants, a low flow valve 72, as shown in FIG. 3, is operably connected to an inlet port 6 not being used for the delivery of primary inert gas. Low volumes of air bleed into the tunnel at one or more pre-selected locations, thereby controlling the parts per million of oxygen mixed with the tunnel gas. Further, as an alternative to the air bleed valves is a direct connection to an oxygen supply can be incorporated in the present system.

[0040] Condensation and filtration chamber 34 used with the present invention is shown in FIG. 4. An inlet port 40 in chamber 34 is connected to the oven 2 through the recirculation ducting 30. A recirculation blower 36 draws the tunnel gas out of the process tunnel 42 through outlet port 8 and into chamber 34 gas containing volatiles. The tunnel gas is drawn past several condensing plates 50 to condense the volatiles into liquid, which will run off the plates 50 into chamber 34 to be drained from the system through drain 52; or particles which will be drawn into and captured by filter 38. The cleaned tunnel gas is then past through recirculation blower 36 exiting the chamber through outlet port 48 and returned as recirculated gas back into process tunnel 42 through recirculation ducting 30. The terms clean gas and recirculated gas or decontaminated gas are used interchangeably throughout the specification, which contains substantially inert gas mixed with low levels of oxygen and other gases. The flow from the recirculation blower 36 is set to a pre-determined value. A venturi flow meter 60 and pressure transmitter 64 senses the flow. The value is calculated by a computer 68 such that as the filter 38 becomes saturated, flow remains constant by speeding up the recirculation blower 36 until a predetermined speed triggers a signal to provide an indication to change the filter 38.

[0041] The tunnel end baffle boxes 54, shown in FIGS. 5A and 5B, are located both top and bottom of both ends of the process tunnel 42. The tunnel end baffle boxes 54 are the second route of the clean gas returning from the condensation and filtration chamber 34 through the recirculation ducting 30 to baffle inlet port 55. The clean gas passes through the tunnel end baffle boxes 54 then out of the oven 2 into the exhaust hood 70, as shown in FIG. 1. Since the baffle box 54 preferably is pressurized by clean recirculated gas at a rate slightly higher than the substantially inert gas, for example nitrogen, input rate plus an expansion factor, the recirculated gas, as shown in FIG. 5A, is metered through an orifice plate 57 with a series of small holes 61 onto a flat angled surface 59 to create a series of low-level laminar flows. Clean recirculated gas blown into these tunnel end baffle boxes 54 causes a high pressure area which impedes the flow of air from the outside environment from entering the process tunnel 42, which would cause deterioration of the inert atmosphere therein and higher volumes of process gas to be required. This flow further impedes the encroachment of outside environment into the inert atmosphere. These flows will be from both top and bottom of the reflow oven. The outer most flows will be allowed to draft out of the oven to be picked up by a low level flow exhaust hoods at the ends of the tunnel constituting the entire exhaust volume. The innermost flows will cause a pressurized space to keep expansion gas from the tunnel from flowing through the baffle space. Further, with all exhaust gas exiting the process tunnel 42 through the tunnel end baffle boxes 54, the flows inside the process tunnel 42 are isolated from outside influences, thereby enhancing the temperature control for the reflow process.

[0042] A further preferred embodiment of the present invention, 51, where similar components use identical reference numbers as shown in FIG. 1 is illustrated in FIG. 6. The basic components of the oven 2 of FIG. 1 are employed here. The difference between the embodiments is the method of the introduction of the substantially inert and recirculated gas into the process tunnel 42 and the removal of tunnel gas. The oven 51 introduces gases directly into the process tunnel 42 bypassing the temperature cell 4, and removes gases directly from the process tunnel 42 bypassing the temperature cell 4. The location of these selected inlet ports 106 are such that the flows caused in the process tunnel 42 are generally from the cool region 24 to the high temperature region 22, from cooler to hotter areas within the high temperature region 22, to impede the flow of volatile laden gas from migrating to the cooler regions where it will condense. The method of using a tube or equivalent provides flexibility to adapt substantially all existing reflow solder ovens with a gas management system as disclosed in the alternative embodiment 2, as well as being the used in future reflow solder oven designs.

[0043] The temperature cell 104 of the second embodiment, illustrated in FIG. 7, shown as a top and bottom pair, is primarily the same temperature cell 4 of the first embodiment with the exception of an inlet 106. Inlet 106 may be provided near the return channel 16 such that gas introduced at the inlet 106 will enter the plenum 26 and be mixed with gas in the process tunnel 42.

[0044] In the oven 51, outlets 8, described in the preferred embodiment 2, are replaced with barrier vent 132, as shown in FIG. 6. Barrier vent 132 is located in the high temperature region 22 to control the flow direction of the expanding gas from the process gas reservoir 46 and is connected to the recirculation ducting 30 for cleaning by the condensation and filtration chamber 34. Gas is condensed, filtered, and reintroduced to the process tunnel 42 is the same manner as in the first embodiment 2. Similar to inlet 106, the method of using a main return duct like barrier vent 132 provides a flexibility to adapt existing reflow solder ovens with a gas management system as well as being the used in future reflow solder oven designs.

[0045] In a further alternative embodiment, low oxygen content in the tunnel gas is desired to improve the system performance, for example filtration of contaminants. A low flow valve, same as or similar to the low flow valve 72 of FIG. 3, is operably connected to an inlet port similar to inlet port 106 used for the delivery of primary inert gas. Low volumes of air bleed into the tunnel at one or more pre-selected locations, thereby controlling the parts per million of oxygen mixed with the tunnel gas. An alternative to the air bleed valves is a direct connection to an oxygen supply.

[0046] Although all of the detailed descriptions described above are directed toward an oven with temperature cells on the top and bottom of the process tunnel, it is understood that the present invention can also be implemented on an oven that either just top or just bottom temperature cells. It is further understood that the location, top or bottom of inlets and outlets is substantially unimportant to the function of the invention, as turbulence in the temperature cell and the process tunnel homogenize these flows instantly.

[0047] It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.