Patent Publication Number: US-2021178307-A1

Title: Exhaust Gas Purification Device

Description:
RELATED APPLICATIONS 
     The present application claims the benefit of Chinese Patent Application No. 201911273380.5, filed Dec. 12, 2019, entitled “EXHAUST GAS PURIFICATION DEVICE.” The entirety of Chinese Patent Application No. 201911273380.5 is expressly incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an exhaust gas treatment system for a reflow oven, an in particular to an exhaust gas purification device for purifying exhaust gas in a reflow oven hearth. 
     BACKGROUND 
     During the manufacture of printed circuit boards, electronic elements are mounted on the circuit boards generally by using a process known as “reflow soldering”. In a typical reflow soldering process, solder paste (e.g., tin paste) is deposited onto selected areas of a circuit board, and wires of one or more electronic elements are inserted into the deposited solder paste. The circuit board then passes through a reflow oven where the solder paste reflows in a heating zone (i.e., being heated to a melting or reflow temperature) and is then cooled in a cooling zone to electrically and mechanically connect the wires of the electronic elements to the circuit board. As used herein, the term “circuit board” includes a substrate assembly of any type of electronic elements, for example, including a wafer substrate. In the reflow oven, air or a substantially inert gas (e.g., nitrogen) is generally used as a working gas, and different working gases are used for circuit boards having different process requirements. A hearth of the reflow oven is full of the working gas, and the circuit board is soldered in the working gas while being transported through the hearth by a conveyer device. 
     In the reflow oven, the solder paste includes not only a solder, but also soldering flux which promotes wetting of the solder and provides a good solder joint. Other additives such as solvents and catalysts may also be included. After the solder paste is deposited on the circuit board, the circuit board is conveyed on a conveyer to pass through a plurality of heating zones of the reflow oven. The heat in the heating zones melts the soldering paste, and volatile organic compounds (VOCs) mainly including the soldering flux are vaporized into vapor, thereby forming “contaminants”. The accumulation of these pollutants in the reflow oven may cause some problems. For example, if the contaminants travel to a cooling zone, they will be condensed on the circuit board and contaminate the circuit board; consequently, subsequent cleaning steps have to be performed. The pollutants may also be condensed on a surface of a cooler of the reflow oven, thereby blocking gas holes. Moreover, condensates may also drip onto subsequent circuit boards; consequently, the elements on the circuit boards may be compromised, or the contaminated circuit boards have to be cleaned in subsequent steps. 
     Therefore, it is necessary to discharge exhaust gas containing contaminants in a reflow oven hearth out of the hearth, in order to keep a clean working atmosphere in the reflow oven hearth, so that the contaminants are prevented from entering a cooling zone of the reflow oven and causing the above-mentioned problems in the reflow oven. 
     Generally speaking, the exhaust gas containing contaminants needs to be purified by an exhaust gas purification device to remove the contaminants therein, and then can be discharged into the external atmosphere or conveyed back to the reflow oven hearth. However, these contaminants will remain in the exhaust gas purification device and affect the purification efficiency of the exhaust gas purification device. Therefore, there is a need for regular maintenance and cleaning of the exhaust gas purification device. 
     SUMMARY OF THE DISCLOSURE 
     In order to solve at least one of the above-mentioned problems, the present disclosure provides an exhaust gas purification device for purifying exhaust gas in a reflow oven hearth. The exhaust gas purification device can not only facilitate maintenance and cleaning, but can also prolong the maintenance interval. 
     In order to achieve the above objective, a first aspect of the present disclosure provides an exhaust gas purification device for purifying exhaust gas in a reflow oven hearth, the exhaust gas purification device comprising: a filtering unit; a filtering component provided in the filtering unit; a controllable gas passage that controllably communicates an interior of the filtering unit with an external environment containing oxygen, the controllable gas passage being configured to controllably introduce gas containing oxygen into the filtering unit; a heating component provided in the filtering unit, wherein the heating component is configured to be capable of heating the gas in the filtering unit to an oxidation temperature at which carbon in adherent substances on the filtering component is capable of chemically reacting to produce a gas. 
     According to the first aspect described above, the exhaust gas purification device has an exhaust gas inlet and a purified gas outlet, a downstream side of the filtering component is in fluid communication with the exhaust gas inlet, and an upstream side of the filtering component is in communication with the purified gas outlet; wherein the heating component is disposed on the downstream side of the filtering component. 
     According to the first aspect described above, the heating component is further configured to be capable of heating the gas in the filtering unit to a self-cleaning temperature at which a soldering flux in the adherent substances on the filtering component is capable of being physically changed into liquid or gas. 
     According to the first aspect described above, the exhaust gas purification device has a self-cleaning mode and a maintenance mode; in the self-cleaning mode, the controllable gas passage is closed, the heating component heats the gas in the filtering unit to the self-cleaning temperature, such that the soldering flux in the adherent substances on the filtering component is physically changed into a liquid or a gas, and carbon is generated in the attachments; and in the maintenance mode, the controllable gas passage is opened, and the heating component heats the gas in the filtering unit to the oxidation temperature, such that the carbon in the adherent substances on the filtering component chemically react to produce a gas. 
     According to the first aspect described above, the exhaust gas purification device further has an exhaust gas purification mode; in the exhaust gas purification mode, the controllable gas passage is closed, the exhaust gas in the reflow oven hearth is filtered through the filtering component, and during the filtering, adherent substances including soldering flux deposit on the filtering component. 
     According to the first aspect described above, the controllable gas passage is configured such that an outlet of the controllable gas passage is close to the filtering component. 
     According to the first aspect described above, after the carbon in the adherent substances on the filtering component chemically react to produce a gas, the exhaust gas purification device is cleaned by a mechanical device. 
     According to the first aspect described above, the exhaust gas purification device further comprises a control device, which is capable of controlling a heating temperature of the heating component, such that the heating component is capable of heating the gas in the filtering unit to the oxidation temperature or the self-cleaning temperature. 
     According to the first aspect described above, the oxidation temperature is higher than the self-cleaning temperature. 
     According to the first aspect described above, the oxidation temperature is 400˜450° C. 
     According to the first aspect described above, the self-cleaning temperature is 150˜170° C. 
     According to the first aspect described above, at the oxidation temperature, a portion of an organic matter in the adherent substances on the filtering component is also capable of chemically reacting to produce a gas. 
     The concept, specific structure and resulting technical effect of the present disclosure are further described below in conjunction with the drawings to fully understand the object, features and effects of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a simplified structural block diagram of an exhaust gas purification device according to an embodiment of the present disclosure; 
         FIG. 1B  is a block diagram showing a gas flow path when the exhaust gas purification device in  FIG. 1A  is in an exhaust gas purification mode; 
         FIG. 1C  is a block diagram showing a gas flow path when the exhaust gas purification device in  FIG. 1A  is in a self-cleaning mode; 
         FIG. 1D  is a block diagram showing a gas flow path when the exhaust gas purification device in  FIG. 1A  is in a maintenance mode; and 
         FIG. 2  is a structural diagram of a control device of the exhaust gas purification device in  FIG. 1A . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Specific embodiments of the present disclosure are described below with reference to the accompanying drawings which constitute part of this description. It should be understood that although the terms, such as “front”, “rear”, “upper”, “lower”, “left”, “right”, “top”, “bottom” and “side”, indicating orientations are used in the present disclosure to describe structural parts and elements in various examples of the present disclosure, these terms are used herein only for ease of illustration and are determined based on the exemplary orientations shown in the accompanying drawings. Since the embodiments disclosed in the present disclosure can be arranged in different directions, these terms indicating directions are only illustrative and should not be considered as limitations. 
     It should be understood by those skilled in the art that exhaust gas or gas described in an embodiment of the present disclosure means the exhaust gas or gas principally constituted by gaseous ingredient, and it may also partially contain a mist-like or granular ingredient. 
       FIG. 1A  shows a simplified structural block diagram of an exhaust gas purification device according to an embodiment of the present disclosure, for showing the connection relationships between various parts of the exhaust gas purification device  100 . As shown in  FIG. 1A , the exhaust gas purification device  100  is disposed outside a hearth  118  of the reflow oven and is connected to the hearth  118  of the reflow oven. When the reflow oven uses a substantially inert gas, e.g. nitrogen, as a working gas, the exhaust gas purification device  100  receives exhaust gas discharged from the hearth  118  of the reflow oven and conveys the purified gas back to the hearth  118 . When the reflow oven uses air as a working gas, the exhaust gas purification device  100  receives exhaust gas discharged from the hearth  118  of the reflow oven, and the purified gas may be conveyed back to the hearth  118 , or may be discharged to the outside of the hearth  118  instead of being conveyed back to the hearth  118 . As shown in  FIG. 1A , the exhaust gas purification device  100  conveys the purified gas back to the hearth  118 . 
     As shown in  FIG. 1A , the exhaust gas purification device  100  comprises a first stage cooling unit  110 , a second stage cooling unit  120 , and a filtering unit  130 , which are connected in sequence and are connected to the hearth  118 . The first stage cooling unit  110  and the second stage cooling unit  120  are used to cool gas therein to a certain temperature, the filtering unit is used to filter the gas such that a part of the substance in the gas is adhered to a filtering component, and the first stage cooling unit  110 , the second stage cooling unit  120  and the filtering unit  130  are connected via pipelines. The exhaust gas purification device may work in an exhaust gas purification mode and in a self-cleaning mode and may be subjected to maintenance in a maintenance mode by controlling a gas flow path in the exhaust gas purification device. A control device (see  FIG. 2 ) controls the exhaust gas purification device  100  to switch between the exhaust gas purification mode, the self-cleaning mode, and the maintenance mode. In the exhaust gas purification mode, the exhaust gas purification device  100  may purify the exhaust gas discharged from the hearth  118  and convey the purified gas back to the hearth  118 . In the self-cleaning mode, the exhaust gas purification device  100  may perform self-cleaning on the first stage cooling unit  110 , the second stage cooling unit  120 , the filtering unit  130  and passages communicated therebetween. In the maintenance mode, the exhaust gas purification device  100  may remove carbon and some organic matters deposited in the filtering unit  130 , to facilitate the subsequent complete disassembling of the exhaust gas purification device  100  for further cleaning. 
     Specifically, the first stage cooling unit  110  is provided with an exhaust gas inlet  111 . 1 , a self-cleaning gas inlet  114 , a gas outlet  111 . 2 , and a first waste liquid outlet  141 . 1 , and the second stage cooling unit  120  is provided with a gas inlet  121 . 1 , a gas outlet  121 . 2 , and a waste liquid outlet  141 . 2 , wherein the exhaust gas inlet  111 . 1  and the self-cleaning gas inlet  114  are connected to an inlet side  184  of the first stage cooling unit  110 , and the gas outlet  121 . 2  is connected to an outlet side  185  of the second stage cooling unit  120 . The filtering unit  130  is provided with a gas inlet  131 . 1 , a self-cleaning gas outlet  134 , a purified gas outlet  131 . 2 , and a waste liquid outlet  141 . 3 . 
     The exhaust gas inlet  111 . 1  of the first stage cooling unit  110  is in controllable fluid communication with a high-temperature zone of the hearth  118  via a valve component  117 . 1 . The gas outlet  111 . 2  of the first stage cooling unit  110  is in fluid communication with the gas inlet  121 . 1  of the second stage cooling unit  120  via a connecting passage  125 . 1 . The gas outlet  121 . 2  of the second stage cooling unit  120  is in fluid communication with the gas inlet  131 . 1  of the filtering unit  130  via a connecting passage  125 . 2 , and the purified gas outlet  131 . 2  of the filtering unit  130  is in controllable fluid communication with a low-temperature zone of the hearth  118  via a valve component  117 . 2 . In this way, the exhaust gas discharged from the hearth  118  can be purified by passing through the first stage cooling unit  110 , the second stage cooling unit  120 , and the filtering unit  130  in sequence and then returns to the hearth  118 . 
     In addition, the self-cleaning gas outlet  134  of the filtering unit  130  is connected to the gas inlet  114  of the first stage cooling unit  110  via a connecting passage  135 , and a passage switching component  117 . 5  is disposed on the connecting passage  135  to controllably and fluidly communicate the self-cleaning gas outlet  134  of the filtering unit  130  with the gas inlet  114  of the first stage cooling unit  110 . In this way, the gas discharged from the self-cleaning gas outlet  134  of the filtering unit  130  can enter the first stage cooling unit  110 , and can flow through the first stage cooling unit  110  and the second stage cooling unit  120  in sequence, and then return to the filtering unit  130 , thereby creating a self-cleaning gas circulation in the exhaust gas purification device  100 . 
     According to an embodiment of the present disclosure, on the first stage cooling unit  110 , instead of disposing the self-cleaning gas inlet  114  that is separate from the exhaust gas inlet  111 . 1 , one inlet may be disposed as both the exhaust gas inlet and the self-cleaning gas inlet. Likewise, on the filtering unit  130 , instead of disposing the self-cleaning gas outlet  134  that is separate from the purified gas outlet  131 . 2 , one outlet may also be disposed as both the self-cleaning gas outlet  134  and the purified gas outlet  131 . 2 . 
     The exhaust gas purification device  100  further comprises a gas replenishing port  112  disposed on the first stage cooling unit  110 , a gas discharge port  132  disposed on the filtering unit  130 , and a gas concentration detection component for detecting a gas concentration in the filtering unit  130 . As an example, the gas concentration detection component is an oxygen concentration detection component  155 , which obtains a concentration of a working gas by detecting an oxygen concentration. The oxygen concentration detection component  155  is disposed near the gas discharge port  132 . The gas replenishing port  112  is controllably opened and closed by means of a valve component  117 . 3 , and the gas discharge port  132  is controllably opened and closed by means of a valve component  117 . 4 . When the reflow oven uses a substantially inert gas (e.g., nitrogen) as a working gas, the working gas, namely, the substantially inert gas (e.g., nitrogen) may be replenished through the gas replenishing port  112  to the exhaust gas purification device  100 , and the gas discharge port  132  is configured to cooperate with the gas replenishing port  112  when the gas replenishing port  112  is working. By providing the gas replenishing port  112  and the gas discharge port  132 , the concentration of the working gas in the exhaust gas purification device  100  may be adjusted to match the concentration of the working gas in the hearth  118 . The gas replenishing port  112  may be in controllable fluid communication with a working gas source, namely, a substantially inert gas (e.g., nitrogen) via the valve component  117 . 3 , and the gas discharge port  132  is in controllable fluid communication with the atmosphere via the valve component  117 . 4 . 
     A filtering component  136  is disposed in the filtering unit  130 . The gas inlet  131 . 1  of the filtering unit  130  is disposed on an upstream side of the filtering component  136 , and the self-cleaning gas outlet  134  and the purified gas outlet  131 . 2  are disposed on a downstream side of the filtering component  136 . It should be noted that “upstream” and “downstream” here are described with respect to a gas flowing direction in the exhaust gas purification device  100 . The filtering component  136  may be a steel ball filter screen, a paper filter screen, etc. 
     A heating component  133  is further disposed in the filtering unit  130 , and the heating component  133  is located below the filtering component  136  and is configured to heat the filtering component  136 . In the exhaust gas purification mode, the heating component  133  does not perform heating. In the self-cleaning mode, the heating component  133  can heat the gas in the filtering unit  130  to a self-cleaning temperature, such that the contaminants, including soldering flux, adhered to the filtering component  136  are physically changed into a liquid or a gas. In the maintenance mode, the heating component  133  can heat the gas in the filtering unit  130  to an oxidation temperature, such that the carbon adhered to the filtering component  136  undergoes a chemical reaction (such as an oxidation reaction) to produce a gas. At this temperature, other organic matters adhered to the filtering component  136  will also chemically react to produce a gas. The heating component  133  may heat the gas in the filtering unit  130  to the self-cleaning temperature or the oxidation temperature by controlling the heating temperature of the heating component  133 . As a specific example, the self-cleaning temperature may be about 150˜170° C., and the oxidation temperature may be about 400˜450° C. When the heating temperature of the heating component  133  is about 400° C., the gas in the filtering unit  130  may be heated to the self-cleaning temperature; and when the heating temperature of the heating component  133  is about 700° C., the gas in the filtering unit  130  may be heated to the oxidation temperature. It should be noted that the self-cleaning temperature and the oxidation temperature may also be set to other specific temperatures, as long as the above functions can be satisfied. Those skilled in the art should understand that, according to the different specific configurations of the filtering unit  130 , such as the size of the filtering unit  130  and the separation distance between the filtering component  136  and the heating component  133 , the heating component  133  may also be set to have different heating temperatures to heat the gas in the filtering unit to the self-cleaning temperature and the oxidation temperature. 
     The exhaust gas purification device  100  further comprises a controllable gas passage  181 , and the controllable gas passage  181  is connected to the filtering unit  130  via a valve component  182 . The valve member  182  is opened only when the exhaust gas purification device  100  is in the maintenance mode, and is used to controllably introduce a gas containing oxygen into the filtering unit  130 . As an example, an outlet of the gas passage  181  is disposed close to the filtering component  136 . An inlet of the controllable gas passage  181  is directly connected to the atmosphere, or connected to a gas source, such as an oxygen cylinder, which is capable of supplying oxygen. 
     It should be noted that “close to” here means that the outlet of the controllable gas passage  181  is not far away from the filtering component  136 , or the outlet of the controllable gas passage  181  is in direct fluid communication with the filtering component  136 . Therefore, it is possible to facilitate the chemical reaction between the oxygen introduced into the filtering unit  130  from the outlet of the controllable gas passage  181  and the carbon and some organic matters deposited on the filtering component  136 . 
     The exhaust gas purification device  100  further comprises a fan  124  configured to drive gas flow in the exhaust gas purification device  100 . In the embodiment shown in  FIG. 1A , the fan  124  is disposed in the filtering unit  130 . Specifically, the fan  124  is disposed on the upstream side of the filtering component  136 , an air inlet side of the fan  124  is in fluid communication with a cavity in the filtering unit  130 , and an air outlet side of the fan  124  is in fluid communication with the purified gas outlet  131 . 2 , the self-cleaning gas outlet  134 , and the gas discharge port  132  of the filtering unit  130 . In another embodiment, another fluid power supplying device (such as an air blower and a pump) may also be used to replace the fan  124  in the embodiment shown in  FIG. 1A , as long as the gas in the exhaust gas purification device  100  can be driven to flow according to an expected path. 
     The exhaust gas purification device  100  further comprises a collecting unit  140 . The waste liquid outlet  141 . 1  of the first stage cooling unit  110 , the waste liquid outlet  141 . 2  of the second stage cooling unit  120 , and the waste liquid outlet  141 . 3  of the filtering unit  130  are all in communication with the collecting unit  140 , so that the liquid discharged from the first stage cooling unit  110 , the second stage cooling unit  120 , and the filtering unit  130  can flow into the collecting unit  140 . A valve component  117 . 6  is disposed at an inlet of the collecting unit  140 . When the collecting unit  140  needs to be replaced or the fluid in the collecting unit  140  needs to be discharged, the valve component  117 . 6  may be closed to disconnect the collecting unit  140  from the first stage cooling unit  110 , the second stage cooling unit  120 , and the filtering unit  130 . 
     The exhaust gas purification device  100  further comprises temperature detection components  151 ,  152  configured to respectively detect the temperatures of the gases in the first stage cooling unit  110  and the second stage cooling unit  120 , and a temperature detection component  161  for detecting the temperature of the gas in the filtering unit  130 . 
     It should be noted that, in the embodiment shown in  FIG. 1A , the exhaust gas purification device  100  comprises two stages of cooling units, the two stages of cooling units being in fluid communication via the connecting passage  125 . 1 . In other embodiments, the exhaust gas purification device may also comprise only one cooling unit, as long as the upstream side of the filtering unit is in fluid communication with an inlet side of the cooling unit, and the downstream side of the filtering unit is in fluid communication with an outlet side of the cooling unit. 
     As described above, the exhaust gas purification device  100  has an exhaust gas purification mode, a self-cleaning mode, and a maintenance mode. In the exhaust gas purification mode, the exhaust gas purification device  100  purifies the gas discharged from the hearth  118  of the reflow oven. In the self-cleaning mode, the exhaust gas purification device  100  performs the self-cleaning on the various units and connecting pipelines inside the exhaust gas purification device  100 . In the maintenance mode, the exhaust gas purification device  100  no longer receives the gas discharged from the hearth  118  of the reflow oven, but makes the carbon and some organic matters in the adherent substances of the filtering component  136  undergo an oxidation reaction, so as to facilitate subsequent disassembling of the exhaust gas purification device  100  for further cleaned by a mechanical device. By controlling the opening and closing of the respective valve components  117 . 1 ,  117 . 2 ,  117 . 3 ,  117 . 4 ,  117 . 5 ,  117 . 6 ,  182 , the exhaust gas purification device  100  can be switched between the three modes, that is, the exhaust gas purification mode, the self-cleaning mode, and the maintenance mode. Gas flow paths in the three working modes of the exhaust gas purification device  100  of the present disclosure will be described below by taking a reflow oven that uses a substantially inert gas (e.g., nitrogen) as a working gas, as an example. 
       FIG. 1B  shows a gas flow path when the exhaust gas purification device  100  shown in  FIG. 1A  is in an exhaust gas purification mode. As shown in  FIG. 1B , when the exhaust gas purification device  100  is in the exhaust gas purification mode, the valve components  117 . 1 ,  117 . 2 ,  117 . 6  are open, the valve component  117 . 3  and the valve component  117 . 4  are closed, the passage switching component  117 . 5  may be closed or at least partially opened, and the valve component  182  is closed to close the gas passage  181 . Exhaust gas (having a temperature of about 170° C.) containing contaminants in the hearth  118  of the reflow oven, after being discharged from a high-temperature zone of the hearth  118 , firstly passes through the first stage cooling unit  110  and is cooled to a first temperature of 110˜130° C., for example. At this temperature, organic soldering fluxes such as rosin in the contaminants of the exhaust gas in the first stage cooling unit  110  are condensed from a gaseous phase into liquid phase and may enter the collecting unit  140  through the waste liquid outlet  141 . 1  of the first stage cooling unit  110 , and the remaining exhaust gas is then conveyed to the second stage cooling unit  120  for further cooling. The gas that has entered the second stage cooling unit  120  is cooled in the second stage cooling unit  120  to a second temperature of 60˜80° C., for example, such that other contaminant organic substances (for example, low-condensation-point acidic or ester or ether organic substances) in the exhaust gas are condensed from a gaseous phase into liquid phase and enter the collecting unit  140  through the waste liquid outlet  141 . 2  of the second stage cooling unit  120 , and the remaining exhaust gas is conveyed to the filtering unit  130  for filtration and purification. After the exhaust gas entering the filtering unit  130  is filtered, it becomes purified gas, wherein the granular and mist-like organic matters are adhered to the filtering component  136 . Finally, the purified gas is conveyed back to a low-temperature zone of the hearth  118  of the reflow oven, thereby completing the purification of the exhaust gas. 
     When the exhaust gas purification device  100  is in the exhaust gas purification mode, if the valve component  117 . 5  is closed, the purified gas filtered by the filtering unit  130  cannot return to the first stage cooling unit  110  through the connecting passage  135 . If the passage switching component  117 . 5  is open or partially open, part of the purified gas filtered by the filtering unit  130  can return to the first stage cooling unit  110  through the connecting passage  135 . In this way, the gas in the first stage cooling unit  110  may be cooled by the purified gas having a lower temperature in the filtering unit  130 , thereby saving a cooling medium used for the heat exchange in the first stage cooling unit  110 . 
       FIG. 1C  shows a gas flow path when the exhaust gas purification device  100  is in the self-cleaning mode. As shown in  FIG. 1C , when the exhaust gas purification device  100  is in the self-cleaning mode, the valve components  117 . 3 ,  117 . 4  are closed, the valve component  117 . 6  and the passage switching component  117 . 5  are opened, the valve component  182  is closed to close the gas passage  181 , and the valve components  117 . 1 ,  117 . 2  may be opened or closed. The state in which the valve components  117 . 1 ,  117 . 2  are open is shown in the figure. In this case, the control device (see  FIG. 2 ) controls the heating temperature of the heating component  133  in the filtering unit  130 , for example, to raise the temperature of the heating component  133  to about 400° C., so as to heat the gas in the filtering unit  130  and raise the temperature of the gas inside the filtering unit  130  to about 150° C. to 170° C., for example. At this temperature, a part of the solid contaminants (e.g., soldering flux) adhered to the filtering component  136  is physically changed to convert into liquid, and another part is converted into gas, wherein the liquid can be discharged through the waste liquid outlet  141 . 3  of the filtering unit  130 , while the gas, at a higher temperature, is conveyed to the first stage cooling unit  110  and the second stage cooling unit  120  through the connecting passage  135 . As like the above exhaust gas purification process, the gases in the first stage cooling unit  110  and the second stage cooling unit  120  flow back to the filtering unit  130  through the connecting passages  125 . 1 ,  125 . 2 , such that the solid contaminant organic substances adhered to the components in the first stage cooling unit  110  and the second stage cooling unit  120  and to inner walls of the connecting passage  125 . 1  and the cooling passage  125 . 2  are heated again into liquid or gas, and the liquid is discharged through the waste liquid outlet  141 . 1  of the first stage cooling unit  110  and the waste liquid outlet  141 . 2  of the second stage cooling unit  120 , while the gas is conveyed back to the filtering unit  130  to comple a self-cleaning gas cycle. In this self-cleaning gas cycle, the filtering component  136  has a higher local temperature at a section close to the heating component  133 , such that a part of the adherent substances on the filtering component  136  will undergo a carbonization reaction to obtain a carbonized product. The carbonized product comprises carbon and some small-molecule organic matters. 
     In the self-cleaning gas cycle shown in  FIG. 1C , if the valve components  117 . 1  and  117 . 2  are closed, the exhaust gas in the hearth of the reflow oven cannot enter and exit the exhaust gas purification device  100 , and the exhaust gas purification device  100  can perform internal self-cleaning in a relatively independent internal circulation process. If the valve components  117 . 1  and  117 . 2  are opened, the exhaust gas in the hearth of the reflow oven can still enter and exit the exhaust gas purification device  100 . During the internal self-cleaning process of the exhaust gas purification device  100 , the exhaust gas in the hearth of the reflow oven may also be subjected to the exhaust gas purification process as shown in  FIG. 1B . Therefore, whether the valve components  117 . 1  and  117 . 2  are closed or opened does not affect the operation of the reflow oven. In other words, even if the reflow oven is in operation, the exhaust gas purification device  100  can be in the self-cleaning mode to perform self-clean operation therein. According to the actual production requirements, the exhaust gas purification device  100  can be controlled to enter the self-cleaning mode at a certain interval of time, and the exhaust gas purification device  100  can be cleaned to some extent without affecting the operation of the reflow oven. 
       FIG. 1D  shows a gas flow path when the exhaust gas purification device  100  is in the maintenance mode. As shown in  FIG. 1D , when the exhaust gas purification device  100  is in the maintenance mode, the valve components  117 . 1 ,  117 . 2 ,  117 . 3 ,  117 . 4 ,  117 . 6  are closed, the passage switching component  117 . 5  is opened, and the valve component  182  is opened to open the gas passage  181 . In this case, the heating component  133  in the filtering unit  130  may be heated, for example, to about 700° C., to heat the gas in the filtering unit  130 , and raise the temperature of the gas inside the filtering unit  130  to about 400° C. to 450° C., for example. At this temperature, the carbon in the carbonized product deposited on the filtering component  136  can undergo an oxidation reaction with the oxygen introduced in the gas channel  181 , so that the carbon becomes a gas. Also at this temperature, the small-molecule organic matters in the carbonized product deposited on the filtering component  136  and a part of the soldering flux contaminants may also undergo an oxidation reaction with the oxygen to produce a gas, thereby reducing the amount of organic matters adhered to the filtering component  136 . These gases are conveyed to the first stage cooling unit  110  and the second stage cooling unit  120  through the connecting channel  135 . The gases in the first stage cooling unit  110  and the second stage cooling unit  110  flow back to the filtering unit  130  through the connecting passages  125 . 1 ,  125 . 2  according to the above mentioned self-cleaning gas circulation process. The control device (see  FIG. 2 ) controls the heating component  133  to continue heating for a period of time and then stop heating. After the gas in the exhaust gas purification device  100  is cooled to the normal temperature, the exhaust gas purification device  100  may be disassembled, and the exhaust gas purification device  100  may be completely cleaned by a mechanical device by means of ultrasonic cleaning, etc. The desired continuous heating time of the heating component  133  is related to the size of the exhaust gas purification device  100  and the amount of carbon deposited on the filtering component  136 , and may be preset by those skilled in the art as required. 
     After observation and analysis, the inventor found that during the self-cleaning gas circulation process, the heating component  133  needs to heat the gas in the filtering unit  130  to the self-cleaning temperature, such that the soldering flux and other contaminants in the adherent substances on the filtering component  136  are physically changed. However, during this process, a local carbonization reaction may also occur on the filtering component  136  to produce the carbonized product including carbon in the adherent substances. The accumulated carbon and other adherent substances including soldering flux remaining on the filtering component  136  may cause the filtering component  136  to be blocked to some extent, which on one hand leads to a poor gas circulation in the exhaust gas purification device  100 , and on the other hand prevents the temperature of the gas in the cooling unit of the exhaust gas purification device  100  from reaching the expected temperature, thereby affecting the exhaust gas purification and self-cleaning effects of the exhaust gas purification device  100 , and also affecting the soldering effect of the reflow oven. 
     In addition, after observation and analysis, the inventor found that as the contaminants in the exhaust gas purification device  100  continue to accumulate after a period of use, the entire exhaust gas purification device  100  needs to be cleaned. In order to achieve mechanized operation, a machine device is often used for cleaning. For example, a vibration-type machine device is used to clean the various components of the exhaust gas purification device  100  in a solvent. However, when the carbon is adhered to the filtering component  136 , the cleaning effect of the machine device may not meet the cleaning requirements due to the poor solubility of the carbon, and thus manual cleaning may be also needed. 
     However, in the exhaust gas purification device  100  of the present disclosure, after the self-cleaning mode, the oxygen is first introduced into the filtering unit  130 , and the heating component  133  is used to perform a high-temperature treatment on the carbon deposited on the filtering component  136 , such that the carbon chemically reacts with the introduced oxygen to remove the carbon from the adherent substances on the filtering component  136 . In addition, during this process, some of the other organic matters (including soldering flux and some small-molecule organic matters, etc.) adhered to the filtering component  136  will also undergo an oxidation reaction to produce a gas. Then, an operator can continue to clean the exhaust gas purification device  100  in a mechanized manner, for example, using an ultrasonic machine device to perform subsequent cleaning of the remaining organic matters in the exhaust gas purification device, which reduces the difficulty of the cleaning, and can also reduce the amount of solvent used in the cleaning process to some extent. 
     It thus can be seen that, by controlling the heating component  133  to heat to different heating temperatures, the heating component  133  not only may heat the gas in the filtering unit  130  to the self-cleaning temperature in the self-cleaning mode such that the soldering flux and other contaminants in the adherent substances on the filtering component  136  are physically changed, but also may heat the gas in the filtering unit  130  to the oxidation temperature in the maintenance mode such that the carbon deposited on the filtering component  136  undergoes a chemical reaction. 
     In the present disclosure, the gas in the filtering unit  130  is heated to different temperatures by controlling the controllable gas passage  181 , the various valve components and the switching component, and by using different heating temperatures for the same heating component  133 , thereby achieving three working modes for one exhaust gas purification device  100 . Moreover, since the exhaust gas purification device  100  of the present disclosure does not need to shut down the reflow oven in the self-cleaning mode, the combination of the self-cleaning mode and the maintenance mode can prolong the period until the reflow oven needs to be shut down for maintenance. 
     As an exemplary technical effect, the period of the self-cleaning mode of the exhaust gas purification device  100  of the present disclosure may be about once a week, and the period of the maintenance mode may be about once a month. It can be seen that the exhaust gas purification device  100  of the present disclosure has a longer maintenance period, which can greatly reduce the maintenance cost. 
     When the hearth  118  of the reflow oven uses a substantially inert gas (e.g., nitrogen) as a working gas, a concentration of the working gas should be kept within a certain range in order to meet process requirements. A unit (e.g., a unit for replenishing the working gas) for regulating the concentration of the working gas in the hearth  118  is generally disposed in the reflow oven. When the exhaust gas purification device  100  is in the exhaust gas purification mode, the gas in the hearth  118  of the reflow oven is continuously purified by the exhaust gas purification device  100  and is re-conveyed back to the hearth  118  of the reflow oven. Therefore, the working gas in the exhaust gas purification device  100  that is in the exhaust gas purification mode has a concentration close to that in the unit for regulating the concentration of the working gas in the hearth  118  of the reflow oven. However, after the exhaust gas purification device  100  goes through the maintenance mode and is disassembled for cleaning, the working gas in the exhaust gas purification device  100  has a concentration lower than the concentration of the working gas in the hearth  118 . Therefore, according to the present disclosure, after the maintenance mode and the cleaning of the exhaust gas purification device  100  are completed, a certain amount of working gas may be replenished to the exhaust gas purification device  100  before the exhaust gas purification device  100  is re-communicated to the hearth  118  of the reflow oven, such that the working gas in the exhaust gas purification device  100  reaches a concentration that is the same as or close to the concentration of the working gas in the reflow oven. For this purpose, the working gas is replenished through the gas replenishing port  112  to the exhaust gas purification device  100  and, at the same time, the gas in the exhaust gas purification device  100  is discharged through the gas discharge port  132 , until it is determined, by the oxygen concentration detection component  155 , that the protective gas in the exhaust gas purification device  100  has reached the concentration of the protective gas in the reflow oven. 
     When the exhaust gas purification device  100  is used in a reflow oven that uses air as the working gas, gas purified by the exhaust gas purification device  100  may be conveyed back to the hearth  118  or, may be directly discharged into the atmosphere instead of being conveyed back to the hearth  118 . If the gas purified by the exhaust gas purification device  100  is directly discharged into the atmosphere, the purified gas outlet  131 . 2  of the filtering unit  130  shown in  FIG. 1B  is in controllable fluid communication with the atmosphere via the valve component  117 . 2 , instead of being connected to the hearth  118 . 
     According to the present disclosure, the first stage cooling unit  110  and the second stage cooling unit  120  in the exhaust gas purification device  100  may use any known type of heat exchange device. 
     According to the present disclosure, the first stage cooling unit  110 , the second stage cooling unit  120 , and the filtering unit  130  of the exhaust gas purification device  100  may be integrated , so that the whole exhaust gas purification device  100  forms a box-type exhaust gas purification device to facilitate use with the reflow oven. 
       FIG. 2  shows a structural diagram of a control device  260  of the exhaust gas purification device  100 . The control device  260  comprises a bus  262 , a processor  264 , an input interface  266 , an output interface  268 , and a memory  274  with control programs  276 . The processor  264 , the input interface  266 , the output interface  268 , and the memory  274  are communicatively connected via the bus  262 , such that the processor  264  can control the operations of the input interface  266 , the output interface  268 , and the memory  274 . The memory  274  is used to store programs, instructions, and data. The processor  264  reads programs, instructions, and data from the memory  274 , and can write data to the memory  274 . 
     The input interface  266  receives signals and data through a connection  278 , such as temperature signals detected by the temperature detection components  151 ,  152 ,  161 , oxygen concentration signals detected by the oxygen concentration detection component  155 , and various manually input parameters and control commands. The output interface  268  sends signals and data through a connection  272 , such as control signals for controlling the closing and opening of the various valve components  117 . 1 ,  117 . 2 ,  117 . 3 ,  117 . 4  and  182  and the passage switching component  117 . 5 , and control signals for controlling the heating temperature of the heating component  133 . The memory  274  stores control programs, data, and the like. Various parameters may be preset in the process of production and manufacturing, or may be set by manual input or data import when in use. The processor  264  obtains various signals, data, programs, and instructions from the input interface  266  and the memory  274 , performs corresponding processing on them, and outputs them through the output interface  268 . 
     Although the present disclosure is described with reference to the specific embodiments shown in the accompanying drawings, it should be understood that the exhaust gas purification device of the present disclosure can have many variations without departing from the spirit and scope of the teaching of the present disclosure. A person of ordinary skill in the art will also realize that there are different ways to alter the arrangements in the embodiments disclosed in the present disclosure, all the alterations falling within the spirit and scope of the present disclosure and the claims.