Patent Publication Number: US-2021176867-A1

Title: Reflow oven

Description:
RELATED APPLICATIONS 
     The present application claims the benefit of Chinese Patent Application No. 201911257935.7, filed Dec. 10, 2019, entitled “REFLOW OVEN.” The entirety of Chinese Patent Application No. 201911257935.7 is expressly incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to the technical field of reflow ovens. 
     BACKGROUND 
     Reflow ovens are mainly configured to solder circuit boards where electronic elements have been mounted. When a reflow oven works, solder paste on the circuit board where an electronic element has been mounted is melted by means of a heating zone of the reflow oven, such that the electronic element and the solder joint of the circuit board are fused and soldered together. The solder paste on the circuit board where the electronic element has been mounted is cooled by means of a cooling zone of the reflow oven, such that the electronic element and the solder joint are solidified and connected together. The heating zone of the reflow oven has a predetermined temperature interval in the working process to provide the heat required for heating the solder paste to a melting temperature or a reflow temperature. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure provides a reflow oven, comprising a heating zone, a plurality of heating devices and a start-stop device. The heating zone comprises a plurality of heating sub-zones, and the plurality of heating sub-zones are sequentially arranged in a length direction of the reflow oven. The plurality of heating devices are arranged in corresponding heating sub-zones of the plurality of heating sub-zones, and each of the plurality of heating devices is configured such that a working temperature of the corresponding heating sub-zone is in a predetermined temperature interval. The start-stop device is configured to activate or deactivate the plurality of heating devices, and the start-stop device is configured in such a way that the start-stop device activates or deactivates the plurality of heating devices according to predetermined time intervals in a process during which the circuit board sequentially passes through the plurality of heating sub-zones, such that a working temperature of each of the plurality of heating sub-zones is in a corresponding predetermined temperature interval. In the present disclosure, the start-stop device is incorporated into the reflow oven. This is suitable for the operation of high-power heating devices in the heating zone, and avoids overheating of a hearth caused by thermal inertia of the high-power heating devices. Therefore, in addition to meeting requirements for soldering and processing of both large-sized circuit boards and small-sized circuit boards, the reflow oven according to present disclosure can further not only improve the processing efficiency of the large-sized circuit boards, but also ensure the processing quality of the large-sized circuit boards. 
     In the reflow oven as described above, the start-stop device activates or deactivates the plurality of heating devices according to predetermined time intervals and a predetermined sequence. 
     In the reflow oven as described above, the reflow oven is suitable for soldering circuit boards with different sizes and different amounts of soldering heat. 
     In the reflow oven as described above, the reflow oven comprises a position sensor, and the position sensor is located at an inlet position of the heating zone; 
     the heating sub-zone located at the inlet position of the heating hearth is referred to as first heating sub-zone, the N-th heating sub-zone counted from the inlet position of the heating hearth is referred to as N-th heating sub-zone, the start-stop device is configured to start counting time when the position sensor monitors that the circuit board enters the heating zone, and the start-stop device stops the operation of the heating device located in the first heating sub-zone when a cumulative counting time t reaches t 1 ; and the start-stop device stops the operation of the heating device located in the N-th heating sub-zone when the cumulative counting time t reaches t 1 +(N−1)·Δt 2 , wherein N is a natural number greater than 1. 
     In the reflow oven as described above, the circuit board has a traveling speed of v in the heating zone, each of the heating sub-zones has a length of H extending in the length direction of the reflow oven, and Δt 2 =H/v. 
     In the reflow oven as described above, the start-stop device is further configured in such a way that the start-stop device resumes the operation of the heating device located in the first heating sub-zone when the cumulative counting time t reaches t 1 +Δt 3 ; and the start-stop device resumes the operation of the heating device located in the N-th heating sub-zone when the cumulative counting time t reaches t 1 +N·Δt 3 . 
     In the reflow oven as described above, the circuit board has a traveling speed of v in the heating zone, each of the heating sub-zones has a length of H extending in the length direction of the reflow oven, and Δt 3 =m*H/v, wherein 1≤m&lt;N. 
     In the reflow oven as described above, the circuit board has a length of L extending in the length direction of the reflow oven, and t 1 =L/v. 
     In the reflow oven as described above, the value of Δt 3  is greater than that of Δt 2 . 
     In the reflow oven as described above, the extending length L of the circuit board is greater than the extending length H of the heating sub-zone. 
     Since a larger circuit board absorbs more heat while a smaller circuit board absorbs less heat, a temperature control system of the conventional reflow oven cannot meet heat requirements of soldering both large-sized circuit boards and small-sized circuit boards. When the temperature control system of the conventional reflow oven which is suitable for soldering small-sized circuit boards is used to solder large-sized circuit boards, it takes a very long time for the reflow oven to heat the hearth of the heating zone to a predetermined temperature range. This easily leads to excessively low circuit board production efficiency and cannot meet production requirements. After observation and research, the inventor found that when the reflow oven uses a heating device with smaller power in the heating zone, the temperature control system of the reflow oven can effectively control the temperature of the heating zone within a working temperature range because of the small heating inertia of the heating device, and when the reflow oven uses a high-power heating device in the heating zone, although the heating device can meet temperature requirements for processing circuit boards with high heat absorption in the outset, due to the “inertia of temperature rise” of the high-power heating device, the temperature of the hearth then will continue to rise until it exceeds a predetermined temperature interval if the heating device is controlled by using the conventional temperature control system. As a result, it cannot meet the soldering temperature requirements. 
     In order to meet the processing requirements of large-sized circuit boards, the reflow oven of the present disclosure is suitable for the operation of high-power heating devices. Besides, in order to suppress the overheating of the hearth caused by thermal inertia of the high-power heating devices, in the present disclosure, the reflow oven is provided with the start-stop device such that the operating states of the heating devices in individual heating sub-zones in the reflow oven are regularly controlled by the start-stop device at predetermined time intervals. This effectively maintains temperature of the hearth in each heating sub-zone within the predetermined temperature interval thereof, and ensures that the large-sized circuit boards are properly and stably soldered and processed in the reflow oven. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a reflow oven  100  according to the present disclosure; 
         FIG. 2  is a schematic diagram of a heating hearth  101  of the reflow oven  100  in  FIG. 1 ; 
         FIG. 3  is a schematic diagram showing the control of a plurality of heating devices  220  by a start-stop device  300  of the reflow oven  100  in  FIG. 1 ; 
         FIG. 4  is a schematic diagram of an internal structure of the start-stop device  300  in  FIG. 3 ; 
         FIGS. 5A to 5D  show an embodiment in which the operation of heating devices  221  is stopped by using the start-stop device  300  in  FIG. 3 ; and 
         FIGS. 6A and 6B  show an embodiment in which the operation of the heating devices  221  is resumed by using the start-stop device  300  in  FIG. 3 . 
     
    
    
     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”, and “right” indicating directions in the present disclosure are used to describe various exemplary structural parts and elements in the present disclosure, these terms used herein are merely used for ease of description and are determined based on the exemplary orientation 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. If possible, the same or similar reference numerals used in the present disclosure refer to the same components. 
       FIG. 1  is a schematic diagram of a reflow oven  100  according to the present disclosure, showing the internal structure seen from a side of the reflow oven  100 . As shown in  FIG. 1 , the reflow oven  100  comprises a heating zone  101  and a cooling zone  102 , and the heating zone  101  and the cooling zone  102  are in communication with each other. A hearth  103  is provided by running through the whole heating zone  101  and cooling zone  102 , an extending direction of the hearth  103  is consistent with a length direction of the reflow oven  100 , and the hearth is used to provide a space for soldering and processing a circuit board. The hearth  103  is located at a middle position in a height direction of the reflow oven  100 , and divides each of the heating zone  101  and the cooling zone  102  into an upper portion and a lower portion. A conveying device  104  is arranged inside the hearth  103 . The conveying device  104  extends in the whole length direction of the hearth  103  and is configured to bear the circuit board and help to convey the circuit board in the whole hearth  103 . The above arrangement enables the reflow oven  100  to perform soldering and processing on the circuit board in upper and lower directions. When the reflow oven  100  starts to work, the circuit board is placed on the conveying device  104 , enters the reflow oven  100  from an inlet of the heating zone  101  in the direction indicated by the arrow in  FIG. 1 , and is conveyed in the heating zone  101  with the conveying device  104 . During conveying circuit board in the heating zone  101 , the heat in the heating zone  101  gradually melts solder paste distributed on the circuit board. After the circuit board is transferred from the heating zone  101  to the cooling zone  102  by the conveying device  104 , due to the low temperature in the cooling zone  102 , the solder paste on a soldering area of the circuit board is solidified due to being cooled. This causes an electronic element to be connected to the circuit board. After passing through the cooling zone  102 , the circuit board is transferred out of the reflow oven  100  by the conveying device  104 , and the reflow oven  100  completes the soldering and processing on the circuit board. In this embodiment, the conveying device  104  conveys the circuit board at a constant speed v in the reflow oven  100 , wherein the conveying speed of the conveying device  104  is 30-100 cm/min. In other embodiments, the conveying device  104  may also be set to other conveying speeds. 
     In this embodiment, the reflow oven  100  comprises two zones: the heating zone  101  and the cooling zone  102 . In other embodiments, the reflow oven  100  may be further provided with an isolation and exhaust zone between the heating zone  101  and the cooling zone  102 , and in this case, the heating zone  101 , the cooling zone  102  and the isolation and exhaust zone are also in fluid communication with one another. The arrangement of the isolation and exhaust zone can not only have a heat isolation function between the high-temperature heating zone  101  and the low-temperature cooling zone  102 , but can also extract gas from the hearth  103  of the heating zone  101  and discharge the extracted gas out of the hearth  103 , thereby preventing the gas containing volatile pollutants from the hearth  103  of the heating zone  101  from entering the hearth  103  of the cooling zone  102 . 
       FIG. 2  is a schematic diagram of the heating zone  101  of the reflow oven  100  in  FIG. 1 . For ease of illustration, compared with  FIG. 1 , the conveying device  104  is omitted from  FIG. 2 . As shown in  FIG. 2 , the heating zone  101  comprises a plurality of heating sub-zones  200 , and the plurality of heating sub-zones  200  are sequentially arranged in the length direction of the reflow oven  100 . The heating sub-zone located at the inlet position of the heating zone  101  is referred to as first heating sub-zone  201 , the N-th heating sub-zone counted from the inlet position of the heating zone  101  is referred to as N-th heating sub-zone, and so on. A front end of the first heating sub-zone  201  is provided with a position sensor  213  for sensing the position of the circuit board entering this zone. In this embodiment, there are twelve heating sub-zones  200  in total in the heating zone  101 , and each heating sub-zone  200  has the same length of H extending in the length direction of the reflow oven  100 . The twelve heating sub-zones  200  are sequentially arranged from left to right from the inlet position of the heating zone  101  as follows: the first heating sub-zone  201 , a second heating sub-zone  202 , a third heating sub-zone  203 , a fourth heating sub-zone  204 , a fifth heating sub-zone  205 , a sixth heating sub-zone  206 , a seventh heating sub-zone  207 , an eighth heating sub-zone  208 , a ninth heating sub-zone  209 , a tenth heating sub-zone  210 , an eleventh heating sub-zone  211 , and a twelfth heating sub-zone  212 . It is worth noting that the number of the heating sub-zones  200  of the reflow oven  100  may be changed according to products to be soldered, and is not only limited to the embodiment shown in  FIG. 2 . For example, in some other embodiments, for a certain type of circuit boards, ten heating sub-zones  200  may be provided in the reflow oven  100 . 
     As shown in  FIG. 2 , the hearth  103  runs through all the heating sub-zones  200  in the arrangement direction of the heating sub-zones  200 , and each heating sub-zone is divided into an upper portion and a lower portion by the hearth  103 . Both the upper portion and the lower portion of each heating sub-zone  200  are provided with a heating device  220 , and the upper and lower heating devices  220  cooperatively control the temperature in the heating sub-zone  200 . That is, two first heating devices  221  are provided in the first heating sub-zone  201 , and the two first heating devices  221  are located above and below the hearth  103  in the first heating sub-zone  201 , respectively. Two second heating devices  222  are arranged in the second heating sub-zone  202 , and the two second heating devices  222  are located above and below the hearth  103  in the second heating sub-zone  202 , respectively. in a similar fashion, two N-th heating devices  220  are provided in the N-th heating sub-zone  200 , and the two N-th heating devices  220  are located above and below the hearth  103  in the N-th heating sub-zone  200 , respectively, wherein N is a natural number less than or equal to 12. Corresponding to the twelve heating sub-zones  200 , there are totally twenty-four heating devices  220  provided in the heating zone  101  in this embodiment. 
     In order to ensure the processing effect of the circuit board in the heating zone  101 , the working temperature of each heating sub-zone  200  of the plurality of heating sub-zones  200  is in a predetermined temperature interval. The upper and lower heating devices  200  located in the same heating sub-zone  200  work cooperatively to keep the working temperature of each heating sub-zone  200  within the corresponding predetermined temperature interval thereof. In this embodiment, the heating devices  220  perform heating by using heating resistors, and each heating device  220  has an independent input interface, so that each heating sub-zone  200  can work independently, and different heating sub-zones  200  can satisfy different predetermined temperature intervals. In addition, each independent heating device  220  corresponds to an independent temperature control mode, such that the heating device  220  can be independently controlled. In this embodiment, the temperature control mode of the heating device  220  is as follows: the heating device  220  located at the upper portion of the heating sub-zone  200  is provided with a temperature sensor at the top of the hearth  103  corresponding to the heating sub-zone  200 , the heating device  220  at the upper portion stops heating when the temperature of the top of the hearth  103  is higher than the predetermined temperature interval of the heating sub-zone  200 , and the heating device  220  at the upper portion resumes heating when the temperature of the top of the hearth  103  is lower than the predetermined temperature interval of the heating sub-zone  200 ; and the heating device  220  located at the lower portion of the heating sub-zone  200  is provided with a temperature sensor at the bottom of the hearth  103  corresponding to the heating sub-zone  200 , the heating device  220  at the lower portion stops heating when the temperature of the bottom of the hearth  103  is higher than the predetermined temperature interval thereof, and the heating device  220  at the lower portion resumes heating when the temperature of the bottom of the hearth  103  is lower than the predetermined temperature interval of the heating sub-zone  200 . The provision of separately controllable heating devices  220  in the upper portion and the lower portion of the same heating sub-zone  200  is helpful to promote the uniform distribution of the temperature in the hearth  103  corresponding to each heating sub-zone  200 , and ensures that the heating devices  220  accurately control the temperature of the hearth  103  in the corresponding heating sub-zone  200 . 
     In this embodiment, the heating device  220  uses a heating resistor to heat the hearth  103 , and the working temperature of each heating sub-zone  200  is approximately 100-300° C. Since the heat absorption amount of small-sized circuit boards is small and the heating temperature rising rate is fast, while the heat absorption amount of large-sized circuit boards is large and the heating temperature rising rate is slow, in order to meet heating requirements of different sizes of circuit boards, the reflow oven  100  of the present disclosure has adjustable heating resistance power. Different resistance power of the reflow oven  100  can be implemented by adjusting the heating resistance. When processing large-sized circuit boards, the heating device  220  uses a high-power resistor for heating; and when processing small-sized circuit boards, the heating device  220  uses a low-power resistor for heating. However, when the power of the heating resistor is relative high, the relative high heating power will bring greater inertia of temperature rise. In this case, relying on only inherent temperature control logic of the heating device  220  will lead to the occurrence of overheating of the hearth  103 . Therefore, in order to meet requirements of processing circuit boards with different sizes, in the present disclosure, a start-stop device  300  is incorporated into the reflow oven  100 . The provision of the start-stop device  300  can implement additional control over the heating device  220  besides the temperature control logic of the heating device  220  itself. 
     When a small-sized circuit board such as a circuit board with a size of 200 mm*300 mm is processed, the resistance power of the heating device  220  of the reflow oven  100  is set to 3-5 kW. Since the thermal inertia of the heating device  220  is small under a low-power working condition, there is no need additional control to the heating device  220  in this case, and the requirements for the working temperature of each heating sub-zone  200  can be met by using only the temperature control logic of the heating device itself (that is, heating is stopped when the temperature in the hearth  103  corresponding to each heating sub-zone  200  is higher than the predetermined temperature interval thereof, and heating is resumed when the temperature is lower than the predetermined temperature interval thereof). 
     When a large-sized circuit board is processed, in order to improve the temperature rise efficiency of each heating sub-zone  200  to the circuit board, the heating device  220  in the reflow oven  100  is set to a higher resistance power. For example, when a circuit board becomes a 5G circuit board with the size increased to 600 mm*800 mm, the resistance power of the heating device  220  is increased to 8-10 kW, such as 9.5 kW. Under the temperature control logic of the heating device  220  itself, when the temperature sensor monitors that the temperature in the hearth  103  is at the upper limit of the predetermined temperature interval, the heating device  220  will stop the heating of the hearth  103  by the heating resistor of the corresponding heating sub-zone  200 . However, since the heating resistor of the heating device  220  works with higher heating power before the heating is stopped, in this case, even if the heating is stopped immediately, the residual heat of the heating resistor will still make the temperature of the hearth  103  rise. This makes the temperature of the hearth  103  exceed the predetermined heating interval of the corresponding heating sub-zone  200 . That is, when the reflow oven  100  works with relative high heating power, the large inertia of temperature rise will cause overheating of the hearth  103 , and the temperature control logic of the heating device  220  itself cannot meet requirements of processing of large-sized circuit boards. 
     In order to meet the requirements of processing of large-size circuit boards, in the present disclosure, an additional start-stop device  300  is incorporated into the reflow oven  100 . When the heating device  220  works with a high-power resistor, the start-stop device  300  enables the reflow oven  100  to stop a temperature control operation of the corresponding heating device  220  on the hearth  103  before the overheating of the hearth  103  in the heating zone  101  occurs. Therefore, in the reflow oven  100  of the present disclosure, even under the working condition of using high-power resistors, the control temperature of the hearth  103  can always meet the working temperature requirements of the processed circuit board for each heating sub-zone  200 , thereby avoiding overheating of the hearth  103 . 
       FIG. 3  is a schematic diagram showing the control of the plurality of heating devices  220  by the start-stop device  300  of the reflow oven  100  in  FIG. 1 . As shown in  FIG. 3 , the start-stop device  300  communicates with the position sensor  213 , such that the start-stop device  300  can receive signals from the position sensor  213 . Besides, the start-stop device  300  further communicates with the heating devices  220  in the plurality of heating sub-zones  200 , wherein the heating devices  220  comprise the first heating devices  221  to the twelfth heating devices  232 . Since the temperature of the hearth  103  in the same heating sub-zone  200  is controlled by two heating devices  220  at the same time, the start-stop device  300  controls two heating devices  220  installed in the same heating sub-zone  200  in a consistent way. That is, the start-stop device  300  can implement the start-stop control over the two heating devices  220  at the same time. The start-stop control includes two control modes: stopping the operation of the heating device  220  and resuming the operation of the heating device  220 . When the operation of the heating device  220  is stopped, the heating device  220  immediately stops heating and no longer works according to the original temperature control logic of the heating device  220 . In some embodiments, when the operation of the heating device  220  is stopped, the temperature sensor corresponding to the heating device  220  can continue to monitor the temperature at the corresponding position in the hearth  103 , but even the temperature in the hearth  103  is lower than a predetermined temperature interval at this time, the heating device  220  will not re-activate the heating resistor for heating. When the operation of the heating device  220  is resumed, the heating device  220  activates to work according to the original temperature control logic. In this case, if the temperature of the corresponding hearth  103  monitored by the temperature sensor is lower than the predetermined temperature interval thereof, the heating resistor immediately resumes heating. If the temperature of the corresponding hearth  103  monitored by the temperature sensor is higher than the predetermined temperature interval thereof, the heating resistor is still kept in a stopped state and will not resume to the heating state until the temperature of the corresponding hearth  103  is lower than the predetermined temperature interval thereof. 
       FIG. 4  is a schematic diagram of an internal structure of the start-stop device  300  in  FIG. 3 . As shown in  FIG. 4 , the start-stop device  300  comprises a processor  401 , an input interface  402 , an output interface  403  and a memory  404 . The input interface  402  is configured to receive signals from the position sensor  213 , the output interface is configured to send control signals to the first heating devices  221  to the twelfth heating devices  232 , the memory  404  is configured to store control programs of the start-stop device  300  and signals received by the start-stop device  300 , and the processor  401  can process the signals received by the input interface  402  and run the control programs stored in the memory  404  in response to the signals from the input interface  402 . 
       FIGS. 5A to 5D  show an embodiment in which the operation of the heating devices  221  is stopped by using the start-stop device  300  in  FIG. 3 . As shown in  FIG. 5A , a circuit board  501  placed on the conveying device  104  (not shown in  FIGS. 5A to 5D ) just enters the heating zone  101 . When a front end of the circuit board  501  just enters an inlet of the heating zone  101 , the position sensor  213  sends a counting time signal to the input interface  402  of the start-stop device  300  upon monitoring the circuit board  501 . After receiving the timing signal, the processor  401  starts counting time, and in this case the time is recorded as t=0. Subsequently, the circuit board  501  enters the first heating zone  201  with the conveying device  104  at a constant speed v. 
     When a cumulative courting time t reaches t 1 , the circuit board  501  enters a position shown in  FIG. 5B , and the output interface  403  of the start-stop device  300  sends a stop signal to the first heating devices  221  to stop the operation of the first heating devices  221 . In this case, the heating resistor of first heating device  221  stops working and no longer works according to the original temperature control logic of the first heating device  221 . As shown in  FIG. 5B , in this embodiment, when the cumulative counting time t reaches t 1 , an rear end of the circuit board  501  just enters the inlet of the first heating zone  201 , and the whole circuit board  501  completely enters the reflow oven  100 . Within the time interval t 1 , the circuit board is conveyed by a distance equal to its own length L, that is, t 1 =L/v. 
     When the cumulative counting time t reaches t 1 +Δt 2 , the circuit board  501  enters a position shown in  FIG. 5C , and the output interface  403  of the start-stop device  300  sends a stop signal to the second heating devices  222  to stop the operation of the second heating devices  222 . In this case, the heating resistor of second heating device  222  stops working and no longer works according to the original temperature control logic of the second heating device  222 . As shown in  FIG. 5C , in this embodiment, when the cumulative counting time t reaches t 1 +Δt 2 , the rear end of the circuit board  501  just enters an inlet of the second heating zone  202 . Within the time interval ≢t 2 , the circuit board is conveyed by a distance equal to the extending length H of the first heating sub-zone  201 , that is, Δt 2 =H/v. 
     When the cumulative counting time t reaches t 1 +2*Δt 2 , the circuit board  501  enters a position shown in  FIG. 5D , and the output interface  403  of the start-stop device  300  instantly sends a stop signal to the third heating devices  223  to stop the operation of the third heating devices  223 . In this case, the heating resistor of third heating device  223  stops working and no longer works according to the original temperature control logic of the third heating device  223 . As shown in  FIG. 5D , in this embodiment, when the cumulative counting time t reaches t 1 +2*Δt 2 , the rear end of the circuit board  501  just enters an inlet of the third heating zone  203 . Within the time interval 2*Δt 2 , the circuit board in conveyed by a distance equal to the extending length of the first heating sub-zone  201  and the second heating sub-zone  202 . 
     In a similar fashion, when the cumulative counting time t reaches t 1 +(N−1)*Δt 2 , the output interface  403  of the start-stop device  300  sends a stop signal to the N-th heating devices  220  to stop the operation of the N-th heating devices  220 . In this case, the heating resistor of each N-th heating device  220  stops working and no longer works according to the original temperature control logic of the N-th heating device  220 . 
     The time interval for the start-stop device  300  to control the heating device  220  to stop operation can be determined according to the size of the circuit board. Since the large-sized circuit board absorbs more heat, while the small-sized circuit board absorbs less heat, different sizes of circuit boards have different heat requirements. The start-stop device  300  used in the reflow oven  100  of the present disclosure can be suitable for soldering and processing circuit boards with different sizes. In order to match the heat absorbed by circuit boards with different sizes, in other embodiments, the start-stop device  300  may also use other time interval modes to sequentially control the first heating devices  221  to the twelfth heating devices to stop the operation thereof, that is, in the equation t=t 1 +(N−1)*Δt 2  expressing the cumulative counting time at which the N-th heating device  220  is controlled to stop operation, t 1  and Δt 2  may be set to other suitable values. 
       FIGS. 6A and 6B  show an embodiment in which the operation of the heating devices  221  is resumed by using the start-stop device  300  in  FIG. 3 . As the cumulative time is counted by the processor  401 , when the cumulative time t reaches t 1 +Δt 3 , the circuit board  501  enters a position shown in  FIG. 6A . In this case, the output interface  403  of the start-stop device  300  sends a re-activate signal to the first heating device  221  to resume the operation of the first heating devices  221 , such that the first heating devices  221  work according to the original temperature control logic thereof. As shown in  FIG. 6A , in this embodiment, when the cumulative counting time t reaches t 1 +Δt 3 , the rear end of the circuit board  501  just enters an inlet of the third heating zone  203 . Since the rear end of the circuit board just enters the inlet of the first heating zone  201  when the cumulative counting time t is t 1 , within the time Δt 3 , the circuit board is conveyed by a distance equal to the extending lengths  2 H of two heating sub-zones  201 , that is, Δt 3 =2H/v. 
     When the cumulative counting time t reaches t 1 +2*Δt 3 , the circuit board  501  enters a position shown in  FIG. 6B . In this case, the output interface  403  of the start-stop device  300  sends a re-activate signal to the second heating devices  222  to resume the operation of the second heating devices  222 , so that the second heating devices  222  work according to the original temperature control logic thereof. As shown in  FIG. 6B , in this embodiment, when the cumulative counting time t reaches t 1 +2*Δt 3 , the rear end of the circuit board  501  just enters an inlet of the fifth heating sub-zone  205 . Within the time interval 2*Δt 3 , the circuit board is conveyed by a distance equal to the extending lengths  4 H of four heating sub-zones  200 . 
     In a similar fashion, when the cumulative counting time t reaches t 1 +N*Δt 3 , the output interface  403  of the start-stop device  300  sends a re-activate signal to the N-th heating devices  220  to resume the operation of the N-th heating devices  220 . In this case, the N-th heating devices  220  work according to the original temperature control logic thereof. The time interval for the start-stop device  300  to control the re-activate of the heating device  220  may be determined according to the size of the circuit board. In other embodiments, the start-stop device  300  may also use other time interval modes to sequentially control the first heating devices  221  to the twelfth heating devices to re-activate the operation thereof. For example, in the equation t 1 +N*Δt 3  expressing the time interval during which the N-th heating devices  220  are re-activate, Δt 3  is set to Δt 3 =m*H/v, wherein 1≤m&lt;N. In some embodiments, the start-stop device  300  sets Δt 3  to be always greater than Δt 2 , thereby ensuring that a moment at which the operation of a heating device  220  is resumed is always later than a moment at which operation of the heating device  220  is stopped. 
     In order to ensure the working efficiency of soldering and processing, when circuit boards with large heat absorption (large surface area and large size) are soldered, it is necessary to use heating devices with large power, but the problem lies in: because of the high power, the inertia of temperature rise of heating is large, and it is not easy to control the temperature of each heating sub-zone within the preset temperature interval thereof. One of the technical effects of the present disclosure is: according to the heat absorption and the speed of a heating board traveling in the hearth, the overheating in the hearth is prevented by the method of time-based start-stop, so as to ensure the normal soldering processing of the large-sized circuit boards. 
     The present disclosure has another technical effect that the reflow oven  100  of the present disclosure is suitable for processing circuit boards with different heat absorption amount, and can be applied to both circuit boards with larger heat absorption amount (larger surface area and larger size) and circuit boards with smaller heat absorption amount(smaller surface area and smaller size). Specifically, in the reflow oven  100  of the present disclosure, when the heating device  220  with higher power is selected to work in the heating zone  101 , the heating device  220  generates an enough amount of heat to heat the circuit board with higher heat absorption amount, and when the heating device  220  with lower power is selected to work, the amount of heat generated by the heating device  220  is suitable for heating the circuit board with lower heat absorption amount. When a small-sized circuit board is soldered, the heating device is activated or deactivated according to the temperature control logic of the heating device  220  itself, such that the hearth temperature does not exceed the predetermined temperature range; on the other hand, when a large-sized circuit board is soldered, it is necessary to use the start-stop device  300  to perform additional start-stop control on the heating device  220  in addition to the temperature control logic of the heating device  220  itself, such that the temperature of the hearth does not exceed the working temperature range. Therefore, the start-stop solution of the present invention can properly control the heating inertia of the heating device  220  under high-power working conditions, and is also suitable for processing large-sized circuit boards and small-sized circuit boards. In addition, since the additional control over the heating device  220  by using the start-stop device  300  in the present disclosure is achieved by using time intervals, instead of comparing temperature parameters acquired by using the temperature sensor, to control the activation and deactivation of the heating device  220 , this method for controlling the reflow oven  100  by incorporating an additional start-stop device  300  therefore implements a simple structure and reliable control.