Abstract:
A method and apparatus for heat treating objects in an enclosure, where the enclosure has an inlet passage, an outlet passage, and a treatment zone. Gaseous first fluid is injected into the treatment zone and the objects to be treated are passed through the treatment zone. A second fluid, with a density greater than that of the first fluid, is injected near the inlet and outlet passages.

Description:
BACKGROUND 
   The present invention relates to a process for the heat treatment of a series of objects in an enclosure, situated in a surrounding gaseous fluid, particularly air, and defining an internal treatment zone, the said enclosure comprising at least one entrance passage and at least one exit passage, the said passages being provided between the treatment zone and the exterior of the enclosure; the process being of the type that comprises the following steps:
     (a) the treatment zone is heated;   (b) a gaseous treatment fluid is injected into the treatment zone;   (c) the said objects are transported through the said treatment zone; and   (d) a gaseous isolating fluid is injected in the vicinity of at least one of the said passages.   

   This process applies to the assembling of electronic boards by soldering. 
   In a process of the abovementioned type (WO 91/11284), components to be assembled are placed on a board in areas covered with a solder cream. The board is then introduced into a oven where it undergoes a thermal cycle which carries out the soldering. 
   To improve the quality of this soldering and in particular avoid defects on boards of complex structure caused by the small size of the components, a stream of inerting nitrogen is injected into the oven. The oxygen concentration in the oven is thus brought down below a limit value of a few hundred ppm (parts per million). This operation, which can be described as “inerting”, under a nitrogen atmosphere, prevents in particular oxidation of the parts to be soldered during the heating. 
   Such processes are not completely satisfactory. Specifically, in order to keep the oxygen content below a few hundred ppm in the oven, and thereby maintain the quality of the soldering, a high nitrogen flow rate is required (some 20 to 40 m 3 /h) 
   SUMMARY 
   The main object of the invention is to solve this problem, that is to create a heat treatment process that performs a high quality treatment at low cost. 
   To this end, the subject of the invention is a process of the abovementioned type, characterized in that, in step (d), the density of the said gaseous isolating fluid at the temperature found in the one or more of the said passages is greater than the density of the gaseous treatment fluid inside the oven, more particularly in the vicinity of the one or more of the said passages. 
   The process according to the invention may comprise one or more of the following features, taken in isolation or in all technically possible combinations:
         the density of the said gaseous isolating fluid at the temperature found in the one or more of the said passages is approximately equal to the density of the surrounding gaseous fluid at the temperature found outside the enclosure.   the density of the said isolating gaseous fluid at the temperature found in the one or more of the said passages is greater than the density of the surrounding gaseous fluid at the temperature found outside the enclosure.   in step (d), the density of the gaseous isolating fluid is controlled as a function of the temperature found at at least one point in the enclosure.   in step (d), the said density is controlled by producing the isolating fluid from at least two sources of auxiliary gaseous fluid, the density of at least one of the auxiliary gaseous fluids, measured at a given temperature, being greater than the density of the gaseous treatment fluid measured at the said given temperature.       

   The present invention also relates to an apparatus for the heat treatment of objects, of the type that comprises:
         an enclosure, intended to be located in a surrounding gaseous fluid, particularly air, and defining an internal treatment zone; the said enclosure comprising at least one entrance passage and at least one exit passage, the said passages being provided between the treatment zone and the exterior of the enclosure;   means for heating all or part of the treatment zone;   means for transporting the said objects through the said treatment zone;   means for injecting a gaseous treatment fluid into the treatment zone; and   means for injecting a gaseous isolating fluid into at least one of the said passages;   which is characterized in that the said means for injecting a gaseous isolating fluid comprise a source of gaseous isolating fluid whose density at the temperature found in the one or more of the said passages is greater than the density of the gaseous treatment fluid inside the oven, in the vicinity of the one or more of the said passages.       

   The apparatus according to the invention may also include one or more of the following features:
         the density of the gaseous isolating fluid at the temperature found in the one or more of the said passages is approximately equal to the density of the surrounding gaseous fluid at the temperature found outside the enclosure.   the density of the said gaseous isolating fluid at the temperature found in the one or more of the said passages is greater than the density of the surrounding gaseous fluid at the temperature found outside the enclosure.   it also includes means for controlling the density of the gaseous isolating fluid, operated by means for measuring the temperature found at at least one point in the enclosure.   the control means comprise means of injecting at least one auxiliary gaseous fluid.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein: 
       FIG. 1  illustrates a schematic, mid-plane sectional view of one embodiment of an apparatus according to the present invention; and 
       FIG. 2  illustrates a detail view of  FIG. 1 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The heat-treatment apparatus  11  illustrated in  FIGS. 1 and 2  is designed for assembling electronic boards  12  in a continuous reflow soldering process. 
   This apparatus  11  comprises a soldering oven  13 , a conveyor  15 , and gaseous barriers  17 A and  17 B located at the entrance and exit, respectively, of the soldering oven  13 . 
   Throughout the remainder of this text, the terms “upstream”, “downstream” and “upper” refer to the direction of movement of the electronic boards  12  on the conveyor  15  (from left to right in  FIG. 1 ). 
   The soldering oven  13  comprises an enclosure  21  and, arranged inside the enclosure  21 , a treatment gas injector  23  and heating means  25 . 
   The enclosure  21  extends along a first longitudinal axis X-X′ between an upstream end  27  and a downstream end  29 . The walls of this enclosure  21  define an internal heat-treatment zone  31 . 
   Upstream and downstream passages  32  and  33  are provided within the enclosure  21  at the upstream and downstream ends  27  and  29 , respectively, of the enclosure  21 . These passages  32  and  33  connect the exterior of the enclosure  21  to the treatment zone  31 . 
   The enclosure  21  is situated in a surrounding gas comprising at least one oxidizing gas, particularly oxygen. In the example which follows, the surrounding gas is air at room temperature. 
   The treatment gas injector  23  feeds into the enclosure  21 . This injector  23  comprises a source  35  of treatment gas (in the present case an inerting gas) and a pipe  37  which admits this gas into the treatment zone  31  and has a valve  39 . 
   The treatment gas contained in the source  35  may be, by way of illustration, nitrogen, helium, hydrogen, argon, carbon dioxide or mixtures of these. According to the invention, helium-based mixtures that have a higher coefficient of thermal conductivity than air or nitrogen are of more particular interest. 
   By injecting the inerting gas into the enclosure  21 , the oxygen concentration within the treatment zone  31  can be kept at a value below a few hundred ppm. 
   The heating means  25  are mounted in a stationary position on an upper wall  41  of the enclosure  21 , in the treatment zone  31 , facing the conveyor  15 . In the present case, they comprise at least one convective heating member  43 . 
   The conveyor  15  comprises a transport belt  45  that passes longitudinally through the enclosure  21  along the axis X-X′, and extends between an upstream end  47  and a downstream end  49 . The upstream and downstream ends  47  and  49  of the belt  45  project out of the enclosure  21  to allow the electronic boards  12  to be loaded and unloaded. 
   At its ends  47  and  49 , the belt  45  passes around drive rollers  51  and  53 . It is thus able to move translationally along the axis X-X′ from upstream to downstream. 
   The boards  12  are placed on the belt  45  at regular intervals between its upstream end  47  and its downstream end  49 . 
   Each board  12  comprises a support  63  and a component  65  that is to be mounted on this support, resting on a layer  67  of solder paste applied to the support  63 . 
   The upstream and downstream gaseous barriers  17 A and  17 B are located at the respective upstream and downstream ends  27  and  29  of the enclosure  21 . 
   The upstream barrier  17 A and the downstream barrier  17 B are analogous in structure. Only the upstream barrier  17 A will therefore be described below. 
   The upstream barrier  17 A comprises an injector  71  connected to two gas sources  73  and  75 , a temperature probe  77 , and control means  79 . 
   The injector  71  comprises an admission pipe that feeds into the upstream passage  32  of the enclosure  21 . The admission pipe is also connected to gas sources  73  and  75 . 
   In the embodiment illustrated here, the first gas source  73  contains a gas whose density at a given temperature is less than or equal to the density of air, at this given temperature. The second gas source  75  contains a gas whose density, at a given temperature, is greater than the density of air at this given temperature. 
   The gas contained in the first source  73  is preferably nitrogen. Examples of gases contained in the second source  75  are carbon dioxide and argon. 
   Each gas source  73 ,  75  is connected to the injector  71  by a pipe equipped with a control valve  81 ,  83  for controlling the flow rate and composition of the isolating gas delivered to the injector  71 , and so controlling the density of this isolating gas. 
   The temperature probe  77  is positioned in the upstream passage  32 , in the vicinity of the injector  71  outlet. 
   The control valves  81  and  83  and the temperature probe  77  are connected electrically to the control means  79 , so that the flow rate and composition of the isolating gas delivered by the gaseous barrier is controlled as a function of the temperature measured by the probe  77 . 
   Of course, although the illustration here is of a structure for injecting isolating gas into the entrance/exit passages arriving through the bottom only of the passages, other arrangements are possible and often practised including overhead injection, or injection into both the top and bottom of each passage. 
   An example showing the operation of the apparatus  11  according to the invention during an electronic board  12  assembly operation will now be described below. 
   The heating means  25  are first activated in order to establish a soldering temperature in the treatment zone  31 . 
   In addition, the treatment (inerting) gas is injected into the oven through the injector  23  in order to reduce oxygen levels in the enclosure  21  to less than a few hundred ppm. 
   As the temperature in the enclosure  21  rises, or once the temperature in the enclosure  21  is stabilized, the upstream and downstream gaseous barriers  17 A and  17 B are activated. 
   The operation of the upstream barrier  17 A will now be described. The operation of the downstream barrier  17 B is analogous to the operation of the upstream barrier  17 A. 
   The temperature at the upstream end  27  of the inerting zone  31  is measured by the probe  77 . Each of the control valves  81  and  83  is actuated by the control means  79 . The composition of the isolating gas injected by the injector  71  of the upstream barrier  17 A is thus controlled in such a way that this gas possesses a density, at the temperature found in the upstream passage  32  of the enclosure  21 , it is approximately equal to the density of air at room temperature on the outside of the soldering oven  13 . 
   The conveyor  15  is then started up and boards  12  are deposited at regular intervals on the belt  45 . 
   As illustrated in  FIG. 2 , the densities of the isolating gas in the upstream barrier  17 A and of the surrounding gas on the outside of this barrier  17 A are approximately equal, so the displacement of the gaseous fluid from the barrier  17 A in the outward direction is approximately laminar and parallel to the horizontal axis X-X′. 
   This movement ensures that air does not enter the enclosure  21 , as would occur if the relatively hotter isolating gas passing out of the barrier  17 A were less dense than the air at room temperature, particularly when a board  12  enters the enclosure  21 . 
   Subsequently, the flow rate of isolating gas injected into the upstream barrier  17 A may be appreciably reduced. 
   The boards  12  are then conveyed towards the downstream end  29  of the enclosure  21  and undergo heat treatment as they pass the heating means  25 . 
   The boards  12  then pass through the downstream barrier  17 B and are unloaded from the belt  45  after cooling. 
   As stated earlier, the operation of the downstream barrier  17 B is analogous to that of the upstream barrier  17 A. 
   Similarly, the composition of the isolating gas delivered by the downstream barrier  17 B is controlled in such a way that this gas has a density, at the temperature found in the downstream passage  33 , that is approximately equal to the density of the air at room temperature on the outside of the soldering oven  13 . 
   Similarly according to the invention, the composition of the isolating gas delivered by the upstream and downstream barriers is so controlled that this gas has a density, at the temperature found in the passages, that is greater than the density of the inerting gas at the upstream or downstream end  27  or  29  of the enclosure  21 , at the temperature found at this end  27  or  29 . 
   As an illustration, the gases used in the different zones of the installation are as follows:
         for the inerting gas: thermally advantageous mixtures such as helium or helium-based mixtures, mixtures containing hydrogen such as nitrogen/hydrogen mixtures;   for the isolating gas: higher-density gases or mixtures such as CO 2 , CO 2 -based mixtures such as He—CO 2 , N 2 —CO 2 , Ar—CO 2 , etc.       

   The invention described above gives access to a very economical high-quality heat treatment process. In particular, the process considerably limits the wastage and therefore the consumption of inerting gas, which is particularly advantageous, not to say necessary, where using thermally advantageous but expensive inerting gases. 
   It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.