INTEGRATES A DRY-WAY PRE-TREATMENT OPERATION The present invention relates to surge welding operations, for electronic circuits. Therefore, it is especially applied to the following operations: - wave welding, to fix the components in a circuit (both in the case of inserted components and components mounted on the surface); - Welding by waves, to fix contact bands on electronic supports, which allows the connection of the support in question, on another support (in this case, it can include the case of a hybrid circuit or a printed circuit which will proceed to insert, through these contacts, in a printed circuit, or also the case of a hybrid or printed circuit that can be plugged in, thanks to this comb, into a connector). The role of surface pre-conditioning is then to prepare the metal surfaces to be welded (performing operations such as degreasing, deoxidation, decontamination of the adsorbed layers, or other preparation of the surface), this, in order to facilitate wetting further processing of said surfaces by welding, but also to remove the oxides that could be formed at the temperature of the solder alloy. Most often, this pre-conditioning operation is carried out by chemical products, often obtained from resinous bases specially supplemented by acidic compounds. After welding, some residues of the chemical used remain on the piece, which often forces the manufacturer to carry out a cleaning operation, usually with chlorinated components, which is the object of intense controversy as it falls within the scope of the "Montreal Protocol" and its subsequent revisions, in which these solvents are the object of a very strict regulation that for some countries amounts to a plain and simple prohibition. The design of the wave welding machines is such that the pieces to be welded are put in contact with one or several waves of liquid welding material, obtained by circulating the bath of the welding material contained in a container, by means of one or several nozzles. Previously, the pieces (for example the electronic circuits on which the components have been presented in their final position, without fixing them), will have been generally submitted to a superficial pre-treatment in an area located upstream (from the production line) with respect to the welding machine, by means of a spray or spraying of the chemical or with a chemical foam, said surface pre-treatment operation being, generally followed by a pre-heating operation which is performed to activate the chemical products previously deposited on the circuit and to preheat the circuits before their arrival in the hot zone of the soldier. A transport system is used to move the pieces from one area to another of the machine.
The problem of cleaning objects after welding, has been the cause of numerous efforts worldwide in recent years, in order to propose a solution that allows to abandon the use of these compounds. Among the possible solutions considered, mention may be made of the dry surface pre-treatment method such as the pretreatment of the surfaces with plasma, before their welding, thereby avoiding the use of chemical products and therefore the need for a proper cleaning operation downstream in the production line. The mixtures considered include especially hydrogen. In this field, it is worth mentioning the document EP-A-0427020, in which it is proposed to carry out a treatment of the pieces to be assembled by welding, by means of the plasma of a process gas (synthetic or semi-synthetic), recommending the use of low pressures for this treatment, "in order to avoid thermal damage in the pieces to be assembled". All the examples provided, in relation to the figures shown, refer to pressure conditions in the range of 30 to 100 Pa. The same observation is made for EP-A 0371693, which refers to a procedure for the pre -treatment of metal surfaces before welding, by micro-wave plasma containing hydrogen. Also in this case it is recommended to use low pressures "in order to limit the level of residual oxygen in the plasma". This unanimity regarding the use of low pressure conditions to perform these surface pre-treatments with plasma, despite the drawbacks, especially linked to the cost of obtaining these pressures or also the difficulty to implement the corresponding infra-structures In an industrial production line, it is undoubtedly linked to the technical and technological difficulty in obtaining plasmas at atmospheric pressure that allow obtaining performances comparable to those traditionally obtained at low pressure. In this context, in the document FR-A-2,697,456 the Applicant had proposed a procedure of surface pre-treatment with plasma for the metal surfaces before welding, at atmospheric pressure, being used to create the plasma, a source of micro-waves or also a discharge with dielectric barrier transferred by means of lights placed in a suitable way in a dielectric layer placed above the piece to be treated. Although this document represents an interesting answer to the problem in question, the Applicant has highlighted the fact that the proposed procedure could be improved, especially as regards the following: - its performance (relation between the power introduced to create the plasma and the density of the species produced that interact effectively with the support to be treated), or also the admissible density of the energy (in the case of discharge with dielectric barrier, it only reaches a few W per cm of dielectric material); if it were possible to raise these indexes, it would be possible to shorten the treatment times; - and it would also be possible to influence the limiting "geometric" factors: in the case of corona discharge, the distance between the electrode and the sample is of critical importance and must be kept at a very low value, which can be problematic in the case of substrates whose relief is relatively irregular; in the case of microwave discharge, it gives rise to the formation of a localized plasmogen point, whose dimensions are determined and limited by the source of the plasma; - on the other hand, a plasma such as the one created in said document, contains, by definition, ionic species and electrons (therefore, they are electrically charged species), whose use on electronic components is always delicate. By continuing its investigations on this subject, the Applicant has more recently proposed, in the French Patent Application filed on December 15, 1993 under the number FR-9315112, an improved procedure for the surface pre-treatment by dry route of a surface metal before welding or tinning by means of an alloy, a process which is remarkable in that it comprises the following steps: a) an initial gas mixture comprising an inert gas and / or a reducing gas and / or an oxidizing gas is passed through, at least one apparatus for forming excited or unstable gaseous species, in order to obtain, at the outlet of the apparatus, a primary gas mixture; b) the surface to be pre-treated is subjected to a pressure close to atmospheric pressure, by means of a gaseous atmosphere of treatment comprising excited or unstable species and that is substantially devoid of electrically charged species, obtained from the gas mixture considered primary The examples developed in said document clearly demonstrated that it is specifically possible, thanks to said procedure: operating at a pressure which is appreciably atmospheric pressure; - obtain a great flexibility as regards the distances between the object to be treated and the device used to carry out said treatment; - avoid contact of the pieces with the loaded species; - offer a better energy density, which allows to increase the speed of the treatment. On the other hand, according to said document, the treatment atmosphere obtained from the primary gas mixture, in turn obtained at the exit of the gas from an apparatus for forming gaseous, excited or unstable species, may also comprise, if necessary, an adjacent gas mixture that has not passed through the appliance. Therefore, said configuration can be classified as "post-discharge", in that the primary component of the treatment atmosphere, which comprises excited or unstable gaseous species, is obtained at the outlet of the apparatus, which ensures the substantial absence of any electrically charged species in said primary component. The adjacent component of the treatment atmosphere, which has not passed through the apparatus, is a fortiori devoid of any electrically charged species. On the other hand, this configuration allows to clearly separate the place of the generation of the primary component of the atmosphere, with respect to the place of its use, which presents an advantage not negligible in terms of pollution created by the apparatus (to prevent the various emanations resulting from the operation of the surface pre-treatment will contaminate the interior of the apparatus, for example its electrodes), but also the advantage of the reproducibility of the atmosphere found by the piece. Finally, the part, which is not treated inside the apparatus (for example, in the environment of the discharge between the electrodes), benefits from a much higher flexibility in terms of the "distance" aspect, already mentioned in what precedes In said document, said method had been exemplified and illustrated in detail by means of a particular device for the formation of excited or unstable gaseous molecules, operating substantially at atmospheric pressure, a device which the Applicant had previously developed and which is described in the document. FR-A-2,692,730.
The Applicant has continued his investigations on this subject, in order to further improve the process of pre-treatment by dry dry surface, of a metal surface before its welding or tinning according to the document FR-9315112, and in particular its adaptation to the specific case of wave welding, in which case sometimes double-sided circuits must be welded, provided with electronic components of the "surface-mounted" type and the "inserted-to-wire" type. These works have shown that in the absence of chemical surface pre-treatment, replaced by a dry surface treatment, the welding of the wire components, presented significant difficulties, which eventually lead to the presentation of defects in the rise of the welding in the metallized holes. Said works have demonstrated that in the case of wave welding, it was particularly advantageous to put, upstream of the weld pool, each of the surfaces of the circuit, with the primary gaseous mixture obtained at the outlet of the gas. at least one apparatus for forming excited or unstable gaseous molecules, the upper face of the circuit being preferably placed in contact with the gas outlets of at least two excited or unstable gaseous molecule-forming apparatuses arranged in series. The wave soldering process of a circuit, according to the present invention, during which the circuit is contacted with at least one surge of liquid solder alloy, obtained by pumping a liquid alloy bath contained in A container, by means of a nozzle or nozzle, is characterized in that, prior to said putting into contact, each of the faces of the circuit has been brought, at a pressure close to atmospheric pressure, against the gas outlet of the circuit. at least one apparatus for producing excited or unstable gaseous species in which at least one apparatus is passed through an initial gas mixture comprising an inert gas and / or a reducing gas and / or an oxidizing gas, so as to obtain, after the transformation, at the outlet of the gas from the apparatus, a primary gas mixture comprising excited or unstable gaseous species, which gaseous mixture is substantially devoid of species electrically charged. It is advantageous if the upper face of the circuit is successively carried out in front of the gas outlets of at least two devices for producing excited or unstable gaseous species, arranged in series. The "circuit" according to the invention may consist of any assembly that may be involved in said welding operations, whether it is for example an electronic circuit on which the components have been attached for the purpose of welding them (whether or not it treats of components inserted like of components mounted in surface), as if it treats of a coupe: band of contacts / electronic support (welding of the band on the support). The "alloy" according to the invention will be constituted by any composition that can be used for an operation of this type, such as for example Sn-Pb, Sn-Pb-Ag, etc. The expression "pressure close to atmospheric pressure" according to the invention refers to a pressure advantageously situated in the range of [0.1 x 10 Pa, 3 x 10 Pa], therefore between 0.1 and 3 bar. The term "electrically charged species" according to the invention refers to ions or electrons. The treatment atmosphere found in the circuit, obtained from the primary mixture, therefore differs from the plasma atmospheres used in the prior art literature, due to the fact that it is substantially devoid of electrically charged species, it is say, of ions or electrons. The atmosphere for the treatment of the circuitIt may also comprise, if necessary, an adjacent gas mixture, obtaining the primary gaseous mixture at the exit of the gases from the device forming excited or unstable gaseous species in which it has been obtained from the primary gas mixture. The initial gas mixture has been transformed and the adjacent gas mixture itself has not passed through the apparatus. As already described in the paragraph dedicated to document FR-9315112, this configuration can be considered as "post-discharge", with all the advantages that result from it and that have already been listed. The inert gas can for example consist of nitrogen, argon, helium or a discharge of said inert gases. The reducing gas can, for example, consist of hydrogen, CH 4 or also ammonia or a mixture of said reducing gases. The oxidizing gas itself may consist of oxygen, or C02, or also N20, H20, or a mixture of said oxidizing gases. Of course, the list of gases given in each category is only indicative, not limiting. The apparatus according to the invention is constituted by any device that allows to "excite" an initial gaseous mixture, so as to obtain at the gas exit from the apparatus, another gaseous mixture (which in this case is qualified as "primary") , which comprises unstable or excited gaseous species, said last gas mixture being substantially devoid of electrically charged species. An excitation of this type may be obtained, for example, by means of an electric discharge, for example of the corona discharge type. FR-A 2,692,730, in the name of the Applicant, already mentioned, describes an apparatus for the formation of gaseous molecules, excited or unstable, suitable for carrying out the process according to the present invention, in practice. The adjacent mixture, according to the invention, may be constituted by any gas, or mixture of gases, for example an inert gas or a mixture of inert gases, which allows to maintain, if necessary, a protective atmosphere around the samples, or it may also be constituted by a reducing gas or by an oxidizing gas, and even by a mixture of gases belonging to one of these three categories. According to one aspect of the invention, the adjacent mixture contains SiH4 silane. The presence of an adjacent mixture of this type, which contains silane, is of advantageous use due to its reducing action with respect to certain metallic oxides present on the surface of the piece to be treated, but also, according to the gas mixture Initially used as "capturing" or "sequestering" (guetter or piegueur) of oxygen, that is, in its interaction with the residual oxygen of the atmosphere present above the circuit, in order to reduce, when it is considered necessary, said level of residual oxygen, to a minimum. According to another aspect of the present invention, during the time in which the circuit is in front of the gas outlet from at least one of said apparatuses, the circuit is brought to a temperature comprised between the ambient temperature and the temperature of fusion of the alloy used to carry out the subsequent operation of welding by waves. This high limit will therefore depend on the use used; it will be for example in the vicinity of 180 ° C in the case of the alloys Sn63-Pb37 or Sn62-Pb36-Ag2, of classic use. It is advantageous, and depending on the case of each type of piece or support subjected to treatment, in order to limit the increase of the thermic, try to adopt a temperature that is not too close to the melting temperature of the alloy used; for example, it is not advisable to exceed 160 ° C in the case of alloys Sb63-Pb37 or Sn62-Pb36-Ag2. According to one of the aspects of the invention, the circuit is brought to the front of the gas exits from the apparatuses, by means of a transport system that crosses an interior space delimited by an enclosing structure (for example, a tunnel or a set of elementary enclosures), isolated with respect to the surrounding atmosphere, the structure being connected in a sealed manner to the apparatuses, or with the apparatuses included within the structure. According to one of the embodiments of the invention, at least one of the apparatuses, in which an initial gas mixture is transformed, is the seat of an electric discharge created between a first electrode and a second electrode, there being a a layer of a dielectric material disposed on the surface of at least one of the electrodes, with respect to the other electrode; The initial gaseous mixture transformed in said apparatus crosses the discharge transversely with respect to the electrodes. The energy applied in the apparatus, referred to the surface unit of dielectric material, will then be advantageously greater than or equal to 1 W / cm, preferably greater than or equal to 10 W / cm, and will more often be comprised in the range [10]. W / cm, 100 W / cm2]. According to one of the embodiments of the invention, the initial gaseous mixture transformed into at least one of the apparatuses comprises steam; in this case, it is advantageous that the water vapor content of the initial gas mixture considered to comprise water vapor is in the range [50 ppm, 6%], preferably in the range [100 ppm, 1%] , but more preferably in the range [500 ppm, 5000 ppm]. According to another embodiment of the invention, the initial gaseous mixture transformed into at least one of the apparatuses comprises oxygen. In this case, it is preferable that the oxygen content of the initial gas mixture be maintained at a value of less than a few hundred ppm. According to another embodiment of the invention, the initial gaseous mixture transformed into at least one of the apparatuses consists of a nitrogen / hydrogen / water vapor mixture. According to another embodiment of the invention, the initial gaseous mixture transformed into at least one of the apparatuses consists of a nitrogen / hydrogen / water vapor / oxygen mixture. According to one of the embodiments of the invention, the initial gaseous mixture transformed into at least one of the apparatuses comprises hydrogen; in such case, its hydrogen content will preferably be in the range [1000 ppm, 50%], but will preferably be less than or equal to 10%. According to one of the embodiments of the invention, a zoning of the atmosphere found successively by the circuit along the conveyor is carried out in the following manner: a) at least one of the forming devices of excited or unstable gas species, transforms an initial gaseous mixture different from that transformed by the apparatus that precedes it in said structure, and / or: b) the adjacent gas mixture applied at the level of at least one of the gaseous species forming apparatuses excited or unstable, is different from that applied to the level of the apparatus that precedes it in said structure. According to one of the embodiments of the invention, steps a) and b) above may involve the same device. According to one aspect of the invention, current '• up or downstream of the cluster of apparatuses found by the circuit (therefore, upstream or downstream of the treatment operation effected by said operation), the circuit is subjected to a pre-heating operation. In this way it will be possible, for example, to proceed to the following successions of steps: a treatment (surface pre-treatment) according to the invention (cold or hot), followed by a preheating operation (followed by welding); - a pre-heating operation, followed by a treatment according to the invention (cold or hot) (followed by welding); - a treatment according to the invention (cold), followed by a pre-heating operation, followed by another treatment according to the invention (hot) (followed by welding). Of course, this list of successions is only illustrative of the numerous possibilities offered by the invention. Downstream of the contacting of the circuit with the welding wave (s), ie, downstream of the welding operation itself, it may, if necessary, carry at least one of the faces of the circuit in front of the gas outlet from at least one apparatus (which may be considered as "downstream") i forming excited or unstable gaseous species in which an initial gas mixture is passed (which in this case may be designated as "downstream") ") comprising an inert gas and / or a reducing gas and / or an oxidizing gas, obtaining at the outlet of the gas from the apparatus, a downstream primary gaseous mixture comprising excitable or unstable species and which is substantially devoid of species electrically loaded, which allows, if necessary, carry out a post-cleaning of the circuit surface. It can also be faced that during all or part of the contacting with the wave (s) of welding material, at least one of the faces of the circuit is put in contact with the gas outlet from at least one apparatus that can be qualified as "complementary" for forming excited or unstable gaseous species, in which apparatus an initial gaseous mixture (which may also be designated as "complementary") is passed, comprising an inert gas and / or a reducing gas and / or a oxidizing gas, obtaining at the outlet of the gas from the apparatus, a "complementary" primary gaseous mixture comprising excited or unstable gaseous species and that is substantially devoid of electrically charged species. As will be apparent to the person skilled in the art, said "complementary" treatment, carried out during the actual welding, may seem very advantageous especially as regards the substantial elimination of the oxides that could be formed at the level of the welding. According to one of the aspects of the invention, the implementation of a regime that could be qualified as "wakeful" of the treatment carried out before the welding is contemplated, when the welding operation must be temporarily stopped. In this way, it is possible to provide according to the invention, situations in which, for example as a consequence of a voluntary action of the user, or also due to the fact that no circuit has been detected at the entrance of the structure during a For the period of time that has been predefined, at least one of the following measures is activated: - the introduction of the primary gas mixture is stopped in each of the apparatuses; - a small flow of primary gas mixture circulating in the considered apparatus is maintained in each of the apparatuses (for example, from 1 to 9%, or some tens of%, of the flow rate circulating in the normal regime used for the treatment);
- when an adjacent gas mixture is applied at the level of at least one of the apparatuses, the arrival of the gaseous mixture adjacent to the level of at least one of these apparatuses is stopped; - in each of the apparatuses, a primary gaseous mixture which can be classified as "waiting" (for example a neutral gas or a neutral gas) is placed in place and at the site of the primary gaseous mixture circulating in the considered apparatus. still a mixture: neutral gas / hydrogen ...); - each of the apparatuses is passed, from the treatment regime in which it was located, to a waiting regime in which the density of the energy applied therein is only a few W / cm. The invention also refers to to a device for the welding of circuits, by waves, by means of a liquid alloy, comprising: an enclosing structure, which defines an interior space crossed by means for transporting the circuits, isolated from the surrounding atmosphere, and which defines a route for transporting the circuits inside the structure; - at least two excited or unstable gaseous species forming apparatus, assembled in series, comprising at least one tubular passage for gases having an axis, formed between an outer electrode and an inner electrode, carrying at least one of the electrodes, facing the other, a dielectric coating, the electrodes being linked to a source of high voltage and high frequency; the outer electrode surrounds the dielectric material and comprises at least one inlet for the gases, called "initial" and at least one outlet for the gases, called "primary elongate", parallel to the axis and substantially diametrically opposite; said outlet for the gas, flows into the interior of the structure; the structure surrounds or connects in a sealed manner to the appliances; at least one of the apparatuses is located above the path, and at least one of the apparatuses is located below the path, and: - a vessel capable of containing an alloy bath, located downstream of the structure ( therefore, outside the structure) or downstream of the apparatuses, in which case the structure surrounds the container or is sealed in a sealed manner therewith. According to one embodiment, the structure is provided with at least one means for injecting a gas called "adjacent" that does not pass through at least one of the apparatuses. According to one aspect of the invention, the device comprises, upstream or downstream of the apparatuses, means for heating the circuits. Other features and advantages of the present invention will come to light through the following description of some embodiments, given by way of illustration but not limitation, made in connection with the accompanying drawings, in which: - Figure 1 is a schematic view of an enclosing structure according to the invention that is connected to three apparatuses of excited or unstable gaseous species, suitable for the implementation of the method according to the invention; Figure 2 schematically represents, in section, an example of an apparatus for forming excited or unstable gaseous species, suitable for implementing the method according to the invention; - Figure 3 is a schematic representation of a wave welding device (machine), suitable for the implementation of the invention. In Figure 1 it can be seen the presence of a circuit 1 to be welded (represented by a simple thickness, the electronic components have not been represented in this Figure), carried by a transport system not represented for reasons of clarity (for example, of the type comprising two chains of fingers that capture the circuit from one side of the tunnel to the other) along a transport path 2, in front of the gas outlets from three excited or unstable gaseous species forming apparatuses:, 12, 11. With the reference number 6 the gas outlet from the first device has been indicated. The transport path 2 passes through an interior space 13, delimited by an enclosing structure (in this case, a tunnel 3, it being advantageous for said structure to be connected in a leak-tight manner to the apparatuses.) It has been schematized in 8, 31, 32 , the primary gaseous mixtures obtained at the outlets of the gases from the apparatuses and successively found by the circuit, obtained by the transformation in each of the apparatuses, of the respective initial mixtures 7, 29 and 30.
With the reference number 5, the gas inlet 4 has been recorded in the apparatus 4. In the embodiment shown in FIG. 1, the presence of inputs for the adjacent gas mixtures 9, 10 can also be observed at the level of the apparatus 4. On the other hand, the installation has been provided, if necessary, with a means for heating circuit 1, not shown in Figure 1. For this heating means, for example infra-red lamps can be used in the tunnel , or a convex heating (tunnel with hot walls), or also the fact that the piece is placed on a heating substrate holder. The structure of Figure 1 has been represented independently of any wave welding machine, but as has been extensively developed in the foregoing, it is possible to face numerous possible arrangements for this structure: - the fact that for example on exit of said tunnel structure, 3, the circuit enters a wave soldering machine
(referenced address: 33), maintaining, if necessary, the circuit between the exit of the tunnel and the entrance of the machine, under a protective atmosphere; - or also the fact that the treatment according to the invention and the welding operation are carried out within the same tunnel structure 3 (in such case, the container of the welding material, located downstream of the last found apparatus, here: 12). In this case, the direction of the entrance of the structure, and by the reference number 34, the sense of the location of the weld pool has been symbolized by the reference number 12. As already detailed above, the assembly A consisting of the tunnel portion including the three excited or unstable gaseous species forming devices 4, 12, 11, will then necessarily be located downstream of the weld pool , but may be located, depending on the cases, downstream or downstream of a circuit pre-heating stage. As will be apparent to the person skilled in the art, if Figure 1 illustrates a particular configuration for the relative positioning of the apparatuses, all possible separation configurations between the apparatuses are feasible according to the invention, including the fact that two devices are on top of each other. As represented in the embodiment of the Figure
2, the apparatus for this embodiment has a cylindrical geometry; It comprises a first tubular electrode 14, formed for example by an internal face of a metal block 15 and in which a set of a tube made of a dielectric material 16, for example in ceramic material, is arranged concentrically on the inside face of the which has been deposited, by metallization, a second electrode, 17, exaggeratedly thickened in Figure 2, for a better understanding. The set of the dielectric material 16 and the second electrode 17 thus defined, together with the first electrode 14, a tubular passage 18 for the gases, and internally, an interior volume 19 in which a coolant is circulated, advantageously freon gas for its electronegative character or also permuted water. The interior passage 18 for gases has an axial extension of less than 1 m, typically less than 50 cm, and its radial thickness does not exceed 3 mm, and is typically less than 2.5 mm. The block 15 comprises, diametrically opposed, two longitudinal grooves 20 and 21 forming respectively the entry of the initial gas to be excited in step 18 and the outlet of the primary gas flow comprising excited or unstable gaseous species. The slits 20 and 21 extend over the entire axial length of the cavity 18, and have a width that for the embodiment shown in Figure 2, does not exceed the thickness "e" and is typically substantially identical to the latter (could be other configurations are conceivable such as, for example, a width slightly greater than the thickness "e"). It is advantageous if the body 15 comprises at the periphery of the first electrode 14, a plurality of conduits 22 for the passage of a refrigerant, for example water. The inlet 20 for the gases communicates with a homogenization chamber 23, formed in a trunk 24 attached to the block 15 and comprising a branch pipe 25 for the initial gas supply, coming from an initial gas source 26, and therefore at a pressure that may vary according to said source, typically between some bars and 100 or 200 bars. The electrodes 14 and 17 are connected to a high-voltage and high-frequency electric generator 27, which operates at a frequency advantageously greater than 15 kHz, which delivers a power which, for example, is of the order of 10 kW. On the other hand, it is possible and advantageous to express said power supplied by the generator, referring it to the surface of dielectric material. The gaseous flow containing the excited species, available at the exit 21 of the gases, is directed to a user station, 28, for example for the surface treatment of metal surfaces. Figure 3 illustrates schematically one of the embodiments of a wave welding machine, according to the invention. Said machine can be divided into three zones: - a pre-heating zone 35 of the circuits (by means 39 located in the lower part of this zone); - a zone 36 for processing the circuits before welding (by means of three apparatuses 4, 12, 11, with the apparatuses 4 and
12, located opposite the upper face of each of the circuits that transits the machine, while the apparatus 11 is located opposite its lower face); - an area 37 welding by waves (the surge being formed by pumping a bath of welding material contained in a container 46, by means of a nozzle or nozzle 44). The circuit 40 (in which electronic components of the type inserted into wire 41 have been attached, such as those of the surface-mounted type, 42), passes from one area to the other of the machine, which is tunnelled (3) in all its length, by means of a transport system comprising two chains of fingers that grip the circuit from part to part of the tunnel. It has simply been represented in the Figure, the transport path 2 of the circuits inside the machine, as well as the systems 43 that realize the displacement of the transport chains. Between the preheating zone and the welding zone, each of the circuits undergoes a treatment by putting them in successive contact with the primary gas mixtures obtained at the gas outlets of the apparatuses 4 (upper face of the circuit), and then 11 (lower face of the circuit) and finally 12 (again, upper face). For this embodiment, no adjacent mixing input has been mentioned in the tunnel. By means of the reference number 38, the treatment atmosphere found by the circuit in this area has been schematized. A device such as the one described in relation to Figure 3, which integrates three apparatuses such as that described in relation to Figure 2, has been used for the realization of implementation examples of the invention. The common conditions for the implementation of these examples are the following: - each of the appliances requires a power of the order of 3 kW, which corresponds to a power density of the order of 35 W / cm2; - the pre-heating zone maintains a temperature at the level of the circuits, in the order of 150 to 160 ° C: - the transport speed of the circuits in zone 36, is 8 cm / min; - at the level of each of the three devices, the circuit is heated (temperature of the circuits, maintained at a level of the order of 150 to 160 ° C) by the face-to-face presence of each of the devices, of a tube radiant (at the level of each apparatus, the circuit is therefore located "sandwich" between the apparatus and a radiant tube not shown in Figure 3); - the circuits tested were of the type of pre-tinned printed circuit boards (PCB), double-sided with metallic holes, comprising components of two categories "surface mounted" and "inserted by wire"; - For each example, for each welded circuit, the number of welding defects on the wire components, whose treatment is especially delicate (defect of rise of the weld in the metallized holes) has been listed. Therefore, in each example the average of the number of defects observed per card is recorded, at least 10 welded cards, knowing that no card carries 100 welding points of the "metallized orifice" type. For the first embodiment, the operating conditions and the results obtained are the following: initial mixture transformed into the first and second apparatus: 17 m3 / hour of a N2 / H2 mixture (4% hydrogen); - initial mixture transformed into the third apparatus (12): 17 m3 / hour of a mixture N2 / H2 / H20, with 4% hydrogen and 1000 ppm H20; - Average defect rate: less than 10%. For the second comparative example, only one (1) device was used in zone 36, in front of the lower face of the circuit. For the type of circuit treated, the defect index for the particular welding points that constitute the metallized holes, was close to 60%, whether the initial mixture treated in the only apparatus is the N2 / H2 / H20 mixture used in the Example 1, as if it were a N2 / H2 mixture at 4% hydrogen. The results just described illustrate the very sensitive improvement of the results observed in the points of the hardest circuit to weld (wire components, metalized holes) by using at least one device in front of each of the faces of the circuit, in In this case, two devices in front of the upper face.
The results obtained by means of a single apparatus on the upper face (in addition to the apparatus located on the lower face) have proved to be certainly satisfactory for the circuit tested, but they required reducing the speed of the passage of the circuit under the apparatus (by comparison with the speed of passage). which can be used when two devices are positioned in the upper face position). It is therefore understood that it is necessary to search for each specific case of the user (treated circuits, desired productivity, ...) the best compromise. Although the present invention has been described in relation to specific embodiments, it is not limited thereto; on the contrary, the invention is susceptible to undergoing modifications and variants that will occur to the person with skill in the art, within the scope of the following claims.