MXPA96001753A - Procedure for dry pre-treatment metal disposals before their owned welding, using an atmosphere containing a vapor - Google Patents

Abstract

The present invention relates to a process for the dry surface pre-treatment of at least one metallic surface before its welding or tinning by means of an alloy, characterized in that: a) an initial gas mixture comprising an inert gas is passed through and / or a reducing gas, and comprising an oxidizing gaseous mixture comprising water vapor, in at least one apparatus forming gaseous species excited or unstable in order to obtain after the transformation in the apparatus, the gas outlet from the apparatus, a primary gaseous mixture, the content of the water vapor being present in the initial gas mixture, located in the range of [50 ppm, 6%], b) the surface to be pre-treated is subjected to a pressure close to atmospheric pressure, by means of a gaseous atmosphere of treatment that comprises excited or unstable species and that is substantially devoid of electrically charged species, obtained from di primed gas mixture

Description

, WATER VAPOR The present invention relates to the operation of pre-conditioning or pre-treatment of metal surfaces, which generally takes place before welding or tinning operations, for electronic applications. Therefore, it applies especially to the pre-treatment carried out before the following operations: - welding operations of the components in a circuit (both in the case of inserted components and components mounted on the surface); - welding operations of the contact bands on electronic supports, which allow the connection of the support in question, on another support (in this case, the example of a hybrid circuit or of a printed circuit to which it will proceed to insert can be included) , by means of these contacts, in a printed circuit, or also the case of a hybrid or printed circuit that can be plugged, thanks to this comb, into a connector); - welding operations of circuits in the bottoms of the trunks (which takes place during the encapsulation of said circuits); - welding operations that take place during the closing procedures of the encapsulation boxes; - welding operations of the naked chips on supports such as printed circuits, hybrid circuits or also multilayer interconnection substrates such as the substrates usually called "MCMM (Multi Chip Modules); - tinning circuits or component termination operations The expression "tin-plating" can be understood as the deposition operations of layers whose composition can be very varied (which includes, but is not limited to, the deposition of tin-lead type layers).

Two of the most common methods of use for carrying out said welding operations (or tinning), are the so-called "welding (or tinning) by surge" (brasage ou étamage à la vague) and "welding by refusion" (brasage par rerusion) . In the first case (wave soldering machines), the design of these machines is such that the pieces to be welded or tinned are put in contact with one or several waves of a liquid welding material, obtained by circulating the welding material contained in a container, through one or several nozzles or nozzles. In the case of the second type of method (reflow soldering), which on the other hand groups several techniques with this name in common, a solder bath is no longer used but a solder paste, which contains the solder alloy, the which alloy is deposited on the support (for example, an electronic circuit before the deposition of the components, the edges of a lockable trunk or also the bottom of a trunk), and to which alloy is incorporated a certain amount of heat that allows obtain the fusion of the metallic alloy. The most frequent is that this thermal transfer is carried out in a continuous furnace. The role of the surface pre-conditioning operation (fluxage) before welding, is then to prepare the metal surfaces to be welded or tinned (performing operations such as degreasing, deoxidation, decontamination of the adsorbed layers, or other preparation of the surface), this, in order to facilitate the subsequent wetting of said surfaces by welding, but also to eliminate the oxides that could be formed due to the alloy of the weld. Most often, this pre-conditioning operation is carried out by chemical products, often obtained from resinous bases specially supplemented by acidic compounds. Said operation is carried out, in the case of a wave welding machine, in an area located upstream of the machine (ie, before the support to be treated comes into contact with the surge of welding material), or well, in the case of reflow soldering, by the fact that the chemical for the surface pre-treatment enters the composition of the solder paste, and the phase of the surface pre-treatment then takes place during a first part of the thermal cycle used. 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 subject of intense controversy as it falls within the scope of the "Montreal Protocol" and its subsequent revisions. Among the possible methods considered to find a replacement solution to the use of these compounds, the dry surface pre-treatment methods such as the pretreatment of the surfaces with plasma, before their welding, can be mentioned, 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 numerical values mentioned, 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 metallic 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 infrastructures in An industrial production line is undoubtedly linked to the technical and technological difficulty of 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, only a few W per cm are reached 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 has a very critical importance and must be maintained at a very low value, which it 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 superficial pre-treatment by dry route, of a metallic surface before welding or tinning by means of an alloy, a process which is notable because it encompasses 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, in at least one apparatus forming excited or unstable gaseous species, obtaining 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 this document, clearly demonstrated that it is specifically possible, thanks to this procedure: - operate at a pressure that is appreciably atmospheric pressure; - obtain a great flexibility as regards the distance 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 apparatus. Therefore, such configuration can be classified as "after discharge", in that the primary component of the treatment atmosphere, which comprises excited or unstable gaseous species, is obtained at the output 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 dry surface pre-treatment procedure of a metallic surface before its welding or tinning according to document FR-9315112, especially with the objective of further reducing the percentage of failures of the products (for example, of the printed circuits), obtained at the exit of the subsequent welding or tinning stage. These works have then shown in a clear, and totally unexpected, way that the "water vapor" H20 species could play a specific and particularly advantageous role in the obtained surface pre-treatment performances and therefore also in the performances of the subsequent welding (for example, by waves). The method of dry surface pre-treatment of a metal surface before welding or tin-plating by an alloy, according to the present invention, is characterized in that: a) an initial gas mixture comprising an inert gas and / or a reducing gas, and comprising an oxidizing gaseous mixture comprising steam, in at least one apparatus for forming excited or unstable gaseous species, obtaining at the outlet of the apparatus, a primary gaseous mixture, the content of the vapor being of water present in the initial gas mixture, in the order of [50 ppm, 6 «/ o]; b) the surface to be pre-conditioned is treated, under a pressure close to atmospheric pressure, by means of a gaseous atmosphere of treatment comprising excited or unstable gaseous species and substantially devoid of species provided with an electric charge, obtained from of the primary gas mixture. By the expression "metal surface" according to the invention, is meant any type of metal surface that can intervene in such welding or tinning operations; it may be steel, copper, aluminum, tin, lead, tin-lead, tin-lead-silver, or also alloys such as Kovar; Of course, this list is not limiting, but indicative. It is also possible to evoke the different types of electronic circuit terminations such as pre-tinning, termination with Ni-Au, or also passivated copper. The part comprising the part to be pre-treated superficially, may consist of any part suitable to intervene in said welding or tinning operations, whether it is for example an electronic circuit in which the components have been attached to the object of welding them (both if they are inserted components and surface mounted components), as if they were a coupe: contact band / electronic support (welding the band on the support); they can also be supports such as printed circuits, hybrid circuits or also multilayer interconnection substrates such as the substrates usually called MCM (Multi Chip Modules) on which the bare chips have been attached for the purpose of welding them; It can also be electronic components that must be tinned. The soldering or soldering "alloy" according to the invention will be constituted by any composition that can be used for operations of this type (for example, for reflow welding operations or in a wave welding machine, or also for the operation of tinning by waves), such as for example Sn-Pb, Sn-Pb-Ag, Pb-In, 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 105 Pa, 3 x 105 Pa]. The term "electrically charged species" according to the invention refers to ions or electrons. As explained above, the treatment atmosphere according to the invention therefore differs from the plasma atmospheres previously described in the prior art literature, by the fact that it is substantially devoid of species electrically charged, that is, of ions or electrons. The atmosphere for the treatment, which is obtained from the primary gas mixture, can also comprise, if necessary, an adjacent gas mixture, the primary gas mixture being obtained at the exit of the gases from the apparatus for generating gaseous excited species or unstable in which the initial gaseous mixture has been transformed while the adjacent gas mixture itself has not passed through the apparatus. As already explained 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. In addition to water vapor, the oxidizing gaseous mixture can for example also contain oxygen, or C02, or else N2O, L ^ O, or a mixture of said species. Of course, the list of species 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 (qualified as "primary"), comprising gaseous species unstable or excited, 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. As will be evident to the man of the art thanks to the reading of the preceding description, the content or content of the water vapor present in the initial gas mixture must be adapted in each case, especially to the content of the rest of the initial gas mixture ( for example, its content in reducing gas, for example in hydrogen), but it must also be adapted to the type of piece to be treated (for example, to the type of condition or surface condition, and therefore, to the termination of the circuit electronic to be pre-treated before its welding by waves). However, it has been found to be advantageous if the water vapor content, present in the initial gas mixture, is in the order of [50 ppm, 6%], preferably in the order of [100 ppm, 1%] , but more preferably still, in the order of [500 ppm, 5000 ppm]. As already indicated above, the initial gas mixture may comprise, in addition to water vapor, another oxidizing species, for example, oxygen. In the latter case, it is preferable that the oxygen content present in 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 consists of a nitrogen / hydrogen / water vapor mixture. According to another embodiment of the invention, the initial gaseous mixture consists of a nitrogen / hydrogen / water vapor / oxygen mixture. When the initial gas mixture comprises hydrogen, it is advantageous that the hydrogen content be in the order of [1000 ppm, 50%], but preferably less than or equal to 10%. The pre-treatment process according to the invention allows the treatment to be carried out by means of the primary mixture obtained at the exit of a single apparatus, or of several devices placed in parallel in the width of the piece to be treated, or also successively by means of primary mixtures obtained in the gas outlets of several devices placed in series. Likewise, as will be evident to the man of the trade, the procedure according to the invention is then applied according to the needs expressed by the user as regards the treatment of only one of the faces of the piece to be treated, or to the case in which the piece must be pre-treated on both sides. In this last case, it will be convenient to arrange the required devices, in front of each of the faces of the piece. 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 another aspect of the present invention, the surface to be treated is brought to a temperature comprised between the ambient temperature and the melting temperature of the alloy used, to carry out the further operation of welding or subsequent tinning. 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, and in order to limit the increase of the intermetallic parts, strive 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 the alloys Sb63-Pb37 or Sn62-Pb36-Ag2 used. According to one aspect of the invention, the piece comprising the metal surface (s) to be treated is brought in front of the gas outlet from the apparatus, or in front of the gas outlets of several devices placed in parallel in the width of the piece and / or successively in front of the gas outlets of several devices placed in series, 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 apparatus, or to the apparatuses, or with the apparatus, or the apparatuses, included within the structure. There is the same observation as that expressed above, in terms of double-sided treatment (also in this case, it is sufficient to implement the required devices, in quantity and arrangement, in front of each of the faces of the piece). When the piece comprising the metal surface (s) to be treated (s) is brought in front of the gas outlets of several devices placed in parallel in the width of the piece and / or successively in front of the gas outlets of several devices placed in series, at least one of said apparatuses transforms an initial gaseous mixture comprising an inert gas and / or a reducing gas, and an oxidizing gaseous mixture, comprising steam. The initial gaseous mixture transformed at the level of the other apparatuses, may then comprise an inert gas and / or a reducing gas and / or an oxidising gas. According to one of the embodiments of the invention, the apparatus (or at least one of the apparatuses), in which the initial gaseous mixture is transformed, is the seat of an electric discharge created between a first electrode and a second electrode, having a layer of a dielectric material disposed on the surface of at least one of the electrodes, facing the other electrode; the initial gaseous mixture crosses the discharge transversely with respect to the electrodes. The energy applied in the apparatus, referred to the surface unit of the dielectric material, will then be advantageously greater than or equal to 1.

W / cm 9, preferably greater than or equal to 10 W / cm 9, and will be more frequently included in the range [10 W / cm, 100 W / cm]. According to one of the embodiments of the invention, a zoning of the treatment atmosphere found successively by the piece to be treated, along the conveyor, is carried out in the following manner: a) at least one of the apparatuses of excited or unstable gas species, transforms an initial gaseous mixture different from the one 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 Apparatus forming excited or unstable gaseous species 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. In this way, it is possible to use mixtures of increasing reducing power, from one device to another. According to one of the aspects of the invention, at the exit of the structure the piece enters a machine in which the welding or tinning operation is carried out; If necessary, between the entrance and exit of the machine, the piece is maintained under a protective atmosphere.

The term "protective atmosphere" then refers to an essentially inert atmosphere, the residual oxygen concentration of which does not exceed a few hundred ppm, or does not exceed 100 ppm. According to another aspect of the invention, the welding or tinning operation is carried out inside the enclosing structure (for example, a tunnel), downstream of the apparatus. According to one of the embodiments of the invention, the welding operation is tin, before which the pretreatment according to the invention takes place, is effected by waves, and the piece comprising the metal surface to be treated, is an electronic circuit, each of whose faces is brought in front of the gas outlet of at least one apparatus forming gaseous excited or unstable species, by the transport system. Advantageously, the upper face of the electronic circuit to be treated is carried out successively in front of the gas outlets, of at least two apparatuses of excited or unstable gaseous species. In this case of the application of the invention to the case of surface pre-treatment prior to wave welding, it should be noted (whether the welding operation is carried out in a separate machine located downstream or if said operation is performed inside the machine. the same enclosure structure), that the operation of the surface pretreatment according to the invention, may be coupled to the pre-heating stage of the circuits or supports, usually present in the wave welding machines between the chemical pre-treatment stage and the welding pot. In this case, according to the invention, the following types of successions may be considered: a surface pre-treatment according to the invention (cold or hot), followed by a pre-heating operation (followed by welding) or tinned); - a pre-heating operation, followed by a treatment according to the invention (cold or hot) (followed by welding or tinning); - a surface treatment according to the invention (cold), followed by a pre-heating operation, followed by a surface treatment according to the invention (hot) (followed by welding or tinning). Of course, this list of successions is only illustrative of the numerous possibilities offered by the invention, and is not of a limiting nature. Downstream of the contacting of the circuit with the welding wave (s), ie, downstream of the welding operation itself, it will be possible, if necessary, to carry at least one of the faces of the circuit in front of the gas outlet of at least one apparatus (which can be considered as "downstream") for forming excited or unstable gaseous species, in which is passed an initial gas mixture (which in this case can be designated as "downstream") comprising an inert gas and / or a reducing gas and / or an oxidizing gas, obtained 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 electrically charged species, which allows, if necessary, post-cleaning the surface of the circuit. It can also be envisaged 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 brought into 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 is passed an initial gas mixture (which may also be designated as "complementary") 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 "complementary" primary gas 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 welding or tinning, when the welding or tinning operation must be temporarily stopped, is foreseen. 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 time period 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 devices; - a small flow of primary gaseous mixture circulating in the device considered (for example, from 1 to 9%, or a few tens of%) of the flow circulating in the normal regime used to maintain each device is maintained. the treatment); - when an adjacent gas mixture is applied at the level of at least one of the apparatuses, the arrival of the adjacent gas mixture is stopped, at the level of at least one of these apparatuses; - a primary gas mixture which can be classified as "standby" (for example a neutral gas or otherwise) is exchanged in each device in place and at the site of the primary gas mixture circulating in the device in question; 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 there is only a few W / cm. Other characteristics and advantages of the present invention will come to light thanks to the following description of some embodiments, given by way of illustration but not limitation, made in relation to the accompanying drawings, in which: - Figure 1 is a schematic view of a suitable installation 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 another installation suitable for the implementation of the invention, in which installation, an electronic circuit to be subsequently welded in a wave welding machine, successively, in an enclosure structure that in this case is of the tunnel type, three apparatuses that form excitable or unstable gaseous species in series, two of these apparatuses being on its upper face, and the third apparatus is on its lower face. In Figure 1 it can be seen the presence of a piece 1 comprising the surface (s) to be pre-treated (s), carried by a conveyor belt 2, in front of the gas outlet 6 of a forming apparatus 4 of excited or unstable gaseous species. The transport system 2 passes through an interior space 31, delimited by a tunnel 3, it being advantageous that it is connected in a leaktight manner to the apparatus 4. Through the reference number 8, the primary gaseous mixture obtained at the outlet 6 of the apparatus has been schematized. The primary gaseous mixture 8 is obtained from an initial gas mixture 7 entering the apparatus at the level of its gas inlet 5. In the embodiment shown in Figure 1, the presence of entries for the mixtures is also observed adjacent sodas, 9, 10.

The gaseous atmosphere obtained from the adjacent gas mixtures 9, 10, and from the primary gas mixture 8, constitutes the treatment atmosphere 30 according to the invention. In the embodiment shown in Figure 1, the presence of excitable or unstable gaseous species forming apparatuses, not represented, 11 and 12, in series with the first apparatus 4 and found successively by the piece 1 can be observed. If necessary, it complements the installation with other inputs for the adjacent gas mixture, such as those represented in 13 and 29.

On the other hand, the association is the pi? Vibla, cu? üc u iicu apu, cun a means to heat the piece 1, not shown in Figure 1. For this heating medium, can be used for example infra-red lamps present in the tunnel, or a convex heating (tunnel with walls hot), or also the fact that the piece is placed on a heating substrate holder. As shown in the embodiment of Figure 2, the excitable or unstable gaseous species forming 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 assembly of the dielectric material 16 and the second electrode 17 thus defines, 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 because of 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 that . -f comprises 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. It is advantageous if the body 15 comprises, at the periphery of the first electrode 14, a plurality of ducts 22 for the passage of a coolant, for example water. The inlet 20 for the gases communicates with a homogenization chamber 23, formed in a trunk or crankcase 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 subjected to a pressure that may vary according to said source, typically between some bars and 100 or 200 bars. The electrodes 14 and 17 are linked to a high-voltage, high-frequency electric generator, 27, which operates at a frequency that is advantageously greater than 15 kHz, and 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 the metal surfaces according to the invention. The installation schematized in Figure 3 illustrates the case of a surface pre-treatment installation according to the invention, which intervenes before a subsequent operation, which may be, for example, a wave welding operation. Each of the circuits to be welded, is in this case transported into a tunnel 32, along a direction 42. The system of capturing or securing the circuits (for example, of the type comprising two chains of fingers that grab the circuit from part to part of the tunnel), has not been represented for reasons of clarity; but said transport systems are widely known by the man of the art who knows the machines of welding by waves. The circuit then finds successively the gas outlets of three excitable or unstable gaseous species forming devices, 33, 34, 35 (in which the gaseous mixtures of the initial type 36, 37, 38 are treated respectively), and more precisely the mixtures resulting primary, 39, 41 and 40. The primary mixes 39 and 40 more specifically treat the upper face of the circuit, while the primary mix 41 deals more specifically with the lower face of this circuit. The installation of Figure 3 has been represented independently of any wave soldering machine, but, as discussed extensively in the foregoing, it is possible to adopt among many possible arrangements of this installation: - the fact that for example to the exit of said installation and of the tunnel structure, 32 including, the circuit enters a welding machine by waves (referenced address: 43), 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 surface pre-treatment according to the invention and the welding or tinning operation are carried out within the same tunnel structure 32 (the container of the welding material being located downstream of the pre-treatment surface and therefore downstream of the last found apparatus, 34). In this case, it has been symbolized by the reference number 44, the direction of the input of the machine, and by the reference number 43, the direction of the location of the weld pool. As already detailed above, the assembly A constituted by the tunnel portion that includes the three apparatuses of excited or unstable gaseous species 33, 34, 35, will then necessarily be located upstream of the bath of welding material , but may be located, depending on the cases, upstream or downstream of a circuit pre-heating stage. An installation such as that described in relation to Figure 3, which integrates three apparatuses such as described in relation to Figure 2, has been used for the realization of examples of implementation of the invention, as well as for the implementation of comparative examples. , consigned in what follows. The common conditions for the implementation of these examples are the following: - the installation of Figure 3 was integrated inside a wave soldering machine (whose installation for the chemical surface pre-treatment had been suppressed), tunnelled in all its length, between pre-heating zone (which maintained a temperature at circuit level, of the order of 150 to 160 ° C) and weld pot Sn63-Pb37; - the transport speed of the circuits, at the level of each of the devices: 8 cm / min; - each device has a power of the order of 3 kW, which corresponded to a power density, of the order of 35 W / cm; - at the level of each of the three apparatuses, the circuit was heated (temperature of the circuits, maintained at a level of the order of 150 to 160 ° C) by the presence in front of each of the apparatuses, of a radiant tube ( at the level of each apparatus, the circuit is therefore located "in sandwich" between the apparatus and a radiant tube not shown in Figure 3); - the circuits pre-treated superficially and welded by waves, were of the type of printed circuits (PCB) pre-tinned, double-sided with metallic holes, which comprised components of two categories: "surface mounted" (hills on the surface) and " inserted by thread "(insérés á fil); - for each example described below, the number of welding defects on the wire components has been listed for each welded circuit, whose treatment is especially delicate (defect of rise of the weld in the metallized holes). Therefore, in each example the average of the number of defects observed per card or plate is recorded, for at least 10 welded cards, it being known that one card carries 100 welding points of the "metallized orifice" type. For the first implementation example, the operating conditions and the results obtained are the following: - initial mixture transformed into the first and second devices: 17 m / hour of a N2 / H2 mixture with 4% hydrogen; • 3 - initial mixture transformed into the third apparatus (34): 17 m / hour of a mixture N2 / H2 / H20, with 4% hydrogen and 1000 ppm H20; - average defect rate: less than 10%. For the second example, the operating conditions and the results obtained are the following: - initial mixture transformed in each of the three apparatuses: 17 m / hour of a N2 / H2 mixture with 4% hydrogen; - average defect rate: less than 10%. For the third example, the operating conditions and the results obtained are the following: - initial mixture transformed into the first and second apparatus: 17 m / hour of a N2 / H2 mixture with 4% hydrogen; • 3 - initial mixture transformed into the third apparatus (34): 17 m / hour of a N2 / H2 / H2O mixture, with 4% hydrogen and 6000 ppm H20; - average defect rate: approx. 65% For the fourth example, the operating conditions and the results obtained are the following: - initial mixture transformed into the first and second apparatus: 17 • 3 m / hour of a N2 / H2 mixture with 4% hydrogen; - initial mixture transformed in the third apparatus (34): 17 m / hour of a mixture N2 / H20, with 400 ppm of H20; - average defect rate: close to 100%. The results just described illustrate the very sensitive improvement of the results observed in the points of the hardest circuit to weld (wire components, metallized holes) in the presence of water vapor in the initial mixture treated in one of the apparatuses. As already mentioned, the water vapor content in the initial gas mixture must be adapted in each case, especially with respect to the content of the rest of the initial gas mixture (in this case, with respect to the wording adopted for the hydrogen), but also with respect to the type of part to be treated (in this case, with respect to the complexity of the weld joints to be made and with respect to the desired termination for the electronic circuit). In the case of the present, if the set of implemented conditions is taken into account, it seems to be advantageous to place the water vapor content of the initial gas mixture, in the vicinity of 1000 ppm, therefore included in the range of [500 ppm, 5000 ppm]. It is also found that for the proposed case, it is desirable to ensure the presence of a small percentage of hydrogen in the initial gas mixture; 4% represents a very reasonable and acceptable concentration, both from an economic point of view and for safety.

The comparative tests carried out (keeping all other conditions constant), for this same circuit, have allowed demonstrating that an increase in the hydrogen content (for example, taking it to 20%), did not improve the results significantly. 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.

Claims (24)

1. CLAIMS 1.- Process for the dry surface pre-treatment of at least one metallic surface before its welding or tin-plating by means of an alloy, characterized in that: a) an initial gas mixture comprising an inert gas and / or a reducing gas, and comprising an oxidizing gaseous mixture comprising water vapor, in at least one apparatus forming excited or unstable gaseous species, in order to obtain after the transformation in the apparatus, the gas outlet from the apparatus, a primary gaseous mixture, the content of the water vapor being present in the initial gas mixture, located in the range of [50 ppm, 6%]; b) the surface to be pre-treated, at a pressure close to atmospheric pressure, is subjected by a gaseous atmosphere of treatment comprising excited or unstable species and that is substantially devoid of electrically charged species, obtained from said gaseous mixture primary.
2. Method according to claim 1, characterized in that said gaseous atmosphere of treatment is obtained from said primary gas mixture and from an adjacent gas mixture, which has not passed through said apparatus.
3. 3. Method according to any one of claims 1 or 2, characterized in that the content of the water vapor in the initial gas mixture is located in the order of [100 ppm, 1%].
4. 4. Method according to claim 3, characterized in that the content of the steam in the initial gas mixture is in the range [500 ppm, 5000 ppm].
5. 5. Method according to any one of claims 1 to 4, characterized in that said oxidative gas mixture also comprises water vapor, oxygen.
6. 6. Process according to any one of claims 1 to 4, characterized in that said initial gas mixture consists of a nitrogen / hydrogen / water vapor mixture.
7. 7. Process according to claim 5, characterized in that said initial gas mixture consists of a mixture nitrogen- / hydrogen / water vapor / oxygen.
8. 8. Process according to any one of claims 1 to 5, characterized in that said initial gas mixture comprises hydrogen and because the content of the hydrogen present in said initial gas mixture is located in the order of [1000 ppm, 50%], preferably less than or equal to 10%.
9. 9. Process according to any one of claims 1 to 8. characterized in that during the treatment, the piece to be treated, is brought to a temperature between the ambient temperature and the melting temperature of the alloy used to effect the welding operation or subsequent tinning.
10. 10. Method according to claim 9, characterized in that the melting temperature of said alloy is close to 180 ° C, and because said temperature at which the piece to be treated is carried, does not exceed 160 ° C. .
11. 11. Method according to any one of claims 1 to 10, characterized in that the piece comprising said at least one metal surface to be treated, is brought in front of the gas outlet of said apparatus, by means of a transport system. passing through an interior space delimited by an enclosing structure isolated with respect to the surrounding atmosphere, said structure being connected in a sealed manner to said apparatus, or said apparatus being included in said structure.
12. 12. Method according to any one of claims 1 to 10, characterized in that the piece comprising said at least one metal surface to be treated, is brought in front of the gas outlets of several devices placed in parallel on the width of the piece and / or successively in front of the gas outlets of several apparatuses, placed in series, by means of a transport system that crosses an interior space delimited by an enclosure structure isolated with respect to the surrounding atmosphere, said structure being connected in sealed manner to said apparatuses, or said apparatuses being included in said structure.
13. 13. Process according to claim 12, characterized in that at least one of said apparatuses transforms an initial gaseous mixture comprising an inert gas and / or a reducing gas, and an oxidizing gaseous mixture comprising steam.
14. 14. Method according to any one of claims 11 or 12, characterized in that said apparatus or at least one of said apparatuses is the seat of an electric discharge created between a first electrode and a second electrode, having a layer of a dielectric material disposed on the surface of at least one of the electrodes, in front of the other electrode, and because the initial gaseous mixture that is transformed in this apparatus, traverses the discharge transversely with respect to the electrodes.
15. 15. Method according to claim 14, characterized in that the power applied in said apparatus, referred to the surface unit of dielectric material, is greater than or equal to 1 W / cm, preferably greater than or equal to 10 W / cm.
16. 16. Method according to claim 12 and related to claim 1, characterized in that a zoning of the treatment atmosphere found successively by the piece to be treated along the conveyor is carried out in the following manner: at least one of the excitable or unstable gaseous species forming apparatuses, transforms an initial gaseous mixture different from that transformed by the preceding apparatus in said structure.
17. 17. Process according to claim 12 and related to claim 2, characterized in that a zoning of the atmosphere successively found by the piece to be treated along the conveyor is carried out in the following manner: a) at least one of the apparatuses of excited or unstable gaseous species, transforms an initial gaseous mixture different from the one transformed by the apparatus that precedes it in said structure, and / or: b) the adjacent gaseous mixture applied at the level of less one of the apparatus for forming excited or unstable gaseous species is different from the gas mixture applied to the level of the apparatus that precedes it in said structure.
18. 18. Method according to claim 17, characterized in that steps a) and b) take place at the level of the same apparatus.
19. 19. Method according to any one of claims 11 to 18, characterized in that the piece exits from said structure to enter a machine in which said welding or tinning operation is carried out, in which procedure, in case of be necessary the piece is maintained under atmospheric pressure between the exit of said structure and the entrance of said machine.
20. 20. Method according to any one of claims 11 to 18, characterized in that said welding or tin operation is carried out within the same structure, downstream of said apparatus or said apparatuses.
21. 21. Method according to claim 19 or 20, characterized in that the part comprising the metal surface to be treated, is an electronic circuit and because said subsequent welding or tinning operation is carried out by waves by putting in contacting the circuit with at least one surge of a liquid solder alloy, each face of the circuit being previously carried in front of the gas outlet of at least one of said excitable or unstable gaseous species forming apparatus, said transportation system.
22. 22. Method according to claim 21, characterized in that downstream of said contacting with the wave (s) of welding material, at least one of the faces of the circuit is brought in front of the gas outlet of at least one of the faces of the circuit. less a downstream apparatus forming excited or unstable gaseous species in which an initial downstream gas mixture is passed which comprises an inert gas and / or a reducing gas and / or an oxidizing gas, obtaining at the gas outlet of the apparatus, a downstream primary gaseous mixture comprising excited or unstable gaseous species, mixes this substantially devoid of electrically charged species.
23. 23. Method according to any one of claims 21 or 22, characterized in that during all or part of said contacting with the surge or with the waves of welding material, at least one of the faces of the circuit is found in contact with the gas outlet of at least one complementary apparatus forming excited or unstable gaseous species in which apparatus passes a complementary initial gas mixture comprising an inert gas and / or a reducing gas and / or an oxidizing gas , obtaining at the gas outlet of the apparatus, a complementary primary gaseous mixture comprising excited or unstable gaseous species, mixes this substantially devoid of electrically charged species.
24. 24. Method according to any one of claims 21 to 23, characterized in that means are implemented, upstream or at the entrance of said structure, for detecting the arrival of the circuit at the entrance of said structure, and as a consequence of an intentional action of the user or by the fact that no circuit has been detected in the entry of said structure for a predefined period of time, at least one of the following measures is activated: - the arrival of the primary gaseous mixture, to each of said apparatuses; - a reduced flow rate of the primary gas mixture circulating in the apparatus considered is maintained in each of the apparatuses; - in each of the apparatuses, it is passed, in replacement and in the place of the primary gaseous mixture circulating in the considered apparatus, a primary gaseous waiting mixture; - the arrival of the adjacent gas mixture applied is interrupted, at the level of at least one of said apparatuses; - each of the devices is placed in a waiting regime in which the load density applied to it is only a few W / cm2.