WO2005058538A1

WO2005058538A1 - Method of recirculating high-performance gases for a reflow oven

Info

Publication number: WO2005058538A1
Application number: PCT/FR2004/050663
Authority: WO
Inventors: Patrick Kae-Nune; Jérôme Perrin; Claude Carsac
Original assignee: L'Air Liquide Société Anonyme à Directoire et Conseil de Surveillance pour l'Etude et l'Exploitation des Procédés Georges Claude
Priority date: 2003-12-17
Filing date: 2004-12-08
Publication date: 2005-06-30
Also published as: FR2863922A1; FR2863922B1

Abstract

The invention relates to a method of recirculating the reflow gas used in a reflow soldering oven, whereby the reflow gas comprises at least one high-thermal-performance gas. The inventive method comprises the following steps consisting in: a) drawing off at least one portion of the gas contained inside the oven; b) pre-filtering the gas thus drawn off (filtration 1); c) recompressing the gas resulting from the aforementioned pre-filtration and, subsequently, directing the gas thus recompressed towards a fine filtration operation (filtration 2); d) treating the gas resulting from the fine filtration using a cleaning operation consisting of one or each of the following successive steps, namely an oxygen cleaning step and a membrane separation step; e) adding at least one of the above-mentioned high-performance gases to the gas resulting from the cleaning step; and f) re-injecting the gas from step (e) into the oven.

Description

DERECYCLAGEDEGAZHAUTESPERFORMANCESPOURFOUR DEREFUSION PROCESS

The present invention relates to the field of assembly of electronic cards by the reflow soldering process. In this process, the electronic card on which the components have been previously positioned on pads covered with solder cream is introduced into a passage oven. This solder cream contains the solder and the flux used to make the solder, but also ensures the maintenance of the components on the board before soldering. As will be seen below in connection with FIG. 1, such a reflow oven generally consists of a tunnel, open at both ends, inside which the electronic card runs, carried by a chain conveyor system or carpet. The oven generally comprises a succession of convective heating zones where the electronic card undergoes a thermal cycle ensuring brazing. This thermal cycle generally comprises a phase of gradual rise in temperature, whether or not followed by a plateau at a temperature known as preheating, then a rapid rise at a temperature called reflow peak of value greater than the melting temperature of the solder, and finally rapid cooling of the card to a temperature close to ambient temperature. To improve the quality of this soldering and in particular to avoid faults on complex cards and / or difficult to solder because of their design or the difficulty of wetting by soldering of certain surfaces to be soldered, in some cases injected with inside the oven an inert gas with high thermal performance, for example helium or helium-based mixtures. The use of such a high performance inert gas is not, for many applications, economically justified, since the consumption of this gas, which must be constantly renewed, represents too great a cost. In fact, the permanent renewal of the gas inside the pass-through oven is essential, mainly because: - Inevitable air inlets through the open ends of the oven. These air inlets, and more particularly oxygen, would gradually make the atmosphere of the furnace too rich in oxygen, without renewal, and therefore unsuitable for the conditions required for brazing in an inert atmosphere. - Pollution of the furnace atmosphere by the fumes and vapors released by the soldering cream, the card and the components brought to high temperature, especially during the transition to the reflow peak. These fumes and vapors are gradually deposited on the cold parts of the oven, hampering its normal operation. Usually we get rid of these fumes and vapors by increasing the gas flow inside the oven, which increases the gas consumption and therefore the operating cost of the operation. The present invention aims to provide a method and a device for efficiently recycling an inert gas with high performance inside a reflow furnace, thereby making it possible to significantly lower the cost of using such a gas. inert with high thermal performance. To do this, there is proposed according to the present invention, a new principle of a high performance gas recycling circuit successively comprising the following steps (which will be detailed in connection with FIG. 3 below): - A suction device gas inside the oven. - A pre-filtration or "coarse" filtration step (Filtration 1). - A compressor. - A fine filtration stage (Filtration 2). Advantageously, a step of analyzing the oxygen content, in order to direct downstream of the purification cycle only a gas mixture having an oxygen content below a predefined limit. - A purification operation comprising at least one of the following stages: an oxygen purification stage, a membrane separation stage. - A stage of enrichment in high performance gas of the gas resulting from the previous purification operation. - A re-injection device in the oven. The gas suction device inside the oven comprises suction orifices preferably arranged in the vicinity of the inlets and outlets of the oven so as to capture the maximum of parasitic air inlets into the oven. The purpose of the pre-filtration stage is to eliminate most of the fumes and vapors emitted during the remelting. It uses all known technologies for eliminating such fumes and vapors, and in particular coalescing filters and / or condensation on a cold wall. This pre-filtration step is preferably designed, according to known methods, so as to be able to easily eliminate the fumes and vapors condensed by standard exchange of cartridge and / or of the pre-filtration device itself, condensation in an easily removable container. , or any other process allowing rapid and easy elimination without interrupting production. The compressor ensures the circulation of gas in the recycling circuit and provides the pressure necessary for the operation of the second filtration step. It is preferably placed, on the recycling circuit, between the two filtration stages. The purpose of this provision is to protect it from the fumes and vapors emitted in the oven. Indeed, the gradual accumulation of these fumes and vapors in the compressor would be likely to seriously affect its proper functioning. The unconventional fine filtration step (2d filtration) aims to protect and increase the reliability of the following purification steps. It is carried out for example: - by condensation on one or more cold walls which are maintained at a judiciously chosen temperature. Maintaining the cold wall (s) at this temperature is obtained by any known method such as mechanical cold, circulation of cryogenic fluid, Pelletier effect or any other. This temperature is chosen so that the vapor pressure of the organic compounds emitted during the reflow peak, at this temperature, is low enough for the membrane to be able to function in optimal conditions. It is preferable to have a two-way system, one for the condensation and the other for evacuating the condensed liquids or solids - By trapping the organic compounds in an activated carbon tower. A dust filter is fitted at the outlet of the tower in order to stop any carbon particles coming from the tower. - By washing with water in a cyclonic washer of known technology. A step of drying the residual water in the gas mixture is then carried out by desiccant or automatic drain. The analysis step - particularly recommended - makes it possible to determine the oxygen content of the impure high-performance gas recovered at the outlet of second filtration and to compare this value with a pre-set threshold value so as not to recycle this gas towards the stages subsequent purification only when this value is lower than the threshold value and to purge in the ambient atmosphere if it exceeds this threshold value. Indeed, the gaseous mixture obtained at the output of second filtration certainly includes high performance gas but also residues in number and in particular air (nitrogen and oxygen). The purpose of the oxygen purification step located downstream of the second filtration is to allow an oxygen concentration inside the furnace not to exceed a limit value of the order of 500 ppm volume and , preferably, of 200 ppm volume. This step can be carried out by any known means, for example adsorption on a copper oxide and manganese bed or any other known adsorbent or also by any “deoxo” type system, preferably operating on methane. It is preferable to have a two-way system, one for adsorption and the other for regenerating said adsorbent. The control of this two-way system can for example be controlled by the oxygen analyzer which is generally equipped with a reflow oven operating under an inert atmosphere. In the case of a “deoxo”, the water and the carbon dioxide possibly resulting from the combustion of oxygen can be filtered conventionally by an automatic drainer and a lime filter. The gaseous mixture obtained at the outlet of the purification stage comprises high-performance gas, but also oxygen (in general less than 200 ppm vol., Or even less than a few tens of ppm), without also forgetting in general of nitrogen (depending on the performance of the tunnel entry-exit barriers). The purpose of the membrane separation step is therefore in particular to separate the high-performance gas, originating from the purification step, from the nitrogen in the air which could enter the passage oven and from the gas from the gas barriers. which serve to prevent the ingress of air into the oven passing through the inlet and outlet openings. Indeed, production constraints favoring so-called "continuous" ovens, the use of such gaseous barriers is essential. In addition, this step also makes it possible to eliminate the residual methane in the case where a deoxo operating on methane was used in the previous step, or else the CO ₂ resulting from such a deoxo, which has an additional advantage in terms of operational safety of the recycling system. However, the efficiency of gas barriers and the tightness of passage ovens are dependent on the technologies implemented by the various furnace manufacturers. Thus, air contamination is subject to variation and will therefore require the implementation of one or each of the means mentioned above: - if the effectiveness of the gas barriers and the tightness of the furnace are optimal, we could consider use only the membrane separation step, in particular to separate the high-performance gas from the barrier gas. - On the other hand, if the efficiency of the gas barriers and / or the tightness of the furnace are average and that consequently air enters the furnace, it will be recommended to implement the first stage of purification of the followed by the membrane separation step. This step also has the advantage of effectively separating the nitrogen from the air from the high performance gas. The high performance gas enrichment step, which is particularly recommended, is intended to limit the acceptable rate any contaminants likely to accumulate gradually in the oven and which would not be eliminated by the recycling system and / or to compensate for any losses of gas linked to the purification. The final stage of the process which is the subject of the invention involves re-injecting the purified high-performance gas into the furnace. This is done using a re-injection device preferably designed so that the purified gas is injected into an area of the furnace close to that corresponding to the reflow peak. The present invention therefore relates to a process for recycling the reflow gas implemented in a reflow soldering oven, reflow gas comprising at least one gas with high thermal performance, according to which: a) suction is carried out at less part of the gas contained inside the oven; b) a pre-filtration of the gas thus aspirated is carried out (filtration 1); c) the gas from said pre-filtration is recompressed before directing the gas thus recompressed to a fine filtration operation (filtration 2). d) the gas from said fine filtration is treated by a purification operation comprising one or each of the following stages arranged in series: - an oxygen purification stage, - a membrane separation stage. e) a step of adding at least one of said high-performance gases to the gas resulting from said purification operation is carried out. f)) the gas from step e) is reinjected into the oven. The method according to the invention can also adopt one or more of the following technical characteristics: - a step of analyzing the oxygen content of the gas from the second filtration step is carried out, the value thus measured is compared with a preset threshold value, and the following measures are implemented: i) if the measured value is less than or equal to the preset threshold value, the gas from the second filtration is directed to said purification operation; j) if the measured value is greater than the prefixed threshold value, the gas from the second filtration is directed towards the ambient atmosphere. - the oxygen purification step is carried out by adsorption. - the oxygen purification step is carried out by passage through a deoxo system. - Said at least one high performance gas is helium. The invention will be better understood on reading the description which follows, given solely by way of example and made with reference to the accompanying drawings, in which: - Figure 1 provides a schematic representation of a continuous brazing furnace by reflow. - Figure 2 provides a schematic representation of an example of installation for recycling the reflow gas implemented in a reflow soldering furnace (comparison). - Figure 3 provides a schematic representation of an example of installation for recycling the reflow gas implemented in a reflow soldering oven according to the invention. In fact, during the development of the present invention, the Applicant first proposed a first recycling scheme, that shown in FIG. 2. This preliminary solution implemented a gas recycling circuit comprising successively the following steps: - A gas suction device inside the oven. - A pre-filtration stage (Filtration 1). - A compressor. - A fine filtration step (filtration 2 in the figure). - A separation membrane. - A step of adsorption of residual traces of oxygen. - A gas enrichment step from the high performance gas adsorption step. - A device for re-injecting the gas thus formed in the furnace. This technical solution certainly had many merits, however, more detailed studies show that it has drawbacks in terms of operating pressure for the membrane separation step. In fact, to overcome the pressure drop linked to the passage of the gas mixture in the oxygen adsorption step, the pressure at the outlet of the membrane - on the permeat side (rich in high performance gases) must be a few bars, preferably between 1 and 3 bars. This increase in pressure consequently causes a reduction in the separation performance of the membrane. A technological solution would have been to put a second compressor between the outlet of the membrane - permeate side and the adsorption stage, but this has the disadvantage of requiring an additional and significant financial investment. FIG. 3 then illustrates an installation in accordance with the invention, using helium or a nitrogen / helium mixture as a brazing gas. The following elements or steps are recognized in the figure: - A sampling point for at least part of the gas inside the oven. - the gas thus sampled (comprising in particular helium, fumes, and traces of air which are inevitable given the air inlets into the oven (depending on the efficiency of the gas barriers with which this oven is equipped) is directed towards a pre-filtration stage (Filtration 1 in the figure). This pre-filtration stage aims in particular to eliminate most of the fumes and vapors emitted during reflow. It uses all the known technologies of elimination of such fumes and vapors and in particular coalescing filters or even condensation on a cold wall. This pre-filtration step is preferably designed so as to be able to easily eliminate the fumes and vapors condensed by standard exchange of cartridge and / or from the pre-filtration device itself, condensation in an easily removable container, or any other process allowing rapid and easy elimination without interrupting the process. - a compressor, located just downstream of the pre-filtration The compressor is preferably placed between the two filtration stages 1 and 2, not only to provide the pressure necessary for the correct operation of the second filtration step but also in order to protect it from the fumes and other vapors emitted in the oven, the progressive accumulation of these fumes and vapors in the compressor would be likely to seriously impair its proper functioning. - the fine filtration step, designated filtration 2 in the figure. This unconventional step aims to protect and increase the reliability of the subsequent purification steps. It is preferably carried out: -> by condensation on one or more cold walls which are maintained at a judiciously chosen temperature. Maintaining the cold wall (s) at this temperature is obtained by any known method such as mechanical cold, circulation of cryogenic fluid, Pelletier effect or any other. This temperature is chosen so that the vapor pressure of the organic compounds emitted during the reflow peak, at this temperature, is low enough for the membrane to be able to function under optimal conditions. It is preferable to have a two-way system, one for condensing and the other for evacuating condensed liquids or solids -> By trapping organic compounds in an activated carbon tower. A dust filter is advantageously mounted at the outlet of the tower in order to stop any carbon particles coming from the tower. -> By washing with water in a cyclonic washer of known technology. A step of drying the residual water in the gas mixture is then carried out by dissicant or automatic drainer. At the end of the fine filtration, a mixture is obtained here comprising in particular helium, but also air. a step of analysis of the oxygen content obtained at the end of fine filtration making it possible to compare the value thus measured with a prefixed threshold value, and to make the following choice: i) if the measured value is less than or equal to the prefixed threshold value, the gas from the second filtration is directed to the purification operation located downstream; j) if the measured value is greater than the prefixed threshold value, the gas from the second filtration is directed towards the ambient atmosphere as shown by the down arrow in the figure. - At the end of the analysis, and in series, there is a step of purifying oxygen (for example by adsorption or again by deoxo reaction on methane or hydrogen) then a membrane separator. The step of purifying oxygen by adsorption provides a gas typically comprising less than 200 ppm of oxygen or even less than a few tens of ppm. The subsequent membrane separation step is very particularly advantageous for eliminating other residues, with, as the case may be: nitrogen due to the air inlets in the oven, methane coming from deoxo, or H ₂ O, CO ₂ etc ... also according to the composition of the gaseous mixture used in the furnace itself but also in the gaseous barriers, the two mixtures being able to be of different nature (one could refer on this subject to patent application FR-03 50664 of October 9, 2003 in the name of the Claimant). - It is advantageously injected into the gas from the membrane separator, on the permeat side, of helium (the high performance gas used in the embodiment exemplified here). The purpose of this high-performance gas injection step is to limit to an acceptable rate the possible contaminants likely to gradually accumulate in the oven and which would not be eliminated by the recycling system and / or to compensate for the possible losses of gases related to purification. - The gas thus obtained is then re-injected into the oven, preferably in an area of the oven close to that corresponding to the reflow peak where the high performance gas fully plays its role.

Claims

1. Process for recycling the reflow gas used in a reflow soldering oven, reflow gas comprising at least one gas with high thermal performance, according to which: a) suction is carried out of at least a portion of the gas inside the oven; b) a pre-filtration of the gas thus aspirated is carried out (filtration 1); c) the gas from said pre-filtration is recompressed before directing the gas thus recompressed to a fine filtration operation (filtration 2). d) the gas from said fine filtration is treated by a purification operation comprising one or each of the following stages arranged in series: - an oxygen purification stage, - a membrane separation stage. e) a step of adding at least one of said high-performance gases to the gas resulting from said purification step is carried out. f) the gas from step e) is reinjected into the oven.

2. Recycling method according to claim 1, characterized in that one proceeds to a step of analysis of the oxygen content of the gas from the second filtration step, the value thus measured is compared to a threshold value prefixed, and the following measures are implemented: i) if the measured value is less than or equal to the prefixed threshold value, the gas from the second filtration is directed to said purification operation; j) if the measured value is greater than the prefixed threshold value, the gas from the second filtration is directed towards the ambient atmosphere.

3. Recycling method according to claim 1 or 2, characterized in that said oxygen purification step is carried out by adsorption.

4. Recycling method according to claim 1 or 2, characterized in that said oxygen purification step is carried out by passage through a deoxo system.

5. Recycling method according to one of the preceding claims, characterized in that said at least one high performance gas is helium.