FR2720018A1 - Method and appts. for soldering electronic components using paste solder and preheat and reflow in oven - Google Patents

Abstract

A method for soldering an electronic component comprises subjecting paste solder to a preheat treatment and a reflow treatment in turn with time in an oven constructed into a non-closed structure having an inlet section and an outlet section which allows flowing of an ambient atmosphere into the oven body, resulting in fixing the electronic component on a printed board by soldering using the paste solder; and introducing dehumidified air into the oven while an air balance in the oven is kept by allowing an ambient atmosphere to flow into and out of the oven body through the inlet and outlet sections to provide an interior of the oven with a dehumidified atmosphere.

Description

The present invention relates to a method and an apparatus for welding

  electronic components and more particularly a method and an apparatus for welding electronic components in which the preheating and reflow are carried out successively in an oven whose unclosed structure allows the circulation of the ambient atmosphere in the oven, so as to obtain the mounting of an electronic component on a printed circuit board by welding

using a solder paste.

  A nitrogen reflow soldering device has been developed with the aim of eliminating the cleaning after soldering of an electronic component using a solder paste. The reflow soldering apparatus known in the art is designed to perform soldering in a non-oxidizing atmosphere inside a furnace supplied with nitrogen to prevent oxidation of the soldering alloy and soldered connection pads in order to eliminate the use of Freon (registered trademark), trichlorethylene or the like in the cleaning operation

subsequent.

  To this end, the conventional reflow welding apparatus includes a closed structure furnace body to maintain the oxygen concentration or the atmosphere inside the furnace body as low as 10 to ppm to minimize the nitrogen consumption. More particularly, the furnace body is divided into a nitrogen purge chamber inlet section, a preheating and reflow section and a nitrogen purge chamber outlet section, among which the inlet sections and outlet are each provided with a two-stage shutter to isolate the welding area inside the furnace body from the ambient atmosphere. In addition, to minimize the outflow of nitrogen from the furnace body, a section of bearings is provided for rotary shafts of fans arranged inside the furnace body, each of the shafts being located in a closed structure, and a conveyor or transport system is arranged so as to automatically control, in synchronism with the operation of the shutters, the speed at which a printed circuit board is introduced into the inlet section of the nitrogen purge chamber and is discharged through the section of

nitrogen purge chamber outlet.

  Although the conventional reflow soldering apparatus constructed in the manner described above allows satisfactory soldering of electronic components, it causes an increase in the cost of the apparatuses because it is necessary to construct the furnace body in the form of a closed structure. In addition, it requires not only a large amount of nitrogen but also the inconvenient replacement of a nitrogen bottle when such a bottle constitutes the nitrogen source. Furthermore, when a nitrogen-producing unit is provided in association with the reflow soldering apparatus, this leads to an additional increase in the cost of the apparatus. However, when the furnace body is constructed as an unclosed structure, the conventional reflow welder is unable to prevent the appearance of weld holes and the formation of an oxide layer on the surface. welding alloy because it does not prevent

  the ambient atmosphere from entering the furnace body.

  The object of the present invention is to remedy the drawback mentioned above. To this end, it has been observed by studying the effect of the ambient atmosphere on the solder alloy and the flux contained in the solder paste that the humidity contained in the ambient atmosphere has a harmful effect on the activity. of the flow. Thus, the present invention excludes the replacement of oxygen by nitrogen in a body

closed structure furnace.

  The object of the present invention is to propose a method of welding an electronic component which is capable of achieving satisfactory welding even in an oven of unclosed structure allowing the ambient atmosphere to

enter the oven.

  The present invention also aims to provide an apparatus for

  solder an electronic component which is capable of performing satisfactory welding

  even in an oven with an unclosed structure allowing the atmosphere

  room to enter the oven.

  According to one of its aspects, the present invention relates to a method of welding an electronic component. This process comprises the stages consisting in subjecting a solder paste successively to a preheating treatment and to a reflow treatment in an oven of unclosed structure which allows the ambient atmosphere to penetrate into the body of the oven, so as to fix the electronic component on a printed circuit board by soldering using solder paste and introducing dehumidified air into the oven to establish a

  dehumidified atmosphere inside the oven.

  In general, the absorption of moisture or water by an activator, a resin or other content in the flux present in the solder used to solder an electronic component leads to a decrease in activity of the flux. In the present invention, dehumidified air is introduced into the oven to establish a dehumidified atmosphere inside the oven to prevent deterioration of the activity of the solder paste flux, so that the flux forms a diaper

  satisfactory protective coating on the solder paste.

  In a preferred embodiment of the present invention, the dehumidified air is an air-nitrogen mixture formed by mixing the dehumidified ambient atmosphere and nitrogen. The nitrogen contained in the air-nitrogen mixture prevents oxidation of the connection pads of a conductor network and the solder alloy, which guarantees satisfactory soldering of the electronic component. In a preferred embodiment of the present invention, the preheating treatment and the reflow treatment are carried out in separate chambers which are defined inside the oven. Such a construction allows the efficient introduction of dehumidified air into any desired section of the oven and maintains the temperature profile which extends from preheating to reflow

  and which is intended for heating the solder paste.

  According to another aspect of the present invention, there is provided an apparatus for soldering an electronic component. This appliance includes a

  unclosed structure which allows the ambient atmosphere to enter the oven.

  The oven includes a preheating device and a reflow device allowing a heat treatment to fix the electronic component on a printed circuit board by soldering using a solder paste. The apparatus also includes a dehumidified air inlet section provided on the oven, for introducing dehumidified air into the oven. Thus, the apparatus according to this

  invention limits increases in equipment costs.

  In a preferred embodiment of the present invention, the dehumidified air inlet section includes a mixture inlet section

  air-nitrogen. The inlet section of the air-nitrogen mixture includes an intro passage

  dehumidified ambient atmosphere duction, a nitrogen introduction passage and an agitator for connecting the atmosphere introduction passage to each other

  dehumidified room and the nitrogen introduction passage. So the air-

  dehumidified nitrogen can be efficiently introduced into the furnace in any desired position. In a preferred embodiment of the present invention, the preheating device and the reflow device are separated from each other

  by a separation device provided in the oven.

  Other characteristics and advantages of the invention will appear better

  in the detailed description which follows and refers to the attached drawings, given

  by way of example only, and in which: Figure 1 is a schematic sectional view of an apparatus for soldering an electronic component according to the present invention; Figure 2 is a graphical representation of the relationship between temperature and time in the welding area of the apparatus of Figure 1; and FIG. 3 is a schematic sectional view of a welding alloy on which the flux forms a protective layer during a heat treatment

  made in the apparatus of Figure 1.

  Referring first to Figure 1, there is shown a reflow oven used to mount an electronic component on the surface of a printed circuit board according to the method of soldering electronic components of the present invention. The reflow oven receives a printed circuit board on which an electronic component has been temporarily temporarily held by applying a solder paste on the connection pads of a network of conductors located on the printed circuit board, each of the electrodes external of the component being placed between predetermined connection pads, to which solder has been applied, while the contact between these elements

is maintained.

  As shown in Figure 1, the reflow oven comprises an oven body 10 of unclosed structure which allows the ambient atmosphere to enter the oven. More particularly, the oven body 10 is constructed in such a way that its inlet and outlet sections respectively allow the introduction of a printed circuit board into the oven body 10 and the evacuation thereof from the furnace body 10 are opened to allow the air present in the furnace body 10 to reach a state of equilibrium by the penetration of the ambient atmosphere into the furnace body 10 by the inlet and outlet sections. exit. The oven body 10 is provided with a conveyor 12 such as a chain conveyor which transports the printed circuit board into a soldering area defined in the oven body 10. The interior of the oven body 10 is divided into a preheating section 14 constituting the first stage, a preheating section 16 constituting the second stage and a reflow section 18. The sections 14, 16 and 18 of the oven body 10 are each provided with two heating devices 20 vertically opposite the one to another. The temperature obtained by the heating devices 20 in each of the sections is determined as a function of the composition of the solder paste used. For example, the heaters 20 may be arranged so that the temperature of the preheating section 14 of the first stage is about 150C, that the temperature of the preheating section 16 of the second stage is approximately 170 to 175 ' C and that the temperature of the reflow section 18 is about 183C. In the embodiment shown, only two preheating sections 14 and 16 are provided. However, it is possible to provide, if necessary, three sections of

preheating of this type or more.

  The reflow oven constructed as described above is also provided on its upper side with a dehumidified air inlet section 22 for

  introduce dehumidified air into the furnace body 10. In the embodiment

  As shown, the dehumidified air inlet section 22 includes an inlet section of an air-nitrogen mixture formed by mixing the dehumidified ambient atmosphere and nitrogen. However, it should be noted that in the present invention it is not absolutely necessary to add nitrogen to the ambient atmosphere and that the dehumidified air can be constituted only by the dehumidified ambient atmosphere. In the embodiment shown, the air-nitrogen mixture inlet section 22 comprises an ambient atmosphere introduction passage 24 connected to an ambient atmosphere introduction device such as a fan or the like and a passage nitrogen introduction 26 connected to a nitrogen source such as a nitrogen bottle. The ambient atmosphere introduction passage 24 is provided with a desiccant which acts as a dehumidification device 28. Thus, the passage 24 serves as a passage for the introduction of dehumidified ambient atmosphere. The ambient atmosphere introduction passage 24 and the nitrogen introduction passage 26 are connected by a mixer or agitator 30 to a series of air-nitrogen mixture introduction passages 32 which are arranged so as to be branched . The agitator 30 has the function of mixing together the dehumidified ambient atmosphere and the nitrogen to form the air-nitrogen mixture. The air-nitrogen mixture introduction passages 32 are each connected to the oven body 10 thereby establishing communication with the interior of the oven body 10. Thus, the air-nitrogen mixture introduction passages 32 are each designed to guide the air-nitrogen mixture towards the furnace body 10. The air-nitrogen mixture introduction passages 32 are each provided with a device

  heater 34 which is used to heat the air-nitrogen mixture to a predetermined temperature

  mined, after which this mixture is introduced into the furnace body. Or alternatively, such a heating device 34 may be common to the passages 32. In the embodiment shown, the desiccant is arranged so as to act as a dehumidifier device 28. However, the dehumidifier device 28 can include a device dehumidified air introduction system which can be designed to introduce compressed air or the ambient atmosphere. In a modification, the dehumidified air introduction device can be designed to introduce air or the ambient atmosphere through a filter. In another modification, it is

  can be designed to circulate the ambient atmosphere in hollow fibers.

  The dehumidified air inlet section 22 is arranged so as to introduce the dehumidified air comprising the dehumidified air-nitrogen mixture or only the dehumidified ambient atmosphere into an area of the oven body 10 which extends from the preheating sections 14 and 16 to the reflow section 18. In the embodiment shown, the dehumidified air inlet section or the dehumidified air-nitrogen mixture inlet section 22 is arranged so as to introduce the mixture in a concentrated manner air-nitrogen dehumidified in the furnace body 10 in positions corresponding to the positions in which the flux contained in the solder paste forms a protective layer between the preheating of the solder paste which adheres to the surface of the printed circuit board under the action of the heating devices 20 of the preheating sections 14 and 16 and the melting of the solder alloy contained in the solder paste. Preferably the air

  dehumidified has a relative humidity of 20% or less. In addition, the air-

  dehumidified nitrogen preferably has an oxygen concentration of 10 to 100 ppm.

  In the embodiment shown, the dehumidified air inlet section 22 is arranged above the reflow section 18 so that the dehumidified air-nitrogen mixture is introduced in a concentrated manner into the reflow section taking into account the temperature to which the solder paste is heated. Alternatively, the dehumidified air inlet section 22 can be arranged so as to concentrate the air-nitrogen mixture in a concentrated manner in an area of the furnace body which extends between the last stage of the multiple preheating sections and the reflow section 18, depending on the conditions under which the flux forms a layer

protective.

  The preheating section 14, the preheating section 16 and the reflow section 18 are substantially separated or isolated from one another by partitions 36 and 38 so that they are in the form of chambers which are substantially isolated or independent from each other while ensuring the passage of the conveyor 12 in the oven body 10 through the partitions 36 and 38. Such an arrangement of the sections or chambers 14, 16 and 18 not only makes it possible to independently control the atmosphere prevailing in each of the chambers and keep it at the desired temperature thanks to the heating devices 20 but also to introduce the dehumidified air-nitrogen mixture into each of the chambers

  through each of the branched air-nitrogen mixture introduction passages 32.

  We will now describe the operating mode of the

  reflow constructed as described above.

  A printed circuit board on which an electronic component has been temporarily fixed before is transferred into the furnace body 10 by being carried on the conveyor 12. Therefore, the printed circuit board is subjected to a heat treatment in the area of welding which extends from the preheating section 14 of the first stage to the reflow section 18 passing through the preheating section 16 of the second stage. This heat treatment is carried out with a temperature profile such as that shown in FIG. 2 because the sections 14, 16 and 18 are in the form of chambers separated from each other by the partitions 36 and 38. FIG. 2 shows the temperature profile obtained when a eutectic solder is used as solder paste. The temperature profiles of the printed circuit board which are obtained when the latter is placed in the preheating section 14 and in the preheating section 16 are designated by the reference signs A and B,

  respectively, in FIG. 2. In the embodiment shown, the temperature

  erasure TO of profile B in the preheating section 16 is approximately 170 to 180 C and the time To which elapses from the start of A to the end of B is approximately 70 to 90 s. The temperature profile of the printed circuit board which is obtained in the reflow section 18 is located in the area represented by X and in the subsequent area in FIG. 2. Thus, during the heat treatment carried out with this temperature profile, the solder paste is first preheated to a certain temperature in the preheating sections 14 and 16 so that the flux contained in the solder paste begins to precipitate or separate on the surface of the solder alloy contained in the solder paste and / or on the surface of each of the connection pads of a network of conductors of the printed circuit board. Then, the heat treatment to which the printed circuit board is subjected during its transfer to the reflow section 18 is carried out at a higher temperature so that the solder alloy begins to melt. At point X (FIG. 2) at which the flux separates or precipitates to form a protective layer before the fusion of the solder alloy, the furnace body 10 is supplied in a concentrated manner with air-nitrogen mixture dehumidified by the section dehumidified air inlet 22 so that a dehumidified atmosphere is formed in the furnace body 10. Thus, the dehumidified atmosphere results

  the constant introduction into the furnace body 10 of the air-nitrogen mixture dehumidified

  midified by the air inlet section 22. However, the dehumidified atmosphere can be maintained by intermittent introduction of the dehumidified air-nitrogen mixture

  in the oven body 10 according to predetermined cycles.

  The dehumidified atmosphere thus formed in the furnace body 10 substantially prevents the degradation of the activity of the flux due to the absorption of moisture or water by the flux, so that the flux can form on the alloy of weld 40 a satisfactory protective layer 42. The protective layer 42 can be formed so as to cover substantially the entire surface of the solder alloy 40 just before the start of the melting of the solder alloy. The formation of this protective layer 42 effectively prevents oxidation of the solder alloy due to melting and allows the molten solder alloy to harden while retaining its wettability. In addition, the nitrogen contained in the dehumidified air-nitrogen mixture prevents the formation of an oxide layer on the welding alloy.

  and on the connection areas of the conductor network.

  Furthermore, the formation of the protective layer 42 by the flux and the melting and hardening of the solder alloy 40 described above effectively prevent the formation of solder balls and / or bridges due to the evaporation of the solder. water and welding defects due to the formation of an oxide layer on the welding alloy, which guarantees satisfactory welding. It is possible to reduce the content of resin (rosin), solvent and flux activator to avoid degradation of the activity of the flow due to the absorption of moisture and the formation of an oxide layer on the surface. welding alloy. This also helps to eliminate any

  cleaning operation after welding.

  As can be seen from the above, the present invention effectively prevents deterioration of the flux activity of a solder paste and the formation of solder balls in an oven body, even when that -it has an unclosed structure which allows the ambient atmosphere to enter it, and it avoids the formation of an oxide layer on the solder alloy of the solder paste so as to guarantee the reliable soldering of an electronic component on a printed circuit board. Furthermore, the present invention

  avoids an increase in fitting costs.

Claims (10)

  1. A method of welding an electronic component, characterized in that it comprises the steps consisting in subjecting a solder paste successively to a preheating treatment and to a reflow treatment in an oven having an unclosed structure which allows the ambient atmosphere to penetrate into the oven body (10), so as to fix the electronic component on a printed circuit board by soldering using solder paste, and to   introducing dehumidified air into said oven to create a dehumidified atmosphere   midified inside said oven.   2. Method according to claim 1, characterized in that said dehumidified air comprises an air-nitrogen mixture formed by mixing of atmosphere   dehumidified room and nitrogen.   3. Method according to claim 1, characterized in that said preheating treatment and said reflow treatment are carried out in chambers (14, 16, 18) defined in said oven so as to be separated from each other others.   4. Method according to claim 2, characterized in that said treatment   preheating and said reflow treatment are carried out in chambers (14, 16, 18) defined in said oven so as to be separated from each other others.   5. Method according to claim 1, characterized in that said air   dehumidified has a relative humidity of 20% or less.   6. Method according to claim 2, characterized in that said air   dehumidified has a relative humidity of 20% or less.   7. Apparatus for welding an electronic component, characterized in that   that it includes an oven with an unclosed structure which allows the atmos-   ambient sphere to enter said oven, said oven comprising a preheating device (14, 16) and a reflow device (18) in which a heat treatment is carried out to fix the electronic component to a printed circuit board by soldering using a solder paste, and a dehumidified air inlet section (22) provided on said oven for introducing dehumidified air into said oven.   8. Apparatus according to claim 7, characterized in that said section   dehumidified air inlet (22) includes an air- mix inlet section   nitrogen, said air-nitrogen mixture inlet section comprising an intro passage   duction of dehumidified ambient atmosphere (24), a nitrogen introduction passage (26) and an agitator (30) for connecting said introduction passage   dehumidified ambient atmosphere and said nitrogen introduction passage.   9. Apparatus according to claim 7, characterized in that said preheating and reflow devices are separated by partitions (36, 38) provided in said oven.   10. Apparatus according to claim 8, characterized in that said preheating and reflow devices are separated by partitions (36, 38) provided in said oven.