WO1996004095A1

WO1996004095A1 - Soldering apparatus

Info

Publication number: WO1996004095A1
Application number: PCT/JP1995/001540
Authority: WO
Inventors: Toshiaki Yamasaki; Yoshiaki Yoshida
Original assignee: Teisan Kabushiki Kaisha
Priority date: 1994-08-04
Filing date: 1995-08-03
Publication date: 1996-02-15
Also published as: JPH0846346A

Abstract

A soldering apparatus containing a preheating zone and a soldering zone, which are adjacent to each other, conveyor means for articles to be soldered, which passes through these two zones, means for introducing nitrogen gas containing oxygen into the preheating zone as an atmosphere gas, means for introducing nitrogen gas into the soldering zone as an atmosphere gas, and means for forming a first nitrogen gas curtain and a second nitrogen gas curtain between both the preheating and soldering zones and on the exit side of the soldering zone, respectively.

Description

D E S C R I P T I O N

"SOLDERING APPARATUS'

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a soldering apparatus, and, in particular, to a soldering apparatus for soldering printed boards having electronic parts attached thereon, after a flux is applied thereto. Description of the Background

In the electronics industry, printed-wiring boards (also called "boards") remaining after soldering, and which are intended for use in electronic equipment, are usually washed in order to remove flux residue therefrom. The washing liquid typically used in this washing is a chlorofluorocarbon (CFC), such as freon 113, or methyl chloroform, both of which are environmen¬ tally harmful as both have been implicated in the destruction of the ozone layer.

Conventionally, there are two methods known for avoiding the use of such harmful washing liquids in the washing of printed boards. A first method entails actually changing the washing system and the second entails a non-washing. As far as the method of changing the washing system, the following methodologies are known. First, on soldering, a water-soluble flux may be used, and water-washing may be effected.

Also, a conventional flux may be used, and on washing, water admixed with a saponifying agent or another organic solvent, such as an alcohol, may be used.

Alternatively, a substitute freon may be used which does not adversely affect the ozone layer.

However, it is increasingly preferred, particularly in Japan, to improve the flux and the soldering apparatus in order to utilize non-washing, i.e., to avoid washing.

One such method, considered to be one of the most promising, entails soldering in an atmosphere of nitrogen gas. For this method, an apparatus for obtaining an excellent quality solder, even with a low activity flux, must be developed. I fact, such a method has already been put partially into practice, and such a flux and apparatus have been sold by some companies.

Generally, methods for automatically soldering printed boards may be classified into two systems, a flow system or a reflow system. In either case, the inside of the soldering apparatus is deactivated by nitrogen gas to restrict the oxygen content therein, with the result that excellent soldering can be realized even with a flux having a poor soldering feature, by preventing reoxidation in the inside of the apparatus.

Conventionally, the kinds of fluxes used in soldering, as well as the respective oxygen demands inherent in these fluxes, are based upon the degree of oxidation of the printed boards to be soldered.

In the soldering field, it is important to attain an excellent wettability of the solder. Conventionally, the wettability of the solder is controlled by changing the quantity of the solder used and the quantity of the activator contained therein.

The term "wettability" of a solder as used herein refers to the ability of the solder to conform to the soldering surface of parts when the activating temperature of a flux and the dissolving temperature of a solder are balanced.

However, it remains very difficult to attain an excellent solder wettability and, accordingly, fluxes used for soldering printed boards must be changed so as to be adaptable thereto. Furthermore, although it is desirable to maintain the concentration of oxygen present in the atmospheric gas in a furnace at a constant level, including the preheating zone and soldering zone of the soldering apparatus, this is difficult to achieve as the temperature distribution in the furnace of a conven¬ tional soldering apparatus is unstable and subject to thermal convection phenomena. This instability is due to the fact that the temperature in the furnace increases due to heat from the preheater and from the heat of molten solder in the solder tank. On the other hand, outside air may invade the furnace from both the entrance opening for admitting printed boards and from the exit opening for the removal thereof.

Accordingly, the distribution density of oxygen in the atmospheric gas in a furnace becomes uneven and, hence, it becomes difficult to maintain the homogeneous distribution of oxygen in the furnace.

This heterogeneous distribution of oxygen and heat in the furnace adversely effects soldering precision, and results in the formation of flux residues on the printed board products. In attempting to address these problems, there is presently known, for example, a soldering apparatus as disclosed in the official gazette of Japanese Patent Application Laid-open No. 29,655/1994, in which inert gas supply nozzles are arranged in turn in a chamber and the quantity of an inert gas to be supplied to the respective inert gas supply nozzles is increased or decreased in accordance with the temperature of the atmosphere in the chamber.

Another soldering apparatus is disclosed in the official gazette of Japanese Utility Model Laid-open No. 28,558/1993, in which a shutter mechanism and a curtain mechanism, which is a curtain structure made of a film plate or the like or a gas curtain of an inert gas, such as nitrogen, are attached at an entrance for admitting printed boards and an exit for removing the same so as to provide a partition wall, respectively. Unfortunately, none of these conventional soldering apparati afford a relatively constant level of oxygen concentration in the furnace atmosphere. Thereby, it remains difficult to prevent the formation of flux residues on printed boards. Thus, a need exists for a soldering apparatus which is capable of maintaining a relatively constant level of oxygen concentration in the atmosphere of a furnace, whereby the formation of flux residues on printed boards can be prevented. SUMMARY OF THE INVENTION

In accordance with the present invention, a soldering apparatus is provided which overcomes the above-noted disadvantages.

In more detail, the present invention provides a soldering apparatus having a preheating zone and a soldering zone, whereby a relatively constant level of oxygen concentration may be maintained in the furnace atmosphere.

Furthermore, the present invention provides a soldering apparatus, the use of which avoids the formation of flux residues on printed boards.

This object and others are provided by a soldering apparatus, which contains: a preheating zone and a soldering zone, which are adjacent to each other; conveyor means for articles to be soldered, which passes through these two zones; means for introducing nitrogen gas containing oxygen into the preheating zone as an atmospheric gas; means for introducing nitrogen into the soldering zone as an atmospheric gas; and means for forming a first nitrogen gas curtain and a second nitrogen gas curtain between both preheating and soldering zones and on the exit side of the soldering zone, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory longitudinal sectional view of the soldering apparatus according to the present invention.

FIG. 2 is an explanatory plane view of the soldering apparatus according to the present invention.

FIG. 3 is a perspective view of the duct in the soldering apparatus according to the present invention. FIG. 4 is an explanatory view of the atmosphere gas supply unit used in the soldering apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, it has been discovered that the addition of oxygen into a nitrogen atmosphere in accordance with the activity of a flux changes the concentration of oxygen contained therein, whereby it becomes possible to obtain a relatively constant level of oxygen concentration in the furnace atmosphere.

It has also been discovered in accordance with the present invention that by blowing nitrogen gas from the side of an exit for carrying out printed boards toward the side of an entrance therefore invasion by outside air can be avoided, whereby the concentration of oxygen in the atmospheric gas in a furnace can be maintained at a constant level, whereby formation of flux residues in printed boards may be avoided.

The present invention is predicated upon each of these discoveries.

The soldering apparatus according to the present invention is contained in a housing and is generally characterized by a preheating zone and a soldering zone, which are adjacent to each other; a conveyor means for articles to be soldered, which passes through these two zones; means for introducing nitrogen gas containing oxygen into the preheating zone as an atmospheric gas; a means for introducing nitrogen gas into the soldering zone as an atmospheric gas; and means for forming a first nitrogen gas curtain and a second nitrogen gas curtain between both the preheating and soldering zones and on the exit side of said soldering zone, respectively.

Each of the first and second nitrogen gas curtain forming means of the present invention contain a double pipe composed of an outer pipe and an inner pipe, each having a slit extending in its axial direction, and the respective slits are positioned on the sides opposite to each other.

The slit of the second nitrogen gas curtain forming means forms a nitrogen gas curtain inclined at an angle of about 15° to 90° downward from the horizontal plane toward the direction of the preheating zone. The means for introducing the nitrogen gas containing oxygen of the present invention includes a means for measuring the quantity of oxygen in the preheating zone and regulating said quantity of oxygen in accordance with the kind of a flux used in the soldering.

The present invention will now be further described by reference to the accompanying drawings.

The following legend is used in conjunction with Figures 1-4: 1) an automatic soldering mechanism, 2) conveyor belt, 3) housing, 4) preheater, having a plurality of heating elements, 5) jet type solder tank, 6) molten solder basin, 7) hood, 8) hood, 9) conveyance entrance, 10) exit, 11) release pipe, 11') opening, 12) release pipe, 12') opening, 13) diffusion structure, 14) diffusion structure, 15) gas curtain gushing duct, 16) hood, 17) gas curtain gushing duct, 18) slit opening, 19) outer pipe, 20) slit opening, and 21) inner pipe.

In Figures 1 and 2, the reference numeral (1) designates an automatic soldering mechanism for carrying out the soldering of printed circuit assemblies after a flux is applied on their soldering surface. Reference numeral (2) designates a conveyor belt for articles to be soldered such as printed boards coated with a flux. Reference numeral (3) designates a housing. Reference numeral (4) designates a preheater, and reference numeral (5) designates a jet type solder tank.

Reference numeral (6) designates a molten solder basin, and reference numeral (7) designates the upper portion (hood) of a structural front member disposed just above a preheating zone having said preheater (4). Reference numeral (8) designates the upper portion (hood) of a structural center member disposed just above a soldering zone opposed to said solder tank (5). Reference numeral (9) designates an entrance for conveyance of the printed boards formed upstream of said soldering zone, and reference numeral (10) designates an exit for conveyance of the printed boards formed downstream of said soldering zone.

In the present invention, a given flow rate of nitrogen gas, in which a preferred amount of oxygen gas, for example, oxygen gas in a fine amount of about

30 N liters/h or about 0.3 Nm³/h, is mixed, in accor¬ dance with the kind of a flux, in nitrogen gas at a flow rate of about 12 Nm³/h, is introduced into release pipe (11) disposed in the hood (7) though its opening (11'), and nitrogen gas is introduced into release pipe (12) disposed in hood (8) through its opening (12'). These gases are uniformly blown downward by way of net-shaped diffusion structures (13) and (14) stretched just above a conveyor passage for the printed boards. Between hoods (7) and (8) is disposed a nitrogen gas curtain gushing duct (15), and between the portion of the exit (10) for the printed board in hood (8) and the upper portion (hood) (16) of a structural rear member is also disposed nitrogen gas curtain gushing duct (17) .

Through the gas curtain gushing duct (15), only nitrogen gas, for example, at about 12.5 Nm³/h, is released vertically downwards under a pressure, for example, of about 3.5 kg/cm², so as to form a gas curtain, and through the gas curtain gushing duct (17), nitrogen gas is also released at an angle inclined about 15°-90° downward from the horizontal plane towards exit (10) in said hood (8), so as to form a gas curtain.

Each of gas curtain gushing ducts (15) and (17) of the present invention contains a double pipe composed of an outer pipe (19) having a slit opening (18) in its axial direction and an inner pipe (21) also having a siit opening (20) in its axial direction, as shown in Figure 3, and the outer and inner pipes (19) and (21) are arranged so that their slit openings (18) and (20) are at 180^c-angled positions different from each other.

In addition, outer pipe (19) of ducts (15) and (17) is arranged so that the direction of its slit opening (18) is perpendicular to the conveying direction of said printed boards.

For instance, a nitrogen atmosphere gas containing oxygen, which is released from release pipe (11) into the duct (7) of the present invention, may be prepared as described below with reference to Figure 4.

Figure 4 shows an atmospheric gas supply unit used in the soldering apparatus according to the present invention. The nitrogen atmosphere gas containing a fine amount of oxygen gas to be supplied to the release pipe (11) is introduced thereinto through a flow meter G4 and a pipe line H, after its mixing ratio is determined by a combination of anyone of pipe lines El, E2 and E3 connected with an oxygen gas source (not shown) by way of plural flow meters Gl - G3, which are different in flow rate and each has a stop valve DI, D2 or D3 inserted therein, and a pipe line C connected to a nitrogen gas source (not shown) . Into release pipe (12), on the other hand, nitrogen gas is introduced from pipe line (F) through flow meter G5 and a pipe line J. To the nitrogen gas curtain gushing duct (15), nitrogen gas whose flow rate is regulated, for example, to a flow rate of about 12.5 Nm³/h by means of a flow meter G6, is sent out under a pressure of about 3.5 kg/cm², so as to form a gas curtain, thereby intercepting the nitrogen atmosphere gas containing a fine amount of oxygen blown in the preheating zone and the nitrogen atmosphere gas blown in the soldering zone. In addition, also to the nitrogen gas curtain gushing duct (17), nitrogen gas whose flow rate is regulated by means of a flow meter G7 is sent out.

In Figure 4, reference letter K designates a mixer, L designates an oxygen analyzer and M designates a recorder.

In the soldering apparatus according to the present invention, due to the aforementioned construction, nitrogen gas containing a fine amount of oxygen, which is optimal in accordance with the kind of a flux, can be supplied into hood (7) forming the preheating zone for printed boards, whereby it becomes possible to use several kinds of fluxes in one soldering step. Furthermore, in the same soldering apparatus, there is a large profit capable of contributing to an improvement in the atmosphere participating in the wettability of a solder.

Moreover, in the soldering apparatus according to the present invention, due to the fact that gas is prevented from invading from the preheating zone into the upper zone of soldering tank (5) by means of a gas curtain formed by gas curtain gushing duct (15) and - 14 -

C L A I S 1. A soldering apparatus, comprising: preheating zone and a soldering zone, which are adjacent to each other; conveyor means for articles to be soldered, which passes through said two zones; means for introducing nitrogen gas containing oxygen into said preheating zone as an atmospheric gas; means for introducing nitrogen gas into the soldering zone as an atmospheric gas; and means for forming a first nitrogen gas curtain and a second nitrogen gas curtain between both said pre¬ heating and soldering zones and on the exit side of said soldering zone, respectively. 2. The soldering apparatus of claim 1, wherein said first and second nitrogen gas curtain forming means comprise a double pipe composed of an outer pipe and an inner pipe, each having a slit extending in its axial direction, and the respective slits are positioned on the sides opposite to each other.

3. The soldering apparatus of claim 2, in which the slit of said second nitrogen gas curtain forming means forms a nitrogen gas curtain inclined at an angle of about 15°-90° downward from the horizontal plane toward the direction of said preheating zone.

4. The soldering apparatus of claim 1, wherein said means for introducing the nitrogen gas containing

Claims

- 1 3 -

a gas curtain is obliquely formed from exit (10) for conveyance of the printed boards towards the upper zone of soldering tank (5) by way of gas curtain gushing duct (17), the upper zone may be filled with nitrogen gas, whereby external disturbances caused by the flowing of air from the outside or the like can be completely avoided.

Furthermore, due to the fact that the ducts in the soldering apparatus of the present invention contain a double pipe composed of an outer pipe and an inner pipe, each having a slit, and that these outer and inner pipes are arranged so that their slits are in 180°-angled directions different from each other, wherein gas is introduced to the inner pipe and this gas is released from the slit of the outer pipe through the slit of the inner pipe, the quantity of the gas released from the slit of the outer pipe becomes even along the same slit, even if the gas is supplied into the inner pipe from its one end. Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. oxygen includes a means for measuring the quantity of oxygen in said preheating zone and regulating said quantity of oxygen in accordance with the kind of a flux used in the soldering. 5. The soldering apparatus of claim 1, which is further contained in a housing.

6. The soldering apparatus of claim 1, wherein said preheating zone contains a plurality of heating elements. 7. The soldering apparatus of claim 1, wherein said first and second nitrogen gas contain forming means are ducts.

8. The soldering apparatus of claim 7, which further comprises an atmospheric gas supply unit in fluid connection with each of said first and second nitrogen gas curtain ducts.

9. The soldering apparatus of claim 8, wherein said atmospheric gas supply unit comprises a nitrogen source and an oxygen source, which form a junction upstream of two flow metering means, each of which in turn is upstream of said first and said second nitrogen gas curtain ducts.

10. The soldering apparatus of claim 9, wherein said oxygen source is in fluid connection with a plurality of piping means, each equipped with a flow metering means affording a flow rate different from the others, said piping means being downstream of said oxygen source and upstream of said junction of said nitrogen source and said oxygen source.

11. A method of soldering a printed board whereby formation of flux residues there is avoided, which comprises soldering said printed board in a soldering apparatus, which comprises: a preheating zone and a soldering zone, which are adjacent to each other; conveyor means for articles to be soldered, which passes through said two zones; means for introducing nitrogen gas containing oxygen into said preheating zone as an atmospheric gas; means for introducing nitrogen gas into the soldering zone as an atmospheric gas; and means for forming a first nitrogen gas curtain and a second nitrogen gas curtain between both said pre¬ heating and soldering zones and on the exit side of said soldering zone, respectively.

12. The method of claim 11, wherein said first and second nitrogen gas curtain forming means comprise a double pipe composed of an outer pipe and an inner pipe, each having a slit extending in its axial direction, and the respective slits are positioned on the sides opposite to each other. 13. The method of claim 11, in which the slit of said second nitrogen gas curtain forming means forms a nitrogen gas curtain inclined at an angle of about 15°-90° downward from the horizontal plane toward the direction of said preheating zone.

14. The method of claim 11, wherein said means for introducing the nitrogen gas containing oxygen includes a means for measuring the quantity of oxygen in said preheating zone and regulating said quantity of oxygen in accordance with the kind of a flux used in the soldering.

15. The method of claim 11, which is further contained in a housing.

16. The method of claim 11, wherein said preheating zone contains a plurality of heating elements.

17. The method of claim 11, wherein said first and second nitrogen gas contain forming means are ducts. 18. The method of claim 17, which further comprises an atmospheric gas supply unit in fluid connection with each of said first and second nitrogen gas curtain ducts.

19. The method of claim 18, wherein said atmospheric gas supply unit comprises a nitrogen source and an oxygen source, which form a junction upstream of two flow metering means, each of which in turn is upstream of said first and said second nitrogen gas curtain ducts. 20. The method of claim 19, wherein said oxygen source is in fluid connection with a plurality of piping means, each equipped with a flow metering means affording a flow rate different from the others, said piping means being downstream of said oxygen source and upstream of said junction of said nitrogen source and said oxygen source.