A soldering apparatus

Abstract: A soldering apparatus is disclosed which comprises a conveyor for moving print circuit boards along a predetermined path, an inert gas-filling chamber provided with a solder tank wherein the print circuit board is soldered in the inert gas atmosphere, primary and secondry, external air shutoff chambers positioned in front of and in rear of the inert gas-filling chamber, respectively. Both both primary and secondary chambers are provided respectively with a first shutter having a mechanism for open/close action and capable of isolating the inside of the chambers from external air and with a second shutter having a mechanism for open/close action and capable of preventing release of the inert gas in the inert gas-filling chamber and are connected through a valve to a vacuum tank capable of evacuating the primary and secondary chambers. The inert gas-filling chamber is connected through a valve to each of the primary and secondary chambers for introducing the inert gas in the inert gas-filling chamber into the evacuated primary and secondary chambers.

Description: The present invention relates to a new soldering apparatus for printed circuit boards carrying electronic parts and/or tip parts thereon, and more particularly to a soldering apparatus wherein a printed circuit board carrying electronic parts and/or tip parts temporarily fixed thereto is soldered under non-oxidative conditions while being conveyed in an atmosphere of an inert gas. There is known a soldering apparatus for soldering a printed circuit board carrying electronic parts and/or tip parts temporarily fixed thereto in an inert gas atmosphere which comprises an inert gas-filling chamber provided with a soldering bath and filled with an inert gas supplied from a gas reservoir such as a gas bomb, an external air shutoff chamber provided at each of the front and rear part of the inert gas-filling chamber along the moving direction of the print circuit board, and a pair of shutters fitted to the external air shutoff chamber, one shutter being capable of isolating external air and the other shutter being capable of isolating an air-contaminated inert gas in the external air shutoff chamber. The air in a soldering space of the apparatus is replaced by an inert gas to prevent oxidation of the solder paste with oxygen at a high temperature on soldering. In such conventional soldering apparatus provided with an external air shutoff chamber, a shutter is opened or closed at the front end through which printed circuit boards are introduced or at the rear end through which soldered printed circuit boards are taken out, but there is no device for positively inhibiting release of the inert gas from the external air shutoff chamber through the opened shutter. Accordingly, external air including oxygen intrudes into the external gas shutoff chamber, and also into the inert gas-filling chamber filled with the inert gas by opening the shutter between both chambers, so that the printed circuit boards cannot be soldered in a pure inert gas atmosphere, thus causing a problem that molten solder cannot be prevented from oxidation or the concentration of flux cannot be lowered to facilitate cleaning of the printed circuit boards. In order to effect soldering of the boards in inert gas which is not contaminated with external air, therefore, the pressure of the inert gas in the inert gas-filling chamber has to be elevated lest external air should be intruded into the chamber. In this case, however, a considerable amount of the inert gas is released from the chamber whenever the shutter is opened. Accordingly, the amount of the inert gas to be supplied has to be increased, to make the soldering operation economically unattractive. On the other hand, if a very small amount of external air is allowed to exist in the chamber, the printed circuit boards have to be pre-painted with a flux. However, the flux is heated to evolve smoke by molten solder in the apparatus. Thus, there is an additional problem that compulsory expelling of smoke out of the apparatus accompanies concurrent expelling of the inert gas with the smoke. In the above circumstances, there is a great demand in this art to develop a new soldering apparatus which can overcome the above-mentioned problems. It is known from DE-U-8520254 that both the intrusion of external air into the inert gas-filling chamber filled with an inert gas and the release of the inert gas from the chamber can be inhibited by providing the external gas shutoff chambers with separately functioning pairs of shutters and with a vacuum tank for rapidly evacuating the inert gas contaminated with external air from the chambers, thus enabling the soldering operation of printed circuit boards to be carried out in a purer inert gas atmosphere in the inert gas-filling chamber. The present invention is defined in Claim 1. It is an advantage of the present apparatus that printed circuit board can be soldered in an inert gas-filling chamber filled with uncontaminated inert gas and that the amount of inert gas released from the chamber can be rather small even when the second shutter is opened for introducing the board into the chamber. It is another advantage of the present apparatus that the inert gas contaminated with external air in the external air shutoff chambers can rapidly be evacuated and that the inert gas is introduced into the evacuated external air shutoff chambers from the inert gas-filling chamber filled with an inert gas so that the inner pressure of the external air shutoff chambers may be equal to that of the inert gas-filling chamber. In case the inner pressure of the external air shutoff chambers is equal to that of the inert gas-filling chamber, the open/close action of the second shutter becomes easy and smooth. The inert gas used in the apparatus of this invention should not be reactive with either the electronic tip parts or the solder at the soldering temperature. A rare gas, such as neon or argon, carbon dioxide and nitrogen may be suitably used. The present invention will more fully be understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration, and thus are not limitative of the present invention, and wherein: Fig. 1 is a side view of a printed circuit board to be soldered according to the present invention; Fig. 2 is a schematic explanatory drawing showing an example of the soldering apparatus of this invention; Fig. 3 is a schematic diagram showing a pipe line system of the apparatus shown in Fig. 2; and Fig. 4 is a side cross section showing the main part of the apparatus shown in Fig. 2. In Fig. 1 showing a printed circuit board to be treated with the apparatus of this invention, the reference numeral 1 is a printed circuit board, 2 electronic parts with lead wires and 3 electronic tip parts. These electronic parts 2 and/or tip parts 3 are temporarily fixed to the board 1 by way of an adhesive. In Fig. 2 showing the structure of one example of the soldering apparatus of this invention, the reference numeral 10 is a whole body of the soldering apparatus, 11 a carrier mounting the printed circuit board 1 thereto, 12 a mounting section where the work for mounting the printed circuit board 1 to the carrier 11 is done, 13 a conveying means in the mounting section 12, 14 a turn table where the moving direction of the carrier 11 shown by the arrow mark A is turned at right angles as shown by the arrow mark B to the direction shown by the arrow mark C, 15 a conveying means for the turn table 14 moving in the direction shown by the arrow mark C, 16 a fluxer where the board 1 is treated with a flux, 17 a conveying means in the fluxer section capable of moving the carrier in the direction shown by the arrow mark D, 18 a primary external air shutoff chamber provided with a first shutter 19 and a second shutter 20, 21 a conveying means in the primary external air shutoff chamber 18 capable of moving the carrier in the direction shown by the arrow mark E, and 22 an L-shaped inert gas-filling or treatment chamber. The chamber 22 is usually filled with an inert gas such as nitrogen and includes a preheater 23, a cart 24 provided with the preheater 23, a conveying means 25 capable of moving the cart 24 in the direction shown by the arrow mark F, a screw rod 26 engaged with the lower part of the cart 24, a guide shaft 27 fitted to the cart 24, a solder tank 28 a part of which is enclosed in the chamber 22, a first ejecting tank 29A of the solder tank 28, a second ejecting tank 29B of the solder tank 28, a molten solder 30, and a conveying means 31 in the solder tank 28 capable of moving the carrier in the direction shown by the arrow marks H and I. In the chamber 22, the print circuit board 1 is heated up to the temperature of molten solder 30 whereby the soldering operation of the board is finished. The reference numeral 32 is a secondary external air shutoff chamber provided with a third shutter 33 and a fourth shutter 34, 35 a conveying means in the chamber 32 capable of moving the carrier in the direction shown by the arrow mark I, 36 a turn table where the carrier is turned through right angles as shown by the arrow mark K and 37 a conveying means in the turn table 36. The conveying means 13, 15, 17, 21, 25, 31, 35 and 37 are engaged with the carrier 11 and are capable of moving the carrier in the indicated directions. A typical example of the conveying means is a chain conveyer. The shutters 19, 20, 33 and 34 are devised to open only when the carrier is passed therethrough. In Fig. 3, the same reference numerals as shown in Figs. 1 and 2 have the same meanings as given above. The reference numeral 41 is a gas bomb or the like gas reservoir as a supply source of an inert gas, 42 a cock of the bomb or reservoir, 43 a safety valve, and 44 a vacuum tank for evacuating the external air shutoff chambers 18 and 32 thereby rapidly eliminating the inert gas contaminated with external air occupied in the chambers. The vacuum tank 44 is connected to a vacuum pump P₁ which is capable of sucking air from the tank to maintain it under vacuum at all times. In addition to the vacuum tank 44, evacuating pumps P₂ and P₃ are provided to evacuate any remaining gas in the external air shutoff chambers 18 and 32, respectively. Pipe lines connecting these pumps, chambers and tanks are provided with valves V₁ - V₈ capable of regulating the rate of gas flow. In Fig. 4, the same reference numerals as shown in Figs. 1-3 have the same meanings as given above. The reference numerals 51 and 52 are drive chains capable of open/close action of the shutters 19, 20, 33 and 34, 53 a packing, and 54 eccentric shafts rotatable for pressing the shutters 19, 20, 33 and 34, in the closed position, against the packing 53 to maintain each of the external air shutoff chambers 18 and 32 air-tight. This shaft has an elliptical form in cross section. An ordinary rod provided with a cam of an elliptical form in cross section may also be used as the shaft 54. Referring again to Figs. 2-4, the process for soldering of printed circuit boards in the soldering apparatus 10 in accordance with this invention will now be described. In Fig. 2, the print circuit board 1 is mounted onto the carrier 11 in the mounting section 12 and the carrier 11 is moved in the direction shown by the arrow mark A and transferred to the chain conveyer 15 of the turn table 14. Next, the turn table 14 is rotated through right angles in the direction shown by the arrow mark B, whereby the moving direction of the carrier 11 is also rotated through 90°. The carrier 11 is transferred to the chain conveyer 17 and moved in the direction shown by the arrow mark C to the fluxer 16 where the board 1 is subjected to a flux treatment. After confirming that the carrier has not yet been entered in the primary external air shutoff chamber 18 by way of a sensor (not shown), the first shutter 19 is opened to move the carrier 11 in the direction shown by the arrow mark D and the carrier 11 is allowed to stop after transferring the chain conveyer from 17 to 21. At this stage, the first shutter 19 is closed and the valve V₁ shown in Fig. 3 is opened whereby the air in the chamber 18 is removed rapidly by suction to the vacuum tank 44 which has previously been evacuated by the action of the pump P₁. In addition, the pump P₂ is driven and the valve V₃ is opened to eliminate any remaining air from the chamber 18. The valves V₁ and V₃ are then closed to maintain the chamber 18 under reduced pressure. At this stage, the valve V₅ is opened to introduce an inert gas into the chamber 18 from the inert gas-filling chamber 22 previously filled with the inert gas supplied from the gas bomb or reservoir 41 through the valve V₈ whereby the gas pressure in the chamber 18 becomes equal to that in the chamber 22, thus making the difference in pressure between both chambers null to facilitate opening of the second shutter 20. The inert gas can directly be introduced into the chamber 18 from the gas bomb or reservoir 41. In this case, however, a device for measuring the pressure will be necessary in order to make the pressure in the chamber 18 equal to that in the chamber 22. Accordingly, the direct introduction of an inert gas into the chamber 18 from the gas bomb 41 is not desirable in view of the increase in cost and labor. The second shutter 20 is then opened to move the carrier 11 in the direction shown by the arrow mark E, and the carrier is transferred to the chain conveyer 25 moving in the direction shown by the arrow mark F from the chain conveyer 21. The carrier 11 is placed on the cart 24 provided with the preheater 23 and is preliminarily heated while being moved on the cart. A motor (not shown) is then driven to rotate the screw rod 26 whereby the cart 24 is moved in the direction shown by the arrow mark G along the guide shaft 27. After the cart 24 is stopped in a predetermined position, the carrier 11 is transferred to the chain conveyer 31 capable of moving in the direction shown by the arrow mark H, i.e. in the reverse direction to the arrow mark F, and is conveyed to the soldering tank 28 where the board 1 is contacted with waves of molten solder 30 formed by the first and second solder applicators 29A and 29B. The third shutter 33 of the secondary external air shutoff chamber 32 is then opened and the carrier 11 is moved towards the direction shown by the arrow mark I and transferred to the chain conveyer 35 in the chamber 32 previously filled with the inert gas in the same manner as described above in the chamber 18. The third shutter 33 is closed at this stage and the fourth shutter 34 is opened. The carrier 11 is moved out of the chamber 32 in the direction shown by the arrow mark J and placed on the turn table driven by the chain conveyer 37. After rotating the turn table through right angles in the direction shown by the curved arrow mark K, the carrier 11 is moved in the direction shown by the arrow mark L and transferred to the original chain conveyer 13. A cycle of the movement of the carrier 11 in the apparatus 10 is finished at this stage and the soldered printed circuit board 1 is thus replaced by a new printed circuit board. When the fourth shutter 34 is opened, external air is intruded into the chamber 32 and contaminates the inert gas occupied therein. Accordingly, the third and fourth shutters 33 and 34 are closed after the exit of the printed circuit board and the chamber 32 is evacuated by opening the valves V₂ and V₄ under the action of the pump P₃ whereby the inert gas contaminated with external air flows to the vacuum tank 44 and to the pump P₃ for venting. When the valves V₅ and V₆ are opened, the inert gas in the chamber 22 is supplied to the evacuated external air shutoff chambers 18 and 32 whereby the inner pressure of the chambers 18 and 32 are kept equal to that of the chamber 22. When a sensor (not shown) detects reduction in the pressure of the inert gas in the chamber 22, the valves V₇ and V₈ are opened to supply the inert gas from the gas bomb or reservoir 41. The temperature of the soldered printed circuit boards is depressed and the molten solder is solidified as soon as they are taken out of the apparatus so that the electronic parts and/or tip parts are finally fixed to the boards. In the event the printed circuit boards thus soldered have lead wires extended from the bottom surface thereof, the boards can be treated in a conventional manner for cutting the extending lead wires. It is a characteristic feature of the apparatus first described that the inert gas-filling chamber where the soldering operation of printed circuit boards is carried out is provided in both front and rear stages thereof with external air shutoff chambers. It is another characteristic feature of the illustrated apparatus that the primary and secondary external air shutoff chambers are respectively provided with two shutters; one being capable of inhibiting intrusion of external air into the chamber and the other being capable of inhibiting release of the inert gas from the inert gas-filling chamber. It is still another characteristic feature of the illustrated apparatus that the external gas shutoff chambers can be evacuated rapidly by using the vacuum tank and the vacuum pumps so that the inert gas contaminated with external gas which is intruded every time the first and third shutters are opened is removed from the chambers. These characteristic features bring about various merits. Firstly, the amount of the inert gas to be supplied to the inert gas-filling chamber can be minimized as the second and fourth shutters prevent release of a large amount of the inert gas as a result of a combination play of the first and third shutters. Secondly, the chambers filled with the inert gas contaminated with external air can be evacuated rapidly by utilizing the vacuum tank and the vacuum pump, so that the time required for moving the printed circuit boards and for the soldering operation itself can be significantly shortened. In addition, the valve connecting the inert gas-filling chamber and the external air shutoff chambers can be opened to make the difference in pressure between these chambers null so that the open/close operation of the second and fourth shutters can be made smoothly and no further device nor expense becomes necessary for measuring the pressure.

Claims: A soldering apparatus comprising: means (13,15,17,21,25,31,35,37) for conveying a printed circuit board (11,30) through a predetermined path; an air-tight treatment chamber (22) located in said path for soldering said printed circuit board in an inert gas atmosphere during passage through said treatment chamber; a source (41) of an inert gas; a supply conduit extending between said treatment chamber (22) and said inert gas source and provided with a feed valve (V8) operable to permit the inert gas to be fed from said inert gas source to said treatment chamber and to maintain the pressure within said treatment chamber within a predetermined range; means (28,29A,29B) provided within said treatment chamber for applying a molten solder to a printed circuit board travelling through said path; a primary chamber (18) disposed upstream of and contiguously to said treatment chamber; a secondary chamber (32) disposed downstream of and contiguously to said treatment chamber; evacuating means (44,P1) a first conduit extending between said primary chamber and said evacuating means and provided with first valve means (V1) operable to permit and prevent gas communication between said primary chamber and said evacuating means for evacuating said primary chamber when said printed circuit board is present therewithin; a second conduit extending between said secondary chamber and said evacuating means and provided with second valve (V2) means operable to permit and prevent gas communication between said secondary chamber and said evacuating means for evacuating said secondary chamber when said printed circuit board is present therewithin; a third conduit extending between said primary chamber and said treatment chamber and provided with third valve means (V5) operable to permit and prevent gas communication between said primary chamber and said treatment chamber for equalizing the pressure within said primary chamber with that within said treatment chamber; a fourth conduit extending between said secondary chamber and said treatment chamber and provided with fourth valve means (V6) operable to permit and prevent gas communication between said secondary chamber and said treatment chamber for equalizing the pressure within said secondary chamber with that within said treatment chamber; a first shutter (19) provided in the upstream end of said primary chamber and movable between open and closed positions so that the printed circuit board can enter said primary chamber when said first shutter is in said open position and ambient air is prevented from flowing into said primary chamber therethrough when said first shutter is in said closed position; a second shutter (20) provided between said primary chamber and said treatment chamber and movable between open and closed positions so that the printed circuit board in said primary chamber can pass to said treatment chamber therethrough when said second shutter is in said open position and the inert gas within the treatment chamber is prevented from flowing into said primary chamber therethrough when said second shutter is in said closed position; a third shutter (33) provided between said secondary chamber and said treatment chamber and movable between open and closed positions so that the printed circuit board in said treatment chamber can pass to said secondary chamber therethrough when said third shutter is in said open position and the inert gas within the treatment chamber is prevented from flowing into said secondary chamber therethrough when said third shutter is in said closed position; and a fourth shutter (34) provided in the downstream end of said secondary chamber and movable between open and closed positions so that the printed circuit board can exit said secondary chamber when said fourth shutter is in said open position and ambient air is prevented from flowing into said secondary chamber therethrough when said fourth shutter is in said closed position.

Inventors: KONDO KENSHI, KONDO, KENSHI
Applicants: NIPPON DENNETSU KEIKI KK, NIHON DEN-NETSU KEIKI CO., LTD.
Application year: 1992
Main classification: H05K3
IPC codes: B23K1, H05K3