Soldering device with shield of gas substantially excluding oxygen

Abstract: The invention relates to a soldering device comprising: a solder tower; transporting means for carrying the objects for soldering along the upper part of the solder tower; means for causing a solder wave to issue from the solder tower, which wave comes into contact with the under-side of the objects for soldering, wherein feed means are arranged for supplying gas substantially excluding oxygen to the direct vicinity of the solder wave generated by the solder tower. Using this device the gas substantially excluding oxygen is only supplied to the place where the soldering process is taking place so that the quantity of supplied gas is limited. According to a preferred embodiment secondary feed means are arranged to supply gas substantially excluding oxygen to the vicinity of the solder tower. Dispersal of the gas supplied directly to the soldering location is hereby prevented.

Description: The invention relates to a soldering device, comprising: a solder tower; transporting means for carrying objects to be soldered along the upper part of the solder tower; means for causing a solder wave to flow from the solder tower, which wave contacts the underside of the objects to be soldered; supply means for supplying gas substantially excluding oxygen to the direct vicinity of the solder wave generated by the solder tower, wherein the supply means are arranged for directing the gas stream emerging from the supply means towards the the dividing line between the objects to be soldered and the solder wave emerging from the tower. Such a soldering device is known from DE-A-2063284. In the soldering device disclosed in said document, the solder tower is adapted to have the solder flowing down at the front side of the solder tower, or continuously at both the front and rear side of the solder tower. The supply means are adapted to direct the gas stream emerging from the supply means substantially parallel to the direction of flow of the solder wave emerging from the tower. A preheating of the gas flow does take place continuously. In the soldering device according to the invention, the solder tower is adapted to have the solder also flowing down at the rear side of the solder tower, and then only when the solder wave is pushed up by a passing object, and further is, according to the invention, the direction of the movement of the gas flow emerging from the supply means a component contrary to the direction of the flow of the solder wave emerging at the rear side of the solder tower or in the solder waves pushed up by a passing object to be soldered. According to the invention thus a preheating is only obtained when the solder wave is pushed up by a passing object, which is of course the object to be soldered, so that the heating of the gas flow only takes place when there is indeed an object to be soldered. Further, the invention provides in bringing the oxygen free gas at the location where oxidation is most critically; at the seperation line between the soldered object and the more or less parallel flowing flow off solder are seperated. At that location the separation takes place slowly because the flow of solder and the direction of movement of the soldered objects are substantially parallel, so that the forming of icicles and other impairing structures is the most likely at that location. This is avoided by the supply of oxygen free gas. The present invention will subsequently be elucidated with reference to the following embodiments, wherein: fig. 1 shows a perspective view of a soldering machine in which the device according to the present invention is applied; and fig. 2 shows a sectional view of the soldering device according to the present invention which is accommodated in the soldering machine depicted in fig. 1. The soldering machine shown in fig. 1 is placed in a housing 1 inside which is arranged a conveyor belt 2 along which the printed circuit boards 3 for soldering are moved forward. The conveyor belt is formed in the present embodiment by a so-called finger conveyor belt, but this is not essential; it is possible to employ other types of transporting means. Arranged in housing 1 is a window 4 through which the conveyor belt 2 is visible. The conveyor belt is further guided through a soldering device 5. Further forming part of the soldering machine are a flux device 6 located upstream and a pre-drying device (not shown in the drawing), which otherwise do not form subject-matter of the present invention and will not be further discussed here. A control panel 7 is fixed to the housing 1. The soldering device 5 is shown in detail in fig. 2. The soldering device 5 is formed by a pot 8 inside which is placed liquid solder 9. The solder 9 is heated by a heating device 10 present on the side of the pot so that it becomes and remains liquid. A solder tower 11 is arranged in the pot 8, wherein the solder 9 present in the pot is pumped upward through the solder tower 11 by means of a pump (not shown in the drawing), wherein the solder flows downward over a front weir 12 and a rear weir 13. The conveyor belt 2 extends above the thus created solder wave so that the printed circuit boards 3 transported by the conveyor belt are soldered on their underside. The above represents only a summary description of the soldering device, since this per se forms no part of the present invention. As shown in fig. 1, at the location of pot 8 two divided side plates 14, 15 are arranged above the pot on each of the sides of the conveyor belt 2. At the top both upper plates 15 are connected by a top plate 16 in which an opening 17 is arranged which can be covered by means of a removable window 18. The latter is fastened by means of fixation devices 19. The lower plates 14 are connected to the solder pot 8. This construction makes it possible to move the conveyor belt upward and then to move the solder pot sideways out of the machine. Both lower side plates 14 are connected at their bottom by diverse underplates 20. The lower side plates 14 are further provided with the downward extending portions which, as shown in fig. 2, extend in the normal position into the pot 8. The sides of both downward extending portions are joined by plates 21 which likewise extend into the pot. A tunnel 22 is thus formed which is entirely closed on both sides and at the top and bottom. The front and rear sides of the tunnel are closed off as far as possible by flexible strips or flaps 23, 24 which enable the entry and exit respectively of printed circuit boards for soldering and soldered boards. It is thus possible to create a space that is closed off as much as possible, in which the soldering process can be performed while a protective gas is supplied. The feed device for supplying protective gas is integrated into the tower. The feed device is formed by an elongatedly formed box 28 arranged on the outflow side of the tower 11. The box 28 is provided on its underside with an opening 29 and provided at the top with an outflow opening 30 on one side. The box is otherwise wholly closed. The box is provided with a stepped upper part 31. Arranged from the top is a downward extending middle wall 32. The latter divides the box into a compartment with a substantially triangular section 33 and a compartment 34 with a substantially rectangular section. Arranged in the compartment 33 of triangular section is a feed tube 35 provided with a number of openings distributed over the whole length. In the embodiment shown the openings are arranged such that they are oriented diagonally towards the apex of the compartment of triangular section. The diameter of the tube 35 is further chosen such that between the tube 35 and the middle wall, respectively the outer wall of the box there is a narrow passage through which the protective gas must pass; this results in improved distribution in the breadth of the gas substantially excluding oxygen. The gas substantially excluding oxygen leaving this tube 35 moves downward, as shown by the arrows in the drawing, through a passage between the dividing wall 32 and the side wall of the box 28 and thus passes into the compartment 34. The gas substantially excluding oxygen will there move upward and flow out against the underside of the upper wall 31 so that it arrives at the place where the printed circuit board is separated from the solder wave. The outflowing gas substantially excluding oxygen is thus pre-heated by the box. The gas substantially excluding oxygen will of course flow out of the outlet opening 30 at the top. It will then move over the solder wave to the place where the printed circuit board is separated from the solder wave. Thus is achieved that the separation between the printed circuit board and the solder wave is wholly covered with gas substantially excluding oxygen. The occurrence of solder whiskers is in any case not inconceivable partly as a result of the fact that the printed circuit board and the solder wave have a movement component in the same direction. This is avoided to a great extent by supplying gas substantially excluding oxygen. This second embodiment otherwise has the advantage that the covering of the upper part of the solder wave with gas substantially excluding oxygen is to a great extent ensured so that oxidation is prevented. The greater part of the solder flows over the front weir 12 counter to the direction of movement of the printed circuit boards and flows downward again, while only a small portion of the solder flows downward through the compartment 34, and then only when the solder wave is pushed up by a passing printed circuit board. The gas substantially excluding oxygen supplied to the circuit board is thus pre-heated, which has a favourable effect on the soldering process through the absence of temperature gradients. Gas substantially excluding oxygen is fed to the environment. For this purpose three gas diffusor pipes 27 are arranged above the conveyor belt 2. Finally, a separate chip soldering device 40, which otherwise forms no part of the invention and which will not be described further, is arranged upstream of the solder tower. It will otherwise be apparent that diverse components of the two embodiments shown can be combined or used separately. It is important that the gas substantially excluding oxygen is supplied to the location where the printed circuit board leaves the solder wave. A high temperature prevails at this location so that the danger of oxidation is great there. By thus locally supplying gas substantially excluding oxygen the total quantity of protective gas required is quite small; this is of importance because protective gas must be supplied continuously during the whole soldering process. Although it is possible to employ such a device without cover, the results will improve if the cover is present in the form of a tunnel or otherwise.

Claims: Soldering device (5), comprising: a solder tower (11); transporting means (2) for carrying objects (3) to be soldered along the upper part of the solder tower (11); means for causing a solder wave to flow from the solder tower (11), which wave contacts the underside of the objects (3) to be soldered; supply means (31-35) for supplying gas substantially excluding oxygen to the direct vicinity of the solder wave generated by the solder tower (11), wherein the supply means (31-35) are arranged for directing the gas stream emerging from the supply means (31-35) towards the dividing line between the objects to be soldered (3) and the solder wave emerging from the tower (11), wherein: the solder tower is adapted to have the solder also flowing down at the rear side of the solder tower (11) and then only when the solder wave is pushed up by a passing object, and the direction of movement of the gas flow emerging from the supply means has a component contrary to the direction of the flow of the solder wave emerging at the rear side of the solder tower when the solder wave is pushed up by a passing object to be soldered. Soldering device as claimed in claim 1, characterized in that the supply means (31-35) comprise a channel (34) that is guided such that the gas flowing through the channel (34) is heated by the solder flowing through the shaft (11) of the solder tower. Soldering device as claimed in claim 1 or 2, characterized in that the outflow openings (30) of the supply means (35) for the gas substantially excluding oxygen are located at the uppermost level of the solder wave. Soldering device as claimed in any of the preceding claims, characterized by secondary supply means (27) for supplying gas substantially excluding oxygen to the vicinity of the solder tower (12). Soldering device as claimed in claim 4, characterized in that the secondary supply means (27) comprise at least a supply member for gas substantially excluding oxygen placed above the transporting means (2) for the objects (3) for transporting. Soldering device as claimed in claim 5, characterized in that the secondary supply means are formed by separate supply members (27) which are each placed in pairs above and below the conveyor belt, respectively upstream and downstream of the solder tower. Soldering device as claimed in any of the foregoing claims, characterized in that an envelope (14-20) is placed round the solder tower (12), the transporting means (2) and the supply means (31-35).

Inventors: DEN DOPPER ROLF ARTHUR, DEN DOPPER, ROLF ARTHUR
Applicants: SOLTEC BV, SOLTEC B.V.
Application year: 1992
Main classification: B23K1
IPC codes: B23K3, B23K1, H05K3