WAVESOLDERING DEVICE AND PROCEDURE

Abstract: The description refers to a device and a procedure for the wavesoldering of printed circuits, in particular for the soldering of components suitable for the surface mounting. As claimed in the invention the soldering wave - or a portion of the same - is formed by molten solder flows progressing in an orbital direction. Said flows can progress horizontally (0) with a direction that cyclically moves between 0 and 360 DEG , causing the removing of gas/air bubbles - in case they are trapped near a component (11) - in any position they stagnate. Any soldering failure due to ''dry soldering'' is therefore avoided.

Description: AVESOLDERING DEVICE AND PROCEDURE DESCRIPTION This invention relates to a wavesoldering procedure for printed circuit boards and to a suitable device for its implementation. As known the wavesoldering methods are so shaped as to convey the printed circuit boards towards one or more waves of molten solder (it is usually used a tin/lead solder carried to a fluid state by means of heating) which "bath" the feet (or the rheophores) of the components laying on the boards as well as the positioning therefor arranged. Once the solder gets cold, the feet are soldered to the surface of the boards. The coming of the surface mounting components (known as "SMD") has caused the rise of some problems which are known in the technique as "dry soldering" or as "solder skips". The procedure and the device according to the invention are hereinafter described referring to the soldering of SMD units by means of a single wave device, but it can be successfully used for the soldering of any components by means of either a single wave or a double wave device (in one or in both waves). The surface mounting units are characterized in that the feet are so shaped as to allow the soldering on the same face of the printed circuit where the components are placed. The phenomenon known as "dry soldering" occurs when air and/or gas bubbles are trapped between two contiguous components, i.e. near to a component, and inhibit the molten solder from bathing the feet and the positioning, even if the component is touched by the wave for a long period of time. The European Patent Application No. 159.425 describes a plurality of solutions for the problem specified above. Said solutions provide molten solder flows progressing in an alternative movement which is oblique in relation to the direction of the printed circuit. The progressive transverse direction of said molten solder flows allows the removal of air or gas bubbles, by partly reducing the soldering failures due to dry soldering. From an analysis carried out by the applicant, it resulted that the transverse movement of said molten solder flows is positive to the extent of the removal of the air/gas bubbles present on two of the component's sides (front and rear side in relation to the progressing direction of the component/printed circuit), while air/gas bubbles remain on the other sides (on the sides of the component) . Purpose of the present invention is a procedure and a device suitable to remove said air/gas bubbles from any of the component's sides, in order to cancel solder failures due to this reason. Object of the present invention is therefore a procedure for the wavesoldering where the printed circuits and their components are "bathed" by a molten solder wave. According to the invention the wave is formed by a plurality of molten solder flows having an orbital movement. According to an alternative embodiment, a first portion of the wave can be constituted by molten solder flows having an orbital movement, while the remaining part of the wave is of traditional type. Another object of the present invention is a device for the wavesoldering of printed circuit boards of the type which comprises suitable means to produce a molten solder wave which "baths" the printed circuit boards and the components there mounted. The invention provides the presence of means suitable to resolve the wave into a plurality of molten solder flows having an orbital movement. According to a preferred embodiment of the invention the orbital movement of the molten solder flows can be obtained by joining (directly or through coupling means) to the connecting rod small end of a first and a second cam the ends of a body which can be crossed by molten solder flows in vertical direction. When at least one of the said cams progresses in a rotative direction, the body joined to it performs an orbital movement which on its turn set the molten solder flows crossing it in a horizontal movement. As the direction of said horizontal movement component cyclically varies in an amplitude between 0 and 360 degrees, the air/gas bubbles are removed in any position they are. Further characteristics of the invention can be taken from the below description which refers to a non-limiting example shown in the enclosed drawings in which: fig. 1 schematically shows a device for the wavesoldering according to the present invention; fig. 2 shows in details the wavesoldering device 5 of fig. 1 in order to illustrate the procedure according to the invention; fig. 3 illustrates the diagrams of the forces which act on the molten solder flows according to the invention; fig. 4 is a view schematically showing a first example of the wavesoldering device 5 of fig. 1, implemented according to the invention; - fig. 5 is a view schematically showing a second example of a wavesoldering device 5 of fig. 1, according to the invention; fig. 6 schematically shows a third _mbo i___ι_ι± of the wavesoldering unit 5 of fig. 1, according to the invention; fig. 7 illustrates a second example of a body 15 of fig. 2, 4, 5 and 6; fig. 8 illustrates a third example of body 15 of fig. 2, 4, 5 and 6. Fig. 1 shows a wavesoldering device so shaped as to comprise: a transport unit 1, suitable to drive the printed circuit boards, where the components to be soldered are mounted, towards the functional units hereinafter listed, everyone of them performing a defined processing; a flux unit 2, suitable to spray deoxidizing fluids (known as flux oils) in order to eliminate the oxide from the parts which are interested by the soldering process and to improve the "wettability"; a drying unit 3, suitable to eliminate the diluent contained in the fuxing oils deposited on the printed circuit boards; a preheating unit 4, suitable to increase the temperature of the printed circuit boards up to an optimal value to the extent of the perfo ance of the soldering process; a soldering unit 5, suitable to produce one or more soldering waves which baths the printed circuit boards causing the soldering of the components there located. The unit of fig. 1 has been naturally cited as a non-limiting example, as the procedure according to the invention can be applied to any wavesoldering unit (table- or bench type), using in total or partially the functional units mentioned above. Figure 2a illustrates a section of the soldering unit 5, perpendicular to the main axis of the wave produced, in order to point out the travel path along which the molten solder moves following to the operation of the pumping means. Figure 2b shows an axonometry of the soldering unit 5 without the molten solder, in order to evidence the configuration of the elements which compose it. Figures 2a and 2b show that a couple of walls 7 and 8 are located inside the well 6 and delimit a duct 9 through which the solder, in a fluid state, flows operated by the pumping means, here not illustrated as they are well known. The pumping means forces the molten solder along the duct 9 and it comes out from the upper opening, creating in this way a wave towards which the printed circuit boards 10, with their components 11, are conveyed. In order to limit the height of the jump of the molten solder at it ' s fall back to well 6 (and to contain the scruffs due to the solder oxidation), a further pair of walls 12 and 13 are placed next to the external side of the walls 7 and 8 which delimit the duct 9. The walls 12 and 13 are so shaped as to contain the molten solder and to allow a gradual transfer into the well 6 through an opening 14 provided between the lower end of the walls 12 and 13 and the external sides of walls 7 and 8. Figures 2a and 2b illustrate the soldering procedure, which is purpose of the present invention. According to this procedure the soldering wave is formed by a plurality of molter solder flows having an orbital movement. Said flows may be obtained by placing a body 15, provided with a plurality of openings 16, against the upper nozzle of duct 9 giving to said body an orbital movement. Openings 16 on body 15 cause the wave resolution into a plurality of flows: when body 15 progresses in an orbital direction, the same direction is transmitted to the molten solder flows which delimit the components mounted on the printed boards and cause the removal of said air/gas bubbles from any side of the component. From the same figure 2 it is possible to note that body 15 has a width slightly inferior to the opening of duct 9, thus allowing to have a wave completely formed by molten solder flows having an orbital movement. According to the invention it is even possible to enlarge the opening of the duct in order to have a wave with a portion formed according to the invention (i.e. fomed by molten solder flows progressing in an orbital direction), while the remaining portion is of traditional type. Figure 3a shows a diagram of the forces which operate on the molten solder flowing through said openings 16. In particular it is possible to note that said molten solder flows has a vertical movement component V (caused by the pumping means) and a horizontal movement component 0 (produced by the walls of said openings 16 when body 15 progresses in an orbital direction). The direction of said horizontal component cyclically varies and assumes values between 0 and 360°, as shown in figure 3b. With said component a force F is reached, which impress a thrust to the air/gas bubbles near one of the common components 11 progressins in a direction cyclically variable as shown in figure 3c. It results that the air/gas bubbles are moved and removed in any position they are. Figure 4 schematically shows a first embodiment of the soldering unit 5, implemented according to the invention. In said figure 4, as well in figures 5 and 6, the walls 12 and 13 are not illustrated due to graphic reasons. On the other hand the transport means provided for the orbital movement of body 15 are illustrated in detail: said means are associated to well 6 and include a motor 17 suitable to cause the rotation of the corresponding cam 18, which is joined through transmission means (such as belt drive), to a second cam 19, on which a gear 20 is mounted. Gear 20 is joined to a first cam 21 which is supported by a bracket 22 and swings around its corresponding axis. On the upper side, respectively on the lower side, of the first cam 21, a first connecting rod 23 is mounted, respectively a second rod 24, staggered among themselves of 90° as better and schematically shown in figure 4b, Connecting rods 23 and 24 are joined to a second cam 25, which swings around its axis dragged by the first cam 21, by means of connecting rods 23 and 24. The eccentricity E of the small end of said first and second cam 21 and 25 is equal to a predetermined value which corresponds to 2,5 mm about. The first connecting rod 23 supports said body 15 provided by a plurality of openings 16, while the second connecting rod 24 must assure the rotation of the second cam 25 even if the small end of the first connecting rod 23 should stop near its dead center. As the first and second connecting rod are staggered among themselves of 90°, if the cams should stop according to the position of figure 4b (the first rod 23 is near its dead center), the rotation of the cam 25 is assured by a second rod 24 which - in said position - works under the maximum mating condition. When motor 17 causes the rotation of cam 21 and 25, body 15 moves according to a direction having a variable amplitude according to the eccentricity E value: the molten solde . flows, coming out from said openings 16, progress in a plurality of orbits which move around single components 11, mounted on printed board 10. According to the invention and to the embodiment of figure 4, as well as to the embodiments of figures 5 and β, it is possible to use _\ single earn and/or means in anyway suitable to cause an orbital movement. Figure 5 shows a second embodiment of the unit implemented according to the invention, where the transporter means, unlike the unit in figure 4, are located outside well 6. More in particular it is possible to see two cams which operate as cams 21 and 25 of figure 4a and are therefore marked with the same numbers. The rotation of said cams is caused by a motor 17 through one or two belts 26: at least one of said belts - crossing well 6 - operates either on cam 21 or on cam 25. The connecting rod 2 which supports body 15 provided with opening 16 is joined to said cams 21 and 25. The second connecting rod 24 is therefore non-essential, as the second cam 25 is directly operated by motor 17 and the problems mentioned above, for the embodiment where the second cam 25 is transported by the first cam 21, do not occur. Even in this case the rotation of motor cam 17 causes the movement in orbvital direction of body 15, which causes the same movement to the molten solder flows coming out from openings 16. Said flows determine the removal of gas bubbles near components 11. Figure 6 shows a third embodiment of the wavesoldering unit according to the invention, where the transport means are placed on the rear surface of well 6 in relation to the feed direction of printed circuits 10. It is to note that motor 17 is in this case supported by a bracket 27, placed on the rear side of the well, and the corresponding cam causes the rotation of a gear wheel 18 which transports a belt 26. This latter is stretched by a couple of gears 28 (having the function to increase the working angle of the gear wheel 18 on belt 26) and csuses the rcta ion of two cams 21 and 25, on the pins of which a longitudinal body 29 is applied. Body 15 is anchored to said body by means of one or more arms 30. When motor 17 is operated, a rotation of cams 21 and 25 is registered, which causes the orbital progressing of the longitudinal body 29. This latter, by means of arms 30, transfers the orbital movement to body 15 provided with openings 16, which on its turn causes the same movement of the molten solder flows which come out from openings 16. Figure 7 shows a second embodiment of body 15 where, unlike the previous embodiment, it is implemented by assembling, by means of an arm 31, a plurality of plates 32 having the edge perpendicular to the surface shaped by the printed circuit boards provided with tongues 33, bended according to a predetermined angle. According to a preferred embodiment said tongues are bended according to a 60° angle and are alternatively placed with the bending towards right side or left side. When said body moves forward, tongues 33 drives the molten solder flows in spiral direction, which is formed by a combination among the orbital movement of said body, the vertical movement caused by the pumping means and the lateral one by the tongues. Using therefore the body of figure 7, a further movement comes up, caused by the bending of tongues 33. Figure 8 shows a further implementation of body 15 formed by a lamination on which a plurality of circular « holes 16 are made. When a body 15, implemented according to figure 8, is used, the force diagrams illustrated in figure 3 are extremely exact, having the carriers of figure 3b and 3c the same value. Naturally the values of said carriers slightly differs among themselves making use of a body 15 having the openings 16 a square or rectangular shape. In order to adapt procedure and device to the different type of components to be soldered, the invention n rovides for the adjustment f the s n eed of rotation of motor 17 (and conseαuently the tip speed of the molten solder flows) as well as of the orbit amplitude of the solder flows. This latter parameter may be modified adjusting the value of said eccentricity E by moving the small ends of said cams 21 and 25 by screw means. The embodiments described above provide for circular orbit. According to the invention it is therefore possible to move body 15 in an ellictic direction (instead of a circular movement) allowing the get results comparable with the ones of a circular movement. According to what stated above it is clear that the procedure and the device/s implemented in compliance with the invention meet the requirements of the invention, as they cause the removal of air/gas bubbles from any position and the elimination of soldering failures due to dry soldering.

Claims: AMENDED CLAIMS [received by the International Bureau on 17 May 1989 (17.05.89) original claims 1,6 and 7 amended; other claims unchanged (4 pages)] 1. Procedure for the wavesoldering of units such as printed circuit boards of the type where the boards are "bathed" by a molden soldering wave, characterized in that at least a portion of the 5 soldering wave is formed by elementary flows of molten solder having an orbital movement. 2. Procedure as claimed under 1 characterized in that the tip speed of the molten solder flows may be io adjusted. 3. Procedure as claimed under 1, characterized in that which the orbit amplitude of the molten solder flows is adjustable. 15 4. Procedure as claimed under 1, characterized in that the orbit of molten solder flows has a circular form. 20 5. Procedure as claimed under 1, characterized in that the orbit of the molten solder has an elliptic form. 6. Device for the wavesoldering of printed circuit boards, of the type which comprises soldering means 25 suitable to produce a wave of molten solder which "baths" the boards as well as the components (11) mounted on it, characterized in that said soldering means generates a soldering wave at least a portion of which is formed by molten solder flows having an 30 orbital movement. 7. Device as claimed under 6, where the soldering wave is produced inside a well (6) through pumping means, characterized in that said soldering means which produce said molten solder flows are constituted by a body (15) provided with openings (16) which is connected to a first and a second cam (21 and 25), and at least one of them progress in a rotative direction. 8. Device claimed as under 7, characterized in that said first and second cam (21 and 25), placed inside the well (6), by the fact that to the small ends of said first and second cam a fi st cam (23) formed by said body (15) provided with openings (16) is joined, and by the fact that a motor (17) and transmitting means (18,19,20) cause the rotative movement of said first cam. 9. Device as claimed under 8, characterized in that to said first and second cam, a second connecting rod (24), staggered in respect to the first connecting rod (23) of a predetermined angle, is joined. 10. Device as claimed under 9, characterized in that said angle is of 90° about. 11. Device as claimed under 7, characterized in that said first and second cam (21 and 25) are placed at the sides -of the well (6), by the fact that said body (15) provided with openings (16) is joined to the connecting rod of said first and second cam (21 and 25) and by the fact that the first cam has a rotative direction caused by a motor (17) and by transmitting means. 12. Device claimed ad under 7, characterized in that said first and second cam (21 and 25) are placed on the front side, respectively rear side, of the well (6), by the fact that to the connecting rod of said first and second cam (21 and 25) a long-shaped body (15) provided with openings is joined through coupling means (30), by the fact that a motor (17) and transmitting means produce the rotative movement of the first cam (21) . 13. Device as claimed under 11 and 12 characterized in that said motor (17) cause the progressing in rotative direction to the first and second cam (21 and 25) by means of a belt (26). 14. Device as claimed under 7, characterized in that said body (15) provided with openings (16) is grid-shaped, the mesh of which have predermined shape and dimension. 15. Device as claimed under 7 characterized in that said body (15) provided with openings (16) is cam-shaped (31), which supports a plurality of plates (32), the ends of which, perpendicular to the surface of the printed circuit boards (10), have a tongue (33) bended according to a predetermined angle. 16. Device as claimed under 7, characterized in that said body (15) provided with openings (16) is lamina-shaped, provided with a plurality of holes. 17. Device as claimed under 15, characterized in that the tongue (33) has an height inferior to the one of the corresponding edge, by the fact that the bending ■ 8 is of 60° about and by the fact that the tongue is alternatively present on the left, respectively right side of the corresponding plate (32). 18. Device as claimed under 6 characterized in that the tip speed of the molten solder flows is variable depending from the rotation speed of a motor (17). 19. Devices as claimed under 6 and 7, characterized in that the orbit of the molten solder flows is variable depending from the modification of the eccentricity value (E) of said first and second cam (21 and 25). 20. Device as claimed under 19, characterized in that said eccentricity value (E) is of 2,5 mm about. STATEMENT UNDER ARTICLE 19 The amendments made to claims 1, 6 and 7 need no the amendments to the description and to the claims.

Inventors: SCORTA ROBERTO, SCORTA, ROBERTO
Applicants: ITALTEL SPA, ITALTEL SOCIETA ITALIANA TELECOMUNICAZIONI S.P.A.
Application year: 1988
Main classification: B23K1
IPC codes: B23K3