Method and apparatus for soldering under a non-oxidising atmosphere

Abstract: A soldering apparatus 20 includes a conveyor 22 adapted to carry assembled circuit boards through a soldering chamber 24. A wave 26 of solder is created in the chamber 24. The chamber 24 has a curtain 40 at its entrance 21 and a curtain 41 at its exit 23. Gas distributors 32 and 34 are located within the chamber 24 and are able to introduce nitrogen into the chamber 24 in order to create a non-oxidising atmosphere in the vicinity of the wave. In operation, the wave 26 of solder wets connecting points between the components and the board to form permanent joints on solidification of the solder. The bottom edge of the curtain is of a shape complementary to the front elevation of the assembled circuit boards to be soldered and is arranged such that the circuit boards just pass thereunder without making contact with it.

Description: This invention relates to a method and apparatus for soldering. It is particularly concerned with the manufacture of circuit boards. The manufacture of circuit boards is a well known industrial operation. Typically, electrical circuits are printed onto a board, and holes are drilled in the board at chosen locations to enable chosen circuit components to be inserted and to be soldered to electrical conductors on the board. The soldering operation is typically performed automatically or semi-automatically. Boards to be soldered are loaded in sequence onto a conveyor which advances them through an elongate chamber housing a soldering station. The soldering therefore takes place as the boards are advanced through the chamber. There are two commercially important methods of soldering: reflow soldering and wave soldering. In reflow soldering, miniature electronic components are surface mounted on a printed circuit board to which a solder in a creamy or paste-like consistency has been applied by a method such as screen printing, stencilling or dispensing. The printed circuit board is then subjected to a sufficiently high temperature, generally 50°C greater than the melting point or liquidus of the solder alloy, to cause the alloy to liquefy and to contact the components such that upon subsequent cooling of the printed circuit board, the components remain secured in place on the board by the solder. The heat can be supplied by, for example, infra red, vapour phase, heated conveyor belt or convective means. The solder conventionally comprises a soft powdered metal alloy dispersed in a liquid medium containing a flux, an organic solvent and a thickening agent selected to impart the desired consistency to the mixture. Wave soldering differs from reflow soldering in that the solder is not applied to the board in advance of the board's being carried through the soldering chamber. Instead, a vessel containing molten solder is located within the chamber and is arranged in conjunction with a pump to create a wave or waves of the solder. The circuit boards are each advanced through the soldering chamber in such spaced relationship to the wave or waves that appropriate amounts of solder are applied, as desired, to the components and that suitable solder joints are formed on cooling the boards. Typically, the boards are pre-treated with a flux at desired locations so that the solder is able to adhere to the surface at these locations. In recent years, considerable research and development activity has been focused on optimising the atmosphere in which the soldered joints are formed. In, for example, conventional wave soldering, the molten solder is oxidised when exposed to molecules of oxygen. The oxidised solder tends to form a surface oxide layer which is removed typically by a flux applied to the board before it is passed into the soldering chamber. Removal of the oxide enables the solder to wet the components to be soldered. Since the wave has a surface layer of molten solder which is continuously broken, a residue of oxide and solder, known as dross, collects within a vessel in which the solder is contained. Dross generation adds considerably to the cost of the wave soldering process owing to the lost value of the solder and the maintenance requirements it imposes. For example, it is necessary from time to time to remove dross from the vessel and to repair mechanical parts of the wave soldering apparatus damaged by its abrasive action. In addition, the dross itself can be a health hazard if the solder contains toxic components such as lead. It has accordingly been proposed to blanket the entire surface of the wave with an atmosphere which is less reactive than air towards the solder. There is, however, a body of teaching in the art which requires the presence of oxygen in the soldering atmosphere. In particular, US-A-4 610 391 relates to an improvement in a process for wave soldering a workpiece in an atmosphere consisting essentially of air wherein (i) there is a first portion of the solder wave in which fluid motion can be observed, said first portion including an active dross forming area, and (ii) there is a second portion of the solder wave which is the last portion of the solder wave with which the workpiece comes into contact. The improvement comprises replacing the atmosphere in contact with at least 50% of the surface of the active dross forming area with an inert gas, but preventing the atmosphere in contact with the surface of the second portion from becoming inert. Typically, two gas diffusers are employed. A first gas diffuser supplies an inert gas such as nitrogen to the front of the wave. A second gas diffuser applies a gas mixture typically containing from 18 to 50% by volume of oxygen (a balance typically being nitrogen) to the rear of the wave. In EP-A-361 507, it is disclosed that in the presence of a soldering flux it is desirable to maintain an oxygen concentration in the range of from about 10,000 to 20,000 parts by volume per million in the vicinity of the solder wave. EP-A-0 330 867 discloses that this oxygen concentration can be as low as 10 parts by volume per million in a wave soldering process which employs no pre-treatment of the circuit boards or other components with flux. EP-A-0 389 218 discloses that reflow soldering may be carried out in a low oxidising atmosphere, preferably one containing no more than 1500 parts per volume per million of oxygen. Notwithstanding the teaching in EP-A-0 330 867 that wave soldering can be conducted without the pretreatment of the printed circuit board with flux when the soldering atmosphere contains in the order of 10 parts by volume per million of oxygen, manufacturers still prefer to pre-treat the circuit boards with flux. The reasons are two-fold. First, difficulties arise in reliably and repeatedly being able to maintain in the soldering chamber an atmosphere having an oxygen concentration as low as 10 parts per million by volume even if, say, a source of substantially pure nitrogen (containing no more than 5 volumes per million of oxygen) is used to create the soldering atmosphere. Secondly, it is still found that some dross is formed even in low oxygen environments. Although soldering flux is able to cater for the presence of oxygen in a soldering atmosphere, we nonetheless believe that there is a need to keep down the ingress of air into the soldering atmosphere. It is known to provide curtains at both the entrance to and exit from a soldering chamber in order to limit the ingress of air. Such curtains form a physical barrier to the ingress of air when nitrogen is supplied to the interior of the chamber. The curtains typically each comprise a number of fingers or filaments of rubber, thin metal sheet or glass fibres. The curtains typically totally obturate the entrance and exit to the chamber but are able to be displaced by advancing circuit boards to permit their entry to and exit from the chamber. We have surprisingly found that contact between such curtains, even though they are lightweight, and boards passing into the soldering chamber can displace some components sufficiently to impair their soldering and hence the quality of the finished circuit board. Although this problem can be overcome, by removing the curtains and increasing the flow of nitrogen used to form the soldering atmosphere, this practice leads to an undesirably high consumption of nitrogen if it is desired to maintain low oxygen concentrations within the soldering atmosphere. We further believe that it is desirable to maintain a soldering atmosphere having a low oxygen concentration even if the boards are pre-treated with a soldering flux. DE-U1-8520254.1 discloses a wave soldering apparatus having movable doors at its inlet and outlet which, in operation, are opened and closed each and every time a component to be soldered is passed into the soldering chamber. Such a system relies on the use of two actuators and, when the door is fully open, provides no barrier to the escape of expensive inerting atmosphere. There is therefore a need in the art for an improved method and apparatus for soldering which facilitates the creation of soldering atmospheres containing only low levels of oxygen, but which overcome the above-described problem that arises with conventional curtains. The invention aims at meeting this need. According to the present invention there is provided a method of soldering an assembled circuit board including; advancing the assembled circuit board through a soldering chamber having an entrance and an exit; supplying non-oxidising gas to the soldering chamber to create a substantially non-oxidising soldering atmosphere within the soldering chamber and soldering the assembled circuit board within the soldering chamber; characterised by providing the entrance and the exit with at least one fixed curtain having a bottom edge shaped complementary to a front elevation of the assembled circuit board to be soldered; and the assembled circuit board being advanced into the soldering chamber by being passed under the bottom edge of the curtain so that components of the assembled circuit board to be soldered just clear said bottom edge without making contact therewith. The invention also provides soldering apparatus including a soldering chamber having an entrance and an exit, a conveyor for conducting circuit boards to be soldered through the chamber, an inlet for non-oxidising gas whereby a substantially non-oxidising soldering atmosphere can be created in the vicinity of where, in operation, the soldering is performed, a door or curtain at the exit from the chamber and a door or curtain at the entrance to the chamber, the doors or curtains being arranged so as to limit the ingress of air into the chamber, characterised in that the door or curtain at the entrance to the chamber has a bottom edge of shape complementary to the front elevation of an assembled circuit board to be soldered, whereby the circuit board is able to pass under the bottom edge without contacting the door or curtain. It is preferred that the relatively non-oxidising atmosphere contains no more than 200 volumes per million of oxygen. By the term "non-oxidising gas" as used herein is meant a gas or gas mixture containing a concentration of oxygen impurity lower than that of the substantially non-oxidising atmosphere. The method and apparatus according to the invention are particularly suitable for use in wave soldering especially when the circuit board includes soldered joints that have previously been made by reflux soldering. They are also suitable for use for reflow soldering. Contact between a door at the chamber entrance and a circuit board entering the chamber can be avoided by providing the apparatus according to the invention with means for detecting the position of a circuit board about to enter the chamber, such means being operatively associated with means for raising or otherwise opening the door so as to permit entry without there being contact between the door and the circuit board, and with means to lower (or otherwise close) the door, or to allow such lowering to take place, once the board has passed through the door. It is, however, preferred to provide a door or curtain at the chamber entrance with a bottom edge of shape generally complementary to the profile (i.e. front elevation) of an assembled circuit board to be soldered, whereby the circuit board is able to pass under the bottom edge without contacting the door or curtain. Preferably, the position and height of the curtain or door at the entrance to the chamber is such that the shaped bottom edge clears the assembled circuit board by less than 5mm. Accordingly, the rate of ingress of air beneath the bottom edge into the chamber is low compared with a chamber without any door or curtain at its entrance. It is therefore possible to maintain a low concentration of oxygen in the atmosphere in the vicinity of where the solder is applied to the boards in the chamber of a wave soldering apparatus without there being a requirement for an excessive rate of introduction of nitrogen or other non-oxidising gas into the chamber. Preferably, when wave soldering in accordance with the invention, before passing the boards into the chamber, each circuit board has a soldering flux applied to it in the locations where the soldered joints are to be made. A conventional soldering flux may be used (e.g. a MULTICORE soldering flux) and it may be applied by conventional techniques. Even if such a soldering flux is so applied, it is preferred that the concentration of oxygen in the vicinity of the wave or waves created in the chamber is kept below 100 parts per million by volume and more preferably below 50 parts per million by volume. In order to achieve such low levels of oxygen in the soldering region, it is desirable to employ a suitable high purity source of nitrogen. Typically, a modern commercial plant for the separation by rectification of nitrogen from air produces a product containing no more than 5 volumes per million of oxygen impurity which is suitable for use as the source of nitrogen in the method according to the invention. By employing the method according to the invention with nitrogen as the non-oxidising gas, we have achieved soldering atmospheres containing no more than 50 volumes per million of oxygen in the region of the solder wave with a total nitrogen flow rate of not more than 250 chamber volumes per hour. Although the non-oxidising gas is preferably nitrogen, it is alternatively possible to use a noble gas, for example, argon. Another alternative non-oxidising gas at soldering temperatures (which are typically below 250°C) is carbon dioxide. Non-oxidising gas introduced into the chamber in the vicinity of the wave or waves of solder desirably has insufficient velocity to impair the formation of the soldered joints. Accordingly, it is preferred that such non-oxidising gas be introduced through a diffuser having a multiplicity of microscopic gas distribution pores. Non-oxidising gas may be introduced into the chamber at locations spaced apart from the immediate vicinity of where the solder is applied. If it is desired to keep the concentration of oxygen to particularly low levels in the vicinity of the waves, say to less than 40 ppm by volume of oxygen, then it is desirable to supply non-oxidising gas within the chamber at a location near the entrance door or curtain and at a location near the exit door or curtain. Moreover, we have surprisingly found that in commercial wave soldering apparatus, some of the soldered joints are not completely solid when each board leaves the furnace. Accordingly, components may be displaced as each board passes through the outlet door or curtain if there is physical contact between the components and the door or curtain. There are two alternative solutions to this problem. The first solution is to prevent contact between the outlet door or curtain and each circuit board leaving the chamber. Accordingly, the exit door or curtain may be arranged in the same manner as the entrance door or curtain. Alternatively, a stream or streams of non-oxidising gas, for example nitrogen, can be directed at the soldered joints so as to provide additional cooling for them at a region of the chamber intermediate the solder wave or waves and the exit therefrom. It is possible to adapt a conventional soldering chamber for use in a method according to the invention by means of relatively simple modifications including the installation of suitable curtains. The method and apparatus according to the invention make possible the formation of sound soldered joints with only a relatively low concomitant formation of dross. Methods and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which:- Figures 1A to 1E are schematic drawings illustrating the preparation of a circuit board for wave soldering; Figure 2 is a schematic side view of wave soldering apparatus according to the invention; Figure 3 is a schematic front elevation of a curtain for use in the apparatus shown in Figure 2; Figure 4 is a schematic view (from below) of a gas distributor for use in the apparatus shown in Figure 2; Figure 5 is a graph illustrating the variation in oxygen concentration along the length of part of a soldering chamber; Figures 6 and 7 are graphs illustrating the change in temperature of a component mounted on a circuit board as the board is advanced through a soldering chamber; and Figure 8 illustrates an alternative arrangement of curtains to that illustrated in Figure 3. The drawings are not to scale. Referring to the drawings, in Figure 1A there is shown a substrate 2 from which a circuit board is able to be made. The substrate 2 is of suitable insulating material for example glass fibre or paper impregnated with a phenolic resin. Referring to Figure 1B, apertures 4 are drilled in the substrate 2 at locations where it is desired to insert electrical components. The substrate 2 is then copper plated to provide appropriate conductors for the electrical circuit that is to be formed (see Figure 1C). In particular, copper plating 6 is provided through and in the vicinity of each aperture 4. A suitable soldering flux (e.g. TAMURA or MULTICORE flux) is then provided on the surfaces of the copper plating 6. Electrical components 8, for example, resistors, capacitors and/or inductances, are then inserted through the holes 4, as illustrated in Figure 1D. The thus assembled circuit board is then subjected to wave soldering in order to form soldered joints in the positions 10 as shown in Figure 1E. The wave soldering of boards as shown in Figure 1E is conducted in an apparatus as shown in Figure 2. This apparatus typically comprises a conventional wave soldering "machine" of a kind that is commercially available, for example, a machine sold under the Trademark EUROPAK, with certain modifications regarding the provision of curtains at the entrance and exit and the provision of means for introducing nitrogen into the soldering chamber. Referring to Figure 2, the illustrated soldering apparatus 20 includes a conveyor 22 adapted to carry assembled circuit boards (not shown in Figure 2) through a soldering chamber 24. The arrangement is such that a wave 26 of solder vets connecting points between the components and the board to form permanent joints on solidification of the solder. Any conventional solder may be used. It typically comprises an alloy of tin and lead (for example a solder sold under the Trademark MULTICORE Non Eutectic). The wave 26 is created by a suitable pumping means 28 which is preferably located within a vessel 30 for holding a volume of molten solder. Heating means (now shown) may be provided in the vessel 30 to maintain the solder in molten state. The pumping means 28 may take the form of an Archimedean screw. Gas distributors 32 and 34 are located within the chamber 24 for creating a non-oxidising atmosphere in the vicinity of the wave. The distributor 32 is an elongate, hollow member extending within the chamber 24 generally transversely to the longitudinal axis thereof and has a lower face of porous material through which non-oxidising gas, for example, nitrogen, supplied from a source thereof is able to diffuse. The distributor 32 is typically positioned over a forward portion of the solder wave 26. The distributor 34 is preferably also an elongate hollow member. As shown in Figure 4 it has a row of gas distribution orifices 38 in its lower face 36. The distributor 34 is positioned rearwardly of the wave 26 and the orifices 38 face the circuit boards as they pass underneath it. In operation, the distributors 32 and 34 are connected to a source of non-oxidising gas such as nitrogen and is able to provide nitrogen to the interior of the chamber 24 and assist in creating a substantially non-oxidising atmosphere in the vicinity of the solder wave 26. In addition, the distributor 34 directs streams of non-oxidising gas in a downward direction at the soldered components which help to cool such components. The distributor 34 is shown in Figure 2 as being positioned intermediate the solder wave 26 and the exit 23 of the chamber 24. The distributor 34 may alternatively be positioned over the wave 26. In order to limit the ingress of air into the soldering chamber 24 through entrance 21 and exit 23 thereof, a curtain 40 is provided at the entrance 21 and a curtain 41 at the exit 23. The curtain 40 is shown in Figure 3 of the accompanying drawings. The curtain 40 comprises a thin plate or lamina 42 having adjacent its upper edge 44 apertures 46 to enable it to be bolted to the face of the chamber 24 at the entrance 21 thereto. The lower edge 48 is complementary to that of the front elevation of the circuit boards to be soldered. The height of the curtain 40 is chosen so that the clearance between the bottom edge 48 and the components to be soldered is in the order of 3mm. Accordingly, the ingress of air beneath the edge 48 into the chamber 24 is at a rate substantially lower than it would be in the absence of any curtain at the entrance 21. In order to form the shaped lower edge 48 of the plate 42, a front elevation of the circuit board may be drawn to scale, and the resulting drawing used as a template to enable a plate of metallic or non-metallic material to be cut to shape. The curtain 40 is preferably readily demountable from the chamber 24. Typically, it is desired in one production run to produce a large number of identical circuit boards of a first layout, and then in another production run to produce a large number of circuit boards of a different layout. A different curtain 40 having an appropriately shaped bottom edge 48 (see Figure 3) may be used for each such production run, with the curtains being changed between production runs. The curtain 41 shown in Figure 2 may be of a conventional kind comprising filaments or fingers of fibrous or elastomeric material or of the kind described above with reference to Figure 3. If desired, a plurality of such curtains may be employed at the exit 23. In operation, in order to prepare the apparatus 20 for a soldering operation, the vessel 30 is charged with a batch of molten solder (or the solder melted in situ ) and the chamber 24 is purged with nitrogen supplied by the distributors 32 and 34 for a period sufficient to reduce the oxygen concentration to a chosen low level. Operation of the pump 28 is then started to create the wave 26 and the conveyer 22 is actuated. Circuit boards to be soldered are then loaded sequentially onto the conveyer belt and are carried through the chamber 24. Each solder board in turn comes into appropriate soldering contact with the wave 26 and the necessary soldered joints are formed. These joints are cooled by the nitrogen supplied from the distributor 34 and have solidified by the time each board leaves the exit 23 of the chamber 24. The nitrogen is supplied to the distributors 32 and 34 from a source external to the chamber 24. The source is preferably a vacuum-insulated vessel (not shown) containing liquid nitrogen of a purity such that it contains less then five volumes per million of oxygen. The vessel is fitted with a vaporiser to evaporate the liquid nitrogen as it passes therethrough. When using a curtain 40 of the kind shown in Figure 3, at the entrance 21 to the chamber 4, and a total nitrogen flow rate of 517 litres per minute into the chamber 24, we have been able to achieve oxygen levels of less than 50 ppm in the vicinity of the wave 26, the chamber 24 having a free space in the order of O.14m³ (five cubic feet). The effectiveness of the method and apparatus according to the invention in reducing oxygen levels within the chamber 24 is illustrated by the graph that forms Figure 5. In this graph the distance from the entry 21 is plotted on the abscissa and the oxygen level in volumes per million is plotted on the ordinate of the graph. Curve B in the graph shows operation without the presence of a curtain at the entrance 21 of the chamber 24. Curve A shows the results obtained when a curtain of the kind shown in Figure 3 was employed at the entrance 21 of the chamber 24. It can be seen that at, for example, a distance 1.22m (four feet) from the entrance 21, the oxygen level, when the chamber was operated in accordance with the invention, was less than 50 volumes per million but when the curtain was omitted, the oxygen level was 200 volumes per million. The effectiveness of the distributor 34 in cooling soldered components in operation of the apparatus shown in Figure 2 is illustrated by Figures 6 and 7. In the graphs that form Figures 6 and 7, time is plotted along the abscissae and temperature along the ordinates. Figure 6 illustrates the temperature of a component as it passes through an apparatus of the kind shown in Figure 2, when both distributors 32 and 34 are of the kind in which nitrogen diffuses into the chamber 24 from a porous surface. The temperature of the component at the curtain at the exit is 200°C. The solder melts in the temperature range of 183°C to 188°C. Accordingly, the solder has not fully set by the time the circuit board leaves the apparatus, and there is hence a risk of the components being displaced by contact with a conventional curtain at the exit 23. To overcome this problem, a curtain of the kind shown in Figure 3 may be employed at the exit 23 as well as the entrance 21. Alternatively, a distributor 34 of the kind shown in Figure 4 may be used to direct nitrogen gas onto the newly soldered components intermediate the solder wave 26 and the exit 23 of the chamber. Such operation is illustrated in Figure 7. The exit temperature of the soldered component was measured at 162°C, i.e. below the temperature range in which the solder. It will thus be appreciated that the use of coolant streams of nitrogen to reduce the temperature of the soldered components has a considerable effect. Various additions and modifications may be made to the above described soldering method and apparatus according to the invention. For example, instead of using a single curtain 40 at the entry 21 and/or exit 23 to the furnace, a plurality of spaced apart curtains 40, each of the kind illustrated in Figure 3 may be employed. Further, nitrogen or other non-oxidising gas may, as shown in Figure 8, be supplied through a distributor 50 located between the pair of curtains 40. Using such a curtaining arrangement it becomes possible to reduce even further the oxygen level in the vicinity of the solder wave 26. It is additionally or alternatively possible to fit the furnace with doors to enable the ingress of air to be prevented altogether during periods in which the apparatus is being held on stand-by. For example, the doors may be housed internally within the solder chamber (in a raised position) allowing circuit boards to pass beneath them and then allowed to swing down to seal the chamber during non-productive periods. In a further modification, if there is inadequate sealing between the underside of the conveyer 22 and the chamber 24 at the entrance 21 or the exit 23, or both, appropriate sealing strips or other means may be applied at these locations so as to reduce further the rate of ingress of air into the chamber 24 during a soldering operation.

Claims: A method of soldering an assembled circuit board (2) including; advancing the assembled circuit board through soldering chamber (24) having an entrance (21) and an exit (23); supplying non-oxidising gas to the soldering chamber to create a substantially non-oxidising soldering atmosphere within the soldering chamber (24) and soldering the assembled circuit board within the soldering chamber; characterised by providing the entrance and the exit with at least one fixed curtain (40, 41) having a bottom edge (48) shaped complementary to the front elevation of the assembled circuit board (2) to be soldered; and the assembled circuit board (2) being advanced into the soldering chamber (24) by being passed under the bottom edge (48) of the curtain (40, 41) so that components of the assembled circuit board (2) to be soldered just clear said bottom edge without making contact therewith. A method as claimed in claim 1, characterised in that the clearance between the said bottom edge and each assembled circuit board is less than 5mm. A method as claimed in either of claims 1 or 2, characterised in that the rate of flow of non-oxidising gas into the chamber is less than 250 chamber volumes per hour. A method as claimed in any one of claims 1 to 3, characterised by the additional step of directing non-oxidising gas at the soldered joints in a region of the chamber intermediate a solder wave and the chamber exit or over the wave itself. Soldering apparatus (20) including a soldering chamber (24) having an entrance (21) and an exit (23) , a conveyor (22) for conducting circuit boards (2) to be soldered through the chamber, an inlet (32) for non-oxidising gas whereby a substantially non-oxidising soldering atmosphere can be created in the vicinity of where, in operation, the soldering is performed, a door or curtain (41) at the exit from the chamber and a door or curtain (40) at the entrance to the chamber (24), the doors or curtains (40, 41) being arranged so as to limit the ingress of air into the chamber, characterised in that the door or curtain (40, 41) at the entrance to the chamber has a bottom edge (48) of shape complementary to the front elevation of an assembled circuit board (2) to be soldered, whereby the circuit board (2) is able to pass under the bottom edge (48) without contacting the door or curtain (40, 41). Apparatus as claimed in claim 5, in which the position and height of the curtain or door (40, 41) at the entrance to the chamber is such that the shaped bottom edge clears the assembled circuit board by less than 5mm. Apparatus as claimed in any one of claims 5 and 6, additionally including means for directing non-oxidising gas at soldered components.

Inventors: PRECIOUS COLIN JOHN, THOMAS RAYMUND, PRECIOUS, COLIN JOHN, THOMAS, RAYMUND
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1992
Main classification: H05K3
IPC codes: B23K1, H05K3, B23K101