SOLDERING APPARATUS

Abstract: A soldering apparatus has a bellows chamber, formed by a bellows (4) and upper and lower closure pieces (6, 8) for containing molten solder (3). As the volume of the bellows chamber (2) is reduced, solder is expelled through a nozzle (52) and a solder ball (62) is formed. The volume of the solder ball (62) is monitored by an optical sensor (60). The volume of solder expelled from the bellows chamber (2) can be controlled precisely, and a joint to be soldered is brought down into contact with the solder ball (62). The space around the nozzle tip (36) is enclosed by heated nitrogen gas to reduce oxidation of the solder and help pre-heat the joint. A vibrator (32, 34) vibrates the nozzle tip (36) to assist the smooth growth of the solder ball (62).

Description: SOLDERING APPARATUS This invention relates to a soldering apparatus. More particularly, the invention relates to a soldering apparatus for soldering individual components to a printed circuit board (pcb). One prior art soldering apparatus consists of a soldering iron mounted in a robot arm. The arm is manipulated to move the bit of the soldering iron into a suitable position to make a joint. Solder wire is fed to the bit of the soldering iron through pinch rollers. This prior art system has a number of drawbacks. The bit of the iron must be kept clean, and a cleaning station for the bit is provided. Ideally the bit should be cleaned before each joint is made. However it takes an appreciable time to move the bit from a working position to the cleaning station, clean the bit, and return the bit to the working position. It is therefore not possible to clean the bit before making every joint, since this would result in a large ' downtime' , and the quality of the joints is degraded in consequence. If, for example, a pin is being soldered to a pcb, a good joint will only form if the bit contacts both the pcb and the pin. If, however, the bit should touch only the pin or only the pcb a bad joint will result. Accordingly, the bit must be positioned very accurately, and this is difficult. A further disadvantage is that the time taken to form a joint is relatively long. Once the components to be soldered have been heated by the bit, cold, solid solder is applied, thereby cooling the components. It is therefore necessary to re-heat the components, and melt the solder, before a joint can be made. In addition, it is difficult to feed the solder in precise quantities, since the solder wire tends to melt back along its length. US-A-4 651 916 discloses a soldering apparatus having a solder bath, a nozzle and a pump for pumping solder to the nozzle. This prior art soldering apparatus is useful for soldering small components using, in effect, a wave soldering technique. However, the apparatus ' is unsuitable for very fine soldering work on pcb' s, because of the flowing solder and, in particular, because of fluctuations in the rate of flow of the solder. GB-A-1 297 138 discloses a soldering apparatus which has a cylinder for containing molten solder. The cylinder is closed at an upper end by a piston, which is provided with a channel. When a pcb to be soldered comes into contact with the piston the piston is depressed, and solder is forced through the channel to the component. This apparatus is a variant of a wave soldering apparatus, and is not designed for very fine soldering work. One problem is that the amount of solder sent to the component to be soldered is not precisely controlled, because the amount of solder displaced by the piston depends on the movement of the piston which, in turn, depends on the positioning of the component to be soldered. Another disadvantage is that galling, or the accumulation of oxidised solder, may occur between the piston and the cylinder, causing the apparatus to seize up. According to a first aspect of the present invention there is provided a soldering apparatus comprising a nozzle and a positive displacement pump for pumping a predetermined quantity of molten solder through the nozzle. The invention also provides a soldering apparatus having a nozzle and means for forming a drop of molten solder of a predetermined size at an end of the nozzle. A small diameter nozzle can be used to deliver molten solder to a precise area on a pcb. The solder itself can heat the joint to be soldered, and the amount of solder fed to the joint can be controlled. In particular, a ball or drop of molten solder which is substantially stationary can be formed at the nozzle outlet and the joint is then brought into contact with the drop of solder. The present invention further provides a soldering apparatus comprising a container for containing molten solder, the container having a nozzle; and means for reducing the working volume of the container to expel a predetermined quantity of solder from the container through the nozzle. In this soldering apparatus, the amount of solder expelled from the container depends only on the amount by which the volume of the container is reduced, and this can be accurately controlled. This enables the same amount of solder to be expelled in subsequent soldering operations. For example, a partial blockage of the nozzle due to an accumulation of oxidised solder will have no effect on the quantity of solder expelled for a given reduction in the volume of the container. The present invention further provides a soldering apparatus comprising a container for containing molten solder, the container having a nozzle; and means for reducing the working volume of the container to expel solder from the container through the nozzle, wherein there are no relatively sliding surfaces in contact with the solder. Preferably, the container has a flexible wall which is deformed to expel the solder. It is envisaged that soldering apparatus of this invention could be used as a de-soldering apparatus, by heating the joint and increasing the volume of the container so that molten solder is sucked into the container. When a joint is soldered, the amount of solder taken will depend on the joint, and may vary between apparently identical joints. Thus, forcing a constant amount of solder through the nozzle tip may not always result in a ball of predetermined size, because it will depend on the amount of solder taken by the previously soldered joint. In a preferred embodiment the soldering apparatus further comprises a sensor for sensing the volume or height of a "solder ball expelled from the nozzle. The actual volume or height of the ball may be monitored, or, the solder ball can be brought to a predetermined, datum size and then be adjusted further (either larger or smaller) to suit a particular joint. In a particularly preferred embodiment the means for reducing the volume of the container is controlled on the basis of an output signal from the sensor. This enables the amount of solder expelled from the container to be controlled even more precisely. In a preferred embodiment the sensor is an optical sensor comprising a light emitter and a detector. The light emitter may be integral with the detector, which is sensitive to light reflected from the solder expelled from the nozzle. Preferably, the soldering apparatus further comprises a nozzle heater for heating the nozzle. The solder in the container may be kept just above its melting point, thereby reducing the power consumed by the apparatus and also reducing the corrosive effect of the solder. As the solder is expelled from the container, the temperature of the solder is boosted to a suitable soldering temperature by the nozzle heater. In a further preferred embodiment flow means is provided for directing a flow of inert gas such as nitrogen around the nozzle outlet to prevent the solder from oxidising. In a particularly preferred embodiment, the gas is heated and can then be used to pre-heat the area to be soldered. In a further preferred embodiment, a flux is carried by the gas. This avoids the need to flux the pcb before carrying out a soldering operation. It has been found that there is a tendency for the ball of molten solder, formed at the nozzle tip, to increase suddenly in size despite a gradual reduction of the working volume of the pump. It is thought this may be due to surface tension or skinning effects. In a preferred embodiment, means for vibrating the nozzle are provided. The vibrations of the nozzle are transmitted to the molten solder that has been pumped/expelled from the nozzle. It has been found that this reduces the tendency of the ball of solder to increase suddenly in size as the pump is operated. In a particularly preferred embodiment, the nozzle is directed upwardly to form a solder ball at the top of the nozzle, and the PCB is then brought into contact with the solder ball. The invention will now be described by way of example with reference to the accompanying drawings in which: - Figure 1 is a cross-sectional view of a preferred embodiment of a soldering apparatus of this invention; Figure 2 is an enlarged view of a nozzle region of the apparatus of Figure 1. The soldering apparatus shown in Figure 1 has a bellows chamber 2 which acts as a container for molten solder 3 and forms a positive displacement pump when the bellows is compressed. The bellows chamber 2 is formed by a cylindrical bellows 4 which is closed at its ends by an upper casting 6 and a lower closure piece 8. The upper casting 6 has a finger-like recess 10 in which a heater (not shown) and a temperature sensor (not shown) are mounted for melting the solder 3 in the chamber 2. The bellows 4, and upper casting 6 and lower closure piece 8 are made of stainless steel. The bellows 4 is made of stainless steel of thickness 0.2mm to 0.5mm. The thinner the material from which the bellows is made, the easier the bellows is to compress. A typical diameter for the bellows would be 10cm. While Figure 1 shows a bellows chamber, the solder container can be constructed in other ways. For example, the container could be generally rigid and have one wall formed from a resilient diaphragm. Other suitable materials such as, for example, titanium can be used for the bellows 4 and closure pieces 6, 8. The upper casting piece 6 is provided with an outlet aperture 12. A nozzle assembly 14 is releasably attached to the top of the upper casting 6 for receiving molten solder 3 expelled through the aperture 12. Referring to Figure 2, the nozzle assembly 14 will be described in more detail. A nozzle mount 16 has a lower end 18 which is secured in the aperture 12 of the casting 6, the inner bore 20 of the mount being in register with the aperture 12. A nozzle 22 is screwed into the upper end 24 of the nozzle mount 16. A flange 26 on the nozzle 22 holds a thermocouple temperature sensor 28 in place between the flange 26 and the upper end 24 of the nozzle mount 16 and also secures a cylindrical heater 28 in place about the nozzle mount 16. The thermocouple 28 monitors the temperature of the nozzle mount 16, and nozzle 22 for controlling the tubular heater 28. A second flange 30 supports a collar 32 which is attached to a vibrator rod 34 for vibrating the nozzle 22, as will be described hereinafter. The nozzle mount 16 and nozzle 22 are surrounded by a stainless steel jacket 34 which acts as a conduit for delivering heated nitrogen gas to the tip 36 of the nozzle 22, and for collecting excess molten solder which is expelled from the nozzle tip 36. Jacket 34 has a base 38 which is attached to the upper casting 6, for example by screws (not shown). An inner cylinder 40 projects up from the base 38 and surrounds the cylindrical heater 28 forming a cylindrical passage 42 between the heater 28 and cylinder 40. An inlet 44 is provided at the base of the cylinder 40 for feeding nitrogen gas into the passage 42, where it is heated by the cylindrical heater 28. The gas may also be pre-heated before delivery to the inlet 44. An outer cylinder 46 surrounds the inner cylinder 40, providing a jacket for thermal insulation. A conical cap 48 sits on top of the cylinders 40, 46 and tapers upwards to a cylinder 50 which surrounds the nozzle 22 at its upper end 52 and stops just short of the nozzle tip 36. The upper end 52 of the nozzle 22 is cylindrical and tapered at the nozzle tip 36 to form a thin edge at its upper end. The upper end of the cylinder 50 is level with the bottom of the tapered tip region 36 and spaced from the outside of the nozzle 22 to provide a passage for the nitrogen gas. A trough 54 is formed around the outside of the cap 48 at its lower end and has a cut-out 56 which forms a drain, leading to a catchment volume 58. An optical sensor 60 is shown schematically. The sensor 60 directs light onto a solder ball 62 which is formed on the nozzle tip 36, as will be described hereinafter, and detects the amount of light reflected back. The amount of reflected light depends on the size of the solder ball 62, and so sensor 60 gives an indication of the ball size. The sensor 60 is movable between two positions, 60' , 60" , by a pneumatic cylinder (not shown). In the upper position 60' the sensor points down onto the ball from above for measurement, in the lower rest position 60", the sensor 60 is below the level of the nozzle tip 36. Referring again to Figure 1, the mechanism for compressing and extending the bellows 4 will be described. A supporting flange 68 is attached to the upper casting 6 and fixed to a base 70 by vertical studs 72. The lower closure 8 rests on a nut 74 which is held against rotation by a pin 76 which slides vertically n a channel 78. The nut 74 is mounted on a lead screw 80 which is coupled to a pulley 82 and rotated by a reversible stepper motor 84 and toothed pulley belt 86. Thus, motor 84 controls the vertical movement of the nut 74 to compress the bellows 4 as the nut 74 is raised, and allow the bellows 4 to expand, due to its own resilience, as the nut is lowered. A datum switch 88 provides a signal to indicate that the nut 74 is at a pre-determined height. As will be described more fully hereinafter, to make a solder joint, a solder ball 62 is formed by urging the lower closure piece 8 up towards the upper casting 6 to compress the bellows 4, and a pcb is then positioned so that a lead of a component to be soldered to the pcb is brought into contact with the solder ball 62. It will be appreciated that the pcb could be put in position close above the nozzle tip 20 first, and a solder ball 18 formed subsequently. The operation of the embodiment of Figures 1 and 2 will now be described. Solder 3 in the bellows chamber 2 is heated by the heater (not shown) in the recess 10 to just above its melting point. The tubular heater 28 is operated to bring the solder in the nozzle mount 16, and nozzle 22 up to soldering temperature. This reduces the power consumed by the bellows heater, and also reduces the corrosive effect of the molten solder in the bellows chamber 2, (solder becomes more corrosive as its temperature increases) thereby increasing the lifetime of the bellows. For example, for 60/40 Sn/Pb solder a suitable soldering temperature is around 290 * C, while the melting point is around 200"C. The ball 62 is brought to a pre-determined size by monitoring the size with the optical sensor 60 in the upper position 60' . The motor 84 is operated to compress the bellows 2, squeezing solder through the nozzle 22 to form the ball 62. When the sensor indicates that the required size has been reached, the motor 84 is stopped and sensor 60 withdrawn downward to the rest position 60". A printed circuit board or other item having a joint to be soldered is then brought down so that the joint contacts the solder ball 62. In particular, a lead projecting through the underside of the pcb is dipped into the solder ball 62, the ball contacting the printed circuit track around the lead. The lead may penetrate the nozzle tip 36 if the lead is pointing downwards. However, this is not essential and leads which are angled to the vertical may also be soldered provided there is sufficient clearance to allow the lead to contact the solder ball near the joint region between the lead and the associated pad on the PCB. Also, it is not essential that the solder ball contact both the lead and pad. The molten solder will run up the lead a short distance to form a joint. However, a good quality joint will be formed more reliably if the solder ball contacts both the lead and the pad. A heated nitrogen gas stream is fed around the nozzle tip 36 by the cylinder 50. This serves to pre-heated the joint, reducing thermal shock to the joint. Pre-heating is also required if a "no-clean" flux is used. The joint is preferably prefluxed at a fluxing station, for example using a spray flux. However, the flux might be carried in the heated nitrogen gas stream. Often the flux type is dictated by the customers specification and this may require a particular procedure for applying the flux. It has been found that a variety of fluxes can be used. An example of a suitable flux is a water based flux NR300 produced by Alpha Metals Limited of Surrey, U. K. The inert nitrogen gas serves also to inhibit oxidation or dross-formation at the nozzle tip and it is believed that it also helps in the formation of a uniform or reproducible solder ball size by preventing oxidation at the ball surface and reducing the surface tension of the ball. It has been found that the formation of the ball 62 is greatly facilitated if the nozzle 22 is vibrated. This is achieved by a solenoid type vibrator mounted on the arm 34 which is coupled to the nozzle 22 by the collar 32. This vibration reduces the tendency of the ball to increase or "jump" suddenly in size, beyond that required, as the bellows 4 is compressed. The vibration frequency is preferably above about 100Hz and preferably 200Hz or higher. In one embodiment, the nozzle 22 has an outside diameter of 2.5mm and an inside diameter of 2mm, the top edge of the nozzle tip 36 being brought to a point. Various nozzle sizes may be used. Preferably the nozzles outer diameter is kept small (less than about 4mm) to facilitate access to the joints on the underside of the pcb and allow formation of a small solder ball to avoid solder bridging. In principle the ball size can be determined from the amount of compression of the bellows 4, and hence from the rotation of the motor 84. A solder joint will "take" the amount of solder is requires, and this will vary from joint to joint, even among apparently identical joints. Thus, in one method the ball is brought to a datum size by monitoring the size with the sensor 60, and the motor 84 can then be driven further (or reversed) to bring the ball to the required size. In the currently preferred method, the ball size is "measured" by monitoring the analogue output form the sensor 60 which gives a relative indication of the ball size, and the system can then be programmed to form particular sized balls. When an oversized ball 62 is formed, and at the start of a soldering operation, the ball 62 is expanded quickly until it is so large that it drops off the nozzle tip. Provided the cylinder 50 is sufficiently close to the nozzle end 52, the ball rolls across the gap between them and is collected in the trough 54 and drains out to the catchment volume 58. The cylinder 50 is spaced no more than 5mm below the top edge of the nozzle tip 36 and a clearance of less than 3mm is provided around the nozzle end 52. It has been found that the waste solder ball will jump the gap on most occasions if the gap is less than about 4mm, and a gap of about 3mm or less performs reliably. Preferably the gap is between 0.5mm and 1.0mm. A plugged filler hole (not shown) is provided between the base 38 of the catchment area and the upper casting 6 for draining waste solder back into the bellows, and for filling the bellows chamber 2. To fill the bellows, the motor 84 is reversed to bring the nut 74 down to a datum position, indicated by switch 88. The plug is removed and fresh solder and any waste solder is introduced into the bellows 2, the heater in the recess 10 being operated to melt the solder. When the bellows chamber 2 is filled, the plug is replaced and the motor 84 is operated to compress the bellows and drive air out of the system through the nozzle 16, 22. It has been found that there is a tendency for the solder level in the nozzle 22 to bounce when a soldered joint is moved away from the nozzle tip. This tendency can be reduced or eliminated if it is ensured that no air is trapped in the bellows chamber 2. In this connection, the design of the bellows wall is important. The walls 90 of the folds of the bellows should remain substantially flat, to avoid trapping air in the folds when the bellows is filled. Generally, when setting up and/or programming a production line the required size of the ball 62 for a joint for a particular component will be determined by trial and error and the related output from the sensor 60 noted. The size can then be reproduced during production. The pcb is carried on an XYZ platform for moving the pcb in the horizontal plane to position a joint over a spray fluxing station and then over the nozzle tip 36, and to move the pcb vertically down to the nozzle. The XYZ platform is programmed to move the board to position each joint in turn over the spray nozzle for fluxing and then move the fluxed joint to the soldering nozzle 22. Once the program has been set for a board, subsequent boards can be fluxed/soldered automatically. The vertical movement of the pcb is adjusted to allow clearance for the optical sensor 60 when it is in the raised position 60' , as the ball 62 is being formed and to bring the pcb " down so that the lead being soldered to the board will penetrate the formed solder ball, and the solder ball 62 will touch the pad on the board, i. e. the tinned area around the lead. For soldering a through hole in the board, the board is brought down to contact the solder ball 62. If necessary, additional solder could be pumped while the through hole is being soldered. However, it has been found that the hole itself can generally take sufficient solder from the nozzle 22 without the need for further pumping of solder into the hole. In the embodiment shown, the nozzle tip 36 is chamferred to provide a sharp edge. The tip 36 of the nozzle 22 may be flat, in which case the nozzle can conveniently be formed by squaring-off a length of tubing. The tip 36 of the nozzle may also be shaped to provide a concave recess. The nozzle 22 can be removed, and replaced by another nozzle of a different size to suit different soldering applications. The nozzle 22 is preferably formed from a material which is not wetted by molten solder. Titanium or stainless steel are examples of materials which could be used for the nozzle. Any suitable conventional sensor can be used as the optical sensor 60. For example, the "optical-fibre photoelectric" unit shown at pages 218-9 of the IMO catalogue, under reference No. E3S-X3, can be used. In an alternative embodiment, a separate detector and light source may be used. However, there is more likelihood of components on the PCB blocking the optical path between the source and detector as the pcb is brought down to the nozzle tip 36. Furthermore, some stray light may be reflected to the detector by the pcb. The sensor output will therefore be influenced by movement of the pcb as well as by changes in the size of the solder ball 62. It may also be possible to use the apparatus as a de-soldering apparatus to remove solder from a pcb. The solder joint is heated and the volume of the bellows chamber 2 is increased to suck solder into the nozzle 22.

Claims: CLAI MS: 1. A soldering apparatus comprising a nozzle and a positive displacement pump for pumping a predetermined quantity of molten solder through the nozzle. 2. A soldering apparatus having a nozzle and means for forming a drop of molten solder of a predetermined size at an end of the nozzle. 3. A soldering apparatus comprising a container for containing molten solder, the container having a nozzle; and means for reducing the working volume of the container to expel a predetermined quantity of solder from the container through the nozzle. 4. A soldering apparatus comprising a container for containing molten solder, the container having a nozzle; and means for reducing the working volume of the container to expel solder from the container through the nozzle, wherein there are no relatively sliding surfaces in contact with the molten solder. 5. A soldering apparatus as claimed in claim 3 or 4 wherein the container has a flexible wall which is deformed to expel the solder. 6. A soldering apparatus as claimed in any preceding claim and further comprising a sensor for sensing the volume of solder expelled from the nozzle. 7. A soldering apparatus as claimed in any one of claims 1 to 5 and further comprising a sensor for sensing the height of a ball of solder expelled from the nozzle. 8. A soldering apparatus as claimed in claim 6 or 7 wherein the means for reducing the volume of the container is controlled on the basis of an output signal from the sensor. 9. A soldering apparatus as claimed in claim 6, 7 or 8 wherein the sensor is an optical sensor comprising a light emitter and a detector. 10. A soldering apparatus as claimed in claim 9 wherein the light emitter is integral with the detector. 11. A soldering apparatus as claimed in any preceding claim and further comprising a nozzle heater for heating the nozzle. 12. A soldering apparatus as claimed in any preceding claim wherein an outlet end of the nozzle is chamfered to provide a sharp edge. 13. A soldering apparatus as claimed in any preceding claim and further comprising flow means for directing a flow of nitrogen gas onto solder expelled from the container. 14. A soldering apparatus as claimed in claim 13 wherein the flow means is adapted to heat the nitrogen gas. 15. A soldering apparatus as claimed in claim 13 or 14 wherein the nozzle is surrounded by a cylinder and the nitrogen gas is fed between the nozzle and cylinder. 16. A soldering apparatus as claimed in claim 13, 14 or 15, wherein the flow means is adapted to introduce a flux to the nitrogen gas. 17. A soldering apparatus as claimed in any preceding ■ claim and further comprising means for vibrating the nozzle. 18. A soldering apparatus comprising a nozzle and means for pumping molten solder through the ( nozzle to form a ball of solder at the nozzle tip, wherein the size of the ball of solder is detected by means of reflected light. 19. A soldering apparatus comprising a nozzle and means for pumping molten solder through the nozzle to form a ball of solder at the nozzle tip, wherein means is provided to vibrate the nozzle tip as the ball of solder is formed at the nozzle tip. 20. A soldering apparatus as claimed in claim 19, wherein a vibrator is mounted directly on the nozzle.

Inventors: CINIGLIO ALEXANDER JAMES, SQUIRE NEIL CHRISTOPHER, TOMBS MICHAEL, CINIGLIO, ALEXANDER, JAMES, SQUIRE, NEIL, CHRISTOPHER, TOMBS, MICHAEL
Applicants: CINIGLIO A J, EVENOAK LTD, SQUIRE NEIL CHRISTOPHER, TOMBS MICHAEL, CINIGLIO, ALEXANDER, JAMES, EVENOAK LIMITED, SQUIRE, NEIL, CHRISTOPHER, TOMBS, MICHAEL
Application year: 1995
Main classification: B23K3
IPC codes: B23K3, H05K3