GB2167991A - Circuit reworking apparatus - Google Patents

Abstract

Apparatus for removing a soldered electrical component from a circuit which includes a workhead with two chambers 22, 23 in piston/cylinder relationship, having an aperture 24 for positioning adjacent the component 25 to be removed and an inlet end for attachement to a gas supply nozzle 19 which provides pressurised hot gas or a suction. In use hot gas is blown onto the component to melt the solder whereupon the blowing action is replaced by a sucking action to cause the component to become attached to the aperture 24 and the workhead to contract by compression of spring 27 thereby to lift the component away from the circuit. <IMAGE>

Description

SPECIFICATION Circuit reworking apparatus This invention relates to apparatus for desoldering and removing electrical circuit components from a support surface such as a printed circuit board or the ceramic substrate of a thick-film circuit. The operation is called 'reworking' of the circuit.

It is known to use a jet of hot non-oxidising gas such as nitrogen to solder components onto printed circuit boards and thick-film substrates and proprietary apparatus is available for the purpose, for example the 'GB7 Hot gas soldering system' made by the firm Planar Products Ltd. Such apparatus could also be used for melting the solder fixing a previously mounted component, the component then being removed with a pair of tweezers or a hand-held vacuum cup say. Considerable manual dexterity would be required. It is also known to melt the solder fixing a component and to then try to remove the molten solder by blowing it away with a blast of high-pressure air or drawing it into a suction device. Spring-loaded solder suction pumps are well known while US Patent No. 3,834,605 discloses the use of an air-blast.In each case, the operation is somewhat heavy-handed and would not be suitable for use in the case say of high density circuits with delicate chip components closely surrounding the one to be removed.

According to one aspect of the invention, there is provided apparatus for removing an electrical component which is fixed to a surface by solder, the apparatus including: support means for supporting the surface from which the component is to be removed; and a work head mounted for relative linear movement with respect to the support means and which includes a chamber having an aperture therein for being applied to said component, hot gas supply means for supplying hot gas via the chamber and its aperture to melt the solder on the component, suction means for drawing gas through said aperture into said chamber thereby to cause said component to become attached to said aperture by suction whilst the solder is in its melted state, and control means for controlling the supply of hot gas and the suction means.

According to another aspect of the invention, there is provided apparatus for removing an electrical component fixed by solder to a support surface, the apparatus comprising support means and a work head mounted for movement in a linear direction with respect to the support means, the head defining a chamber having an aperture therein for being applied to said component and the apparatus further comprising gas supply means for supplying gas to said chamber for said gas to flow from said aperture and to pass at least near said solder, heating means for heating said gas priorto its flow from said aperture, pressure reducing means for producing a reduction in pressure within said chamber to cause said component to become attached by suction to said aperture, control means for controlling the supply of gas to and the production of said reduction in pressure within the chamber and enabling firstly the heated gas to be applied via said aperture to melt said solder and then said reduction in pressure to be produced causing the component to become attached as aforesaid, and translation means for moving said work head in said linear direction with respect to the support means thereby drawing said chamber along with the attached component away from said support surface.

Preferably, said translation means comprises cylinder means within which a piston defining portion of said work head is slidably mounted, the pressure reducing means being coupled to said cylinder means for a reduction in pressure to be produced therein under the control of said control means thereby drawing said piston defining portion further within the cylinder means and hence moving the work head.

In a particularly advantageous embodiment, said work head comprises a hollow chamber having an aperture at one side and being open at the opposite side, the chamber being slidably mounted at least partly within an open-en ded further chamber forming said cylinder means with the aperture in the first-mentioned chamber exposed at the open end of said further chamber, and said pressure reducing means being coupled to said further chamber for producing a reduction in pressure within both chambers causing said component to become attached by suction to said aperture and causing said first-mentioned chamber to be drawn further into the further chamber.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a diagrammatic side view of apparatus for reworking a circuit comprising electrical components soldered onto a ceramic substrate, and Figures 2 and 3 are respective sectional views of a work head and translation mechanism used in the figure 1 apparatus, figure 2 showing the head applied to a component to be removed from the ceramic substrate and figure 3 showing the head retracted by the translation mechanism and the component lifted off the substrate.

The illustrated apparatus comprises a platform base 1 upon which is mounted a fixed, upright support column 2 and a heatable platen 3 upon which there is arranged the circuit 4 to be reworked.

The column 2 carries a support block 5 which can be slid up and down the column to adjust its height above base 1 and, when in the correct position, clamped to the column using clamp screw 6. Extending forwardly from the block 5 is a carrier bar 7 carrying a stereoscopic microscope and illuminating unit 8 arranged to illuminate and provide an enlarged view of the circuit 4. Between the unit 8 and block 5, the carrier bar 7 supports a gas supply and component lifting mechanism 9. This mechanism consists of an elongate cylindrical container 10 containing a heating element (not shown) and mounted with its axis vertical to a slideway 11, which slideway is fixed to the bar 7. By turning thumb wheel 12, the container 10 can be moved up or down with respect to the slideway 11 to finely adjust its height relative to the circuit 4.The container 10 communicates, via tube 13 and changeover valve 14, with a nitrogen gas supply unit 15 and a vacuum reservoir 16. A vacuum is maintained within the reservoir 16 by a vacuum pump 17 controlled by pressure switch 18 arranged to sense the pressure within the reservoir. At the lower end of container 10 there is an outlet nozzle 19 to which there is fixed, by clamping ring 20, an operating head and translation mechanism 21. The mechanism 21 comprises a hollow cylindrical chamber 22 having walls 22a made of stainless steel and the upper end of which is shaped to fit onto the end of the outlet nozzle 19 and to receive clamping ring 20.Slidably mounted within chamber 22 is the actual operating head, this taking the form of a cylindrical chamber 23 of which the walls 23a are made of hard brass, which is open at its upper side, ie open to the interior of chamber 22, and of which the lower side is exposed at the lower end of chamber 22. The lower side of chamber 23 is also open to form a gas outlet aperture 24. The upper part of the wall of chamber 23 has a shape and exterior dimensions matching those of the interior surface of the wall of chamber 22, le so that chamber 23 can slide within chamber 22, while the lower part of the wall of chamber 23 is stepped down in size and adapted as regards its cross-sectional shape so that the opening or aperture 24 at its lower side matches the shape and is just smallerthan the component 25 to be removed from the substrate 26 of circuit 4.A compression spring 27 is positioned within the chambers 22 and 23 to bias the chamber 23 downwardly, the downward movement of this chamber relative to chamber 22 being limited by an abutment lip 28 at the lower end of chamber 22.

In operation, the circuit 4 is mounted on platen 3 and held in place by any suitable means, for example some form of clamp (not shown), and the position of microscope unit 8 is set to give a good view of the circuit. The position of the circuit is adjusted to bring the component 25 accurately beneath the aperture 24. This could be done manually, ie by unclamping the circuit 4, sliding it to the proper position on platen 3 and then reclamping it, but for preference the apparatus includes some form of fine XYtranslation mechanism (not shown) for adjusting the position of the platen 3 sideways and back and forth with respect to the base 1.The circuit 4 is then permitted to come up to the temperature of the heated platen 3, which temperature is carefully set so that it is not sufficient to melt the solder fixing the components of the circuit 4 in place or to damage these components but is sufficient to engender an appreciable reduction in the amount of heat which it is necessary to apply to the solder fixing component 25 in order to melt this solder. The thumb wheel 12 is then turned to bring the periphery of aperture 24 to just above the component 25 as shown in figure 2 and the valve 14 is operated to admit nitrogen gas from supply unit 15 to the interior of container 10 where it is heated to a controlled temperature (say between 1 500C and 700so) and thence via outlet nozzle 19 to the chambers 22 and 23.The gas flows out of aperture 24 over the edges of component 25, at which edges are the component connector portions (not shown) and the solder joints 29 fixing these portions to the conductor tracks (not shown) on the substrate 26. The temperature of the edges of component 25 and the solder joints 29 is thereby increased until eventually the solder becomes molten. As will be appreciated, although the hot gas comes into contact with the central portion of component 25, since most of the gas flow takes place at the edges of the component, these edges and the solder joints will be heated more quickly.

Thus, provided the operation is properly carried out, the central part of the component can be kept sufficiently cool so that the component is not damaged - for example, the component may have a soldered-on lid which it is desired should remain fixed in place.

When the solder has melted, the valve 14 is operated so as to cut off the supply of gas to the container 10 and to instead connect the container to the vacuum reservoir 16. As a result, a reduction in pressure is produced within the chambers 22 and 23.

This causes the component 25 to move and become attached, by suction, to the periphery of the aperture 24 (this is possible because the solder previously fixing the component to substrate 26 is still molten) and virtually immediately afterwards causes the chamber 23 to be drawn up into the chamber 22, in the manner of a piston and cylinder actuator, as shown in figure 3.

One of the functions of the spring 27 is to adjust the relative values of the force tending to lift the chamber 23 when the reduced pressure is produced in chamber 22 and the force attaching the component 25 to aperture 24. In its absence, an oscillatory movement of the chamber 23 may occur, ie the suction attracting the component 25 (which has to overcome the weight of the component plus any tendencyforthe component to stick to the substrate) may be insufficient relative to the force tending to lift the chamber so that the chamber tends to be lifted away from the component - this raises the pressure within chambers 22 and 23 so the chamber drops down again and so on. This effect is dependent upon various factors, however, so the spring 27 may not always be needed.For example, the mass of the chamber 23 opposes the lifting force so if this mass is appropriate the effect may not appear. Also, since the lifting force is dependent upon the crosssectional area of the opening at the top of chamber 23 while the force attaching component 25 is dependent on the area of the aperture 24, an adjustment of the relative sizes of the opening and the aperture, perhaps even to the point where chamber 23 is Iargeratthe bottom than atthetop, may negate the requirement for spring 27.

The chamber walls 22a and 23a do not have to be made of stainless steel and brass respectively or even necessarily of metal - at least the chamber 23 could be of ceramic material perhaps. Both chambers could be made of aluminium although then the clearance between the engaging surfaces of the two chambers may have to be made largerto prevent binding. On the other hand, some clearance between these surfaces is not necessarily a disadvantage any leakage of gas here has to be allowed for of course but leakage of nitrogen during the heating phase might form a kind of curtain of the gas all round the component being heated, this possibly helping to reduce oxidation of the solder joints.

The valve 14 could be hand-operated or operated by a solenoid controlled by a switch. Instead of a change-over valve two on-off valves could be used.

Avarying degree of automaticity may be introduced, eg by having valve 14 controlled automatically by a timer.

In order to ensure that the aperture 24 is brought to the right height above the component 25 for the heating phase, the rim around the aperture could be formed with downwardly extending projections which are brought into actual contact with the component and have a length equal to the desired spacing between aperture 24 and component 25.

The shape of the aperture 24 does not have to match that of the component - for example, the aperture could be circular even though the component may be square or rectangular. Naturally, the component should be large enough relative to the aperture so that at least some portions of it underlie the aperture edges and it can properly attach to the aperture as shown in figure 3. Meanwhile, the aperture should be large enough relative to the component to ensure that heat is applied reasonably directly to the solder joints rather than to the central area of the component. Thus, at least some matching of component and aperture is desirable. To permit uniformity of manufacture, the chamber 23 could be made with a closed flat bottom, ie cupshaped.The purchaser could then customise the chamber to his particular requirements by forming a hole of appropriate size in the bottom of the chamber and perhaps silver-soldering a short length of appropriately shaped tubing to the chamber bottom around the hole therein.

Instead of being heated within the container 10, the nitrogen gas could be heated by a heating element within the chamber 23. For example, the chamber 23 could contain porous sintered material which is heated by a suitable electric element and which heats the gas as it passes through the material.

The chambers 22 and 23 can be attached to nozzle 19 by any suitable means other than the illustrated clamp, for example they could be screwed in place or possibly the nozzle 19 and the chambers could be sold as a unit.

As noted earlier, proprietary hot gas soldering equipment is available such as the Planar Products Ltd 'GB7' apparatus. This already comprises the parts shown in figure 1 except for the work head and translation mechanism 21 and the vacuum reservoir and associated items. Thus, such apparatus could be readily adapted to form an example of the invention simply by providing and connecting up the vacuum equipment and by replacing the jet nozzle which would normally be at the bottom of container 10 with a suitably designed version of the work head and translation mechanism 21. When required, the mechanism 21 could then be interchanged with a jet nozzle to permit soldering or reworking as desired.

Although, as shown, the apparatus is generally intended for removing flat leadless components such as discrete chip capacitors and resistors or integrated circuits made up in the form of leadless chip carriers (LCC's), it could be adapted for other component shapes, eg by suitably shaping the periphery of aperture 24.

The heated platen 3 may not be necessary if sufficient heat is available via the hot gas aperture 24 to melt the solder without use of the platen.

Claims (7)

1. Apparatus for removing an electrical component which is fixed to a surface by solder, the apparatus including: support means for supporting the surface from which the component is to be removed; and a work head mounted for relative linear movement with respect to the support means and which includes a chamber having an aperture therein for being applied to said component, hot gas supply means for supplying hot gas via the chamber and its aperture to melt the solder on the component, suction means for drawing gas through said aperture into said chamber thereby to cause said compo nentto become attached to said aperture by suction whilst the solder is in its melted state, and control means for controlling the supply of hot gas and the suction means.

2. Apparatus for removing an electrical component fixed bysolderto a support surface, the apparatus comprising support means and a work head mounted for movement in a linear direction with respect to the support means, the head defining a chamber having an aperture therein for being applied to said component and the apparatus further comprising gas supply means for supplying gas to said chamber for said gas to flow from said aperture and to pass at least near said solder, heating means for heating said gas prior to its flow from said aperture, pressure reducing means for producing a reduction in pressure within said chamber to cause said component to become attached by suction to said aperture, control means for controlling the supply of gas to and the production of said reduction in pressure within the chamber and enabling firstly the heated gas to be applied via said aperture to melt said solder and then said reduction in pressure to be produced causing the component to become attached as aforesaid, and translation means for moving said work head in said linear direction with respect to the support means thereby drawing said chamber along with the attached component away from said support surface.

3. Apparatus according to claim 2, wherein said translation means comprises cylinder means within which a piston defining portion of said work head is slidably mounted, the pressure reducing means being coupled to said cylinder means for a reduction in pressure to be produced therein under the control of said control means thereby drawing said piston defining portion further within the cylinder means and hence moving the work head.

4. Apparatus according to any of the preceding claims, wherein said work head comprises a hollow chamber having an aperture at one side and being at the opposite side, the chamber being slidably mounted at least partly within an open-ended further chamber forming said cylinder means with the aperture in the first-mentioned chamber exposed at the open end of said further chamber, and said pressure reducing means being coupled to said further chamber for producing a reduction in pressure within both chambers causing said component to become attached by suction to said aperture and causing said first-mentioned chamber to be drawn further into the further chamber.

5. Apparatus according to claim 3, wherein a compression spring is positioned between said first-mentioned chamber and said further chamber to bias said first-mentioned chamber towards said component.

6. Apparatus according to any preceding claim, wherein the support means is heated to reduce the amount of heat to be supplied by the hot gas supply.

7. Apparatus substantially as hereinbefore described with reference to the accompanying drawing.