GB2155368A

GB2155368A - Method and apparatus for correcting printed circuit boards

Info

Publication number: GB2155368A
Application number: GB08506525A
Authority: GB
Inventors: M Jean Claude Wery; Michel Jehay; Andre Job
Original assignee: Burroughs Corp
Current assignee: Unisys Corp
Priority date: 1982-08-03
Filing date: 1985-03-13
Publication date: 1985-09-25
Also published as: GB8506525D0; GB2155368B

Abstract

After cutting a conductive pad P1 or P2 bearing adhesive on one surface and an electrically conductive layer on the other surface from a strip of pads, the pad is bonded to a printed circuit board by passing a current between two electrodes to cause softening of the adhesive. A wire is then welded to the pad by urging a wire against the conductive pad using one of the electrodes E1 and passing a further current between the two electrodes. Vacuum holders receive the pad from the cutter and position it on the circuit board. <IMAGE>

Description

SPECIFICATION Method and apparatus for correcting printed circuit boards Background of the invention Field of The Invention The present invention relates to a method and apparatus for enabling the modification or correction of printed circuit boards by adding interconnections to existing boards. It relates particularly to apparatus, herein called stitching-pads, for use on printed circuit boards to enable new electrical connections to be made. It relates further to apparatus and methods for fabricating such pads and for using such pads to provide bases on printed circuit boards to which connections may be welded in order to modify interconnections between existing electrical circuitry on such boards.

Description of the prior art In order to correct an existing printed circuit board, or to implement engineering instructions for changes to such an existing printed circuit board, it is necessary either to change connections, as by adding or deleting connections, between certain connection points on the printed circuit board or to redo the artwork for the printed circuit board. Adding wire connections from point-to-point on a board appears, on its face, to involve simple and straight-forward procedures, but has not been done satisfactorily in the past. Redoing the artwork is expensive, in that existing inventory must be scrapped, and it is time consuming, in that normal lead times approximate 14-16 weeks. Redoing the artwork is, in any case, not practical for only one or a few boards.

A system currently on the market for making corrections to existing printed circuit boards by redoing the artwork is marketed by Additive Technology Corporation, a Massachusetts company. In that system the printed circuit board is re-etched on a dielectric applied over existing circuitry to implement the corrections or engineering instructions.

That method is expensive in that it is priced at several dollars per board with an initial set-up charge of several hundred dollars. It has been determined that that system is not cost effective for runs of fewer than 50,000 boards. The turn-around time is too long for many purposes, since it is approximately three weeks.

A system for adding connections is marketed by Augat. This system employs a printed circuit board of special construction which is faced by a stainless steel sheet to which wire is stitched, as by welding. The wire utilized is formed of nickel with a teflon coating and the wire stitching employs resistive welding. Fig. 1 is a sectional view illustrating the application of this prior art system which involves three distinct steps, or procedures. In step 1 an upper Tungsten electrode 26 is lowered until the teflon (RTM) insulation 24 encasing nickel wire 22 is in contact with an etched pad of stainless steel. In step 2 the upper electrode is advanced furthor against the teflon which breaks at its contact with the stainless steel as well as at its contact with the Tungsten edge.In step 3, a potential is applied across electrodes 26 and 28 causing the nickel wire and the stainless steel to be melted together to form a weld at 30 as a consequence of the excessive heat developed in the resistance at the nickel-stainless steel junction. It will be noted that the current flow between the aluminum electrode and the copper sheet causes little heat because of the high conductivity of both metals. In addition, it should be noted that the teflon flows back into the break at the top and thus is self-healing.

The printed circuit boards according to the Augat system with its special boards have application primarily in the prototype stage. Augat markets a small wire stitching machine for use with its printed circuit boards for approximately two to three thousand dollars which permits placement and attachment of wires by hand. A more expensive semi-automatic machine is available utilizing an x-y table for positioning the printed circuit board. However, the positioning of the wire, the cutting of the wire, the routing of the wire, and other similar operations are performed manually.

This system is objectionable for use in the correction of production runs because it requires the use of special boards and is labor intensive.

Two patents which appear to relate to the subject matter of the Augat system have been granted to Larry A. Conley. These are U.S. Patent No.'s 3,786,172 and 3,850,711. In each of these, a printed circuit board is disclosed which has both copper and stainless steel layers with segments of each exposed enabling solder to be applied on the copper surface, as by wave soldering, and welding to be performed on the stainless steel surfaces.

Clearly, a system using this special printed circuit board will not be useful for the correction of other types of existing boards.

Summary of The Invention The invention relates to apparatus and methods for correcting existing standard printed circuit boards. The apparatus includes selectively positionable stitching pads formed by a plurality of layers of materials bonded together to form a sandwich-like structure for use with printed circuit boards. The pads include one outer layer of adhesive material of a resin epoxy which will adhere to a surface, such as the face of a printed circuit board, when heat is applied to the pad. Once the epoxy resin has been bonded to a printed circuit board, heat can be applied again without disturbing the bond. The pads include a layer of stainless steel forming all, or part, of the second outer layer of the pad to which suitable wire, such as nickel wire, can be welded, thus enabling connections to be completed through nickel wire between a succession of pads.A layer, or sheet, of copper may be located between the layer, or sheet, of resin epoxy and the layer, or sheet of stainless steel in a preferred embodiment.

A pad may be of a type called a "pivot pad" herein, consisting simply of layers of each mate rial, with or without intermediate copper and other layers, to enable the epoxy face of the pad to be bonded to a printed circuit board while the opposite face of stainless steel is available for welding to suitable wire so that electrical connections can be made between pads.

A more complex form of pad according to the invention is one called a connecting pad herein. A "connecting pad" is a stitching pad of two or more layers with a hole through the pad along an axis substantially perpendicular to the layers of materials. The hole is plated-through with a layer of copper to which a layer of solder (a tin-lead alloy) is bonded. The plated-through copper layer generally is connected to any layers of copper present between the stainless steel and epoxy layers of the pad. The plated through layer of copper and the overlying layer of solder are extended beyond the ends of the hole to form rims of solder-on-cbpper around the ends of the hole.As a consequence, when solder is applied, as in a flow-solder operation, it will readily form an adhesive bond to the walls of the hole and to connectors on the printed circuit board as well as to terminals of electrical components, when such terminals are positioned in the holes.

Another aspect of the invention relates to apparatus and methods for using positionable stitching pads, which may be pivot pads and/or connecting pads to form new electrical connections on printed circuit boards. It involves apparatus for placing a plurality of positionable pads adhesive-side down in contact with preselected spots on the face of a printed circuit board, heating the pads to cause the heat-responsive adhesive sides thereof to adhere to the face of the board, placing a wire in contact with the stitching side of each pad and applying heat to weld the wire to the pad. Each of the pads may be connected to the others in this way.

Still another aspect of the invention involves the use of connecting pads, i.e. those having plated through holes. For this type of pad, additional apparatus is employed according to a proven method to center the hole in each pad over a preselected hole in a printed circuit board. Additional connections may be completed then, by flowing solder into the holes, between the stainless steel face of the pad and circuitry on the opposite face of the printed circuit board. Connections may also be completed in this way to electrical components, when their terminals extend into the holes.

Brief Description of The Drawings Figure 1 is a sectional view showing the structure of a prior art printed circuit board of special construction and illustrating how wire is stitched (welded) to the stainless steel surface of such a special printed circuit board; Figure 2A and 2B are a top view and a sectional view, respectively, of a stitching pad having a plated-through hole (called a connecting pad herein) in accordance with the invention; Figure 3 shows the layout of a preferred arrangement of stitching pads prior to their application to printed circuit boards, where groups of pads having plated-through holes, or connecting pads, alternate with groups of pads without holes, or pivot pads;; Figure 4 is a view partially in section showing a pad stamper for separating stitching pads, which are assembled in a ribbon such as that shown in Fig. 3, into separate pads; Figure 5 is a view in partial section of apparatus of use in placing the individual stitching pads in position on a printed circuit board; Figure 6 is a view in perspective showing a printed circuit board supported on an x-y table, with a number of stitching pads affixed to the board and with welding equipment in position to stitch wire to the pads; Figure 7 is a sectional view of a printed circuit board, showing a stitching pad in place on the board, a component inserted through and soldered into a plated-through hole, and a nickel wire stitched (welded) to the stainless steel face of the pad;; Figures 8-15 illustrate a series of steps employed in positioning wire, stitching wire and cutting wire to add new connections to a printed circuit board.

Figure 16 shows the configuration of electrodes El and E2 and the relationship of El to the wire 24.

Figure 17 is an elevation showing details of construction of a preferred stitching head, according to the invention, which may be employed in the performance of the operations shown schematically in Figures 8-15.

Figure 18 shows a portion of the stitching head according to Fig. 17 as it would be positioned for welding.

Further Description of Exemplary Prior Art Turn to Fig. 1 where a sectional view of a portion of a special printed circuit board according to the prior art is shown at 10. This board consists of a sandwich of four layers including two layers, or sheets, of copper 12 and 14 separated by a layer of epoxy 16 with a sheet of stainless steel 18 affixed to one sheet of copper 14. A hole 20 through the printed circuit board is lined by a thin layer of copper 21 which is plated-through the hole for use in mounting electrical components by flow soldering and the like.

In order to stitch, or weld, an insulated wire to the stainless steel surface it is necessary to use wire of suitable materials and follow an effective procedure. In this example, a nickel wire 22, insulated by a layer of teflon 24, is placed in contact with the surface of the stainless steel sheet 18. A Tungsten electrode 26 is placed in contact with the teflon coating and force is applied with the view to rupturing the teflon coating where it contacts the stainless steel surface as well as where it contacts the Tungsten electrode. An electrode 28 made of highly conductive material, and preferably of aluminum, is placed against the surface of copper sheet 12.

A low voltage source of electricity having high current capacity is applied across the electrodes 26, 28, causing the nickel wire and the surface of the stainless steel sheet to melt together to form a weld, as indicated by the dark spot at 30. It will be seen that the weld is formed due to the excessive heat developed in the area of the high resistance occuring at the junction of the nickel and stainless steel. The teflon coating has the characteristic property of flowing as it cools so that it flows back into the break to seal insulation over the point of the weld. Very little heat is developed at the junction of the aluminum electrode 28 and the copper sheet since this junction is of very low resistance.

As previously indicated, this prior art apparatus and method are restricted to specially designed printed circuit boards and are not useable in the modification of other printed circuit boards. They are of interest as prior art especially because they illustrate certain principles which apply in the application of the present invention.

Description Of A Preferred Embodiment Turn now to Figs. 2A and 2B for a description of a zero profile soldering and welding pad, herein called a connecting pad, according to the invention. The pads are in the form of a sandwich construction having an adhesive layer in the form of a resin epoxy at 32 which is secured to a layer, or sheet, of epoxy material 34, which in a typical case would be of approximately 100 microns thickness.

Attached to the epoxy material, on the side opposite the adhesive, is a layer of copper 36, approximately 15 microns in thickness, and attached to the copper is a stainless steel layer 38 which is approximately 100 microns in thickness. A plated through hole 40 is plated at 42 with an alloy of tin and lead (solder) over a copper layer 44. The plating in the holes is extended beyond the edges of the holes to overlap the face of the epoxy layer 34 and the face of the stainless steel layer 38 by approximately 0.015 inches. In addition to connecting pads through which plated holes are provided, the invention encompasses pivot pads (page 2, Fig. 3) which are like the connecting pads but lack the plated-through holes.

Connecting pads P1 such as are illustrated in Figs. 2A, 2B and pivot pads P2 (with no holes) are provided preferably in a ribbon or strip 46, as indicated in Fig. 3. As shown in this example, connecting pads are grouped in pairs separated by one or two pivot pads. This arrangement was selected as convenient in a particular instance to facilitate cutting pads apart and mounting them on printed circuit boards by use of automatic equipment. it will be recognized, of course, that other arrangements of pivot pads and connecting pads are possible without departing from the spirit of the invention.

In a particular case where a large number of printed circuit boards are to be modified, the strip 46 may have the connecting pads and the pivot pads grouped in a different way so that connecting pads to enable connections to the board and pivot pads through which no connections to the board can be made are distributed exactly as needed over the board in a fashion to optimize the operation of apparatus according to the invention.

The ribbon or strip-of-pads 46 is placed in a pad stamper 48, shown in partial section in Fig. 4, which positions the strip, clamps it, cuts it into individual pads and delivers the individual pads to a spoon holder SH. As indicated in detail in Fig. 4, a strip 46 is positioned between pinch rollers R2, R4 of the pad stamper 48 which advances the strip-ofpads a predetermined distance which may be established by a human operator or by computer means.When a plated-through hole in a pivot pad is positioned appropriately, a centering needle CN may be inserted into the hole to better establish the position of the strip, impede further movement of the strip of positionable pads, and in cooperation with a pad clamper CL, which moves along the line marked by a two headed arrow A2, to clamp the strip in place and permit a pad of predetermined length to be severed from the strip by means of a cutter CU. When a connecting pad is positioned under the centering pin or centering needle, and there is no hole to receive the centering pin, the centering pin remains in the withdrawn position and the pad clamper CL is moved, along the path indicated by the two headed arrow A2, into position against the strip 46 to secure the strip.While the strip is secured a pad of predetermined length is severed from the strip by means of the cutter CU moving along the path indicated by two headed arrow A4.

The severed pad falls on a receiving member, or pad spoon, RM which is supported by the spoon holder SH which, in turn, is movable in the direction indicated by the associated two-headed arrow A6. The receiving member RM is coupled via tube 50 to a source of variable air pressure (not shown) so that it can either hold the pad or repel it.

The pad is held in place by the receiving member RM making use of low air pressure. The holder SH is then moved in a direction along the horizontal indicated by arrow A6 to transport the pad to a position near a positioner PE as indicated in Fig. 5.

When the pad is in place beneath the positioner, air pressure is reversed through 50 to lift the pad off of the pad spoon RM and a vacuum is applied through 52 to lift the pad into contact with a pasting electrode PE. Pad holders at P4, P6 are then urged, by means not shown, into contact with the pad to hold it.

A printed circuit board is then moved under the positioner containing the positionable pad with the movement of the printed circuit board being coarsely determined by an x-y table or other positioning means, and the fine positioning being accomplished with the use of a light beam and a four-quadrant sensor. If the pad is a connecting pad (with a plated-through hole) the light beam is used to position the selected plated-through hole of the printed circuit board in line with the platedthrough hole of the positionable pad to ensure the alignment of both plated-through holes. If the pad is a pivot pad (with no plated-through hole) its position on the printed circuit board is determined with sufficient accuracy by the x-y table. In either case, the positionable pad is then affixed to the printed circuit board by utilizing pressure and two electrodes to cause a current flow. The flow of current develops heat in the positionable pad and causes the adhesive on the positionable pad to bond to the printed circuit board.

Fig. 6 represents a printed circuit board positioned on an x-y table showing a plurality of connecting pads P1 and pivot pads P2 secured to the printed circuit board and interconnected by wires in accordance with further principles of the invention. As indicated above, the pads P1 and P2 are secured to the board when electricity is applied across two electrodes contacting one face of the pad and producing heat to fuze the adhesive epoxy to the printed circuit board. The blocks C3, C5 shown in Fig. 6, as will be explained more fully below, terminate in electrodes El, E2 of use in stitching wire to the pads Pl, P2.It will be understood, however that these electrodes, one of which may be the pasting electrode PE, may be used to heat a pad for the purpose of causing the adhesive epoxy, and thereby the pad, to adhere to the printed circuit board.

Fig. 7 illustrates a sectional view through a printed circuit board PC and an associated connecting pad, P1 on the board. A component C1 is shown in position where it has been inserted through and soldered into a plated-through hole which extends through the PC board and the pad.

A nickel wire 22 is shown stitched (welded) to the stainless steel face of the pad. Other connections, not shown, may be completed to the solder on the lower side of the PC board using conventional flow-solder techniques.

Method Of Stitching Wire To Pads A preferred method, in accordance with the invention, for preparing wire for stitching (welding) and then stitching wire to positionable pads is shown in Figs. 8-13. This method includes the following steps: 1. As shown in Fig. 8, a wire positioner comprised of two electrodes El, E2, the first electrode El (the inner electrode in the drawing) being preferably of Tungsten and the second electrode E2 (the outer electrode in the drawing) being preferably of aluminum or copper, positions the wire over the positionable pad P1.

2. As the first electrode El (which carries the wire) moves toward the positionable pad as seen in Fig. 9, a movable plunger PL is moved toward the right, as shown, causing a right angle to be formed in the wire and positioning the wire between the end of the first electrode and the positionable pad.

3. As shown in Fig. 10, the second electrode E2 is lowered then until it extends over the end of the first electrode El.

4. As shown in Fig. 11, the electrodes are lowered together then until the second electrode E2 comes into contact with the positionable pad P1.

5. As shown in Fig. 12, the first electrode El is lowered then into contact with the positionable pad, with the wire interposed between the positionable pad and the first electrode. A sufficient force is then applied to mechanically break the insulation on the wire.

6. With the electrodes positioned as in Fig. 12, current flow is initiated then between the first electrode and the second electrode of sufficient magnitude to resistance weld the nickel wire to the stainless steel surface of the positionable pad.

7. In Fig. 13, the first and second electrodes are shown after being removed from the positionable pad. In this view, apparatus is indicated at DY for detecting and measuring the motion of the wire.

8. In Fig. 14, the wire is shown being routed by electrode El which carries the wire to a succession of positionable pads where steps one through six are repeated in order to stitch the wire to each pad.

9. As a final step, after the wire connections have been made to all the pads, the wire is cut by cutter means indicated at CU2, CU4 in Fig. 15.

A further showing of the relationships between the electrodes El and E2 and the wire is indicated in Fig. 16, which represents an enlarged view of the tips of the electrodes El, E2 with wire 24 extending over the end of El as they would appear from a vantage point along line 16-16 near the face of the pad in Fig. 12.

In the practice of this invention, a voltage of less than two volts and about 60 joules of energy is utilized for resistance welding. The energy to weld is dissipated through the electrodes by discharging a capacitor. The amount of energy is voltage and time dependent based upon the materials utilized.

Fig. 17 is an elevation showing details of a preferred stitching head of use in performing the operations set forth schematically in Figs. 8-15.

In Fig. 17, the Tungsten electrode El and aluminum electrode E2 are shown in a position corresponding to that of Fig. 9 (but as a mirror image).

In this view, a wire shooter WS serves the same purpose as the plunger PL of Fig. 9. It will be seen that a camming surface at PL1 engages a cam at PL2 which causes the wire shooter to move against a mechanical bias provided by a spring SP to bend the wire 24 as indicated. The wire 24 is fed through a needle hole 25 in the slab holder 27 and out through the electrode El, as shown. A slab holder 29 supports the electrode E2 and the wire shooter WS. The slab holders, in a preferred embodiment, are positioned by air pressure systems operating through cylinders the shafts of which are indicated at 31 and 33.

Fig. 18 is a representation of a portion of the stitching head showing the electrodes El and E2 in a position corresponding to that of Fig. 12 while stitching, or welding, is performed. The electrodes in both Fig. 18 and Fig. 12 occupy the relative positions, side-by-side, indicated in Fig. 16.

The foregoing apparatus may be controlled by a microprocessor which relies on inputs from various actuators and sensors and is responsive to instructions stored in RAMS or ROMS. The microprocessor may also receive instructions from a datacom which is connected to a central processor and from a floppy disk. The software contained on the floppy disk is software unique to the printed circuit board being worked upon as it defines the exact location of wires, routings, etc.

According to a first method of operation, the po sitioning of the positionable pads and the wire stitching may be controlled manually using a joy stick or the joy stick may be utilized to create software to effect control. A second method may make use of a graphic terminal which utilizes the data for the printed circuit board and displays the actual artwork. By employing a cursor, the positionable pad and wire can be positioned, routed and terminated graphically. Once the operator is satisfied with the graphic corrections, such corrections, after being subject to certain review techniques and procedures, would be implemented in software. The positioning may also be automatically performed through use of a design file which compares the design with subsequent revisions; accordingly, the entire procedure can be automated.

Other enhancements and improvements include replacing the x-y table with a robot control to provide a completely unattended machine. A further improvement is to provide a coating or abrasive tool for the purpose of removing or breaking existing connections on a printed circuit board, thereby permitting non-additive changes in the same operation.

Claims

CLAIMS 1. A selectively positionable stitching pad, formed by a plurality of layers of materials, for use in changing electrical connections on printed circuit boards, comprising: a plurality of layers of materials bonded together to form a sandwich-like structure, in which a first face of the pad includes a layer of adhesive material and a second face of the pad includes a layer of weldable material; said layer of adhesive material being responsive to the application of heat at a selected level to adhere to a surface with which it is placed in contact, thereby bonding the pad to the surface in a fashion which will retain the bond during subsequent applications of heat; and said layer of weldable material being responsive to the application of heat at a selected level to form a weld with suitable wire with which it is placed in contact. 2. The invention as claimed in Claim 1, in which: an additional layer of material is positioned between the layer of adhesive material and the layer of weldable material; said additional layer being formed substantially of copper. 3. The invention as claimed in Claims 1 or 2, in which: a hole is extended through the stitching pad along an axis substantially perpendicular to the layers of material; said hole is plated through with a layer of copper to which a layer of solder is bonded; and said layer of copper and said layer of solder are extended beyond the end of the hole through the weldable layer and the copper is bonded to the weldable layer to form a rim of solder on copper around the periphery of the hole; said rim of solder on copper covering a portion of the weldable layer and leaving a substantial portion of the weldable layer exposed and available to receive a weld; whereby solder, applied to the pad, will readily form an adhesive bond within the hole and to the rim of solder in place on copper around the end of the hole. 4. The invention as claimed in Claim 3, in which the pad includes: a layer of epoxy material positioned adjacent the adhesive layer in position between the other layers of the pad and the adhesive layer; said layer of epoxy material forming a lip around one end of the hole; said adhesive layer extending over a portion of the epoxy layer, but leaving said lip exposed; said layer of copper and said layer of solder extending beyond the end of the hole through the epoxy layer with the copper bonded to the epoxy to form a rim of solder on copper extending over said lip; whereby solder applied, as in a flow-solder operation, will readily form an adhesive bond within the hole and to the rim of solder in place on copper extending over said lip. 5. The invention as claimed in Claim 3, in which: a layer of epoxy, through which the hole extends, is positioned between the weldable layer and the layer of adhesive material; the layer of copper to which a layer of solder is bonded is extended beyond both ends of the hole; the layer of copper extending beyond the layer of epoxy is bonded to the face of the layers of epoxy and together with the layer of solder forms a rim extending over the epoxy around the face of the hole through the epoxy; and said adhesive layer forms a portion of the face of the pad extending outwardly from the edge of the rim. 6. The invention as claimed in Claims 1 or 2 in which: Othe stitching pad is formed as one of a plurality of stitching pads which collectively form a sheet of stitching pads; said sheet being susceptible of separation into individual stitching pads for application to printed circuit boards. 7. The invention as claimed in Claim 3 in which: the stitching pad is formed as one of a plurality of stitching pads which collectively form a sheet of stitching pads; said sheet being susceptible of separation into individual stitching pads for application to printed circuit boards. 8. A composite sheet of material which may be separated into a plurality of stitching pads, comprising: a plurality of layers of materials in which a first face includes a layer of adhesive material for bonding to a selected surface and a second face includes a layer of material to which a weld may be made; first and second pads separable from said sheet; said first pad and said second pad each comprising a sandwich-like structure including a first face and a second face; and said second pad including a hole therethrough along an axis substantially perpendicular to said first face and said second face, said hole being plated-through with materials to which solder will adhere enabling connections to be established through the hole. 9. A method for changing electrical connections on printed circuit boards, said method involving the use of positionable pads having a heat responsive adhesive side and a stitching side and wiring to interconnect the positionable pads, comprising: placing positionable pads adhesive side down in contact with the face of a printed circuit board; heating the positionable pads to cause heat responsive adhesive sides thereof to adhere to the board, thereby securing the positionable pads to the board; and placing a wire in contact with the stitching side of a pad attached to the board and welding said wire to said pad to establish an electrical connection to the pad. 10. The method according to Claim 9 in which the positionable pads are presented as an integral ribbon which first must be cut into individual pads, comprising: advancing the ribbon along a rigid support into a position to be cut; cutting the ribbon to produce a positionable pad; moving said freshly cut positionable pad into a selected position adhesive side down in contact with the face of a printed circuit board; and repeating the cutting and positioning steps for however many positionable pads are needed. 11. The method as claimed in Claims 9 or 10, in which at least one of the positionable pads includes a plated-through-hole, and the step of plac ing positionable pads on the face of a printed circuit board includes: centering the plated-through-hole over a hole through the circuit board whereby connections may be made from the face of the pad through the holes to circuitry on the printed circuit board. 12. The method as claimed in Claims 9 or 10, in which the step of welding the wire to a pad in cludes: Oplacing a wire positioner, comprising a first electrode carrying wire and a second elec trode, over a positionable pad; moving the first electrode toward the pad; moving a plunger against the wire causing a right angle to be formed in the wire in order to po sition the wire between the end of the first elec trode and the positionable pad; moving the second electrode into position until it extends over the end of the first electrode and con tacts the pad; moving the first electrode toward the pad until the wire is interposed between the pad and the in ner electrode; applying force to the electrode against the wire to mechanicaliy break the insulation on the wire;; and initiating current flow between the first and second electrode to weld the wire to the pad. 13. The method as claimed in Claim 12, including the additional steps, comprising: removing the inner and outer electrodes from the pad; and moving the electrode carrying wire to another positionable pad for the purpose of repeating said claimed steps and welding said wire to the additional pad. 14. The method as claimed in Claim 11, in which the step of welding the wire to a pad includes: placing a wire positioner, comprising a first electrode carrying wire and a second electrode, over a positionable pad; moving the first electrode toward the pad; moving a plunger against the wire causing a right angle to be formed in the wire in order to position the wire between the end of the first electrode and the positionable pad; moving the second electrode into position until it extends over the end of the first electrode and contacts the pad; moving the first electrode toward the pad until the wire is interposed between the pad and the inner electrode: applying force to the electrode against the wire to mechanically break the insulation on the wire; and initiating current flow between the first and second electrode to weld the wire to the pad. 15. The method as claimed in Claim 14, including the additional steps, comprising; removing the inner and outer electrodes from the pad; and moving the electrode carrying wire to another positionable pad for the purpose of repeating said claimed steps and welding said wire to the additional pad. 16. A method for stitching wire to positionable pads comprising the following steps: positioning a first electrode bearing a wire, over a first positionable pad; moving a plunger to cause a right angle to be formed in the wire, and thus to position a portion of the wire between the end of the first electrode and the positionable pad; lowering a second electrode associated with the first until the end of the second electrode extends over the end of the first electrode; lowering the electrodes together until the second electrode comes into contact with the positionable pad; lowering the first electrode until the wire is inter posed between the positionable pad and the first electrode; applying a sufficient force to the first electrode to mechanically break insulation on the wire; ; initiating current flow between the first and sec ond electrode of sufficient magnitude to resistance weld the wire to the stainless steel surface of the positionable pad; and removing the first and second electrodes from the positionable pad. 17. A method employing a wire positioner for stitching wire to positionable pads, where the wire positioner includes a first electrode of tungsten and a second electrode of a good metalic conductor, said first electrode including means supporting a wire, the method comprising the following steps: moving the first electrode, which includes means supporting the wire, toward a positionable pad; moving a movable plunger to cause a right angle to be formed near the end of the wire so as to position the end of the wire between the end of the first electrode and the positionable pad; lowering the second electrode until it extends over the end of the first electrode; lowering the electrodes together until the second electrode comes into contact with the positionable pad;; lowering the first electrode into contact with the positionable pad, with the wire interposed between the positionable pad and the first electrode; applying a sufficient force to mechanically break the insulation on the wire; providing an electrical potential between the first and second electrodes of sufficient magnitude to resistance weld the nickel wire to the stainless steel surface of the positionable pad; moving the first and second electrodes from contact with the positionable pad; routing the wire to a second and successive positionable pads for the purpose of repeating the foregoing steps; and after the desired wire connections have been made to all the pads, cutting the wire. 18. A selectively positionable stitching pad substantially as hereinbefore described with reference to, and as illustrated in, the accompanying diagrammatic drawings. 19. A composite sheet of material which may be separated into a plurality of stitching pads substantially as hereinbefore described with reference to, and as illustrated in, the accompanying diagrammatic drawings. 20. A method for changing electrical connections on printed circuit boards substantially as hereinbefore described with reference to, and as illustrated in, the accompanying diagrammatic drawings. 21. A method for stitching wire to positionable pads substantially as hereinbefore described with reference to, and as illustrated in, the accompanying diagrammatic drawings. 22. A method employing a wire positioner for stitching wire to positionable pads substantially as hereinbefore described with reference to, and as il lustrated in, the accompanying diagrammatic drawings. 23. Any features of novelty, taken singly or in combination of the embodiments of the invention as hereinbefore described with reference to the accompanying diagrammatic drawings. Amendments to the claims have been filed, and have the following effect: (a) Claims 1-23 above have been deleted. (b) New or textually amended claims have been filed as follows: CLAIMS

1. An apparatus for correcting printed circuit boards, said apparatus comprising: a pad dispenser, operative to dispense a stitching pad to be attached to a printed circuit board, said stitching pad comprising a heat activated adhesive layer on a first face thereof and an electrically conductive weldable layer on a second face thereof; a pad positioner operable to receive a stitching pad from said pad dispenser and to apply said first face of the pad to a selectable location on a printed circuit board; and first and second electrodes operable thereafter to provide electrical contact with said weldable layer to pass a first electrical current therebetween to cause said weldable layer to generate heat for transfer to said adhesive layer to cause said adhesive layer to bond the pad to the printed circuit board and to remain so bonded during subsequent heating; where said first electrode is operable thereafter to urge a wire against said weldable layer and said second electrode is operable to provide electrical contact with said weldable layer for a second electrical contact to be passed between said first and second electrodes to provide an electrical resistance weld between the wire and said weldable layer on the pad.

2. An apparatus according to Claim 1 wherein said pad dispenser comprises a pad stamper for cutting the terminal one from a strip of pads as the pad to be dispensed.

3. An apparatus according to Claim 2 for use where said strip of pads comprises pivot pads having a hole through-penetrative thereof, wherein said pad dispenser comprises means for advancing said strip of pads a predetermined distance between subsequent operations of said pad stamper, wherein said pad dispenser comprises a centring needle operable to be urged towards or away from said terminal one of said strip of pads, and wherein, in the event of said terminal one of said strip of pads being a pivot pad, said centring needle is advanced into the hole in the pivot pad to establish the precise position of said strip of pads, said centring needle being withdrawn from the hole subsequently thereto to allow dispensing of the pad.

4. An apparatus according to any of the preceding claims wherein said pad positioner comprises: a receiving member for receiving the dispensed pad from said pad dispenser; and a positioning member for positioning the dispensed pad on the printed circuit board; said receiving member being operable to move the dispensed pad to said positioning member and to transfer the dispensed pad to said positioning member.

5. An apparatus according to Claim 4 wherein said receiving member comprises a first source of variable air pressure, said first source of variable air pressure being operative to hold the dispensed pad on said receiving member during receipt of the dispensed pad from said pad dispenser and movement of the dispensed pad towards said posi tioning member, and being operative thereafter to urge the dispensed pad towards said positioning member.

6. An apparatus according to Claim 5 wherein said positioning member comprises a second source of variable air pressure operative to hold the dispensed pad on said positioning member until said selected position on the printed circuit board is reached and operable thereafter to transfer the dispensed pad to the printed circuit board.

7. An apparatus according to Claims 4, 5 or 6 wherein said positioning member comprises a light beam and a four-quadrant optlcal sensor operable, in the event of the dispensed pad being a pivot pad, to line up said hole in dispensed pad with a corresponding hole in the printed circuit board.

8. An apparatus according to Claims 4,5,6 or 7 wherein said positioning member comprises an X Y table for positioning the printed circuit board.

9. An apparatus according to Claim 3 or according to any of Claims 4 to 7 when dependent upon Claim 3 for use where said hole in the dispensed pad when said dispensed pad is a pivot pad is a plated-through hole.

10. An apparatus according to Claim 9 when dependent upon Claim 7 for use where said platedthrough hole and said hole in the printed circuit board are co-operative to receive a leg of an electrical or electronic component.

11. An apparatus according to any of the preceding claims wherein said first electrode is positionable over a pad and bears a wire; wherein said apparatus comprises: a plunger moveable to cause a right angle to be formed in the wire to position a portion of the wire between the end of said first electrode and the positionable pad; wherein said second electrode is lowerable until the end of said second electrode extends beyond the end of said first electrode in the direction of the pad; wherein said first and second electrodes are together lowerable until said second electrode comes into contact with the positionable pad; wherein said apparatus comprises means operative thereafter to apply mechanical force towards said pad to said first electrode to break insulation on the wire for the wire to have electrical connection both with the pad and with said first electrode; wherein said apparatus comprises means operative thereafter for initiating current flow between said first and said second electrodes of sufficient magnitude to resistance weld the wire to the surface of the positionable pad; and wherein said apparatus comprises means for removing said first and second electrodes from the positionable pad.

12. An apparatus according to Claim 11 wherein said first and said second electrodes are supported by said positioning member to be moved thereby towards the pad and to pay out the wire between pads as said first and second electrodes are moved between pads.

13. An apparatus according to any of the preceding claims wherein said first electrode is provided within a recess in said second electrode, said first electrode being free to move towards and away from said pad and being electrically insulated from said second electrode.

14. An apparatus according to any of the preceding claims wherein said second electric current is derived from discharge of a capacitor.

15. An apparatus according to Claim 14 wherein said capacitor is charged with a voltage less than two volts to store an energy of 60 joules.

16. An apparatus according to any of the preceding claims for use where said pad comprises a layer of copper intermediate between said adhesive layer and said weldable layer.

17. An apparatus according to Claim 3 or according to any of Claims 4 to 16 when dependent upon Claim 3 for use where said hole in the pad is plated through with a layer of copper lining to whicha layer of solder is bonded; and where said layer of copper lining and said layer of solder together extend beyond a first end of said hole proximate to said second face of said pad through said weldable layer, said copper lining being bonded to said weldable layer for said solder to form a rim of solder on copper around the periphery of said hole to cover a first portion of said weldable layer and to leave a second portion of said weldable layer exposed and available to receive a weld; whereby solder, applied to the pad, will readily form a solder bond both within said hole and on said rim of solder on copper around said first end of said hole.

18. An apparatus according to Claim 17 for use where the pad comprises a layer of epoxy material adjacent to said adhesive layer between the other layers of said pad and said adhesive layer and forming a lip around a second end of said hole proximate to said first surface of said pad; where said adhesive layer extends over a portion of said epoxy layer, leaving said lip of said epoxy layer exposed; where said copper lining and said layer of solder together extend beyond said second end of said hole through said epoxy layer; and where said copper lining is bonded to said lip of said epoxy layer to form a second rim of solder on copper extending over said lip in said epoxy layer; whereby solder applied, as in a flow-solder operation, will readily form a solder bond within said hole and to said second rim of solder on copper extending over said lip on said epoxy layer.

19. An apparatus according to Claim 3 or according to any of Claims 4 to 16 when dependent upon Claim 3 for use where the pad comprises a layer of epoxy, through which said hold extends intermediate between said weldable layer and said layer of adhesive layer adjacent to said adhesive layer; a copper lining having a layer of solder bonded thereto extending beyond a second end of said hole proximate to said first surface of said pad, where said copper lining extending beyond said layer of epoxy is bonded at said first face of said paid to said layer of epoxy to form a second solder rim extending over said epoxy layer around said second end of said hole through the epoxy; and where said adhesive layer forms a portion of said first face of said pad extending outwardly from the edge of said second solder rim.

20. An apparatus according to any of the preceding claims for use where said weldable layer is a stainless steel layer.

21. An apparatus according to any of the preceding claims for use where said adhesive layer comprises a thermo-setting resin.

22. An apparatus according to Claim 8 for use where said thermo-setting resin is an epoxy resin.

23. An apparatus according to any of the preceding claims for use where said weldable layer is 100 microns in thickness.

24. An apparatus according to Claim 16 or according to any of Claims 17 to 23 when dependent upon Claim 16 for use where said copper layer is 15 microns thick.

25. An apparatus according to Claims 18 or 19 or 5 or according to any of Claims 6 to 12 when dependent upon Claims 18 or 19 for use where said epoxy layer is 100 microns thick.

26. An apparatus according to any of the preceding claims wherein said wire is a nickel wire.

27. An apparatus according to Claim 11, or according to any of Claims 12 to 26 when dependent upon Claim 11 for use where said insulation on said wire is polytetrafluoroethylene.

28. An apparatus according to any of the preceding claims wherein said first electrode is tungsten.

29. An apparatus according to any of the preceding claims wherein said second electrode is ai- uminium.

30. An apparatus substantially as described with reference to Figs. 4, 5 and 6 and 8 to 18 of the appended drawings.

31. An apparatus according to Claim 30 for use with a pad substantially as described with reference to Figs. 2A, 28, 3 and 7 of the appended drawings.

32. A method for correcting printed circuit boards, said method comprising the steps of: employing a pad dispenser to dispense a stitching pad to be attached to a printed circuit board, said stitching pad comprising a heat activated adhesive layer on a first face thereof and an electrically conductive weldable layer on a second face thereof; employing a pad positioner to receive a stitching pad from said pad dispenser and to apply said first face of the pad to a selectable location on a printed circuit board; thereafter moving first and second electrodes to provide electrical contact with said weldable layer; passing a first electrical current between said first and second electrodes to cause said weldable layer to generate heat for transfer to said adhesive layer to cause said adhesive layer to bond the pad to the printed circuit board and to remain so bonded during subsequent heating; thereafter urging said first electrode towards the pad to urge a wire against said weldable layer; providing electrical contact with said weldable layer with said second electrode; and passing a second electric current between said first and second electrodes to provide an electrical resistance weld between the wire and said weldable layer on the pad.

33. A method according to Claim 32 wherein said pad dispenser comprises a pad stamper for cutting the terminal one from a strip of pads as the pad to be dispensed.

34. A method according to Claim 33 for use where said strip of pads comprises pivot pads having a hole through-penetrative thereof, including the steps of: advancing said strip of pads a predetermined distance between subsequent operations of said pad stamper, providing a centring needle operable to be urged towards or away from said terminal one of said strip of pads; in the event of said terminal one of said strip of pads being a pivot pad, advancing said centring needle into the hole in the pivot pad to establish the precise position of said strip of pads; and withdrawing said centring needle from the hole subsequently thereto to allow dispensing the of pad.

35. A method according to any of Claims 32 to 34 wherein said bad positioner comprises a receiving member for receiving the dispensed pad from said pad dispenser; and a positioning member for positioning the dispensed pad on the printed circuit board; said receiving member being operable to move the dispensed pad to said positioning member and to transfer the dispensed pad to said positioning member.

36. A method according to Claim 35 wherein said receiving member comprises a first source of variable air pressure, said first source of variable air pressure being operative to hold the dispensed pad on said receiving member during receipt of the dispensed pad from said pad dispenser and movement of the dispensed pad towards said positioning member, and being operable thereafter to urge the dispensed pad towards said positioning member.

37. A method according to Claim 36 wherein said positioning member comprises a second source of variable air pressure operative to hold the dispensed pad on said positioning member until said selected position on the printed circuit board is reached and operable thereafter to transfer the dispensed pad to the printed circuit board.

38. A method according to Claims 35,36 or 37 wherein said positioning member comprises a light beam and four-quadrant optical sensor operable, in the event of the dispensed pad being a pivot pad, to line up said hole in dispensed pad with a corresponding hole in the printed circuit board.

39. A method according to Claims 35,36,37 or 38 including the use of an X-Y table for positioning the printed circuit board.

40. A method according to Claim 34 or according to any of Claims 35 to 39 when dependent upon Claim 34 for use where said hole in the dispensed pad when said dispensed pad is a pivot pad is a plated-through hole.

41. An apparatus according to Claim 40 when dependent upon Claim 38 for use where said plated-through hole and said hole in the printed circuit board are co-operative to receive a leg of an electrical or electronic component.

42. A method according to any of Claims 32 to 41 wherein said first electrode is positionable over a pad and bears a wire; wherein said apparatus comprises a plunger moveable to cause a right angle to be formed in the wire to position a portion of the wire between the end of said first electrode and the positionable pad; wherein said second electrode is lowerable until the end of said second electrode extends beyond the end of said first electrode in the direction of the pad; wherein said first and second electrodes are together lowerable until said second electrode comes into contact with the positionable pad; wherein said apparatus comprises means operative thereafter to apply mechanical force towards said pad to said first electrode to break insulation on the wire for the wire to have electrical connection both with the pad and with said first electrode; wherein said apparatus comprises means operative thereafter for initiating current flow between said first and said second electrodes of sufficient magnitude to resistance weld the wire to the surface of the positionable pad; and wherein said apparatus comprises means for removing said first and second electrodes from the positionable pad.

43. A method according to Claim 42 wherein said first and second electrodes are supported by said positioning member to be moved thereby towards the pad and to pay out the wire between pads as said first and second electrodes are moved between pads.

44. A method according to any of Claims 32 to 43 wherein said first electrode is provided within a recess in said second electrode, said first electrode being free to move towards and away from said pad and being electrically insulated from said pad and being electrically insulated from said second electrode.

45. A method according to any of Claims 32 to 44 wherein said second electric current is derived from discharge of a capacitor.

46. A method according to Claim 45 wherein said capacitor is charged with a voltage less than two volts to store an energy of 60 joules.

47. A method according to any of Claims 32 to 46 for use where said pad comprises a layer of copper intermediate between said adhesive layer and said weldable layer.

48. A method according to Claim 34 or according to any of Claims 35 to 47 when dependent upon Claim 34 for use where said hole in the pad is plated through with a layer of copper lining to which a layer of solder is bonded; and where said layer of copper lining and said layer of solder together extend beyond a first end of said hole proximate to said second face of said pad through said weldable layer, said copper lining being bonded to said weldable layer for said solder to form a rim of solder on copper around the periphery of said hole to cover a first portion of said weldable layer and to leave a second portion of said weldable layer exposed and available to receive a weld; whereby solder, applied to the pad, will readily form a solder bond both within said hole and on said rim of solder on copper around said first end of said hole.

49. A method according to Claim 48 for use where the pad comprises a layer of epoxy material adjacent to said adhesive layer between the other layers of said pad and said adhesive layer and forming a lip around a second end of said hole proximate to said first surface of said pad; where said adhesive layer extends over a portion of said epoxy layer, leaving said lip of said epoxy layer exposed; where said copper lining and said layer of solder together extend beyond said second end of said hole through said epoxy layer; and where said copper lining is bonded to said lip of said epoxy layer to form a second rim of solder on copper extending over said lip in said epoxy layer; whereby solder applied, as in a flow-solder operation, will readily form a solder bond within said hole and to said second rim of solder on copper extending over said lip in said epoxy layer.

50. A method according to Claim 34 or according to any of Claims 35 to 47 when dependent upon Claim 34 for use where the pad comprises: a layer of epoxy, through which said hole extends intermediate between said weldable layer and said layer of adhesive layer adjacent to said adhesive layer; a copper lining having a layer of solder bonded thereto extending beyond a second end of said hole proximate to said first surface of said pad, where said copper lining extending beyond said layer of epoxy is bonded at said first face of said pad to said layer of epoxy to form a second solder rim extending over said epoxy layer around said second end of said hole through the epoxy; and where said adhesive layer forms a portion of said first face of said pad extending outwardly from the edge of said second solder rim.

51. A method according to any of Claims 32 to 50 for use where said weldable layer is a stainless steel layer.

52. A method according to any of Claims 32 to 51 for use where said adhesive layer comprises a thermo-setting resin.

53. A method according to Claim 52 for use where thermo-setting resin is an epoxy resin.

54. A method according to any of Claims 32 to 53 for use where said weldable layer is 100 microns in thickness.

55. A method according to Claim 47 or according to any of Claims 48 to 53 when dependent upon Claim 47 for use where said copper layer is 15 microns thick.

56. A method according to Claims 49 or 50 or according to any of Claims 51 to 55 when dependent upon Claims 49 or 50 for use where said epoxy layer is 100 microns thick.

57. A method according to any of Claims 32 to 56 wherein said wire is a nickel wire.

58. A method according to Claim 42 or according to any of Claims 43 to 57 when dependent upon Claim 42 for use where said insulation on said wire is polytetrafluoroethylene.

59. A method according to any of Claims 32 to 58 wherein said first electrode is tungsten.

60. A method according to any of Claims 32 to 59 wherein said second electrode is aluminium.

61. A method substantially as described with reference to Figs. 4,5 and 6 and 8 to 18 of the appended drawings.

62. A method according to Claim 61 for use with a pad substantially as described with reference to Figs. 2A,2B,3 and 7 of the appended drawings.