DEVICE FOR CONNECTING ELECTRICAL CONDUCTORS

Abstract: A device for electrically connecting at least one conductor (54) to at least one other conductor comprises a dimensionally heat-stable polymeric layer (48) having at least one window (56) therein and a quantity of fusible electrically conductive material (58), e.g. solder disposed with respect to the window or windows (56) to allow it to flow through the window and to form a conductive connection between conductors positioned on opposite sides of the polymeric layer. The device includes one or more hollow positioning elements, which are preferably formed as pockets (52) between the heat-stable layer (48) and a further polymeric layer (50). The positioning elements are used to retain conductors (54) in position in the device before and during formation of the connection.

Description: This invention relates to devices for connecting electrical conductors, and especially for terminating a plurality of closely spaced conductors to a conecctor or other substrate. Joining flat cable or ribbon cable to connectors can be accomplished by a number of different methods, with soldering being probably the most reliable. However, if all the solder terminations must be done by hand, the costs involved and the time required to perform a multiplicity of repetitive soldering operations far outweigh any gains in reliability. In addition, as connector pin spacing decreases, due to higher interconnection densities, the reliability of hand soldering decreases because of the possibility of solder bridging terminations and shorting out adjacent connectors. It would therefore be desirable to have a solder system in which all leads can be soldered to the connector simultaneously, rapidly and reliably for a wide range of a number of terminations and spacings. In the past, various systems have been developed for simultaneously applying a plurality of bodies of solder. One such system is that disclosed in U.S. Patent number 3,396,894, which discloses the prepackaging of metered amounts of flux and solder in a heat-recoverable preformed polymer sheet which forces the solder into place. The patent teaches the use of discrete pieces of solder which must be precisely positioned above regions to be soldered. As heat is applied, the polymer sheet returns to its original flat configuration prior to solder melting. U.S. Patent 3,719,981 discloses an alternative method of applying solder balls, which are appropriately spaced on the tacky surface of a pressuresensitive tape, to solder bumps used for connections. Both of the above methods rely on the positioning of the small pieces of solder immediately adjacent to the terminals which are to be soldered, and, because small pieces of solder are used, each of the solder systems disclosed is difficult to manufacture. U.S. Patent number 3,750,252 discloses the use of a single continuous piece of solder to simultaneously solder a large number of terminals. The solder wire extends along a terminal strip and, on heating, the solder melts and coalesces on the individual contacts to form independent connections. With the increasing use of ribbon or n flexprintn cable, especially multilayer cable, which is analogous to flexible printed circuit board, and the decreasing size of many electronic assemblies, so that bulky connectors are disadvantageous, a method of terminating flat cables to flat substrates has also become extremely desirable. According to one aspect, the present invention provides a device for electrically connecting at least one first conductor to at least one second conductor, which comprises a dimensionally heat stable polymeric layer having at least one window therein, a quantity of fusible electrically conductive material, preferably in strip form, disposed with respect to the window or windows to allow the conductive material to flow through the or each window and to form a conductive connection between conductors positioned on opposite sides of the polymeric layer, and one or more hollow positioning elements defined at least partly by the polymeric layer capable of receiving the or each first conductor and retaining the or each first conductor in position in the device. According to a preferred aspect, the invention provides a device for electrically connecting electrical conductors of a set of first conductors to electrical conductors of a set of second conductors, which comprises a dimensionally heat-stable polymeric layer having windows therein, a quantity of fusible electrically conductive material, preferably in strip form, disposed with respect to the windows to allow the conductive material to flow through the windows when heated and to form a conductive connection between conductors positioned on opposite sides of the polymeric layer, and hollow positioning elements defined at least partly by the polymeric layer, which elements are capable of receiving the first conductors and retaining them in position in the device. Preferably the device includes a further polymeric layer that is bonded to the heat-stable polymeric layer to form at least one pocket which contains the or at least one of the windows and which constitutes a hollow positioning element. This form of the device has the advantage that the further polymeric layer may be employed to insulate the connection and to strain relieve the first conductors. This is especially true if, as is preferred, the said further polymeric layer is dimensionally heat-recoverable so as to grip the or each first conductor when heated. In addition, the use of a heat-recoverable layer has the advantage that the pockets may be made sufficiently large to faciliate insertion of the conductors therein without loss of alignment. The fusible, electrically conductive material used to form the strip may be formed from any of a number of materials, e.g. fusible plastics materials that have a high loading of a metal filler in the form of particles or flake. Preferably, however the fusible, conductive material comprises solder. As used herein, solder means any metal or metallic alloy used to join metallic surfaces by melting that metal or metallic alloy and then allowing it to cool. A solder strip, as used herein, means an elongated, continuous element of solder of any cross-section including, but not limited to, round, square, flat, or any other cross-section. Such a solder strip may contain a flux core and/or may be coated on all or a part of its outer surface with a flux coating. The strip may be perforated to enable better flow of a flux core. The polymeric layers should preferably be made of materials capable of resisting elevated temperatures for the time needed to melt and flow the solder, generally about 15 seconds. Suitable materials include, for example and not by way of limitation, polyvihylidene fluoride, poly(parabanic acid), and poly(pyromelittimide) or other high-temperature polyamides or -imides. These polymers may be cross-linked by either chemicals or radiation to improve their high-temperature properties. Lower-temperature polymers, such as polyethylene, etc., may be used depending on the particular use to which the terminator is to be put, and the choice of suitable materials is considered to bve within the scope of one skilled in the art in view of this disclosure. While the provision of adhesive and sealant layers is contemplated by this invention, and will be discussed in more detail below with respect to certain preferred areas, it is also contemplated that the polymeric layers may themselves have adhesive properties, even though they are generally required to be non-melting in use. Such properties may be particularly desirable for the window layer, and may be provided, for example, by extensive cross-linking of an adhesive material. By this means, the device may be adhered to a substrate in use. While the window layer is heat-stable, the further polymeric layer which forms the pockets is, as stated above, preferably heat recoverable. Heat recoverable polymers and methods of rendering polymeric articles heat recoverable, are well known in the art. These are generally cross-linked, at least partially crystalline polymers, or mixtures incorporating them. The polymeric materials can be tailored to suit the intended use by the addition of fillers, e.g. flame-retardants, plasticizers, pigments, stabilizers, co-curing agents to facilitate cross-linking, etc. The polymeric materials used in the construction of this invention are important in the transfer of thermal energy. While the primary heat transfer occurs through the metallic elements, including the conductors, terminals, and solder, a significant amount of heat is transferred through the window in a parallel manner. Additionally, the flux trapped within the window spaces is a significant source of circulatory liquid and vapor mass heat transfer. The presence of these additional heat transfer paths contributes to the overall speed and quality of the desired solder connections. An advantage of the terminator of this invention is that it readily permits "tapping" a flat cable or similar substrate at a plurality of points. The insulation may be stripped from one side only of the cable, and a terminator according to this invention used to make connections between the thus-exposed conductors of the cable and other conductors. While the terminator would normally be placed parallel to the cable, if a stair-step configuration of the windows is used, it may be placed perpendicular to the cable to allow an extremely convenient tap. According to another aspect, the invention provides a method of electrically connecting at least one first conductor to at least one second conductor by means of a device comprising a dimensionally heatstable polymeric layer having at least one window therein and at least one strip of fusible electrically conductive material disposed with respect to the window or windows to allow the conductive material to flow through the or each window when heated, the device including one or more hollow positioning elements defined at least partly by the polymeric layer for receiving the or each first conductor and retaining the or each first conductor in position in the device, which method comprises positioning the or each first conductor on one side of the heat-stable polymeric layer so that the or each first conductor is received by a positioning element, positioning the or each second conductor on the opposite side of the heatstable polymeric layer so that at least one first conductor and at least one second conductor are in register with at least one window, and heating the device to melt the electrically conductive material so that the electrically conductive material forms a conductive connection between the first and second conductor. Several forms of device according to the invention and their use 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 first embodiment of this invention used to terminate a wire to a flat substrate. Figure 2 is a cross-section through line 2-2 and Figure 3 a cross-section through line 3-3 of Figure 1. Figure 4 is a perspective view of a second embodiment of this invention used to terminate twisted pairs. Figure 5 is a view of this second embodiment from beneath, while Figures 6 and 7 are cross-sections through lines 6-6 and 7-7, respectively, of Figure 5. Figure 8 is a perspective view of a heater platen for use with the embodiment of Figure 4. Figure 9 is a perspective view of a third embodiment designed for terminating triaxial cable. Figure 10 is a cross-section through line 10-10 of Figure 9, also showing an inserted triaxial cable. Figure 11 is similar to Figure 10 but shows an alternative method of positioning the center conductor, while Figure 12 is an underneath view of the embodiment of Figure 11. Figures 13 and 14 depict the operation of the polymer layers and the solder during termination. Figures 15 and 16 depicit a method bf connection according to the invention. Figure 17 depicts yet another method in which the backing layer has holes through which the conductors may protrude, and Figure 18 depicts that embodiment with a cable ready for soldering. Figure 19 depicts the use of windows shaped to retain the conductors of a cable, wh-ile Figure 20 is a side view of Figure 19; Figures 21 and 22 depict a device for use in another method; and Figures 23 and 24 shown another form of device Figure 1 is a cross-sectional view through a pocket of a first embodiment of this invention where the- terminator shown generally at 20 is being used to terminate a conductor to a flat substrate, such as a printed circuit board shown generally at 22 and comprising an insulating support 24 and conductive region 26. The wire 28 having first conductor 30 has been inserted into terminator 20. The terminator comprises a first polymeric layer 32 having a window 34 therein (the window layer), a solder strip 36 positioned adjacent the window 34, and a polymeric backing layer which is shown here as two regions 38, over the conductor 30, and 40, over the insulation of wire 28. If these regions were joined by the region 42 shown in phantom, a unitary backing layer would be formed. This backing layer is heat recoverable. The window layer is adhered to the substrate 22 by adhesive layer 44 and to the backing layer by adhesive layer 46. Alternatively, the window layer may be itself adhesive and/or be fusion bonded or otherwise attached to the backing layer. When the conductor is inserted so that it overlays the window 34 and heat and pressure are applied, solder 36 fuses to form an electrical connection between the first conductor 30 and conductive region or second conductor 26 through the window 36, and the backing layer recovers (shrinks) to encapsulate the wire, thereby insulating and strain-relieving the connection thus formed. Figure 2 is a cross-section through line 2-2 of Figure 1 illustrating the wire 28 in the pocket formed between layers 32 and 40, while Figure 3, which is a cross-section through line 3-3 of Figure 1, illustrates the conductor in position above the solder 36 and windows 34. Figure 4 depicts in perspective a second embodiment useful for the termination of twisted pair wires. The terminator comprises a window layer 48 and backing layer 50 bonded together appropriately, with pockets 52 formed in backing layer 50. Wires 54 are shown inserted into the pockets, which are shaped such that the ends of wires 54, cut and stripped to different lengths, enter separate sub-pockets for termination. These sub-pockets end above two rows of windows 56 as shown in Figure 5, which is a bottom view of the terminator of Figure 4. Window layer 48 has two rows of windows 56, with a solder strip 58 shown mostly in phantom, between layers 48 and 50 and adjacent each window. Figure 6 is a cross-section through line 6-6 of Figure 5, and depicts one of the pair 54, in a manner similar to Figure 1. Figure 7 is a cross-section through line 7-7 of Figure 5, and shows one row of windows 56, with the corresponding solder strip 58 and the pair of sub-pockets of pocket 52. Soldering the wires through the windows is accomplished by application of heat and pressure, which will also cause the pad to become properly heated for effective soldering. The spaces along the solder strips whose length is indicated by "x" in Figure 5 may be left unheated, and will then remain unmelted and free from contact with anything but the polymer layers surrounding them. The solder in and around the window will have been heated and propelled into the window by force from a heating tool, which is shown in Figure 8 in perspective view from underneath. In Figure 8, the shaded region is in the plane of the window layers, causing the heating and separation of the solder, while relief area 60 defines a cavity for the wires in the pockets. Relief area 62, if present, allows for the unmelted solder regions discussed above, but if that relief is absent, all the solder would be forced from between the wire cavities into the window areas. A smaller center-to-center spacing or alternative tool design could produce the same effect. Some variations can be made in the invention to accomodate the general physical characteristics of coaxial, triaxial or other multiaxial wire cables. Most have a centrally located center wire surrounded with a thick wall insulation to provide a large dimensional separation from the next coaxial wire. This next wire is also wrapped with a thin electrically conductive foil to create a shield. Sometimes this second wire is replaced entirely by a multitude of smaller, braided wires forming a net like shield. This can be followed by a third wire, braid, etc., to provide additional electrical and/or mechanical shielding. Thus there are two or more electrical connections to make per multiaxial cable. In addition the geometric dimension considerations present unusual problems when it is desired to electrically and mechanically join these cables to flat surfaces. The following variation of the invention as shown in Figures 9 through 12 will illustrate its usefulness. Figure 9 is a top perspective view of a terminator designed for coaxial wire; The window layer 48 and backing layer 50 form pockets 52. Figure 10 is a cross-section through one pocket of the embodiment of Figure 9, with a triaxial cable 64 inserted in pocket 50. Here, each of the three conductors to be terminated, outer conductor 6# (which has been wound back over insulation 68), middle conductor 70 (which has been wound back over insulation 72), and center conductor 74 (protruding from insulation 76), are shown emplaced above solder strips 58 and windows 56 formed in window layer 48. Termination is again accomplished by heat and pressure, using an appropriately-shaped heating tool. Figure 11 is a view similar to Figure 10 except that 4 windows are present per pocket. Three of the windows are used in controlling the placement of solder and the formation of inter connections between the conductors and substrates, while the fourth permits the passage of a center conductor 74. In this way, the thin center conductor 74 is brought into direct contact with the substrate when the terminator is in place, helping to ensure a good solder joint. In this figure, also, the outer conductor has been shown as a braid 78, as is common with multiaxial wire. Figure 12 is a bottom view of the embodiment of Figure 11, when the center conductor 74 can be clearly seen below the window layer 48. This system is of course applicable to coaxial cables, where one less window and solder strip is required. Strain and relief for the connection formed is provide by recovery of pocket 52 about the wire 64 while either polymeric layer may have adhesive associated therewith, as discussed elsewhere. Figures 13 and 14, which show a single pocket and solder strip for simplicity, clarify the operation of the invention. Window layer 80 and backing layer 82, with pocket 84 therein, are bonded together except over area 86 (to form the pocket) and in the vicinity of solder strip 88. This permits the use of a continuous solder strip in manufacture. During soldering, force is applied to all areas immediately around the pocket, which forces the solder into the window region 88 shown in phantom and seals the edges of layers 80 and 82 together. The device used in the method shown in figure 15 and 16 comprises a polymeric heat-stable layer 91 having an array of elongate windows 93 therein a strip of solder 97 extending accross the windows 93 and a temporary backing layer 95. As shown in Figure 15 a cable 89 with stripped conductors 90 is placed on the device. The backing layer 95 may adhere to the cable, and the system is placed initially so that the windows 93 are aligned with the conductors as they emerge from the cable insulation. As shown in Figure 16, a burnisher 92 or similar tool is stroked along the conductors in the direction of arrow A, away from the#cable insulation, to align the misaligned conductors. Downward pressure is also applied to cause the wires to move in the direction of arrow B and enter the windows 93, thus aligning them. Pressure may then be applied to the assembly to cause good adhesion of the aligned components, and alignment should thus be retained until soldering, thus preventing deformation of individual wires within the cable during handling, etc. Figure 17 depicts an another embodiment of this invention which also facilitates conductor alignment. If the backing layer 95 is placed so as to expose window 93 to the back via a small aperture 94 (or if the backing layer has suitable perforations), a hole for each wire is created. The misaligned wires may be straightened with a comb-like device 96 and the tips of the conductors 90 threaded through the apertures 94. The comb may then be removed and the cable 89 bent in the direction of arrow produce afford the arrangement of Figure 27. If desired, before soldering occurs the cable may be pulled back (once the assembly is emplaced against the termination substrate) to leave no protruding conductor ends. An additional feature of the window means of the invention is that they may keep conductors aligned during actual soldering. In particular, when soldering a multiplicity of small diameter wires to a printed wiring board, the wires may tend to fall off during application of heat and force. the windows may be further configured to minimize this tendency. Maintenance of conductor alignment is shown in Figures 19 and 20 by one of many possible window shape variations. The windows 93 in window layer 91 have localized areas-of restricted width ("grips") 98 on the precise locations desired which provide the positioning elements. The window layer portions adjacent to each end of the windows provide structural rigidity and location accuracy for every conductor. In the case shown, the grip width is an interference fit to the diameter of the conductors 90, and partly overlays the solder strip 97, shown shaded. In such a case, the grips are preferably staggered so that the potential growth incurred when the conductors are forced into place may be absorbed by simple bending of the window frames. The balance of the window remains wide enough to permit formation of a good solder fillet along the remaining conductor length, and the solder is placed to correctly guide the flow into each window. Figure 20 illustrates this in side view. Two further forms of device suitable for the method of the invention are shown in figures 21 and 22. In these forms of device a solder strip 95 is either, as shown in figure 21, placed on the heat stable layer 91 over the windows 93 or, as shown in figure -22, the solder strip is sandwiched between two layers which together form a heat stable layer and have aligned windows herein. These forms of device may for example be used by placing a flat cable on the device so that the conductors extend through the windows 93, positioning the device on the substrate e.g. a printed circuit board, and heating the device to melt the solder strip 97. Another device is shown in figures 23 and 24 in which a heat stable polymeric layer 110 has a number of windows 112 each of which is formed with a narrow portion or neck 116 for gripping a conductor, the neck constituting a positioning element. The polymeric layer is placed over the substrate so that the windows 112 are in register with conductive regions of the substrate. The underside of the layer 110 is preferably adhesive in nature so that it will adhere to the substrate. The layer 110 has a tear-off portion 119 which is then separated from the remainder of the layer along a line of weakness 120, e.g. a score-line or a line of perforations, and conductors 118 are then placed over the layer 110 so that they are gripped by the necks 116 of the windows and thereby held in alignment as shown in figure 24. The device may then be heated to fuse the solder strip 114 and form a connection between the conductors 118 and the con ductive regions of the underlying substrate. Advantageously, the upper surface of the layer 110 is also adhesive in nature to assist in holding the conductors 118 in place. Whilst the device preferably is formed with a tear-off strip 119, it is also possible to form the device initially as shown in figure 24.

Claims: CLAIMS 1. A device for electrically connecting at least one first conductor to at least one second conductor, which comprises a dimensionally heat stable polymeric layer having at least one window therein, a quantity of fusible electrically conductive material disposed with respect to the window or windows to allow the conductive material to flow through the or each window and to form a conductive connection between conductors positioned on opposite sides of the polymeric layer, and one or more hollow positioning elements defined at least partly by the polymeric layer capable of receiving the or each first conductor and retaining the or each first conductor in position in the device. 2. A device for electrically connecting electrical conductors of a set of first conductors to electrical conductors of a set of second conductors, which comprises a dimensionally heat-stable polymeric layer having windows therein, a quantity of fusible electrically conductive material disposed with respect to the windows to allow the conductive material to flow through the windows when heated and to form a conductive connection between conductors positioned on opposite sides of the polymeric layer, and hollow positioning elements defined at least partly by the polymeric layer, which elements are capable of receiving the first conductors and retaining them in position in the device. 3. A device as claimed in Claim 1 or Claim 2, wherein the electrically conductive material comprises solder. 4. A device as claimed in any one of Claims 1 to 3, wherein the or each positioning element is in the form of an aperture in the polymeric layer. 5. A device as claimed in any one of Claims 1 to 3 wherein the or each positioning element is provided by a portion of the or one of the windows in the polymeric layer which is capable of gripping a conductor. 6. A device as claimed in any one of Claims 1 to 3, which includes a further polymeric layer that is bonded to the heat-stable polymeric layer to form at least one pocket which contains the or at least one of the windows and which constitutes a hollow positioning element. 7. A device as claimed in Claim 6, wherein the or at least part of the electrically conductive material is disposed between the heat-stable layer and the further polymeric layer. 8. A device as claimed in Claim 6 or Claim 7, wherein the said further polymeric layer is dimensionally heat-recoverable so as to grip the or each first conductor when heated. 9. A device as claimed in any one of Claims 6 to 8, wherein the or each pocket contains a plurality of windows, each of the plurality of windows having a strip of electrically conductive material associated therewith to enable the device to connect multiconductor wires. 10. A device as claimed in any one of Claims 6 to 9, which has a plurality of the pockets. 11. A device as claimed in any one of Claims 1 to 10, wherein the or each window has a'generally rectangular configuration. 12. A device as claimed in Claim 5, wherein the or at least one window that provides a positioning element has a neck portion in which a conductor may be gripped without the conductor extending through the window. 13. A device as claimed in any one of Claims 1 to 12, wherein the heat-stable polymeric layer and/or the further polymeric layer, if present, has adhesive character. 14. A device as claimed in any one of Claims 1 to 13, which includes a temporary backing layer located on the heat-stable polymeric layer, which backing layer is capable of being removed after the or each first conductor has been positioned in the device. 15. A device as claimed in any one of Claims 1 to 14 wherein the fusible electrically conductive material is in the form of one or more strips. 16. A method of electrically connecting at least one first conductor to at least one second conductor by means of a device comprising a dimensionally heatstable polymeric layer having at least one window therein and a quantity of fusible electrically conductive material disposed with respect to the window or windows to allow the conductive material to flow through the or each window when heated, the device including one or more hollow positioning elements defined at least partly by the polymeric layer for receiving the or each first conductor and retaining the or each first conductor in position in the device, which method comprises positioning the or each first conductor on one side of the heat-stable polymeric layer so that the or each first conductor is received by a positioning element, positioning the or each second conductor on the opposite side of the heatstable polymeric layer so that at least one first conductor and at least one second conductor are in register with at least one window, and heating the device to melt the electrically conductive material so that the electrically conductive material forms a conductive connection between the first and second conductor. 17. A method as claimed in Claim 16, wherein the device includes a temporary backing layer located on the heat-stable polymeric layer, the method including the step of removing the backing layer after the or each first conductor has been positioned in the device and before the or-each second conductor has been positioned on the opposite side of the polymeric layer. 18. A method as claimed in Claim 16 or Claim 17, wherein the or at least one positioning element is provided by a window in the heat-stable polymeric layer, which method includes the step of burnishing the or at least one first conductor to force it into the window. 19. A method as claimed in Claim 18, wherein the or each first conductor is aligned with the device by the burnishing step.

Inventors: ALLEN, LESLIE JOHN, GRAUSSAUER, WILLE KARL, ROBINSON, WILLIAM MORRIS
Applicants: RAYCHEM CORPORATION
Application year: 1982
Main classification: H01R4
IPC codes: H01R9, H01R43, H05K3, H01R4