COMPOSITE SOLDER ARTICLE

Abstract: A composite solder article comprises: (a) a quantity of a base solder which comprises a non-eutectic alloy; and (b) a temperature control component comprising one or more metals and having a melting point that is higher than the solidus temperature of the non-eutectic alloy, the relative quantities and compositions of the two components being such that when the base solder and the temperature control component have been fused together the resulting solder alloy has a smaller solidus/liquidus temperature difference than the base solder. Preferably the temperature control component is formed from a pure metal that is present in the base solder and especially in such an amount that the overall composition is in the region of the eutectic point. The article may be used for forming solder connections to standard electrical conductors and can exhibit a reduced tendency to flow or ''wick'' along the conductor.

Description: COMPOSITE SOLDER ARTICLE This invention relates to solder and in particular to so-called "soft solder", that is to say, solder normally containing tin and/or lead and having a relatively low melting point., e.g. less than 430°C, as distinct from hard solder used for brazing. Solder has been used' for many y ar.3 for the protection of strong, high quality electrical and mech¬ anical connections. Recently electrical connector devices have been made in which a quantity of solder is located within a dimensionally heat-recoverable article, usually in the form of a sleeve, optionally together with one or more fusible plastics inserts for sealing the connection from ingress of water. The article may be placed over the objects to be connected and may then be heated to cause the solder to fuse and form a connection between the objects and to cause the article to recover about the objects and provide electrical insulation for the connection. These articles are described for example in US patent specifications Nos. 3,243,211, 4,282,396 and 4,283,596 and in British patent specification No. 1,470,049, the disclosures of which are incorporated herein by reference, and are sold by Raychem Corporation, Menlo Park, California under the trade name "Solder Sleeve" and others. In international patent application Mo. (FP038) it has been proposed to employ a composite solder insert in such devices, the composite solder insert comprising a relatively low melting point solder and a relatively high melting point solder, so that, during installation of the device, the device will be heated until the relatively high melting point solder has melted, thereby ensuring that the relatively low melting point solder has received sufficient heat to cause it to melt and flow. Although- such articles function well in most instances, it has been observed that in certain cases whare the device is employed with highly solder receptive substrates such as with silver coated wires, so-called "wicking" may occur. icking is a phenomenon in which a stranded . or braided conductor to be connected can draw molten solder away from the joint area by capillary action in the manner of a wick, and so cause the joint area to be depleted of solder. This is often associated with undue stiffness in the portion of stranded or braided conductor adjacent to the joint due to the presence of the solder. Depletion of the solder in the joint area can cause the joint to have an unacceptably high resistance, or can cause mechanical failure of the joint, especially where the connected conductors have become stiff. The tendency of conductors to cause wicking generally increases with temperature, and it appears that continued heating of the device to cause the high temperature solder to melt can exacerbate the problem. According to the present invention, there is provided a composite solder article which comprises: (a) a quantity of a oase solder which comprises a non-eutectic alloy of two or more metals; and (b) a temperature control component comprising one or more metals, and having a melting point that is higher than the solidus temperature of the non- eutectic alloy, the temperature control component navir.g sucn a composition, and the relative quantities of the temperature control element and the base solder being such, that when the 'ruse solder and the temperature control component have been fused together the resulting solder alloy has a smaller difference between its liquidus and solidus temperatures than has the base solder. In use, when a solder joint is formed, the composite solder is heated, and, instead cf melting, the Dase solder becomes pasty as the temperature rises above the solidus temperature of the r.on-eutectic composition, the pasty nature of the base solder preventing or retarding the tendency of the solder to be drawn or to "wick" along any of the conductors. As heating of the solder is continued, it has been observed that the temperature control component can dissolve in the fused or pasty base solder at a temperature significantly below its melting point, thereby reducing the probability that the joint will be heated to such an extent that substantial wicking _ a _ occurs. Even though the temperature control component may never reach its melting point before it disappears, the disappearance of it may nevertheless be used as an indication that the base solder has melted, fully because the liquidus temperature of the non-eutectic alloy falls as more of the temperature control component dissolves in it. Preferably the melting point of the temperature control component is preferably higher than the liquidus temperature of the base solder, e.g. at least 5°C higher and especially at least 10°C higher than the liquidus temperature, but preferably not more than 35°C higher, and especially not more than 25°C higher than the liquidus -point. In many instances it may be desirable for the temperature control component to exhibit a substantially well defined melting point, in which case a substantially pure elemental metal may be chosen or a eutectic alloy. Preferably the temperature control component comprises a metal that is also present in the base solder. For example the base solder may have compositions that lie. on opposite sides of the eutectic point in the phase diagram of the base solder components. One example of such a system is a composite solder having a base solder comprising a non eutectic tin lead alloy having less than the eutectic quantity (63* by weight) of tin, and a temperature control component having a relatively high tin content, e.g. substantially pure tin or a eutectic 96.5% tin 3.51 silver alloy. It may well be desirable for the compositions and the relative proportions of the base solder and the temperature control component to be such that the overall composite solder article has a composition falling substantially at the eutectic point. In this way it can be ensured that all the solder has melted at the lowest possible temperature. Normally the quantity of base solder will be greater than that of the temperature control component, usually significantly greater, e.g. in a proportion of at least 2:1, preferably 3:1 and especially at least 5:1 (all percentages, ratios and the like expressed herein being measured by weight), in which case the temperature control ring will usually be formed from a pure metal that is used in the non-eutectic alloy, while the base solder will have a composition that is relatively close to the eutectic composition, e.g. not more than 30$, more preferably not more than 10$ and especially not more than 10$ removed from the eutectic point (e.g. in the case of a two component system). In the case of a tin/lead solder composition, which has a eutectic point at 63$ tin, 37$ lead (m.p. 183°C) the composite solder preferably comprises a strip of lase solder comprising not more than 62$ tin, more preferably not more than 61$ tin, but preferably at least 40$ . and especially at least 50$ tin, the remainder being lead, and a thin strip of pure tin bonded to the base solder. The base solder may be formed from a number of metals in addition to, or instead of, tin and lead, e.g. bismuth, indium or cadmium. Optionally quantities of other metals such as silver may be present in addition. The two portions of the composite solder may be joined together by any of a number of means. For example they may be bonded together for example by means of an adhesive or by means of the solder flux, or they may be soldered together, either by means of a third solder material or by means of the same solder as is used to form either of the two portions of the device. Alternatively the solder portions may be bonded together by cold working. This may be achieved for example in the case of an annular insert by a stamping operation in which one portion, e.g. in the form of a collar, is bonded to the second portion in the form of an annular flange as the second portion is stamped out of a blank sheet. Another method of cold working the two portions together that may be used to form the composite solder insert is a cold rolling method in which two or more strips of solder material, including at least one strip each of the high and low melting point solder are juxtaposed or overlapped, or one is placed upon the other, and then rolled to bond the strips together. The composite strip so formed can then be cut to length and the cut lengths wrapped around an appropriately shaped mandrel to form a wrapped solder insert or may be wrapped around the mandrel and- then cut to length. Yet another method of forming the composite solder includes forming separate appropriately shaped inserts and then press fitting inserts formed from different solder material together. Whichever form of composite solder is used, it is preferred that the individual, discrete, portions formed from a least one of the solder materials are formed only at the same time as the composite solder insert is formed. That is to say, the composite insert is preferably formed by an operation that does not involve the prior formation of discrete solder portions of both solder materials. Such operations have the advantage that not more than one type of individual solder insert is employed at any one time, with considerable simplification of the handling of the solder. Examples of such operations include cold rolling of solder strips followed by chopping and wrapping the strips, and stamping operations in which one portion is stamped out of a Dlank sheet at the same time as it is bonded to the other portion. The insert is preferably formed from a strip of solder that is formed into an appropriate shape for the solder device, for example by wrapping it around a mandrel. When forming this type of insert it is often necessary for the two strips of solder that make up the composite strip to be attached together by some means at least until the discrete composite insert has been formed. Often, for example where one solder strip is positioned in a groove in the other solder strip the two strips will be retained together by their shape when the strip has been wrapped round the mandrel to form the discrete device. The insert is preferably formed in such a way that no material is present in the insert other than solder or flux, or that, at most, only an insignificant quantity of such material is present. This form of solder insert may be formed in a number of ways: the two different solder strips may be assembled together with no adhesive to form a composite solder strip, and then the strips may be stuck together by means of a removable adhesive applied to the' surface only of the composite strip. After the strip nas been wrapped around the mandrel to form the composite insert, so that the adhesive is no longer necessary, the adhesive may be removed, e.g. by heat or i frared radiation or by application of an appropriate solvent, and the manufacturing process may be continued in the conventional manner. Alternatively the two strips of solder may be held together by means of an adhesive coated paper layer that can be peeled away from the composite strip just prior to wrapping the strip about the mandrel. • In another form of device the two strips of solder may be welded together for example by an electrical welding method ' in which a pair of electrodes is pressed against opposite sides of the composite solder strip and a current is passed through the composite strip that welds the strips together by joule heating. In yet another form of device the two strips are joined together by a train of laser welds in which a laser is fired at a spot on the strip and forms a hole through the composite strip and, at the same time, bonds the two strips together. This form of joining the strips together has the advantage- that it is very rapid, and that a numb'er .of small holes are formed in the insert through which solder can flow when the device is heated. A further form of device can incoporate an insert that has been formed from a co-extruded strip of solder. Another method in which the composite insert can be formed without the prior formation of discrete solder portions is one in which the insert is formed from compacted, preferably sintered solder powders. Thus, for example, the low cr high melting point solder powder may be placed in a mould and sintered under pressure, and then the other solder powder may be added to the same mould and itself sintered under pressure and at a temperature of up to 150°C to sinter both powders together. The composite solder may be employed in a heat- shrinkable article as mentioned above, in which case it is preferable, although not essential, for the base solder and the temperature contol component to be bonded together. Thus, according to another aspect, the invention provides a device for forming a solder connection between a plurality of elongate objects, which comprises a dimensionally heat-recoverable sleeve which contains: (a) a quantity of a base solder which comprises a non-eutectic alloy of two or more metals; and (b) a temperature control component comprising one or more metals, and having a melting point that is higher than the solidus temperature of the non- eutectic alloy, the temperature control component having such a composition, and the relative quantities of the temperature control element and the base solder being such, that when the base solder and the temperature control component have been fused together the resulting solder alloy has a smaller difference between its liquidus and solidus temperatures than has the base solder. The neat-recoverable sleeve may be formed from any polymeric material that can be rendered dimensionally heat-recoverable by crosslinking and expansion as described in the patent specifications described above. Preferred materials include ethylene homo- and copolymers such as low, medium or high density polyethylene, copolymers of ethylene with C3 to C5 alpha olefins and ethylene/vinyl acetate copolymers. Preferred materials include fluorocarbon polymers, for example polyvinylidine fluoride. An example of an article and device in. accordance with the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic view of part of a composite solder article in exploded form; Figure 2 is a • schematic view of the assembled solder article of figure 1; and Figure 3 is a side sectional elevation of a heat- shrinkable connector article in accordance with the invention. Referring to the accompanying drawings, figures 1 and 2 show schematically the steps in the manufacture of a solder device that employs an insert from a - 1 1 wrapped composite strip 1. As shown, the strip 1 comprises a major strip 2 of non-eutectic solder and a smaller strip 3 of pure tin which are held together by means of a small quantity ■ ' ■ of a water soluble or cyanoacrylate adhesive. Alternatively and preferably the two strips 2 and 3 may be bonded together by means of rosin flux that is employed in the solder. The solder strip is formed from a binary alloy comprising 60$ tin, 40$ lead, which has a solidus temperature equal to the Sng^ Pbqγ eutectic temperature of 183°C and a liquidus temperature of about 188°C. The pure tin strip has a melting point of 231°C. After the composite strip has been formed, it is wrapped around a mandrel and cut (not necessarily in that order) in order to form an annular insert for a solder connection device. Such a connection device is shown in figure 3« The device comprises a heat- shrinkable polyvinylidine fluoride sleeve 6 that has been partially recovered about the composite solder ring 1 and about two uncrosslinked sealing rings 7 and 8 which may be formed from a conventional hot-melt adhesive e.g. polyethylene, an ethylene vinyl acetate copolymer, a polyamide or a fluoropolymer such as polyvinylidine fluoride. In use, a pair of objects to be connected, e.g. stranded electrical wires or a wire and a coaxial cable having an exposed braid, may be inserted in the device and the device is simply heated to cause the sleeve 6 to recover about the objects, the composite solder strip to melt and flow about the objects and, the sealing rings to fuse and provide a seal against moisture ingress. As the device is heated the strip 2 of base solder will rise in temperature above its solidus temperature and will become pasty, the pasty nature restricting the degree to which the solder will flow along to objects. As the temperature rises the strip 2 rises above its liquidus temperature and begins to dissolve the tin temperature control strip 3 so that at about 215 to 220°C the tin strip 3 has dissolved and the sleeve has completely recovered. If desired- an alloy of 50$ tin 50$ lead r.ay be used for the base solder strip 2 together with a slightly larger temperature control strip 3, in which case the strip ?. will remain pasty until the liquidus temperature of. about 212°C has been reached, thereby reducing further the propensity of the fused solder composition to flow along the objects.

Claims: CLAIMS : 1. A composite solder article which comprises: (a) a quantity of a base solder which comprises a non-eutectic alloy of two or more metals; and (b) a temperature control component comprising one ;r more metals, and having a melting point that is higher than the solidus temperature of the non- eutectic alloy, the temperature control component having such a composition, and the relative quantities of the temperature control element and the base solder being such, that when the base solder and the temperature control component have been fused together the resulting solder alloy has a smaller difference between its liquidus and solidus temperatures than has the base solder. 2. An article as claimed in claim 1 , wherein the melting point of the temperature control component is higher than the liquidus temperature of the base solder. 3« An article as claimed in claim 1 or 'claim 2, wherein the melting point of the temperature control component is at least 10°C higher than the liquidus temperature of the base solder. 4. An article as claimed in any one of claims 1 to 3, wherein the quantity of base solder is greater than that of the temperature control component. 5. An article as claimed in --- , wherein the proportion of base solder to temperature control component is at least 5:1. 6. An article as claimed in any one of claims 1 to 5, wherein the temperature control component comprises a substantially pure metal or a eutectic alloy. 7. An article as claimed in any one of claims 1 to 6, wherein the temperature control component comprises a metal that is present in the base solder. 8. An article as claimed in claim 7, wherein the base solder and the temperature control component have compositions that lie on opposite sides of the eutectic point in the phase diagram of the base solder components. 9. .An article as claimed in any one of claims 1 to 8, wherein the base solder * comprises any one or more of the following:- tin, lead, indium or bismuth. 10. An article as claimed in claim 9, wherein the base solder comprises an alloy of tin and lead. 11. An article as claimed in claim 10, wherein the base solder has a composition in the range of from 62$ tin 38$ lead to 40$ tin 60$ lead. 12. An article as claimed in claim 11, wnerein the base solder has a composition in the range of from 61$ tin 39$ lead to 50$ tin 50$ lead. 13. An article as claimed in any one of claims 1 to 12, wherein the temperature control component comprises substantially pure tin or a 96.5$ tin 3.5$ silver eutectic alloy. 14. An article as claimed in any one of claims 1 to 13, which is in the form of a strip. 15. A device for forming an electrical or mechanical connection, whic.. • jomprises a heat-shrinkable polymeric sleeve and a composite solder article as claimed in any one of claims 1 to 14 retained within the sleeve. * * * * * *

Inventors: DELLALE JACQUES, DELLALE, JACQUES
Applicants: RAYCHEM LTD, RAYCHEM PONTOISE SA, RAYCHEM LIMITED, RAYCHEM PONTOISE S.A.
Application year: 1990
Main classification: B23K35
IPC codes: B23K35, H05K3