This invention relates to solder and in particular to so-called "soft solder", that is to say, solder nor¬ mally containing tin and/or lead and having a relati¬ vely low melting point, e.g. less than 430°C, as distinct from hard solder used for brazing.

Solder has been used for many, years for the pro¬ tection 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 speci¬ fications 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.

The choice of solder that is used to form the con¬ nection will depend, among other things, on the objects to be connected and the temperature to which they can be subjected. For example, electrical wiring having certain forms of electrical insulation e.g. polyvinyl chloride insulation or foamed insulation require a relatively low melting point solder. However the choice of solder compositions is generally very limited, and the choice of low melting point solders is extremely limited, only one solder, Sn5i _# 2Pb3o.6 ^Cα *18.2 having a eutectic melting point of 145°C, normally being employed. This solder composition has the disad¬ vantage that it contains cadmium which is toxic, and it is therefore desirable to be able to employ an alter¬ native low temperature solder.

According to one aspect, the present invention provides an articl 1e that is formed from a non-eutectic solder composition and which includes a temperature i indicating component that provides a relatively well- defined indication that the non-eutectic solder com¬ position has been heated to a temperature above the liquidus point thereof. i

According to another aspect the invention provides a device for forming a solder connection which includes a solder article in accordance with the invention.

The present ήinvention is based on the principle that any of a number of non-eutectic metal alloys may

be used to form a reliable solder connection so long as means are provided for determining when sufficient heat has been applied to the solder to melt it completely. When a non-eutectic alloy is heated it enters a pasty phase as its temperature passes through the solidus point of the alloy, the pasty phase comprising two dif¬ ferent alloys, one solid and one liquid. As the tem¬ perature rises the paste becomes softer until the liquidus point of the composition is reached at which point the solder is fully molten. Thus, with a non- eutectic alloy, there is no clear physical change that occurs at a single well defined temperature that can be used to indicate that sufficient heat has been applied to form a reliable solder connection. By "well-defined" is meant that the temperature indicator undergoes a clearly visible transformation within about 3, and especially within 1°C and normally within about 5 seconds and especially within 1 second at that tem¬ perature. In its broadest aspect a number of different means may be used to indicate that the correct tem¬ perature has been reached. Thus, for example, the solder article may be provided with a quantity of ther¬ mochromic material as described for example in British patent specification No. 2,109 ,418A. Preferably, however, the temperature indicating component comprises a fusible material, and especially it comprises a eutectic solder composition. Thus, in this preferred aspect the solder of the connection devicei may contain no additional components other than solder flux.

The device for forming the solder connection may have any of a number of constructions . IjjFor example it may be in the form of a dimensionally stable array of

windows through which the solder is intended to flow, for example as described in our British patent specifi¬ cation No. 2,113,134. However, it is preferred for the connector to be dimensionally heat-recoverable as described above. A heat-recoverable connector is nor¬ mally formed by partially recovering a heat-recoverable sleeve onto a solder insert e.g. in the form of a ring so that an annular protrusion extends around the cir¬ cumference of the sleeve caused by the solder. In use, the sleeve is positioned over the appropriate conduc¬ tors and heated. Initially the sleeve shrinks radially about the enclosed conductors, the central annular protrusion remaining until the solder ring melts and flows, whereupon the protrusion quickly collapses, thereby indicating that the solder has melted and that enough heat has been supplied to the article. ^' If the solder is replaced by a non-eutectic solder ring the collapsing of the protrusion may occur suddenly as is the case with a eutectic solder or may not do so, but in either case the collapsing of the protrusion does not indicate that all the solder has melted but rather that the solder paste formed by heating the solder has insufficient strength to resist the recovery forces of the sleeve.

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 con¬ siderable 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 blank 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 pre¬ sent. 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 has 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 infrared 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 number 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 or 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.

Preferably composite solder has a larger quantity of the non-eutectic solder than of the eutectic solder, the proportion of non-eutectic solder to eutectic solder being .at least 2:1, more preferably at least 3:1 and especially at least 5:1 so that the composition of the final solder joint is determined mainly by the com¬ position of the non-eutectic solder. The solder and device according to the invention are particularly use¬ ful for forming low temperature solder connections, in which case the non-eutectic solder preferably has a solidus temperature in the range of from 90 to 180°C, more preferably from 120 to 160°C, and especially from 130°C to 150°C although it is quite possible for solders having melting points outside these ranges to be used. The non-eutectic solder preferably has a liquidus point at least 5°C above, more preferably at least 10°C above and especially at least 15°C above its solidus temperature but usually not more than 60°C and especially not more than 40°C above the solidus tem¬ perature. The non-eutectic solder is not normally cho¬ sen, however, for its liquidus point but rather for is solidus point and its chemical composition. For example it may be advantageous for the solder to con¬ tain substantially no cadmium for toxicity reasons. In other instances, for example with higher temperature solder where bismuth-containing solders have often been used (m.p. about 310°C), it is possible to employ solder according to the invention as an alternative to the conventional solder compositions such as bismuth- containing solders or Sngg _β 5 g3.5. The melting point of the eutectic solder is preferably at least as high as the liquidus point of the non-eutectic solder in order to provide an adequate indication that all non-

eutectic solder has fused, more preferably at least 10°C above the liquidus point, and especially from 15 to 25°C above the liquidus point.

The preferred non-eutectic solders normally comprise alloys of tin and/or lead with one or more of the following elements: bismuth, germanium, antimony and tellurium, indium, silver, gallium and especially bismuth. Examples of such solder alloys include Sn4gPb42Biιo. The high temperature, eutectic solder may be formed from conventional solder compositions e.g. Sng3 P***37 (m.p. 183°C) or Sngg _# 5 g3 _# 5 m.p. 220°C).

Several forms of solder insert and connection device according to the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is an axial section through a solder connection device according to the invention;

Figure 2 is a perspective view of the composite solder insert of figure 1;

Figure 3 is a perspective view of another form of composite solder insert;

Figure 4 is a perspective view of part of a further solder insert during manufacture;

Figure 5 is a schematic view of another form of composite solder strip according to the invention during manufacture;

Figures 6 and 7 are schema/tic views showing the manufacture of yet another form of composite solder strip according to the invention;

Figure 8 is a cross-sectional view through yet another form of composite solder strip; and

Figure 9 is an isometric view of a further form of composite solder strip.

Referring to the accompanying drawings, figures 1 and 2 show a solder connection device according to the present invention, which comprises a hollow, open-ended transparent heat-shrinkable sleeve 1, preferably formed from crosslinked polyvinylidine fluoride crosslinked low density polyethylene or polytetrafluoroethylene. Inside the sleeve 1 is a composite solder insert 2 and a pair of uncrosslinked polyethylene sealing rings 3 for forming an environmental seal to the bodies, e.g. electrical wires or coaxial cables to be enclosed.

The composite solder insert comprises a main por¬ tion 4 formed from a low melting point solder alloy, e.g. a Sn4g b42 Biio non-eutectic alloy which has a solidus point at about 140°C and a liquidus point at about 160°C. The composite solder insert has an edge portion 5 that is formed from a eutectic solder material, e.g. a Sn 3 PB37 solder alloy which melts at 220°C. The two portions have a lap-joint region 6 in which each portion has an overlapping flange of half the wall thickness . This form of composite solder insert may be formed by manufacturing the separate com¬ ponents and then push-fitting them together or by

stamping if annular composite inserts are required Alternatively strips of the different solder material may be cold rolled together and then chopped and wrapped around a mandrel to form a wrapped insert.

Other solder compositions that may be employed include Sn43Pb43Bi]_4 (solidus 143°C - liquidus 163°C), Sn ^* L5Pb33Bi52, or Sn]_ Pb3 Bi38 (solidus 140°C - liquidus 162°C).

Figure 3 shows a similar form of solder insert as that shown in figure 2 in which a short tube or collar 4 of the non-eutectic solder has been joined to a ring of the eutectic solder by a stamping process at the same time as the ring 5 is stamped out of the solder blank.

Figure 4 shows a composite strip of solder during manufacture of a composite solder insert. In one form of device a central strip 6 of eutectic high melting point Sn 3Pb37 solder is cold rolled with a larger strip 7 of low melting point non eutectic Sn48Pb42Bi ^* LQ solder so that the strip 7 is located on both sides of strip 6. This strip may then be chopped into discrete lengths and wrapped with the high melting point solder strip 6 facing outwards. Alternatively it is possible for the composite insert to be formed with the central strip 6 formed from the lower melting point solder and facing outwards so that the portions of the larger strip 7 of higher melting point solder lying on each side of the central strip 6 act as barriers to prevent fused low melting point solder squirting out of the article when the article is heated. However, in this

- re ¬

arrangement, as in others, it is preferred for the non- eutectic solder to constitute the major proportion of the solder.

Figure 5 shows schematically the steps in the manufacture of a solder device that employs an insert from a wrapped composite strip. As shown in figure 5 the strip comprises a major strip 7 of non-eutectic solder described above and a minor strip 6 of high melting point eutectic solder which are held together by means of a small quantity 8 of a water-soluble and/or cyanoacrylate adhesive.

After the composite strip has been wrapped around a mandrel and cut to form an insert for a solder con¬ nection device it can be briefly heated, for example by means of an infrared heater to remove the layer 8 of adhesive before it is inserted into a polyvinylidine fluoride sleeve to form the device.

Figures 6 and 7 show schematically an alternative method of attaching the strip 6 of high melting point solder to the strip 7 of low melting point adhesive in which a high current is passed through the composite strip periodically by means of electrodes 10 in order to form welds 9 between the high and low melting point solders.

Figure 8 is a cross-section through a composite strip comprising low melting point solder 7, high melting point solder 6 and flux 11 showing yet another method of bonding the two strips together in which a hole 12 is formed by means of a laser and the areas 13

adjacent to the hole are welded together. A laser such as a pulsed neodymium YAG infrared laser may be used to form an array of holes of about 0.2 to 0.3mm diameter in the composite strip. In the hole 12 a very thin solder wall 14 is formed that will prevent premature escape of solder flux, but will allow early escape of flux via the hole 12 when the device is heated in use.

Figure 9 is a schematic isometric view of yet another form of composite solder strip in which a peripheral jacket of low melting point solder 7. has been coextruded about a thin central strip 6 of high melting point solder.