GB2057781A

GB2057781A - Electrical connector assemblies

Info

Publication number: GB2057781A
Application number: GB7929041A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1979-08-21
Filing date: 1979-08-21
Publication date: 1981-04-01
Also published as: GB2057781B

Abstract

A method of connecting a connector body member (19) to a metallic sheath (14) of a semi-rigid electric cable used, for example, in a microwave limiter amplifier, comprises locking member (19) to sheath (14) by welding, Fig. 6, or by interlocking cooperating parts (23,24), Fig. 7. In Fig. 6 the end of the sheath (14) is shaped to be a slide fit in the connector body member (19) and a pulsed laser beam (20) is used to weld the sheath (14) and member (19) together, the member and sheath being rotated to form a series of overlapping welds. The sheath (14) may have nickel tape applied to its end as well as or instead of being shaped to be a slide fit prior to welding. <IMAGE>

Description

SPECIFICATION Electrical connector assemblies This invention relates to semi-rigid cables, and in particular to the assembly of connectors to semi-rigid electric cables.

Semi-rigid cables are used as interconnections in, for example, microwave limiter amplifiers. One form of semi-rigid cable comprises a copper central conductor insulated with solid polytetrafluorethylene (PTFE) dielectric and with a copper outer conductor or sheath surrounding the P.T.F.E. dielectric. RF connectors have to be fitted to each end of a short length of semi-rigid cable (typically six cm) which may have to have a U-shaped or hooked-shaped (for example) configuration in order to connect the required terminals. The cable diameter is commonly about 0.2 cm or 0.36 cm.

A conventional joint between an RF connector and a semi-rigid cable is shown in the section of Fig. 1. A semi-rigid cable 1 comprises a copper central conductor 2, P.T.F.E.

dielectric 3 and a copper outer conductor 4.

An RF connector 5 comprises a stainless steel nut 6 rotatably retained on a nickel plated stainless steel hollow stepped connector body 7 by means of a ring member 8. The central conductor 2 is soldered into a bore of a centre pin 9 of the connector 5. The outer conductor 4 and dielectric 3 extend up to the internal step of the body 7 and the outer conductor 4 is soldered to the body 7 such as to produce a fillet of solder as at 10 with solder penetrating into the joint, that is into the clearance between the internal wall of the body 7 and the adjacent external surface of the outer conductor 4. A Teflon (Registered Trade Mark) insert or plug 11 is arranged on the pin 9.

It has been found that during the soldering operations there is a tendency for the P.T.F.E.

to "creep", with the result that in use of semi-rigid cable connector assemblies, particularly under rigorous environmental conditions, solder joint fractures subsequently occur with resultant adverse effects on the RF performance of the assemblies. Consequently it was proposed to use a treatment programme of temperature cycling (- 40"C to + 1 20 .C with a cycle time of approximately 24 hrs.) of the cable and connector at certain stages during the manufacture of the cable connector assemblies, so that the effect of "creep" was minimised on subsequent temperature cycling during use of the finished assembly, at least to an extent acceptable at that time when the assemblies were used in Gain Tracking Travelling Wave Tubes. Typically the treatment programme consisted of 1.Temperature cycle 60cm lengths of semi-rigid cable as follows: (a) + 1 20 C for 1 hour (b) room temperature for 1 hour (c) - 40 C for 1 hour (d) room temperature for 1 hour 2. Repeat 1(a), (b), (c) and (d) and leave for 24 hours.

3. Cut cable to required length, fit one RF connector and bend cable to required shape.

4. Repeat 1(a), (b), (c) and (d).

5. Fit other RF connector.

With the advent of microwave limiter amplifiers the same assembly treatment programme was employed. However under rigorous environmental testing procedures (temperature cycling between - 50"C and + 95"C) fractures of the solder joint 10 between the connector shoulder and the cable itself became apparent after only a few cycles.' The cause was initially believed to be due to expansion of the P.T.F.E. dielectric 3, and so the inner end of the Teflon insert 11 was counter-bored to a depth insufficient to significantly degrade the RF performance in an attempt to relieve the stress. However this merely postponed joint fracture by a few more cycles.Several other assembly procedures were attempted, such as using different solders, drilling the neck of the connector to release gas and ensure that the solder reaches the bottom of the joint, and strengthening the solder joint by soldering a brass sleeve over the joint and the connector neck. None of these procedures however resulted in a finished assembly which could reliably withstand continuous temperature cycling (-- 55"C to + 1 20 C with an approximately 24 hour cycle) with both ends of each assembly torqued up to a mating connector, even when the upper limit was reduced to 11 0 C.

Tests were subsequently performed on failed assemblies and it was concluded that most assemblies had failed because of poor solder joints which, after weakening during the high temperature part of the cycle, were unable to restrain the longitudinal force of the dielectric expansion. Soldering faults were the result of lack of penetration into the joint, insufficient wetting of surfaces, poor adhesion of the nickel plating to the connector body and excessive joint clearances.

According to the present invention there is provided a method of assembling a connector to a semi-rigid electric cable having a metallic sheath and a central conductor insulated from the sheath by solid dielectric, including the steps of mechanically locking the sheath to a body member of the connector, such that relative movement therebetween is not permitted, by welding or interlocking co-operating parts and providing electrical connection therebetween.

The mechanical lock should be such as to lock at least the front edge of the copper outer conductor to the body of the connector so that there can be no relative movement of the cable and the connector due to dielectric expansion.

Embodiments of the invention will now be described with reference to Figs. 2 to 8 of the accompanying drawings, in which Figs. 2 to 8 show, somewhat schematically, sections through a semi-rigid cable and part of a connector body at various stages during a method of mechanically locking them together according to one embodiment of the present invention using laser welding.

A semi-rigid cable end is prepared such that the central conductor 1 2 extends a predetermined distance beyond the dielectric 1 3 and the outer conductor 14 (Fig. 2). The dielectric 1 3 is then undercut as at 1 5 in Fig. 3. The outer end (front edge) of the outer conductor 1 4 is spread by a suitable tool 1 6 (Fig. 4) and swaged (Fig. 5) by a suitable tool 1 7 so that it is a push fit in the body, rather than there being clearance therebetween as indicated in Fig. 1.A centre pin 1 8 is fitted on the central conductor 1 2 in a conventional manner by soldering, and a connector body 1 9 (only part of which is shown) slid into position on the prepared cable end (Fig. 6).

The assembly is then mounted in a rotatable jig under a laser welding head. The laser beam 20 is directed at a point on the circular junction between the cable outer conductor 14 and the connector body 19, and by rotating the jig, in steps, a succession of overlapping welds around the entire circular junction is obtained. The laser may be a gas or solid state laser welder adjusted to operate automatically at 1 pulse per second, each pulse being of 3 milliseconds duration and 1.5 joules output. This gives a weld diameter of 0.012". The rotation of the jig may be performed manually or automated using the automatic timer in the welder itself. The weld is cleaned up by removing any "splash" debris to leave a flush finish, and a P.T.F.E. bush or plug equivalent to plug 11 of Fig. 1 inserted.

The assembly is then preferably soldered at 21 (Fig. 6) as previously described with reference to Fig. 1 in order to give protection against RF leakage and to give additional strength to the connector 1 cable joint. Whilst a small amount of differential expansion might still occur at the solder fillet, it was considered that the overall strain on the joint would be much reduced. The finished assembly is then RF tested.

Temperature cycling of assemblies manufactured using the above described laser welding technique (- 55"C to + 11 0 C, as described above) has shown that this technique produces much stronger assemblies than the previous solder-only techniques, and that a laser welded assembly still possesses considerable strength even if the solder joint becomes severely cracked during temperature cycling.

In order to facilitate the laser welding it is additionally possible to arrange nickel tape in the gap between the cable and the connector, in which case the spreading and swaging of the outer conductor may be omitted or per formed to a lesser extent. Whilst only laser welding has been described other forms of welding could alternatively be used.

Other forms of direct mechanical locking between at least the free end of the outer conductor and the connector body, such that relative movement therebetween is not permit ted, are possible. For example, by providing the connector body with an internal thread of such an overall diameter and such that it will cut into the outer circumference of the copper outer conductor when applied thereto with a screwing action, in the manner of a self tapping nut, thus providing a continuous lock along the whole length of the connector bore.

In this case a finishing solder fillet would still be needed to prevent RF leakage. Alterna tively the end of the outer conductor and the bore of the connector body may be provided with interlocking screw threads or other inter locking co-operating parts, as shown in Figs.

7 and 8. Here the connector body 21 and copper conductor 22 have co-operating threads 23 and 24 and a solder fillet 25 is subsequently applied after assembly.

Claims

1. A method of assembling a connector to a semi-rigid electric cable having a metallic sheath and a central conductor insulated from the sheath by solid dielectric, including the steps of mechanically locking the sheath to a body member of the connector, such that relative movement therebetween is not permit ted, by welding or interlocking co-operating parts, and providing electrical connection therebetween.

2. A method as claimed in claim 1, further including the step of soldering the connector body member to the metallic sheath at a position spaced apart from the end of the metallic sheath.

3. A method as claimed in claim 1 or 2, wherein the connector body member is a slide fit on the metallic sheath and laser welding is used to weld the end of the metallic sheath to an adjacent portion of the connector body member.

4. A method as claimed in claim 1 or 2, wherein nickel tape is applid to the end of the metallic sheath, the connector body member being a slide fit on the nickel tape, and wherein laser welding is used to weld the end of the metallic sheath to an adjacent portion of the connector body via the nickel tape which facilitates the laser welding.

5. A method as claimed in claim 3, in cluding the steps of exposing a length of the central conductor at the end of the semi-rigid cable, shaping the end of the metallic sheath such that it is a slide fit in a bore of the connector body member which is of a larger diameter than the metallic sheath diameter, arranging the connector body member on the shaped end of the metallic sheath and laser welding the end of the metallic sheath to the adjacent of the connector body member.

6. A method as claimed in claim 1 or 2, wherein the metallic sheath is of copper and the connector body member includes a bore with a self-tapping thread and is of such a diameter that upon screwing of the connector body member onto the copper sheath the connector body member and the copper sheath become interlocked.

7. A method as claimed in claim 1 or 2, wherein at least an end porton of the external circumference of the metallic sheath and an internal bore of the connector body member have interlocking screw threads.

8. A method of assembling a connector to a semi-rigid electric cable as claimed in claim 1 and substantially as herein described with and as illustrated in Figs. 2 to 6 of the accompanying drawings.

9. A connection between a connector and a semi-rigid electric cable made by a method as claimed in any one of the preceding claims.

CLAIMS (22 January, 1980)

1. A method of assembling a connector to a semi-rigid electric cable having a metallic sheath and a central conductor insulated from the sheath by solid dielectric, including the steps of mechanically locking the sheath to a body member of the connector, such that relative movement therebetween is not permitted, by welding.