Termination of a small coaxial cable

A small ferrule having inwardly projecting lances is clenched about a small coaxial cable. The tips of the lances pierce the outer plastic covering of the cable and mechanically engage the underlying metal shield layer. Each lance includes an opening adjacent thereto through which a laser beam is directed to disintegrate a portion of the plastic layer adjacent the lance. Solder paste is then deposited into the opening and heated to cause the solder to reflow and form a low-resistance contact between each lance and an adjacent portion of the shield.

The present invention is related to an electrical termination of an outer 
shield of a fine coaxial wire. 
BACKGROUND OF THE INVENTION 
In the medical industry, the use of very sophisticated electronic equipment 
is becoming commonplace. The instrumentation of this equipment, especially 
in the ultrasound area, is requiring large numbers of terminations of very 
small diameter interconnecting wires. For example, 1,000 conductor cables 
may have to be terminated to a crystal in an area measuring only 
3/8.times.3/4 inches. In such a case up to 400 positions may have to be 
arranged within an area of about 0.062 square inches. An additional 
complexity, resulting from the need to minimize leakage of the low level 
signals carried by these conductors, the conductors are usually of the 
coaxial type. These coaxial cables may be as small as 0.008 inch in 
diameter. In practice, the transducer and equipment are interconnected by 
means of these cables. 
In the process of diagnostic examination of a patient, a particular cable 
assembly is selected from a rack containing a number of cable assemblies 
each having a unique type of transducer attached to one end. The other 
end, which is terminated to a connector, is then plugged into the 
diagnostic equipment. During the normal course of an examination, as many 
as 20 different transducer-cables may be alternately used. 
At the present state of the art in the industry, these fine coaxial cables 
are being terminated manually. The procedure is done under a microscope by 
a skilled technician who must strip the outer plastic cover from the 
cable, wipe the braid or helical shield back, and tin the shield and core 
wire, all being done prior to terminating the coaxial conductor to the 
transducer or connector. It takes about 12 hours of a technician's time to 
terminate a 128 conductor shielded cable in this manner. It is the purpose 
of the present invention to eliminate most of the manual labor involved in 
this work and to provide a method and apparatus for automation of these 
time consuming manual steps. 
SUMMARY OF THE INVENTION 
The present invention is a method and apparatus for terminating a fine 
coaxial cable to respective terminals in a transducer or connector. The 
termination of the shield of the coaxial cable includes a ferrule having a 
plurality of lances projecting inwardly from an inner surface of a wall of 
the ferrule. Each lance has a tip in mechanical contact with the shield. 
There is a first plurality of openings through the wall that are disposed 
so that at least one opening is adjacent each tip. An electrically 
conductive material is disposed in the openings in low-resistance contact 
with both the tips and respective portions of the shield adjacent the tip.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT 
There is shown in FIG. 1 a coaxial cable 10 having an inner conductor 12, a 
dielectric layer 14, a shield 16, and an outer protective covering 18 that 
is a dielectric as well as abrasion resistant. The center conductor 12 may 
be a single strand as small as 0.002 inch in diameter or it may be 
multi-strand, as for example, 7 strands each being as small as 0.0005 
inch. The layer 14 is usually a material having a low dielectric constant 
such as, for example, Teflon or foamed Teflon. The shield 16 may be 
constructed of thin metal braid or shallow helical wound thin metal wires 
having a diameter of about 0.0005 inch. The outer protective covering 18 
has a thickness of about 0.001 inch. The tedious and time consuming task 
of terminating such fine cables will be appreciated by those skilled in 
the art. The procedures for doing this include stripping a portion of the 
outer covering 18 from the end exposing the shield 16, as best seen in 
FIG. 2. A portion of the shield 18 is stripped away to expose the end of 
the layer 14, as shown in FIG. 2, and that end is also stripped away to 
expose the end of the central conductor 12 as shown in FIG. 3. A portion 
20 of the shield is then carefully folded back over the outer covering 18, 
as shown in FIG. 3, and tinned. The end of the conductor 12 is usually 
tinned at this time as well. Heretofore, all of these operations were done 
manually with tweezers under a microscope. 
The present invention provides a terminal structure for the shield 16, as 
illustrated in FIG. 4, which permits automation of the process of 
terminating the cable 10. A ferrule 30, made of a conductive metal and 
having a plurality of lances 32 formed therein, is clenched about the 
cable 10 so that tips 33 of the lances pierce the covering 18 and engage 
the underlying shield 16. As is shown in FIG. 5, there is a small opening 
34 adjacent each lance 32 exposing a portion 36 of the covering 18 
adjacent the lance 32. This portion 36 is removed by any suitable means, 
including mechanically cutting using a tool, chemical etching, sputtering, 
or vaporizing. In the present example, a laser 40 is used to vaporize the 
exposed plastic portion 36. A beam 42 of light is directed from the laser 
40 into the opening 34 and focused on the portion 36. The intensity of the 
laser beam is sufficient to vaporize the plastic of the covering 18 that 
is exposed in the opening 34, but not sufficiently intense to damage the 
fine wires of the shield 16. By way of example, a CO.sub.2 gas discharge 
laser would be suitable for this purpose. The reason for removing the 
plastic is to expose the shield adjacent the tip 33 so that a good 
electrical connection can be made between the lance and the shield. This 
connection is accomplished by reflowing solder, or similar conductive 
material into the opening 34 to form a low-resistance electrical 
connection, as shown in FIG. 6. 
The ferrules 30 are made from copper or copper alloy sheet 50 having, in 
the present example, a thickness of about 0.002 inch and a width of about 
0.125 inch. The sheet 50 may be maintained on a large supply roll and fed 
into a set of roll dies, not shown, for continuously forming the lances 
32, three abreast as shown in FIG. 7. As can be seen in FIG. 8 the tips 33 
are very sharp so that they can penetrate the plastic covering 18 during 
the clenching operation. The tips 33 project from the surface 52 
approximately 0.001 to 0.0015 inch. Dies suitable for forming the lances 
32 are well known in the industry and therefore will not be further 
described here. At this point the formed sheet 50 may be wound onto a 
storage reel for future use, or it may be fed into a machine where a 
measured length is severed from the sheet and rolled into a ferrule 30 and 
clenched onto the cable 10. An alternative to the lances 32 of the portion 
54, shown in FIG. 8A, is that the cut ends 56 have turned down edges 44 
having sharp tips 33. The tips 33 are very sharp so that they will 
penetrate the protective covering 18 and engage the shield 16 during the 
clenching operation. 
FIG. 9 schematically shows this clenching operation where the cut portion 
54 of the sheet 50 is formed into a cylindrical shape about the cable 10. 
As the cut ends 56 are brought together, the tips 33 are forced into and 
through the outer covering 18 and into mechanical engagement with the 
metal shield 16. This is accomplished by any suitable clenching die in a 
manner that is well known in the industry. The cable 10 and clenched 
ferrule 30 are then exposed to the laser 40 in the manner set forth above 
and depicted in FIG. 5 to vaporize the plastic portions 36. Solder 58, or 
another suitable reflowable conductive material is then deposited in the 
openings 34 which may be accomplished, for example, by injecting solder 
paste into the openings under pressure by means of an extrusion head 60, 
as shown in FIG. 10. The cable and attached ferrule 30 are then positioned 
adjacent a heater 62 that directs sufficient heat to the ferrule to reflow 
the solder 58 within the openings 34 so that the solder joins the lances 
32 with respective portions of the shield 16 in low resistance electrical 
connections. The heater 62 may be an electrical resistance heater, an 
electron beam gun, an RF generator, or any other suitable device that 
provides sufficient heat to the ferrule, solder, and shield. In the case 
of the portion 54 having the turned down edges 44, the cut ends 56, after 
clenching, form an opening therebetween which exposes a portion of the 
plastic protective covering 18 adjacent the edges 44. This opening is 
analogous to the openings 34. The plastic exposed between the ends 56 is 
removed as described above with respect to the openings 34, and solder 
deposited therein and reflowed to form a low resistance electrical contact 
between the shield 16 and the edges 44. 
At this point the end of the cable 10 may be striped as shown in FIG. 4 and 
terminated in the usual manner to a connector or transducer. If desired, 
the cable 10 with the ferrules in place may be wound onto a reel 64, as 
shown in FIG. 12, for subsequent processing. 
As will be appreciated by those skilled in the art, the present apparatus 
and method eliminates the tedious manual operations that must be performed 
under a microscope. This greatly reduces manufacturing costs and increases 
yield of the final product. Additionally, utilizing the teachings of the 
present invention, the entire process of cutting a fine coaxial cable to 
length and terminating both the shield and signal conductor of each end to 
a desired connector or transducer can be automated for further cost 
savings. A further advantage is that the ferrule 30 provides a clean, trim 
termination of the fine wire braid or helical shield 18 without loose ends 
of the fine wire projecting outwardly as with the prior art termination 
shown in FIG. 3.