Elongated printed circuit flexible cables and method of making the same

Elongated, flexible printed circuit board coaxial cable which is made using a pattern on a flexible PCB substrate on which coaxial cables are formed is described. A series of slits and folds allows one or more elongated cables to be formed in each pattern. Such cables are particularly useful for connecting multi-connector apparatus of the type which requires coaxial cables interconnected in very close spaces, such as the type used in ultrasound transducers. The method for manufacturing such elongated, flexible printed circuit board coaxial cable is also described. The method includes forming a pattern on a flexible PCB substrate on which coaxial cables are formed. Thereafter, a series of slits and folds allows one or more elongated cables to be formed in each pattern. Such cables are particularly useful for connecting multi-connector apparatus of the type which requires coaxial cables interconnected in very close spaces, such as the type used in ultrasound transducers.

BACKGROUND OF THE INVENTION 
The present invention relates to elongated flexible printed circuit board 
cables. In particular, the invention relates to a method for making 
elongated, flexible, coaxial cables which are substantially longer than 
those which have heretofore been available. 
Long, flexible printed circuit board (PCB) coax cables are used in a number 
of applications. Such PCB coax cables are typically printed on or etched 
from a flexible substrate having an electrically conductive covering, such 
as copper. The substrate is typically comprised of a polyimide film. One 
such substrate is comprised of Dupont's Kapton brand polyimide film. In 
particular, such PCB coax cables are used in connection with ultrasound 
transducers, such as linear and phased array transducers, which have 
upwards of 64 elements, each of which must be connected to signal 
processor and display electronics by a coaxial, controlled impedence 
cable. 
In order to keep cable size, weight, and stiffness to a minimum, very small 
coaxial wires have been developed. These coaxial wires typically have 
center conductors of 36 AWG or smaller. Consequently, they are very 
expensive to manufacture and to terminate. This is particularly true when 
such cables are used to connect to an ultrasound transducer in a scanhead 
where space is at a premium. Heretofore, the major part of the 
manufacturing cost of the scanhead utilizing a linear or phased array 
transducer has been in the cable and in its terminations. 
A possible approach to eliminating the expense associated with attaching 
coaxial cables to an ultrasound transducer is to use flexible, PCB coax 
cables. Unfortunately, heretofore such flexible PCB coax cables have been 
limited in length to approximately 24 inches as a result of the 
manufacturing processes and equipment used to make them. In order to 
successfully utilize such cables in connection with ultrasound 
transducers, it is necessary for the cables to be approximately 6 to 8 
feet long. Heretofore, no such cables were available. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an improved method for laying out 
and manufacturing flexible PCB coax cables is described. Utilizing the 
present invention, it is possible to make long PCB coax (i.e. ground and 
signal) cables on existing flexible PCB production equipment. 
The long, flexible printed circuit board cable of the present invention 
comprises a pattern of cables on a flexible substrate. The pattern 
includes at least two substantially parallel, elongated sections joined by 
a perpendicular connecting section. This is a fold through one of the 
elongated sections on a fold line which is substantially perpendicular to 
the slit, and there is another fold through the connecting section along a 
second fold line substantially perpendicular to the first fold line. The 
second fold line is substantially aligned with the slit. 
In accordance with the invention, the method of making a long, flexible 
printed circuit board cable comprises first forming a pattern of cables on 
a flexible substrate. The pattern must have at least two substantially 
parallel, elongated sections which are joined by a perpendicular 
connecting section. A fold is made through one of the elongated sections 
on a fold line which is substantially perpendicular to the slit separating 
the elongated sections. Then another fold is made through the connecting 
section along a second fold line which is substantially perpendicular to 
the first fold line and which is substantially aligned with the slit. The 
folded portions are then preferably glued to hold them in the appropriate 
position.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
Referring to FIG. 1, a pattern 10 of the type used in the preferred 
embodiment of the present invention for manufacturing long, continuous, 
flexible PCB coax cable is illustrated. In the center of the pattern 10 
there is an opening 12 corresponding to the location where an ultrasound 
transducer will ultimately be mounted. A series of lines 14, corresponding 
to the coax cables are illustrated in the area of the transducer mounting 
location 12. In order to prevent obscuring the present invention, these 
lines 14, are not illustrated over the remainder of the pattern 10, 
although they are actually present. 
The pattern 10 is printed over an area which corresponds to that which is 
within present manufacturing capabilities. The pattern 10 includes a 
number of slits 16 each of which preferably ends with a circular opening 
18, which acts as a stress relief point. In the present embodiment of the 
invention, four different sections of flexible PCB coax cable end, 
respectively, in termination pads 20, 22, 24, 26. These correspond to four 
groups of cables 14 which extend between the pads 20-26 and the transducer 
mounting location 12. While this particular configuration is used to 
illutrate the invention, it will be understood by those skilled in the art 
that a greater or lesser number of groups of elongated, flexible PCB coax 
cables 14 could be manufactured without departing from the present 
invention. 
Referring to FIG. 2, an exploded portion 28 of a part of the pattern 10 of 
FIG. 1 is shown. The exploded portion 28 includes a section of the pattern 
12 having a slit 16 which ends in a stress relief opening 18. In FIG. 2, 
the coax cables 14 are illustrated. As shown, the coax cables 14 are 
printed onto the flexible backing material in substantially parallel runs 
on elongated sections 25, 27. The cables 14 from the elongated sections 
25, 27 are connected together via a connecting section 29. The cables 14 
in the connecting section 29 are also substantially parallel to one 
another, but the cable runs in a connecting section 29 are perpendicular 
to the cable runs in the elongated sections 25, 27. The cables 14 are 
formed, and then they are insulated by a protective, transparent layer. 
Referring now to FIG. 3, the elongated section 25 is first folded along a 
line 30 which is perpendicular to the slit 16, with the fold 30 being made 
at the stress relief opening 18. Thereafter, with reference to FIG. 4, a 
fold is made along a line 32 which is aligned with the slit 16 (and which 
extends through the center of the connecting section 29) whereby a long, 
continuous PCB coax cable is created. In the preferred embodiment of the 
invention, the flexible material is preferably glued to maintain the shape 
at the folds 30, 32 whereby a long, continuous PCB coax cable is 
manufactured. 
As will be recognized by those skilled in the art, the manner of connecting 
the flexible coax cable to a transducer, in the case of an ultrasonic 
scanhead application, is via a reflow soldering technique of the type 
heretofore used. Such a method is particularly advantageous in that the 
cables made using the present invention have integral connectors formed on 
them. Also, the present technique can be employed to obtain long cables 
which are not coaxial in electrical characteristics.