Abstract:
A flexible interconnect circuit includes a plurality of substantially flat flex circuits. Each flex circuit has a length substantially greater than its corresponding width. The plurality of flex circuits are folded parallel to their long axes and configured together to provide a layered flex interconnect circuit structure in which at least one ground flex circuit is interposed with one or more signal flex circuits.

Description:
BACKGROUND 
       [0001]    The invention relates generally to flexible circuits. In particular, the invention relates to a high density, flexible, foldable interconnect circuit that is particularly suited for applications requiring long, compact interconnect assemblies such as catheters and endoscopes. 
         [0002]    Processes for assembling a catheter interconnect presently require that an interconnect stack be assembled from individual signal and ground (GND) layers, e.g., 4 signal layers and 5 GND layers arranged in an alternating fashion. Each signal layer must be separated and unfolded from a panel containing many signal layers in a serpentine shape such as depicted in  FIG. 1  that illustrates a flex circuit structure  10  known in the art. The GND layers are cut to length from a spool. The interconnect assembly process requires careful attention to ensure that the layers remain in order and do not become twisted. Further, since each of the signal layers contains termination sites, they must be exactly aligned to their corresponding termination sites, a tedious process that requires differential adjustment of the lengthwise positions of the signal layers relative to one another. 
         [0003]    Several of the flexible interconnects depicted in  FIG. 1  may be required for arrays requiring a large number of interconnections such as depicted in  FIG. 2  that illustrates a flex circuit array structure cross-section  20  known in the art. Each of the flex circuits  24  must therefore be cut from a panel, unfolded, interspersed with ground (GND) layers  22 , and assembled into a stack in the correct layered order without any twists, a very tedious, time-consuming process. 
         [0004]    A need therefore exists for a simplified high density, flexible, foldable interconnect circuit structure that simplifies assembly of interconnect stacks conventionally assembled from individual signal and GND layers, eliminates twisting generally associated with interconnect stacks assembled from individual signal and GND layers, eliminates layer re-shifting requirements generally necessary during assembly of interconnect stacks assembled from individual signal and GND layers, and substantially reduces the time and expense of assembling interconnect stacks assembled from individual signal and GND layers. 
       BRIEF DESCRIPTION 
       [0005]    According to one embodiment, a flexible interconnect circuit comprises a plurality of substantially flat flex circuits, each flex circuit having a length substantially greater than its corresponding width, wherein the plurality of substantially flat flex circuits are configured together in a folded parallel to their long axis to provide a layered flex interconnect circuit structure comprising at least one ground flex circuit interposed with one or more signal flex circuits. 
         [0006]    According to another embodiment, a flexible interconnect circuit comprises: 
         [0007]    one or more signal flex circuits disposed on a first single substantially flat substrate, each signal flex circuit having a length substantially greater its corresponding width; 
         [0008]    at least one ground flex circuit disposed on a second single substantially flat substrate, each ground flex circuit having a length substantially greater than its corresponding width; 
         [0009]    wherein the one or more signal flex circuits and at least one ground flex circuit are folded parallel to their long axis to provide a layered flex interconnect circuit structure comprising one or more ground flex circuits interposed with one or more signal flex circuits. 
     
    
     
       DRAWINGS 
         [0010]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0011]      FIG. 1  illustrates a flex circuit structure known in the art; 
           [0012]      FIG. 2  illustrates a flex circuit array structure known in the art; 
           [0013]      FIG. 3  illustrates a flexible interconnect circuit structure with alternating signal-ground circuits in accordance with one embodiment of the present invention; 
           [0014]      FIG. 4  illustrates electrical shield layers added to the flexible interconnect circuit structure depicted in  FIG. 3  according to one aspect of the present invention; 
           [0015]      FIG. 5  illustrates a flexible interconnect circuit structure with a plurality of flex circuit widths in accordance with another embodiment of the present invention; 
           [0016]      FIG. 6  illustrates a flexible interconnect circuit structure configured from distinct and separate flex circuits in accordance with another embodiment of the present invention; 
           [0017]      FIG. 7  illustrates a flexible interconnect circuit structure configured with signal flex circuits, ground flex circuits, and ground-shield circuits in accordance with another embodiment of the present invention; 
           [0018]      FIG. 8  illustrates a flexible interconnect circuit structure configured with a deflection section according to one embodiment of the present invention; 
           [0019]      FIG. 9  illustrates flexible interconnect circuit folding features in accordance with one embodiment of the present invention; and 
           [0020]      FIG. 10  illustrates a flexible interconnect circuit structure with a removable section in accordance with another embodiment of the present invention. 
       
    
    
       [0021]    While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0022]    The embodiments described herein with reference to  FIGS. 3-9  are directed to structures and processes for constructing a high density, flexible, foldable interconnect circuit that is particularly suited for applications requiring long, compact interconnect assemblies such as catheters and endoscopes. Some embodiments comprise one or more long flex circuits containing adjacent signal and GND segments, such that when folded parallel to their long axis, an alternating signal-GND layered structure is achieved, which is desirable for electrical crosstalk isolation. 
         [0023]    The presence of a GND layer between every signal layer is not required however to implement a high density flexible foldable interconnect according to the principles described herein. One embodiment, for example, comprises multiple adjacent signal layers with ground layers only on the outside. 
         [0024]    At least one embodiment described herein comprises EMI shielding layers. The interconnect structures can be configured to provide a specific cross-sectional shape subsequent to folding, such as a circle, which is desirable for efficient use of available space in such applications as catheters. 
         [0025]    The embodiments described herein greatly simplify the interconnect assembly process, leading to reduced cost, ease of termination of the interconnect ends, and adaptability of the interconnect to a specific shape. 
         [0026]      FIG. 3  illustrates a flexible interconnect circuit structure  30  in accordance with one embodiment of the present invention. The flex interconnect circuit structure  30  is fabricated from a single full-length sheet without any serpentine arrangement, and incorporates both signal  32  and GND  34  stripes that may be configured to alternate as shown. When the flexible sheet comprising interconnect circuit structure  30  is folded lengthwise along the dotted lines  36 , the desired alternating signal-GND structure is achieved. Cutting out individual signal and GND layers is therefore no longer required, greatly simplifying the assembly process. The corresponding substrate  38  that the flexible interconnect circuit  30  is fabricated on (typically polyimide), may be modified along the lengths where the folds  36  occur, e.g., by perforation or thinning, to ease the folding process. 
         [0027]      FIG. 4  illustrates electrical shield layers  40  added to the flex interconnect circuit structure  30  depicted in  FIG. 3  according to one embodiment of the present invention. These electrical shield layers  40  are added to the signal and ground layers  32 ,  34  such that when they are folded, the shield layers  40  surround the resultant flex stack comprising the alternating signal and GND flex layers  32 ,  34 . The shield layers  40  may or may not also include the regions where the folds  36  occur, depending upon the desired application. 
         [0028]      FIG. 5  illustrates a flexible interconnect circuit structure  50  in accordance with another embodiment of the present invention. The signal and GND stripes  32 ,  34  may have non-uniform widths such that when folded, specific geometries are created. The right side of  FIG. 5  illustrates that a circular cross-section is created subsequent to folding which may advantageously utilize a greater percentage of available space for certain application such as catheters or endoscopes. 
         [0029]      FIG. 6  illustrates a flexible interconnect circuit structure (flex stack)  60  in accordance with another embodiment of the present invention. The flex stack  60  may be assembled from multiple flex circuits. The flex stack  60  depicted in  FIG. 6  comprises a single signal flex interconnect structure  62  and two GND flex interconnect structures  64 . The signal flex interconnect  62  comprises three signal flex stripes  32  while each GND flex interconnect  64  comprises two GND flex stripes  34 . The signal flex interconnect structure  62  is folded in a serpentine fashion. Each GND flex interconnect structure  64  is folded once and then inserted into the spaces between the resultant serpentine structure as shown to form the desired flexible interconnect circuit structure  60 . 
         [0030]      FIG. 7  illustrates a flexible interconnect circuit structure (flex stack)  70  in accordance with another embodiment of the present invention. Flex stack  70  may similarly be assembled from multiple flex circuits. The flex stack  70  depicted in  FIG. 7  comprises a single signal flex interconnect structure  72 , two GND flex interconnect structures  74 , and two GND-shield flex structures  76 . The signal flex interconnect  72  comprises seven signal flex stripes  32  while the GND flex interconnect  74  comprises two GND flex stripes  34 , and the GND-shield flex structure  76  comprises a GND flex stripe  34  and a shield flex stripe  78 . The signal flex interconnect structure  72  is folded in a serpentine fashion. The double GND flex interconnect structure  74  is folded once and then inserted into the spaces between the resultant serpentine structure as shown. The double GND flex interconnect structure  74  may be configured to surround a desired number of signal flex circuits  32 . GND flex interconnect structure  74 , for example, is configured to surround one pair of signal flex circuits  32 . One or more GND-shield flex structures  76  are folded and inserted into the resultant serpentine structure as shown to form the desired flexible interconnect circuit structure  70 . 
         [0031]      FIG. 8  illustrates a flexible interconnect circuit structure  80  in accordance with another embodiment of the present invention. The base substrate material  88  is perforated or removed in desired portions  86  of one or more deflection sections  82  of the flex interconnect circuit  80  that are most subject to bending. Catheters for example, often require deflection at the tip of the catheter. Removing the substrate  88  between layers in the deflection section  82  would allow the layers  32 ,  34  to slide relative to one another during deflection. End tabs  84  allow the flex circuits  32 ,  34  to remain as a single piece during the folding process, but could optionally be later removed from the flex interconnect circuit structure  80  as desired for a particular application. 
         [0032]      FIG. 9  illustrates flexible interconnect circuit folding features that facilitate easy folding of the flex interconnect circuit where desired, in accordance with one embodiment of the present invention. More specifically,  FIG. 9  depicts an end view of a flex interconnect circuit structure  90 , where a thinned region  92  is devoid of metal or cover layers  94 , signal flex traces  32 , and GND flex metal  34 , making it easier to fold the flex interconnect circuit  90  along those paths. Other embodiments may employ features including without limitation, one or more of perforations, mechanical scoring, or chemical etching, for example, to facilitate easier folding of the flex interconnect circuit structure  90 . 
         [0033]      FIG. 10  illustrates a flexible interconnect circuit structure  100  in accordance with another embodiment of the present invention. According to one embodiment, the substrate material  88  employed by flex interconnect circuit structure  100  comprises a removable section  102  that is formed as a tear away strip through use of one or more tear strips  104  and corresponding rip stops  106 . According to one embodiment, the tear strip  104  comprises a section of the substrate  88  that is specifically designed to be mechanically weaker than the rest of the substrate  88 , e.g., by thinning. The tear strips  104  can be removed once the flex interconnect circuit  100  has been folded in order to provide increased flexibility to a specific portion of the flex interconnect circuit  100 , e.g., the deflection section of a catheter. The rip stop  106  terminates the tear strip  104 . The rip stop  106  may comprise, for example, a simple through hole. 
         [0034]    In summary explanation, structures and processes are described for constructing a high density, flexible, foldable interconnect circuit that is particularly suited for applications requiring long, compact interconnect lengths such as catheters and endoscopes. Particular embodiments comprise one or more long flex circuits containing adjacent signal and GND stripes such that when folded parallel to their long axis, a layered structure comprising signal and GND layers is achieved, which is desirable for electrical crosstalk isolation. The embodiments described herein greatly simplify the interconnect assembly process, leading to reduced cost, ease in termination of the interconnect ends, and adaptability of the interconnect to a specific shape. Other advantages include without limitation, the ability to shield interconnects using the same folded structure, the ability to implement different cross section interconnect stack shapes and elimination or substantial reduction of twisting of flex layers. 
         [0035]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.