Abstract:
A flexible circuit that includes mounted electrical components, where bonding wires providing an electrical connection to the electrical components are aligned perpendicularly to the primary plane in which the flexible circuit bends and multiple redundant vias for electrical and thermal connections. The flexible circuit may include an array of light emitting diodes “(LEDs”) that are positioned length-wise in a flexile LED strip as well as flexible printed circuits having a plurality of electrical components attached thereto, where the electrical components may include LEDs. Methods of improving the reliability and thermal dissipation of a flexible circuit and producing a flexible circuit with re-aligned bonding wires and multiple vias for electrical and thermal connections are also provided.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The definition of a flexible circuit found in the IPC-T-50-F: Terms and Definitions for Interconnecting and Packaging Electronic Circuits, Revision F (June 1996), is: “A patterned arrangement of printed wiring utilizing flexible base material with or without flexible cover lay.” From this definition, there are a number of basic material elements that make up a flexible circuit: a dielectric substrate material, electrical conductors, a protective finish, and adhesives to bond the various materials together, with the adhesives being optional because there are alternatives to utilizing adhesives to bond the various materials together. 
         [0002]    In general, the dielectric material may be either a polymide film, such as Kapton®, Apical®, or Upilex®, or a polyester. As for the electrical conductor, generally the material of choice is copper, which is available in several types. As for the flexible circuit itself, there are several generic variations, such as single-sided construction, double-sided construction, multilayer construction, and rigid-flex construction. In the case of rigid-flex circuits, rigid and flexible substrates may be laminated together, where the rigid substrate is typically FR-4, the material usually found in Printed Circuit Boards (“PCBs”). Together these materials form a basic flexible-circuit laminate that may be utilized as a simple wiring assembly or as a flexible final circuit assembly after mounting additional devices directly on the flexible-circuit laminate. 
         [0003]    The advantages of flexible circuits are that they are significantly thinner and lighter than standard rigid PCBs, and may be utilized where space is at a premium, by bending the flexible circuit around corners or over itself in order to fit within a much smaller device enclosure than would be required for a rigid PCB. Flexible circuits have found use in many types of applications, and their design in based in part on the number of flex cycles required during its expected lifetime, from a few times during assembly, i.e., a one-time bending for fit or assembly (bend statically), to multiple flexes, i.e., repeated flexing over many cycles (bend dynamically). 
         [0004]    With advances in Surface Mount Technology (“SMT”), there is the capability of mounting numerous other electrical components such as resistors, capacitors, current driver integrated circuits (“ICs”), controller and other ICs on a flexible-circuit laminate. Also included within such electrical components are light emitting diodes (“LEDs”). LEDs are, in general, miniature semiconductor devices that employ a form of electroluminescence resulting from the electronic excitation of a semiconductor material to produce visible light. Initially, the use of these devices was limited mainly to display functions on electronic appliances and the colors emitted were red and green. As the technology has improved, LEDs have become more powerful and are now available in a wide spectrum of colors, including blue and white. 
         [0005]    With the capability of producing white light, there is now the possibility of using LEDs for illumination in place of incandescent and fluorescent lamps, including use in outdoor lighting applications. The advantages of using LEDs for illumination is that they are far more efficient than conventional lighting, are rugged and very compact, and can last much longer than incandescent or fluorescent light bulbs or lamps. 
         [0006]    Given these properties of LEDs and the various colors that are now available, LEDs are finding usage in many more applications, including application that utilize flexible circuits. An example of a flexible circuit is the flexible LED array (or “flex-LED”), which is an array of LEDs aligned length-wise, where each LED in the flex-LED is electrically connected to the adjacent LEDs, thus completing an electrical connection whereby each LED in the flex-LED has a bias voltage. The flex-LED may also contain an encapsulant that covers each LED, which may be any encapsulant used in an LED, such as an optically clear epoxy resin or silicone system, and a flexible substrate on which the LEDs are attached. The flex-LED may also be enclosed in a waterproof/weatherproof, transparent casing, which may be made from any polymeric transparent material. The flex-LED may be commercially available in various standardized lengths of a light strip such that these light strips may be cut or drilled so that the user can connect multiple light strips and adapt the flex-LED to his particular installation requirements. 
         [0007]    In  FIG. 1A , a schematic diagram illustrating an example of a section of a known flex-LED  100  along its length is shown. Flex-LED  100  is a section of a flex-LED strip that may be of a standard length, that is, flex-LED  100  shows only a portion of a longer strip that contains a plurality of LEDs positioned equidistantly throughout the flex-LED strip. This flex-LED strip may be cut to obtain the desired length and multiple strips may then be joined together using standardized connectors (not shown). 
         [0008]    The flex-LED  100  may include a substrate  102 , which may be flexible-circuit laminate that includes a flexible dielectric and electrical conductors. Attached to the substrate  102  is a plurality of LEDs  104 . A bonding wire  106  may provide one of the two electrical connections required for each of the LEDs  104 , for example, an anode connection. A cathode connection may then be located on the bottom surface of each LED  104 , in the form of backside metallization (not shown), which may be implemented by attaching a conducting material to the bottom of each LED  104 . The entire assembly of the LEDs  104  and the substrate  102  may then be encapsulated in an encapsulant  108  applied to the surface of the assembly. Additionally, the entire assembly including the encapsulant  108  may also be enclosed in a transparent casing (not shown). 
         [0009]      FIG. 1B  shows a cross-sectional side view of the flex-LED  100  across its width. In  FIG. 1B , the flex-LED  100  includes an LED  104  attached to a substrate  102 . This package may be covered by an encapsulant  108 . The bonding wire  106  completes the electrical connection to an electrical conductor (not shown). In  FIG. 1A , the arrows  112  indicate the primary direction of the flexing of the flex-LED  100 . As the flex-LED  100  flexes in the direction of the arrows  112 , the bonding wires  106  will tend to stretch, and with repeated flexing of the flex-LED  100 , there is an increased possibility of the bonding wires  106  breaking or failing. 
         [0010]    In  FIG. 1C , a schematic diagram illustrating another example of a section of a known flex-LED  100  along its length is shown. As in  FIG. 1A , flex-LED  100  is a section of a flex-LED strip that may be of a standard length, that is, flex-LED  100  shows only a portion of a longer strip that contains a plurality of LEDs positioned equidistantly throughout the flex-LED strip. The flex-LED  100  may include a substrate  102  that may include a flexible dielectric and electrical conductors. Attached to the substrate  102  is a plurality of LEDs  104 . A bonding wire  106  may provide one of the two electrical connections required for each of the LEDs  104 , for example, an anode connection. The other connection, in this case, a cathode connection for each LED  104 , may then be provided by bonding wire  110 . The entire assembly of the LEDs  104  and the substrate  102  may then be encapsulated in an encapsulant  108  applied to the surface of the assembly. Additionally, the entire assembly including the encapsulant  108  may also be enclosed in a transparent casing (not shown). 
         [0011]    The arrows  112  indicate the primary direction of the flexing of the flex-LED  100 . As the flex-LED  100  flexes in the direction of the arrows  112 , the bonding wires  106  and  108  will tend to stretch, and with repeated flexing of the flex-LED  100 , there is an increased possibility of the bonding wires  106  and  110  breaking or failing. 
         [0012]    A similar problem in both flexible and rigid circuits is present with respect to vias. In general, vias are holes drilled through a flexible circuit or a PCB, which are plated and then filled with a polymer, which may be conductive or non-conductive, to provide a vertical electrical or thermal connection between different layers of the flexible circuit or PCB. In  FIG. 2 , a schematic diagram illustrating an example of a section of a known flexible circuit is shown. The flexible circuit  200  may include a dielectric  202  laminated with a top conductor  204  on the top and a bottom conductor  206  on the bottom. The flexible circuit  200  may also include a via  212 , which may be plated  210  and filled with a filler  208 . 
         [0013]    When the flexible circuit  200  flexes in the direction of the arrows  214 , the filler  212  may tend to separate from the edge of the via  208 , thus creating cracks or fissures within the area denoted by the circle  216 . With repeated flexing of the flexible circuit  200 , there is an increased possibility of the cracks appearing in the flexible circuit  200 , which may eventually cause its failure. In particular, if the via is utilized for thermal dissipation, the tendency for such cracks to appear may be even more likely. 
         [0014]    In  FIG. 3 , a schematic diagram illustrating an example of a section of a known flexible printed circuit (“FPC”)  300  is shown. FPC  300  shows a side view of a section of a flexible circuit that may be of a standard length, that is, FPC  300  shows only a portion of a longer strip containing an array bonded to a thin, flexible dielectric. A dielectric  302  is positioned between a top metal layer  304  and a bottom metal layer  306 . Attached to the top metal layer  304  may be a component  308 , which may be, as an example, an LED. A bonding wire  310  may provide one of the two electrical connections required for the component  308 , for example, an anode connection. A cathode connection may then be located on the bottom surface of the component  308 , in the form of backside metallization (not shown), which may be implemented by attaching a conducting material to the bottom of component  308 . The entire assembly may then be covered by an encapsulant  312 . 
         [0015]    In addition to the problems caused by the repeated flexing of the flex-LEDs and flexible circuits, these devices also have the problem of thermal dissipation. In particular, LEDs generate heat and in the LED arrays found in flex circuits, this problem is even more critical. In general, LED devices are commonly prone to damage caused by buildup of heat generated from within the devices, as well as heat from sunlight in the case of outside lighting applications. Although metallized LED substrates are useful design elements that can be incorporated into LED devices and may serve to dissipate heat, these elements are often inadequate to maintain reasonably moderate temperatures in the devices. Excessive heat buildup can nevertheless cause deterioration of the materials used in the LED devices, such as encapsulants for the LED. When LEDS are attached to flexible-circuit laminates that may also include other electrical components, the heat dissipation problems are greatly increased. 
         [0016]    Consequently, there is a continuing need to improve the design of flexible circuits and flex-LEDs to reduce damage to these devices caused by repeated flexing or sharp bending, which may occur during manufacture, testing, installation, or operation, as well as to improve the thermal dissipation properties of these devices. 
       SUMMARY 
       [0017]    In general, a system and a method of improving the reliability of flexible circuits and their thermal dissipation properties by re-aligning the wire bonding in these devices and by adding and repositioning redundant vias in the flexible-circuit laminate utilized for thermal dissipation are disclosed. In a flex-LED device, the wire bonding for each LED is aligned perpendicular to the direction of primary flexing, i.e., perpendicular to the length-wise axis of the flex-LED. Additionally, a flexible circuit may include multiple vias utilized for thermal dissipation positioned near a component attached to the flexible-circuit laminate to improve thermal dissipation 
         [0018]    In another example of an implementation, an electrical connection may be made without a via, by utilizing an electrical connection on the top surface of the flexible-circuit laminate. 
         [0019]    Other systems, methods and features of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention can be better understood with reference to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
           [0021]      FIG. 1A  shows a cross-sectional side view illustrating an example of an implementation of a known flex-LED. 
           [0022]      FIG. 1B  shows a cross-sectional end view of the known flex-LED device shown in  FIG. 1A . 
           [0023]      FIG. 1C  shows a cross-sectional side view illustrating another example of an implementation of a known flex-LED with two bonding wires per LED. 
           [0024]      FIG. 2  shows a cross-sectional side view illustrating an example of a known implementation of a printed circuit board. 
           [0025]      FIG. 3  shows a cross-sectional side view illustrating an example of a known flexible circuit. 
           [0026]      FIG. 4A  shows a cross-sectional side view illustrating an example of an implementation of a flex-LED in accordance with the invention. 
           [0027]      FIG. 4B  shows a cross-sectional end view of the flex-LED shown in  FIG. 4A . 
           [0028]      FIG. 4C  shows a cross-sectional end view of the flex-LED with a filled via. 
           [0029]      FIG. 5A  shows a perspective view of a section of an example of an implementation of a flexible circuit having multiple vias. 
           [0030]      FIG. 5B  shows a perspective view of a section of another example of an implementation of a flexible circuit having multiple vias. 
           [0031]      FIG. 6  shows a cross-sectional side view illustrating an example of an implementation of a flexible circuit having multiple vias for thermal and electrical connections. 
           [0032]      FIG. 7A  shows a perspective view illustrating an example of an implementation of a flexible circuit having multiple vias. 
           [0033]      FIG. 7B  shows a top view of an example of a via layout for the flexible circuit shown in  FIG. 7A . 
           [0034]      FIG. 8  shows a perspective view illustrating another example of an implementation of a flexible circuit without vias. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    In the following description of examples of implementations, reference is made to the accompanying drawings that form a part hereof, and which show, by way of illustration, specific implementations of the invention that may be utilized. Other implementations may be utilized and structural changes may be made without departing from the scope of the present invention. 
         [0036]    In general, a system and a method of improving the thermal dissipation properties of flexible circuits by adding and repositioning vias utilized for thermal dissipation and the reliability of these devices by adding multiple electrical vias and by re-aligning the wire bonding in these devices is disclosed. Turning to  FIG. 4A , a cross-sectional side view illustrating an example of an implementation of a flex-LED  400  in accordance with the invention is shown. The flex-LED  400  may include a substrate  402  that may include a flexible dielectric and electrical conductors. Attached to the substrate  402  is a plurality of LEDs  404 . A bonding wire  406  may provide one of the two electrical connections required for each of the LEDs  404 , for example, an anode connection. A cathode connection may then be located on the bottom surface of the LED  404 , in the form of backside metallization (not shown), which may be implemented by attaching a conducting material to the bottom of each LED  404  and mounting LED  404  on the electrode and thermal pad  414 . 
         [0037]    In  FIG. 4A , the wire bonding wire  406  for each LED  404  is aligned perpendicularly to the direction of primary flexing, i.e., perpendicular to the length-wise axis of the flex-LED  400 . In another embodiment, a two-wire bond LED chip may be implemented where both anode and cathode electrode contacts are on the same side of the LED chip, i.e., the top surface. Where there are two bond wires per LED chip, both bond wires are positioned to be substantially perpendicular to the longitudinal axis of the flex strip. 
         [0038]    The entire assembly may be encapsulated in an encapsulant  408 . In another embodiment, the encapsulant and the assembly may be enclosed in a transparent casing (not shown).  FIG. 4B  shows a cross-sectional end view of the flex-LED  400  across its width. In  FIG. 4B , the flex-LED  400  includes an LED  404  attached to a substrate  402 . This package may be encapsulated in an encapsulant  408 . The bonding wire  406  completes an electrical connection to a second electrode  416  in the substrate  402 . The other electrical connection is to a first electrode  414 , which may be a fully-filled via, e.g., a blind via, or a filled via, i.e., a via created by a hole drilled through the substrate  402 , then plated with a conductive metal such as copper, silver, etc., and filled with a resin/plug material. 
         [0039]    In  FIG. 4A , the arrows  412  indicate the primary direction of the flexing of the flex-LED  400 . In  FIG. 4B , the bonding wire  406  is affixed to the LED  406  and connected to the second electrode  416  in an orientation that is perpendicular to the plane of the direction of primary flexing, that is, the plane defined by the arrows  412 . Thus, any flexing in this plane will not effect or cause any stress on the bonding wire  406 . 
         [0040]    In  FIG. 4C , a cross-sectional end view illustrating another example of an implementation of the flex-LED device shown in  FIG. 4A  is shown. As in  FIG. 4B , the bonding wire  406  is affixed to the LED  404  and connected to the second electrode  416  in an orientation that is perpendicular to the plane of the direction of primary flexing, that is, the plane defined by the arrows  412 . The connection to the first electrode  414  is made through via  418 , which is positioned under the LED  404 . Thus, any flexing in this plane will not effect or cause any stress on the bonding wire  406  or the via  418 . 
         [0041]    In  FIG. 5A , a perspective view of a section illustrating an example of a flexible circuit  500  having multiple vias in accordance with the invention is shown. Flexible circuit  500  may include a substrate  502  on which a component  504 , such as an LED, may be attached. A bonding wire  506  may provide an electrical connection for the component  504  to an anode pad  510 , with a connection to a cathode pad  508  made on the bottom surface of the component  504  utilizing a backside metallization (not shown). 
         [0042]    Flexible circuit  500  may also include 4 vias  514  that may be positioned near the LED attached to the flexible circuit. As an example, the flexible circuit may include multiple, redundant vias  514  utilized for thermal dissipation positioned near the component  504  at approximately equal distances therefrom. With this configuration of multiple thermal vias utilized for thermal dissipation, at least two of these vias will not be subjected to stress caused by repeated flexing or sharp bending of the flexible circuit of which it is a part. 
         [0043]    In  FIG. 5B , a perspective view of a section illustrating another example of a flexible circuit  500  having multiple vias in accordance with the invention is shown. A component  504 , such as an LED, may be attached to a substrate  502 , with a bonding wire  506  providing an electrical connection for the component  504  to an anode pad  510 , and a bonding wire  512  providing an electrical a connection to a cathode pad  508 . As in  FIG. 5A , the flexible circuit  500  may include multiple, redundant vias  514  utilized for thermal dissipation positioned near the component  504  at approximately equal distances therefrom, thus allowing to avoid the stress caused by repeated flexing or sharp bending of the flexible circuit of which it is a part. 
         [0044]    Turning to  FIG. 6 , a cross-sectional side view illustrating an example of an implementation of a flexible circuit having multiple vias for thermal and electrical connections is shown. Flexible printed circuit (“FPC”)  600  may include a dielectric  604  that is positioned between a top metal layer  606  and a bottom metal layer  602 . Attached to the top metal layer  606  may be a component  608 , which may be, as an example, an LED. A bonding wire  610  may provide one of the two electrical connections required for the component  608 , for example, an anode connection. A cathode connection may then be located on the bottom surface of the component  608  in the form of backside metallization (not shown), which may be implemented by attaching a conducting material to the bottom of component  608 . The entire assembly may then be covered by an encapsulant  612 . 
         [0045]    FPC  600  may also include vias  614  and  616  for thermal dissipation that pass through the dielectric  604  and dissipate heat from the component  608  through the top layer  606  to the bottom layer  602 , which may be an aluminum or copper plate. For the other electrical connection, i.e., the cathode connection in this example, the FPC  600  may include a blind via  618  under the component  608  that provides an electrical connection to the bottom layer  602 . 
         [0046]    In  FIG. 7A , a perspective view illustrating an example of another implementation of a flex-circuit having multiple vias is shown. Flex-circuit  700  may include a substrate  702  on which a component  704 , such as an LED, may be attached. A bonding wire  706  may provide an electrical connection for the component  704  to an anode pad  710 , with a connection to a cathode pad  708  made on the bottom surface of the component  704  utilizing a backside metallization (not shown). In one embodiment, the substrate  702  and the LED  704  may be encapsulated with encapsulant  712 . In another embodiment, the substrate  702  may be enclosed within a transparent casing (not shown), which may then be filled with an encapsulant. 
         [0047]    Flex-circuit  700  may include multiple vias drilled through the substrate  702 , such as vias  714 , which may be in electrical connection with the anode pad  710 , and vias  716 , which may be in electrical connection with the cathode pad  708 . These vias may be also configured to provide a path for thermal dissipation from the component  708  by being filled with a thermally conductive material. 
         [0048]    Flex-circuit  700  may further include a blind via (not shown) located under the component  704  that provides an electrical connection from the component  704  to a ground plane (not shown) below the substrate  702 .  FIG. 7B  shows a top view illustrating an example of a via layout for the flex-circuit  700  shown in  FIG. 7A . Vias  714  and  716  may be configured for thermal connections, while blind via  718  may be configured for an electrical connection for the component (not shown) attached to the cathode post  708 . 
         [0049]    In  FIG. 8 , a perspective view illustrating an example of an implementation of a flex-circuit without vias is shown. Flex-circuit  800  may include a substrate  802  on which a component  804 , such as an LED, may be attached. A bonding wire  806  may provide an electrical connection for the component  804  to an anode pad  810 , with a connection to a cathode pad  808  made on the bottom surface of the component  804  utilizing a backside metallization (not shown). In a first embodiment, the substrate  802  and the LED  804  may be encapsulated with encapsulant  812 . In a second embodiment, the substrate  802  and the LED  804  may be enclosed within a transparent casing (not shown), which may then be filled with an encapsulant. 
         [0050]    In flex-circuit  800 , the electrical connection is taken out of the flex-circuit  800  by external terminations  814  and  816  of electrical terminals without utilizing vias or solder pads. The electrical terminals may be positioned so that they are taken out of the flex-circuit  800  on the same side of the flex-circuit  800 . 
         [0051]    While the foregoing descriptions refer to the use of an LED as the component attached to a flex-LED and a flexible circuit, the subject matter is not limited to LEDs as the component utilized in a flexible circuit or to flex-LEDs or flexible printed circuits as the substrate. Any electronic component and any type of substrate that could benefit from the functionality provided by the components described above may be implemented as the elements of the invention. The flexible circuits described above applies to thin laminated circuits having low thickness compared to conventional PCBs and may or may not be subject to being flexed or bended many multiple times in end applications. In end applications, it may be in a final state or shape of being bent, or curved to conform to a particular shape with straight sections, curved sections or combination thereof. 
         [0052]    Moreover, it will be understood that the foregoing description of numerous implementations has been presented for purposes of illustration and description. It is not exhaustive and does not limit the claimed inventions to the precise forms disclosed. Modifications and variations are possible in light of the above description or may be acquired from practicing the invention. The claims and their equivalents define the scope of the invention.