Abstract:
A flexible circuit having improved tear resistance is provided. A flexible circuit, made of polyimide film, includes at least one extension which needs to be folded. To prevent tearing, an annular piece of metal, like an exposed copper pad for example, is placed at the apex of the bend angle. A second metal is then deposited atop the annular piece of metal, thereby reinforcing the annular piece of metal. The reinforced annular piece of metal helps to prevent the flexible circuit from tearing when shearing forces are applied to the extension. Experimental results have shown that the invention provides as much as a 285% increase in tear resistance when compared to prior art flexible circuits.

Description:
BACKGROUND 
     1. Technical Field 
     This invention relates generally to flexible circuit substrates, and more specifically to a tear resistant flexible circuit substrate. 
     2. Background Art 
     Over one hundred and forty million Americans now use a cellular telephone and another fifteen million or so are expected to subscribe in the coming year. Sales of cellular phones have risen faster than those of facsimile machines, subscriptions to cable television service, and sales of videocassette recorders. Cellular telephones have become a feature across both the business and recreational landscapes. Customers have come to expect, and demand, steady increases in reliability and portability of the telephones. They have also come to demand the constant reductions in cost of the telephones. 
     A critical aspect of the reliability of the cellular telephone is the reliability of its power source, the battery. The battery may well be the single most important feature in a cellular telephone, or for that matter, in other types of electronic devices, including two-way radios. Consumer surveys have shown that talk time is the feature valued by users of cellular telephones and two-way radios above all else. 
     At the same time, consumers are demanding smaller and smaller phones. Consequently, the non-cell components of the corresponding battery pack, like the safety circuitry, charging circuitry and fuel gauging circuitry, have become smaller to deliver greater talk time without increasing the size of the battery. The advent of flexible circuits has been integral in the size reduction of these circuits. 
     By way of background, not too long ago, electronic circuits were mounted upon printed circuit boards. These printed circuit boards were rigid, flat boards made of layers of fiberglass with copper pads and traces disposed atop and between these layers. Printed circuit boards were difficult to work with in the small confines of a batter pack in that they were both bulky and rigid. The thickness of the printed circuit boards increased the overall thickness of the pack, and the rigidity prevented the printed circuit board from folding about the cell. 
     To remedy these issues, a new substrate, known as a “flexible circuit substrate” was developed. These substrates, affectionately known as “flexes”, are generally manufactured from polyimide films, like Kapton®, manufactured by the DuPont Company (See, e.g., www.dupont.com/kapton/). Kapton® is a semi-transparent film that is durable, flexible and heat resistant, and is used in applications ranging from circuit substrates to automotive wiring harnesses to solar cell and space exploration applications. 
     By depositing conductive copper pads and traces atop and between layers of Kapton®, durable flexible circuits are made. The use of flexible circuits in battery packs is well known. For example, commonly assigned U.S. Pat. No. 6,153,834, entitled “Flexible Circuit with Tabs for Interconnection to Battery Cells, issued Nov. 28, 2000, incorporated by reference herein, teaches the use of a flexible circuit in conjunction with a battery pack. 
     One problem that exists with polyimide films like Kapton® involves tearing. Much like a bag of potato chips that is initially difficult to open, but once torn tears very easily, polyimide films tend to tear very easily once a tear has been initiated. As many battery pack designs, like that taught in the &#39;834 patent mentioned above, employ flexes that have extensions that are bent in different directions, any tearing of the flex may render the battery inoperable. The reason for this is that tears in the flex may in turn tear critical conductive traces within the flex. 
     There is thus a need for an improved flexible circuit substrate that is resistant to tearing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art flex. 
         FIG. 2  illustrates a prior art flex that has torn due to bending. 
         FIG. 3  illustrates a flex in accordance with the invention. 
         FIG. 4  illustrates a flex in accordance with the invention after folding an extension. 
         FIG. 5  illustrates a rechargeable battery pack employing a flex in accordance with the invention. 
         FIGS. 6–9  illustrated exemplary flexes in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
     Referring now to  FIG. 1 , illustrated therein is a prior art flexible circuit substrate  10 , or “flex”. The flex  10  is made from a polyimide film, like Kapton® for example. Due to design requirements, such a flex is often required to bend in many applications. As such, the flex  10  includes a central portion  100  with a first extension  101  and a second extension  102  extending from the central portion  100  of the flex  10 . 
     Between the two extensions  101 , 102  is as parting line  105 . The parting line  105  may be as simple as a slit cut into the flex to facilitate bending of the extensions  101 , 102  in different directions, or may be more complex shapes. In either case, the parting line  105  ends at a termination point  106 . 
     For exemplary purposes, a simple, single fold will be used as an illustration. To make this simple, single fold, extension  101  will be folded across extension  102  and the central member  100  in the direction of line  104 . Folding line  103  illustrates where the flex  10  is intended to bend when extension  101  is folded. 
     Referring now to  FIG. 2 , illustrated therein is the problem associated with prior art flexes. When extension  101  is folded across the central member  100 , the folding forces, exerted naturally against the termination point  106 , will often cause the flex  10  to tear. For example, force F 1  and F 2  act as a shearing force against the flex  10 . Robust materials, like Kapton®, will initially resist tearing, so long as the shearing force remains below about 0.6 lbs. However, once this limit is exceeded, a tear  200  will result. Once the tear has started, forces less than 0.1 lbs can increase the length of the tear  200 . 
     Referring now to  FIG. 3 , illustrated therein is a tear-resistant flex  30  in accordance with the invention. The flex  30  is made of at least two layers of flexible material, like polyimide films. Between and/or atop the layers, electrically conductive traces  307  may be disposed. These traces may be manufactured from copper, copper alloys, aluminum, or other equivalent conductors. Electrically conductive pads  308  may be disposed atop the layers. The pads  308  may couple to the traces  307  so as to form a circuit network. Electrical components (not shown) may be coupled to the pads  308 . 
     The flex  30  includes a central member  304  from which at least one extension  305  extends. In keeping with the illustrative example of  FIGS. 1 and 2 , the flex  30  is shown with two extensions  305 , 306 , although the invention is not so limited. It will be clear to those of ordinary skill in the art who have the benefit of this disclosure that any number of extensions, in any number of shapes, may extend in any number of directions from the central member. 
     Between the two extensions  305 , 306  is as parting line  309 . The parting line  309  may be as simple as a slit cut into the flex to facilitate bending of the extensions  305 , 306  in different directions, or may be more complex, cut-away shapes as noted in subsequent paragraphs. In either case, the parting line  309  ends at a termination point, shown here as point  310 . 
     An annular member  300  is disposed about the termination point  310 . The annular member  300  is preferably made of metal. To reduce cost of the overall flex  30 , it is often desirable to construct the annular member  300  from the same metal as the conductive traces  307 . The annular member  300  is thus preferably constructed from copper, copper alloys, aluminum or other conductors. 
     A second metal  303  is deposited upon the annular member  300 . The second metal  303  serves to reinforce and strengthen the annular member  300 . To reduce overall cost, the second metal  303  is preferably solder that is deposited upon the annular member  300  by way of reflow soldering, hand soldering, wave soldering or other equivalent method. While soldering works well, other methods, like vapor deposition or plating would also suffice. 
     If solder is employed as the second metal  303 , it is desirable to include an aperture  301  in at least one of the film layers of the flex  30 . The aperture  301  allows the annular member to contact solder during conventional manufacturing processes, like reflow soldering for example. To properly retain the annular member  300  between the film layers, the area of the aperture  301  should be less than the area of the annular member  300 . For a single sided flex (pads only on one side of the flex), the aperture  301  may only pierce the top layer of film, whereas for double sided flexes (pads on both the top and bottom of the flex), apertures may be found through both the top and bottom layers of film, thereby allowing the second metal to be deposited on both the top and bottom of the annular member  300 . 
     For convenience in folding, a second aperture  302  may be added about the termination point. The second aperture  302  is essentially a hole that passes through all the layers of the flex  30 , as well as through the annular member  300 . The second aperture  302  is added to the annular member  300  prior to the deposition of the second metal  303 . 
     The length of the annular member  300  will vary depending upon the application. For this exemplary embodiment, where one extension  305  is being folded in a perpendicular fashion with respect to the central member  304 , a fold line  307  indicates where the fold will be made. The annular member  300  runs approximately 225 degrees, in that it runs from the parting line to the fold line. Other applications, as will be discussed with respect to  FIGS. 6–8 , may require greater or lesser angles. 
     Referring now to  FIG. 4 , illustrated therein is the substrate of  FIG. 3  after the extension  305  is folded. Note that the annular member  300 , with the second metal  303  deposited atop, functions as a mechanical restraint that prevents the flex  30  from tearing when shearing forces are applied to the extensions  305 , 306 . Experimental results have shown that for both single sided (pads only on one side of the flex) and double sided (pads on both the top and bottom of the flex), employing the annular member with a second metal deposited atop has greatly improved the tear strength against shearing forces. Consider the following table: 
     
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Test 
                 Tear Force 
                 (lbs.) 
                   
                   
                   
                   
                   
                 Std. Dev. 
                 Avg. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 0.56 
                 0.57 
                 0.57 
                 0.28 
                 0.57 
                 0.57 
                 0.56 
                 0.108452 
                 0.525714 
               
               
                 2 
                 1.14 
                 1.13 
                 1.13 
                 1.13 
                 1.13 
                 1.13 
                 1.14 
                 0.00488 
                 1.132857 
               
               
                 3 
                 1.98 
                 2.27 
                 1.65 
                 1.42 
                 1.72 
                 1.42 
                 1.99 
                 0.31725 
                 1.778571 
               
               
                 4 
                 0.57 
                 0.85 
                 0.85 
                 0.56 
                 0.85 
                 0.57 
                 1.13 
                 0.213263 
                 0.768571 
               
               
                 5 
                 2.01 
                 2 
                 2.28 
                 2.27 
                 2.29 
                 2.58 
                 3.17 
                 0.403131 
                 2.371429 
               
               
                 6 
                 2.3 
                 2.3 
                 2.47 
                 3.14 
                 3.75 
                 3.43 
                 3.46 
                 0.610503 
                 2.978571 
               
               
                   
               
             
          
         
       
     
     In table 1, tests 1 and 4 represent a single sided flex and double sided flex, respectively, that includes only a parting line between extensions (similar to  FIG. 1 ). In these two tests, the pull strength to tear the flex was less than 1 lb. Tests 2 and 5 are single and double sided flexes, respectively with only an annular member. In other words, there is no second metal deposited atop the annular member for these tests. 
     Tests 3 and 6 correspond with the present invention, in that test 3 is a single sided flex with an annular member disposed about the termination point of the parting line, the annular member having a second metal deposited thereon. Test 4, correspondingly, is a double sided flex with an annular member having the second metal deposited upon both sides. Note that test 3 increases tear resistance by 240% over the flex alone, and by 56% over the flex with only an annular ring. Test 6 increases the tear resistance by 285% over the flex alone, and by 25% over the flex with only an annular ring. 
     Referring now to  FIGS. 6–8 , illustrated therein are some of the various folding applications to which the invention may be applied.  FIGS. 6–8  do not represent all of the applications, but rather are for exemplary purposes only. It will be clear to those of ordinary skill in the art who have the benefit of this disclosure that other scenarios also exist. 
     With respect to  FIG. 6 , the parting line  601  is U-shaped, thereby allowing a U-shaped extension  602  to fold across the central member  600  of the flex  60  along folding line  604 . In this embodiment, the annular members  603  may run 270 degrees in length. 
     With respect to  FIG. 7 , the parting line  701  is an elongated L-shape, thereby allowing the extension  702  and central member  700  to form a T-shape. The extension  702  would then fold along folding line  704  atop the central member  700  of the flex  70 . In this embodiment, the annular members  703  may run up to 270 degrees in length. 
     With respect to  FIG. 8 , the parting line  801  is also L-shaped, but is disposed within the boundaries of the central member  800 , thereby allowing the extension  802  to extend within the overall limits of the flex  80 . As such, the extension  802  becomes an L-shaped triangle, folding atop the central member  800  across folding line  804 . In such an embodiment, the annular members  803  may run up to 315 degrees in length. 
     With respect to  FIG. 9 , the parting line  901  is again L-shaped, but is configured differently from the flex of  FIG. 8 . The flex  90  of  FIG. 9  is designed to allow the extension  902  to “flop” below the central member  900  by folding along line  904 . This is often desirable when one component is much larger than the rest. A flopping extension  902  compensates for the additional height of the component. 
     In this configuration, the annular members  903  are positioned at the vertex of the parting line  901 , and at the termination point. An annular member  903  at the vertex is preferable because force  905  tends to shear flexes comprising right angle cuts. 
     Referring now to  FIG. 5 , illustrated therein is one exemplary application for a flex in accordance with the invention. This exemplary application is that of a rechargeable battery pack  50 . A cell  505  is positioned within a housing  504 . As noted above, rechargeable cells require certain circuits, like safety circuits, charging circuits, etc., for operation. Such a circuit may be constructed on a flexible circuit in accordance with the invention. 
     The flex  507  includes a central member  500  and a folded extension  501 . In keeping with the exemplary geometry of the preceding figures, a parting line  508  exists between the first extension  901  and the second extension. An annular member  503  with a second metal deposited atop is disposed about the termination point of the parting line  508 . A circuit is constructed on the flex  507  by coupling electrical components  506  to the pads of the flex. While a battery is one application, it will be clear to those of ordinary skill in the art having benefit of this disclosure that other applications work equally well with the flex of the present invention. 
     While the preferred embodiments of the invention have been illustrated and described, it is clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the following claims.