Abstract:
The invention removes copper from the concave side of a flex circuit around a bendable region and replaces it with a conductive epoxy to allow it to be formed to tighter bend radii than would otherwise be possible. After the flex circuit is shaped in a tight radius and attached to a mechanical structure, the conductive epoxy is cured to act as functional replacement of the removed copper.

Description:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with government support. The government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    This invention is directed to flexible printed circuit boards, and particularly, to a device and method for a flexible printed circuit board incorporating stripline or microstrip transmission lines that pass through a small radius bend. 
         [0004]    2. Related Art 
         [0005]    Flexible printed circuit boards or “flex” circuits are used in a wide variety of applications, where an electrical circuit must bend around corners or be flexed during operation. Typically, flex circuits are thin, light weight, flexible, and exhibit high routability. Generally, a flex circuit may be used as an interconnecting medium in a phased array architecture. In some cases, particularly when microwave signals are present, design considerations mandate that the flex circuit is a stripline construction of certain minimum thickness; which typically consists of a central trace sandwiched between two ground planes, which are spaced a certain distance apart. Two interposing low-loss dielectric material layers are used as insulators. Alternately, the flex circuit may feature a microstrip construction; which typically includes a trace and a single ground plane, spaced a specific distance apart, with a low-loss dielectric material as an insulating interposer. 
         [0006]    Generally, there is a minimum bend radius to which flex circuits may be formed without damaging the flex circuit. The minimum bend radius is a function of several aspects of the flex circuit geometry and the materials used, but the distance between the outermost metal layers of the flex circuit is a key parameter limiting the minimum bend radius. 
         [0007]    Many flex circuits have only one metal layer, or the distance between the outermost metal layers is minimized, so that the minimum allowable bend radius may also be minimized. Unfortunately, in some cases the distance between the outermost metal layers cannot be decreased below a particular value due to electrical design considerations or manufacturing limitations. This is often the case with flex circuits that incorporate a stripline or microstrip construction. 
         [0008]    When a flex circuit having two or more metal layers is formed to a bend radius that is less than allowable minimum, the external copper layers of the circuit tend to crack or buckle. Internal delamination has also been observed. In some cases concerning a flex circuit with a stripline construction, one or more central traces have broken, resulting in open circuits. This results in low manufacturing yields, and raises serious long-term reliability concerns. Typically, the copper ground plane on the convex side of the flex circuit cracks while the copper ground plane on the concave side buckles. When no cracking occurs, it is often because internal delamination has provided strain relief, sufficient to prevent cracking, but such delamination leads to additional reliability problems. 
         [0009]    What is needed is a structure and method that allow bending of the flex circuit around a small radius while preserving both the mechanical and electrical integrity of the design. 
       SUMMARY 
       [0010]    The invention provides a device and method for forming a flexible printed circuit board to a smaller bend radius than would otherwise be possible without damaging the circuit. This is done by removing copper from the concave side of the flex circuit in the bend region and replacing it with conductive epoxy in an uncured or semi-cured state. After the flex circuit is formed into a small radius bend, the conductive epoxy is cured to act as a functional replacement of the removed copper. 
         [0011]    In one aspect of the invention, a method is provided for forming a conformable circuit element. The method includes depositing a conductive layer on a first side of a flex circuit; etching the conductive layer to form an etched region; depositing a conductive epoxy on the etched region; bending the flex circuit along a bending axis to form a concave surface on the first side; and curing the conductive epoxy. 
         [0012]    In another aspect of the present invention, a flexible circuit is provided including at least an outside metal layer and an inside metal layer. A first dielectric layer is interposed between the outside metal layer and the inside metal layer. The inside metal layer includes an etched-out area. A layer of conductive epoxy is deposited on the inside metal layer having the etched-out area. 
         [0013]    This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention may be obtained by reference to the following detailed description of embodiments thereof in connection with the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The foregoing features and other features of the present invention will now be described with reference to the drawings. In the drawings, the same components have the same reference numerals. The illustrated embodiment is intended to illustrate, but not to limit the invention. The drawings include the following Figures: 
           [0015]      FIG. 1  illustrates a three dimensional packaging architecture for a Phased Array Antenna Element, in which a flexible printed circuit is typically used; 
           [0016]      FIGS. 2A  and PB shows a stackup of a multi-layer flex, in accordance with an embodiment of the present invention; 
           [0017]      FIG. 3  shows a bending geometry of the multi-layer flex of  FIGS. 2A and 2B  in accordance with an embodiment of the present invention; 
           [0018]      FIG. 4  shows locations of the trouble spots associated with prior art solution; 
           [0019]      FIG. 5A  shows a side view of a bent flex circuit in accordance with an embodiment of the present invention; 
           [0020]      FIG. 5B  shows a front view of the bent flex circuit (having a stripline configuration) of  FIG. 5A  in accordance with an embodiment of the present invention; 
           [0021]      FIG. 6B  shows a front view of the bent flex circuit (having a microstrip configuration) of  FIG. 6A  in accordance with an embodiment of the present invention; 
           [0022]      FIG. 7  shows a plot of an insertion loss captured on a network analyzer in accordance with an embodiment of the present invention; 
           [0023]      FIG. 8  shows a plot of a return loss captured on a network analyzer in accordance with an embodiment of the present invention; and 
           [0024]      FIG. 9  is a flowchart showing a method of producing a flex circuit in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    As shown in  FIG. 1  a multi-chip, three-dimensional packaging architecture  100  (hereinafter “module  100 ”), includes a pair of chip carries  110  and  110 A mechanically attached to a mandrel  104 . Electrically and mechanically coupled to the chip carries  110  and  110 A are bridge PWBs  112  and  112 A, respectively. A bent flex circuit  102  provides electrical connection to bridge PWBs  112  and  112 A. Guard shims  108  and  100 A are attached between chip carriers  110  and  110 A and bridge PWBs  112  and  112 A, respectively, and lids  106  and  106 A are used to cover the exposed surface of chip carriers  110  and  110 A. An aperture integrated wiring board (AIPWB)  114  is attached to mandrel  104  and electrically connected to chip carriers  110  and  110 A. 
         [0026]    In one embodiment, bent flex circuit  102  may be delivered in an unbent form, having the stack-up shown in  FIG. 2 . Flex circuit  102  includes a conductive paste epoxy  202  (hereinafter “epoxy  202 ”) used to form a base of flex circuit  102 , and used to contact mandrel  104  ( FIG. 1 ). Epoxy  202  is cured into a semi-solid state referred to as “b-stage”. 
         [0027]    Stacked on epoxy  202  is first metal layer  204 , first dielectric layer  206 , prepreg layer  208 , second metal layer  210 , second dielectric layer  212 , and third metal layer  214 . The metal layers may include any suitable metal material, such as copper. 
         [0028]    In one embodiment, flex circuit  102  may be formed to a bending profile, as shown in  FIG. 3 , where r 0   302  and r 1   304  are the internal bend radii. In this embodiment, flex circuit  102  is aligned and clamped to mandrel  104 . Mandrel  104  with flex circuit  102  are then inserted into a forming tool (not shown). In one embodiment, the forming tool has spring-loaded rollers that gently bend flex circuit  102  conforming it to the shape of mandrel  104 . Additional clamps are placed on the outside of the bent flex circuit  102  and the assembly is placed in an oven to finish curing b-stage epoxy  202 . Bent flex circuit  102  is attached to mandrel  104 , which provides the mechanical structure for module  100  ( FIG. 1 ). 
         [0029]    In one embodiment, the internal bend radius of flex circuit  102  may be between about 0.040 and 0.060 inches to accommodate half-lambda (λ/2) element spacing, where λis the wavelength of the antenna frequency. For example, the λ/2 element spacing dictates a module spacing that in turn dictates a bend radius of about 0.050 inches at 30 GHz. The bend radius is scaleable with the inverse of antenna frequency. However, in practice the larger, lower frequency antennas have additional requirements for multi-beam capability that require more space for interconnects. As a result, the internal bend radius required to meet operational objectives has remained relatively constant over a frequency range of 8 GHz to 30 GHz. In the current example, 0.056 inches is satisfactory. 
         [0030]    The thickness of flex circuit  102  may be determined by the spacing required between the outer ground planes; which is in turn determined by the dielectric constant of the substrate, the width of the internal transmission lines, and the desired characteristic impedance of the transmission lines. In one embodiment, practical limits on these parameters dictate that flex circuit  102  be about 0.013 inches thick, excluding the thickness of the exterior ground planes. The thickness of the exterior ground planes is on the order of 0.001 inches, thus most of the thickness of the flex circuit is due to the spacing between the exterior ground planes. 
         [0031]    Historically, problems occur when a flex of the thickness noted above is formed to the previously described internal bend radius. The problems include cracks on the surface, after the flex circuit is formed around mandrel  104 . In addition, metal can pull away from the dielectric causing delamination. In addition, buckling of the backside metal can develop.  FIG. 4  shows the typical location where these problem areas occur. 
         [0032]    The Institute for Interconnecting and Packaging Electronic Circuits maintains IPC-2223 as the design standard for flex circuit construction. Section 5.2.3.4.2 and  FIG. 5-7  of the Nov. 1998 edition set limits on the strain the copper can sustain in different situations. This standard also provides means of estimating the minimum bend radius that corresponds to the limiting strain. Table 1 from IPC-2223 lists applicable strain limits for rolled annealed copper and electrodeposited copper. The value for rolled annealed copper is applicable only if rolled annealed copper foil is used, and if no copper is electroplated over the top of the foil. In one embodiment, flex circuit  102  features electrodeposited copper foil with electroplated copper over the top. Thus the smaller strain limit may be applied in this example. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Maximum Permissible Strain 
               
               
                   
                   
                 in Copper when Flex is Formed 
               
               
                   
                 Case 
                 into Place 
               
               
                   
                   
               
             
             
               
                   
                 Rolled annealed copper 
                 ≦16% 
               
               
                   
                 Electrodeposited copper 
                 ≦11% 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 Resulting 
               
               
                   
                   
                   
                   
                 Minimum 
               
               
                   
                   
                   
                   
                 Internal Bend 
               
               
                   
                 Internal 
                 Effective 
                 Ground 
                 Radius for 11% 
               
               
                   
                 Bend 
                 Substrate 
                 Plane 
                 Strain Limit 
               
               
                   
                 Radius 
                 Thickness 
                 Thickness 
                 and No Cover 
               
               
                 Case 
                 (inches) 
                 (inches) 
                 (inches) 
                 Layers (inches) 
               
               
                   
               
             
             
               
                 Prior Art 
                 0.056 
                 0.013 
                 .002 
                 0.069 
               
               
                 Invention 
                 0.056 
                 0.005 
                 .002 
                 0.036 
               
               
                   
               
             
          
         
       
     
         [0033]    Table 2 shows the geometric parameters of the prior art. The computed minimum bend radius of 0.069 inches is greater than the previously described example of 0.040 to 0.060 inch range. Thus, theory agrees with experiment that the flex should crack under the design parameters of the prior art. 
         [0034]      FIGS. 5A and 5B  illustrate a stackup  500  in accordance with the present invention. Stackup  500  at bend region  504  includes outside metal layer  214  and intermediate metal layer  210 ; first dielectric layer  212  interposed between outside metal layer  214  and intermediate metal layer  210  and second dielectric layer  206  interposed between intermediate metal layer  210  and inside metal layer  204  all stacked upon epoxy layer  202 . In one embodiment, inside metal layer  204  is a stripline ground-plane. 
         [0035]    In one embodiment, a portion  502  of inside metal layer  204  in bend region  504  is etched away The area corresponding to portion  502  of metal layer  204  thus removed, is then re-filled with conductive epoxy  506 . 
         [0036]      FIGS. 6A and 6B  illustrate a stackup  600  in accordance with the present invention. Stackup  600  at bend region  614  includes outside metal layer  602  and second metal layer  606 ; dielectric layer  604  interposed between metal layer  602  and metal layer  606  metal layer  606  stacked upon epoxy layer  610 . In this embodiment, metal layer  606  is a microstrip ground-plane. 
         [0037]    In this embodiment, a portion  612  of metal layer  606  in bend region  608  is etched away. The area corresponding to portion  612  of metal layer  606  thus removed, is then re-filled with conductive epoxy  608 . 
         [0038]      FIG. 9  is a flowchart illustrating a method  900  of forming a bent flex circuit  102  in accordance with the present invention. 
         [0039]    Referring now to  FIGS. 5A ,  6 A and  9 , in operation, a circuit  102  is formed including at least an outside metal layer  214  or  602  and an inside metal layer  204  or  606 , such as copper metal layers. In step S 902  a portion of inside metal layer  204  or  606  is removed, such as by etching metal layer  204  or  606 , from bend region  504  or  614 . After portion  502  or  612  has been removed, in step S 904  a deposition process is used to re-fill bend region  504  or  614 , without using copper, to restore the electrical continuity of metal layer  204  or  606 . In one embodiment, the fill material is a conductive epoxy, like Epoxy Technologies EE149-6. 
         [0040]    At step S 906 , the fill material is subjected to B-stage curing. 
         [0041]    Thereafter, in step S 908 , flex circuit  109  may be formed around mandrel  104  to create the desired bend radius. The “bent” flex circuit  102  may then be cured to cause epoxy  202  or  610  to become structural and conductive. Beneficially, epoxy  202  or  610  can be selected to duplicate the electrical functions of the portion  502  or  612  of metal layer  204  or  606  that was removed. Although, epoxy  202  or  610  was previously present, it was used to bond the copper ground plane to mandrel  104 , and was not a direct part of the RF transmission structure. 
         [0042]    This approach is advantageous because the copper is a much stiffer material than either the dielectric materials or the b-stage epoxy during both elastic and plastic deformation. The difference is so pronounced that the mechanical characteristics of flex circuit  102  are almost entirely determined by the copper metal layer. 
         [0043]    Referring to  FIGS. 4 ,  5 A and  5 B when portion  502  is removed from metal layer  204 , the neutral axis  402  shifts from the center of stackup  500  (see  FIG. 4 ) to somewhere between the two remaining metal layers  210  and  214  as shown in  FIG. 5A . Mechanically, flex circuit  102  bends almost as if it included only copper layers  210  and  214 , and second dielectric  212 , even though prepreg layer  208  and first dielectric  206  are still present. The effective thickness of stackup  500  can be viewed as the combined thicknesses of second dielectric  212  and copper layers  214  and  210 , which in one embodiment is about 0.009 inches. As demonstrated in Table 2, when the pertinent parameters are applied to the IPC model, the resulting minimum bend radius becomes about 0.036 inches. 
         [0044]    A series of electrical measurements were performed on representative flex circuits, before and after the inside metal layer was removed.  FIGS. 6 and 7  show the results of these measurements. Traces  602  and  702  are the insertion and return loss signals, respectively, as measured on a typical flex circuit, Traces  604  and  704  are the insertion and return loss signals, respectively, as measured on a modified flex circuit  102  with the copper removed in accordance with the present invention. In the intended environment, the removed copper that forms part of the stripline ground-plane is replaced by the b-stage epoxy. However, for the purpose of verification, the parts were tested with no epoxy added; this condition represents the worst-case condition. The measurements demonstrated there is significant change to the electrical performance. 
         [0045]    Implementation of this invention allows flex circuit  102  to be formed to a tighter bend radii than would otherwise be possible, and allows the use of a broader range of materials, such as the use of electrodeposited copper rather than rolled annealed copper. 
         [0046]    In an alternate embodiment, the flex circuit is a microstrip construction. The microstrip construction may include a single layer of dielectric with conductors laminated to either side. The conductor on one side is etched into one or more conducting traces, while the copper on the other side is a monolithic ground plane. The procedure previously described is equally applicable to the microstrip construction when the epoxy substitution approach is applied to the ground plane side of the flex circuit. 
         [0047]    Although the present invention has been described with reference to specific embodiments, these embodiments are illustrative only and not limiting. Many other applications and embodiments of the present invention will be apparent in light of this disclosure and the following claims.