Abstract:
A rigid flex circuit board and a method of fabricating a rigid flex circuit board. The method comprising forming a stack of at least two layers of at least one of a flexible material, prepreg material, insulative material, or conductive material over a flexible core to form a structure, wherein the structure comprises a first rigid portion, a second rigid portion, a flexible portion extending between the first and second rigid portions, and a removable rigid portion extending between the first rigid portion and the second rigid portion, processing the structure to form interconnects; and removing the removable rigid portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit of U.S. provisional patent application Ser. No. 60/823,679, filed Aug. 28, 2006, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Embodiments of the present invention generally relate to a multilayer rigid flex printed circuit board.  
         [0004]     2. Description of the Related Art  
         [0005]     Rigid flex circuit boards comprise at least one portion that is rigid and another portion that is flexible such that the rigid portion can be manipulated when installing the circuit board. The rigid portions contain electrical traces that are conductively connected through the flexible portion to interconnect at least two rigid portions. In a typical rigid flex printed circuit board construction, a flexible printed circuit portion extends from one or more edges around the periphery of rigid portion or portions. The rigid portions are typically used for mounting electronic components, connectors and hardware. The flex portion on the other hand serves to connect the various rigid portions while allowing rigid portions to be located in hardware equipment on different planes or at different angular orientations with respect to each other.  
         [0006]     In certain applications, the flexible portion extends from a single rigid portion and terminates into “fingers”; thus forming a flexible cable. The fingers can be used to attach to zero insertion force (ZIF) connectors.  
         [0007]     As the density of electronic circuitry has become greater over the years, more complex multilayer rigid flex circuit boards have evolved with boards now having a dozen or more patterned conductive circuit layers. The fabrication of such boards includes materials such as pre-impregnated (prepreg) fiberglass epoxy sheet spacers or bonding material, in various polyimide, aramid or epoxy/glass copper clad laminates. The use of some materials leads to a number of problems including moisture absorption, cracking, and fractures. Furthermore, a serious problem that arises from some manufacturing techniques is the fracturing of the copper foil when sanding processes are applied to planarize the vias within a rigid portion of the rigid flex circuit board. An unequal sanding force across the rigid-to-flex interface causes the copper foil at the interface to fracture.  
         [0008]     Therefore, there is a need in the art for an improved rigid flex circuit boards as well as an improved method of manufacturing rigid flex circuit boards.  
       SUMMARY OF THE INVENTION  
       [0009]     Embodiments of the present invention comprise a rigid flex circuit board and a method of fabricating a rigid flex circuit board. The method comprising forming a stack of at least two layers of at least one of a flexible material, prepreg material, insulative material, or conductive material over a flexible core to form a structure, wherein the structure comprises a first rigid portion, a second rigid portion, a flexible portion extending between the first and second rigid portions, and a removable rigid portion extending between the first rigid portion and the second rigid portion, processing the structure to form interconnects; and removing the removable rigid portion. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0011]      FIG. 1  depicts a top view of a rigid flex circuit board in accordance with one embodiment of the present invention;  
         [0012]      FIG. 2  depicts a top view of a rigid flex circuit board during manufacture showing the lateral slots having been previously formed in layers of the circuit board in accordance with one embodiment the present invention;  
         [0013]      FIG. 3  depicts a top view of the structure of the rigid flex circuit board indicating where longitudinal slots are cut into the circuit board;  
         [0014]      FIG. 4  depicts a side cross sectional view of a stack of layers that forms the rigid flex circuit board in accordance with the present invention;  
         [0015]      FIG. 5  depicts a cross sectional view of the rigid flex circuit board after final processing;  
         [0016]      FIG. 6  depicts a side cross sectional view of a stack of layers that form a rigid flex cable in accordance with a second embodiment of the invention;  
         [0017]      FIG. 7  depicts a cross sectional view of the embodiment of  FIG. 6  at an intermediary fabrication step;  
         [0018]      FIG. 8  depicts a cross sectional view of the embodiment of  FIG. 6  and  7  after a final fabrication step;  
         [0019]      FIG. 9  depicts a cross sectional view of a stack of layers that form a third embodiment of a rigid flex cable;  
         [0020]      FIG. 10  depicts a cross sectional view of the embodiment of  FIG. 9  at an intermediary fabrication step; and  
         [0021]      FIG. 11  depicts a cross sectional view of the embodiment of  FIGS. 9 and 10  after a final fabrication step.  
     
    
     DETAILED DESCRIPTION  
       [0022]      FIG. 1  depicts a top view of a rigid flex circuit board  100  comprising rigid sections  104  and  106  and a flexible section  102 . To facilitate a transition between the rigid sections  104  and  106  and the flexible section  102  a pair of cantilevers  108  and  110  extend from the rigid sections  104  and  106  onto the flexible section  102 . The cantilevers are formed as a “dual lip” structure as discussed below.  
         [0023]      FIG. 2  and  FIG. 3  should be view together to understand one embodiment of the fabrication process of the present invention.  FIG. 2  depicts a top view of the rigid flex circuit  100  having the rigid sections  104  and  106  and the flexible section  102  spaced therebetween. To facilitate creating the flexible portion, a pair of lateral slots  200  and  202  are routed, milled or otherwise formed into a stack of layers that comprise the rigid flex circuit board  100 . Generally, the lateral slots  200  and  202  are preformed in the layers prior to stacking the layers. After all the processing is complete, longitudinal slots  300  and  302  are routed into the structure to release a central portion  304 . Until the central portion  304  is released, the structure is rigid.  
         [0024]      FIG. 4  depicts a side, cross sectional view of  FIG. 2  along lines  4 - 4 . The stack of materials that form the stack  450  comprise the flexible core  400  generally comprising a flexible polyimide, FR-4, CUTE® or any other flexible core material that is clad with copper  402 A and  402 B on at least one side. Generally, circuit traces or planes are formed from the copper on both sides of the core. These traces/planes carry the electrical signals to/from the rigid portions of the circuit board across the flexible portion  102 . A layer  414 A and  414 B of flexible soldermask is applied to an area that defines the flexible portion  102  such that than insulative layer  414 A and  414 B is formed that protects the copper traces on the flexible core. A layer  404 A and  404 B of prepreg is applied to the assembly, where the layers  404 A and  404 B define an aperture in which is the insulative layer  414 A,  414 B. The layers  404 A and  404 B overlap the edge of the layers  414 A and  414 B. Atop the layer of prepreg is positioned a blank FR4 core  406 A and  406 B that is cured to be stiff. The layers  406 A and  406 B comprise a lateral slot  202 , where the edge of layers  406 A and  406 B partially overlap the edge of layers  404 A and  404 B. This forms the “dual lip” cantilever. In this manner, the dual lip cantilever  110 ,  108  provides a strong edge along which the flexible portion  102  bends. Atop of the cured FR4 core are layers of prepreg  408 A and  408 B. In one embodiment of the invention, these layers  406 A,  406 B and  408 A,  408 B, comprise the lateral slots  200 ,  202  that are preformed in the layers before stacking. Note that the slots are aligned with the dual lip cantilever  108  and  110 .  
         [0025]     Atop those layers is positioned a copper foil  410 A and  410 B in which traces and planes will be etched to facilitate mounting of integrated circuits to the rigid portion of the board. Subsequent to applying the copper foil, the copper foil is etched using photomask layer  412 A and  412 B to define traces for the circuitry on the rigid portions  104  and  106  of the circuit board  100 . Additionally, via holes may be drilled and plated, as well as sanded, as needed. Upon etching the copper foil to form traces, the copper foil in the flexible portion  102  is removed.  
         [0026]      FIG. 3  depicts the longitudinal slots  300  and  302  having been cut on either side of the flexible region. The longitudinal slots  300  and  302  are routed, milled or otherwise formed into the circuit board to contact the lateral slots  200  and  202 . In this manner, the layers  406 A,  406 B,  408 A and  408 B above and below the flexible portion  102  that do not form part of this flexible portion are released from the structure allowing the rigid portions  104  and  106  to be freed and become movable. A cross sectional view of the rigid flex circuit board is shown in  FIG. 5 . The final form is a multilayer, e.g., four layer, rigid flex circuit board  100 . Importantly, the entire circuit board structure is competed prior to releasing the flexible portion. As such, the manufacturing process is performed upon a stable, rigid structure.  
         [0027]     The routing bit that is used for forming the lateral and longitudinal slots is in the range of 0.018 to 0.022 inches in diameter. The lip that forms the cantilever has a length from the rigid portion of approximately 0.010 inches. The flexible core may be CUTE® manufactured by Hitachi, flexible FR-4, a more traditional polyimide material or any other flexible core material. Using the rigid support during manufacturing that spans the flexible portion, sanding and other planarization techniques for the conductive foil on the rigid portions of the board can be performed without causing “kneeing” or other problems with board manufacture.  
         [0028]     To summarize the manufacturing process, the process begins with a flexible core material supporting a pattern of circuit traces on both surfaces, an insulative layer of flexible photomask material is applied over both sides of the flexible portion, and a first layer of prepreg material having a precut opening is positioned over the core material. The opening is aligned with the insulative layer. Additional layers of FR-4 and prepreg material are cut (routed) to form a lateral slot that will be aligned with a lateral edge of the flexible portion. These layers are stacked on both sides of the structure. A layer of foil is applied to both sides of the structure. The entire stack is cured at a pressure, temperature and an amount of time sufficient to harden the materials of the stack (except for the flexible core, its traces, and the flexible insulative layer). Once cured, the foil layer is drilled and blind or buried interconnects are filled. Sanding is performed, if necessary. Then, the through holes are drilled and plated. The circuit traces are patterned and etched into the foil. Longitudinal slots are cut (routed) into the stack to connect the lateral slots such that a region above the flexible portion is removed (released). In this manner, the entire processing of the rigid flex circuit board is performed while the structure is rigid. The last process step releases the flexible portion to complete the rigid flex circuit board.  
         [0029]     One other feature depicted in  FIG. 4  is the use of the prepreg material  408 A and  408 B having an edge that is cut back from the edge of the lateral slot  200 ,  202 , i.e., the slot in layers  408 A,  408 B is wider than the slot inlayers  406 A and  406 B. By using such a prepreg cut back, during curing, the prepreg material will not flow into the lateral slot  200 ,  202 .  
         [0030]     Generally, to align all of the layers that are stacked and then cured to form the rigid flex circuit board  100 , an alignment system such as ACCULINE® of the Multiline Company of Farmingdale, N.Y. which uses a four slot printed circuit board punch and a plurality of pins to hold and retain the circuit board stack during assembly and curing.  
         [0031]     In one embodiment of the invention, the insulating material of the flexible soldermask is an ultraviolet curable material fabricated by Lackwerke Peter GMBH or the heat curable coating preparation sold by ASI-Coates.  
         [0032]     In one embodiment of the invention, after all of the layers have been stacked the plates are aligned on either side of the stack of material and pressure is applied at 300 to 350 psi at a temperature of 350° for several hours to form a hard unitary board structure. These process parameters are exemplary, the parameters will vary depending upon the materials used and the respective thicknesses of the materials. Once cured, the outer copper layers are then drilled, plated, patterned and etched, both to form the desired circuit features in the layer and to remove the copper portion overlying the flexible portion.  
         [0033]      FIGS. 6, 7  and  8  depict cross sectional views of a second embodiment of the invention at various stages during fabrication. This embodiment is a rigid flex cable having a rigid end  800  and a flexible end  802 , where the flexible end  802  is adapted for insertion into a zero insertion force (ZIF) connector.  
         [0034]      FIG. 6  depicts a structure  600  comprising a stack  602  of layers being substantially similar to the stack  450  of  FIG. 400 . The difference between stack  450  and stack  600  is that the flexible insulator layer  604  does not extend completely across the flexible portion  102 . Additionally, gold (or other highly conductive material) is deposited to form at least one pad  606 A and  606 B at the distal end of the traces on the foil layers  402 A,  404 B located on both sides of the flexible portion. The position and deposition of the at least one pad occurs during formation of the traces prior to forming the stack  602 .  
         [0035]     As shown in  FIG. 7 , the structure  600  is processed and longitudinal slots cut to release the flexible portion  102  between the rigid portions  104  and  106 . As shown in  FIG. 8 , the rigid portion  106  is removed from the distal end of the flexible portion  102 . This removal is facilitated by routing, punching, scribing, snapping or otherwise detaching the rigid portion  106 . The result is a rigid flex cable  800  having fingers  802  positioned along the distal end of the flexible portion  102 . These fingers  802  are adapted (sized and shaped) to insert into a double sided zero insertion force (ZIF) connector.  
         [0036]      FIGS. 9, 10  and  11  depict a series of cross sectional views of a third embodiment of the invention during various fabrication steps of making a rigid flex cable used for single sided ZIF connectors. The structure  900  is similar to the structure  600  of  FIG. 6 , except the top and bottom arrangements are asymmetrical. The top portion  904  of a stack  902  contains the at least one conductive contact (pad  606 A). The bottom portion  906  is arranged in a manner similar to the structure  600 , i.e., a rigid portion.  
         [0037]     As shown in  FIG. 10 , when released, the flexible portion  102  spans between rigid portions  104  and  106 . A portion of portion  106  is removed at the pad  606 A. Consequently, the pad  606 A that forms part of a finger  1102  rests upon a rigid portion  1104 . The rigid portion  1104 , flexible portion  102  and rigid support  1104  form a rigid flex cable  1100  that comprises a finger  1102  adapted for insertion into a one-sided ZIF connector.  
         [0038]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.