Abstract:
In order to limit the stress and strain applied to a printed circuit board while still maintaining flexibility, a flexible section of the printed circuit board is configured to have a non-linear portion that functions as a hinge when the flexible section is bent, flexed, twisted or otherwise subjected to a motion related force. The hinge configuration improves durability and flexibility while minimizing ripping and cracking of the printed circuit board, particularly interconnects within the flexible section and a transition region between the flexible section and a rigid section of the printed circuit board. The hinge is configured to have a non-linear shape, such as a serpentine or circuitous path that can include curved portions, different linear portions or some combination of curved and linear portions.

Description:
RELATED APPLICATIONS 
     This Patent Application claims priority under 35 U.S.C. 119(e) of the U.S. provisional patent application, Application No. 61/994,748, filed on May 16, 2014, and entitled “HINGE”, which is hereby incorporated in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is generally directed to printed circuit boards. More specifically, the present invention is directed to flexible printed circuit board configured as a hinge. 
     BACKGROUND OF THE INVENTION 
     Electronic devices are increasingly being developed so as to be worn by a user, such as in wearable electronics. As these wearable electronics gain traction in the marketplace, a new breed of devices that are able to bend, flex and stretch must be developed. These mechanical requirements present reliability challenges on mechanical components, circuit boards and other interconnects, as well as electronic components. For dynamic applications, especially where the desired amount of stretch and strain is unknown, it is important to strengthen the printed circuit board so that it is able to bend and twist without failing. Particularly, twisting and bending of a flexible circuit board can create points of failure of between rigid and flexible sections. It is desired to develop wearable electronics that limit the stress and strain to the constituent components while still maintaining flexibility and functionality. 
     SUMMARY OF THE INVENTION 
     In order to limit the stress and strain applied to a printed circuit board while still maintaining flexibility, a flexible section of the printed circuit board is configured to have a non-linear portion that functions as a hinge when the flexible section is bent, flexed, twisted or otherwise subjected to a motion related force. The hinge configuration improves durability and flexibility while minimizing ripping and cracking of the printed circuit board, particularly interconnects within the flexible section and a transition region between the flexible section and a rigid section of the printed circuit board. The hinge is configured to have a non-linear shape, such as a serpentine or circuitous path that can include curved portions, different linear portions or some combination of curved and linear portions. Examples of such non-linear shapes include, but are not limited to, an “S” shape or a sawtooth shape. 
     In an aspect, a circuit board is disclosed that includes one or more rigid sections, one or more flexible sections coupled to the one or more rigid sections, and one or more radius sections formed within portions of the one or more flexible sections that extend from the one or more rigid sections. In some embodiments, the one or more flexible sections are a flexible circuit board. In some embodiments, the one or more flexible sections are a stretchable circuit board. In some embodiments, each of the one or more radius sections has a radius greater than zero. In some embodiments, the one or more radius sections have a length of greater than zero to ten inches. In some embodiments, a stress applied to the bending, flexing or twisting of the one or more flexible sections is distributed across the one or more radius sections. In some embodiments, the one or more radius sections form a directional change in the portion of the one or more flexible sections in an X-Y direction that corresponds to a length and width of the one or more flexible sections. In some embodiments, the one or more radius sections form a directional change in the portion of the one or more flexible sections in an X-Z direction that corresponds to a length and thickness of the one or more flexible sections. In some embodiments, the one or more radius sections reduce crimping or creasing at a rigid to flexible transition area. 
     In another aspect, a circuit board is disclosed that includes a rigid section, and a flexible section coupled to the rigid section, wherein the flexible section includes a hinge having a non-linear shape with a plurality of directional change points. In some embodiments, when the flexible section is moved relative to the rigid section a stress is applied, and the stress is distributed across the hinge. In some embodiments, the stress is distributed to each of the plurality of directional change points of the hinge. In some embodiments, the flexible section is a flexible circuit board. In some embodiments, the flexible section is a stretchable circuit board. In some embodiments, each of the plurality of directional change points is a corner. In some embodiments, each of the plurality of directional change points is a curve. In some embodiments, each of the plurality of directional change points is either a corner or a curve. In some embodiments, the non-linear shape is a sawtooth pattern. In some embodiments, the non-linear shape is a S-shaped pattern. In some embodiments, the non-linear shape is a serpentine pattern. In some embodiments, the hinge has a length of greater than zero to ten inches. In some embodiments, each of the plurality of directional point changes form a directional change in the flexible section in an X-Y direction that corresponds to a length and a width of the flexible section. In some embodiments, each of the plurality of directional point changes form a directional change in the flexible section in an X-Z direction that corresponds to a length and a thickness of the flexible section. In some embodiments, one or more of the plurality of directional point changes form a directional change in the flexible section in an X-Y direction that corresponds to a length and a width of the flexible section, and one or more of the plurality of directional point changes form a directional change in the flexible section in an X-Z direction that corresponds to the length and a thickness of the flexible section. In some embodiments, the hinge reduces crimping or creasing at a rigid to flexible transition area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Several example embodiments are described with reference to the drawings, wherein like components are provided with like reference numerals. The example embodiments are intended to illustrate, but not to limit, the invention. The drawings include the following figures: 
         FIG. 1  illustrates a side view of a printed circuit board stack according to an embodiment. 
         FIG. 2  illustrates a top down view of printed circuit board according to an embodiment. 
         FIG. 3  illustrates a top down view of a printed circuit board with a flexible section configured as a hinge according to an embodiment. 
         FIG. 4  illustrates a top down view of a printed circuit board with a flexible section configured as a hinge according to another embodiment. 
         FIG. 5  illustrates a top down view of a printed circuit board with a flexible section configured as a hinge according to yet another embodiment. 
         FIG. 6  illustrates a top down view of a printed circuit board with a flexible section configured as a hinge according to still yet another embodiment. 
         FIG. 7  illustrates a side view of a simplified printed circuit board stack having a rigid section and a flexible section. 
         FIG. 8  illustrates a side view of a printed circuit board stack with mechanical strengtheners according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present application are directed to a printed circuit board hinge. Those of ordinary skill in the art will realize that the following detailed description of the printed circuit board hinge is illustrative only and is not intended to be in any way limiting. Other embodiments of the printed circuit board hinge will readily suggest themselves to such skilled persons having the benefit of this disclosure. 
     Reference will now be made in detail to implementations of the printed circuit board hinge as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts. In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer&#39;s specific goals, such as compliance with application and business related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure. 
     A printed circuit board can be configured having a multi-layer body, one or more layers of which include rigid sections and one or more layers of which include flexible sections. As used herein, “rigid” is a relative term and refers to those sections that are more rigid than other sections such as the flexible sections. The rigid sections and flexible sections can be configured in the same vertical stack, such as to form overlapping portions. Interconnects can be formed between the one or more rigid sections and the one or more flexible sections. In some embodiments, the interconnects are electrical interconnects, such as conductive traces. In other embodiments, the interconnects are optical interconnects, such as waveguides. It is understood that other types of interconnects are contemplated. 
       FIG. 1  illustrates a side view of a printed circuit board stack according to an embodiment. The printed circuit board includes a flexible section  30 , a flexible section  40  and a rigid section  50 . The printed circuit board includes a flexible base  4  that extends through the rigid section  50  in addition to the two flexible sections  30  and  40 . In some embodiments, the flexible base  4  is made of polyimide. It is understood that other flexible materials can be used. In other embodiments, the flexible base does not extend through the entire rigid section. For example, the flexible section  30  may include a first flexible base that extends partially into a first end of the rigid section, and the flexible section  40  may include a second flexible base that extends partially into a second end of the rigid section. 
     Interconnects can be formed on either or both surfaces of the flexible base  4 . In the exemplary configuration shown in  FIG. 1 , an interconnect layer  18  is formed on a first surface of the flexible base  4  and an interconnect layer  20  is formed on a second surface of the flexible base  4 . In some embodiments, the interconnect layers  18  and  20  are patterned copper interconnects that form electrically conductive interconnects. It is understood that other electrically conductive materials can be used. In other embodiments, the interconnect layers  18  and  20  form optical interconnects, such as waveguides. 
     In the rigid section, a pre-preg layer is added over each interconnect layer. As shown in  FIG. 1 , in the rigid section  50  a pre-preg  6  is added over the interconnect layer  18  and a pre-preg layer  8  is added over the interconnect layer  20 . Additional interconnect layers and pre-preg layers can be added to the rigid section. An interconnect layer is formed on the top-most pre-preg layer, such as an interconnect layer  22  formed on the pre-preg layer  6 . In the case of a printed circuit board having electronic components mounted onto both sides, for example the top side and the bottom side, of the printed circuit board, an interconnect layer is also formed on the bottom-most pre-preg layer, such as an interconnect layer  24  formed on the pre-preg layer  8 . One or more electronic components are coupled to the printed circuit board in the rigid section  50 . In the exemplary configuration of  FIG. 1 , an electronic component  10  is coupled to the interconnect layer  22  and an electronic component  12  is coupled to the interconnect layer  24 . 
     In each flexible section, a cover layer is added over each interconnect layer. As shown in  FIG. 1 , in the flexible section  30  a cover layer  14  is added over the interconnect layer  18  and a cover layer  16  is added over the interconnect layer  20 . In the flexible section  40  a cover layer  26  is added over the interconnect layer  18  and a cover layer  28  is added over the interconnect layer  20 . The cover layer material can acrylic, polyimide, acrylic-epoxy or other polymeric material with a Glass Transition Temperature (Tg) greater than 100 degrees Celsius. The cover layer material may or may not be filled with micro-particles or nano-particles or a woven reinforced material that can be organic or inorganic. Additional interconnect layers and cover layers can be added to each flexible section. An interconnect layer can be formed on the exposed top-most and/or bottom-most cover layers. 
       FIG. 2  illustrates a top down view of printed circuit board according to an embodiment. The printed circuit board includes a rigid section  60  and a flexible section  62 . The stack of the rigid section  60  can be similar to the stack of the rigid section  50  shown in  FIG. 1 , and the stack of the flexible section  62  can be similar to the stack of the flexible section  40 . Although the printed circuit board is shown in  FIG. 2  as having only a single flexible section  62 , it is understood that one or more additional flexible sections can be coupled to the rigid section  60 . The top down view shows that a footprint of the rigid section  60  has a rectangular shape. It is understood that alternative shapes are also contemplated. The flexible section  62  extends laterally from the rigid section  60 . The flexible section  62  is said to be shaped linearly since a central axis  66  of the flexible section  62  maintains a straight line as the flexible section extends laterally from the rigid section  60 . In other words, the central axis  66  is parallel to the x-axis along its entire length. 
     The interface between the rigid section  60  and the flexible section  62  forms a rigid to flexible transition. When the flexible section extends linearly from the rigid section, as does the flexible section  62 , and the flexible section  62  is subject to bending, flexing, twisting or other related movement relative to the rigid section  60 , stress is concentrated at the interface junction between the rigid section  60  and the flexible section  62 , and stress is particularly concentrated at the corner points  64  at the interface. Concentrated stress points provide points of failure that may ultimately result in damage to the interconnects at these points, such as severing of electrically conductive traces. 
     By configuring all or some of the flexible section in a non-linear shape, the stress is dispersed from the junction interface and is distributed across the length of the non-linear portion. The non-linear portion of the flexible section is referred to as a hinge. The hinge has one or more directional change points that form the non-linear shape. Each directional change, referred to as a hinge loop, can be gradual, such as a bend, arc, or curve, or more pronounced, such as a corner. Examples of such “corners” can include, but are not limited to, a 90 degree transition as in a square or rectangle, as shown in  FIG. 3 , or a transition less or greater than 90 degrees, as in a triangle or trapezoid, as shown in  FIG. 4 . In this manner the hinge can have, for example, a saw-tooth design or offset squares design.  FIG. 3  illustrates a top down view of a printed circuit board with a flexible section configured as a hinge according to an embodiment. A flexible section  72  of the printed circuit board is coupled to a rigid section  70 . The rigid section  70  and the flexible section  72  can have stack configurations similar to those previously described, such as the stacks shown in  FIG. 1 . Alternative stack configurations are also contemplated. A portion of the flexible section  72  is configured as a hinge. In the exemplary configuration shown in  FIG. 3 , the hinge has multiple direction changes, such as at directional change points  74 . The exemplary directional change points  74  form 90 degree corners. In some embodiments, the hinge portion of the flexible section is that portion of the flexible section immediately adjacent to the interface with the rigid section, as shown in  FIG. 3 . In other embodiments, the hinge portion of the flexible section is displaced from the interface. A length of the hinge can be the entire portion of the flexible section or some smaller portion. In an exemplary application, the hinge length is between 0 and 10 inches. 
       FIG. 4  illustrates a top down view of a printed circuit board with a flexible section configured as a hinge according to another embodiment. A flexible section  102  of the printed circuit board is coupled to a rigid section  100 . In the exemplary configuration shown in  FIG. 4 , the hinge has multiple direction changes, each of which forms a corner with an angle greater than 90 degree. Such a configuration forms a saw-tooth type hinge. 
     The directional change points can also be curves or bends.  FIG. 5  illustrates a top down view of a printed circuit board with a flexible section configured as a hinge according to yet another embodiment. A flexible section  82  of the printed circuit board is coupled to a rigid section  80 . In the exemplary configuration shown in  FIG. 4 , the hinge has multiple direction changes, such as directional change points  84 . The exemplary directional change points  84  form 90 degree curves. 
       FIG. 6  illustrates a top down view of a printed circuit board with a flexible section configured as a hinge according to still yet another embodiment. A flexible section  92  of the printed circuit board is coupled to a rigid section  90 . In the exemplary configuration shown in  FIG. 6 , the hinge has multiple direction changes, each of which forms a curve with an angle greater than 90 degree. Such a configuration forms a serpentine type hinge. 
     It is understood that a hinge can be configured having curves, corners, curves and corners, or other types of direction changes that are different than those shown in  FIGS. 3-6 . 
     In some embodiments, the hinge is pre-formed by pressing a flexible circuit into heated inter-locking dies and then cooling. It will relax much of the way, but still retain some bend. This may need to occur after PCBA solder reflow, so it doesn&#39;t interfere with SMT placement. In some embodiments, the flexible circuit is weaved through a plastic clip (with 2 or more openings) just before placement in a mold. The clip should ideally be fairly soft and have a melting point substantially above that of the molding material to remain permanent, or be made of the same material as the mold or with a melting point that is slightly higher, and merge with the molding material during the injection. 
     The hinge in the flexible section enables the flexible section to mechanically move without damaging, or minimize the propensity to damage, the interconnects within the flexible section. Damage can manifest as an open circuit or a higher resistance circuit due to mechanical stress. By creating one or more directional change points within the flexible section, one or more stress points of the flexible section are modified. The one or more directional change points determine where the flexible section bends and are able to disperse stress over a greater length. In this manner, the stress to the printed circuit board may be dispersed or moved to different areas so there is less chance that the interconnects will break and subsequently fail as the flexible section is twisted and bent. Additionally, the one or more directional change points decrease the chance that the flexible section will crimp or crease at the rigid to flexible transition area. 
     In some embodiments, the directional change points are formed in the X-Y direction, where the X-direction corresponds to the length of the flexible circuit and the Y-direction corresponds to the width. In some embodiments, the directional change points are formed in the X-Z direction, where the Z-direction corresponds to a thickness of the flexible section. An example of directional change points configured in the X-Z direction is shown in  FIG. 7 .  FIG. 7  illustrates a side view of a simplified printed circuit board stack having a rigid section  110  and a flexible section  112 . An abbreviated number of layers in the printed circuit board stack are shown in  FIG. 7 . In some embodiments, the directional change points are formed in all three directions, X-Y-Z. Inclusion of directional change points in the Z-direction provides extra length to the flexible section thereby allowing certain degree of in-plane stretchability. This allows the bending, twisting, buckling or any type of combination of these dynamic motions to happen without over challenging the critical areas, such as the junction interface. 
     More specifically, directional change points in all three directions includes almost all in-plane and out-of-plane deformations and their combinations. For example, stretching and compression are in-plane deformation, while bending, twisting and buckling more often happen out-of-plane. Especially when over molded with soft stretchable materials, like soft thermoplastic polyurethane (TPU), the strechability of TPU allows the flexible section to deform according to the environment motion asserted, while the degree of the stretchable percentage of TPU, if designed correctly (less than the stretchable percentage of the flexible section), can protect the flexible section from being fully stretched and hence ruptured. 
     In some embodiments, the one or more directional change points are built into the electronics. 
     The mechanical hinge enables a higher degree of bending, flexing and twisting, stretching at the rigid to flexible transition area to maintain electrical continuity in the flexible section. The mechanical hinge also enables a degree of stretchability along the length of the flexible section. Examples of methods of fabricating the hinge include, but are not limited to, die-cutting, laser cutting, milling, water jet cutting, or photo-definable polyimide patterning. A support film may be applied before or after cutting to aid in handling. This support film may be permanent or temporary depending on the final use case of the product. 
     In some embodiments, the printed circuit board also includes one or more additional mechanical strengtheners, such as a film or woven glass material that is resistant to ripping or cracking. One or more mechanical strengthener layers can be added throughout the body of the printed circuit board. The one or more mechanical strengthener layers strengthen the flexible section so as to minimize or prevent ripping or cracking as the printed circuit board is bent, flexed and twisted. The one or more mechanical strengthener layers can be attached at one or more specific locations through the stack in order to strengthen the printed circuit board. The mechanical strengtheners are used in addition the hinge portion of the flexible section. 
       FIG. 8  illustrates a side view of a printed circuit board stack with mechanical strengtheners according to an embodiment. The printed circuit board of  FIG. 8  is similar to the printed circuit board of  FIG. 1  with the addition of one or more mechanical strengtheners  109 . The mechanical strengtheners  109  can be added onto the outermost cover layer, as shown in  FIG. 8 , and/or added throughout the body of the printed circuit board. The mechanical strengtheners  109  strengthen the printed circuit board so it is not ripped or cracked as the circuit is bent, flexed and twisted. The mechanical strengtheners  109  are attached at one or more specific locations in order to strengthen the printed circuit board. The mechanical strengtheners  109  can be added on the inner layers and/or outer layers of the printed circuit board depending upon the desired application. 
     The present application has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the printed circuit board hinge. Many of the components shown and described in the various figures can be interchanged to achieve the results necessary, and this description should be read to encompass such interchange as well. As such, references herein to specific embodiments and details thereof are not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications can be made to the embodiments chosen for illustration without departing from the spirit and scope of the application.