Abstract:
In order to limit the stress and strain applied to a printed circuit board while still maintaining flexibility, multiple small rigid mesas are formed on a flexible printed circuit, where the rigid mesas are physically isolated by trenches formed around their perimeters. Individual electronic components are attached to the multiple rigid mesas. These trenches form openings at which the printed circuit board is enabled to flex, bend or twist, thereby minimizing, if not eliminating, resulting stress applied to the interconnection between the electronic components and the rigid mesas.

Description:
RELATED APPLICATIONS 
     This Patent Application claims priority under 35 U.S.C. 119(e) of the U.S. provisional patent application, Application No. 61/931,503, filed on Jan. 24, 2014, and entitled “Stress Relief for Rigid Components on Flexible Circuits”, 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 circuits having attached rigid components. 
     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 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, multiple small rigid mesas are formed on a flexible printed circuit, where the rigid mesas are physically isolated by trenches formed around their perimeters. Individual electronic components are attached to the multiple rigid mesas. These trenches form openings at which the printed circuit board is enabled to flex, bend or twist, thereby minimizing, if not eliminating, resulting stress applied to the interconnection between the electronic components and the rigid mesas. 
     In an aspect, a circuit board is disclosed. The circuit board includes a flexible printed circuit comprising a first exterior surface and a second exterior surface opposite the first exterior surface. The circuit board also includes a plurality of rigid sections coupled to the first exterior surface, wherein each rigid section is physically separated from each other rigid section, and each rigid section includes one or more electrical pads. The circuit board also includes a plurality of electronic components, one electrical component coupled to a corresponding one rigid section. In some embodiments, the flexible printed circuit comprises a plurality of stacked layers. In some embodiments, the plurality of stacked layers comprises a plurality of alternating flexible layers and electrically conductive layers. In some embodiments, each flexible layer comprises polyimide or other polymer material. In some embodiments, each electrically conductive layer comprises patterned electrically conductive interconnects. In some embodiments, each rigid section has a higher elastic modulus than each flexible layer. In some embodiments, each rigid section is made of a different material than each flexible layer. In other embodiments, each rigid section is made of the same material as each flexible layer, and each rigid section is thicker than each flexible layer. In some embodiments, the circuit board further comprises a plurality of vias wherein each via is coupled to one of the electrical pads of the rigid section and to an electrically conductive layer in the flexible printed circuit. In some embodiments, each electronic component comprises a plurality of connection pads, one connection pad coupled to a corresponding one electrical pad on the rigid section. In some embodiments, each rigid section consists of one of polyimide or other polymer material, a glass-reinforced polymer, or a printable polymer. In some embodiments, each rigid section has a higher elastic modulus than the flexible printed circuit. In some embodiments, each rigid section is made of a different material than the flexible printed circuit. In some embodiments, the circuit board further comprises one or more additional rigid sections coupled to the second exterior surface of the flexible printed circuit, wherein the one or more additional rigid sections increase an overall rigidity of the circuit board. 
     In some embodiments, the circuit board further comprises another plurality of rigid sections and another plurality of electronic components. The other plurality of rigid sections is coupled to the second exterior surface, wherein each rigid section of the other plurality is physically separated from each other rigid section of the other plurality, and each rigid section of the other plurality includes one or more electrical pads. One electrical component of the other plurality is coupled to a corresponding one rigid section of the other plurality. In some embodiments, the circuit board further comprises another plurality of vias wherein each via of the other plurality is coupled to one of the electrical pads of the rigid section of the other plurality and to an electrically conductive layer in the flexible printed circuit. In some embodiments, each electronic component of the other plurality comprises a plurality of connection pads, one connection pad coupled to a corresponding one electrical pad on the rigid section of the other plurality. 
     In another aspect, another circuit board is disclosed. The circuit board includes a flexible printed circuit comprising an exterior surface. The circuit board also includes a plurality of rigid sections coupled to the exterior surface, wherein each rigid section is physically separated from each other rigid section, and each rigid section includes an electrical pad. The circuit board also includes a plurality of electronic components wherein each electronic component comprises a plurality of connection pads, one connection pad is coupled to the electrical pad of a corresponding one rigid section. 
     In yet another aspect, another circuit board is disclosed. The circuit board includes a flexible printed circuit comprising an exterior surface having a plurality of electrical pads. The circuit board also includes a plurality of rigid sections positioned within the flexible printed circuit, wherein each rigid section is physically separated from each other rigid section. The circuit board also includes a plurality of electronic components wherein each electronic component comprises a plurality of connection pads, wherein one connection pad is coupled to a corresponding electrical pad on the exterior surface of the flexible printed circuit. 
    
    
     
       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 cut out side view of a printed circuit board according to an embodiment. 
         FIG. 2  illustrates a cut out side view of a printed circuit board according to another embodiment. 
         FIG. 3  illustrates a cut out side view of a printed circuit board according to another embodiment. 
         FIG. 4  illustrates a cut out side view of a printed circuit board according to another embodiment. 
         FIG. 5  illustrates a process for fabricating a printed circuit board according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present application are directed to a flexible printed circuit having attached rigid components. Those of ordinary skill in the art will realize that the following detailed description of the flexible printed circuits having attached rigid components is illustrative only and is not intended to be in any way limiting. Other embodiments of the flexible printed circuits having attached rigid components 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 flexible printed circuits having attached rigid components 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 plurality of rigid sections, referred to as rigid mesas, attached to a flexible printed circuit (FPC). The FPC includes a plurality of stacked layers, the layers made of flexible layers and conductive layers. In some embodiments, each flexible layer is made of polyimide. It is understood that other flexible materials can be used. In some embodiments, each conductive layer includes patterned copper interconnects that form electrically conductive interconnects, such as conductive traces. It is understood that other electrically conductive materials can be used. In some embodiments, the stacked layers are arranged as alternating flexible layers and conductive layers. As used herein, “rigid” is a relative term and refers to those sections that are more rigid than other sections such as the flexible layers. Electronic components are electrically and mechanically coupled to the rigid mesas. In some embodiments, one electronic component is coupled to a corresponding one rigid mesa. Each electronic component includes a plurality of connection pads, one connection pad is coupled with a corresponding one electrical pad on the mesa. In other embodiments, each rigid mesa includes a single electrical pad, and each electronic component is coupled to multiple rigid mesas such that each connection pad on the electronic component is coupled to a corresponding one rigid mesa. 
       FIG. 1  illustrates a cut out side view of a printed circuit board according to an embodiment. The printed circuit board  10  includes a FPC  12 , rigid mesas  14  and electronic components  16 . The FPC  12  includes a plurality of stacked layers, the layers made of flexible layers and conductive layers. In some embodiments, each flexible layer is made of polyimide. It is understood that other flexible materials can be used. In some embodiments, each conductive layer includes patterned copper interconnects that form electrically conductive interconnects, such as conductive traces. It is understood that other electrically conductive materials can be used. In some embodiments, the stacked layers are arranged as alternating flexible layers and conductive layers. The rigid mesas  14  are formed on a first exterior surface of the FPC  12 . In the exemplary configuration shown in  FIG. 1 , the first exterior surface is the bottom surface of the FPC stack. Each rigid mesa  14  is physically separated from an adjacent rigid mesa  14  by an opening  22 . In some embodiments, the rigid mesas  14  are formed by laminating a rigid sheet to the first exterior surface of the FPC  12 . The rigid mesas  14  are formed by laser cutting or stamping out the rigid sheet to remove portions of the rigid sheet, such as cutting a trench in the rigid sheet around where each electronic component is mounted, thereby forming the openings where the portions of the rigid sheet are removed. The opening  22  forms a point of flexion where the printed circuit board  10  can bend or twist with reduced stress to the connection formed between the rigid mesa  14  and the electronic component  16 . 
     A plurality of vias  18  are formed through each rigid mesa  14 . In the exemplary configuration shown in  FIG. 1 , there are four vias  18  for each rigid mesa  14 . It is understood that more or less than four vias can be formed through each rigid mesa. Each via  18  is coupled to a conductive layer within the FPC  12 . Although each of the vias  18  is shown in  FIG. 1  as being coupled to the same conductive layer, it is understood that each via can extend to any one of the conductive layers or electrical interconnects formed in the FPC  12 . An electrical pad  20  is coupled to each of the vias  18 , providing an electrical interconnect point. An electronic component  16  is coupled to each mesa  14 . Each electronic component  16  includes a plurality of connection pads (not shown), each one of which is coupled to a corresponding one electrical pad  20  for providing an electrical interconnection. The electrical pad can be mechanically and electrically connected to the connection pad using any conventional means, including but not limited to solder or electrically conductive adhesive. Examples of electronic components include, but are not limited to, LEDs, integrated circuits (ICs), capacitors, resistors and inductors. It is understood that the electronic components  16  can be of the same or different types. Some electronic components may have as few as two connection pads, such as a resistor. Other electronic components, such as an IC, may have a few hundred connection pads. 
     Each rigid mesa  14  is shown in  FIG. 1  to have the same size footprint as the attached electronic component  16 . It is understood that the footprint of the rigid mesa can be larger or smaller than the attached electronic component. Both rigid mesas  14  and electronic components  16  are shown in  FIG. 1  as having the same size and shape. It is understood that each rigid mesa may have a different shape. It is also understood the each rigid mesa may be differently shaped than its corresponding attached electronic component. 
     Each rigid mesa has a higher elastic modulus than the FPC. In some embodiments, each rigid mesa is made from polyimide, or other polymer material, a glass-reinforced polymer such as FR4, or a printable polymer. In some embodiments, the flexible layers of the FPC are made from polyimide or other polymer material. The flexible layers and the rigid mesa can be made of different types of polyimide, where the polyimide used for the rigid mesa has a higher elastic modulus than the polyimide used for the flexible layers. The rigid mesas can alternatively or also be made more rigid than the flexible layers by increasing the thickness layer of the rigid mesa relative to a thickness of the flexible layers of the FPC. It is well known that rigidity increases with increasing thickness. In the case where rigidity equals the cube of the thickness, doubling the thickness of the rigid mesa compared to a thickness of the flexible layer in the FPC, even where the same material is used for both, increases the rigidity of the rigid mesa by eight-fold. As such, the rigidity of certain areas can be adjusted without having to change the material, simply adjust the thickness. Preferably though the rigid mesa is made from a more rigid type of polyimide, such as FR4, than that of the flexible layers in the FPC. In some embodiments, the flexible layers of the FPC are made of a polyimide film with no glass reinforcement. 
     The overall flexibility/rigidity of the printed circuit board can be adjusted by selective use of materials and thicknesses for the rigid mesa and the FPC. The overall flexibility/rigidity can also be adjusted by adding one or more rigid mesas on the opposite exterior surface of the FPC  12 . These rigid mesas may or may not have the same dimensions and/or relative placement on the FPC as compared to the rigid mesas  14 . Electronic components are not attached to theses rigid mesas, as these rigid mesas function strictly for adjusting the overall flexibility/rigidity of the printed circuit board. 
     Forming a plurality of mesas enables mounting of rigid electronic components on flexible substrates while minimizing if not eliminating stress applied to the electronic components that can crack either the solder joints, adhesive or the electronic component itself. In an exemplary application, it is possible to manufacture a flexible display made of LEDs mounted to an array of rigid mesas formed on a flexible substrate. 
       FIG. 1  shows rigid mesas and attached electronic components formed on a single side of the FPC. In some embodiments, rigid mesas and attached electronic components can be formed on both opposing sides of the FPC.  FIG. 2  illustrates a cut out side view of a printed circuit board according to another embodiment. The printed circuit board shown in  FIG. 2  is the printed circuit board  10  from  FIG. 1  with additional rigid mesas  24  formed on the second exterior surface of the FPC  12 . Each rigid mesa  24  is physically separated from an adjacent rigid mesa  24  by an opening  32 . A plurality of vias  28  are formed through each rigid mesa  24 . In the exemplary configuration shown in  FIG. 2 , there are four vias  28  for each rigid mesa  24 . It is understood that more or less than four vias can be formed through each rigid mesa. Each via  28  is coupled to a conductive layer within the FPC  12 . Although each of the vias  28  is shown in  FIG. 2  as being coupled to the same conductive layer, it is understood that each via can extend to any one of the conductive layers or electrical interconnects formed in the FPC  12 . An electrical pad  30  is coupled to each of the vias  28 , providing an electrical interconnect point. An electronic component  26  is coupled to each mesa  14 . Each electronic component  26  includes a plurality of connection pads (not shown), each one of which is coupled to a corresponding one electrical pad  30  for providing an electrical interconnection. It is understood that the electronic components  26  can be of the same or different types. 
     The rigid mesas  24  and attached electronic components  26  are shown as being symmetrically dimensioned and aligned as the rigid mesas  14  and attached electronic components  16 . It is understood that the shape, dimension, and position of each rigid mesa and attached electronic component, as well as the number of rigid mesas and attached electronic components on one exterior surface of the FPC may be different than those on the other exterior surface. 
     In the embodiments shown in  FIGS. 1 and 2 , there is one rigid mesa per electronic component, and each rigid mesa includes a plurality of electrical pads. In other embodiments, one rigid mesa is formed for each electrical pad. In this configuration, there are multiple rigid mesas coupled to a single electronic component, as each electronic component has at least two connection pads.  FIG. 3  illustrates a cut out side view of a printed circuit board according to another embodiment. The printed circuit board  50  shown in  FIG. 3  is similar to the printed circuit board  10  of  FIG. 1  except that the printed circuit board  50  is configured to have one rigid mesa per electrical pad. The printed circuit board  50  includes an FPC  52 , rigid mesas  54  and electronic components  56 . In the exemplary configuration shown in  FIG. 3 , each electronic component  64  has four connection pads, and therefore there are four rigid mesas  54 , each having an electrical pad  60  and via  58 , coupled to each electronic component. The rigid mesas  54  attached to a common electronic component  56  are each physically separated from an adjacent rigid mesa  54  by an opening  64 . Rigid mesas  54  attached to different electronic components  56  are each physically separated by an opening  62 . Typically, the spacing between adjacent connection pads on a common electronic device is smaller than the spacing between adjacent electronic components, and as such the openings  64  are typically smaller than the openings  62 , although this is not necessarily the case. 
     Each via  58  is coupled to a conductive layer within the FPC  52 . The FPC  52  can be similarly configured as the FPC  12  in  FIG. 1 . Although each of the vias  58  is shown in  FIG. 3  as being coupled to the same conductive layer in the FPC  52 , it is understood that each via can extend to any one of the conductive layers or electrical interconnects formed in the FPC  52 . 
     In the embodiments shown in  FIGS. 1-3 , the rigid mesas are formed on an exterior surface of the FPC. In other embodiments, the rigid mesas can be formed within the interior of the FPC, and the electronic components are coupled to electrical pads on an exterior surface of the FPC.  FIG. 4  illustrates a cut out side view of a printed circuit board according to another embodiment. The printed circuit board  80  includes an FPC  82 , rigid mesas  84  and electronic components  86 . The FPC  82  is similar to the FPC  12  in  FIG. 1  in that the FPC  82  includes a plurality of stacked layers, the layers made of flexible layers and conductive layers. However, in the embodiment of  FIG. 4 , the rigid mesas  84  are formed within the interior of the FPC  82 . To effectively mount an electronic component, a rigid support is provided onto which the electronic component is mounted. In the embodiments of  FIGS. 1-3 , an electronic component is mounted directly onto such a rigid support. In the embodiment of  FIG. 4 , an electronic component is mounted indirectly onto such a rigid support, with intervening FPC material between the two. Each rigid mesa  84  is physically separated from an adjacent rigid mesa  84 . In some embodiments, the rigid mesas  14  are pre-formed and added to removed sections within the FPC. In other embodiments, the rigid mesas  84  are added as part of the FPC fabrication process. 
     A plurality of vias  88  are formed through a portion of the FPC  82 . Each via  18  is coupled to a conductive layer within the FPC  12 . Although each of the vias  88  is shown in  FIG. 4  as being coupled to the same conductive layer in the FPC  82 , it is understood that each via can extend to any one of the conductive layers or electrical interconnects formed in the FPC  82 . An electrical pad  90  is coupled to each of the vias  88 , providing an electrical interconnect point. An electronic component  86  is coupled to the FPC  82  via the electrical pads  90 . Each electronic component  86  includes a plurality of connection pads (not shown), each one of which is coupled to a corresponding one electrical pad  90  for providing an electrical interconnection. The electrical pad can be mechanically and electrically connected to the connection pad using any conventional means, including but not limited to solder or electrically conductive adhesive. It is understood that the electronic components  86  can be of the same or different types. In the exemplary configuration shown in  FIG. 4 , there are four vias  88  for each electronic component  86 . It is understood that more or less than four vias can be formed for each electronic component. 
       FIG. 4  shows one rigid mesa  84  corresponding to each electronic component  86 . Alternatively, more than one rigid mesa can correspond to each electronic component. For example, the rigid mesa  84  can be sub-divided into smaller rigid mesas, similar to the rigid mesas  54  in  FIG. 3 . 
     Each rigid mesa  84  is shown in  FIG. 4  to have the same size footprint as the corresponding electronic component  86 . It is understood that the footprint of the rigid mesa can be larger or smaller than the corresponding electronic component. Both rigid mesas  84  and electronic components  86  are shown in  FIG. 4  as having the same size and shape. It is understood that each rigid mesa may have a different shape. It is also understood the each rigid mesa may be differently shaped than its corresponding electronic component. 
       FIG. 5  illustrates a process for fabricating a printed circuit board according to an embodiment. The process described in relation to  FIG. 5  corresponds to the printed circuit board  10  in  FIG. 1 . It is understood that the concepts described in relation to the process of  FIG. 5  can alternatively be used to fabricate the alternative embodiments shown in  FIGS. 2-4 , as well as alternative embodiments not explicitly described in  FIGS. 1-5  but nevertheless enabled by the concepts described in relation to  FIGS. 1-5 . 
     At the step  100 , the FPC  12  is provided and vias  18  are formed. Any conventional method for forming the vias within the FPC can be used. In some embodiments, the vias are made of copper or copper plating. It is understood that alternative conductive materials can be used. Each via  18  is formed to provided an application specific interconnect to a conductive interconnect included within the FPC. 
     At the step  102 , a rigid sheet  14 ′ is coupled to the first exterior surface of the FPC  12 . In some embodiments, the rigid sheet  14 ′ is laminated to the FPC  12 . At the step  104 , selective portions of the rigid sheet  14 ′ are removed to form rigid mesas  14 . The removed portions form openings  22  between adjacent rigid mesas  14 . Although the opening  22  is shown as a channel in the two-dimensional representation of  FIGS. 1 and 5 , the removed portion forms a trench around the entire perimeter of the rigid mesa. In some embodiments, a laser or other cutting device is used to remove portions of the rigid sheet. 
     At the step  106 , vias  18  are formed through each rigid mesa  14  to match up with the portion of the via  18  already formed in the FPC  12 . In alternative embodiments, the vias  18  are not formed in the FPC  12  at the step  102 , but instead the vias  18  are formed through both the rigid mesas  14  and the FPC  12  at the step  106 . After the vias  18  are formed, electrical pads  20  are formed at the exposed surface of the rigid mesas  14  and each electrical pad is connected to a corresponding via  18 . 
     At the step  108 , electronic components  16  are attached to the rigid mesas  14 . In some embodiments, a solder joint is formed between each connection pad on the electronic component  16  and the corresponding electrical pad  20  on the rigid mesa  14 . In other embodiments, an electrically conductive adhesive is used. 
     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 flexible printed circuits having attached rigid components. 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.