Patent Publication Number: US-2023161390-A1

Title: Multi-part device enclosure

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation patent application of U.S. patent application Ser. No. 17/158,480, filed Jan. 26, 2021 and titled “Multi-Part Device Enclosure,” which is a continuation patent application of U.S. patent application Ser. No. 16/145,019, filed Sep. 27, 2018 and titled “Multi-Part Device Enclosure,” now U.S. Pat. No. 10,915,151, which is a nonprovisional patent application of and claims the benefit of U.S. Provisional Patent Application No. 62/566,081, filed Sep. 29, 2017 and titled “Multi-Part Device Enclosure,” the disclosures of which are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD 
     The described embodiments relate generally to electronic devices, and more particularly to electronic devices with multi-part enclosures. 
     BACKGROUND 
     Modern consumer electronic devices take many shapes and forms, and have numerous uses and functions. Smartphones, notebook computers, and tablet computers, for example, provide various ways for users to interact with other people, as well as access information, work, play games, and so forth. Such devices use enclosures to house delicate electrical components, allow a user to easily handle and use the device, and to provide a desired shape, form factor, and overall appearance of the device. Enclosures for electronic devices may be formed in various ways and using various materials. For example, enclosures may be formed of plastic or metal. 
     SUMMARY 
     An electronic device includes an enclosure formed of a plurality of layers cooperating to define an interior volume. The enclosure includes a first layer formed of a first material and defining a user input surface of the enclosure and a first portion of a side surface of the enclosure. The enclosure also includes a second layer, formed of a second material different from the first material, positioned below the first layer and defining a second portion of the side surface of the enclosure. The enclosure also includes a third layer, formed of a third material different from the first and second materials, positioned below the second layer and defining a bottom surface of the enclosure and a third portion of the side surface of the enclosure. The first layer may include a transparent region, and the electronic device may further include a display positioned below the first layer and aligned with the transparent region of the first layer. The side surface may define a curved surface along at least the second portion of the side surface and the third portion of the side surface. 
     The electronic device may further include a fourth layer between the first layer and the second layer and defining a fourth portion of the side surface of the enclosure. The electronic device may further include an electronic assembly within the interior volume and having a non-planar side profile. The second layer and the fourth layer may cooperate to define a non-planar interior wall of the interior volume that conforms to the non-planar side profile of the electronic assembly. The fourth layer may be formed of a fourth material different from the first, second, and third materials. The electronic device may further include a fifth layer defining a fifth portion of the side surface of the enclosure, and a sixth layer defining a sixth portion of the side surface of the enclosure. The fifth and sixth layers of the enclosure may cooperate with the second and fourth layers to define the non-planar interior wall of the interior volume that conforms to the non-planar side profile of the electronic assembly. 
     An electronic device may include a top layer defining a top surface of the electronic device, and a first portion of a side surface of the electronic device. The electronic device may also include an electrically operative layer positioned below the top layer and defining a second portion of the side surface of the electronic device, and a bottom layer positioned below the electrically operative layer and defining a bottom surface of the electronic device, and a third portion of the side surface of the electronic device. The top layer may include an opening in the top surface, and the electronic device may include a button mechanism positioned in the opening. The button mechanism may include a dome switch coupled to the an electrically operative layer. The first, second, and third portions of the side surface may extend around an entire periphery of the electronic device. 
     The electronic device may further include a reinforcing layer attached to the top layer and defining an additional portion of the side surface of the electronic device between the top layer and the an electrically operative layer. The reinforcing layer may include carbon fiber. The top layer may be formed of a material selected from aluminum, stainless steel, plastic, sapphire, glass, or carbon fiber. 
     An electronic device may include a display portion including a display enclosure and a display within the display enclosure. The electronic device may also include a base portion rotatably coupled to the display portion and including a top case defining a top surface of the base portion and a first portion of a side surface of the base portion. The base portion may also include a first intermediate layer, having a first thickness, positioned below the top case and defining a second portion of the side surface of the base portion, a second intermediate layer, having a second thickness different than the first thickness, positioned below the first intermediate layer and defining a third portion of the side surface of the base portion, and a bottom case. The bottom case may define a bottom surface of the base portion and a fourth portion of the side surface of the base portion. 
     The display portion may further include a transparent cover defining a front surface of the display portion and a first portion of a side surface of the display portion. The display portion may further include an intermediate layer between the transparent cover and the back layer and defining a second portion of the side surface of the display portion. The back layer may define a back surface of the display portion, and a third portion of the side surface of the display portion. 
     The intermediate layer may be a polarizer layer of the display. The first and second intermediate layers may be formed of different materials. The second intermediate layer may include a substrate and a conductive trace integrated with the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG.  1    depicts an example electronic device. 
         FIG.  2    depicts a simplified exploded view of the electronic device of  FIG.  1   . 
         FIGS.  3 A- 3 E  depict partial cross-sectional views of various example electronic devices. 
         FIG.  4    depicts another example electronic device. 
         FIG.  5    depicts a simplified exploded view of a base portion of the electronic device of  FIG.  4   . 
         FIG.  6    depicts a partial cross-sectional view of the electronic device of  FIG.  4   . 
         FIG.  7    depicts a simplified exploded view of a base portion of another example electronic device. 
         FIGS.  8 A- 8 E  depict partial cross-sectional views of base portions of various example electronic devices. 
         FIG.  9    depicts a simplified exploded view of a base portion of another example electronic device. 
         FIG.  10 A  depicts a partial cross-sectional view of an example electronic device with multiple layers conforming to an internal component of the electronic device. 
         FIG.  10 B  depicts a partial cross-sectional view of an example electronic device with multiple layers cooperating to define a support for an internal component of the electronic device. 
         FIG.  10 C  depicts a partial cross-sectional view of an example electronic device with multiple layers cooperating to define an opening. 
         FIG.  10 D  depicts a partial cross-sectional view of an example electronic device with multiple layers with individual layers having openings. 
         FIG.  11    depicts a partial exploded view of an example base portion for an electronic device. 
         FIGS.  12 A- 12 E  depict partial cross-sectional views of a composite structure of the base portion of  FIG.  11   . 
         FIG.  13    depicts a partial exploded view of an example base portion for an electronic device. 
         FIG.  14    depicts a partial exploded view of an example base portion for an electronic device. 
         FIGS.  15 A- 15 C  depict partial cross-sectional views of a composite structure of the base portion of  FIG.  14   . 
         FIG.  16    depicts an example composite structure for a base portion of an electronic device. 
         FIGS.  17 A- 17 E  depict partial cross-sectional views of the composite structure of  FIG.  16   . 
         FIG.  18 A  depicts an example composite structure for an electronic device. 
         FIGS.  18 B- 18 G  depict partial cross-sectional views of the composite structure of  FIG.  18 A . 
         FIGS.  19 A- 19 B  depict an example electronic device with a composite structure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     The embodiments described herein are generally directed to electronic device enclosures that include multiple layers that cooperate to form the exterior surfaces of the enclosures. The layers that form the exterior surfaces of an enclosure (e.g., top, bottom, and side surfaces) may be more than just housing components, but may also be functional components of the electronic device. For example, in conventional electronic device enclosures (e.g., for smartphones, tablet computers, wearable electronic devices such as smartwatches, notebook computers, etc.), internal components such as circuit boards, display components, keyboard substrates, touch- and/or force-sensing components and the like are all substantially enclosed in an interior cavity of a metal or plastic housing. Embodiments described herein, by contrast, use such components to perform their traditional functions, as well as to form exterior surfaces (or portions thereof) of an electronic device enclosure. 
     In such cases, multiple functional components of an electronic device (e.g., enclosure members, circuit boards, display components, keyboard or keypad substrates, and the like) may be layered in such a way that the peripheral sides of these components cooperate to define the side surfaces of the enclosure of the electronic device. This construction technique may have several advantages. For example, the laminate structure may be strong and stiff, thereby producing a robust and durable electronic device. Further, as the functional components also form the physical structure of the enclosure, additional shells, covers, frames, or other conventional housing components may be omitted. Also, complex geometries can be formed without machining or other material removal operations by effectively building the geometries one layer at a time. Finally, the layered or laminate-style construction may result in a side surface in which each individual layer is visually distinct, producing visually appealing appearance to the device. 
     As used herein, an enclosure may refer to a component (or components) of a device that define one or more exterior surfaces of the device and also define one or more interior cavities in which components of the electronic device are enclosed. Accordingly, while the electronic devices described herein may use functional components of the device (e.g., circuit boards, display layers, etc.) to define at least part of its outer or exterior surfaces, it will be understood that those components may define or form the enclosure of the device while also performing other functions, such as electrical functions, computing functions, display functions, input functions, or the like. Moreover, it will be understood that an enclosure need not be a separate housing component (such as a plastic or metal shell), but may be formed of multiple components that are not conventionally used to define the exterior surfaces of the device. 
       FIG.  1    shows an electronic device  100  that may include an enclosure formed of multiple layers of functional components, as described above. While the device  100  resembles a smartphone, this is merely one example of an electronic device for which the enclosure construction described herein may be used. Accordingly, it will be understood that the techniques, concepts, and principles described herein with reference to the device  100  are applicable to other devices, such as wearable electronic devices (e.g., smart watches, heart rate monitors, biometric sensors), desktop computers, notebook computers, tablet computers, head-mounted displays, and the like. 
     The device  100  includes an enclosure  101  that defines exterior surfaces of the device  100 , including a top surface  104 , a bottom surface  103 , and side surfaces  108 . The side surfaces  108  may extend from the top surface  104  to the bottom surface  103 , and may define the height of the enclosure  101  (as well as the overall height of the device  100 ). As the top surface  104  may include input devices such as touch- and/or force-sensitive displays, buttons, keyboards, trackpads, touch sensors, etc., the top surface  104  may also be referred to herein as an input surface. 
     The device  100  may include a transparent cover  102  (e.g., a first layer) that covers or otherwise overlies a display, and may define a front face and an input surface  104  of the electronic device  100 . For example, a user may operate the device  100  by touching the input surface  104  to select affordances displayed on the display. The transparent cover  102  may have a transparent region that overlies and is aligned with the display, and opaque or masked regions surrounding the transparent region. The masked or opaque region may cover and obscure internal components of the device, and may visually define the outer boundary and/or shape of the visible portion of the display. 
     The electronic device  100  may also include a button  106 . The button  106  may be movable, such as a mechanical push-button or key, or it may be substantially rigid. In either case, the button  106  may be used to control an operation of the device  100  or otherwise cause the device  100  to perform various functions. The button  106  (or a component of the button  106 ) may be positioned in an opening or aperture in the cover  102 . 
     The electronic device  100  may also include touch- and/or force-sensing systems associated with the cover  102 . Touch- and/or force-sensing systems may include electrode layers (e.g., substrates with electrical traces thereon) that are coupled to touch- and/or force-sensing circuitry to detect electrical changes (e.g., capacitive, resistive, inductive, etc.) due to the proximity or contact of a user&#39;s finger or other implement. These components, along with the display that is positioned below the transparent cover  102 , produce an input surface  104  that may accept various types of physical inputs, such as swipes, gestures, touch inputs, presses (e.g., touch inputs applied to the cover  102  that are above a threshold pressure or force), and the like. Where the device  100  includes touch- and/or force-sensing systems, components of those systems may be positioned below the cover  102 . In some cases, they may be attached to a bottom surface the cover  102 , or otherwise below the cover  102 . 
     In some cases, the device  100  may also include a keypad or keyboard. The keypad may include a plurality of buttons, keys or other input mechanisms. The keypad may be provided in addition to a touch- and/or force-sensitive input devices on a front or top surface of the device  100  (e.g., on a user-interface surface). A keypad may include a substrate, such as a circuit board, on which components of key or button mechanisms may be applied. For example, a keypad substrate may be a printed circuit board on which dome switches or other electrical switching components may be electrically coupled. Other components of button mechanisms may be applied to the keypad substrate, such as button support mechanisms, switch housings, light sources (e.g., for illuminating button glyphs, etc.). In some cases, a keypad may include multiple substrates, such as a printed circuit board, to which electrical components of the button mechanisms may be attached, and a support plate, to which mechanical components of the button mechanisms may be attached. A keypad may also include other components or layers, such as membranes, fabric covers, light guide layers, or the like. As described herein, any of the substrates or layers of a keypad may extend to and define a portion of a side surface of the enclosure of the device  100 . 
     The enclosure  101  of the device  100  (e.g., the components that define one or more exterior surfaces of the device  100 ) may be formed at least partially of functional, electrically operative components of the device. More particularly, components of the device that contribute to the electrical and/or computing functions of the device, such as display components, circuit boards, etc., may extend to the periphery of the device  100  and define portions of a side surface  108  of the device enclosure.  FIG.  1    shows an example in which three layers or components define the exterior side surface of the device  100 . For example, the transparent cover  102  may define a user input surface  104  of the device  100 , as well as a first portion of the side surface  108  of the enclosure. A second layer  110  may be positioned below the transparent cover  102  and above an underlying third layer  112  (which may form a back surface of the device  100 ). The second layer  110  may be any component or component(s), including an electrically operative layer such as a printed circuit board, a display component, a touch- and/or force-sensing layer (e.g., a flex circuit substrate with electrodes deposited thereon), or the like. Where the second layer  110  is a circuit board, it may have various components attached thereto, such as processors, memory, dome switches or other switching mechanisms (e.g., for the button  106  or buttons or keys of a keypad), haptic actuators, or the like. Both the second layer  110  and the third layer  112  also define portions of the side surface  108  of the enclosure. Further, as described above, the cover  102  and the second and third layers  110 ,  112  (and any additional layers that may define the side of the device) extend around an entire periphery of the device  100 . 
     The transparent cover  102 , the second layer  110 , and the third layer  112  may be formed from different materials, thus producing a side surface of the device  100  having layers of contrasting materials. For example, the transparent cover  102  may be formed from glass, while the second layer  110  may be formed from a glass-reinforced polymer or a plastic, and the third layer  112  may be formed from metal. Other materials and combinations of materials are also contemplated. Example materials for any given layer include metal, plastic, carbon fiber, glass, sapphire, ceramic, or the like. In some cases, the layers are symmetrically stacked, such that the first and third layers are one material, and the second layer is another material. This symmetrical arrangement may also be present in enclosures that use more than three layers. For example, in a five layer enclosure, the first and fifth layers may be formed from a first material, and the second and fourth layers may be formed from a second material (different from the first material), and the third layer may be a third material (which may be different from at least the second material, and optionally different from the first material). 
     The transparent cover  102 , the second layer  110 , and the third layer  112  (as well as any optional additional layers) may have side surfaces of any suitable thicknesses. For example, in some cases, the side surfaces of each layer are substantially the same, while in other cases the side surfaces of each layer are different. In yet other cases, some of the layers have the same thickness while others have different thicknesses. The thicknesses of the layers may range from about 100 microns (e.g., in the case of a film or ink layer that is exposed along the side surface of the device  100 ), to 5 mm or 10 mm (e.g., in the case of a shell or housing component that includes a peripheral wall that defines part of an internal cavity). Layers of different thicknesses are also contemplated. 
       FIG.  2    shows an exploded view of the device  100 , showing the cover  102  (e.g., the first or top layer) and the second layer  110  separated from the third layer  112 . As shown, the second layer  110  may extend the full length and width of the device  100 , such that its side surfaces are exposed along and define a portion of the side surface of the device  100 . The second layer  110  may be an electrically operative layer (e.g., a circuit board, one or more display layers, an antenna, etc.), a stiffening or reinforcing member, a battery layer, or any other suitable component. The third layer  112  may include a peripheral wall that defines a portion of the side surface of the device  100  and also defines the sides of an internal cavity (cavity  304 ,  FIG.  3 A ) in which components of the device  100  may be positioned. Such components may include, for example, a battery, processor, circuit board, memory, hard drive, antennas, cameras (which may be aligned with openings in the device  100 ), environmental sensors (e.g., accelerometers, barometric sensors), biometric sensors (e.g., heart rate sensors), and the like. 
       FIGS.  3 A- 3 E  are partial cross-sectional views of example electronic device enclosures, viewed along line A-A in  FIG.  1   , showing various configurations of layers and side shapes of an electronic device (or other electronic device using the construction technique described herein). While the partial cross-sections shown in these figures are viewed at one particular location on a device or enclosure, these cross-sections may be representative of substantially an entire peripheral region of the device. For example, because the components forming the side surface of the device are layers that may extend to the perimeter of the device (e.g., they extend edge-to-edge), the same cross-section may exist at all (or most) locations around the periphery of the device. In some cases, the side surface may have openings formed therein, such as for speakers, microphones, charging ports, electrical/communication connectors (e.g., universal serial bus (USB) ports), heat sinks, cooling fans, disk drives, or other devices. In such cases, the cross-sections in those areas may differ from those shown herein, and the seams between layers may be broken or discontinuous at the openings. Apart from these discontinuities, the layered appearance and construction (e.g., the seams, the side surfaces of each layer, etc.) may extend around substantially the entire periphery of the device. In some cases, the seams and/or sides of the layers extend around more than 80%, more than 90%, or more than 95% of the periphery of the device. 
     Where a device includes openings in a side surface, the openings may be integrally formed with one or more layers of the enclosure. For example, a layer may include an opening or gap along a segment of the layer that otherwise forms a portion of the side of the enclosure. The opening may be aligned with a component (e.g., a charging port, speaker, etc.) to facilitate the function of the component. In some cases, the opening may define a serpentine pattern through the layer. For example, a speaker or microphone opening (or pressure relief opening) may not be defined by a single linear opening extending perpendicularly through the layer. Rather, the opening may be defined by a first aperture opening to the exterior of the enclosure, a second aperture offset from the first aperture and opening to the interior cavity of the enclosure, and a channel through the material of the layer and connecting the first and second apertures along a path that is not perpendicular to the exterior surface. In this way, a path from the outside of the device to the inside of the device may be formed without visually or otherwise directly exposing an internal component through an opening in the housing. 
       FIG.  3 A  shows a partial cross-section of the device  100  shown in  FIGS.  1 - 2   . In particular,  FIG.  3 A  shows the cover  102  (e.g., a first layer), the second layer  110  (e.g., an electrically operative layer, a circuit board, display component, or the like), and the third layer  112  (e.g., an enclosure component), all having exposed side surfaces that form part of the side surface of the device  100 .  FIG.  3 A  also shows how the cover  102  may define a cavity  302  in which components associated with a display and touch- and/or force-sensitive components may be positioned. For example, a display module, including a light source, polarizers, filters, light diffusers, liquid crystals, light emitting diodes (LEDs), organic light emitting diodes (OLEDs) or other components may be positioned within the cavity  302  and optionally coupled to the cover  102 . Where a display module is positioned in the cavity  302 , the second layer  110  may be a circuit board, mounting plate, or other component that provides electrical and/or computing functionality to the device  100 . 
       FIG.  3 B  shows a partial cross sectional view of another example device  310  (which may be an embodiment of the device  100 ) in which the layered components of the device define portions of the side surface of the device  310 . The device  310  may include a transparent cover  312  (e.g., a first layer) and a display stack  314  (e.g., a second or intermediate layer) below the transparent cover  312  and above a bottom portion  316  (e.g., a third layer). The display stack  314  may be attached to the transparent cover  312 , or below the transparent cover  312  without interstitial layers. 
     The display stack  314  may include multiple individual layers, some or all of which may provide an optical function to facilitate the operation of the display. For example, the display stack  314  may include optical filters, polarizers, light guide layers, liquid crystal layers, LED layers. While the display stack  314  is shown as having four layers of equal thickness, this is merely one example configuration, and more or fewer layers (and layers of different sizes) may be used in a display stack. As shown, however, the layers of the display stack  314  extend to the outermost side of the device  310  and define a portion of the side surface of the device  310 . In some cases, less than all of the layers or components of a display stack are exposed on the side surface of a device. 
     Because the display stack  314  extends to the outermost side of the device  310 , it may not be necessary for the transparent cover  312  to have a cavity or recess to accommodate a display module. However, the bottom portion  316  may define a cavity  318 , similar to the cavity  304  in  FIG.  3 A , in which components of the device  310  may be positioned. Such components may include, for example, a battery, processor, circuit board, memory, hard drive, antennas, cameras (which may be aligned with openings in the device  310 ), environmental sensors (e.g., accelerometers, barometric sensors), biometric sensors (e.g., heart rate sensors), and the like. 
       FIG.  3 C  shows a partial cross sectional view of another example device  320  (which may be an embodiment of the device  100 ) in which the layered components of the device define portions of the side surface of the device  320 . The device  320  may include a transparent cover  322  (e.g., a first layer) and a display stack  324  (e.g., a second or intermediate layer) below the transparent cover  322  and above a bottom portion  326  (e.g., a third layer). The display stack  324  may be attached to the transparent cover  322 , or below the transparent cover  322  without interstitial layers. The bottom portion  326  may also define an internal cavity  328 , similar to the cavity  304  in  FIG.  3 A , in which components of the device  320  may be positioned. Such components may include, for example, a battery, processor, circuit board, memory, hard drive, antennas, cameras (which may be aligned with openings in the device  320 ), environmental sensors (e.g., accelerometers, barometric sensors), biometric sensors (e.g., heart rate sensors), and the like. 
     The display stack  324  may include multiple individual layers, some or all of which may provide an optical function to facilitate the operation of the display. For example, the display stack  324  may include optical filters, polarizers, light guide layers, liquid crystal layers, LED layers, or the like. Whereas the side surfaces of the components in the display stack  314  of  FIG.  3 B  are exposed and each define a portion of the side surface of the device, the side surfaces of the components in the display stack  324  are covered or encapsulated by a covering  325 . The covering  325  may seal and/or otherwise protect the components of the display stack  324  from delamination or other damage that may occur if such components are not suitably strong or resistant to damage during normal use of the device  320 . The covering  325  may be transparent so that each layer is visible and visually distinct from the adjacent layers, thus providing a visual appearance similar to that of the device  310 . In other cases, the covering  325  may be opaque. The covering  325  may be any suitable material, including but not limited to epoxy, plastic, glass, adhesive, ink, one or more films, or the like. 
       FIG.  3 D  shows a partial cross sectional view of another example device  330  (which may be an embodiment of the device  100 ) in which the layered components of the device define portions of the side surface of the device  330 . The device  330  may include a transparent cover  332  (e.g., a first layer) and a display stack  334  (e.g., a second or intermediate layer) below the transparent cover  332  and above a bottom portion  336  (e.g., a third layer). The display stack  334  may be attached to the transparent cover  332 , or below the transparent cover  332  without interstitial layers. The display stack  334  is similar to the display stack  314  in  FIG.  3 B , with the side surfaces of the display components defining a portion of the side surface of the enclosure of the device. 
     The device  330  also includes an intermediate layer  335 , which may be below the display stack  334  and above the bottom portion  336 , and which may define a peripheral wall that defines the outer boundaries of a cavity  338 . More particularly, the bottom portion  336  and the intermediate layer  335  (which may have the appearance of a frame) cooperate to define at least part of the internal cavity  338 . The internal cavity  338  is otherwise similar to the cavity  304  in  FIG.  3 A , and components of the device  330  may be positioned in the cavity  338 . Such components may include, for example, a battery, processor, circuit board, memory, hard drive, antennas, cameras (which may be aligned with openings in the device  330 ), environmental sensors (e.g., accelerometers, barometric sensors), biometric sensors (e.g., heart rate sensors), and the like. 
       FIG.  3 E  shows a partial cross sectional view of another example device  340  (which may be an embodiment of the device  100 ). In this case, instead of the layered components of the device defining portions of the side surface of the device  340 , the device  340  includes a side member  343  positioned along the side of the device  340 , covering and protecting the end surfaces of the various layers. The device  340  may include a transparent cover  342  (e.g., a first layer) and a display stack  344  (e.g., a second or intermediate layer) below the transparent cover  342  and above a bottom portion  346  (e.g., a third layer). The display stack  344  may be attached to the transparent cover  342 , or below the transparent cover  342  without interstitial layers. The display stack  344  is similar to the display stack  314  in  FIG.  3 B . 
     The device  340  also includes an intermediate layer  345 , which may be below the display stack  344  and above the bottom portion  346 , and which may define a peripheral wall that defines the outer boundaries of a cavity  348 . More particularly, the bottom portion  346  and the intermediate layer  345  (which may have the appearance of a frame) cooperate to define at least part of the internal cavity  348 . The internal cavity  348  is otherwise similar to the cavity  304  in  FIG.  3 A , and components of the device  340  may be positioned in the cavity  348 . Such components may include, for example, a battery, processor, circuit board, memory, hard drive, antennas, cameras (which may be aligned with openings in the device  340 ), environmental sensors (e.g., accelerometers, barometric sensors), biometric sensors (e.g., heart rate sensors), and the like. 
     The side member  343  may extend around the entire periphery (or substantially the entire periphery) of the device, thus covering and optionally protecting the end surfaces of the various layers. The side member  343  may be attached to the device  340  using an adhesive  347  (which may be an epoxy or any other suitable bonding agent). The layers of the device  340  may also be shaped or otherwise configured to define a cavity  349  along the side of the device  340 . The adhesive  347  may at least partially fill the cavity  349 , thereby increasing the mechanical strength of the bond between the adhesive  347  and the layers, and thus increasing the mechanical strength of the coupling between the side member  343  and the layers of the device  340 . The side member  343  may be any suitable material, such as stainless steel, aluminum, magnesium, titanium, a metal alloy, a polymer, a composite, carbon fiber, or the like. 
     The various layers of the devices  100 ,  310 ,  320 ,  330 , and  340  may be attached to one another in any suitable way, such as those set forth above with respect to the notebook computer implementations. For example, they may be secured using adhesives, bolts, screws, threaded fasteners, rivets, stakes, latches, clips, or any other suitable techniques. 
       FIGS.  3 B and  3 D  illustrate components of a display stack having exposed side surfaces in the context of a smartphone. However, any electronic device that includes a display may have a construction analogous to those shown in  FIGS.  3 B and  3 D , with the side surfaces of one or more display components defining a portion of a side surface of an enclosure. For example, a clamshell phone (e.g., with a base portion having a keypad positioned therein and a display portion hinged to the base portion) or a notebook computer with a display (e.g., in a display portion) may have exposed display components around the peripheral side surface of the enclosure of the display portion. 
       FIGS.  1 - 3 E  show the concepts and construction principles in the context of one example electronic device. Similar concepts and principles may apply equally or by analogy to other devices or device configurations. For example,  FIGS.  4 - 10 D  show how a layered enclosure may be implemented in an electronic device with two portions hinged or otherwise rotatably coupled to one another, such as a clamshell phone or “flip-phone.” Of course, the concepts described with respect to these figures are applicable to other types of electronic devices as well, such as handheld electronic devices with articulable covers (e.g., tablet computers with folding covers), foldable electronic devices, notebook computers, wearable electronic devices, and the like. 
       FIG.  4    depicts an electronic device  400  (or simply “device  400 ”) that may include an enclosure formed of multiple layers of functional components, as described above. The device  400  resembles a clamshell-style phone that has a display portion  402  and a base portion  404  flexibly or rotatably coupled to the display portion  402 . The display portion  402  includes a display  403  that provides a primary means of conveying visual information to the user, such as by displaying text, digits, images, graphical user interfaces, and the like. 
     The base portion  404  may include various types of input mechanisms, such as a keypad  406  (which may include a plurality of buttons, keys, touch-sensitive input devices, or other input devices) and a directional pad  408  (which may also include a plurality of buttons, keys, touch-sensitive input devices, or other input devices). In some cases, the keypad  406  and/or the directional pad  408  may be or may include a touch- and/or force-sensitive input device that is configured to receive various types of inputs, such as touch inputs (e.g., gestures, multi-touch inputs, swipes, taps, etc.), force inputs (e.g., presses or other inputs that satisfy a force or deflection threshold), touch inputs combined with force inputs, and the like. 
     The input mechanisms (as well as other components of the device  400 ) may be housed in or attached to (or otherwise integrated with) an enclosure  405 . The enclosure  405  defines exterior surfaces, including a top surface  416 , a bottom surface  420 , and side surfaces  418 . The side surfaces  418  may extend from the top surface  416  to the bottom surface  420 , and may define the height of the base portion  404 . As the top surface  416  may include input devices such as keypads, directional pads, touch sensors, keyboard, touch- and/or force-sensitive input devices, etc., the top surface  416  may also be referred to as an input surface. 
     The display portion  402  and the base portion  404  may be coupled to one another such that they can be positioned in an open position and a closed position. In the open position, a user may be able to provide inputs to the device  400  via the base portion  404  while simultaneously viewing information on the display portion  402 . In the closed position, the display portion  402  and the base portion  404  are collapsed against one another. More particularly, the display portion  402  and the base portion  404  may be hinged together (e.g., via a pivot or hinge mechanism, or other suitable flexible coupling) to form a clamshell-style device that can be moved (e.g., rotated) between an open and a closed configuration. 
     As noted above, the base portion  404  may include multiple components, in a layered or laminated configuration, that together define the enclosure  405  of the base portion  404 . For example, the base portion  404  may include a top case  410 , an intermediate layer  412 , and a bottom case  414 . The top case  410 , intermediate layer  412 , and bottom case  414  (and any additional layers that may be included) may cooperate to define the exterior surfaces of the enclosure  405 . For example, the top case  410  may define all or part of the exterior top surface  416  of the enclosure  405 , as well as a portion of the side surface  418  of the enclosure  405 . The bottom case  414  may define all or part of the exterior bottom surface  420  of the enclosure  405 , as well as another portion of the side surface  418  of the enclosure  405 . The intermediate layer  412  (as well as any additional intermediate layers between the top and bottom cases) may define yet another portion of the side surface  418  of the enclosure. The side surfaces of each layer may be exposed around substantially the entire side surface of the base portion  404 , producing a layered or laminated appearance around the entire periphery of the base portion  404 . Moreover, the interfaces between adjacent layers may be substantially planar or otherwise configured to producing substantially straight, unbroken seams between the layers around the periphery of the device. 
     In addition to defining a portion of the side surface  418  of the enclosure  405 , the intermediate layer  412  may provide electrical or other computing functionality to the device  400 . For example, the intermediate layer  412  may be an electrically operative component, such as a circuit board (e.g., a printed circuit board) on which electrical components of the device  400  are physically and/or electrically coupled. In such cases, the circuit board (or other electrically operative component) may include conductive traces that are integrated with a substrate material, and may include electrical components coupled thereto, including but not limited to processors, memory, batteries, dome switches (e.g., for keypad buttons or keys), antennas, light sources, display components, haptic actuators, and the like. In other cases, the intermediate layer  412  may be a component other than a circuit board. For example, it may be a reinforcing structure (e.g., a carbon fiber or metal structure that reinforces the top case  410  and/or the bottom case  414 ), a keypad substrate, a light guide layer (e.g., for distributing light to buttons of a keypad), or any other suitable component. While  FIG.  4    shows one intermediate layer  412 , this is merely exemplary, and the device  400  may include multiple intermediate layers. In such cases, the enclosure  405  may include more than the three layers that form (and are visible on) the side surfaces  418  of the enclosure  405 . 
     In addition to providing different functions, the top layer (e.g., the top case  410 ), the intermediate layer  412 , and the bottom layer (e.g., the bottom case  414 ) may be formed of different materials. For example, the top case  410  and the bottom case  414  may be formed of metal (e.g., aluminum, stainless steel, zinc, titanium, etc.), and the intermediate layer  412  may be formed of a different material such as fiberglass, carbon fiber, plastic, polycarbonate, glass, a different kind of metal, or the like. The particular materials selected for each layer may be selected based on various considerations and may be selected to provide various different functions and/or benefits. Example materials for the top and bottom cases  410 ,  414  as well as the intermediate layer  412  include metal, plastic, fiberglass, carbon fiber, glass, reinforced plastics, and so on. Such materials may be used in any suitable combination and/or order to form the enclosure  405 . 
     Further, the top case  410 , the intermediate layer  412 , and the bottom case  414  may have any suitable thicknesses, and in some cases may have different thicknesses (at least at their peripheral sides, which define their respective portions of the side surface  418  of the enclosure  405 ). In some cases, the top case  410  is the thinnest of the layers and the bottom case  414  is the thickest of the layers, though other configurations are also contemplated. 
     In some cases, the display portion  402  may be constructed in a similar manner to the base portion  404 , with multiple layers forming an enclosure portion of the display portion  402 . Also like the base portion  404 , the layers that form the enclosure (e.g., the exterior surfaces of the display portion  402 ) may be more than just enclosure components, and they may provide display-related functions as well as forming the exterior surface of the enclosure. For example, as described herein, the exterior surfaces of the display portion  402  may be defined by a transparent cover  422 , such as a glass or plastic sheet that covers and protects the display  403 , an intermediate layer  424 , and a back layer  426 . The back layer  426  may define a back surface of the display portion  402 , as well as a portion of a side surface of the display portion  402 . The transparent cover  422  may define a front surface of the display portion  402 , and may also define a portion of the side surface of the display portion  402 . The intermediate layer  424 , which may be a display layer (e.g., a polarizer, light guide panel, light diffuser, or the like, or multiple of such layers), may also define a portion of the side surface of the display portion  402 . Like the layers that define the exterior side surfaces of the base portion  404 , the layers that define the exterior side surfaces of the display portion  402  may be different thicknesses, different combinations of materials, and may provide various computing or display functions in addition to defining exterior surfaces of the display portion  402 . Further, while the display portion  402  in  FIG.  4    is shown as having three distinct layers defining its side surfaces, this is merely exemplary, and the display portion  402  may have more or fewer layers that what is shown. 
       FIG.  5    is a partial exploded view of the base portion  404  of the device  400 . As described above, the device  400  includes a top case  410 , an intermediate layer  412 , and a bottom case  414 . The top case  410  may be a first or top layer of the base portion  404 , and may define a top or user input surface of the base portion  404 . The top case may optionally define openings, such as a directional pad opening  504  and a keypad opening  506 . The keypad opening  506  may be a single opening, or it may include a web with multiple segmented openings for individual buttons or keys. The directional pad opening  504  may be configured to receive a directional pad (e.g., a glass or plastic cover that is configured to accept inputs at multiple different locations to perform multiple different actions based on the location of the received input). In some cases, the top case  410  may include different openings or no openings in the top surface. 
     As described above, the top case  410  defines both a top surface of the base portion  404  as well as a portion of the side surface of the base portion  404 . That is, the side surfaces of the top case  410 , which extend around the perimeter of the top case  410 , are exposed and define a top portion of the side surface  418  ( FIG.  4   ) of the base portion. 
     Below the top case  410  is the intermediate layer  412  (e.g., a second layer). As shown in  FIG.  5   , the intermediate layer  412  is a keypad substrate, though this merely one example of the type of component that may provide computing or electrical functions to the device  400  and also define a portion of the side surface of the enclosure. The intermediate layer  412  may be an electrically operative component, such as a circuit board that includes conductive traces, dome switches (e.g., for the keypad and directional pad), solder pads, vias, and the like. The intermediate layer  412  may also include conductive traces or connectors for electrically connecting components attached to the intermediate layer  412  to processors or other computing components. 
     The intermediate layer  412  may include dome switches coupled thereto, which may be collapsed or otherwise contacted by buttons  502  disposed above the dome switches. The buttons  502  may be mechanically attached to the intermediate layer  412 , or they may be mechanically attached to a separate component (e.g., a button or key support plate that is positioned between the top case  410  and the intermediate layer  412 ). Where a support plate is included, it may form an additional intermediate layer that defines yet another portion of the side surface of the enclosure. In such cases, the side surface of the base portion  404  may have four distinct layers or portions. 
     The base portion  404  may also include a bottom case  414 , which similarly defines a portion of the side surface of the base portion  404 . As shown, the bottom case  414  may include a peripheral wall that defines a cavity  510 . In other cases, the bottom case  414  may not define a cavity. For example, it may be a substantially planar sheet. In some cases, as described herein, a base portion  404  may include a substantially planar bottom case and a separate wall component that, when coupled to form the enclosure of the base portion  404 , produces a shape or configuration similar to the bottom case  414  shown in  FIG.  5   , which includes an integrated wall structure. 
     The top case  410 , intermediate layer  412 , and bottom case  414  may be coupled together to form a substantially rigid base portion  404  in any suitable way. For example, the components may be coupled using adhesives, bolts, screws, threaded fasteners, rivets, stakes, latches, clips, or any other suitable technique. In some cases, interstitial layers (e.g., the intermediate layer  412 ) may be held captive between two opposing layers (e.g., the top case  410  and the bottom case  414 ) that are mechanically coupled with fasteners, adhesives, or the like. 
     The layers that define the side surface of the base portion  404  and the display portion  402  may be substantially planar or flat along their peripheral regions. This configuration may result in substantially straight or linear seams between the layers along the side surfaces. In such cases, the seams may appear as unbroken lines or seams around the entire periphery (or substantially the entire periphery) of the enclosure that is defined by the layers. 
     As noted above,  FIG.  5    illustrates one example configuration of a base portion  404  with external surfaces defined by multiple layers of components. While  FIG.  5    shows only three layers, it will be understood that other configurations with more or different layers or components between the top case  410  and the bottom case  414  are also contemplated, where the additional layers or components also define portions of the side surface of the base portion  404 . Examples of additional layers include, without limitation, additional printed circuit boards, flexible circuit substrates, display components, light guide layers, metal sheets, shielding layers, reinforcement layers, electrode layers (e.g., for a touch- and/or force-sensing systems), antennas, magnets, spacers, and the like. Where magnets are incorporated in a layer, they may be positioned in openings in a peripheral portion of the layer. For example, a layer may define one or more openings along a peripheral portion of the layer, and individual magnets may be positioned in the one or more openings. The magnets may be configured to be substantially the same size and shape as the openings (e.g., producing a tolerance fit), such that the magnets and the peripheral portion of the layer have a substantially uniform thickness. The magnets may therefore be integrated into the structure of the layered enclosure. Incorporating magnets into a peripheral portion of a layer may cause the side of the enclosure proximate the magnets to magnetically attract other components or objects. 
       FIG.  6    is a partial cross-sectional view of the device  400  of  FIG.  4   , viewed along line B-B in  FIG.  4   .  FIG.  6    shows the device  400  in a closed configuration, such as where the display portion  402  is rotated about a hinge or other flexible coupling mechanism such that the front surface of the display is facing (e.g., substantially parallel to) the top or user input surface defined by the top case of the base portion.  FIG.  6    illustrates how components of the display and base portions extend to and define the side surfaces of the display and base portions. The resulting side surfaces have a layered appearance, with each layer (e.g., each component) defining a visually and structurally distinct layer. Moreover, the layered structure of both the display portion  402  and the base portion  404  provides a consistent construction and appearance across both portions of the device. 
       FIG.  7    is an exploded view of part of another example base portion  700  that uses a layered construction as described herein. The base portion  700  differs from the base portion  404  in that the base portion  700  includes more layered components that define the side surface of the base portion  700  while also providing computing and electrical functionality to the electronic device. 
     The base portion  700  may include a first layer  702 , which may correspond to a top case of a clamshell-style phone (as shown), or it may be any other layer or component that defines a top surface of the enclosure of the base portion  700 , regardless of the particular type of electronic device with which it is incorporated. The first layer  702  may be similar in construction, function, material, etc., to the top case  410  of  FIGS.  4 - 5   . For example, the first layer  702  may define a keypad opening, a web, a directional pad opening, or the like. The first layer  702  may be formed from any suitable material, such as metal, plastic, carbon fiber, fiberglass, glass, sapphire, ceramic, or the like. In some cases, the first layer  702  may have one or more pigment layers, applied to a bottom and/or a top surface of the first layer  702 . Such pigment layers, which may include inks, pigments, dyes, colored films, etc., may extend to the edges of the first layer  702  and thus may also be visible on (and may define a portion of) the side surfaces of the base portion  700 . 
     The base portion  700  may also include a second layer  704 . The second layer  704  may be a structural reinforcement or brace for the first layer  702  (or an underlying layer). In some cases, the second layer  704  may be sheet with openings that correspond to the openings in the first layer  702  (in cases where the first layer  702  has openings). In some cases, the second layer  704  comprises a series of ribs, lattices, beams, or other structural shapes and/or feature that increase the stiffness, strength, toughness, rigidity, or other physical property of the first layer  702  or the base portion  700  as a whole. The second layer may be formed of any suitable material, such as metal (e.g., aluminum, steel, magnesium, titanium), plastic, fiberglass, carbon fiber, or the like. The second layer  704  may be bonded, adhered, or otherwise attached to the first layer  702  (or any other adjacent layer) in any suitable way, as described above (e.g., including adhesives, fasteners, mechanical interlocks, etc.). 
     The base portion  700  may also include a third layer  706 . The third layer  706  is shown as a circuit board, similar to the example of the intermediate layer  412  shown and described with respect to  FIG.  5   . The third layer  706  may have the same or similar features and functions as the intermediate layer  412 . For example, the third layer  706  may be a printed circuit board substrate having vias, conductive traces, solder pads, dome switches, interconnects, or other electrically functional components incorporated therewith. The third layer  706  may also be a substrate or base structure for the buttons or keys of a keypad, and may thus include electrical and/or mechanical features that enable the button mechanisms (or other suitable input devices) to operate and accept inputs from a user. 
     The base portion  700  may also include a fourth layer  708 . The fourth layer  708  is shown as a spacer layer configured as a rim or frame that defines an internal cavity in which components may be housed. The spacer layer may be used to define an internal cavity of the base portion  700 , in which other components may be positioned. For example, the internal cavity defined by the spacer layer (as well as layers above and below the spacer layer) may house a battery, processor, circuit board, memory, a hard drive, or any other suitable component(s). 
     The base portion  700  may also include a fifth layer  710 , which may correspond to a bottom case of an electronic device. Accordingly, the fifth layer  710  may be similar in construction, function, material, etc., to the bottom case  414  of  FIGS.  4 - 5   . For example, the fifth layer  710  may define a bottom surface of the base portion  700 , as well as a portion of the side surface, and may be formed from metal, plastic, carbon fiber, fiberglass, glass, or any other suitable material. The fifth layer  710  may be a substantially continuous sheet. The fifth layer  710  may be substantially flat or planar, or it may have a contoured shape. For example, a peripheral region of the fifth layer  710  (e.g., the outer periphery of the fifth layer  710 ) may be curved upwards to produce a shape with a concave interior-facing surface and convex exterior-facing surface. The fifth layer  710  may have a substantially continuous thickness, or the thickness may be different in different regions of the fifth layer  710 . For example, the fifth layer  710 , or bottom case, may include a thicker region or frame around a thinner central portion, thus defining a recess in which components may be housed (e.g., similar to the configuration of the bottom case  414  shown in  FIG.  5   ). 
     The multiple layered components shown in  FIG.  7    may have substantially flat or planar interfacing regions along their outer periphery. When assembled, the flat or planar interfacing regions form seams between adjacent layers, with the seams extending around substantially the entire side surface of the resulting base portion  700 . The seams may be substantially flat or linear, as a result of the substantially planar configuration of the interfacing regions of the individual layers. In cases where the peripheral regions of the layers are not substantially flat or planar, the seams may not appear linear. For example, if the peripheral regions of two adjacent layers have wavy or crenate configurations (which may interlock or otherwise align with one another), those seams may have a wavy or crenate path around the periphery of the base portion  700 . 
       FIGS.  8 A- 8 D  are partial cross-sectional views of example base portions, viewed along line A-A in  FIG.  4   , showing various configurations of layers and side shapes of a base portion (or other enclosure or electronic device using the construction technique described herein). While the partial cross-sections shown in these figures are viewed at one particular location on a base portion (or other enclosure), these cross-sections may be representative of substantially an entire peripheral region of the base portion. For example, because the components forming the side surface of the base portion are layers that may extend to the perimeter of the base portion (e.g., they extend edge-to-edge of the device), the same or substantially the same cross-section may exist at all (or most) locations around the periphery of the base portion. In some cases, the side surface may have openings formed therein, such as for buttons, charging ports, electrical/communication connectors (e.g., universal serial bus (USB) ports, display ports), or other components. In such cases, the cross-sections in those areas may differ from those shown herein, and the seams between layers may be broken or discontinuous at the openings. Apart from these discontinuities, the layered appearance and construction may extend around substantially the entire periphery of the base portion. In some cases, the seams and/or sides of the layers extend around more than 80%, more than 90%, or more than 95% of the periphery of the base portion. 
     In some cases, layers may have discontinuities or gaps along the periphery of the layers, and the discontinuities or gaps may be filled with other components, which themselves define or form part of the side surface of the enclosure. For example, a metal layer may have a gap, along its periphery, that is configured to be aligned with an antenna that is positioned within the enclosure. Instead of the gap defining an opening in the side surface of the housing, a dielectric (or other suitably RF transparent) material may be positioned in the gap, thereby forming a continuous and solid side surface while also allowing the antenna to transmit and/or receive signals through the dielectric material. Other types of components may be positioned in gaps or discontinuities of layers, such as antennas, transparent windows (e.g., clear glass or plastic), light sources, light guides, connectors, magnets, or the like. In such cases, those components may define part of the side surface of the enclosure along with any other layers of the enclosure. 
       FIG.  8 A  shows a partial cross-section of the base portion  700  shown in  FIG.  7   . In particular,  FIG.  8 A  shows the first layer  702  (e.g., a top case), the second layer  704  (e.g., a reinforcing layer), the third layer  706  (e.g., a printed circuit board), the fourth layer  708  (e.g., a spacer), and the fifth layer  710  (e.g., a bottom case), all having exposed side surfaces that form part of the side surface of the base portion.  FIG.  8 A  also shows how the fourth layer  708  acts as a spacer to define an internal cavity  802  in which device components (e.g., processors, memory, storage media, circuit boards, etc.) may be positioned. 
       FIG.  8 B  shows a partial cross-section of a base portion  810 . Similar to the base portion  700 , the base portion  810  includes a first layer  812  (e.g., a top case), a second layer  814  (e.g., a reinforcing layer), a third layer  816  (e.g., a printed circuit board), a fourth layer  818  (e.g., a spacer), and a fifth layer  820  (e.g., a bottom case). Whereas the fifth layer  710  in  FIGS.  7  and  8 A  may be substantially flat (at least at the peripheral region), the fifth layer  820  in  FIG.  8 B  may have a peripheral wall  822  (which may be similar to the peripheral wall defining the cavity  510 , as shown in  FIG.  5   ). The peripheral wall  822  may cooperate with the fourth layer  818  (e.g., a spacer layer) to define an internal cavity  824  in which device components may be positioned. The combination of the peripheral wall  822  and the fourth layer  818  may produce a larger internal cavity than that shown in  FIG.  8 A , and illustrates how different configurations of the individual layers may be used to produce different form factors and different sized or shaped interior cavities (as well as different outside dimensions) of the enclosure. 
       FIGS.  8 C and  8 D  illustrate example cross-sections of a layered base portion (similar to the base portion  700 ), in which the side surfaces of the base portion are curved or contoured, rather than being substantially flat or planar (as shown in  FIGS.  8 A- 8 B , for example). In particular, a base portion  830  in  FIG.  8 C  includes a first layer  832  (e.g., a top case), a second layer  834  (e.g., a reinforcing layer), a third layer  836  (e.g., a printed circuit board), a fourth layer  838  (e.g., a spacer), and a fifth layer  840  (e.g., a bottom case). The side surface of the base portion  830  defines a curved surface. The curved surface extends along the side surface of the base portion  830  such that at least two of the side surfaces of the layers are curved to define the overall curve of the base portion  830 . For example, as shown in  FIG.  8 C , the side surfaces of the third layer  836 , fourth layer  838 , and fifth layer  840  may all define portions of the overall curve of the base portion  830 . The curve shown in  FIG.  8 C  is merely one example curve, and other curved shapes may also be formed. For example, a base portion may be constructed to have smaller or larger radii, and/or incorporating or spanning more or fewer layers that what is shown in  FIG.  8 C . In some cases, the curved shape of the side of a base portion is consistent around substantially the entire periphery of the base portion. In other cases, different portions of the side surface of a base portion may have a different shape. For example, a side surface along a back portion of the base portion (e.g., where a display portion may be coupled, via a hinge, to the base portion) may be flat, or may have cutouts, recesses, or other features to facilitate the coupling to the display portion, side surfaces along the lateral and front portions of the base portion may have a curved shape or profile. 
       FIG.  8 D  shows a base portion  850  with a side surface having a different curved profile than that shown in  FIG.  8 C . In particular, the base portion  850  in  FIG.  8 D  includes a first layer  852  (e.g., a top case), a second layer  854  (e.g., a reinforcing layer), a third layer  856  (e.g., a printed circuit board), a fourth layer  858  (e.g., a spacer), and a fifth layer  859  (e.g., a bottom case). The side surface of the base portion  850  defines a curved surface having two curved regions. For example, the first and second layers  852 ,  854  are curved or angled towards the top surface of the base portion  850 , while the third, fourth, and fifth layers  856 ,  858 , and  859  are curved or angled towards the bottom surface of the base portion  850 . As noted above, the curved shape of the side of the base portion  850  may be consistent around substantially the entire periphery of the base portion, or different portions of the side surface may have different curvatures (or no curvature). 
     The curved or contoured side surfaces of the base portions  830 ,  850  in  FIGS.  8 C- 8 D  may be formed in various ways. In some cases, the base portions  830 ,  850  may be assembled by securing some or all of the multiple layers together, and then subjecting the assembled layers to a forming process, such as machining, grinding, cutting, polishing, or any other suitable technique to remove material from multiple layers and define the desired shape of the side surface. In other cases, the side surfaces of each individual layer may be shaped prior to being assembled into the base portion, and once assembled, the shapes of the side surfaces of each individual layer may together form a continuous, curved side surface of the base portion. In some cases, even where the side surfaces of individual layers are shaped prior to assembly, the base portion may be subjected to a material removal operation after assembly, such as a machining or polishing step, to remove any sharp edges or ridges due to misalignment of the layers. Machining, grinding, polishing, and/or other material removal operations may also be used for base portions with straight or flat side surfaces. 
       FIG.  8 E  shows a base portion  860  with a side surface defined by a side member  863 . The side member  863 , which may be similar to the side member  343  ( FIG.  3 E ) and may extend around the entire periphery (or substantially the entire periphery) of the device, thus covering and optionally protecting the end surfaces of the various layers. The base portion  860  in  FIG.  8 E  also includes a first layer  862  (e.g., a top case), a second layer  864  (e.g., a reinforcing layer), a third layer  866  (e.g., a printed circuit board), a fourth layer  868  (e.g., a spacer), and a fifth layer  870  (e.g., a bottom case). 
     The side member  863  may be attached to the base portion  860  using an adhesive  861  (which may be an epoxy or any other suitable bonding agent). The side member  863  may be any suitable material, such as stainless steel, aluminum, magnesium, titanium, a metal alloy, a polymer, a composite, carbon fiber, or the like. The layers of the base portion  860  may also be shaped or otherwise configured to define a cavity  867  along the side of the base portion  860 . The adhesive  861  may at least partially fill the cavity  867 , thereby increasing the mechanical strength of the bond between the adhesive  861  and the layers, and thus increasing the mechanical strength of the coupling between the side member  863  and the layers of the base portion  860 . 
     As noted above, the layers that form the enclosure of an electronic device (e.g., an enclosure of a base portion of a notebook computer) may provide electrical or computing functions in addition to defining the exterior side surfaces of the enclosure. For example, the printed circuit boards described with respect to  FIGS.  5  and  7    may have various electrical components coupled thereto, such as a directional pad, electrical switching components for keypad buttons or keys, and the like.  FIG.  9    shows another example of intermediate layers that define a portion of a side surface of an enclosure while also providing integral electrical functions of the device. 
       FIG.  9    shows an exploded view of several layers that may form part of a base portion of a clamshell-style phone (or any other suitable type of electronic device). In particular,  FIG.  9    shows a first layer  902 , which may be a printed circuit board on which electrical components are coupled. Similar to the printed circuit boards described above, where the first layer  902  is a printed circuit board, the circuit board substrate may define a portion of a side surface of the enclosure or device in which it is incorporated. 
     The first layer  902  may include conductive traces  906  on and/or in the material of the first layer  902 . The conductive traces  906  may be configured to electrically couple to various electrical components on the first layer  902 , such as dome switches (or other switches or touch- and/or force sensitive input devices or components), light sources, sensors, or the like. The conductive traces may carry electrical signals from these components to a processor or other computing component of a notebook computer. 
     The device shown in  FIG.  9    may also include a second layer  908 , which may be a spacer layer (as described above), and which may also define a portion of a side surface of the enclosure or device in which it is incorporated. The second layer  908  may be a circuit board, or any other suitable substrate or material. 
     In addition to helping to define the shape of the enclosure, the second layer  908  may include electrical conductors, traces, and connectors that carry signals from the first layer  902  to a processing component. For example, the second layer  908  may include electrical contacts  910  that that are configured to contact the conductive traces  906  of the first layer  902 . The second layer  908  may also include conductive traces  912  (which may be on a surface of the second layer  908  or embedded within the second layer  908 ) that electrically couple the electrical contacts  910  to an electrical connector  914 . The electrical connector  914  may be configured to couple to a processor module  916  via a complementary connector  918 . By incorporating electrical contacts and traces into the second, spacer layer  908 , valuable space on the first layer  902  may be available for other components, especially in cases where the first layer  902  includes numerous electrical components or electrical components that require numerous signal lines. Further, by incorporating conductive traces and connectors in multiple layers, and allowing those layers to electrically interconnect to one another, additional avenues for electrically connection components are provided. This may facilitate more efficient interconnection of electrical components, and/or may allow more options for the placement of interconnected electrical components. 
     The processor module  916  may include one or more processors and/or memory, or other circuit elements that provide computing functionality to a device. The processor module  916  may be positioned within an internal cavity defined at least in part by the second layer (e.g., spacer)  908  and a third layer  920  (which may be a bottom case, as described above). 
     While  FIG.  9    shows a processor module  916  coupled to electrical traces on another circuit board via conductive traces (or other conductors)  912  in the second layer  908 , this type of construction may be used to interconnect any suitable types of electrical components. For example, traces  912  in a layer may be used to provide power from a battery to any other electrical component(s) in a device. As another example, traces  912  in a layer may be used to electrically couple an antenna structure (e.g., a conductor that radiates and/or receives wireless signals) to antenna circuitry or other communications circuitry. In some cases, an antenna structure may take the form of a conductive material (e.g., a conductive trace, wire, metal strip, or other conductor) that is integrated with or coupled to a layer. For example, with reference to  FIG.  9   , a conductive trace that operates as an antenna structure may be positioned on or in the second layer  908 , and may be electrically coupled to the processor module  916  via a conductive trace similar to the trace  912 . Other components may also or instead by interconnected by conductive traces (or other conductors) that are integrated with layers of the device. In some cases, optical channels (e.g., fiber optics) may be used instead of or in addition to conductive paths. Optical channels may be implemented in the same or similar manner as the electrical connections described above. 
     Moreover, any electrical interconnection using conductors integrated with a layer, as shown in  FIG.  9   , may be configured to send and/or receive any suitable type of signal or power. For example, communications signals (e.g., analog or digital signals) may be carried, as well as direct current and/or alternating current power (e.g., from a battery, invertor, or other power source). 
     While  FIG.  9    shows three layers of a device, these are merely portions of the device, and more layers may be included, with each additional layer (or a subset of the additional layers) also defining part of the side surface of the device. Additional layers may include a top case above the first layer  902 , a reinforcing layer below the top case and above the first layer  902 , and the like. 
     By building enclosures of electronic devices using multiple discrete layers, complex internal geometries can also be produced without material removal operations (or with less material removal operations than would otherwise be necessary to form such geometries). The complex internal geometries may be configured to conform to the shapes of internal components of the device, thereby maximizing the amount of material devoted to forming the enclosure and eliminating empty space, which may otherwise be wasted. 
       FIG.  10 A  shows a partial cross-sectional view of an electronic device  1000  in which multiple layers of the device cooperate to define a non-planar interior wall of the interior volume of a device, where the interior wall conforms to a non-planar side profile of an electronic device assembly. The electronic device  1000  may be any suitable electronic device, such as a smartphone, clamshell-style phone, wearable electronic device, notebook computer, desktop computer, or the like. 
     The electronic device  1000  includes an electronic assembly  1002 , which may include any suitable electronic device components, such as a circuit board, display assembly, battery, processor, haptic actuator, speaker, microphone, light source, or the like. (While the electronic assembly  1002  is shown as a component having four discrete, substantially planar layers, this is merely an example configuration showing an irregular, non-planar side profile. Other example electronic assemblies may include different components having different shapes and/or configurations.) 
     The electronic device  1000  may include multiple layers  1004  that together define an irregular, non-planar interior wall that conforms to the side profile of the electronic assembly  1002 . Notably, because the non-planar interior wall conforms to the side profile of the electronic assembly  1002  (e.g., is in intimate contact with the electronic assembly  1002 , at least along part of the side profile of the assembly  1002 ), empty space between the interior wall of the interior volume may be reduced or eliminated, thus maximizing the thickness of the enclosure while still accommodating the internal components. 
     The multiple layers  1004  that define the interior wall and the exterior side surface of the device  1000  may be any suitable components or layers, including circuit boards, reinforcing layers, spacers, light guide panels, and the like. Also, similar to the other layers described above, the multiple layers  1004  may be substantially continuous around the periphery of the device  1000 , thus defining a continuous, laminated appearance around the side of the device. Furthermore, while the interior wall is shown in  FIG.  10 A  as having one particular shape, the multiple layers  1004  may define differently shaped walls at other regions of the device  1000 . For example, the cross-section shown in  FIG.  10 A  may correspond to a location of the line B-B in  FIG.  4    (e.g., a lateral side of a device). A cross-section along a bottom side of the same device, however, may define a different interior wall shape, such as a substantially planar surface, or a curved surface, or any other non-planar profile that is different from that shown in  FIG.  10 A . Also, the layers shown in  FIG.  10 A  need not be only a frame or gasket-shaped component having material only along a periphery of the device, but may have regions that extend across the internal cavity, forming a substrate or other substantially planar region to which components may be attached. 
     In yet other configurations, different layers of an enclosure may cooperate to define other complex geometries within an internal cavity of a device.  FIG.  10 B , for example, shows a partial cross-sectional view of an electronic device  1010  that includes multiple layers  1014  defining a continuous side surface of the electronic device  1010 , as described above. A subset  1016  of the layers may form a ledge  1018  that supports a component  1012  of the device  1010 . The component  1012  may be any component, such as a circuit board, stiffener, battery, haptic actuator, or the like. The ledge  1018  may also include a mechanical feature to facilitate attachment of the component  1012  to the ledge. For example, the ledge may define an opening (e.g., a smooth hole, a threaded hole, a splined hole, etc.), a protrusion (e.g., a pin or rod), a clip, an undercut, or any other suitable feature. Like the layers  1004  in  FIG.  10 A , and as described throughout this application, the multiple layers  1014  may have functions beyond merely forming the enclosure. For example, they may be circuit boards, display components, stiffeners, reinforcing members, light guides, electrode sheets (e.g., for touch- and/or force-sensing components), and the like. 
     In  FIGS.  10 A- 10 B , the enclosures may be formed by applying multiple layers each defining a portion of the interior geometry. In this manner, irregular, non-planar interior geometries may be formed without (or with less) machining or other material removal operations. This technique may have numerous advantages over material removal operations. For example, it may reduce manufacturing time, reduce wasted material, improve the fitment between the enclosure and internal components (e.g., reducing or eliminating space between the enclosure and internal components), use more efficient manufacturing techniques, and so on. As one particular example, instead of machining a complex, irregular internal geometry from a housing component, a stack of layers (which may be formed quickly from relatively simple forming processes such as stamping) may be laminated to produce the desired geometry. While further machining operations may be performed to finalize the internal geometry (e.g., polishing, threading, finishing, etc.), the amount of machining and post-processing may be substantially less than forming the same geometry without a laminated enclosure. 
     In some cases, an enclosure formed using multiple layers may define internal holes, cavities, or other spaces. In such cases, these spaces may be filled with a filler material. The filler material may increase the strength, stiffness, or other physical property of the enclosure. In some cases, the filler material may also increase thermal conductivity between layers, which may help draw heat away from heat producing components such as batteries, processors, and the like. The spaces and filler material may be specifically configured and positioned to provide thermally conductive paths that draw heat in advantageous directions, such as away from a battery and towards a portion of the device that will radiate the heat without negatively impacting usability of the device (e.g., towards a side of the device, which may cause less discomfort to a user than if it were directed towards a bottom or top of the device). 
       FIGS.  10 C and  10 D  show partial cross-sectional views of electronic devices in which internal spaces are filled with a filler material.  FIG.  10 C , for example, shows a partial cross-sectional view of a device  1020 , which may be or may correspond to any suitable electronic device, such as a smartphone, clamshell-style phone, wearable electronic device, notebook computer, desktop computer, or the like. 
     The device  1020  includes a first layer  1021  (which may correspond to a top case of a clamshell-style phone or notebook computer), a second layer  1022 , a third layer  1023 , a fourth layer  1024 , and a fifth layer  1025  (which may correspond to a bottom case of a clamshell-style phone or a notebook computer). The second, third, and fourth layers  1022 - 1024 , which may be circuit boards, intermediate layers, stiffeners, or any other suitable layer or component as described herein, may each define an opening, and the openings may be aligned with one another or otherwise communicate to form an opening  1026 . The opening  1026  may be filled with a filler material  1027 . The filler material  1027  may be any suitable material, such as an adhesive, epoxy, thermoplastic or thermoset polymer, foam (which may be expanded or otherwise formed into a foam after a material is positioned in the opening  1026 ), metal, or the like. In cases where the filler material  1027  is an adhesive or other bonding agent, the filler material may retain the layers together (alone or in conjunction with other adhesives, fasteners, or the like). 
     As shown, the openings in the layers in the device  1020  are offset from one another, thus defining an irregular shape with undercuts. This shape may help improve the device&#39;s resistance to delamination and/or otherwise more generally increase the strength, stiffness, and/or toughness of the device. 
       FIG.  10 D  shows a partial cross-sectional view of a device  1030 , which may be or may correspond to any suitable electronic device, such as a smartphone, clamshell-style phone, wearable electronic device, notebook computer, desktop computer, or the like. The device  1030  includes a first layer  1031  (which may correspond to a top case of a clamshell-style phone or notebook computer), a second layer  1032 , a third layer  1033 , a fourth layer  1034 , and a fifth layer  1035  (which may correspond to a bottom case of a clamshell-style phone or a notebook computer). The second, third, and fourth layers  1032 - 1034 , which may be circuit boards, intermediate layers, stiffeners, or any other suitable layer or component as described herein, may each define one or more openings. For example, the second layer  1032  may include openings  1036 , and the fourth layer  1034  may include openings  1038 . Unlike the openings in  FIG.  10 C , the openings  1036  and  1038  do not connect to one another. 
     Filler material  1037 ,  1039  may occupy the openings  1036 ,  1038 , respectively. The filler materials  1037 ,  1039  may be any suitable material, such as an adhesive, epoxy, thermoplastic or thermoset polymer, foam, metal, or the like. In cases where the filler materials  1037 ,  1039  are an adhesive or other bonding agent, the filler materials  1037 ,  1039  may retain the layers together (alone or in conjunction with other adhesives, fasteners, or the like). The openings  1036 ,  1038  and filler materials  1037 ,  1039  may serve the same or similar functions as the opening  1026  and filler material  1027  described with respect to  FIG.  10 C . 
     As noted above, the components that define the exterior surfaces of an enclosure may be formed from metal. For example, referring to  FIG.  4   , the top case  410 , which may define an exterior top surface  416  of the base portion  404  of the enclosure  405 , may be formed of a metal material. Similarly, the bottom case  414 , which may define an exterior bottom surface  420  of the enclosure  405 , may also be formed of a metal material. The back layer  426  of the display portion  402  may also be formed of a metal material. Any suitable metal material may be used, such as aluminum, stainless steel, magnesium, amorphous metals, alloys, and the like. In some cases, however, the components that define exterior surfaces, such as the top and bottom cases  410 ,  414  and the back layer  426 , may be formed of a composite structure formed of multiple layers. The composite structures may provide several advantages over a single metal member. For example, a composite structure may have preferred thermal properties, which may help dissipate heat from internal components of a device (e.g., a battery, processor, etc.). As another example, a composite structure may have an increased stiffness relative to a metal layer.  FIGS.  11 - 19 B  depict example configurations of composite structures that may be used in electronic device enclosures (such as any of the enclosures described herein), and may particularly be used as the layers or components that define the exterior surfaces of an enclosure. 
       FIG.  11    is a partial exploded view of a base portion  1100  of an electronic device. The base portion  1100  may be an embodiment of the base portion  404  in  FIG.  4   . Accordingly, aspects of the base portion  404  described above are equally applicable to the base portion  1100 , and are not repeated here. As shown in  FIG.  11   , the base portion  1100  may include a top case  1102 , a bottom case  1104 , and an intermediate layer  1106 . The top case  1102  may define an exterior top surface of the base portion  1100 , and the bottom case  1104  may define an exterior bottom surface of the base portion  1100 . The intermediate layer  1106  may define part of an exterior side surface of the base portion  1100 , as described herein. In some cases, the intermediate layer  1106  may be a circuit board. 
     The base portion  1100  may also include a heat generating component, such as a processor  1108 . The processor  1108  is one example of a heat generating component that may be positioned within the base portion  1100 , and, as used herein, may represent other heat generating components as well, such as a battery, memory module, light source, power convertor, transistor, or the like. Where the intermediate layer  1106  is a circuit board, the processor  1108  may be operatively coupled to the intermediate layer  1106  (e.g., via solder or other suitable electrical connections). As described herein, the processor  1108  may also be thermally coupled to the bottom case  1104 . For example, the processor  1108  may be in contact with an interior surface of the bottom case  1104 , or it may be coupled to the bottom case  1104  via an adhesive, film, bonding pad, or other material. In some cases, the thermal coupling between the bottom case  1104  and the processor  1108  is configured to facilitate removal of heat from the processor  1108 . 
     As noted above, composite structures may be used for components of an enclosure, such as the top case  1102  and/or the bottom case  1104 . In some cases, the composite structures have metal layers defining one or more of the exterior surfaces of the composite structure. Thus, for example, the bottom case  1104  may have a metal bottom layer such that the exterior surface of the enclosure is metal. The composite structures may have multiple layers of different materials that together produce desired structural, thermal, or other properties. Further, the layers that define a composite component may not be uniform across the entire component. For example, the thickness of a given layer may be different at different locations of the component (e.g., a top or bottom case). Further, layers may be discontinuous, and multiple different materials may occupy the space of a single layer. Various examples of composite structures are described herein. 
       FIG.  12 A  depicts a cross-sectional view of an example bottom case  1200 , which may be an embodiment of the bottom case  1104  in  FIG.  11   . The cross-sectional view of the bottom case  1200  corresponds to line C-C in  FIG.  11   . The composite structure of the bottom case  1200  may, however, be used for other layers or components of the enclosures described herein, such as a top case (e.g., the top case  1102  in  FIG.  11   , or any other top case described herein), a back layer of a display portion (e.g., the back layer  426  in  FIG.  4   ), an enclosure component defining a bottom portion of a tablet computer or smartphone (e.g., the third layer  112 ,  FIG.  2   ), or the like. 
     The bottom case  1200  includes a first layer  1201  formed of a metal, a second layer  1202  formed of a different material, and a third layer  1203  formed of a metal. The first and third layers  1201 ,  1203  may be formed from the same metal or different metals. Example metals include but are not limited to steel, stainless steel, aluminum, magnesium, titanium, metal alloys, and the like. 
     The second layer  1202  may be formed of a different material than the first and second layers  1201 ,  1203 . For example, the second layer  1202  may be a foam material, which may be an open cell foam or a closed cell foam (or a combination of open and closed cells). The foam material may be formed from or include any material or materials, including, for example, polyurethane foam, polyethylene terephthalate foam, polyvinylchloride foam, polyisocyanurate foam, reinforced foam (e.g., including a polymer material and reinforcing material such as glass or ceramic fibers), a metal foam, or any other suitable foam material. 
     In some cases, the second layer  1202  may be a graphite material. For example, the second layer  1202  may be thermoset carbon fiber, thermoplastic carbon fiber, pyrolytic carbon, one or more pre-impregnated sheets or layers of carbon fibers, or the like. Graphite materials may be composites formed of graphite (e.g., in the form of fibers, tows, yarns, woven sheets, or the like) and a polymer matrix (e.g., epoxy, polyester, nylon, or the like). In some cases, the second layer  1202  (or indeed any graphite material layers described herein) may be formed of or include multiple layers of graphite sheets. The combination of first and third layers formed of metal with a second layer formed of foam or graphite (or another material) may exhibit advantageous structural and/or thermal properties as compared to a single metal layer of a similar thickness, for example. 
       FIG.  12 B  depicts a cross-sectional view of an example bottom case  1210 , which may be an embodiment of the bottom case  1104  in  FIG.  11   . The cross-sectional view of the bottom case  1210  corresponds to a view along line C-C in  FIG.  11   . The composite structure of the bottom case  1210  may, however, be used for other layers or components of the enclosures described herein, such as a top case (e.g., the top case  1102  in  FIG.  11   , or any other top case described herein), a back layer of a display portion (e.g., the back layer  426  in  FIG.  4   ), an enclosure component defining a bottom portion of a tablet computer or smartphone (e.g., the third layer  112 ,  FIG.  2   ), or the like. 
     The bottom case  1210  includes a first layer  1211  formed of a metal, a second layer  1212  formed of a polymer material, a third layer  1213  formed of a graphite material, a fourth layer  1214  formed of a polymer material (e.g., the same or a different polymer material as the second layer  1212 ), and a fifth layer  1215  formed of a metal (e.g., the same or a different metal as the first layer  1211 ). The metal of the first and fifth layers  1211 ,  1215  may be any suitable metal, including steel, stainless steel, aluminum, magnesium, titanium, metal alloys, and the like. The polymer material of the second and fourth layers  1212 ,  1214  may be any suitable polymer material, such as aramid (e.g., woven aramid fibers), nylon, polyethylene, Vectran, or the like. In the case of polymer fibers (e.g., woven aramid), the polymer material may be cured or stiffened using an epoxy or other resin as a matrix. The graphite material of the third layer  1213  may be any suitable graphite material, such as thermoset carbon fiber, thermoplastic carbon fiber, pyrolytic carbon, one or more pre-impregnated sheets or layers of carbon fibers, or the like. 
       FIG.  12 C  depicts a cross-sectional view of an example bottom case  1220 , which may be an embodiment of the bottom case  1104  in  FIG.  11   . The cross-sectional view of the bottom case  1220  corresponds to a view along line C-C in  FIG.  11   . The composite structure of the bottom case  1220  may, however, be used for other layers or components of the enclosures described herein, such as a top case (e.g., the top case  1102  in  FIG.  11   , or any other top case described herein), a back layer of a display portion (e.g., the back layer  426  in  FIG.  4   ), an enclosure component defining a bottom portion of a tablet computer or smartphone (e.g., the third layer  112 ,  FIG.  2   ), or the like. 
     The bottom case  1220  includes a first layer  1221  formed of a metal, a second layer  1222  formed of a polymer material, a third layer  1223  formed of a foam material, a fourth layer  1224  formed of a graphite material, a fifth layer  1225  formed of a foam material (e.g., the same or a different foam material as the third layer  1223 ), a sixth layer  1226  formed of a polymer material (e.g., the same or a different polymer material as the second layer  1222 ), and a seventh layer  1227  formed of a metal (e.g., the same or a different metal as the first layer  1221 ). The particular metal, polymer, foam, and graphite materials used in the bottom case  1220  may be any of the corresponding materials as described above, and are not separately listed here. 
       FIG.  12 D  depicts a cross-sectional view of an example bottom case  1230 , which may be an embodiment of the bottom case  1104  in  FIG.  11   . The cross-sectional view of the bottom case  1230  corresponds to a view along line C-C in  FIG.  11   . The composite structure of the bottom case  1230  may, however, be used for other layers or components of the enclosures described herein, such as a top case (e.g., the top case  1102  in  FIG.  11   , or any other top case described herein), a back layer of a display portion (e.g., the back layer  426  in  FIG.  4   ), an enclosure component defining a bottom portion of a tablet computer or smartphone (e.g., the third layer  112 ,  FIG.  2   ), or the like. 
     The bottom case  1230  includes a first layer  1231  formed of a metal, a second layer  1232  formed of a polymer material, a third layer  1233  formed of a graphite material, a fourth layer  1234  formed of a foam material, a fifth layer  1235  formed of a graphite material (e.g., the same or a different graphite material as the third layer  1233 ), a sixth layer  1236  formed of a polymer material (e.g., the same or a different polymer material as the second layer  1232 ), and a seventh layer  1237  formed of a metal (e.g., the same or a different metal as the first layer  1231 ). The particular metal, polymer, foam, and graphite materials used in the bottom case  1230  may be any of the corresponding materials as described above, and are not separately listed here. 
     The composite structures in  FIGS.  12 A- 12 D  have vertically symmetrical layer arrangements. However, composite structures for use in enclosures need not have a vertically symmetrical arrangement.  FIG.  12 E  depicts a cross-sectional view of an example bottom case  1240  that is not vertically symmetric. The bottom case  1240  may be an embodiment of the bottom case  1104  in  FIG.  11   . The cross-sectional view of the bottom case  1240  corresponds to a view along line C-C in  FIG.  11   . The composite structure of the bottom case  1240  may, however, be used for other layers or components of the enclosures described herein, such as a top case (e.g., the top case  1102  in  FIG.  11   , or any other top case described herein), a back layer of a display portion (e.g., the back layer  426  in  FIG.  4   ), an enclosure component defining a bottom portion of a tablet computer or smartphone (e.g., the third layer  112 ,  FIG.  2   ), or the like. 
     The bottom case  1240  includes a first layer  1241  formed of a metal, a second layer  1242  formed of a polymer material, a third layer  1243  formed of a foam material, a fourth layer  1244  formed of a graphite material, and a fifth layer  1245  formed of a metal material (e.g., the same or a different metal as the first layer  1241 ). The particular metal, polymer, foam, and graphite materials used in the bottom case  1240  may be any of the corresponding materials as described above, and are not separately listed here. 
     The various layers of the composite structures described with respect to  FIGS.  12 A- 12 D  may have any suitable thickness. In some cases, the overall thickness of the composite structures about one millimeter or less. For example, the overall thickness of a composite material may be about 1.0 mm, 0.9 mm, 0.8 mm, 0.7 mm, 0.6 mm, or 0.5 mm. Each individual layer may have a thickness that is selected based on a desired overall performance or property of the composite material. For example, the in-plane (e.g., horizontal, as shown in  FIGS.  12 A- 12 D ) thermal conductivity may be greater when there is relatively more graphite than foam in the composite structure. Accordingly, in order to increase the in-plane thermal conductivity, a composite may have a graphite layer that is thicker than a foam layer. 
     Table 1, below, shows the layer materials and layer thicknesses of four example composite structures that may be used for electronic device enclosures as described herein. These examples may generally correspond to the layer arrangement shown and described with respect to  FIG.  12 D , and may be vertically symmetric. A layer of zero thickness indicates that that particular material layer may be omitted from the composite structure. (Example 3 may be understood to include two layers of 0.22 mm pyrolytic graphite laminated together, or one layer of 0.44 mm pyrolytic graphite.) 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 1 
                 2 
                 3 
                 4 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Stainless Steel 
                 0.05 mm 
                 0.05 mm 
                 0.05 mm 
                 0.05 mm 
               
               
                 Aramid 
                 0.07 mm 
                 0.07 mm 
                 0.07 mm 
                 0.07 mm 
               
               
                 Pyrolitic Graphite 
                 0.17 mm 
                  0.0 mm 
                 0.22 mm 
                 0.12 mm 
               
               
                 Foam 
                 0.12 mm 
                 0.46 mm 
                  0.0 mm 
                 0.22 mm 
               
               
                 Pyrolitic Graphite 
                 0.17 mm 
                  0.0 mm 
                 0.22 mm 
                 0.12 mm 
               
               
                 Aramid 
                 0.07 mm 
                 0.07 mm 
                 0.07 mm 
                 0.07 mm 
               
               
                 Stainless Steel 
                 0.05 mm 
                 0.05 mm 
                 0.05 mm 
                 0.05 mm 
               
               
                   
               
            
           
         
       
     
     Table 2, below, shows the layer materials and layer thicknesses of four further example composite structures that may be used for electronic device enclosures as described herein. These examples may generally correspond to the layer arrangement shown and described with respect to  FIG.  12 E , and may be not vertically symmetric. A layer of zero thickness indicates that that particular material layer may be omitted from the composite structure. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 5 
                 6 
                 7 
                 8 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Stainless Steel 
                 0.05 mm 
                 0.05 mm 
                 0.05 mm 
                 0.05 mm 
               
               
                 Aramid 
                 0.07 mm 
                 0.07 mm 
                 0.07 mm 
                 0.07 mm 
               
               
                 Foam 
                 0.41 mm 
                 0.53 mm 
                  0.0 mm 
                 0.28 mm 
               
               
                 Pyrolitic Graphite 
                 0.12 mm 
                  0.0 mm 
                 0.47 mm 
                 0.25 mm 
               
               
                 Stainless Steel 
                 0.05 mm 
                 0.05 mm 
                 0.05 mm 
                 0.05 mm 
               
               
                   
               
            
           
         
       
     
     The various layers of the composite structures shown in  FIGS.  12 A- 12 E  may be secured to one another in any suitable way. For example, some or all the layers may be bonded to one another with adhesive or another bonding agent. In some cases, the adhesive or bonding agent may be part of or distributed throughout a given layer. For example, a graphite layer may be impregnated with epoxy or another adhesive or bonding agent, and as such a separate adhesive may not be required to bond the graphite layer with an adjacent layer (e.g., the impregnated epoxy may bond the graphite layer to an adjoining layer while also providing structure to the graphite layer). In some cases, a separate adhesive, epoxy, or bonding agent is applied between two adjacent layers. In some cases, mechanical fastening is used instead of or in addition to adhesives or other bonding agents, such as the mechanical fastening described in  FIGS.  16 - 17 D . 
     The composite structures shown in  FIGS.  12 A- 12 E  also show the ends of each layer exposed along the side surface of the composite structure. That is, each layer defines part of a side surface of the composite structure. When implemented as a bottom case (or other component) of an electronic device enclosure, these layers may remain exposed, or they may be covered by another material or component. For example, a strip or binding may be applied over some or all of the side surfaces of a composite structure to cover and protect the side surfaces of each individual layer. The strip or binding may be a polymer, ink, paint, film, metal, epoxy, or any other suitable material. In some cases, the composite structure is deformed at the periphery (e.g., by crimping, rolling, or the like) so that the side surfaces of the individual layers are not exposed, as described with respect to  FIGS.  18 A and  18 C . 
     While the cross-sectional views in  FIGS.  12 A- 12 E  show the composite structures in one particular orientation, this orientation is merely for illustration, and the composite structures may be rotated or flipped in any given implementation. Further, either exterior layer of a given composite structure may be used to define an exterior surface of an enclosure. For example, if the composite structure shown in  FIG.  12 E  were used as a top case, the first layer  1241  may define the top exterior surface of the top case. Alternatively, the composite structure may be flipped so that the fifth layer  1245  defines the top exterior surface of the top case. 
     As noted above, the layers of a composite structure as described herein need not be uniform over their entire area.  FIG.  13    shows an exploded view of part of a base portion  1300  of a device, including an intermediate layer  1302  (e.g., a circuit board), a bottom case  1304  that is formed of a composite material having different layer configurations at different locations, and a processor  1303  (or other heat generating component) which may be attached to the intermediate layer  1302  and thermally coupled to the bottom case  1304 . 
     The bottom case  1304  may include several different regions, including a main region  1306 , reinforced regions  1308 , and a thermal region  1310 . The main region  1306  may have multiple layers, and may take the form of any of the composite structures described above with respect to  FIGS.  12 A- 12 E . The reinforced regions  1308  may include reinforcing materials that increase the strength, toughness, stiffness, or other property of the composite structure in those regions. The reinforced regions  1308  may be formed by increasing the thickness of one or more layers of the composite structure (while optionally decreasing or removing other layers). For example, if the main region  1306  has a composite structure corresponding to the bottom case  1200  in  FIG.  12 A , the second layer  1202 , which may be foam in the main region  1306 , may be replaced with carbon fiber in the reinforced regions  1308 . Similarly, in the thermal region  1310 , the foam may be replaced with graphite or copper (or another material with a higher thermal conductivity than foam). The graphite or copper in the thermal region  1310  may help conduct heat away from the processor  1303  (or other heat generating component), which may help cool the processor  1303 . 
     In some cases, instead of replacing all of a layer of material with another material, as described above, one or more layers may be added to (or omitted from) the composite structure in that area. For example, instead of replacing all of the foam with carbon fiber in the reinforced regions  1308 , the foam layer may be half as thick as the foam in the main region  1306 , and the carbon fiber may occupy the remaining space left available by the use of the thinner foam. 
       FIG.  14    shows another example bottom case  1400  that uses a non-uniform layer arrangement to facilitate the extraction of heat from a processor  1401  (or other heat generating component). As shown in  FIG.  14   , the processor  1401  is thermally coupled to the bottom case  1400 , though it may be electrically coupled to a circuit board or other component that may be positioned above the bottom case  1400 . As shown in greater detail in  FIGS.  15 B- 15 C , the bottom case  1400  may include thermal conduits  1404  within the composite structure. The thermal conduits  1404  may conduct heat from the processor  1401  to heat sinks  1406 , which may also be positioned within or otherwise integrated with the composite structure. The thermal conduits  1404  may be formed of or include any suitable material, such as graphite, copper, aluminum, steel, metal alloys, composite materials, or the like. In some cases, the material of the thermal conduits  1404  may be selected to have a thermal conductivity that draws heat away from the processor  1401  to the heat sinks  1406  at a sufficient rate to facilitate effective cooling of the processor  1401  (or other heat generating component). 
       FIG.  15 A  depicts a cross-sectional view of the bottom case  1400 , viewed along line D-D in  FIG.  14   . The bottom case  1400  includes a first layer  1501 , a second layer  1502 , and a third layer  1503 . For simplicity, the bottom case  1400  is shown as having only three layers, though the bottom case  1400  may use any suitable composite structure, such as any of the composite structures described herein. As shown, the first layer  1501  may be a metal, the second layer  1502  may be a foam, and the third layer  1503  may be a metal (e.g., the same or a different metal as the first layer  1501 ). In this region of the bottom case  1400 , the first, second, and third layers  1501 ,  1502 ,  1503  extend fully (and uniformly) from one side of the bottom case  1400  to an opposite side of the bottom case  1400 . 
       FIG.  15 B  depicts a cross-sectional view of the bottom case  1400 , viewed along line E-E in  FIG.  14   . This view shows the structure of the bottom case  1400  in the area where the thermal conduits  1404  and the heat sinks  1406  are positioned. The processor  1401  may be thermally coupled to the first layer  1501 , such as by directly contacting the first layer  1501  or via a bonding material  1504  (e.g., a thermal paste, adhesive, epoxy, copper layer, etc.). In this region, all or some of the material of the second layer  1502  may be replaced with a material with a higher thermal conductivity (or having any particular thermal properties, including lower thermal conductivity, anisotropic thermal conductivity profiles, and the like). For example, where the second layer  1502  is a foam material, some or all of the foam material may be replaced with graphite, copper, gold, aluminum, or another thermally conductive material, to form the thermal conduits  1404 . The thermal conduits  1404  may be thermally coupled to the heat sinks  1406 , and may thus conduct heat away from the processor  1401  and towards the heat sinks  1406 , as illustrated by arrows  1505 , where the heat may be expelled from the device. The heat sinks  1406  may be any suitable type of heat sink or thermal mass. For example, the heat sinks  1406  may be vapor chambers, metals or other materials or components with a high heat capacity (e.g., gold, iron, fluid vessels with water or ammonia or other fluids), structures with heat-exchanging fins, or the like. The thermal conduits  1404  and the heat sinks  1406  may assist in cooling the processor  1401 , and may also help distribute heat produced by the processor  1401  more evenly throughout the bottom case  1400 . For example, the composite structure of the bottom case  1400  may have a thermal conductivity profile that results in greater through-plane heat conduction (e.g., in the vertical direction as shown in  FIG.  15 B ) than in-plane heat conduction (e.g., in the horizontal direction as shown in  FIG.  15 B ). This may result in the area of the bottom case  1400  that is directly below the processor  1401  becoming too hot. By conducting the heat to more remote parts of the bottom case  1400 , the heat may be more evenly distributed, thus producing a more even temperature profile along the bottom case  1400 . 
       FIG.  15 C  depicts a cross-sectional view of another example bottom case  1510 , which may be an embodiment of the bottom case  1400  in  FIG.  14   . The cross-sectional view of the bottom case  1510  corresponds to a view along line E-E in  FIG.  14   . The bottom case  1510  may include a laminated graphite second layer  1511  that defines the thermal conduits within the bottom case  1510 . The laminated graphite second layer  1511  may define stepped profiles  1513  in the area underneath or near the processor  1401 , which may aid in the transfer of heat into the layers of graphite. For example, heat may be more readily conducted into the graphite layers of the laminated graphite second layer  1511  through the ends of the layers, rather than through the primary surfaces of the layers (e.g., the top and bottom surfaces, as illustrated in  FIG.  15 C ). Accordingly, the stepped profiles  1513  may expose the ends of the graphite layers to improve the transfer of heat into the graphite second layer  1511 . 
     The bottom case  1510  may also include a conductive mass  1512  that is thermally coupled to the processor  1401  (e.g., via the first layer  1501  and the optional bonding material  1504 ) and to the ends of the graphite layers of the laminated graphite second layer  1511 . More particularly, the conductive mass  1512  may have complementary stepped profiles that correspond to and/or mate with the stepped profiles  1513  of the laminated graphite second layer  1511 . The conductive mass  1512  may thus conduct heat away from the processor  1401 , through the conductive mass  1512 , and into the laminated graphite second layer  1511  via the ends of the layers of graphite (as illustrated by arrow  1514 ). The conductive mass  1512  may be formed of any suitable thermally conductive material, such as metal (e.g., copper, gold, silver, aluminum, etc.), thermally conductive polymers, or the like. 
     The composite structures described herein may be formed of multiple layers that are adhered or otherwise bonded together. In some cases, instead of or in addition to adhesive between adjacent layers, the layers of the composite structures described herein are secured together using other fastening structures.  FIG.  16    depicts an example bottom case  1600 , formed from a composite structure, that has fastening structures (e.g., a fastening structure  1601 ) that secure the layers of the composite structure together, alone or in conjunction with adhesives. The fastening structures may help improve physical properties of the composite structure (and thus the bottom case  1600  or any other component in which the composite structure is used), such as flexural rigidity, tensile stiffness, toughness, resistance to shear deformations, and the like. 
     In some cases, the fastening structures may reduce or otherwise affect the thermal conductivity of the bottom case  1600 , especially along a transverse direction (e.g., in-plane, when the bottom case  1600  defines a plane or a planar region). Accordingly, in cases where the bottom case  1600  is used to conduct and/or dissipate heat from a processor or other heat generating component, the fastening structures may be arranged to define one or more thermal passages (e.g., a thermal passage  1603 ) extending from a heat-generating component region  1602  towards a periphery of the bottom case  1600 . The thermal passages may be substantially or entirely free of fastening structures along a linear path from the heat-generating component region  1602  to a peripheral side of the bottom case  1600 . In some cases, the thermal passages extend radially from the heat-generating component region  1602  towards the peripheral side of the bottom case  1600 . 
     The fastening structures may take several different forms.  FIGS.  17 A- 17 D  depict bottom cases having various different fastening structure configurations. For example,  FIG.  17 A  depicts a partial cross-sectional view of a bottom case  1700 , which may be an embodiment of the bottom case  1600  in  FIG.  16   . The cross-sectional view of the bottom case  1700  corresponds to a view along line F-F in  FIG.  16   . Further, while the fastening structure of the bottom case  1700  is shown in conjunction with a composite structure having a particular arrangement of layers, the same fastening structure may be used for any composite structure described herein, such as those shown and described with respect to  FIGS.  12 A- 12 E . 
     The bottom case  1700  includes a first layer  1701  formed of a metal, a second layer  1702  formed of a polymer, a third layer  1703  formed of a foam or graphite, a fourth layer  1704  formed of a polymer (e.g., the same or a different polymer as the second layer  1702 ), and a fifth layer  1705  formed of a metal (e.g., the same or a different metal as the first layer  1701 ). Any of these layers may include or be replaced by multiple sub-layers. For example, a third layer  1703  formed of graphite may include multiple sub-layers of graphite. As another example, the third layer  1703  may include multiple sub-layers of foam and graphite. 
     The bottom case  1700  also includes a fastening structure  1706  (e.g., an embodiment of the fastening structure  1601 ). The fastening structure  1706  includes an opening  1707  (or a recess) in the third layer  1703  (which may itself include multiple layers), and portions of the second and fourth layers  1702 ,  1704  extending into the opening  1707 . For example, the second and fourth layers  1702 ,  1704  may be formed of or include a polymer material (e.g., an aramid fabric) pre-impregnated or otherwise incorporated with a resin or other adhesive or bonding agent. When forming the bottom case  1700 , a force may be applied to the second and fourth layers  1702 ,  1704  to force some of the polymer and/or bonding agent into the opening  1707 . In some cases, the force may be applied to the second and fourth layers  1702 ,  1704  directly, and then additional layers may be added to the second and fourth layers  1702 ,  1704 . In other cases, the force is applied to the second and fourth layers  1702 ,  1704  through other layers, such as where the first and fifth layers  1701 ,  1705  are applied prior to forming the fastening structure. By having the polymer and/or bonding agent extend into the opening  1707 , the polymer and the third layer  1703  define an interlocking structure (e.g., the polymer and the layer at least partially overlap one another) that aids in maintaining the lateral or in-plane positions of the layers in the bottom case  1700 . This may result in a bottom case with increased flexural rigidity, tensile stiffness, toughness, resistance to shear deformations or strains, or the like. 
       FIG.  17 B  depicts a partial cross-sectional view of a bottom case  1710 , which may be an embodiment of the bottom case  1600  in  FIG.  16   , and which includes another example fastening structure. The cross-sectional view of the bottom case  1710  corresponds to a view along line F-F in  FIG.  16   . Further, while the fastening structure of the bottom case  1710  is shown in conjunction with a composite structure having a particular arrangement of layers, the same fastening structure may be used for nay composite structure described herein, such as those shown and described with respect to  FIGS.  12 A- 12 E . 
     The bottom case  1710  includes a first layer  1711  formed of a metal, a second layer  1712  formed of a polymer, a third layer  1713  formed of a resin or other adhesive or bonding agent, a fourth layer  1714  formed of a foam or graphite, a fifth layer  1715  formed of a resin or other adhesive or bonding agent (e.g., the same or a different resin as the third layer  1713 ), a sixth layer  1716  formed of a polymer (e.g., the same or a different polymer as the second layer  1712 ), and a seventh layer  1717  formed of a metal (e.g., the same or a different metal as the first layer  1711 ). The bottom case  1710  also includes a fastening structure  1718  (e.g., an embodiment of the fastening structure  1601 ). The fastening structure  1718  includes an opening  1719  (or a recess) in the fourth layer  1714  (which may itself include multiple layers), and portions of the third and fifth layers  1713 ,  1715  extending into the opening  1719 . For example, the third and fifth layers  1713 ,  1715  may be formed of or include a resin, adhesive, or other bonding agent that is used to secure the second and sixth layers  1712 ,  1716  (which may be a polymer material such as an aramid fabric) to adjacent layers, and optionally to act as a matrix for the second and sixth layers  1712 ,  1716 . When forming the bottom case  1710 , a force may be applied to the composite structure to force some of the resin into the opening  1719 , or the resin may otherwise be caused to flow into the opening  1719 . In some cases, the force may be applied to the second and fourth layers  1712 ,  1714  directly, and then additional layers may be added to the second and fourth layers  1712 ,  1714 . In other cases, the force is applied to the second and fourth layers  1712 ,  1714  through other layers, such as where the first and fifth layers  1711 ,  1715  are applied prior to forming the fastening structure. By having the resin extend into the opening  1719 , the resin and the fourth layer  1714  define an interlocking structure that aids in maintaining the lateral or in-plane positions of the layers in the bottom case  1710 . This may result in a bottom case with increased flexural rigidity, tensile stiffness, toughness, resistance to shear deformations, or the like. 
       FIG.  17 C  depicts a partial cross-sectional view of a bottom case  1720 , which may be an embodiment of the bottom case  1600  in  FIG.  16   , and which includes another example fastening structure. The cross-sectional view of the bottom case  1720  corresponds to a view along line F-F in  FIG.  16   . Further, while the fastening structure of the bottom case  1720  is shown in conjunction with a composite structure having a particular arrangement of layers, the same fastening structure may be used for nay composite structure described herein, such as those shown and described with respect to  FIGS.  12 A- 12 E . 
     The bottom case  1720  includes a first layer  1721  formed of a metal, a second layer  1722  formed of a foam or graphite, and a third layer  1723  formed of a metal (e.g., the same or a different metal as the first layer  1721 ). Of course, as noted above, the bottom case  1720  may include more or different layers than those shown in  FIG.  17 C . The bottom case  1720  also includes a fastening structure  1724  (e.g., an embodiment of the fastening structure  1601 ). The fastening structure  1724  may be formed by deforming the first and third layers  1721 ,  1723  to force part of the material of the first and third layers  1721 ,  1723  into an opening  1725  in the second layer  1722 . In some cases, the deformed portions of the first and third layers  1721 ,  1723  are welded or otherwise fused together within the opening  1725 . In such cases, the first and third layers  1721 ,  1723  may be spot welded, laser welded (e.g., after the deformed regions are formed), diffusion bonded, or the like. By having the deformed portions of the first and third layers  1721 ,  1723  extend into the opening  1725 , the first and third layers  1721 ,  1723  and the second layer  1722  define an interlocking structure that aids in maintaining the lateral or in-plane positions of the layers in the bottom case  1720 . This may result in a bottom case with increased flexural rigidity, tensile stiffness, toughness, resistance to shear deformations, or the like. 
       FIG.  17 D  depicts a partial cross-sectional view of a bottom case  1730 , which may be an embodiment of the bottom case  1600  in  FIG.  16   , and which includes another example fastening structure. The cross-sectional view of the bottom case  1730  corresponds to a view along line F-F in  FIG.  16   . Further, while the fastening structure  1734  of the bottom case  1730  is shown in conjunction with a composite structure having a particular arrangement of layers, the same fastening structure may be used for nay composite structure described herein, such as those shown and described with respect to  FIGS.  12 A- 12 E . 
     The bottom case  1730  includes a first layer  1731  formed of a metal, a second layer  1732  formed of graphite (optionally including multiple sub-layers of graphite), and a third layer  1733  formed of a metal (e.g., the same or a different metal as the first layer  1731 ). Of course, as noted above, the bottom case  1730  may include more or different layers than those shown in  FIG.  17 D . 
     In order to form the fastening structure  1734  (e.g., an embodiment of the fastening structure  1601 ), a laser beam, plasma beam, or other heating or machining process may be used to form a cavity in the bottom case  1730  and at least partially fuse the ends of the various layers together. For example, by directing a laser beam onto the bottom case  1730 , a cavity may be formed that extends through the metal of the first layer  1731 , through at least part of the second layer  1732  (which may be formed of multiple sub-layers of graphite), and optionally into the third layer  1733 . The heat of the laser (or other type of beam or implement) may cause some of the metal of the first layer  1731  to form a fused surface  1735  along the recess. The fused surface  1735  may be formed from essentially only the metal that is ablated from the first layer  1731 . In other cases, the fused surface  1735  may be a mixture of the metal that is ablated from the first layer  1731  and the material (e.g., graphite) of the second layer  1732 . In yet other cases, the process of forming the fastening structure  1734  results in the ends of the individual sub-layers of graphite fusing directly together (e.g., without substantial integration of metal from the first layer  1731 ). The fused surface  1735  may help secure the various layers of the bottom case  1730  together, thus aiding in maintaining the lateral or in-plane positions of the layers in the bottom case  1730 . This may result in a bottom case with increased flexural rigidity, tensile stiffness, toughness, resistance to shear deformations, or the like. 
       FIG.  17 E  depicts a partial cross-sectional view of a bottom case  1740 , which may be an embodiment of the bottom case  1600  in  FIG.  16   , and which includes another example fastening structure. The cross-sectional view of the bottom case  1740  corresponds to a view along line F-F in  FIG.  16   . Further, while the fastening structure of the bottom case  1740  is shown in conjunction with a composite structure having a particular arrangement of layers, the same fastening structure may be used for nay composite structure described herein, such as those shown and described with respect to  FIGS.  12 A- 12 E . 
     The bottom case  1740  includes a first layer  1741  formed of a metal, a second layer  1742  formed of a polymer, a third layer  1743  formed of a foam or graphite, a fourth layer  1744  formed of a polymer (e.g., the same or a different polymer as the second layer  1742 ), and a fifth layer  1745  formed of a metal (e.g., the same or a different metal as the first layer  1741 ). The bottom case  1740  also includes an interstitial member  1746  positioned in an opening in the third layer  1743 . While the opening is shown as being defined in only one layer, the opening may extend through other layers as well, including any of the layers shown in  FIG.  17 E , or other layers that may be included in other embodiments. 
     The interstitial member  1746  may be any suitable material or combinations of materials. For example, the interstitial member  1746  may be a metal component, such as a metal plate, cylinder, or other component. In other examples, the interstitial member  1746  may be a plastic or polymer, ceramic, alloy, composite, foam, adhesive, or any other suitable material. The interstitial member  1746  may itself be an adhesive that is positioned in the opening during lamination of the bottom case  1740  and then cured. Once cured, the interstitial member  1746  may bond to the second, third, and fourth layers  1742 ,  1743 ,  1744 , thereby securing the layers together. 
     In some cases an adhesive or other bonding agent bonds the interstitial member  1746  to the layers of the bottom case  1740 . The adhesive or other bonding agent may be included in the opening and/or on the interstitial member  1746  when the interstitial member  1746  is positioned in the opening. In some cases, an adhesive or other bonding agent may be applied between the second layer  1742  and the third layer  1743  (and between the third layer  1743  and the fourth layer  1744 ), in which case that same adhesive may help secure the interstitial member  1746  and maintain it in position. 
     The interstitial member  1746  may provide stiffness, strength, or toughness to the bottom case  1740 , for example, by making it more difficult for adjacent layers to slide or shear with respect to one another (e.g., the interstitial member  1746  may increase the shear strength of the bottom case  1740 ). In some cases, the interstitial member  1746  may act as a thermal conduit to help improve the thermal conductivity of the bottom case  1740 . In such cases, the interstitial member  1746  may be formed of a metal or other material that has a greater thermal conductivity than the bottom case  1740  as a whole (or greater than the material of other layers, such as the third layer  1743 ). 
     In some cases, bottom cases (or other components) formed from the composite structures described herein are substantially planar and define featureless (e.g., flat) surfaces. In other cases, however, composite structures may include integrated features such as ribs, recesses, channels, posts, bosses, and the like.  FIGS.  18 A- 18 C  illustrate examples of features that may be formed in a bottom case formed of a composite structure. 
       FIG.  18 A  depicts an example bottom case  1800  that includes shaped features, including ribs  1801  and a rolled edge  1802 . The ribs  1801  and the rolled edge  1802  may increase the stiffness, strength, rigidity, or other physical property of the bottom case  1800 . The rolled edge  1802  may also position the end surfaces of the various layers of the bottom case  1800  so that they are not exposed along the side surface of the bottom case  1800 . This may help protect the layers from delamination or other damage. 
       FIG.  18 B  depicts a partial cross-sectional view of the bottom case  1800 , as viewed along line G-G in  FIG.  18 A . The bottom case  1800  may include a first layer  1811  formed of metal, a second layer  1812  formed of a polymer material, a third layer  1813  formed of a foam or graphite, and a fourth layer  1814  formed of a metal (e.g., the same or a different metal as the first layer  1811 ). Of course, the bottom case  1800  may include more or different layers than those shown in  FIG.  18 B . The rib  1801  is formed into to the first layer  1811 , and may be formed by molding, pressing, forging, hydroforming, machining, or any other suitable technique. As shown, the rib  1801  defines a convex feature along the top exterior surface of the first layer  1811 , and a concave feature along the interior surface of the first layer  1811  (e.g., the surface that is opposite the top exterior surface). Some of the material of the second layer  1812  may extend into the concave feature (e.g., into the rib  1801 ) in order to improve the strength, stiffness, rigidity, or other property of the rib  1801 , or the bottom case  1800  more generally. As noted above, a polymer layer may include a polymer fiber and an epoxy, resin, adhesive, or other curing agent. Accordingly, the material of the second layer  1812  that extends into the concave feature may be the polymer fiber, the epoxy (or other curing agent), or a combination of both materials. The material of the second layer  1812  may extend into the concave feature in response to a force being applied on the first layer  1811  prior to the second layer  1812  becoming cured. In some cases, an adhesive is used between the first layer  1811  and the second layer  1812  (e.g., in addition to or instead of a resin matrix that is integrated with the polymer fibers), and the adhesive may extend into the concave feature in a manner similar to that shown in  FIG.  18 B . 
       FIG.  18 C  depicts a partial cross-sectional view of the bottom case  1800 , as viewed along line H-H in  FIG.  18 A , showing details of the rolled edge  1802 . As noted above, a rolled edge may position the end surfaces of the layers of the bottom case  1800  so that they are facing (and optionally touching) a main surface of the bottom case  1800 , or are otherwise not exposed to the external environment where they may be susceptible to damage. As shown in  FIG.  18 C , all of the layers  1811 - 1814  of the composite structure are rolled or deformed so that the ends  1815  of the layers are facing the top surface of the first layer  1811 . In other cases, the layers may be rolled in the opposite direction, such that the ends  1815  are facing the fourth layer  1814 . Other shapes and configurations may be used instead of or in addition to the rolled configuration shown in  FIG.  18 C , including folds, crimps, pinched ends, or the like. Indeed, any shape or configuration that positions the ends of the layers so that they are not exposed to the external environment during normal operating use of a device may be used. 
       FIG.  18 D  depicts a partial cross-sectional view of the bottom case  1800 , showing another example of a rolled edge  1816  that may be formed in the bottom case  1800 . As shown in  FIG.  18 D , all of the layers  1811 - 1814  of the composite structure are folded, crimped, bent, or otherwise formed so that the ends  1815  of the layers are not facing the top surface of the first layer  1811  (e.g., resembling a single fold hem). 
       FIG.  18 E  depicts a partial cross-sectional view of the bottom case  1800 , showing another example of a rolled edge  1817  that may be formed in the bottom case  1800 . As shown in  FIG.  18 E , all of the layers  1811 - 1814  of the composite structure are folded, crimped, bent, or otherwise formed so that the ends  1815  of the layers are facing a folded portion of the bottom case  1800  (e.g., resembling a double fold hem). 
       FIG.  18 F  depicts a partial cross-sectional view of the bottom case  1800 , showing an example of a crimped edge  1819  that may be formed in the bottom case  1800 . As shown in  FIG.  18 F , the first and fourth layers  1811 ,  1814  may extend beyond the second and third layers  1812 ,  1813 , and the free ends of the first and fourth layers  1811 ,  1814  may be crimped together. The crimped ends may be secured together (e.g., via adhesive, weldments, fasteners, rivets, or the like), or they may simply be in contact with one another. Where the first and fourth layers  1811 ,  1814  are formed from a metal or other plastically deformable material, the crimped ends may retain their deformed shape after crimping. 
       FIG.  18 G  depicts a partial cross-sectional view of the bottom case  1800 , showing another example of a crimped edge  1820  that may be formed in the bottom case  1800 . As shown in  FIG.  18 G , instead of the first and fourth layers  1811 ,  1814  extending beyond the second and third layers  1812 ,  1813 , the first through fourth layers may all extend to the end of the bottom case, and all of the layers may be subjected to a crimping or deforming operation. In some cases, one or more layers of the bottom case  1800  are at least partially crushed, or otherwise thinned, due to the crimping, such as the second and third layers  1812 ,  1813  (as illustrated in  FIG.  18 G ). The crushed or thinned layers may be formed from a material that can be crushed or otherwise thinned during the crimping operation, such as a foam, composite, honeycomb, or the like. The crimped ends may be secured together (e.g., via adhesive, weldments, fasteners, rivets, or the like), or they may simply be in contact with one another. Where the first and fourth layers  1811 ,  1814  are formed from a metal or other plastically deformable material, the crimped ends may retain their deformed shape after crimping. 
     In some cases, the edges of a bottom case, such as the bottom case  1800 , may be both crimped (as in  FIGS.  18 F- 18 G ) and rolled or hemmed (as in  FIGS.  18 C- 18 E ). This may reduce the size of the rolled edge (relative to an un-crimped rolled edge), as the thickness of the portion being rolled or hemmed may be reduced relative to a main portion of the bottom case  1800 . 
     As described above, composite structures may facilitate cooling of electronic device components. For example, composite materials may be configured with thermal conduits, thermally conductive materials, heat sinks, and other materials, structures, and features that help remove and/or dissipate heat.  FIGS.  19 A- 19 B  depict an example electronic device that uses composite structures to facilitate cooling. For example,  FIG.  19 A  depicts an example electronic device  1900  that includes a composite structure integrated into a display portion to facilitate cooling of a display or other components of the electronic device. The device  1900  resembles a clamshell-style phone that has a display portion  1904  and a base portion  1902  flexibly or rotatably coupled to the display portion  1904 . The device  1900  may be an embodiment of other otherwise resemble other devices described herein, such as the device  400 . Accordingly, details of such devices are equally applicable to the device  1900  and will not be repeated here. 
     The display portion  1904  includes a front member  1906 , a back member  1908 , and an air-permeable structure  1910 . The front and back members  1906 ,  1908  may be any suitable materials and/or components, and may be single monolithic structures, or assemblies. For example, the front member  1906  may include a display, a cover, one or more housing components, and so on. The back member  1908  may be metal, plastic, a composite or laminate material, an assembly having multiple components, or the like. 
     The air-permeable structure  1910  may be any suitable structure, such as a series of rods, filaments, shaped (e.g., corrugated) sheets, fibers, an open-cell foam, or the like, that extend between the front member  1906  and the back member  1908 . The rods, filaments, fibers, foam, or shaped sheets may be formed from or include any suitable material, such as carbon fiber, polymer, metal, ceramic, polymer, composite materials, or the like. The air-permeable structure  1910  may structurally and at least semi-rigidly couple the back member  1908  to the front member  1906 . For example, the air-permeable structure  1910  may be sufficiently rigid to prevent substantial movement or flexing of the back member  1908  relative to the front member  1906  during normal use of the device  1900 . 
     The permeability of the air-permeable structure  1910  may allow air to flow through the structure to facilitate cooling of the device  1900 . For example, as shown in  FIG.  19 B , which is a side view of the device  1900  of  FIG.  19 A , air may enter one end of the air-permeable structure  1910  (e.g., at arrow  1914 ) and pass through the air-permeable structure  1910 , exiting at another location (e.g., as shown by arrow  1918 ). The air flow may be due to natural convection or forced air (e.g., from a fan). In the latter case, a fan may be included in the display portion  1904  or the base portion  1902  and configured to direct air flow through the air-permeable structure  1910 . 
     The air-permeable structure  1910  may be configured to assist in cooling the display portion  1904  or the base portion  1902  (or components within or otherwise coupled to the display or base portion). In order to move heat from the base portion  1902  to the air-permeable structure  1910 , the device  1900  may include a thermally conductive conduit  1912  that extends from the base portion  1902  to the display portion  1904  and thermally couples one or more heat generating components within the base portion  1902  to the air-permeable structure  1910  (or near the air-permeable structure  1910 ). The thermally conductive conduit  1912  transfers heat from the base portion  1902  to the display portion  1904  so that it can be removed via the airflow through the air-permeable structure  1910 . 
     The air-permeable structure  1910  may be shaped or otherwise configured so that air can flow through the structure in one or more directions. For example, the structure may allow air flow from a bottom of the display portion  1904  (e.g., proximate the hinge that joins the base portion  1902  and the display portion  1904 ) to the top of the display portion  1904 . Arrows  1914  and  1916  illustrate this passage. In some cases, instead of or in addition to allowing flow from the bottom to the top of the display portion  1094 , the air-permeable structure  1910  allows air flow in a perpendicular direction (e.g., into the page relative to  FIG.  19 B ). In some cases, air can flow through the air-permeable structure  1910  in substantially any direction that is parallel to a plane defined by the front and/or back members  1906 ,  1908  of the display portion  1904 . 
     As noted above, the instant application describes enclosures or device constructions in which side surfaces of functional components of the device (e.g., components providing more functionality than merely forming a housing or enclosure) are used to define distinct portions of a laminated side surface of the enclosure. While such constructions are shown using certain layers or components, it will be understood that more, fewer, or different layers than those shown herein may be used. Additional components that may be used in various types of devices that may form portions of the layered side surface of a device include, but are not limited to, shielding layers (e.g., metal or conductive films, foils, meshes, etc.), membranes, fabrics, coloring layers (e.g., inks, dyes, paints, etc.), lenses, image sensors, antennas (e.g., radiating structures of antennas), electrical insulators (e.g., plastics, foams, rubbers, glass, or other dielectric components), adhesives, and the like. Such layers may be positioned in any suitable position and to achieve various desired functions in the device in addition to forming part of an exterior surface of the device. Further, the foregoing examples describe a construction of a device (or a device enclosure) using various types of handheld electronic devices as example devices. However, as noted above, the concepts described herein may be used for other types of devices as well. For example, a layered device enclosure may be used for tablet computers, watches (e.g., smart watches), smartphones, desktop computers, notebook computers, displays, head-mounted displays, and the like. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. Also, when used herein to refer to positions of components, the terms above and below, or their synonyms, do not necessarily refer to an absolute position relative to an external reference, but instead refer to the relative position of components with reference to the figures.