Patent Description:
Aspects of the present invention are as defined in the accompanying claims.

To extend the useful lifetime of printed circuit boards and inhibit circuit board traces from failing under repeated mechanical deformation, the inventors have conceived of including one or more ultrathin metal layers within a printed circuit board stack that act as a structural element, either throughout the printed circuit board or in particular regions for rigid-flex construction. High tensile strength and modulus of elasticity of the metal layers may absorb mechanical loads, especially in tension, and make the printed circuit more mechanically and electrically robust. As a result, the printed circuit may exhibit an increased useful lifespan, and may even be designed to move, bend, or fold repeatedly during use, without increasing the risk of failure of the circuit board traces.

A printed circuit board <NUM> including a flexible region <NUM> is provided. <FIG> depicts a first aspect of the printed circuit board <NUM> and relates to non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention. The printed circuit board <NUM> may include a first copper layer <NUM>, a first dielectric layer <NUM>, a second copper layer <NUM>, a first adhesive layer <NUM>, and a first metal layer <NUM>, in this order. <FIG> shows the incorporation of this ordered layering within a printed circuit board <NUM>. The first metal layer <NUM> may be a metal film having a tensile strength greater than each of the first and second copper layers <NUM>, <NUM> and greater than the dielectric layer <NUM>. The first metal layer <NUM> may also have a large modulus of elasticity compared to these layers. One potential advantage of this configuration is that the tensile strength of the first metal layer <NUM> reinforces the printed circuit board <NUM>. In particular, by using the first metal layer <NUM> as the load-receiving element in the layered stack, the copper traces in the copper layers <NUM>, <NUM> may not be as susceptible to fracture.

The first metal layer <NUM> may have a thickness in a range of <NUM> to <NUM>. It will be appreciated that the first metal layer <NUM> may be an ultrathin layer, thus while lending tensile strength to printed circuit board <NUM>, the first metal layer <NUM> may be incorporated into thin printed circuit board designs. The first metal layer <NUM> may be a metal film having a tensile strength in a range of <NUM> to <NUM> MPa. The first metal layer <NUM> may have a modulus of elasticity in a range of <NUM> to <NUM> GPa, one such material with these properties being stainless steel. Tensile performance may be improved by <NUM>% on a two-layer flexible printed circuit with a single metal layer <NUM> in thickness and relate to non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention. Alternatively, a <NUM> thick metal layer in a two-layer flexible printed circuit may realize a <NUM>% improvement in tensile strength over a standard two-layer flexible printed circuit. In implementations requiring significant structural strengthening, a <NUM> thick metal layer may be possible, the resulting board retaining some bendability.

One potential advantage of using a metal layer film is an increase in tensile strength without the addition of extra bulk, a problem with many alternative solutions to printed circuit board strengthening. Known solutions often rely on secondary reinforcement, such as polyester strips, glass fiber, plastic stiffeners, and metal shields, to give a few examples. These additions may be bulky, too rigid, or lack the appropriate material qualities desired for a flexible printed circuit. The metal layer film may be manufactured from a copper material such as electro-deposited (ED) copper. Alternatively, copper materials with stronger crystalline structures (e.g., HA, RA) compared to standard ED copper may be used; however, these materials often involve more sophisticated or expensive manufacturing processes. Alternative copper materials or layering extra copper for strength may add only marginal increases in reliability.

The first and second copper layers <NUM>, <NUM> may each have a thickness in a range of <NUM> - <NUM> mils (<NUM> - <NUM>). In an alternative configuration, the copper layers <NUM>, <NUM> may have traces that are <NUM> - <NUM> thick. The first and second copper layers <NUM>, <NUM> may have a modulus of elasticity of <NUM> GPa. It will be appreciated that although thinner traces are less susceptible to shear, they may be more vulnerable to tension. Incorporation of an ultrathin metal layer to withstand tension forces may therefore protect the copper traces.

The first adhesive layer <NUM> may be a thermoset adhesive. The thickness of the first adhesive layer <NUM> may be between <NUM> and <NUM>. The first adhesive layer <NUM> may be an adhesive that is non-conductive and that functions as an insulative layer; alternatively, the first adhesive layer <NUM> may be conductive. It will be appreciated that the first metal layer <NUM> and the second copper layer <NUM> may not be secured by adhesive, but allowed to move independently from one another. This may be advantageous in a design where greater flexibility is desired and the first metal layer <NUM> is utilized as a semi-independent strengthening layer.

The printed circuit board <NUM> may include a rigid region <NUM> including a rigid substrate <NUM>. This configuration is presented in <FIG>. The rigid substrate <NUM> may be a dielectric and may be, for example, a fiberglass material such as glass-reinforced epoxy laminate (e.g., FR4) that is conventionally incorporated into printed circuit boards. The rigid section may have a thickness between <NUM> and <NUM>. However, it will be appreciated that the thickness of the rigid region may vary given design and electrical constraints. In contrast to the rigid substrate <NUM>, the first dielectric layer <NUM> may be a flexible material such as polyimide that is commonly used in flexible printed circuits.

As shown in <FIG>, the first metal layer <NUM> may extend from the flexible region <NUM> into the rigid region <NUM>. Although this configuration represents one example implementation, for some applications it may be beneficial to run the first metal layer <NUM> the entire length of the printed circuit board <NUM>. In this implementation, the tensile load is carried by the first metal layer <NUM> from one end of the printed circuit board <NUM> to the other end. Specifically, continuous extension of the first metal layer <NUM> throughout regions prevents the tensile load from being concentrated in transition areas between rigid region <NUM> and flexible region <NUM>. Continuous extension in this manner offers the potential advantage of reducing the potential failure points, particularly in the transition areas.

Also illustrated in <FIG> is a printed circuit board stack in the rigid region <NUM>, where an additional copper layer, adhesive layer, and metal layer are incorporated. It will be appreciated that the exact layering of materials will accommodate the functions of the desired application. Metal layering may include an extended metal layer such as first metal layer <NUM> in <FIG>, or it may be sectioned only within a particular region, such as the metal layer in the rigid region <NUM> also shown in <FIG>.

An advantage of the printed circuit board <NUM> with at least first metal layer <NUM> is the compatibility of its construction with existing conventional printed circuit board manufacture. At its simplest, a method of manufacture includes lamination of first metal layer <NUM> as a new layer in the stack of printed circuit board <NUM>. As first metal layer <NUM> may be an ultrathin layer, via holes, conductor patterns, and/or keep-outs may be pre-die cut into the first metal layer <NUM>. Conventional equipment may then be used to laminate the first metal layer <NUM>. This is in contrast to known alternative techniques mentioned above that may require additional equipment, costly processes, or more complex alteration of known methods of manufacture. Such sophisticated methods have been often undertaken because standard ED copper may break in as few as <NUM>-<NUM> bend cycles, while copper with costlier crystalline structures (e.g., HA) may last through, for example, <NUM> bend cycles before breaking. Even these benefits may be mitigated when vias or other components are added to layers, since in that case an entire layer may require copper to be electro-deposited. The addition of first metal layer <NUM> may avoid these complications, may reduce the number of layers needed, and may increase the flexibility of printed circuit construction.

The construction of the printed circuit board <NUM> may include providing substrates, heating, lamination, application of adhesives, cutting, bonding, drilling, additive or subtractive processes, and other methods implemented in printed circuit board construction. It will be appreciated that application of the metal layering may be performed at any point during construction, and may involve simply aligning the metal layer and applying heat and/or pressure to an adhesive placed at desired locations to secure the metal layer. More sophisticated methods may be employed. Additionally, metal layering may be employed with normal rigid printed circuit board construction techniques to create inexpensive rigid-flex circuits. For example, metal sheets may be cut into traces and adhered between rigid boards to mechanically and electrically connect them.

The first metal layer <NUM> may be coupled by electrical connections to other layers, as shown in <FIG>. The first metal layer <NUM> is discontinuous and is comprised of strips and relates to non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention. These strips may be utilized as conductive conduits. <FIG> depicts connections <NUM> between the second copper layer <NUM> with the third copper layer <NUM> that pass through the first metal layer <NUM>. The connections <NUM> may be extensions of the first metal layer <NUM>, or they may be vias or other types of connectors. It will be appreciated that the first metal layer <NUM> may be formed in strips, as printed patterns, or may be laser cut, stamped, or punched, in addition to other possible fabrication methods being applied.

An additional advantage to patterning or designing the geometry of the first metal layer <NUM> may be to create a preferential breaking zone. In such a case, should breakage occur within the printed circuit board <NUM>, the first metal layer <NUM> will be constructed in such a way that the breakage will be in certain preferential zones or regions, for example. Consequently, important data lines may be preserved in the event of failure.

Alternatively, the first metal layer <NUM> may be coupled by mechanical connections to other layers. <FIG> show two example implementations of this configuration. In <FIG>, the first metal layer <NUM> is discontinuous and applied in the form of strips. While these strips add mechanical support to the printed circuit board <NUM>, the spaces between the strips allow for connections <NUM> to the second and third copper layers <NUM>, <NUM> separately from the first metal layer <NUM>. However, the first metal layer <NUM> itself is not used as a conductive connection in this case. The connections <NUM> may be, for example, vias. The first metal layer <NUM> may consist of strips arranged in regions to create spaces for multiple connections, as shown in <FIG> and relates to non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention. While the first metal layer <NUM> again functions as a mechanical connection, electrical connections are placed together in a separate space as connections <NUM>.

The printed circuit board <NUM> may be manufactured in a band structure <NUM> to be incorporated, for example, into a wearable device. <FIG> illustrates an example band structure <NUM>, which as shown includes the printed circuit board <NUM> and both a rigid region <NUM> and flexible regions <NUM>. Components requiring a firm foundation may be mounted to rigid region <NUM>, while flexible functionality may be achieved by incorporating flexible regions <NUM> into the band structure <NUM>. <FIG>, which replicates the rigid region <NUM> of <FIG>, shows an example implementation of a cross-section of rigid region <NUM> including a rigid substrate <NUM> in the band structure <NUM>. It will be appreciated that first metal layer <NUM> may continue the length of printed circuit board <NUM>, though rigid region <NUM> and flexible regions <NUM>, or it may be selectively applied to various regions as desired for design and application purposes.

<FIG> also shows the band structure <NUM> incorporating the printed circuit board <NUM>. The printed circuit board <NUM> may include at least one extension <NUM> of the first metal layer <NUM>, as illustrated in <FIG> by a magnification of a section of band structure <NUM>. The extension <NUM> may be a tab to be utilized during or after manufacture of band structure <NUM>. Alternatively, the extension <NUM> may function as a pull-point, a tack point, or an anchor point to secure or connect various components and structures of the band structure <NUM>. In one implementation, the extension <NUM> may function as a heat sink to draw heat away from sensitive components.

The printed circuit board <NUM> may be curved and relates to non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention <FIG> shows band structure <NUM> including printed circuit board <NUM> as shown previously as well as in a side-view, the curved structure apparent in profile. Device components may be mounted to different regions of the curved structure. Curvature may be achieved by a number of methods and employ many combinations of structures, taking particular advantage of the presence of metal layering in printed circuit board <NUM>. For example, a lamination process may be used. Inner and outer layers of the printed circuit board <NUM> including any metal layering may be constructed with various thicknesses to behave differently depending on their respective placement within the band structure <NUM>. As some areas may be more susceptible than others to wearing forces, varying the behavior of different layers may be advantageous to a curved design. Multiple metal sheets of varying thicknesses may reinforce some areas of higher stress in the band structure <NUM>. First metal layer <NUM> may run the length of band structure <NUM> but adhere only to certain portions of the printed circuit board <NUM>, such as in the rigid regions <NUM>. Alternatively, first metal layer <NUM> may only be composed in some regions of the printed circuit board <NUM>, or be applied in strips or cut-out patterns that are anchored to some locations. Physical properties of the patterns may be exploited, for example, by creating hatching, bending, or features to restrict or enhance movement. This may be particularly advantageous in a wearable device.

It will be appreciated that the first and/or second metal layers <NUM>, <NUM>, or any additional metal layers incorporated into the printed circuit board <NUM>, may be advantageously engineered to provide a number of other functions. The first metal layer <NUM> may create a shield for electromagnetic interference and electrostatic discharge. Alternatively, the metal layers may function mechanically to hold a particular shape or restore the device to a preferred shape after deformation, the metal layering being prestressed, annealed, or treated otherwise to achieve similar ends.

Many possible combinations of layers in the stack of printed circuit board <NUM> may be employed, depending on the desired application. Returning to <FIG>, the printed circuit board <NUM> may further include a second adhesive layer <NUM> on the first copper layer <NUM> and a second metal layer <NUM> on the second adhesive layer <NUM>. In this configuration the second adhesive layer <NUM> and the second metal layer <NUM> may be positioned on the side of the printed circuit board <NUM> opposite from the first adhesive layer <NUM> and first metal layer <NUM>. One potential advantage of this configuration is the use of two metal layers on either side of the printed circuit board <NUM> to balance the printed circuit board <NUM>.

In <FIG>, a second adhesive layer <NUM>, a third copper layer <NUM>, a second dielectric layer <NUM>, and a fourth copper layer <NUM> may be applied in this order and relates to non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention. In this configuration, the second adhesive layer <NUM> may be on the first metal layer <NUM> on the side of the printed circuit board <NUM> opposite from the first adhesive layer <NUM>, as shown in <FIG>. One potential advantage of this configuration is the positioning of the first metal layer <NUM> at the center of the stacked printed circuit board <NUM> as a strengthening component, the copper traces toward the outer edges of the printed circuit board <NUM> exposed for functional use.

<FIG> illustrates a method <NUM> for manufacturing a printed circuit board <NUM> including a flexible region <NUM>. At <NUM>, the method <NUM> includes providing a first copper layer <NUM>. The method <NUM> at <NUM> further includes providing a first dielectric layer <NUM>. At <NUM>, the method <NUM> further includes providing a second copper layer <NUM>. The aforementioned layers are provided in the listed order. The method <NUM> further includes at <NUM> providing a first adhesive layer <NUM>. At <NUM>, the method <NUM> further includes adhering a first metal layer <NUM> and the second copper layer <NUM> to the first adhesive layer <NUM>, the first metal layer <NUM> positioned on the side of the printed circuit board <NUM> opposite from the second copper layer <NUM>. As described above, the first metal layer <NUM> is a metal film having a tensile strength greater than each of the first and second copper layers <NUM>, <NUM> and greater than the first dielectric layer <NUM>. The first metal layer <NUM> may also have a greater modulus of elasticity than these layers, one being stainless steel with a modulus of elasticity in a range of <NUM> to <NUM> GPa.

As described above, a rigid substrate <NUM> is provided in a rigid region <NUM> of the printed circuit board <NUM>. Additionally, the first metal layer <NUM> may extend from the flexible region <NUM> into the rigid region <NUM>.

As further described above, a second adhesive layer <NUM> is provided and a second metal layer <NUM> and the first copper layer <NUM> adhered to the second adhesive layer <NUM>. The second metal layer <NUM> may be positioned on the side of the printed circuit board <NUM> opposite from the first copper layer <NUM>.

As also described above, a third copper layer <NUM>, a second dielectric layer <NUM>, and a fourth copper layer <NUM> is provided, in this listed order. A second adhesive layer <NUM> is provided where the first metal layer <NUM> and the third copper layer <NUM> are adhered to the second adhesive layer <NUM>. In this configuration, the third copper layer <NUM> is positioned on the side of the printed circuit board <NUM> opposite from the first metal layer <NUM>.

The first metal layer may be coupled to other layers in the printed circuit board <NUM> via electrical connections, as detailed above. Alternatively or additionally, the first metal layer may be coupled to other layers via mechanical connections.

The invention presented herein significantly addresses a fundamental problem in printed circuit board design. By controlling the position of an ultrathin metal layer within the printed circuit board stack and how it is connected to its environment, novel possibilities for design options in devices employing flex/rigid-flex printed circuit boards are created. A new level of mechanical robustness may be achievable by implementing the invention, which transforms a formerly delicate and protected component, the printed circuit board and its copper traces, into a strengthening structure that may not only be more reliable in and of itself but may be integrated as a load-bearing component in its own right. While previous solutions have been costly or required sophisticated manufacturing, this solution may be given to less expensive mass production.

In some aspects, the methods and processes described herein may be tied to a computing system of one or more computing devices and relatesto non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention.

<FIG> schematically shows a non-limiting aspect of a computing system <NUM> that can enact one or more of the methods and processes described above and relates to non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention. Computing system <NUM> may take the form of one or more personal computers, server computers, tablet computers, home-entertainment computers, network computing devices, gaming devices, mobile computing devices, mobile communication devices (e.g., smartphone), wearable computers, and/or other computing devices.

Computing system <NUM> includes a logic processor <NUM>, volatile memory <NUM>, and a non-volatile storage device <NUM>. Computing system <NUM> may optionally include a display subsystem <NUM>, input subsystem <NUM>, communication subsystem <NUM>, and/or other components not shown in FIG.

For example, the logic processor may be configured to execute instructions that are part of one or more applications, services, programs, routines, libraries, objects, components, data structures, or other logical constructs.

The logic processor may include one or more processors configured to execute software instructions. Additionally or alternatively, the logic processor may include one or more hardware or firmware logic processors configured to execute hardware or firmware instructions. Processors of the logic processor may be single-core or multi-core, and the instructions executed thereon may be configured for sequential, parallel, and/or distributed processing. In such a case, these virtualized aspects may be run on different physical logic processors of various different machines.

The term "program" may be used to describe an aspect of computing system <NUM> typically implemented in software by a processor to perform a particular function using portions of volatile memory, which function involves transformative processing that specially configures the processor to perform the function. Thus, a program may be instantiated via logic processor <NUM> executing instructions held by non-volatile storage device <NUM>, using portions of volatile memory <NUM>. It will be understood that different programs may be instantiated from the same application, service, code block, object, library, routine, API, function, etc. Likewise, the same program may be instantiated by different applications, services, code blocks, objects, routines, APIs, functions, etc. The term "program" may encompass individual or groups of executable files, data files, libraries, drivers, scripts, database records, etc..

In some aspects, the input subsystem may comprise or interface with selected natural user input (NUI) componentry and relate to non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention Such componentry may be integrated or peripheral, and the transduction and/or processing of input actions may be handled on- or off-board.

In some aspects, the communication subsystem may allow computing system <NUM> to send and/or receive messages to and/or from other devices via a network such as the Internet and relate to non-claimed combinations of features which are nevertheless relevant to highlight specific aspects of the invention.

Claim 1:
A rigid-flex printed circuit board (<NUM>), comprising:
a flexible region (<NUM>), including:
a first copper layer (<NUM>),
a first dielectric layer (<NUM>),
a second copper layer (<NUM>),
a first adhesive layer (<NUM>), and
a first metal layer (<NUM>), in this order,
wherein the first metal layer (<NUM>) is a metal film made of stainless steel having a tensile strength greater than each of the first and second copper layers (<NUM>, <NUM>) and greater than the first dielectric layer (<NUM>);
a rigid region (<NUM>) including a rigid substrate (<NUM>), wherein the first metal layer extends through the flexible region and the rigid region; and
wherein components are mounted in the rigid region.