Patent Grant
10624202
Many activities require secure electronic communications. To facilitate secure electronic communications, an encryption/decryption system may be implemented on an electronic assembly or printed circuit board assembly that is included in equipment connected to a communications network. Such an electronic assembly is an enticing target for malefactors since it may contain codes or keys to decrypt intercepted messages, or to encode fraudulent messages. To prevent this, an electronic assembly may be mounted in an enclosure, which is then wrapped in a security sensor and encapsulated with polyurethane resin. A security sensor may be, in one or more embodiments, a web or sheet of insulating material with circuit elements, such as closely-spaced, conductive lines fabricated on it. The circuit elements are disrupted if the sensor is torn, and the tear can be sensed in order to generate an alarm signal. The alarm signal may be conveyed to a monitor circuit in order to reveal an attack on the integrity of the assembly. The alarm signal may also trigger an erasure of encryption/decryption keys stored within the electronic assembly.
In one or more aspects, is a method of fabricating a tamper-respondent assembly is provided, which includes: providing at least one tamper-respondent sensor, the at least one tamper-respondent sensor comprising one or more tamper-detect networks defined, at least in part, by unexposed circuit lines within the at least one tamper-respondent sensor, and the at least one tamper-respondent sensor having at least one external bond region disposed over, in part, the unexposed circuit lines; providing at least one conductive trace forming, at least in part, at least one tamper-detect network of the one or more tamper-detect networks, the at least one conductive trace being exposed, at least in part, on the at least one tamper-respondent sensor within the at least one bond region, including over, in part, the unexposed circuit lines of the tamper-respondent sensor; and providing an adhesive contacting the at least one conductive trace within the at least one bond region of the at least one tamper-respondent sensor, the adhesive facilitating securing the at least one tamper-respondent sensor within he tamper-respondent assembly.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention.
One or more aspects of the present invention are particularly pointed out and distinctly claimed as examples in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
Aspects of the present invention and certain features, advantages, and details thereof, are explained more fully below with reference to the non-limiting example(s) illustrated in the accompanying drawings. Descriptions of well-known materials, fabrication tools, processing techniques, etc., are omitted so as not to unnecessarily obscure the invention in detail. It should be understood, however, that the detailed description and the specific example(s), while indicating aspects of the invention, are given by way of illustration only, and are not by way of limitation. Various substitutions, modifications, additions, and/or arrangements, within the spirit and/or scope of the underlying inventive concepts will be apparent to those skilled in the art for this disclosure. Note further that reference is made below to the drawings, which are not drawn to scale for ease of understanding, wherein the same reference numbers used throughout different figures designate the same or similar components. Also, note that numerous inventive aspects and features are disclosed herein, and unless otherwise inconsistent, each disclosed aspect or feature is combinable with any other disclosed aspect or feature as desired for a particular application, for establishing a secure volume about an electronic component or electronic assembly to be protected.
Reference is first made to
In one or more implementations, a tamper-proof electronic package such as depicted is configured or arranged to detect attempts to tamper-with or penetrate into electronic assembly enclosure 110. Accordingly, electronic assembly enclosure 110 also includes, for instance, a monitor circuit which, if tampering is detected, activates an erase circuit to erase information stored within the associated memory, as well as the encryption and/or decryption module within the communications card. These components may be mounted on, and interconnected by, a multi-layer circuit board, such as a printed circuit board or other multi-layer substrate, and be internally or externally powered via a power supply provided within the electronic assembly enclosure.
In the embodiment illustrated, and as one example only, electronic assembly enclosure 110 may be surrounded by a tamper-respondent sensor 120, an encapsulant 130, and an outer, thermally conductive enclosure 140. In one or more implementations, tamper-respondent sensor 120 may include a tamper-respondent laminate that is folded around electronic assembly enclosure 110, and encapsulant 130 may be provided in the form of a molding. Tamper-respondent sensor 120 may include various detection layers, which are monitored through, for instance, a ribbon cable by the enclosure monitor, against sudden violent attempts to penetrate enclosure 110 and damage the enclosure monitor or erase circuit, before information can be erased from the encryption module. The tamper-respondent sensor may be, for example, any such article commercially available or described in various publications and issued patents, or any enhanced article such as disclosed herein.
By way of example, tamper-respondent sensor 120 may be formed as a tamper-respondent laminate comprising a number of separate layers with, for instance, an outermost lamination-respondent layer including a matrix of, for example, diagonally-extending or sinusoidally-extending, conductive or semi-conductive lines printed onto a regular, thin insulating film. The matrix of lines forms a number of continuous conductors which would be broken if attempts are made to penetrate the film. The lines may be formed, for instance, by printing carbon-loaded Polymer Thick Film (PTF) ink onto the film and selectively connecting the lines on each side, by conductive vias, near the edges of the film. Connections between the lines and an enclosure monitor of the communications card may be provided via, for instance, one or more ribbon cables. The ribbon cable itself may be formed of lines of conductive ink printed onto an extension of the film, if desired. Connections between the matrix and the ribbon cable may be made via connectors formed on one edge of the film. As noted, the laminate may be wrapped around the electronic assembly enclosure to define the tamper-respondent sensor 120 surrounding enclosure 110.
In one or more implementations, the various elements of the laminate may be adhered together and wrapped around enclosure 110, in a similar manner to gift-wrapping a parcel, to define the tamper-respondent sensor shape 120. The assembly may be placed in a mold which is then filled with, for instance, cold-pour polyurethane, and the polyurethane may be cured and hardened to form an encapsulant 130. The encapsulant may, in one or more embodiments, completely surround the tamper-respondent sensor 120 and enclosure 110, and thus form a complete environmental seal, protecting the interior of the enclosure. The hardened polyurethane is resilient and increases robustness of the electronic package in normal use. Outer, thermally conductive enclosure 140 may optionally be provided over encapsulant 130 to, for instance, provide further structural rigidity to the electronic package.
Note that, as an enhancement, within a sealed electronic package, such as the tamper-proof electronic package depicted in
Hollow spacers 213 may be placed below dimples 206 in top metal shell 204, and rivets 214 provided, extending through openings in dimples 206, through hollow spacers 213 and through openings in printed circuit board 210 to base metal shell 202 in order to fixedly secure electronic assembly 208 within the enclosure formed by base and top metal shells 202, 204. A security mesh or tamper-respondent sensor 216 is wrapped around the top, base, and four sides of the enclosure formed by base and top metal shells 202, 204. As illustrated, in one or more embodiments, top metal shell 204 may have an opening through which a bus 220 extends. One end of bus 220 may be connected to conductors (not shown) on printed circuit board 210, and the other end may be connected to conductors (not shown) on a printed circuit board 222. As bus 220 passes through the opening, the bus extends between an inner edge region 223 of the security mesh 216 and an overlapping, outer edge region 224 of the security mesh 216. A group of wires 226 connect, in one embodiment, security mesh 216 to conductors on printed circuit board 210. Circuitry on printed circuit board 210 is responsive to a break or discontinuity in security sensor array 216, in which case, an alarm signal may be emitted on bus 220, and also encryption/decryption keys stored within electronic assembly 208 may be erased.
In one or more implementations, liquid polyurethane resin may be applied to security mesh 216 and cured. An outer, thermally conductive enclosure 228, such as a copper enclosure, may be filled with liquid polyurethane resin with the electronic assembly and inner enclosure and security mesh suspended within it. Upon curing the resin, the electronic assembly and inner enclosure and security mesh become embedded in a polyurethane block or encapsulant 230, as shown. The enclosure 228 is mounted on the printed circuit board 222, which can be accomplished using, for instance, legs 240 which extend through slots in printed circuit board 222 and terminate in flanges 242, which are then bent out of alignment with the slots. Bus 220 may be connected, by way of printed circuit board 222 to connectors 244 located along, for instance, one edge of printed circuit board 222.
When considering tamper-proof packaging, the electronic package needs to maintain defined tamper-proof requirements, such as those set forth in the National Institutes of Standards and Technology (NIST) Publication FIPS 140-2, which is a U.S. Government Computer Security Standard, used to accredit cryptographic modules. The NIST FIPS 140-2 defines four levels of security, named Level 1 to Level 4, with Security Level 1 providing the lowest level of security, and Security Level 4 providing the highest level of security. At Security Level 4, physical security mechanisms are provided to establish a complete envelope of protection around the cryptographic module, with the intent of detecting and responding to any unauthorized attempt at physical access. Penetration of the cryptographic module enclosure from any direction has a very high probability of being detected, resulting in the immediate zeroization of all plain text critical security parameters (CSPs). Security Level 4 cryptographic modules are useful for operation in physically unprotected environments. Security Level 4 also protects a cryptographic module against a security compromise due to environmental conditions or fluctuations outside of the module's normal operating ranges for voltages and temperature. Intentional excursions beyond the normal operating ranges may be used by an attacker to thwart the cryptographic module's defenses. The cryptographic module is required to either include specialized environmental protection features designed to detect fluctuations and zeroize critical security parameters, or to undergo rigorous environmental failure testing to provide reasonable assurance that the module will not be affected by fluctuations outside of the normal operating range in a manner that can compromise the security of the module.
To address the demands of ever-improving anti-intrusion technology, and the higher-performance encryption/decryption functions being provided, enhancements to the tamper-proof, tamper-evident packaging for the electronic assembly at issue are desired. Numerous enhancements are described hereinbelow to, for instance, tamper-respondent assemblies and tamper-respondent sensors. Note that the numerous inventive aspects described herein may be used singly, or in any desired combination. Additionally, in one or more implementations, the enhancements to tamper-proof electronic packaging described herein may be provided to work within defined space limitations for existing packages. For instance, one or more of the concepts described may be configured to work with peripheral component interconnect express (PCIe) size limits, and the limitations resulting from being capsulated in, for instance, an insulating encapsulant.
Thus, disclosed hereinbelow with reference to
As noted, as intrusion technology continues to evolve, anti-intrusion technology needs to continue to improve to stay ahead. In one or more implementations, the above-summarized tamper-respondent sensor 300 of
In one or more aspects, disclosed herein is a tamper-respondent sensor 300 with circuit lines 301 having reduced line widths W1 of, for instance, 200 μm, or less, such as less than or equal to 100 μm, or even more particularly, in the range of 30-70 μm. This is contrasted with conventional trace widths, which are typically on the order of 350 μm or larger. Commensurate with reducing the circuit line width W1, line-to-line spacing width Ws 303 is also reduced to less than or equal to 200 μm, such as less than or equal to 100 μm, or for instance, in a range of 30-70 μm. Advantageously, by reducing the line width W1 and line-to-line spacing Ws of circuit lines 301 within tamper-respondent sensor 300, the circuit line width and pitch is on the same order of magnitude as the smallest intrusion instruments currently available, and therefore, any intrusion attempt will necessarily remove a sufficient amount of a circuit line(s) to cause resistance to change, and thereby the tamper intrusion to be detected. Note that, by making the circuit line width of the smaller dimensions disclosed herein, any cutting or damage to the smaller-dimensioned circuit line will also be more likely to be detected, that is, due to a greater change in resistance. For instance, if an intrusion attempt cuts a 100 μm width line by 50%, it reduces the remaining available line width for conducting current to 50 μm. This change is more likely to result in a detectable change in resistance, compared with, for instance, a 50% reduction in a more conventional line width of 350 μm to, for instance, 175 μm. The smaller the conductive circuit line width becomes, the more likely that a tampering of that line will be detected.
Note also that a variety of materials may advantageously be employed to form the circuit lines. For instance, the circuit lines may be formed of a conductive ink (such as a carbon-loaded conductive ink) printed onto one or both opposite sides of one or more of the flexible layers 302 in a stack of such layers. Alternatively, a metal or metal alloy could be used to form the circuit lines, such as copper, silver, silver carbon, or nickel-phosphorus (NiP), or Omega-Ply®, offered by Omega Technologies, Inc. of Culver City, Calif. (USA), or Ticer™ offered by Ticer Technologies, Chandler, Ariz. (USA). Note that the process employed to form the fine circuit lines or traces on the order described herein is dependent, in part, on the choice of material used for the circuit lines. For instance, if copper circuit lines are being fabricated, then additive processing, such as plating up copper traces, or subtractive processing, such as etching away unwanted copper between trace lines, may be employed. By way of further example, if conductive ink is employed as the circuit line material, fine circuit lines on the order disclosed herein can be achieved by focusing on the rheological properties of the conductive ink formulation. Further, rather than simple pneumatics of pushing conductive ink through an aperture in a stencil with a squeegee, the screen emulsion may be characterized as very thin (for instance, 150 to 200 μm), and a squeegee angle may be used such that the ink is sheared to achieve conductive ink breakaway rather than pumping the conductive ink through the screen apertures. Note that the screen for fine line width printing such as described herein may have the following characteristics in one specific embodiment: a fine polyester thread for both warp and weave on the order of 75 micrometers; a thread count between 250-320 threads per inch; a mesh thickness of, for instance, 150 micrometers; an open area between threads that is at least 1.5× to 2.0× the conductive ink particle size; and to maintain dimensional stability of the print, the screen snap-off is kept to a minimum due the screen strain during squeegee passage.
In one or more implementations, circuit lines 301 of tamper-respondent sensor 300 are electrically connected to define one or more resistive networks. Further, the circuit lines may include one or more resistive circuit lines by selecting the line material, line width W1 and line length L1, to provide a desired resistance per line. As one example, a “resistive circuit line” as used herein may comprise a line with 1000 ohms resistance or greater, end-to-end. In one specific example, a circuit line width of 50 μm, with a circuit line thickness of 10 μm may be used, with the line length L1 and material selected to achieve the desired resistance. At the dimensions described, good electrical conductors such as copper or silver may also be employed and still form a resistive network due to the fine dimensions noted. Alternatively, materials such as conductive ink or the above-noted Omega-Ply® or Ticer™ may be used to define resistive circuit lines.
In a further aspect, the flexible layer 302 itself may be further reduced in thickness from a typical polyester layer by selecting a crystalline polymer to form the flexible layer or substrate. By way of example, the crystalline polymer could comprise polyvinylidene difluoride (PVDF), or Kapton, or other crystalline polymer material. Advantageously, use of a crystalline polymer as the substrate film may reduce thickness of the flexible layer 302 to, for instance, 2 mils thick from a more conventional amorphous polyester layer of, for instance, 8 mils. A crystalline polymer can be made much thinner, while still maintaining structural integrity of the flexible substrate, which advantageously allows for far more folding, and greater reliability of the sensor after folding. Note that the radius of any fold or curvature of the sensor is necessarily constrained by the thickness of the layers comprising the sensor. Thus, by reducing the flexible layer thickness to, for instance, 2 mils, then in a four tamper-respondent layer stack, the stack thickness can be reduced from, for instance, 30 mils in the case of a typical polyester film, to 10 mils or less with the use of crystalline polymer films.
As noted, the circuit lines 301 forming the at least one resistive network may be disposed on either the first side or the second side of the opposite sides of the flexible layer(s) 302 within the tamper-respondent sensor 300, or on both the first and second sides. One embodiment of this depicted in
As illustrated in
An alternate tamper-respondent sensor 300′ is depicted in
In addition, or alternatively, the first circuit lines 301 of the first flexible layer may be formed of a first material, and the second circuit lines 301′ of the second flexible layer may be formed of a second material, where the first material of the first circuit lines 301 may be different from the second material of the second circuit lines 301′. For instance, first circuit lines 301 may be formed of conductive ink, and second circuit lines 301′ may be formed of a metal, such as copper. By providing tamper-respondent sensor 300′ with at least some of the circuit lines formed of a metal material, such as copper, enhanced tamper detection may be obtained. For instance, an intrusion tool passing through one or more layers of circuit lines 301′ formed of a metal could generate debris which may be distributed during the intrusion attempt and result in shorting or otherwise damaging one or more other tamper-respondent layers within the tamper-respondent sensor 300′. If desired, more than two materials may be employed in more than one layers of circuit lines within the tamper-respondent sensor.
Based on the description provided herein, those skilled in the art will understand that the tamper-respondent sensors described above in connection with
In contrast to a prior tamper-respondent sensor which may utilize a single substrate of flexible dielectric with circuit lines, either on the upper or lower surface, or both surfaces, provided herein are tamper-respondent sensors which comprise, in one or more embodiments, multiple layers of materials and circuits to provide an enhanced tamper-proof, tamper-evident packaging, to meet the demands of ever-improving anti-intrusion technology requirements to protect encryption/decryption functions. By way of example,
As illustrated in
Therefore, in one or more embodiments, first sensor layer 410 and third sensor layer 430 may also each comprise a flexible layer of material having circuit lines forming one or more resistive networks disposed on the first and/or second sides thereof. For instance, conductive circuit lines may be provided on both the first and second sides of the flexible layers of the first sensor layer 410, the second sensor layer 420, and the third sensor layer 430, such that a vertical cross-section through the stack of layers intersects multiple layers of circuit lines. In this configuration, forming the second sensor layer 420 with curvatures, for instance, forming the second layer to be corrugated, advantageously enhances protection against physical intrusion, such as by a drill, without detection by the resistive networks by making the location of the circuit lines defining the resistive network(s) harder to identify.
By way of example, the second sensor layer 420 may initially comprise a thin, flexible layer of material, such as a thin, flexible layer with a thickness comparable to the desired minimum radius of the bending curvature for the desired corrugation of the second sensor layer. In one or more implementations, the second sensor layer may be corrugated by obtaining a flat, flexible sensor which is then fed through a set of heated top and bottom rollers, each with mating gear teeth to create the desired sinusoidal pattern in the sensor layer. One or more outer circuit layers or films comprising the circuit lines forming the one or more resistive networks may then be laminated, as desired, to one or both of the first and second sides of the formed layer to define the formed, flexible layer. In one or more implementations, an adhesive may be employed to affix the circuit layers or films comprising the one or more resistive networks to the formed layer. By way of example, the adhesive could include a PSA, epoxy, acrylic, thermoset, thermoplastic, electrically conductive epoxy, thermally conductive epoxy, etc., one or more of which could also be employed to affix the multiple sensor layers 410, 420, 430 together within the stack of layers.
As illustrated in
Connections of the tamper-respondent sensors, and sensor layers, described herein to, for instance, monitor circuitry disposed within the associated secure volume defined by the tamper-respondent electronic circuit structure may comprise input/output contacts or connectors formed on one or more edges of the tamper-respondent sensor (or sensor layer) or, for instance, one or more ribbon cables extending from the tamper-respondent sensor into the secure volume, as will be understood by one skilled in the art.
In
In further embodiments, one or more of the other layers may themselves comprise a flexible dielectric material with circuit lines forming at least one other resistive network on one of the first side or second side thereof.
In the implementation of
In the example of
As a variation,
Note that in the embodiments of
In the embodiment of
Note that although depicted in
As illustrated in
As depicted in
By way of further example,
Referring collectively to
Tamper-proof electronic package 800 further includes an enclosure 820, such as a pedestal-type enclosure, mounted to multilayer circuit board 810 within, for instance, a continuous groove (or trench) 812 formed within an upper surface of multilayer circuit board 810. In one or more embodiments, enclosure 820 may comprise a thermally conductive material and operate as a heat sink for facilitating cooling of the one or more electronic components 802 within the secure volume. A security mesh or tamper-respondent sensor 821, such as the above-described tamper-respondent sensors of
As depicted in
As noted with reference to
Note that the embodiment depicted in
By way of further example,
As illustrated, one or more external signal lines or planes 905 enter secure volume 801 between, in this embodiment, two tamper-respondent mat layers 900, and then electrically connect upwards into the secure volume 801 through one or more conductive vias, arranged in any desired location and pattern. In the configuration depicted, the one or more tamper-respondent frames 901 are disposed at least inside of the area defined by continuous groove 812 accommodating the base of enclosure 820. Together with security sensor 821 associated with enclosure 820, tamper-respondent frames 901 define secure volume 801 where extending, in part, into multilayer circuit board 810. With secure volume 801 defined, at least in part, within multilayer circuit board 810, the external signal line(s) 905 may be securely electrically connected to, for instance, the one or more electronic components 802 (
Added security may be provided by extending tamper-respondent mat layers 900 (and if desired, tamper-respondent frames 901) outward past continuous groove 812 accommodating enclosure 820. In this manner, a line of attack may be made more difficult at the interface between enclosure 820 and multilayer circuit board 810 since the attack would need to clear tamper-respondent mat layers 900, the bottom edge of tamper-respondent sensor 821 associated with enclosure 820, as well as the tamper-respondent frames 901 of the embedded tamper-respondent sensor.
Variations on the multilayer circuit board 810 of
Note also that, once within the secure volume is defined within multilayer circuit board 810, conductive vias within the secure volume between layers of multilayer circuit board 810 may be either aligned, or offset, as desired, dependent upon the implementation. Alignment of conductive vias may facilitate, for instance, providing a shortest connection path, while offsetting conductive vias between layers may further enhance security of the tamper-proof electronic package by making an attack into the secure volume through or around one or more tamper-respondent layers of the multiple tamper-respondent layers more difficult.
The tamper-respondent layers of the embedded tamper-respondent sensor formed within the multilayer circuit board of the electronic circuit or electronic package may include multiple conductive traces or lines formed between, for instance, respective sets of input and output contacts or vias at the trace termination points. Any number of conductive traces or circuits may be employed in defining a tamper-respondent layer or a tamper-respondent circuit zone within a tamper-respondent layer. For instance, 4, 6, 8, etc., conductive traces may be formed in parallel (or otherwise) within a given tamper-respondent layer or circuit zone between the respective sets of input and output contacts to those conductive traces.
In one or more implementations, the multilayer circuit board may be a multilayer wiring board or printed circuit board formed, for instance, by building up the multiple layers of the board.
As illustrated in
A first photoresist 1004 is provided over build-up 1000, and patterned with one or more openings 1005, through which the overlying conductive layer 1003 may be etched. Depending on the materials employed, and the etch processes used, a second etch process may be desired to remove portions of trace material layer 1002 to define the conductive traces of the subject tamper-respondent layer. First photoresist 1004 may then be removed, and a second photoresist 1004′ is provided over the conductive layer 1003 features to remain, such as the input and output contacts. Exposed portions of conductive layer 1003 are then etched, and the second photoresist 1004′ may be removed, with any opening in the layer being filled, for instance, with an adhesive (or pre-preg) and a next build-up layer is provided, as shown. Note that in this implementation, most of overlying conductive layer 1003 is etched away, with only the conductive contacts or vias remaining where desired, for instance, at the terminal points of the traces formed within the layer by the patterning of the trace material layer 1002. Note that any of a variety of materials may be employed to form the conductive lines or traces within a tamper-respondent layer. Nickel-phosphorous (NiP) is particularly advantageous as a material since it is resistant to contact by solder, or use of a conductive adhesive to bond to it, making it harder to bridge from one circuit or trace to the next during an attempt to penetrate into the protected secure volume of the electronic circuit. Other materials which could be employed include OhmegaPly®, offered by Ohmega Technologies, Inc., of Culver City, Calif. (USA), or Ticer′, offered by Ticer Technologies of Chandler, Ariz. (USA).
The trace lines or circuits within all of the tamper-respondent layers, and in particular, the tamper-respondent circuit zones, of the embedded tamper-respondent sensor, along with the tamper-respondent sensor 821, may be electrically connected into monitor or compare circuitry provided, for instance, within secure volume 801 of multilayer circuit board 810. The monitor circuitry may include various bridge or compare circuits, and conventional printed wiring board electrical interconnect inside the secure volume 801, for instance, located within the secure volume defined by the tamper-respondent frames 901 (
Note that advantageously, different tamper-respondent circuit zones on different tamper-respondent layers may be electrically interconnected into, for instance, the same comparator circuit or Wheatstone bridge of the monitor circuitry. Thus, any of a large number of interconnect configurations may be possible. For instance, if each of two tamper-respondent mat layers contains 30 tamper-respondent circuit zones, and each of two tamper-respondent frames contains 4 tamper-respondent circuit zones, then, for instance, the resultant 68 tamper-respondent circuit zones may be connected in any configuration within the secure volume to create the desired arrangement of circuit networks within the secure volume being monitored for changes in resistance or tampering. Note in this regard, that the power supply or battery for the tamper-respondent sensor may be located external to the secure volume, with the sensor being configured to trip and destroy any protected or critical data if the power supply or battery is tampered with.
As depicted in
As illustrated in
One consideration with a tamper-respondent assembly, and more particularly, a tamper-respondent electronic circuit structure such as described herein, arises from the need to transition the inner-sidewall tamper-respondent sensor through one or more inner-sidewall corners of an electronic enclosure such as described. As noted above, in one or more embodiments, the tamper-respondent electronic circuit structure comprises one or more tamper-respondent sensors, which are adhesively mounted or affixed to the inner surfaces of the electronic enclosure. These inner surfaces include an inner main surface, and an inner sidewall surface having, for instance, at least one inner-sidewall corner. As noted, the tamper-respondent sensor(s) may each be formed of one or more flexible layers having circuit lines on one or more layers which define tamper-detect networks, such as resistive networks, that may be connected to monitor circuitry for detection of intrusion attempts into the secure space defined by the tamper-respondent assembly. During fabrication, the flexible layers of the tamper-respondent sensor(s) could stretch and potentially buckle within one or more inner-sidewall corners of the electronic enclosure as the sensor is mounted to the enclosure. This stretching or buckling within the corner(s) could result in breaking one or more circuit lines defining the tamper-detect networks to be monitored, which would destroy the tamper-respondent assembly for its intended use. Further, any buckling of the tamper-respondent sensor(s) over the inner surface, such as at the inner-sidewall corner(s) of an inner sidewall surface, could result in potential breach points, which would cause the tamper-respondent assembly to fail a NIST FIPS 140-2 Level 4 security test. Described hereinbelow with reference to
As one example, flat angled-sidewall portion 1411 may be oriented at a 45° angle to the adjoining first side 1404 and second side 1405 of electronic enclosure 1400, which in one or more embodiments may be perpendicular to each other. In one or more implementations, first and second curved sidewall portions 1412, 1413 may have a similar bend radius, which may be, for instance, approximately five times or greater the thickness of the tamper-respondent sensor being mounted to the inner sidewall surface of the electronic enclosure 1400. In the illustrated example, transition region 1403 between inner sidewall surface 1402 and inner main surface 1401 continues within the inner-sidewall corners 1410, where a lower part 1411′ of flat, angled-sidewall portion 1411 curves outward in transition to inner main surface 1401, and lower portions 1412′, 1413′ of first and second curved-sidewall portions 1412, 1413 also further curve outward in transition to inner main surface 1401, as illustrated. Note that the corner configuration of
Electronic enclosure 1400 may be fabricated of a variety of materials and have a variety of different configurations. In one or more implementations, the enclosure may be a rigid, thermally conductive enclosure (fabricated, for instance, of a metal material) to facilitate conduction of heat from one or more electronic components within the secure volume defined (at least in part) by the tamper-respondent assembly. Note also that the rectangular configuration of electronic enclosure 1400 could be replaced with any of a variety of different enclosure configurations, any one of which may include one or more inner-sidewall corners configured, by way of example, such as illustrated in
As illustrated, in one or more implementations, the tamper-respondent electronic circuit structure associated with electronic enclosure 1400 may include an inner-sidewall tamper-respondent sensor 1510 and an inner main surface tamper-respondent sensor 1520, along with a security band 1530. In the illustrated example, inner-sidewall tamper-respondent sensor 1510 may be formed with an integrated flex ribbon cable or extension 1511 to facilitate electrical connection of the at least one resistive network within inner-sidewall tamper-respondent sensor 1510 to appropriate monitor circuitry (not shown) disposed within, for instance, the secure volume defined, at least in part, by the tamper-respondent assembly of
Note that, in the example provided in
Referring collectively to
In the depicted configuration, multiple corner tabs 1710 are provided, with at least one corner tab 1710 being provided at the at least one inner-sidewall corner. In the exemplary embodiment illustrated, two corner tabs 1710 are provided at each corner of the inner main surface tamper-respondent sensor 1520. These corner tabs 1710 include circuit lines 1705 (
As noted above in connection with
Generally stated, in one or more implementations, disclosed herein is a tamper-respondent assembly which includes an electronic enclosure to enclose, at least in part, at least one electronic component to be protected, wherein the electronic enclosure includes an inner surface. The tamper-respondent assembly also includes a tamper-respondent electronic circuit structure comprising a tamper-respondent sensor lining and covering, at least in part, the inner surface of the electronic enclosure. The tamper-respondent sensor may include a flexible layer having opposite first and second sides, and circuit lines substantially covering at least one of the first side or the second side of the flexible layer, forming at least one tamper-respondent network, such as described herein. The flexible layer of the tamper-respondent sensor could be a non-formed sensor layer or a formed sensor layer, in accordance with one or more of the sensor layer embodiments described herein.
The tamper-respondent assembly further includes a physical security structure, such as at least one security element, that overlies and physically secures in place, at least in part, the tamper-respondent sensor covering, at least in part, the inner surface of the electronic enclosure. In the embodiment of
In the example of
In one or more enhanced embodiments, the security element(s) defining the security band, or more generally, the physical security structure, are formed (for instance, by stamping) a metal material, or metal alloy, such as copper, soft stainless steel, etc. Further, the metal security element(s) may advantageously be electrically connected to ground to further enhance detection capabilities of the tamper-respondent assembly. By forming the security element(s) of a metal that is difficult to drill through, then, if an attempt were made to drill through the security element, metal fragments would be created, which potentially could be pulled into the sensor layer(s) lining the inner surface of the electronic enclosure, which would result in a greater chance of shorting or otherwise damaging the circuit lines forming the one or more tamper-respondent networks of the sensor during the attack, and thus enhance detection capability of the tamper-respondent sensor. Further, by electrically grounding the security element(s), then a drill contacting the grounded security element(s) after drilling through one or more tamper-respondent sensors would be more likely to short one or more of the circuit lines forming the at least one tamper-detect network in the associated tamper-respondent sensor(s). By grounding the security element(s), another path for current to flow is established, which advantageously increases the likelihood of detecting an attempt to tamper with the tamper-respondent assembly. Note that grounding of the security element(s) could be by any means, such as by electrically connecting the elements to one or more ground lines on the electronic assembly being protected by the tamper-respondent assembly, or (in certain of the embodiments disclosed herein) by electrically connecting the elements to one or more ground planes within the multilayer circuit board forming, in part, the secure volume about the electronic assembly being protected. In one or more implementations, the security element(s), or more generally, the security band or physical security structure, may be pre-formed (e.g., by stamping) into the desired shape, for example, to accommodate and overlie the overlap between the inner-sidewall tamper-respondent sensor and the inner main surface tamper-respondent sensor, such as depicted in
As a further enhancement, increased sensor sensitivity to a tamper event may be provided by fabricating the tamper-respondent assembly, and in particular, the tamper-respondent sensor, to include one or more regions with increased fragility or susceptibility to damage from mechanical stress resulting from or associated with a tamper event. By including circuit lines or traces of the tamper-respondent sensor at least in part within the one or more regions of increased susceptibility to damage, then there is an increased likelihood that one or more of the circuit lines may be damaged during the tamper event due the associated mechanical stress, thus enhancing ability to detect the tamper event. The one or more regions of the tamper-respondent sensor having increased susceptibility may be formed using a variety of approaches, such as selective laser ablation or etching of the sensor, configuring the sensor with cutout areas to define stress-concentrating inner edges, and/or directly adhering exposed circuit lines to a rigid surface of the tamper-respondent assembly. By way of example,
Referring to
As illustrated, tamper-respondent assembly 1900 includes a tamper-respondent electronic circuit structure comprising a tamper-respondent sensor 1910. In the depicted embodiment, tamper-respondent sensor 1910 includes one or more flexible layers 1911 having opposite first and second sides 1912, 1913, and circuit lines 1915, 1915′ forming, at least in part, at least one tamper-detect network. For instance, the circuit lines may form, at least in part, at least one resistive network electrically coupled to monitor circuitry such as described herein. By way of example, first circuit lines 1915 are disposed on first side 1912, and second circuit lines 1915′ are disposed on second side 1913 of flexible layer 1911. The circuit lines may be in the same or different patterns, as described above. Further, note that there may be any number of flexible layers 1911, and any number of circuit line 1915, 1915′ layers provided within tamper-respondent sensor 1910 as discussed above, for instance, in connection with
In accordance with one or more aspects of the present invention, circuit lines 1915 at first side 1912 of flexible layer 1911 are exposed, that is, any overlying protective layer, such as a polyimide layer, is at least partially, or fully, removed in the region to be secured to rigid surface 1902 of structure 1901. Note that circuit lines 1915, 1915′ may be formed of a variety of materials including, for instance, a metal or metal alloy. For instance, first circuit lines 1915 could be formed from copper, silver, silver-carbon, nickel-phosphorous. Alternatively, other materials, such as conductive ink or Omega-Ply®, or Ticer™, could be employed.
In the exemplary embodiment, an adhesive 1920, such as a thermoset material, is used to adhere or laminate tamper-respondent sensor 1910 to rigid surface 1902. In particular, in the embodiment depicted, the exposed first circuit lines 1915 on first side 1912 are directly, adhesively secured to rigid surface 1902 of structure 1901. In one or more implementations, adhesive 1920 is selected with a bond strength to secure first circuit lines 1915 to rigid surface 1902 that is equal or greater to a bond strength of first circuit lines 1915 to first side 1912 of one or more flexible layers 1911. In this manner, should a tamper event occur comprising an attempt to physically separate tamper-respondent sensor 1910 from structure 1901, one or more of first circuit lines 1915 will likely separate from flexible layer 1911, and thus be broken, with the resultant broken lines enhancing the ability of the tamper-respondent electronic circuit structure to detect the tamper event.
In one or more embodiments, setoff elements could be provided between rigid surface 1902 of structure 1901 and first side 1912 of tamper-respondent sensor 1910 to ensure the presence of a gap between first side 1912 of tamper-respondent sensor 1910 and the rigid surface 1920, and thus ensure that adhesive 1920 remains between the two surfaces during a fabrication approach where pressure may be applied to force the surfaces together to, for instance, facilitate the laminating of tamper-respondent sensor 1910 to structure 1901. In one or more implementations, the setoff elements could be dispersed throughout adhesive 1920 and may comprise, for instance, spherical elements 1922, such as glass spheres, disposed within the adhesive. Alternatively, in one or more implementations, rigid surface 1902 could be formed with one or more standoffs extending from the surface a desired gap distance.
As illustrated in
In one or more embodiments, tamper-respondent sensor 1910′ may comprise a stack of tamper-respondent layers secured together via an adhesive layer 1925, such as a double-sided adhesive film, similar to the structure described above in connection with
In accordance with one or more aspects of the present invention, a portion of circuit lines 1915 is exposed on the upper side and a portion of circuit lines 1915′ is exposed on the lower side of tamper-respondent sensor 1910′. The exposed circuit lines 1915, 1915′ are respectively laminated using an adhesive layer 1920, 1920′, to rigid surface 1902′ of electronic enclosure 1400, or to rigid surface 1932 of another structure 1930, as illustrated. That portion of tamper-respondent sensor 1910′ adhered to electronic enclosure 1400 and/or to another structure 1930 defines the one or more regions of the tamper-respondent sensor having the increased susceptibility to damage from mechanical stress associated with a tamper event. That is, should a tamper event attempt to delaminate the tamper-respondent sensor from one or both of electronic enclosure 1400 and another structure 1930, then adhesive layers 1920, 1920′ bonding the structures to the first side and the second side of tamper-respondent sensor 1910′, will likely cause one or more of the exposed circuit lines 1915, 1915′ to delaminate from the tamper-respondent sensor as the structures are separated, causing breaks in the circuit lines, and thereby facilitating detection of the tamper event by monitor circuitry of the tamper-respondent electronic circuit structure. As noted, this result can be facilitated by selecting the adhesive 1920, 1920′ to have, for instance, an equal or greater bond strength than the bond strength holding circuit lines 1915, 1915′ to their respective sides of the tamper-respondent sensor 1910′.
As noted, in the embodiments depicted in
Referring to
Multiple regions 2032, 2042 within tamper-respondent sensor 2001 of increased susceptibility to damage from mechanical stress are defined in this example by providing the tamper-respondent sensor with one or more cutout areas 2030, 2040. In the exemplary embodiment, cutout areas 2030 are illustrated as rectangular-shaped cutouts, extending into the sensor from one or more peripheral edges 2002, and cutout areas 2040 are triangular-shaped cutouts, extending into the sensor from one or more peripheral edge 2002 of the tamper-respondent sensor 2001. In each instance, cutout areas 2030, 2040 define stress-concentrating inner edges 2031, 2041, which (in this example) extend vertically through tamper-respondent sensor 2001. These edges 2031, 2041 are stress risers which define the regions 2032, 2042, respectively, of increased susceptibility to damage within tamper-respondent sensor 2001. The regions 2032, 2042 are locations within the sensor 2001 where stress is concentrated due to the provision of the stress-concentrating inner edges 2031, 2041. For instance, circuit lines 2015 may be provided which follow the contours of tamper-respondent sensor 2001 and extend, at least in part, through the respective regions 2032, 2042 of increased susceptibility to damage. In this manner, should a tamper event occur, mechanical stress within the tamper-respondent sensor will be concentrated by the stress-concentrating inner edges 2031, 2041, in regions 2032, 2042 of the sensor, thereby increasing a likelihood of a break in one or more of the circuit lines 2015 within those regions. This increased likelihood of a break facilitates detection of the tamper event by the monitor circuitry of the tamper-respondent electronic circuit structure.
Note that the circuit lines 2015 may be provided in any desired configuration or pattern on the various flexible layers 2011, as described herein. For instance, a rectangular grid of circuit lines could be provided in one or more layers which intersects, at least in part, the regions 2032, 2042 of increased susceptibility to damage from mechanical stress associated with a tamper event. Note also that regions 2032, 2042 of the tamper-respondent sensor are distinct regions of the sensor, separate from the balance of the tamper-respondent sensor, which may be characterized as being a standard robustness region outside of the regions 2032, 2042 created with the increased susceptibility to damage from mechanical stress. Thus, the tamper-respondent sensor may be thought of as being divided into different regions, a standard robustness region, and the regions 2032, 2042 of increased susceptibility to damage resulting from the provision of stress risers within the tamper-respondent sensor.
Note that any stress in one or more of regions 2032, 2042 of increased susceptibility to damage may extend vertically across multiple layers of the tamper-respondent sensor 2001, or be associated with one or more particular tamper-respondent layers (or flexible layers) within the tamper-respondent sensor 2001, dependent, for instance, on the tamper event. In one or more implementations, the cutouts, comprising for instance cutout areas 2030 and/or cutout areas 2040, may extend around a portion or all of the periphery of tamper-respondent sensor 2001, in any desired configuration and number. Further, any cutout configuration may be used that provides stress-concentration regions within the tamper-respondent sensor, for instance, due to the presence of one or more stress-concentrating inner edges (or stress risers). Additional embodiments of this concept are depicted in
Referring to
In the exemplary embodiment of
As illustrated, cutout areas 2060 may also, or alternatively, by provided as one or more channels extending through protective layers 2026 above and/or below the one or more flexible layers 2011 and circuit lines 2015. These cutout areas 2060 again comprise stress-concentrating inner edges 2061, which define regions 2062 adjoining the edges within the tamper-respondent sensor 2001′ that have increased susceptibility to damage from mechanical stress accompanying a tamper event against tamper-respondent sensor 2001′. As in the embodiments described above, circuit lines 2015 may extend, at least in part, through regions 2062 of increased susceptibility to damage, and thus, should a tamper event occur which generates mechanical stress within tamper-respondent sensor 2001′, any circuit lines within the regions of increased susceptibility to damage, for instance, in regions adjoining the stress-concentrating inner edges 2061, will more likely be damaged or break (that is, compared with the balance of the tamper-respondent sensor, characterized above as the standard robustness region of the sensor), thereby enhancing likelihood of detection of the tamper event by the monitor circuitry of the tamper-respondent electronic circuit structure comprising the tamper-respondent sensor 2001′.
Those skilled in the art will note that, disclosed herein are various enhancements to creating a secure volume for accommodating one or more electronic components, such as one or more encryption and/or decryption modules and associated components of a communications card or other electronic assembly. In certain embodiments, the tamper-respondent assembly, or tamper-proof electronic package, includes a tamper-respondent electronic circuit structure comprising one or more tamper-respondent sensors in various configurations disposed, for instance, external to or internal to an electronic enclosure to contain the electronic component(s) or electronic assembly to be protected. The tamper-respondent electronic circuit structure may also include tamper-detect monitor circuitry which monitors, for instance, for changes in resistance in one or more tamper-detect networks, such as one or more resistive networks, defined (at least in part) by circuit lines within the tamper-respondent sensor(s).
In one or more embodiments, the tamper-respondent sensor may be implemented as a fine-pitch flex circuit being formed, for instance, of multiple stacked layers of flexible film, such as the above-described crystalline-polymer material, with Kapton being one specific example. On one or more sides of each flexible layer, circuit lines are provided and electrically connected to facilitate defining one or more tamper-detect networks of the tamper-respondent sensor. In one or more implementations, line width and pitch may be significantly reduced in size from conventional approaches to, for instance, ≤200 μm. The circuit lines may be formed of any appropriate material, including a metal or metal alloy such as copper, silver, nickel-phosphorous (NiP), Omega-Ply®, or Ticer™. The flexible layers with the circuit lines are enclosed, for instance, by polyimide, such that the circuit lines are unexposed within the tamper-respondent sensor, and the tamper-respondent sensor provides a secure defense against a mechanical or physical intrusion through the sensor into a secure volume defined, at least in part, by the tamper-respondent sensor.
A variety of tamper-respondent assembly configurations are disclosed herein which may employ an adhesive in one or more external bond regions on a surface of the tamper-respondent sensor(s) to secure, for instance, a tamper-respondent sensor in an operative position within the tamper-respondent assembly. By way of example, the adhesive may be employed to maintain a particular configuration of the tamper-respondent sensor about an electronic enclosure, or to bond two or more tamper-respondent sensors together in a multi-sensor configuration, or to position a tamper-respondent sensor relative to an electronic enclosure of a tamper-respondent assembly, such as over an inner surface of an electronic enclosure. These external bond regions could be susceptible to chemical attack against the adhesive.
Therefore, by way of further enhancement, increased sensitivity to a tamper event may be provided by fabricating the tamper-respondent assembly to include conductive traces positioned and fabricated to have increased mechanical and/or chemical fragility or susceptibility to damage from a tamper event, and in particular to a tamper event at an external bond or overlap region of the sensor. For instance, one or more conductive traces may be exposed within a bond region(s) of one or more tamper-respondent sensors, and an adhesive provided contacting the conductive trace(s) within the bond region(s) of the tamper-respondent sensor(s). By directly contacting the adhesive to the conductive traces, any attempt to mechanically and/or chemically tamper with the adhesive, to facilitate gaining access to the secure volume within the tamper-respondent assembly, is more likely to damage one or more conductive traces, and thus be detected. Note in this regard, that by forming the conductive traces of a chemically compromisable or dissolvable conductive material during a chemical attack on the adhesive, then the conductive traces will likely be damaged during the attack on the adhesive. In this manner, the exposed conductive traces provide increased fragility or susceptibility to mechanical and/or chemical attack at the external bond region(s).
As noted, the at least one external bond region may be a region of the tamper-respondent sensor(s) where the sensor adheres to another surface, such as the surface of a rigid structure of the tamper-respondent assembly, or the surface of another tamper-respondent sensor, in a multi-tamper-respondent sensor embodiment, or even to another region of the same tamper-respondent assembly, such as depicted (for instance) in
In one or more embodiments, these one or more conductive traces may be distinct conductors from the unexposed circuit lines on the flexible layers within the tamper-respondent sensor(s). For instance, and as noted, conductive traces may be formed of a chemically compromisable or dissolvable material susceptible to damage during a chemical attack of the adhesive within the bond region(s) of the tamper-respondent sensor(s) to facilitate detecting the chemical attack, whereas the unexposed circuit lines forming the tamper-respondent sensor may be of a different conductive material (and may even be of different line width, and/or line-to-line spacing) to facilitate, for instance, securing the one or more electronic components to be protected against a mechanical tamper event through the sensor. In one or more examples, the circuit lines within the tamper-respondent sensor(s) may be smaller and of closer pitch than the conductive traces exposed on the surface of the tamper-respondent sensor(s) within the bond region(s). The one or more conductive traces forming the outer tamper-detect network may be placed on the tamper-respondent sensor(s) in any location susceptible to a chemical attack, such as where adhesive is employed to bond the tamper-respondent sensor to another surface of the tamper-respondent assembly, such as to an electronic enclosure, or to another tamper-respondent sensor of the assembly, or even to itself, depending on the implementation.
Referring to
Note also that, in one or more other implementations, the conductive traces described herein as susceptible to damage during chemical attack of the adhesive, could be disposed between one or more tamper-respondent sensors and an outer surface of a structure, such as an outer surface of an electronic enclosure. In this regard, reference the embodiments of
As illustrated, in one or more implementations, the tamper-respondent electronic circuit structure of the tamper-respondent assembly 2100 may include an inner-sidewall tamper-respondent sensor 1510′ and an inner main surface tamper-respondent sensor 1520′, each comprising one or more tamper-detect networks, such as one or more unexposed tamper-detect networks formed by circuit lines on one or more flexible layers, such as described above. The one or more tamper-detect networks are electrically connected to appropriate monitor circuitry (not shown) disposed within, for instance, the secure volume 801 defined by tamper-respondent assembly 2100. Note that in this example, inner-sidewall tamper-respondent sensor 1510′ and inner main surface tamper-respondent sensor 1520′ are discrete, first and second tamper-respondent sensors that overlap, at least in part, and facilitate defining the secure volume about the at least one electronic component 802 to be protected. For instance, the secure volume may be defined by securing the electronic enclosure to a multilayer circuit board 810 with an embedded tamper-respondent sensor 811, such as described above in connection with
As noted, to provide enhanced tamper-detect protection, one or more conductive traces 2120 may be provided exposed, at least in part, on one or more of the tamper-respondent sensors 1510′, 1520′ of tamper-respondent assembly 2100. For instance, one or more conductive traces 2120 are illustrated in the overlap region 2105 between inner-sidewall tamper-respondent sensor 1510′ and inner main surface tamper-respondent sensor 1520′, by way of example. Additionally, or alternatively, one or more conductive traces 2120′ could be provided, as part of the same or a different tamper-detect network(s), on a surface of inner-sidewall tamper-respondent sensor 1510′ between inner-sidewall tamper-respondent sensor 1510′ and an inner-sidewall of structure 2101, and/or on a surface of inner main surface tamper-respondent sensor 1520′, between inner main surface tamper-respondent sensor 1520′ and rigid surface 2102 of structure 2101. In one or more implementations, the conductive traces 2120, 2120′ in the bond region(s) are formed of a chemically compromisable or dissolvable conductive material susceptible to wetting or other damage during a chemical attack of the adhesive 2110, 2115 in direct contact therewith. The damage may result in dissolving one or more portions of the conductive traces, and thus, one or more portions of the associated tamper-detect network(s) defined (at least in part) by the traces and being monitored by the tamper-respondent electronic circuit structure, thereby facilitating detecting the tamper event. Note that the chemically dissolvable conductor used to form the conductive traces may be the same or a different material than the material used to form the unexposed circuit lines defining the one or more tamper-detect networks of the respective tamper-respondent sensor.
Stated generally, the conductive traces may be formed of a chemically compromisable conductive material, and may be provided in any bond region external to one or more sensors where, for instance, an adhesive bonds the respective tamper-respondent sensor to another surface, such as another surface of the tamper-respondent assembly. By way of example, the chemically dissolvable material used to form the conductive traces may comprise, at least in part, at least one of carbon, silver, or carbon-silver. For instance, the one or more conductive traces of the respective tamper-detect network(s) may be formed of a carbon-loaded conductive material, silver-loaded conductive material, or carbon-silver-loaded conductive material. Note also that different conductive traces may be in the same or different tamper-detect networks, and that conductive traces may be in the same or a different tamper-detect network than the network(s) defined by the sensor's unexposed circuit lines.
In the example of
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), and “contain” (and any form contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises”, “has”, “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more steps or elements. Likewise, a step of a method or an element of a device that “comprises”, “has”, “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way, but may also be configured in ways that are not listed.
The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below, if any, are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of one or more aspects of the invention and the practical application, and to enable others of ordinary skill in the art to understand one or more aspects of the invention for various embodiments with various modifications as are suited to the particular use contemplated.
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| Number | Date | Country | |
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| 20180103538 A1 | Apr 2018 | US |
| Number | Date | Country | |
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| Parent | 14941860 | Nov 2015 | US |
| Child | 15835569 | US | |
| Parent | 14865708 | Sep 2015 | US |
| Child | 14941860 | US |
