Changing the dielectric material to optimize the electrical and mechanical properties of the flex circuit

Abstract

A uniform thickness flex circuit is taught that uses more than one dielectric layer. The first dielectric layer is more flexible and can be reliably bent at a radius of curvature that the second dielectric layer cannot. The second dielectric layer has at least one more desirable electrical property, such as leakage characteristics, than the first dielectric region. The use of the uniform thickness flex circuit to protect sensitive materials within an electronic enclosure is further described.

Description

Technical Field

[0001] The present disclosure generally relates to an electronic system having a flex circuit. The flex circuit may be placed in an electrical enclosure. The flex circuit within the electrical enclosure is bent or formed into a specific curvature having a first radius of curvature in a first region on the flex circuit. A first dielectric of the flex circuit is used in the first region, and a second dielectric of the flex circuit is used in a second region on the flex circuit that has a larger radius of curvature or is a flat region.

Summary of the Invention

[0002] Embodiments of the present invention provide a flex circuit of uniform thickness having a first dielectric having properties that enable bending without damaging the conductors of the flex circuit and a second dielectric having properties that result in less leakage than the leakage of the first dielectric.

[0003] In one embodiment, the first dielectric has a lower modulus of elasticity than the second dielectric, such that the first dielectric can bend with a smaller radius of curvature than the second dielectric. The first dielectric has a greater leakage than the second dielectric.

[0004] In one embodiment of the present invention, by way of non-limiting example, the first dielectric is "FR", where FR relates to a flame-retardant acrylic adhesive material. The second dielectric is "HT" and relates to a class of high-temperature laminate materials characterized by favorable material and electrical properties. Such materials can have a higher glass transition temperature (Tg) and lower loss characteristics than materials such as FR. Metallization is formed in both the first and second dielectric regions, and an AP core can be used that can be etched to provide an electrical wiring path. "AP" is generally used herein in relation to a class of weakly adhesive / high-performance laminate materials that includes a copper-clad laminate and the entire polyimide composite of a polyimide film bonded to a copper foil. In this specification, AP7163E is used as an exemplary "AP" film.

[0005] In one embodiment of the present invention, a conductor pattern is formed at one or more wiring levels of a flexible circuit. The conductor pattern can be a signal line that couples a first semiconductor chip to a second semiconductor chip. A battery can supply energy to the first semiconductor chip and the second semiconductor chip. The conductor pattern can be a meandering pattern that will be cut by a perforation or a tear indicating tampering of the electrical enclosure.

Brief Description of the Drawings

[0006]

Figure 1A

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[0007] The present disclosure generally relates to an electronic system having a flex circuit that needs to be sharply bent or folded. The flex circuit may be placed within an electrical enclosure. The flex circuit within the electrical enclosure is bent or formed into a particular curvature having a first radius of curvature in a first region on the flex circuit. A first dielectric of the flex circuit is used in the first region, and a second dielectric of the flex circuit is used in a second region on the flex circuit that has one of a larger radius of curvature and a flat region. The flex circuit has a uniform thickness.

[0008] The flex circuits described herein can be used when the flex circuit needs to be bent sharper than the radius of curvature supported by the second dielectric. The second dielectric has more desirable properties such as loss, leakage, and coefficient of expansion of high-frequency signals. In such curvatures, the first dielectric is used to support the necessary curvature without causing cracks in the wiring of the flex circuit. In places where such curvature is not required, i.e., in the "flatter" portions of the flex circuit, the second dielectric is used to advantageously utilize the aforementioned more desirable properties.

[0009] A small radius of curvature may be required in various applications. For example, the flex circuit may need to be folded. In another example, the electronic enclosure may contain highly confidential, perhaps encrypted components, and it is necessary to detect tampering such as drilling into the electronic enclosure or prying open the electronic enclosure. The flex circuit can be "formed" within a recess of the electronic enclosure that typically results in a small curvature. The flex circuit can include a pattern of one or more levels of wiring. Drilling into or prying open the electronic enclosure will cut one or more of the wirings within the pattern of the wiring, exposing the tampering. Some drawings of the electronic enclosure having the flex circuit will be described later.

[0010] In the drawings and the detailed description, generally, the same numbers refer to the same components, parts, steps, and processes.

[0011] Referring now to FIGS. 1A, 1B, and 1C, a top view of a flex circuit having an embodiment of the present invention is shown.

[0012] FIG. 1A is a top view of the flex circuit 101. The semiconductor chips 150 and 151 can be mounted on the flex circuit 101 by conventional means, such as wire bonding or surface mount connection. Further, a battery 152 for supplying energy to one or both of the semiconductor chips 150 and 151 can be mounted on the flex circuit 101. A cross-sectional view taken along A-A of FIG. 1A is shown in FIG. 1B.

[0013] FIG. 1B shows a cross-sectional view taken along A-A. Wiring levels 102, 103, and 104 are shown. More or fewer wiring levels are conceivable. Regions 110 (one on the left, one on the right) are regions where the aforementioned dielectric 2 is used and are hereinafter referred to as HT regions. Region 111 uses a dielectric 1 that can surely bend with a small radius of curvature and is hereinafter referred to as an FR region. FIG. 1B shows the flex circuit 101 bent with a radius of curvature 106. The curved region is referred to as a transition zone. The wirings of wiring levels 103 and 104 are shown as continuous. The wiring of wiring level 102 is shown as 102A and 102B, which would be cut as shown by a gap 105 if region 111 were implemented with a more brittle HT dielectric. Thus, region 111 is implemented using an FR dielectric to prevent such cuts in the transition zone.

[0014] FIG. 1C shows a semiconductor chip 150 mounted on a flex circuit 101, with an enlarged view of the chip, interconnects, and wiring patterns shown. The semiconductor chip 150 is mounted on the flex circuit 101 using surface connection elements 161 to the plating 170 and drops down to the upper level wiring signal vias 171. Within an electronic enclosure that protects the sensitive circuitry, the flex circuit 101 is formed in contact with a recessed section of the electronic enclosure where it can be held, for example as shown in FIG. 4A and described later, using an adhesive 465. Attempts to tamper with the electronic enclosure by perforation will cut one or more of the wiring level wiring patterns, thereby detecting the attempt at tampering.

[0015] Figures 4A and 4B show the electronic housing 400, with the top view shown in Figure 4B and the cross-sectional view at B-B shown in Figure 4A. The electronic housing 400 includes a housing top 402, a housing bottom 401, and a cavity 403. The flex circuits 101A and 101B are formed within the recesses of the housing top 402 and the housing bottom 401 and are shown to be adhered using an adhesive 465 to the contours of the housing top 402 and the housing bottom 401. The HT region 110 and the FR region 111 are shown in the top view 4B and the cross-sectional view 4A. In the cross-sectional view 4A, the flex circuits 101A and 101B use the HT region 110 within the double-arrow lines indicated by 110, and the flex circuits 101A and 101B use the FR region 111 within the two double-arrow lines indicated by 111. In the FR region 111, transition regions occur in the housing top 402 and the housing bottom 401, and the curvature is accommodated in those transition regions. In the central flat region of the cavity 403, the HT region is used to obtain the advantages of more desirable dielectric parameters of the HT dielectric. In practice, most of the housing top 402 and the housing bottom 401 are typically covered by the HT region, as shown in Figure 8. Currently, Figures 4A and 4B are drawn to emphasize the curved regions, but in reality, they are typically small compared to the actual size of the electronic housing (see Figure 8 for more typical curved regions). For example, the HT region 110 can be 90% or more of the region in Figure 4B, and the FR region 111 can be 10% or less of the region in Figure 4B. The concentric rectangular arrangement shown in Figure 4B facilitates the lamination of two different materials (i.e., the HT region 110 and the FR region 111) at different lamination temperatures. The HT region 110 is laminated at approximately 300 °C, and the FR region 111 is laminated at approximately 200 °C.

[0016] Figure 4C shows the electronic housing 400 and includes a cross-sectional view of the printed circuit board (PCB) 470 sandwiched between the flexible circuit 101A and the flexible circuit 101B as shown. The PCB 470 can be adhered to the flexible circuit 101A and the flexible circuit 101B using an adhesive 465. The PCB 470 can have a semiconductor chip 471 and a battery 472 electrically and mechanically attached thereto. Confidential information such as an encryption key can be stored in the semiconductor chip 471. The battery 472 can supply energy to destroy the confidential information of the semiconductor chip 471 when tampering of the electronic housing 400 is detected.

[0017] Figure 5 shows an example of the electronic housing 500. The flexible circuit 101 is shown as having a 180-degree bend. The electronic housing 500 is shown in cross-section and includes a housing bottom 501, a housing top 502, and the flexible circuit 101. It should be understood that semiconductor chips and other electronic components can be mounted on the flexible circuit 101 but are not shown in Figure 5 for simplicity. The flexible circuit 101 has a 180-degree bend and is shown as being folded back on itself in a transition zone where folding occurs, creating a bend with a small radius of curvature. The HT region 110 is shown as being used in a flat section of the flexible circuit 101, and the FR region 111 is shown in the bend. As before, this provides a large area of the flexible circuit 101 with the desirable properties of the HT dielectric and a relatively small area of the flexible circuit 101 that does not have the desirable properties of the HT dielectric but provides the flexibility of the FR dielectric in the curved region. The flexible circuit 101 is coupled to the housing bottom 501 and the housing top 502 using an adhesive 465. The housing top 502 and the housing bottom 501 can be attached at the location where they meet by an adhesive, screws, or other known attachment methods.

[0018] Next, referring to FIG. 2A, the raw material for region 111 of the flex circuit 101 is shown as the raw material before etching and lamination. The dielectric region 111 is a "FR" dielectric region used where the flex circuit 101 needs to be bent with a small radius of curvature. "Small radius of curvature" means that region 111 can be reliably bent at that radius of curvature, while region 110 cannot be reliably bent at that radius of curvature. The exemplary flex circuit 101 used for illustrative purposes in FIGS. 2A, 2B, 2C (dielectric of FR region 111) and FIGS. 3A, 3B, and 3C shows three cores, although more or fewer cores are also conceivable.

[0019] FIG. 2A shows the numbered layers and the thickness of each layer.

[0020] "Core 1" 201 includes 0.35 - mil copper (Cu) metallizations 202 and 204 and 1.0 - mil polyimide 203. As described above, core 201 can be AP7163E.

[0021] "Bonding film 1" 231 includes 0.5 - mil FR adhesive layers 232 and 234 and 0.5 - mil polyimide 233.

[0022] "Core 2" 211 includes 0.35 - mil metallization layers 212 and 214 and 1.0 - mil polyimide 213. Core 211 can be AP7163E.

[0023] "Bonding film 2" 241 includes 0.5 - mil FR adhesive layers 242 and 244 and 0.5 - mil polyimide 243.

[0024] "Core 3" 221 includes 0.35 - mil metallization layers 222 and 224 and 1.0 - mil polyimide 223. Core 221 can be AP7163E.

[0025] Figure 2B shows a cross-sectional view after etching and metallization of the raw material layer of Figure 2A. As shown, 50% etching is performed to generate the fine line pattern. In one embodiment, the fine line pattern has wirings proceeding in the same direction, while an alternative wiring layer has wirings and spaces alternately as shown. This can be useful for preventing perforation through the flexible circuit 101 without cutting the wirings of the wiring layer. The upper metallization layer 202 and the bottom metallization layer 224 are shown as not being etched in Figure 2B, but portions of the metallization layers 202 and 224 can be etched in a pattern to provide metal connections to circuits such as semiconductor chips 150 and 151.

[0026] Figure 2C shows a cross-sectional view after lamination of the raw materials of Figure 2B. Note that the flexible circuit 101 has a thickness of 6.7 mils. The metallization layers 202 and 224 are removed from this cross-sectional view. The reference numerals refer to the same levels as those shown in Figures 2A and 2B.

[0027] Referring next to Figure 3A, the raw materials of region 110 of the flexible circuit 101 are shown as the raw materials before etching and lamination. The dielectric region 110 is an "HT" dielectric region, which is used in places where the flexible circuit 101 can be bent with a relatively large radius of curvature or in flat regions. Figure 3A shows the numbered layers and the thickness of each layer.

[0028] "Core 1" 301 in Figure 3A is the same as "Core 1" 201 in Figure 2A but is called "Core 1" 301 and includes a 0.35-mil Cu metallization layer 302, 304 and a 1.0-mil polyimide layer 303. Core 301 can be AP7163E such as Pyralux® AP (AP7163E, DuPont).

[0029] "Bonding layer 1" 331 includes a 1.5-mil HT polyimide 333.

[0030] "Core 2" 311 includes 0.35 - mil Cu layers 312, 314 and 1.0 - mil polyimide 313. Core 311 can be AP7163E.

[0031] "Bonding film 2" 341 includes 1.50 - mil HT polyimide 343.

[0032] "Core 3" 321 includes 0.35 - mil Cu layers 322, 324 and 1.0 - mil polyimide 323. Core 321 can be AP7163E.

[0033] Figure 3B shows a raw material having the same reference numbers as in Figure 3A and having patterns etched in the metallized layers 304, 312, 314, and 322. Similar to the aforementioned FR region cross - sectional view, the metallized layers 302 and 324 may or may not be etched.

[0034] Figure 3C shows a cross - sectional view of the flexible circuit 101 in the "HT" region of section 110. The reference numbers are the same as those mentioned so far.

[0035] The thickness after lamination of the HT region 110 of the flexible circuit is 6.7 mils as shown in Figure 3C. This is the same thickness as the FR section 111 of the flexible circuit shown in Figure 2C. The core sections are of the same material and the same thickness for regions 111 and 110. The adhesive bond ply sections for the FR region 111 and the HT region 110 need to be defined so that the thickness of the flexible circuit 101 is uniform after lamination.

[0036] For simplicity, the cores are shown separately (i.e., cores 201, 211, and 222 are separate from cores 301, 311, and 321), but these cores include all the wiring within the flex circuit 101 and these cores are continuous. For example, cores 201 and 301 are the same core extending along the length of the flex circuit 101, and similarly, cores 211 and 311 are the same core extending along the length of the flex circuit 101, and cores 221 and 321 are the same core extending along the length of the flex circuit 101. In contrast, the bonding films for the FR regions 111 (reference 231 and 241) and the HT regions 110 (reference 331 and 341) are separate and need to contact each other at the junction where the FR region 111 and the HT region 110 are joined.

[0037] Next, referring to FIG. 6, an embodiment is shown that increases the likelihood of tearing and cutting of one or more wirings of the flex circuit 101 when the electronic housing is pried open during a tampering event. The electronic housing 400 (FIGS. 4A, 4B, and 4C), shown for simplicity as a portion of the electronic housing 400 beyond the dotted line 640, has the wiring in at least one of the regions 111 and 110 cut when pried open. An adhesive 465, such as epoxy or other strong adhesive, resists prying and causes the wiring of the flex circuit 101 to be cut. The FR region 111 is shown as a curved region 631 corresponding to the FR region 631 with a small radius of curvature as the first layer 611 and the second layer 621. The curved region 631 can occur when the flex circuit 101 is formed in the cavity 403 as shown in FIG. 4A. As shown in FIG. 6, the region of the FR region 111 of the first layer 611 overlaps the region of the HT region 110 of the second layer 621 as indicated by the dotted line 630, and the FR region 111 of the first layer 611 overlaps the boundary between the FR region 111 and the HT region 110 of the second layer. When the electronic housing is pried open, the overlapping structure will greatly increase the likelihood of tearing in both the FR region 111 and the HT region 110, and detect the tampering of the electronic housing by cutting the wiring.

[0038] FIG. 7 shows another embodiment that increases the likelihood of tearing and cutting of the wiring flex circuit 101 when the electronic housing 400 is pried open. Again, for simplicity, only a portion of the layer 711 cut by the dotted line 740 is shown. In this embodiment, in a single layer of the flex circuit 101, portions of the FR region 111 and the HT region 110 alternate and extend to the end of the flex circuit 101 including over the curved region 731. It is understood that in the electronic housing 400 in this embodiment, the radius of curvature needs to correspond to the minimum radius of curvature of the HT region 110.

[0039] FIG. 8 shows another embodiment that increases the possibility of wiring tearing and cutting when the electronic housing 400 is pried open. This embodiment can also be included as a variation of the foregoing embodiments. A top view of the housing bottom 401 (of the electronic housing 400 in FIGS. 4A and 4B) is shown. The enlarged section 801 shows a wiring pattern that meanders within the HT region 110 in a flat area near the end of the housing bottom 401. The gap 802 is a "slit" in one or more layers having the wiring of the flex circuit 101. The FR region 111 was shown in FIGS. 4A and 4B as having the end of the flex circuit 101 at the end of the electronic housing 400. In the embodiment of FIG. 8, as shown in the figure, since the end of the housing bottom 401 is flat at the periphery, the HT region 110 can be used for the flat portion to utilize its desirable electrical characteristics, and the FR region 111 is used to handle a small radius of curvature with respect to the flat bottom of the housing bottom 401 of the housing 400 (FIGS. 4A, 4B). As shown in FIGS. 4A and 4B, the HT region 110 will also be used for the flat bottom. The lamination process will again manage to laminate the HT region 110 using a temperature of about 300° C. and laminate the FR region 111 ring inside the periphery of the housing bottom 401 of the electronic housing 400 at a temperature of about 200° C. By laminating the FR region 111 having a lower temperature of about 200° C. after laminating the HT region 110 having a higher temperature of about 300° C., different lamination temperatures can be performed sequentially. Alternatively, two-temperature lamination can be performed using two plates at different temperatures that correlate with the pattern between the HT region 110 and the FR region 111.

[0040] FIG. 9 shows an exemplary plot of dielectric leakage in decibels (dB) versus frequency in kilohertz (kHz). For example, the vertical axis can range from about -92 dB at the bottom to -70 dB at the top. Similarly, the horizontal axis can range from about 0 kHz to 1.0 kHz. It may be necessary for the wiring in any layer of the flex circuit 101 to carry a relatively high-frequency signal, or an occasional pulse having high-frequency components. The plot shows very large power losses in the FR region 111, shown as the FR dielectric 902 in FIG. 9, as compared to the HT110 region, shown as the HT dielectric 902 in FIG. 9. In an application for detecting tampering, a battery (e.g., battery 152 in FIGS. 1A and 1B) inside the electronic enclosure 400 may be required to destroy confidential information on semiconductor chips (150, 151) inside the electronic enclosure 400. The confidential information can be, for example, an encryption key. An action to signal a wire cut can include deleting the encryption key inside the electronic enclosure. In some embodiments, the minimum operating life of the battery can be defined by a published cryptographic security specification such as the U.S. Government Federal Information Processing Standard (FIPS) 140-2 Security Requirement for Cryptographic Modules for the cryptographic module. In such applications, conservation of the energy stored in the battery is important.

[0041] FIG. 10 shows a method 100 that can be used to create a flex circuit using an HT region where an HT dielectric can be reliably used and an FR region where an HT dielectric cannot be reliably used.

[0042] In step 1010, a core material having a conductive material on at least one side that can be etched to form wiring is selected.

[0043] In step 1012, a first bonding layer is selected that includes a dielectric and an adhesive on at least one surface of the dielectric and supports a first radius of curvature. The first bonding layer has a thickness when laminated to the core material.

[0044] In step 1014, a second bonding layer is selected. The second bonding layer has at least one electrical property (e.g., leakage) or mechanical property (e.g., coefficient of thermal expansion) that is more desirable than the same electrical or mechanical properties of the first bonding layer. The second bonding layer cannot be reliably bent at the first radius of curvature. The second bonding layer has a thickness when laminated to the core material.

[0045] In step 1016, in the electronic housing, one or more regions where a curvature of the first radius of curvature is required are determined.

[0046] In step 1018, in the electronic housing, regions where the second bonding layer can be reliably used are determined.

[0047] In step 1020, the first bonding layer is laminated to the core material in one or more regions where a curvature of the first radius of curvature is required, and the second bonding layer is laminated to the core material in one or more regions where the second bonding layer can be reliably used.

Claims

1. A core having a polyimide layer with a surface to which a wiring pattern is attached, A first dielectric region attached to the surface of the core where a transition zone occurs, A second dielectric region attached to the surface of the core where the first dielectric is not attached, having less leakage than the first dielectric region, Comprising, The transition zone is a region including a curvature of a specific radius that can reliably bend the first dielectric region and cannot reliably bend the second dielectric region, Equipped with a flexible circuit, The thickness of the flexible circuit is uniform over the length of the flexible circuit, Device.

2. The first dielectric region is a flame-retardant acrylic adhesive material (FR), and the second dielectric region is a high-temperature lamination material (HT), the device according to claim 1.

3. Further comprising one or more electronic components mechanically and electrically coupled to the flexible circuit, the device according to claim 1.

4. Further comprising an electronic housing for accommodating at least a part of the flexible circuit, the electronic housing comprising a recessed region having the transition zone where a part of the flexible circuit is formed, the device according to claim 1.

5. The electronic housing further comprises a housing bottom including the recessed region, and the flexible circuit is attached to the surface of the housing bottom by an adhesive, the device according to claim 4.

6. The electronic housing further comprises a housing top, and the flexible circuit is attached to the surface of the housing top by the adhesive, the device according to claim 5.

7. The wiring pattern has overlapping wiring levels so as to cut at least one wiring of the wiring pattern by perforation in a tampering event, the device according to claim 6.

8. The dielectric region selected from at least one of the first dielectric region and the second dielectric region has a pattern for tearing and cutting at least one wiring of the wiring pattern in a tampering event of prying open the housing top and the housing bottom, the device according to claim 6.

9. The wiring pattern for tearing includes a meandering wiring pattern having slits from the ends of the dielectric region to strengthen the tearing, the device according to claim 8.

10. The apparatus according to claim 8, wherein the first pattern on the first dielectric region and the second pattern on the second dielectric region overlap between the first pattern and the second pattern arranged to cause tearing.

11. selecting a core material having a conductive material on its surface; selecting a first dielectric layer that supports a curvature having a first radius of curvature, wherein the first dielectric layer has a thickness when laminated with the core material; selecting a second dielectric layer that has more desirable electrical properties than the first dielectric layer but cannot reliably support the curvature at the first radius of curvature, wherein the second dielectric layer has the thickness when laminated with the core material and has less leakage than the first dielectric layer; determining a first region of the flexible circuit that must be bent at the first radius of curvature; determining a second region of the flexible circuit that does not need to be bent at the first radius of curvature; laminating the core material with the first dielectric layer in the first region of the flexible circuit and laminating the remaining portion of the core material with the second dielectric layer A method of creating a flexible circuit, comprising:

12. The method according to claim 11, further comprising selecting "AP core" for the core material, wherein "AP core" is a class of high-performance laminated materials without adhesiveness.

13. selecting an "FR" bonding layer as the first dielectric layer, wherein "FR" is a flame-retardant acrylic adhesive material in the flexible circuit; selecting an "HT" bonding layer as the second dielectric layer, wherein "HT" is a class of high-temperature laminated materials The method according to claim 11, further comprising:

14. A method of encapsulating a confidential material in an electronic housing, comprising: creating a housing bottom of the electronic housing, wherein the electronic housing has a recessed region and a transition zone between an end of the housing bottom and a bottom of the recess; creating a housing top of the electronic housing; A step of adhering a flexible circuit that covers the inner surfaces of the upper housing and the bottom housing, the flexible circuit being for detecting tampering of the electronic housing, the adhering step; comprising; the flexible circuit has a first dielectric region that surely crosses the transition zone; the flexible circuit has a second dielectric region for covering a region not covered by the first dielectric region; the second dielectric region has less leakage than the first dielectric region; a method.

15. The method according to claim 14, further comprising using a flame-retardant acrylic adhesive material "FR" as the first dielectric region and using a high-temperature laminate material "HT" as the second dielectric region.

16. The method according to claim 14, further comprising using a wiring pattern of the flexible circuit configured to detect tampering of the electronic housing.

17. The method according to claim 14, further comprising using a layout pattern of the first and second dielectric regions to strengthen tearing and cutting of the wiring of the flexible circuit when the upper housing and the bottom housing are pried open.

18. The method according to claim 14, further comprising disposing a printed circuit board (PCB) between the upper housing and the bottom housing, the PCB further having a semiconductor chip including the confidential material.

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