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Title:
METHODS AND APPARATUS FOR FORMING LIQUID CRYSTAL POLYMER FLEXIBLE CIRCUITS
Document Type and Number:
WIPO Patent Application WO/2019/217053
Kind Code:
A1
Abstract:
A flexible circuit (100) having a liquid crystal polymer (LCP) substrate (102) and an electrically conductive layer (104) laminated to the substrate is provided. The flexible circuit is formed in a hot pressing step utilizing a flexible graphite sheet (110) adapted to form an improved press surface for distributing press forces and press temperatures to the LCP film forming the laminated substrate. The graphite sheet can be a press pad for use in a hot press. The graphite sheet can be integrated into a press member thereby forming a press surface of a hot press press member. A plurality of LCP assemblies (101) each including an LCP material and an electrically conductive material forming an electrical circuit can be laminated to form a high layer count flexible circuit.

Inventors:
TRIMMER BRET A (US)
TAYLOR JONATHAN A (US)
BEYERLE RICHARD A (US)
Application Number:
PCT/US2019/028114
Publication Date:
November 14, 2019
Filing Date:
April 18, 2019
Export Citation:
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Assignee:
NEOGRAF SOLUTIONS LLC (US)
International Classes:
H05K1/03; B29D7/01; B30B15/06; B32B9/00; H05K3/00; H05K3/02; H05K3/46
Domestic Patent References:
WO2018039402A12018-03-01
Foreign References:
US20050079355A12005-04-14
US20130280470A12013-10-24
US20050067739A12005-03-31
US20040012754A12004-01-22
EP1428417A12004-06-16
US6963387B22005-11-08
Attorney, Agent or Firm:
SLEPECKY, Adam P. et al. (US)
Download PDF:
Claims:
CLAIMS

What is claimed is:

1. A method of making a flexible circuit comprising

hot pressing at least one assembly of a liquid crystal polymer film and an electrically conductive material with at least one press member and at least one sheet of compressed particles of exfoliated graphite having a density of at least 30 lbs./ft3.

2. The method of making a flexible circuit of claim 1 wherein the sheet of compressed particles of exfoliated graphite is disposed against the at least one of the press member, the liquid crystal polymer film and the electrically conductive material.

3. The method of claim 1 wherein the hot pressing further comprises adhering the electrically conductive material to the liquid crystal polymer film to form a laminated stack.

4. The method of 1 wherein the electrically conductive material comprises at least one of a copper foil, a silver foil and an aluminum foil.

5. The method of claim 4 wherein the at least one assembly comprises a plurality of assemblies each including a liquid crystal polymer film in contact with an electrically conductive material.

6. The method of claim 5 wherein the at least one assembly comprises no more than 50 assemblies.

7. The method of claim 3 wherein the adhering further comprises adhering the liquid crystal polymer film to the electrically conductive material without performing a second hot pressing step.

8. The method of claim 3 wherein the hot pressing occurs without an adhesive disposed between the electrically conductive material and the liquid crystal polymer film.

9. The method of claim 1 wherein the hot pressing occurs without the presence of a polyimide sheet.

10. The method of claim 1, wherein the sheet of compressed particles of exfoliated graphite has a density of between 40 lbs./ft3 and 110 lbs./ft3.

11. The method of claim 1 wherein the liquid crystal polymer film has a thickness of no more than 65 microns.

12. The method of claim 2 wherein the assembly is devoid of an intermediate layer between the conductive material and the liquid crystal polymer film.

13. The method of claim 1 wherein a release material is disposed between the at least one assembly and the at least one sheet of compressed particles of exfoliated graphite.

14. The method of claim 1 wherein the hot pressing further comprises hot pressing a cover layer of liquid crystal polymer film on top of the electrically conductive material.

15. A method of pressing liquid crystal polymer comprising:

bringing a liquid crystal polymer into contact with a flexible graphite sheet; and

hot pressing the liquid crystal polymer.

16. The method of claim 15 wherein the flexible graphite sheet is in contact with one or more press platen members.

17. A hot press comprising:

a press member having a graphite sheet of compressed particles of exfoliated graphite with a density of at least 30 lbs./ft3, the graphite sheet having a major surface, the major surface forming a press member press surface.

18. The hot press of claim 17 wherein the graphite sheet comprises less than 1% ash particles.

19. The hot press of claim 18 wherein the ash particles have a Mohs hardness of less than 5.

20. The hot press of claim 19 wherein the ash particles having a size of less than 1000 microns.

21. A press pad adapted to be disposed between a heated press member and a liquid crystal polymer layer, the press pad comprising a sheet of compressed particles of exfoliated graphite with a density of at least 30 lbs./ft3 and having a major surface adapted to be disposed against at least one of the press member, the liquid crystal polymer layer and a release material.

22. The press pad of claim 21, wherein the sheet of compressed particles of exfoliated graphite has a density of between 40 lbs./ft3 and 110 lbs./ft3.

23. The press pad of claim 21, wherein the sheet of compressed particles of exfoliated graphite is adapted to be disposed against the liquid crystal polymer layer during pressing.

24. The press pad of claim 21, wherein the flexible graphite sheet comprises ash particles having a Mohs hardness of less than 5.

25. The press pad of claim 24, wherein a size of the ash particles comprises less than 1000 microns.

26. A press pad assembly adapted to be disposed between a heated press member and a liquid crystal polymer layer comprising:

a sheet of compressed particles of exfoliated graphite with a density of at least 30 lbs./ft3 having a major surface adapted to be disposed against the press member; and

a release material adapted to be disposed between the liquid crystal polymer layer and the sheet of compressed particles of exfoliated graphite.

Description:
METHODS AND APPARATUS FOR FORMING

LIQUID CRYSTAL POLYMER FLEXIBLE CIRCUITS

CROSS REFERENCE TO RELATED APPLICATIONS

[001] This application claims priority to and the benefit of U.S. Application

No. 62/668, 194 filed May 7, 2018 and U.S. Application No. 62/700, 109 filed July 18, 2018, the entire contents of both of which are incorporated herein by reference.

TECHNICAL FIELD

[002] The present invention relates to a method of forming a flexible circuit by hot pressing liquid crystal polymer (LCP) and more specifically placing LCP in physical/functional contact with one or more graphite sheets and hot pressing the LCP to form the flexible circuit.

BACKGROUND

[003] Flexible circuits have traditionally been made with a substrate formed of polyimide films, but new processes have recently been developed using with an alternative substrate - liquid crystal polymer (LCP). Compared to polyimide, LCP films have about one-tenth of the moisture uptake and both lower dielectric constant and a lower loss (dissipation) factor. Use of LCP film as substrates for flexible circuits improves high frequency performance of these circuits.

[004] Potential applications for LCP substrates forming flexible circuits include medical technologies, telecom apparatus including smartphone, high frequency interconnect systems, optoelectronics, COF(chip on flex) and CSP(chip scale packaging) or anywhere high frequency and/or dimensional stability are desirable.

[005] Characteristics of LCP films include electrical insulation, moisture absorption less than 0.5% at saturation, a coefficient of thermal expansion approaching that of the copper used for plating through holes, and a dielectric constant not to exceed 3.5 over the functional frequency range of lkHz to 45GHz at 23°C, as disclosed in EP1428417A1.

[006] As the development of improved process techniques for laminating LCP continues, a need for improved hot pressing materials and processes for hot pressing LCP is addressed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[007] Fig 1 a is a side view of a LCP material stack and graphite sheet used to form a flexible circuit as described herein;

[008] Fig. lb is a side view of an optional embodiment of the LCP material stack and graphite sheet including a release layer used to form a flexible circuit as described herein;

[009] Fig 2 is a side view of a laminated LCP flexible circuit formed in a manner as described herein;

[0010] Fig 3 a is a side view of a LCP assembly disposed in a platen press together with a pair of flexible graphite sheet forming press pads;

[0011] Fig 3b is a side view of a platen press having a flexible graphite sheet forming press surfaces; and

[0012] Fig 4 is a side view of a roller press having a flexible graphite sheet forming a press surface. DETAILED DESCRIPTION OF EMBODIMENTS

[0013] With reference now to Figs. 1 and 2, a stack of laminate materials used to form a flexible circuit is shown generally at 100. The stack 100 includes an assembly 101 formed of a liquid crystal polymer film 102 disposed beneath and adjacent to an electrically conductive material 104. The stack 100 can include one assembly 101 or a plurality of assemblies 101, only two of which are shown for simplicity. It should be noted that for the sake of clarity not all the components and elements of the stack 100 may be shown and/or marked in all the drawings. Also, as used in this description, the terms "up," "down," "top," "bottom," etc. refer to stack 100, circuit 200, and laminated stack 206 when in the orientation shown in Figs. 1 and 2 respectively. However, the skilled artisan will understand that circuit 200 can adopt any particular orientation when in use.

[0014] The stack 100 is hot pressed together in a press having a press member surface formed of a sheet of flexible graphite 110, as shall be described in further detail below, to form a flexible circuit shown generally at 200 in Fig. 2. Alternatively, one or more sheets of flexible graphite 110 can be used as one or more press pads which are not attached to a press member, as shall be described in further detail below. Alternatively, flexible graphite sheet(s) may be removably attached to one or more press members, as shall be described in further detail below.

[0015] Flexible circuit 200 includes a hot pressed liquid crystal polymer (LCP) substrate 202. The LCP substrate 202 is essentially a thermally stable thermoplastic material formed by hot pressing the LCP film 102, as described herein. Examples of materials used to form the LCP film include, but are not limited to, thermotropic polymer films identified by the brand- names VECTRA ® (naphthalene based, available from Hoechst Celanese Corp.) and XYDAR (biphenol based, available from Amoco Performance Products). FELIOS LCP available from Panasonic, and VECSTAR a liquid crystalline polymer films, such as CT-Z and FB films, developed by Kuraray Co, LTD. Other examples include the LCP films described in LTS 6,963.387, the contents of which are incorporated herein by reference.

[0016] The LCP film 102 can have a thickness up to no more than 150 pm, including thicknesses up to no more than 100 pm, no more than 75 pm, no more than 65 pm, no more than 50 pm, and including up to no more than 35 pm. In another example, the LCP film 102 thickness can range from about 5 pm to 65 pm.

[0017] The flexible circuit 200 also includes an electrically conductive layer 204, which is formed of the electrically conductive material shown as 104 (prior to pressing), which has been hot pressed to the LCP substrate 202 to form a laminated stack 206. The LCP substrate 202 acts as an insulating layer as well as a bonding layer for use with the electrically conductive layer 204. As used as described herein, the LCP substrate avoids the need for an adhesive layer thereby reducing the thickness of the flexible circuit 200.

[0018] In one or more embodiments of the flexible circuit 200, the electrically conductive layer 204 is patterned to form an electrical circuit. The electrically conductive layer 204 can include electrically conductive materials 104 including but not limited to copper, silver, aluminum, and foils formed of one or more of these metals. In another example, the electrically conductive material 104 is a conductor paste, such as for example copper conductor paste, silver conductor paste or aluminum conductor paste suitable for hot pressing. Further examples of materials that may be considered include platinum, gold, mercury iron, brass, bronze and combinations thereof.

[0019] As shown in Fig. 1, the electrically conductive material 104 is patterned to form an electrical circuit having very small circuit trace widths 106 and spacing 108 as small as 100 pm, in other examples as small as 50 pm, and in still other examples as small as 10 pm, and in other examples smaller than 100 pm, and in still other examples smaller than 50 pm, and in still other examples smaller than 20 pm. The circuit patterns can be formed by photolithography, by etching, or other suitable known processes. The circuit patterns are not shown in Figs. 2-4 for simplicity.

[0020] The electrically conductive material 104 can have a thickness which can range from about 5 pm to about 100 pm. In another example, the electrically conductive material 104 thickness can range from about 5 pm to 50 pm, in another example, from about 5 pm to 20 pm and in another example from about 15 pm to 20 pm.

[0021] The electrically conductive layer 204 can be flexible, capable of being bent or otherwise distorted from a planar configuration while maintaining electrically conductive properties suitable for functioning as a flexible circuit 200. The electrically conductive layer 204 can be as flexible as the LCP substrate 202.

[0022] In one or more embodiments of the flexible circuit 200, the electrically conductive layer 204 is laminated onto the outer surface of the LCP substrate 202. In examples described in further details below, the assembly 101 which includes the LCP film 102 and an electrically conductive material 104 is hot pressed together to form a laminated stack 206. The flexible circuit 200 can include a laminated stack 206 formed of a plurality of such assemblies 101, only two of which are shown for simplicity. Examples of the number of assemblies 101 used to form the laminated stack 206 forming the flexible circuit 200 can include more than 50 assemblies, other examples can include 50 or fewer assemblies, other examples can include 25 or fewer assemblies.

[0023] In one or more embodiments, the assemblies 101 can each include a LCP film 102 formed of the same material. In one or more other embodiments, the assemblies 101 can include LCP films 102 formed of different materials. In other embodiments, more than one assembly 101 can include LCP films 102 formed of the same material.

[0024] In one or more embodiments, the assemblies 101 can each include electrically conductive materials 104 formed of the same material. In one or more other embodiments, the assemblies 101 can include electrically conductive materials 104 formed of different materials. In other embodiments, more than one assembly 101 of the plurality of assemblies can include electrically conductive layers formed of the same material.

[0025] The flexible circuit 200 can include an optional LCP cover layer 208 covering the electrically conductive layer 204. The cover layer 208 can be formed by hot pressing an LCP film on top of the electrically conductive material 104 used to form the top electrically conductive layer 204 in a hot pressing step utilizing a flexible graphite sheet 110 as described in further detail below.

[0026] Methods and apparatus in accordance with the present invention generally provide a method and apparatus for forming the high layer count multi-layered flexible circuit 200. The subject invention is specifically directed to methods of hot pressing one or more LCP film layers 102 together with one or more electrically conductive materials 104 using one or more sheets of flexible graphite 110 forming one or more press pads. The graphite press pad can be used in a platen press or a roller press for pressing the LCP film 102 against the electrically conductive material 104 to form the laminated stack 200. As discussed above, flexible circuits have traditionally been made with a substrate formed of polyimide films. By using LCP films, the flexible circuits 200 disclosed herein and the corresponding methods of making the flexible circuits by hot pressing are done without a polyimide substrate (e.g., without the presence of a polyimide sheet). [0027] Also discussed herein are embodiments of hot presses, also known as hot roller presses, using one or more sheets of flexible graphite 110 to form one or more press member press surfaces for abutting at least one of LCP film 102 and the electrically conductive material 104 during hot pressing. One example of the press includes a platen hot press having top and bottom platens capable of applying pressure to an assembly 101 of a LCP film 102 and electrically conductive material 104 arranged in a stack and located between the platens as discussed in further detail below. In one or more embodiments, one or more flexible graphite sheets 110 are used to form the platen press surfaces for abutting the assembly 101 during hot pressing operations. Other embodiments include a roller press having spaced apart rollers with one or more graphite sheets 110 forming roller press surfaces for hot pressing the assembly 101 to form the flexible circuit 200 as discussed in further detail below.

[0028] It should be understood by those skilled in the art that any type of press using a flexible graphite sheet 110 of compressed exfoliated graphite to form a press surface may be used in accordance with the invention. For example, the press may be a mechanical/electrical press, a mechanical hot oil press, a mechanical hot steam press, or any other type of press that is capable of applying pressure at the press surface and heating the press surface to a sufficient temperature for hot pressing the assembly 101 to form a flexible circuit.

[0029] In an optional embodiment shown in Fig. lb, flexible graphite sheet 110 may be used as an assembly in combination with another material 122. One example of another material that may be combined with flexible graphite sheet 110 is a release material 120. A non-limiting example of a release material is polytetrafluoroethylene (“PTFE”). A commonly known type of PTFE is known as Teflon. Other types of release materials may include polyethylene or polypropylene. A further example of the release material may be a metal foil, such as aluminum foil or copper foil. Other options to address sticking of the flexible graphite if it is a concern, include heat treating the flexible graphite sheet. The flexible graphite sheet may be annealed to a temperature of up to about 2000°C in an inert atmosphere or to up to about 400°C in an oxygen containing environment. A second technique may be the application of a release material to one or both sides of the flexible graphite sheet. As such, the release material (not shown) may be disposed between the flexible graphite sheet 110 and the press platen member 312, 412 (discussed below). One such release coating is a clay-based release coating. Another suitable release coating is graphite release coating such as those available from Dylon Industries of Cleveland, Ohio.

[0030] Referring now to Fig. 3a, a platen hot press is shown generally at 300 for hot pressing an LCP assembly 101 to form a flexible circuit 200 as described herein. The platen hot press 300 includes a pair of spaced apart platen press members 3 l2a, 3 l2b, referred to generally at 312. The platen press members 312 are formed of a rigid material capable of providing a press force (as shown by arrows) suitable for hot pressing the LCP to form the flexible circuit 200 as described herein. Alternatively, instead of each platen moving, one of the platens may remain stationary and the other platen will move axially towards or away from the stationary, depending on if the pressure is being applied or relieved. For example in one embodiment the top platen may move vertically up and down, and the bottom platen will remain stationary or vice versa. The rigid platen press members 312 are formed of a material suitable for heating having a low thermal conductivity to reduce the formation of hot spots on the surface of the press members 312. Examples of materials used to form the platen press members 312 can include but are not limited to steel or other rigid materials adapted for heating to temperatures sufficient for hot pressing LCP.

[0031] The material stack 100 is placed between the press platens 312. In the example shown, only one LCP assembly 101 is shown for simplicity, though it should be appreciated that any suitable number can be used. A graphite sheet 110 forming an upper graphite press pad 3 lOa is placed between the LCP assembly 101 and the top press platen 3 l2a. A second graphite sheet 110 forming a lower graphite press pad 3 l0b is placed between the LCP assembly 101 and the bottom press platen 3 l2b. An optional release material, for example, referred to generally at 320 and as described above at 120 can be placed between the graphite sheets 3 l0a, 3 l0b and the assembly 101 (shown in Figs lb and 3a, 3b), and/or between the graphite sheets 3 l0a, 3 l0b and the respective press platen members 3 l2a and 3 l2b (not shown). The release material 320 can be disposed between the graphite sheet 3 l0a and the electrically conductive material 104, between the graphite sheet 310b and the LCP film 102, or between the graphite sheet 3 l0a and the electrically conductive material 104 and between the graphite sheet 3 lOb and the LCP film 102.

[0032] The upper and lower press platens are heated to a suitable press temperature for hot pressing LCP. An example range of temperatures is between from 240°C to 450°C. In other examples, a range of temperatures is between 280°C to 4lO°C. In still other examples, the range of temperatures is between 300°C to 400°C.

[0033] The press platens 312a, 312b are pressed together, against the press pads 3 lOa, 3 lOb which then press the LCP assembly 101 together in a hot pressing step to form the laminated stack 206. As discussed above, a plurality of LCP assemblies 101 can be hot pressed to form the laminated stack 206. A press pad 310 can be placed between each LCP assembly 101, in the material stack 100, if so desired. In other examples, a press pad 310 can be placed between every other LCP assembly for hot pressing a plurality of LCP assemblies. In other examples, a graphite press pad 310 can be placed between every 3 rd LCP assembly 101. In still other examples, a graphite press pad 310 can be placed between every Xth LCP assembly 101 in the material stack 100, where X = 4 to 20. [0034] The graphite sheet 110 includes graphite, in the form of one or more sheets of compressed particles of expanded natural graphite, such as for example exfoliated graphite, referred to herein as flexible graphite.

[0035] The flexible graphite sheet 110 may have a thickness up to 4.0 mm. An example of a minimum thickness may be at least 1.0 mm. Exemplary thickness ranges may include 1.0 to 4.0 mm, 1.25 to 3.5 mm or over 1.25 to 3.25 mm. When used as press pad 310, the flexible graphite sheet 110 having a thickness of up to 4.0 mm may be used in conjunction with an optional metallic sacrificial material. Example of the sacrificial material may include a metallic sheet or foil. A particular metal of interest may be aluminum. In the press pad application, the sacrificial material may be in between the press pad 310 and the platen press member 3 l2a or 3 l2b. In practice, it is believed that flexible graphite residue of the press pad 310 may remain on the sacrificial material after the pressing step. The flexible graphite sheet 110 may be reused until it is sufficiently densified and has a reduced thickness that it will not function as a press pad. Typically, the sacrificial material will be used no more than ten (10) times; in some cases it may only be used once.

[0036] An example of a flexible graphite sheet 110 formed of a natural graphite that may be used in accordance with the present invention is EGRAF HITHERM thermal interface materials or EGRAF SPREADERSHIELD heat spreaders both available from NEOGRAF Solutions, LLC of Lakewood, OH.

[0037] The graphite sheet 110 material possess a high degree of anisotropy with respect to thermal conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from high compression, making it especially useful in the heat spreading application for use as press surfaces in hot presses. [0038] The graphite sheet 110 provides high temperature stability for use as described herein. The graphite sheet 110 has a low coefficient of thermal expansion (CTE) suitable for use in a wide range of temperature. The graphite sheet 110 also has a low oxidation at temperatures used for hot pressing. The graphite sheet provide compressibility and conformability which provide useful advantages as a press pad and/or press member surface for cushioning the LCP assembly 101 during pressing to compensate of any height irregularities in the press member surface.

[0039] The graphite sheet 110 also exhibits high purity for use as described herein, for ash particles having a Mohs hardness of 5 or more and a particle size of 150 microns or more are present at a concentration of less than 500 ppm. Examples of preferred concentrations of such ash particles include less than 100 ppm, less than 50 ppm, less than 25 ppm, less than 10 ppm, and no more than 1 ppm. It has been found that a graphite sheet 110 having a low ash level performs better as a press pad/press surface.

[0040] In a preferred embodiment, the graphite sheet has no more than nominal outgassing at the operating temperatures, more preferably such outgassing is at or below detectable limits.

[0041] Regarding other types of ash particles, suitable graphite sheets having less than 1% ash particles having a Mohs hardness of less than 5, preferably 3 or less, more preferably less than 3, most preferably less than 2 are desirable. These particles can have a size of less than 1000 microns, and in other examples less than 500 microns.

[0042] The graphite sheet 110 is formed from flakes of graphite which have been intercalated and exfoliated and compressed to form a sheet compressed particles of exfoliated graphite having a density of at least 30 lbs. /ft 3 . In examples, the graphite sheet 110 has a density of between 30 lbs./ft 3 and 115 lbs. /ft 3 . In other examples, the graphite sheet 110 has a density of between 40 lbs./ft 3 and 80 lbs. /ft 3 In still other examples, the graphite sheet 110 has a density of between 40 lbs. /ft 3 and H5lbs./ft 3 .

[0043] In at least one example, the flexible graphite sheet 110 is substantially resin-free, wherein resin-free is defined as being below conventional detection limits. In other examples, the flexible graphite sheet 110 has less than 5% by weight resin, in other examples, less than 2% by weight resin, and still other examples less than 1% by weight resin.

[0044] The flexible graphite sheet 110 can have a relatively small amount of binder, or no binder. In at least one example, the flexible graphite 110 sheet can have less than 10% by weight of binder. In another example the flexible graphite sheet 110 can have less than 5% by weight of binder. In at least one other example, the flexible graphite sheet 110 can be substantially binder- free, wherein binder-free is defined as being below conventional detection limits.

[0045] In accordance with the present invention, the flexible graphite sheet 110 has a thickness ranging from about 0.05 mm to about 5.0 mm. In examples, the flexible graphite sheet has a thickness ranging from about 0.5 mm to about 2 mm, in other examples from about 1 mm to about 2 mm. In other examples, the flexible graphite sheet has a thickness ranging from about 0.127 mm to about l.5mm, in other examples from about 0.5 mm to about l.25mm, in still other examples from about 0.5 mm to about 1.0 mm, and in still other examples from about 0.120 mm to about 0.250 mm.

[0046] The graphite sheet 110 has anisotropic characteristics, such as anisotropic ratio, which provides an improved temperature distribution over the LCP assembly 101. The graphite sheet 110 of compressed particles of exfoliated graphite are anisotropic in nature; that is, the thermal conductivity of the sheets is greater in the in-plane, or“a” directions, as opposed to the through- sheet, or“c” direction. In this way, the anisotropic nature of the graphite sheet directs the heat along the planar direction of the thermal solution (z.e., in the“a” direction along the graphite sheet). Such a sheet 110 generally has a thermal conductivity in the in-plane direction of at least about 140, more preferably at least about 200, and most preferably at least about 250 W/m*K and in the through-plane direction of no greater than about 12, more preferably no greater than about 10, and most preferably no greater than about 6 W/m*K. Thus, non-limiting examples of the heat dispersing graphite material 110 have a thermal anisotropic ratio (that is, the ratio of in-plane thermal conductivity to through -plane thermal conductivity) of no less than about 10 for use as a pad and/or hot press member press surface.

[0047] Referring now to Fig. 3b, the graphite sheet 110 is disposed on the surface of the upper and lower platen press members 3 l2a, 3 l2b. The graphite sheet 110 has a surface l lOa forming the platen press member press surfaces 315 which is adapted to be brought into contact with the LCP assembly 101 with a suitable force and suitable temperature (as discussed above) for hot pressing the LCP film 102 together with the electrically conductive material 104 to form the laminated stack 206 comprising the flexible circuit 200. In other embodiments, the platen press member press surfaces 315 which are adapted to be brought into contact with the one or both of the optional release material 320a, 320b for hot pressing the LCP film 102 together with the electrically conductive material 104 to form the laminated stack 206 comprising the flexible circuit 200

[0048] Referring now to Fig. 4, another example a hot press is a cylindrical roller hot press shown generally at 400. The press 400 includes at least one cylindrical roller press member 412 having outer surface 4l2a. The roller press member 412 is formed of a rigid material capable of providing suitable press forces (illustrated by arrows FIG. 4) to the materials pressed therebetween. [0049] The roller press member 412 is formed of a material suitable for heating having a low thermal conductivity to reduce the formation of hot spots on the surface of the press members 4l2a. An example of a material suitable for forming the roller press member can include, but is not limited to metal, such as steel, though other suitable rigid materials are contemplated. A graphite sheet 110 is disposed on the radially outer surface 4l2a of the press roller 412. The graphite sheet 110 has a surface 1 lOa forming a press surface 4l4a of the press member 412.

[0050] The press 400 can include a pair of spaced apart roller press members 412 adapted for rotation about parallel axes of rotation such that the graphite sheets 110 form cylindrical, spaced apart press surfaces 414a arranged to form a nip N located at closest point of proximity between the press members.

[0051] The press members 412 are heated so that the graphite press surfaces 414a reach a suitable temperature for hot pressing the LCP assemblies 101 together to form a laminated stack 206 formed of a LCP substrate 202 and electrically conductive layer 204 to form the flexible circuit 200 as described above.

[0052] A method of making a flexible circuit 100 using a LCP shall now be described. The method includes forming a LCP assembly 101 by placing an electrically conductive material 104 adjacent and above a LCP film 102. The LCP assembly 101 is hot pressed by hot press apparatus 300, 400 having a press member 312, 412 with a sheet of compressed particles of exfoliated graphite 110 forming a press surface 315, 414. The press member 312, 412 is heated to a temperature suitable for hot pressing the LCP assembly 101. The sheet of compressed particles of exfoliated graphite 110 distributes the heat of the heated press member evenly across press surface 315,414 during the hot pressing step to laminate the assembly 101 together to form a laminated stack 206 forming the flexible circuit 200. [0053] As previously stated, the material stack 100 may comprise a high number of LCP assemblies 101 each having a LCP film 102 and an electrically conductive material 104 patterned to form an electrical circuit. In various embodiments, the LCP assembly 101 is devoid of an intermediate layer between the conductive material 104 and the LCP film 102. Preferably, no adhesive is placed between the LCP films layer(s) and the electrically conductive material layer(s). The hot pressing is sufficient to adhere the electrically conductive layer(s) 204 to the LCP substrate layer(s) 202 and to bond the LCP layers together to form the laminated stack 206. In various embodiments, this hot pressing step is sufficient to adhere the electrically conductive layer(s) 204 to the LCP substrate layer(s) 202 and to bond the LCP layers together to form the laminated stack 206 without performing any additional hot pressing steps (e.g., a second hot pressing step). An optional LCP adhesive layer may be placed between one or more of the LCP film layers 102 and conductive layers 104 to create the material stack 100 which is then hot pressed and laminated to form a high layer flexible circuit 200.

[0054] During hot pressing heat and pressure is applied to the material stack 100 by press members 312, 412, and the graphite sheet 110 spreads the heat to form an evenly heated press surface for pressing the LCP film 102 against the electrically conductive material 104 and laminating the assembly without the need for adhesives. Lamination of the material stack 100 is done by bonding the layers of the LCP film 102 with heat and pressure. It should be noted that press members 312, 412 also provide a heat source which is transmitted and dispersed by the graphite sheet 110 to the press surface 315, 414 for elevating the temperature of the LCP layers 102 in the material stack 100. When using multiple LCP assemblies 101 in material stack 100, lamination temperatures and pressures are selected to bond the LCP layers together and/or to bond the electrically conductive material 104 to the LCP 102 while keeping the temperature below the temperature at which the LCP film and conductive layer will deteriorate.

[0055] In the various embodiments contemplated for this invention, the flexible graphite may be used one time or alternatively, the flexible graphite may be reusable. A flexible graphite is reusable if it is used for one or more subsequent hot pressing steps. In one example, the flexible graphite press pad may be used for up to about one-hundred (100) cycles. Preferably the flexible graphite press pad is reused for at least four (4) cycles. In various embodiments of the invention, the press pad is removably attached to the press members 312, 412.

[0056] One method disclosed herein includes a method of making a flexible circuit comprising hot pressing at least one assembly of a liquid crystal polymer film and an electrically conductive material with at least one press member and at least one sheet of flexible graphite. The flexible graphite sheet has a density as disclosed herein, such as at least 30 lbs./ft 3 . The flexible graphite sheet may have been used at least once in a prior hot pressing step. Stated another way, in this method the flexible graphite sheet is being reused from a prior hot pressing step.

[0057] A further method disclosed herein includes pressing a liquid crystal polymer comprising bringing a liquid crystal polymer in contact with a flexible graphite sheet and hot pressing the liquid crystal polymer, wherein the flexible graphite sheet was used in a prior hot pressing step.

[0058] A further embodiment included herein is one of a press pad adapted to be disposed between a heated press member and a liquid crystal polymer layer. The press pad comprises flexible graphite having a density as disclosed herein, such as at least 30 lbs./ft 3 . A suitable type of flexible graphite is compressed particles of exfoliated graphite. Optionally the flexible graphite may be in the form of a composite of the flexible graphite and a release material. It is further preferred that the press pad is suitable for multiple hot pressing steps prior to deteriorating or becoming obsolete.

[0059] All cited patents and publications referred to in this application are incorporated by reference in their entirety.

[0060] The invention thus being described, it will clear that it may be varied in many ways. Modifications and alterations will occur to others upon reading and understanding the preceding specification. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.