Large printed circuit board for RF rotary joint

Abstract

A copper clad laminate (500) has at least a first copper sheet layer (510) and a second copper sheet layer (520) laminated on opposite sides of a dielectric layer (530) comprising syndiotactic polystyrene, and the dielectric layer (530) comprises a filler comprising at least 3 vol.% of hexagonal boron nitride particles (540), where the first copper sheet layer (510) and the second copper sheet layer (520) are laminated on opposite sides of the dielectric layer (530), where the first copper sheet layer (510) and the second copper sheet layer (520) are laminated on opposite sides of the dielectric layer (530), and where the first copper sheet layer (510) and the second copper sheet layer (520) are laminated on opposite sides of the dielectric layer (530). More than 50% of the hexagonal boron nitride particles (540) are arranged parallel to the first copper sheet layer (510).

Description

Technical Field

[0001] This invention relates to a printed circuit board that can be manufactured in large sizes and used in RF rotary connectors with data rates exceeding 10 Gbit / s. It also relates to RF rotary connectors using such a printed circuit board. Background Technology

[0002] Printed circuit boards (PCBs) play a crucial role in RF rotary joints, especially in applications requiring the transmission of radio frequency (RF) signals between rotating and stationary components. RF rotary joints are used in a variety of systems, such as CT scanners, radar systems, satellite communication systems, and antenna arrays, where continuous and reliable RF signal transmission is required even when components rotate relative to each other.

[0003] RF signals are sensitive to factors such as impedance mismatch, reflection, and signal attenuation. PCBs must provide geometry-controlled and consistent traces and dielectric materials to allow for precise impedance matching and minimize signal loss as RF signals pass through connectors. This helps maintain signal integrity and prevent distortion.

[0004] PCB materials can be specifically selected based on their dielectric properties, which affect the performance of RF signals at higher frequencies. Materials with low dielectric constants and low dielectric losses are preferred for RF applications because they reduce signal propagation delay and minimize signal loss.

[0005] Low-loss PCB materials can include PTFE or ceramic materials. Both are expensive and require complex processing. PTFE may cause environmental problems and may therefore be avoided in future projects. Ceramic materials are relatively rigid and difficult to fit rotary joints of different diameters. PTFE exhibits low adhesion, which requires the copper layer to have a higher surface roughness to achieve sufficient adhesion. Alternative materials, such as cross-linked polyethylene disclosed in US 2002 / 0004125 A1, soften at higher temperatures and cause changes in the dielectric properties of the PCB.

[0006] Known PCB manufacturing processes are limited to sizes smaller than 48'' or 1.2m, as this is sufficient for typical batches of PCBs carrying electronic circuitry. The availability of longer substrates is very rare and limited to 108'' or 2.4m. For large rotary joints, such as CT scanners, PCBs with circumferences up to 5m may be required. Furthermore, material consistency over such long lengths is far more important than on smaller PCBs, as signal errors accumulate over such lengths. Summary of the Invention

[0007] The problem this invention addresses is to provide a PCB (Printed Circuit Board) and PCB material suitable for RF applications at frequencies above 1 GHz or above 10 GHz, providing low dielectric loss at these frequencies. The PCB should be cost-effective, easy to manufacture, and provide constant and reproducible electrical parameters. The PCB should be able to be manufactured in large sizes for large rotary joint diameters and have high material consistency. The material should be flexible, with a bending radius of approximately 0.5 m.

[0008] The solution to this problem is described in the independent claims. The dependent claims relate to further improvements to the invention.

[0009] In this embodiment, the copper-clad laminate (CCL), which can be a printed circuit board substrate, comprises a pair of copper sheets laminated on opposite sides of a dielectric material layer. For multilayer PCBs, multiple layers may also be present. The dielectric material includes syndiotactic polystyrene (sPS). Syndiotactic polystyrene is a specific type of polystyrene in which styrene monomer units are arranged in a regular, alternating pattern along the polymer chain. Syndiotactic polystyrene exhibits higher heat resistance, better chemical resistance, and improved dimensional stability. It also tends to have higher stiffness and strength, making it suitable for applications requiring more robust performance. Most importantly, syndiotactic polystyrene exhibits excellent dielectric properties at high frequencies (er ≤ 2.4, tan δ ≤ 0.0003), similar to the dielectric properties of PTFE and PTFE-based substrates at high frequencies (er ≤ 2.2, tan δ ≤ 0.0009). Therefore, syndiotactic polystyrene is best suited for applications transmitting RF signals at high frequencies—e.g., above 10 GHz. Syndiotactic polystyrene is an ideal base material for striplines, microstrips, differential lines, or waveguides—all of which can be used for contactless, high-speed data transmission.

[0010] Alternative materials such as those using woven glass-reinforced hydrocarbon materials / ceramics (er≤3.5, tan δ≤0.0035) exhibit higher losses and are therefore limited in use at high frequencies.

[0011] This type of printed circuit board substrate is more flexible than other known printed circuit board substrates—for example, those including PTFE or FR4 or other commonly used materials. Here, the dielectric material can be deformed uniformly over a long length (e.g., over 1 m) to avoid variations in thickness and uniformity. No cracks or voids are allowed, as these can significantly affect the dielectric properties and thus signal transmission. Typically, with a bending radius greater than 100 times the dielectric layer thickness, the dielectric constant should vary by no less than 10%, 5%, or 2% over a 1 m length. The thickness of this PCB is typically about 1.5 mm, but can range from 0.2 mm to 5 mm. Copper sheets typically have a thickness of 9 µm to 35 µm; the copper can be etched thinner or added via electroplating.

[0012] Typically, in contactless data link applications with long printed circuit boards—potentially exceeding 1m and reaching 3m-8m—constant characteristic impedance along its length is paramount. If parallel and differentially driven lines exist, it is crucial that the two lines are symmetrical and possess identical characteristics. This can only be achieved through very high material uniformity and consistency of the dielectric material, along with very low geometric tolerances in the substrate and subsequent PCB. Material uniformity refers to the uniformity and predictability of properties within the material. Using sPS, excellent uniformity in dielectric properties (dielectric constant and dielectric loss), which primarily define RF properties, can be achieved. Tests show that sPS exhibits higher RF characteristic uniformity than other materials.

[0013] In the field of rotary joints, printed circuit boards (PCBs) are typically bent only once when mounted into the joint. Multiple bending is avoided. Multiple bending can reduce mechanical stability and, consequently, material uniformity. Furthermore, the thermoplastic nature of sPS allows PCBs to be bent while heated, making them more flexible. For example, the PCB can be heated to a temperature between 100°C and 150°C before bending. This bending can result in plastic deformation, allowing the PCB to remain in a bent or flexed state. Additionally, multiple substrates can be soldered together. This provides a strong connection, which can facilitate the fabrication of the closing loop for the rotary joint.

[0014] sPS is more economical than PTFE because sPS is at least slowly biodegradable, while PTFE never degrades. sPS-based materials can be easily recycled through melting. Therefore, separation from the copper component can be easily achieved. Furthermore, the energy required to produce sPS materials is approximately 30% of that required to produce PTFE materials.

[0015] Materials like FR4 can experience a decline in RF characteristics due to water absorption. At frequencies above 10 GHz, water has a very high dielectric constant of approximately 50 and exhibits high losses. Therefore, water absorption is highly undesirable as it leads to significant degradation of the printed circuit board. sPS, on the other hand, has very low water absorption and thus avoids water-based degradation.

[0016] For most industrial and medical applications, sPS-based substrates have demonstrated sufficient robustness in flammability testing. While PTFE and FR4 exhibit lower flammability, PTFE, in particular, produces highly toxic fumes when burned. The flammability of sPS-based substrates can be further improved by adding flame-retardant materials to meet flammability requirements by slowing flame spread and reducing smoke generation during combustion.

[0017] Compared to conventional polystyrene (PS), the regular molecular structure of syndiotactic polystyrene (sPS) makes it more difficult to process. Specialized processing techniques may be required to achieve the desired properties. Typically, sPS is processed only through injection molding. All currently available substrates—whether based on thermosetting materials (FR4), PTFE, or hydrocarbon materials—are laminated and therefore limited in size. sPS-based printed circuit board substrates can be manufactured using a continuous extrusion process, which is highly efficient, produces consistent quality, and allows for the production of long substrates required for large rotary joints—for example, in CT scanners. Lengths can far exceed the current 108'' limit of commercially available substrates or laminates. Furthermore, the extrusion process allows for the production of curved substrates that can be adapted to radii or curvatures used later. This allows for the production of tension-free substrates.

[0018] The dielectric layer includes a filler comprising at least 3 vol-% hexagonal boron nitride particles. It may contain more than 5 vol-%, 10 vol-%, or 20 vol-%, or between 5 vol-% and 50 vol-%, or between 10 vol-% and 40 vol-%. Furthermore, more than 50% of the hexagonal boron nitride particles are arranged parallel to the first copper layer. They may be parallel between the copper layers. Alternatively, more than 30%, 40%, 60%, or 70% of the hexagonal boron nitride particles may be arranged in parallel.

[0019] Boron nitride (BN) is a heat- and chemically resistant refractory compound of boron and nitrogen. Compared to sPS, BN increases the dielectric constant of the material while significantly reducing thermal expansion and increasing mechanical strength. Due to its high thermal conductivity, PCB heat dissipation can be improved, but the increase in dielectric constant caused by BN is less than that caused by ceramic fillers. In this embodiment, hexagonal α-BN (boron nitride) is used. It may be contaminated with less than 10%, 5%, or 1% cubic β-BN. Boron nitride particles arranged parallel to the copper layer can provide higher electrical insulation resistance. Heat transfer in directions parallel to the copper plate, such as towards the sides of the strip, is improved. Furthermore, particles orthogonal to the copper layer are undesirable because they increase the tendency to generate microcracks in the plastic material between layers in the vertical direction.

[0020] Hexagonal boron (BN), also known as α-BN, consists of layers with a planar hexagonal honeycomb structure in which boron (B) and nitrogen (N) atoms alternate. Hexagonal BN particles are plate-like, allowing them to have two substantially flat surfaces that can be substantially parallel.

[0021] sPS is best suited for CT scanner applications because it has high X-ray tolerance, far superior to PTFE. BN is also highly X-ray resistant and can provide some shielding against sPS.

[0022] In the embodiments, blends of syndiotactic polystyrene (sPS) and PPE (polyphenylene oxide) (poly(oxy-2,6-dimethyl-1,4-phenylene) – an amorphous high-performance plastic) simplify the extrusion process because the blend provides dimensionally more stable extrudates and a wider thermal processing window. The blend also improves the temperature dimensional stability of the substrate. Furthermore, flame retardancy is enhanced by the inherent flame-retardant properties of PPE. The disadvantages may be a higher dielectric constant and higher dielectric loss. Specifically, a very consistent material with highly constant electrical properties over a long length can be produced by the following method. If a blend of sPS and PPE is used, the PPE can range from 1% to 50% by weight of the particles that form part of the mixture.

[0023] In embodiments, the filler may further include at least one of talc, hollow glass spheres, quartz, and fused silica. Boron nitride may be hexagonal or cubic, with hexagonal boron nitride being preferred due to its better thermal conductivity, lower hardness, and therefore better machinability. Another option is a filler with good dielectric properties, a low dielectric constant of 2.2 to 2.5, and approximately 2 × 10⁻⁶. -4Low dielectric loss fillers can be aluminum hydroxide (Al₂(OH)₃), which is also a very effective flame retardant because it generates H₂O when heated to 200°C. Combinations of different fillers are also possible.

[0024] Talc (Mg3H2(SiO3)4) is a clay mineral composed of hydrated magnesium silicate. Talc has a slightly higher dielectric constant than sPS, which increases the overall dielectric constant. However, talc improves the mechanical stability of sPS, thus enhancing its mechanical properties and machinability.

[0025] Hollow glass spheres or glass microspheres increase mechanical strength. Hollow spheres reduce the dielectric constant, but must have small dimensions (e.g., D50 < 25 μm and D95 ​​< 50 μm) to not hinder the through-hole (THT) process and the fabrication of vias. In the through-hole THT process, copper is plated onto the sidewalls of the drilled hole to connect copper traces, and the via electrically connects the two copper-plated sides of the substrate material.

[0026] Glass can be added as short fibers, and one or more pieces of woven glass can also be used as filler.

[0027] Quartz is a hard crystalline mineral composed of silica (SiO2, silicon dioxide). It also improves mechanical strength. Fused silica is an amorphous silica.

[0028] The total filler content can be as high as 50% of the volume, and is usually shared between two fillers, with each filler accounting for 10% to 20%.

[0029] In addition, flame-retardant materials can be added to improve flame retardancy. Furthermore, the filler content improves flammability because the content of thermoplastic SPS and PPE in the substrate is reduced, and the aforementioned fillers are non-flammable.

[0030] In an embodiment, the printed circuit board includes a printed circuit board substrate according to any one of the preceding claims. The printed circuit board may include at least one antenna structure or waveguide.

[0031] In an embodiment, the contactless data link includes a printed circuit board as described in the preceding claim. The contactless data link may have a circular shape.

[0032] In this embodiment, the CT scanner includes the non-contact data link described herein. The non-contact data link may have a circular shape and may be axially or radially oriented.

[0033] The process of laminating copper and producing finished printed circuit board substrates—known as copper-clad laminates (CCLs)—can be performed using a combination of a single-screw extruder, an in-situ laminating tool, and a calender. For mixing, sPS granules or a premixed blend of sPS and PPE granules can first be melted and mixed, with the addition of a flame retardant and at least one of several fillers, and then fed to an extruder. The mixture can be formed into a strip via a nozzle and cooled in air or water, then cut into segments called granules for later processing by remelting and extrusion. Alternatively, the mixture can be used directly by being extruded through a nozzle while still molten. Two sheets of copper material are fed substantially parallel to each other on either side of a channel nozzle, the width of which corresponds to the width of the copper sheets, and the height of which can correspond to the thickness of the substrate. In any case, the nozzle height can be less than or greater than this distance. The channel nozzle can provide some parallel alignment of the BN granules. The mixture is plasticized and remixed by the extruder and then fed to the nozzle, distributing the material across the width of the copper sheet material. At the outlet, the extrudate can be stretched to better align the BN particles and laminated with copper layers on both sides. The lamination process further improves the alignment. The substrate can be fed through a calender to calibrate the substrate thickness and provide a flat surface with even better alignment of the BN particles, especially on longer sections of the substrate. A mixer can be used instead of an extruder or added to the extruder.

[0034] Furthermore, woven glass can be supplied substantially parallel to one or both sides of the slotted nozzle to become part of the substrate and serve as a filler, enhancing the substrate's resistance to lateral forces. Additionally, woven glass can be added to the substrate in the first step, and a copper layer can be added in subsequent steps.

[0035] This method is a continuous process that can be used for large substrate lengths. It requires a thermoplastic material as the dielectric material that can be extruded, such as sPS. Compared to processes that manufacture individual pieces of the substrate sequentially, the continuous process ensures constant electrical properties over longer lengths—for example, exceeding 108''.

[0036] Extrusion allows the production of dielectric layers of varying sizes or thicknesses without significant modifications to the laminating apparatus. To adjust and / or change the thickness of the dielectric layer, only the width of the extrusion slit die and / or the distance between the copper sheets can be adjusted, for example, by adjusting the distance between the rollers that guide and / or hold the copper sheets. Other adjustments can be made by modifying the calender.

[0037] Extrusion also allows for the production of curved substrates that are adapted to the radius of curvature required in subsequent applications—such as rotary joints.

[0038] As sPS is known to be a dielectric material used in injection-molded components, it would not be considered as a suitable material for PCBs. However, the excellent RF characteristics of sPS, especially its uniformity and unexpected material consistency at high frequencies, make it an ideal base material for long RF PCBs that can be used in waveguides and contactless data links.

[0039] A PCB can be fabricated from a substrate with laminated copper sheets by transferring the circuit layout to a laminate, etching away some of the copper to form traces and microstrips, and then using the remaining copper. Through-holes can be drilled through the PCB stack formed by the copper sheets and dielectric layers. Unplated holes can be used for mechanical mounting purposes, while plated holes can connect copper traces on both sides. Solder resist can be applied to allow reflow soldering. During reflow, the PCB with solder paste and components can be heated to 250°C for a short time, thus connecting the components and traces to the circuitry.

[0040] The sPS-based substrate can be laminated onto an FR4 pre-etched board as an additional layer with better RF properties and processed into a PCB as mentioned above, which benefits from the mechanical properties of FR4, for example, for backplane applications. Attached Figure Description

[0041] In the following description, examples of embodiments of the invention will be illustrated by way of example with reference to the accompanying drawings without limiting the overall inventive concept.

[0042] Figure 1 The printed circuit board substrate is shown in a side view.

[0043] Figure 2 The printed circuit board is shown in a side view.

[0044] Figure 3 A front view of the printed circuit board is shown.

[0045] Figure 4 A top view of the previous attached figure is shown.

[0046] Figure 5 The modified waveguide section is shown.

[0047] Figure 6 The curved waveguide section is shown.

[0048] Figure 7 Another embodiment of a curved waveguide segment is shown.

[0049] Figure 8 The CT (Computed Tomography) scanner rack is shown schematically.

[0050] Figure 9 A laminator is illustrated with an example. Detailed Implementation

[0051] Figure 1 A printed circuit board substrate 500 is shown in a side view. A first copper layer 510 and a second copper layer 520 are laminated on opposite sides of a dielectric layer 530 comprising syndiotactic polystyrene (sPS). Hexagonal boron nitride particles 540 are encapsulated within the dielectric layer 530 and arranged substantially parallel to the first copper layer 510. For better illustration, these particles are shown magnified. In practice, they have millimeter, sub-millimeter, or micrometer-scale dimensions. They may have a D50 between 20 micrometers and 80 micrometers or between 50 micrometers and 60 micrometers.

[0052] Figure 2 A printed circuit board 110 is shown in a side view. The printed circuit board 110 is based on a printed circuit board substrate having shaped conductors formed of copper layers. A dielectric layer 111 has a first copper layer 113 and a second copper layer 116 opposite to the first copper layer 113.

[0053] Figure 3 A front view of a printed circuit board 110 is shown, which can be used for contactless data transmission, such as in a CT scanner. This embodiment or similar embodiments can also be called waveguides because they are suitable for guiding electromagnetic waves. Here, two elongated conductors 112, 113 are formed by a first copper layer 113. These conductors are similar to striplines or microstrips (striplines or microstrips can also be used for data transmission). In fact, for high-speed contactless data transmission, differential signals driven on two parallel lines perform better.

[0054] Figure 4 A top view of the previous attached figure is shown. Figure 4 An embodiment of a waveguide segment 120 is shown, which has elongated conductors 112 and 113 with a specific zigzag pattern. This pattern provides higher interference immunity compared to microstrip lines. Essentially, this waveguide segment may have a first end segment 141 and a second end segment 142 opposite to the first end segment 141. Furthermore, a coordinate system is shown, wherein, as shown in the figure, the x-direction points from the right side of the figure to the left, the y-direction points from the center to the top of the figure, and the z-direction points into the plane of the figure.

[0055] Figure 5 The modified waveguide segment 130 with microstrip conductors 114 and 115 is shown.

[0056] Figure 6A bent waveguide segment is shown. Here, waveguide segment 131 is bent with a radius 151 in the xz plane, such that an elongated conductor is located on the outer side of the cylindrical shape formed by the bending. In an alternative embodiment, the bending can be performed in other ways, such that the elongated conductor is located on the inner side. Here, the dielectric material layer must be deformed uniformly over a long length (e.g., exceeding 1 m) to avoid variations in thickness and uniformity. No cracks or voids should be present, as these can significantly affect the dielectric properties and thus signal transmission.

[0057] Figure 7 Another embodiment of the curved waveguide segment 132 is shown. Here, the waveguide segment is curved with a radius 152 in the xy plane to form a disk-shaped embodiment, in which an elongated conductor is located on one side of the disk. Again, uniform deformation of the dielectric material layer is essential here.

[0058] Figure 8 A CT (Computed Tomography) scanner gantry is schematically shown. A fixed portion is suspended within a large frame 810. A rotating portion 809 of the gantry is rotatably mounted relative to the fixed portion and rotates in a direction of rotation 808. The rotating portion 809 supports an X-ray tube 801 configured to generate an X-ray beam 802 that irradiates a patient 804 lying on a table 807, and is intercepted by a detector 803 and converted into electrical signals and imaging data. Power from a power supply unit 811 can be transmitted to the rotating portion via a slip ring (not shown). Data acquired by the detector 803 is transmitted to an evaluation unit 806 via a non-contact data link 800 or a non-contact rotary joint through a data bus or network 805.

[0059] Figure 9 A laminator 200 and an extruder 300 used together for manufacturing printed circuit boards are illustrated by way of example. The laminator 200 has a body 210 for holding a plurality of rollers 220 that guide copper sheets 340, 350 and compress a substrate 380. Plasticized sPS material 310 flows in direction 320 through an extrusion nozzle 300 and is laminated together with the copper sheets 340, 350 to form a substrate 380 that moves in direction 390. A calender 400 including opposing rollers 410 can be provided to calibrate the thickness of the substrate 380. A cooling unit (not shown) may be present behind the calender.

[0060] List of reference numerals 110 Printed Circuit Board 111 Dielectric material layer 112, 113 Conductors of the first copper layer 114 and 115 are slender conductors used as microstrip conductors. 116 Second Copper Layer 120 waveguide section 130 Waveguide Section 131 Waveguide Section 132 Waveguide Section 141 First end section 142 Second end section 151 radius in the xz plane 152 radius in the xy plane 200 laminator 210 Laminator Unit 220 rolls 300 Extrusion Nozzle 310 Plasticized SPS Material 320 sPS flow direction 340, 350 copper sheets 380 substrate 390 Substrate moving direction 400 calender 410 Calender Roll 500 Printed Circuit Board Substrate 510 First copper layer 520 Second Copper Layer 530 includes a dielectric layer of syndiotactic polystyrene (sPS). 540 Hexagonal Boron Nitride Particles 800 Non-contact Rotary Joint 801 X-ray tube 802 X-ray beam 803 X-ray detector Patient 804 805 Network 806 Evaluation Unit 807 Patient Desk 808 Rotation Direction 809 Rotating Part 810 Frame 811 Power Supply Unit

Claims

1. A copper-clad laminate (500), the copper-clad laminate (500) comprising at least a first copper sheet layer (510) and a second copper sheet layer (520), the second copper sheet layer (520) being laminated parallel to the first copper sheet layer (510) on opposite sides of a dielectric layer (530) comprising syndiotactic polystyrene (sPS), Its features are, The dielectric layer (530) includes a filler comprising at least 3 vol-% of hexagonal boron nitride particles (540), wherein more than 50% of the hexagonal boron nitride particles (540) are arranged parallel to the first copper sheet layer (510).

2. The copper-clad laminate (500) according to the preceding claim. Its features are, The dielectric layer (530) also includes a blend of syndiotactic polystyrene and polyphenylene ether.

3. The copper-clad laminate (500) according to any one of the preceding claims. Its features are, The filler also includes at least one of talc, glass fiber, hollow glass spheres, quartz, and fused silica.

4. The copper-clad laminate (500) according to any one of the preceding claims. Its features are, More than 70% of the hexagonal boron nitride particles (540) are arranged parallel to the first copper sheet layer (510).

5. The copper-clad laminate (500) according to any one of the preceding claims. Its features are, The filler comprises hexagonal boron nitride particles (540) ranging from 5 Vol-% to 50 Vol-% or from 10 Vol-% to 40 Vol-% 6. The copper-clad laminate (500) according to any one of the preceding claims. Its features are, The filler includes aluminum hydroxide.

7. A printed circuit board (110) comprising a copper-clad laminate (500) according to any one of the preceding claims.

8. A contactless data link, the contactless data link comprising a printed circuit board (110) according to the preceding claim.

9. A CT scanner comprising a non-contact data link as described in the preceding claim.

10. A method for manufacturing a copper-clad laminate according to any one of claims 1 to 6, the method comprising the following steps: - A mixture of sPS with fillers and flame retardants, with sPS optionally blended with PPE. Two pieces of copper material are supplied parallel to each other on both sides of a groove-shaped nozzle, the width of the nozzle corresponding to the width of the copper pieces. - The material used to form the dielectric layer by heating and plasticizing the mixture using an extruder. - The material is supplied to the nozzle and distributed across the width of the copper sheet material.

11. The method for manufacturing a copper-clad laminate according to claim 10 or 11, the method further comprising the following steps: After the steps of feeding the material into the nozzle and distributing the material across the width of the copper sheet material, the material is stretched in the extrusion direction.

12. The method for manufacturing a copper-clad laminate according to claim 10 or 11, the method further comprising the following steps: The substrate is fed through a rolling mill to calibrate its thickness and provide a flat surface.

13. The method for manufacturing a copper-clad laminate according to the preceding claim, the method further comprising the following steps: Prior to the copper foil feeding step, woven glass is fed parallel to the plasticized material on both sides of the grooved nozzle so that the woven glass becomes part of the dielectric layer.

Images