Technical Field: Electronic devices, particularly those with rigid-flexible circuit boards. More specifically, it relates to rigid-flexible circuit boards with signal lines and grounding areas on both sides and below some conductive layers in the flexible circuit board area to reduce impedance variation.
Background Technology Overview:
With the increasing integration of electronic devices and the popularity of wireless communication, devices are becoming thinner, more compact, and denser, leading to higher demands for the integration and lightweight nature of printed circuit boards (PCBs). Rigid-flexible PCBs combine the characteristics of rigid PCBs and flexible PCBs, often using multilayer structures to achieve circuit integration.
However, in the flexible circuit board area, some layers in the multilayer structure are stacked together but not bonded. When the flexible area bends or deforms, this lack of bonding can cause some layers to bend in reverse. This reverse bending can lead to impedance changes in high-speed wiring signal lines (especially RF signal lines), thereby increasing signal loss.
Technical Problem:
As described in the background technology, multilayer rigid-flexible PCBs are prone to reverse bending in the flexible area due to the lack of bonding between layers, which can lead to impedance changes in high-speed wiring signal lines (such as RF signal lines), thus increasing loss.
Invention Content (Solution to the Problem):
Pengding proposes a novel electronic device that includes an improved rigid-flexible circuit board designed to address the aforementioned technical issues, capable of reducing RF impedance variation even when the flexible circuit board area is bent or deformed, thereby reducing loss.
The key to the core solution lies in the layer structure design of the flexible circuit board area, particularly the arrangement of signal lines and grounding areas on both sides and below the signal lines.
Main Technical Solutions and Important Ideas/Facts:
Rigid-Flexible Printed Circuit Board Structure:
The PCB includes a rigid circuit board area and an adjacent flexible circuit board area.
The entire structure consists of multiple conductive layers and insulating layers.
The flexible circuit board area is specifically designed to maintain low RF impedance variation even when bent or deformed.
Layer Structure of the Flexible Circuit Board Area:
The fifth conductive layer (upper grounding): includes a second signal line and first and second grounding lines spaced apart on either side of the second signal line.
“Wherein at least a portion of each of the first grounding line and the second grounding line provides grounding for the second signal line.”
The second signal line is configured to transmit and/or receive RF signals.
“Wherein the second signal line is coplanar with the first grounding line and the second grounding line.” (Claim 9) This implies a coplanar waveguide (CPW) type of routing.
The spacing between the signal line and the grounding lines on both sides can range from 0.05mm to 0.12mm (Claim 10).
The sixth conductive layer (signal and side grounding): (According to the description of Claim 7, this should be the second conductive layer of the flexible circuit board, which includes signal lines and grounding lines on both sides, facing the third conductive layer of the flexible circuit board (ground). However, the layer numbering in Claim 7 differs from the illustrations in the specification. Based on Claim 1 and the description in the specification, the signal lines and side grounding in the flexible area may be on the same layer, with a grounding layer below and possibly a disconnected grounding layer above. We will integrate the claims and the specification for understanding.)
The specification illustration (e.g., Figure 7B) shows that the flexible circuit board contains the fifth, sixth, and seventh conductive layers. The sixth conductive layer includes the second signal line 533c and the second-first grounding line 533a and the second-second grounding line 533b on both sides. The seventh conductive layer 535 is formed as a grounding plane. The fifth conductive layer 531 has a disconnected portion above the second signal line.
The sixth conductive layer: includes a second signal line (e.g., RF signal line) arranged along the centerline and grounding lines arranged on both sides of the second signal line. This structure provides a coplanar waveguide (CPW) type of routing path. “For example, the RF signal line (e.g., second signal line 533c) and the grounding lines (e.g., second-first grounding line 533a and second-second grounding line 533b) formed in the same layer (e.g., sixth conductive layer 533) can be formed as CPW type.” (Paragraph [0129])
The seventh conductive layer (lower grounding): formed as a grounding facing the sixth conductive layer, typically as a grounding plane.
“The sixth conductive layer is formed as a grounding facing the fifth conductive layer, with multiple insulating layers arranged between the fifth conductive layer and the sixth conductive layer, wherein the multiple insulating layers contact the sixth conductive layer and are separated from each other by air gaps;” (Claim 7)
The specification illustration (Figure 7B) shows that the seventh conductive layer is a grounding plane, with a fourth insulating layer between the fifth and sixth layers, and a fifth insulating layer between the sixth and seventh layers, with the fifth insulating layer containing layers separated by air gaps.
The seventh conductive layer can be formed as a single plate or include disconnected portions to provide a grid-like grounding (Paragraphs [0131]-[0132]). Grid-like grounding can control grounding impedance, allowing for increased RF line width and reduced loss.
Insulating Layers and Air Gaps:
The flexible circuit board includes insulating layers arranged between conductive layers.
An important structural feature is that at least some insulating layers (e.g., the fifth insulating layer 575/575b in Figures 5B and 7B, the fifth insulating layer 775 in Figures 10B and 12B) are separated from adjacent insulating layers by air gaps.
“The third insulating layer is located between the second insulating layer and the second conductive layer’s second area while contacting the second layer and separated from the second insulating layer by an air gap” (Abstract)
“The third insulating layer is located in the second area between the second insulating layer and the third conductive layer while contacting the third conductive layer and separated from the second insulating layer by an air gap;” (Claim 1)
This design of air gaps may help reduce interlayer stress when the flexible area bends and control the dielectric constant, thereby stabilizing impedance.
Different insulating layers can use different materials (Claims 4, 5). For example, PPG, black cover layers, cover layers, polyimide, etc.
Conductive Vias:
The rigid circuit board area includes conductive vias.
These conductive vias are formed to pass through insulating layers, electrically connecting different conductive layers within the rigid area, and/or connecting conductive layers of the rigid area with those of the flexible area.
An important positional limitation is: at least one conductive via is set at a distance from the second conductive layer in the flexible area (the layer containing the signal line), and the distance is equal to less than 1/4 of the first wavelength (λ) (Claim 1, 7).
This close positioning of vias may help provide a stable grounding path for RF signal lines, especially at the transition between the rigid and flexible areas.
Vias can directly pass from one layer to another (Figure 13A) or can be offset in different layers for electrical connection (Figure 13B).
RF Signals and Impedance Matching:
The electronic device includes wireless communication circuits electrically connected to the signal lines and configured to transmit and/or receive RF signals.
RF signals have a specific wavelength (λ).
By setting grounding areas (CPW + grounding plane) below and on both sides of the signal lines, and placing vias at critical positions (less than 1/4λ), this structure aims to reduce RF impedance variation, especially when the flexible area bends or deforms.
Figures 17 and 19 show that compared to traditional structures, the disclosed structure exhibits minimal differences in return loss (less than 0.1dB) before and after bending of the flexible circuit board, and the impedance remains within a predetermined range (approximately 50Ω), significantly reducing loss and improving signal integrity.
“One aspect of the disclosure provides an electronic device with a rigid-flexible circuit board designed to reduce RF impedance variation, even when the flexible circuit board area is bent or deformed, thereby reducing loss.” (Invention Benefits [0013])
“Another aspect of the disclosure provides a rigid-flexible circuit board, which has signal lines and grounding areas on both sides and below some conductive layers in the flexible circuit board area to reduce impedance variation.” (Invention Benefits [0014])
Applications of the Electronic Device:
This rigid-flexible circuit board can be used in various electronic devices, particularly those requiring high integration, compactness, and wireless communication capabilities (such as smartphones, wearable devices, etc.).
It is particularly suitable for millimeter-wave communication devices (e.g., devices communicating in the 6GHz and above bands), as high-frequency signals are very sensitive to impedance variations (Paragraphs [0178], [0188]).
The circuit board can be used to connect communication devices (e.g., PCBs containing wireless communication circuits and antennas) with the main circuit board (containing processors and RF transceivers) (Figure 14), or to connect different parts of communication devices (e.g., PCBs containing different antenna arrays) (Figure 15).
Important Citations:
“This disclosure generally relates to electronic devices with rigid-flexible circuit boards, and more specifically, to rigid-flexible circuit boards with signal lines and grounding areas on both sides and below some conductive layers in the flexible circuit board area to reduce impedance variation.” (Background Technology [0001])
“When the flexible PCB area bends or deforms, due to the lack of bonding, some layers may bend in reverse. Reverse bending may increase loss due to impedance changes in high-speed wiring signal lines (e.g., RF signal lines) on the flexible PCB area.” (Technical Problem [0007])
“Rigid-flexible printed circuit board PCB, defined as having a first area where a rigid circuit board is arranged and an adjacent second area where a flexible circuit board is arranged, and includes:… (describing specific layer structures, including air gaps)” (Claim 1)
“Wherein at least one first conductive via is set at a distance from the second conductive layer in the second area, and the distance is equal to less than 1/4 of the first wavelength (1/4λ).” (Claim 1)
“The multiple conductive layers of the flexible circuit board include: a fifth conductive layer, including a second signal line and first and second grounding lines spaced apart on either side of the second signal line, wherein at least a portion of each of the first grounding line and the second grounding line provides grounding for the second signal line; and a sixth conductive layer, formed as a grounding facing the fifth conductive layer, with multiple insulating layers arranged between the fifth conductive layer and the sixth conductive layer, wherein the multiple insulating layers contact the sixth conductive layer and are separated from each other by air gaps;” (Claim 7)
“Wherein the second signal line is coplanar with the first grounding line and the second grounding line.” (Claim 9)
“The comparison of impedance variation between lines L6 and L7 indicates that even when the flexible circuit board bends, the structure according to this disclosure (line L7) does not cause significant impedance variation, but maintains impedance within a predetermined range (approximately 50Ω), thus reducing RF impedance variation and achieving reduced loss despite the deformation of the flexible circuit board.” (Specific Implementation [0210])
Summary:
Pengding has proposed an improved rigid-flexible circuit board structure to address the RF signal loss issues caused by bending or deformation in the flexible area. The invention stabilizes signal integrity and impedance, especially in high-frequency RF signal transmission, by employing coplanar waveguide (CPW) type routing in the layer where the signal lines are located in the flexible area (with grounding lines on both sides), and by setting a grounding plane below, combined with a special insulating structure (including air gaps) and conductive vias set at less than 1/4 wavelength near the boundary of the rigid-flexible area. This technology is significant for electronic devices requiring compactness, high integration, and high-performance wireless communication capabilities (such as millimeter-wave communication modules in smartphones).
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