1. Ultra-high speeds on flex and rigid-flex PCB boards are inevitable as these boards are increasingly utilized in electronic applications.
2. These systems necessitate ground planes for effective isolation and to separate RF and digital references within wireless protocols.
3. High speeds and frequencies introduce the potential for signal integrity challenges, many of which are associated with the positioning and geometry of ground planes in a PCB.
4. A common approach to maintain a reliable 0 V reference on flex and rigid-flex boards is to implement a hatched or grid-like ground plane on the flex ribbon.
5. This design provides a substantial conductor that offers shielding over a broad frequency range, while still permitting the flexible tape to bend and fold without causing excessive rigidity.
6. Nonetheless, signal integrity issues emerge in two key areas: ensuring consistent trace impedance, effective shielding and isolation, and mitigating fiber-braid-like effects within hatch structures.
**Grid Ground Plane Design**
1. In a basic sense, hatching functions similarly to any other ground plane. It is designed to provide a consistent reference, enabling traces to be designed with the desired impedance. Any common transmission line geometry (microstrip, stripline, or waveguide) can be implemented on a rigid-flex or flex PCB board with a meshed ground plane.
2. Placing a hatched copper area on the surface layer of the flex tape provides nearly the same effect as solid copper at low frequencies. Common configurations for stripline and microstrip routing on flex tape with a mesh ground plane are used. This mesh can be utilized on rigid boards, though it is rare and no client has specifically requested it. Instead, a mesh pattern is applied in flex/rigid-flex boards to balance the need for impedance control with the requirement for reasonable flexibility.
**Impedance Control**
3. One approach for using single-ended or differential pairs is to place solid copper in the plane layer just below the traces and use the mesh elsewhere in the circuit. If routing becomes very dense, it will be necessary to employ meshes throughout. Opting for a mesh provides more flexibility, but the shield isolation will be lower and impedance control conditions will vary.
4. The mesh plane structure has two geometric parameters: L and W. These parameters can be combined as a fill factor or as a fraction of the mesh area covered by copper. Changing these parameters has the following effects: Assuming other factors remain constant, increasing the mesh openings (by increasing L) raises the impedance and makes the ribbon easier to bend (requiring less force). Conversely, increasing W while keeping other parameters constant closes the mesh area, thereby increasing the impedance and making the ribbon pattern harder to bend (requiring more force). Other parameters that control the impedance in standard geometries have similar effects when using a meshed ground plane. At high frequencies, non-EM modes around the transmission line may be excited, potentially resulting in a fiber weave-like effect.
**Is There a Fiber Weave Effect in the Flex Ribbon?**
5. Grid ground planes on PCB boards are particularly intriguing because the grid pattern can start to resemble the glass weave pattern used in FR4 and other laminates. This means that we must consider the effect of fiber weaving in what is normally a smooth, relatively uniform substrate. These effects occur when the bandwidth of the traveling signal overlaps with one or more resonances in the mesh. For L = 60 mils on polyimide, the first resonance will be at 50 GHz. Whether on rigid or flexible PCB board substrates, these hatch structures can generate strong radiation as digital signals travel along the grid ground plane tracks. With the rise of high-frequency Flex applications, these effects are expected to be more pronounced in a Flex ribbon with a grid ground plane.
**High Q Resonance**
6. Similar to conventional glass-woven substrates, the meshes form a cavity structure that can support resonance at specific frequencies. These resonant cavities in the grid ground plane will exhibit very high Q values due to the high conductivity of the cavity walls (copper). This results in lower losses and higher Q resonance, leading to increased cavity emission and resonant power losses.
**Open Mesh with Low Isolation**
7. A mesh ground plane typically ensures that any radiated EMI from the fiber braid cavity is emitted along the board’s edge. Due to the open cavities in the mesh, isolation is reduced, and radiation can also occur along the surface of the flexible ribbon. This has the opposite effect: while the trace is more prone to radiate, it is also more susceptible to external EMI. To address these issues, use a tighter mesh, similar to how a tighter glass weave is used to mitigate the fiber weave effect. Flexible and rigid-flex PCBs will continue to evolve and become more advanced with newer manufacturing capabilities.
2. These systems necessitate ground planes for effective isolation and to separate RF and digital references within wireless protocols.
3. High speeds and frequencies introduce the potential for signal integrity challenges, many of which are associated with the positioning and geometry of ground planes in a PCB.
4. A common approach to maintain a reliable 0 V reference on flex and rigid-flex boards is to implement a hatched or grid-like ground plane on the flex ribbon.
5. This design provides a substantial conductor that offers shielding over a broad frequency range, while still permitting the flexible tape to bend and fold without causing excessive rigidity.
6. Nonetheless, signal integrity issues emerge in two key areas: ensuring consistent trace impedance, effective shielding and isolation, and mitigating fiber-braid-like effects within hatch structures.
**Grid Ground Plane Design**
1. In a basic sense, hatching functions similarly to any other ground plane. It is designed to provide a consistent reference, enabling traces to be designed with the desired impedance. Any common transmission line geometry (microstrip, stripline, or waveguide) can be implemented on a rigid-flex or flex PCB board with a meshed ground plane.
2. Placing a hatched copper area on the surface layer of the flex tape provides nearly the same effect as solid copper at low frequencies. Common configurations for stripline and microstrip routing on flex tape with a mesh ground plane are used. This mesh can be utilized on rigid boards, though it is rare and no client has specifically requested it. Instead, a mesh pattern is applied in flex/rigid-flex boards to balance the need for impedance control with the requirement for reasonable flexibility.
**Impedance Control**
3. One approach for using single-ended or differential pairs is to place solid copper in the plane layer just below the traces and use the mesh elsewhere in the circuit. If routing becomes very dense, it will be necessary to employ meshes throughout. Opting for a mesh provides more flexibility, but the shield isolation will be lower and impedance control conditions will vary.
4. The mesh plane structure has two geometric parameters: L and W. These parameters can be combined as a fill factor or as a fraction of the mesh area covered by copper. Changing these parameters has the following effects: Assuming other factors remain constant, increasing the mesh openings (by increasing L) raises the impedance and makes the ribbon easier to bend (requiring less force). Conversely, increasing W while keeping other parameters constant closes the mesh area, thereby increasing the impedance and making the ribbon pattern harder to bend (requiring more force). Other parameters that control the impedance in standard geometries have similar effects when using a meshed ground plane. At high frequencies, non-EM modes around the transmission line may be excited, potentially resulting in a fiber weave-like effect.
**Is There a Fiber Weave Effect in the Flex Ribbon?**
5. Grid ground planes on PCB boards are particularly intriguing because the grid pattern can start to resemble the glass weave pattern used in FR4 and other laminates. This means that we must consider the effect of fiber weaving in what is normally a smooth, relatively uniform substrate. These effects occur when the bandwidth of the traveling signal overlaps with one or more resonances in the mesh. For L = 60 mils on polyimide, the first resonance will be at 50 GHz. Whether on rigid or flexible PCB board substrates, these hatch structures can generate strong radiation as digital signals travel along the grid ground plane tracks. With the rise of high-frequency Flex applications, these effects are expected to be more pronounced in a Flex ribbon with a grid ground plane.
**High Q Resonance**
6. Similar to conventional glass-woven substrates, the meshes form a cavity structure that can support resonance at specific frequencies. These resonant cavities in the grid ground plane will exhibit very high Q values due to the high conductivity of the cavity walls (copper). This results in lower losses and higher Q resonance, leading to increased cavity emission and resonant power losses.
**Open Mesh with Low Isolation**
7. A mesh ground plane typically ensures that any radiated EMI from the fiber braid cavity is emitted along the board’s edge. Due to the open cavities in the mesh, isolation is reduced, and radiation can also occur along the surface of the flexible ribbon. This has the opposite effect: while the trace is more prone to radiate, it is also more susceptible to external EMI. To address these issues, use a tighter mesh, similar to how a tighter glass weave is used to mitigate the fiber weave effect. Flexible and rigid-flex PCBs will continue to evolve and become more advanced with newer manufacturing capabilities.
