Analysis of Signal Integrity in PCB Design

Flexible and rigid - ultra high speed on flexible plates is inevitable because these plates are increasingly used in advanced electronic products. These systems also require a ground plane for isolation and separation of RF and digital references for wireless protocols. High speed and high frequency bring the possibility of signal integrity problems, many of which are related to the location and geometry of the ground plane in the PCB.

A common way to provide a consistent 0 V reference on flexible and rigid flexible plates is to use shaded lines or grid ground planes on flexible ribbons. This provides a large conductor that can still provide shielding over a wide frequency range while still allowing the flexible belt to bend and fold without excessive rigidity. However, signal integrity problems arise in two areas:

Ensure consistent routing impedance, shielding and isolation, and prevent similar fiber weaving effects in the incubation structure.

Grid ground plane design

In the most basic sense, the section line works the same as any other ground plane. It is intended to provide a consistent reference so that traces can be designed to have the required impedance. Any common transmission line geometry (microstrip line, stripline or waveguide) can be placed in a hard flexible or flexible PCB with a mesh ground plane. The copper area where hatched lines are placed on the surface layer of the flexible belt can provide almost the same effect as low-frequency solid copper.

Common configurations of stripline and microstrip wiring on flexible tape with mesh ground plane are as follows.

Grid grounding plane pattern on elastic ribbon.

This mesh structure can be used on rigid plates, but in fact I have never seen it and no customer asked for it. Instead, mesh patterns are used in flexible / rigid flexible plates to balance the needs of impedance control and reasonable flexible belts. Whether you are designing traces or hatch patterns, follow best practices for static and dynamic flexible ribbons and the IPC 2223 standard.

Impedance control

One option to use single ended or differential pairs is to place solid copper in a planar layer directly below the wiring and place the mesh elsewhere in the circuit. If routes become very dense, you will need to use grids everywhere. If the grid is selected, it will have greater flexibility, but the shielding isolation is low, and the impedance control conditions will change.

As shown above, the grid plane structure has two geometric parameters: l and W. These two parameters can be combined into a filling factor or a part of the grid area covered by copper. Changing these parameters has the following effects:

Assuming that other parameters remain unchanged, opening the grid area (increasing the grid opening by increasing L) will increase the impedance. This also makes the ribbon easier to bend (less force).

While keeping other parameters unchanged, increasing w will close the grid area and increase the impedance. This also makes the ribbon pattern difficult to bend (with greater force).

When using the grid ground plane, other parameters that control the impedance of the standard geometry have the same effect. After entering the high frequency, you will excite the non TEM mode around the transmission line, and you may even see an effect similar to fiber weaving.

Is there fiber weaving effect in flex ribbon?

This is a very interesting place for the grid ground plane on the PCB because the grid pattern can begin to be similar to the glass braided pattern used in FR4 and other laminates. As a result, we now return to a situation where we have to worry about the fiber weaving effect in a normally smooth and relatively uniform substrate. These effects occur when the bandwidth of the travel signal overlaps one or more resonances in the mesh structure. For L = 60 mil on polyimide, the lowest order resonance will be 50 GHz.

Whether on rigid pcb or flexible PCB, these shadow structures can produce strong radiation when digital signals propagate along the track of grid grounding plane. As more and more flex applications open at higher frequencies, for some reason, I hope these effects will be worse in flex ribbons with grid ground planes.

High Q resonance

As in a conventional glass braided substrate, the mesh forms a cavity structure that can support resonance when excited at a specific frequency. Since the walls of the cavities have high conductivity (copper), these resonant cavities in the grid ground plane will have very high Q values. Therefore, it will have lower loss and higher Q resonance. This leads to increased cavity emission and resonant power loss.

Low isolation of open grid

The mesh ground plane usually ensures that any radiated EMI emitted from the fiber braided cavity is emitted along the edge of the plate. Because the grid has an open cavity, the isolation is small, and it can also radiate along the surface of the flexible carbon belt. This has the opposite effect: when the wiring is more prone to radiation, it is also more vulnerable to external EMI.

To solve these problems, use a tighter mesh, just like using a tighter glass braid to prevent fiber braiding effects. Flexible and rigid - Flexible PCBs will continue to be part of the PCB field and become more advanced with newer manufacturing capabilities.

Editing: hfy

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