Best High-Speed Routing Practices on Hatched Planes

Rigid-flex and flex PCBs sometimes require a different approach compared to standard PCBs, and this can be seen when looking at the placement of ground. A hatched pattern of copper can be used for ground in a flex PCB, but the use of hatched copper creates an incomplete ground region with many gaps. Radiation can escape, high speed boards can experience excessive return loss, but the benefit is repeated flexing.

If you know anything about high-speed design and routing for digital signals, then you know that complete ground planes are the best option for supporting high-speed signals. Does this mean that high-speed digital components cannot be used on hatched planes? The reality is that there are some simple design choices that can help ensure signal integrity on flex sections with hatched planes, even in some of the newest computing interfaces.

What’s Wrong With Hatched Planes?

Technically, there is nothing wrong with a hatched plane. Rigid-flex and flex boards need to have a hatched copper structure because they would not be able to flex dynamically with a fully solid copper plane. While flex sections can be fabricated with solid copper and later bent to a fixed angle, this is not conducive to an application that requires repeated flexing. Therefore, you may find that certain applications require hatching on ground pour rather than solid copper pour for ground.

So, if you want to route high-speed traces over a flex section with a hatched plane, there are some simple design choices that bring ground back into the region around your traces. The idea is to compensate for openings in the hatched plane and thereby bring the input impedance along the trace to be much closer to the desired characteristic impedance value.

Coplanar Ground Traces

Coplanar lines can be used around a signal trace to provide additional ground. These don’t need to be large pour regions, they can be larger traces that are connected to the GND reference (the hatched plane). This provides the same effect as using coplanar ground on a rigid board.

GND traces near a signal trace can modify impedance of the trace if brought close enough to the signal trace.

In this way, the trace sees a nearby ground that can set the impedance value without using an excessively wide trace. This means it could be easier to route into a component on the flex section, or into a plated contact on an FPC ribbon.

Route Only Over Solid Sections

Using this method, you could route only over a region that contains solid copper, and it could provide the same results as using a ground trace near the signal trace. This forces more careful routing, and for very high speed signals it will require routing along only a straight line. If you route only over the copper sections, take note of these points:

• This is best used on single-ended traces
• This works best when the copper in the hatch is wider than the trace
• This might require a thinner polyimide insulation layer between the trace and hatched region

Closely Spaced Differential Pairs

Finally, because most high-speed interfaces are differential, the distance between the two traces in the pair will determine the impedance of the pair. To prevent the need for very wide traces in a differential pair, the spacing between the two traces can be made smaller. This will set the differential impedance to the target value without requiring very wide traces on the flex ribbon. The same strategy can be used with thicker polyimide layers or on rigid boards with thicker dielectric layers.

Bends Affect Impedance and Losses

When the flex region containing high-speed signals is bent, the underlying polyimide can become compressed. This will bring the hatched plane closer to the traces, which will lower the impedance of those transmission lines.

If you plan to route high-speed signals through a bend region, the closer ground below the traces may create excessive insertion loss at higher frequencies. This would need to be simulated to determine the exact impact, or it should be measured. Unfortunately, simple impedance solvers can’t handle this kind of problem, and a 3D EM field solver would be needed to perform such a simulation.

At Some Point, You’ll Need an EM Field Solver

Since most of the above points rely on getting an accurate impedance calculation out to very high frequencies, you will need to verify impedance and losses up to some minimum frequency limit.

Most simple solvers and impedance calculators are not designed to calculate impedances for hatched planes. This is because the impedance will become a function of signal frequency and the size of the openings in the hatched plane. To handle these complex systems, an EM field solver is needed that can account for the openings in plane layers. This will apply to single-ended traces and differential pairs in high-speed interfaces.

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