Barry, that's a great blog post right there!!
Feel free to repeat what you said on the Altium forums.
I would also add that if you must use a crosshatch, you can place reference
traces (for return paths) directly under the diff pairs, and/or make sure
your crosshatch is not 90/45 angled with respect to the diff pair
directions so that they have statistically equal return paths.
On Fri, Apr 11, 2014 at 3:14 PM, Barry Olney <[log in to unmask]> wrote:
> Hi Dave,
>
> Cross hatch planes increase series inductance and reduce shunt capacitance
> -
> this raises the impedance of the referenced traces. The hatches must be
> much
> smaller than the length of the rising edge of the propagating signal.
> Generally a 50/50 copper/gap hatch is sufficient to provide flex and
> prevent
> cracking of the plane. Increasing the gap past this level will increase
> inductance in the signal return path to a point whereby it is detrimental
> to
> signal propagation increasing delay, crosstalk and EMI.
>
> The impedance of cross hatched planes can vary by up to 18% depending on
> whether the trace crosses copper or space in the copper. According to IPC,
> layer to layer registration has 3% accuracy so a trace may fall over a
> solid
> copper area or copper/gap and there is no way of determining the exact
> location of the trace with respect to the cross hatch plane regardless of
> the pattern.
>
> No gap (solid 100% Cu) is Z+0%
> Little gaps (60% Cu) are Z+15%
> Big gaps (40% Cu) are Z+18%
>
> The amount of flexibility gained by using crosshatching will be directly
> proportional to the percentage of copper removed from the plane. A cross
> hatched plane pattern can also increase bond strength between that layer
> and
> the adjacent layer since adhesives bond better to polyimide than to copper.
> This method should be used in cases where the plane functions primarily to
> control EMI. The percentage of copper that can be removed will depend on
> the
> frequency of the noise that the plane is functioning to keep out of the
> circuit. It should be noted that reducing the copper plane coverage will
> significantly impact the impedance of any signals using that plane as a
> return path.
>
> Unfortunately, CAD support for meshed planes is poor at best, allowing
> little control of where the holes are placed with respect to traces. This
> is what leads to a significant impedance control and crosstalk problem.
> Pass
> two traces over the identical row of mesh holes, across a significant
> length, and you'll see crosstalk skyrocket. Or pass one side of a
> differential pair across mesh holes and the other side across a solid
> plane,
> and watch the differential impedance and skew go way out.
>
> If the tolerance of the 2 sections is independent of each other, and if the
> flex tolerance is +/-20%, assuming a standard +/-10% tolerance on the rigid
> boards, this could create a substantial impedance discontinuity, e.g:
> 120-Ohm on the Flex and 90-Ohm on the Rigids.
>
> So in answer to your question: it is best to use solid copper for impedance
> control applications.
>
> Cheers,
> Barry
>
> -----Original Message-----
> From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of David
> Baldwin
> Sent: Saturday, 12 April 2014 6:16 AM
> To: [log in to unmask]
> Subject: [DC] Rigid Flex Plane Cross-Hatching & Impedance
>
> I have a rigid-flex design that is too difficult to bend and
> form. We have 100 ohm DP's going across the flex against solid
> reference planes. We want to cross hatch the planes in the flex
> regions in an attempt to make it more flexible (along with other
> adjustments).
>
> 1. What are the guidelines for cross-hatching the copper (50%,
> diagonal, orthogonal?)
> 2. How badly will it affect the impedance? Should I keep the copper
> solid under the DP's?
>
> Thanks,
>
> Dave
>
>
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