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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