46 The PCB Design Magazine • May 2017
THE DARK SIDE – RETURN OF THE SIGNAL
• As the frequency increases, the current is
forced into the outer surface of the copper,
due to the skin effect.
• Skin depth decreases with increased
frequency.
• At high frequencies, the thickness of the
copper plane is irrelevant—½ oz. and
3 oz. copper will have the same surface
conduction area.
• There is a sweet spot where the total
energy stored in the electromagnetic field
surrounding the trace is optimized.
• The signal return currents generate EM
fields. Those EM fields, in turn, induce
voltages (crosstalk) into other signals.
• The easiest way to reduce crosstalk is to
increase the spacing between the signals
in question.
• Crosstalk can also be controlled by
varying the trace height, above the plane.
A tight coupling (less height) results in
less crosstalk.
• The return current distribution of two
parallel traces shows an overlap of current
in the surface of a microstrip plane.
• To minimize crosstalk do not to
intermingle dissimilar technologies but
rather keep them isolated.
• Synchronous buses, as typically used in
DDRx designs, benefit from an
extraordinary immunity to crosstalk.
• Ensure that the required setup and hold
times are provided at the receiver.
References
1. Barry Olney's Beyond Design columns:
Return Path Discontinuities, The Dumping
Ground, Controlling the Beast, Effects of Sur-
face Roughness on High-speed PCBs, Uncom-
mon Sense.
2. High-Speed Digital System Design, by Ste-
ven H. Hall, Garrett W. Hall, and James A. McCall
3. High-Speed Digital Design, by Howard
Johnson and Martin Graham
Barry Olney is managing director
of In-Circuit Design Pty Ltd (iCD),
Australia. The company developed
the iCD Design Integrity software
incorporating the iCD Stackup,
PDN and CPW Planner, is a PCB
design service bureau and special-
izes in board-level simulation. The software can
be downloaded from www.icd.com.au. To
contact him or read past columns, click here.
Researchers of Karlsruhe Institute of Technology
(KIT) have now developed a method to erase the
ink used for 3D printing. In this way, small struc-
tures of up to 100 nm in size can be erased and
rewritten repeatedly. This development opens up
many new applications of 3D fabrication in biol-
ogy or materials sciences.
"Developing an ink that can be erased again
was one of the big challenges in di-
rect laser writing," Professor Chris-
topher Barner-Kowollik of KIT's
Institute for Chemical Technology
and Polymer Chemistry says.
The process was developed in
close cooperation with the group
of Professor Martin Wegener at the
Institute of Applied Physics and the
Institute of Nanotechnology of KIT.
Structures written with erasable ink can be in-
tegrated into structures made of non-erasable ink:
Support constructions can be produced by 3D
printing, which are similar to those used when
building bridges and removed later on. Recently,
such structures were designed by KIT to grow cell
cultures in three dimensions on the laboratory
scale.
"During cell growth, parts of the
3D microscaffold could be removed
again to study how the cells react
to the changed environment," We-
gener explains. According to the sci-
entists, it is also feasible to produce
reversible wire bonds from erasable
conducting structures in the future.
Erasable Ink for 3D Printing
