April 2015
24
T
OOLS
& S
OFTWARE
The ever-changing world
of high speed digital design
By Lee Ritchey,
Speeding Edge
This article describes the development
of modern electronics in the last
decade, which has brought not only
advantages but also some issues. To
deal with these issues developers can
attend a seminar which shows how to
design the required high speed PCBs.
Advances in semiconductor technology in
the rst decade of the 21st century have had a
dramatic e ect on what can be done in a sin-
gle integrated circuit. Transistor gate lengths
as small as 22 nanometers have made possible
integrated circuits with performance at ultra-
high speeds and incredible transistor densities.
At the turn of the 21st century the upper limit
on transistor count of an integrated circuit
was 200 million transistors with data speeds
as high as 2.5 gigabit per second over a sin-
gle data path. Currently, integrated circuits
are commonly made with more than a billion
transistors with data rates as high as 28 giga-
bits per second over a single data path.
ese advances in integrated circuit technol-
ogy have made possible products that could
only have been imagined a few years ago. In
addition to all of the products that have been
in use for many years, two very diverse new
product categories have developed around
these advances. ese are the ultra-dense cell
phones and tablets that have crept into every
aspect of our lives and, driven by these same
devices, the ultra-high performance routers,
switches and servers that provide all of the
Internet services they require.
e rst of these has driven component pack-
ages to lead pitches as small as 0.4 mm or 16
mils with components on both sides of the
PCB substrate. is has given rise to build
up PCBs and laser-drilled blind vias as small
as 0.1 mm or 4 mils. is technology had
driven laminate manufacturers to develop
ultra-thin laminates and pre-pregs which
have glass weaves that are uniformly spread
out to improve the quality of the laser-drilled
blind vias. e e ect of these changes is not
apparent when one looks at the size of the
cell phones and tablets as the external pack-
ages remain relatively the same. What these
changes have done is make it possible to put
entire systems, such as GPS, in the same pack-
age with the phone as well as allow the user to
surf the web and watch real time TV and mov-
ies on these same devices. On top of all this,
high quality cameras are included that allow
the user to easily take movies and still photos.
is is remarkable innovation made possible
by improvements in IC technology.
e second of these has driven the perfor-
mance of routers, switches and servers used
in the cloud to unprecedented highs in a very
short time period. Let´s imagine a terabit
router introduced in 2002. It uses half a rack,
weights 350 pounds (160 kilograms) and con-
sumes seven kilowatts of power. Five years
later in 2007, the same terabit router is only
1U high, weighs 22 pounds (10 kilograms)
and consumes 700 watts. More recently, the
capacity of the 1U router has increased to
2.5 terabits per second in the same size pack-
age with 32 each 40 gigabit per second ports
across the front panel. Where these new tech-
nologies takes performance seems to have no
limits.
e fastest signal on a PCB in the 2002 router
was 2.5 gigabits per second. e fastest signal
on a PCB in the 2007 router is 10 gigabits per
second. e routers developed in 2014 con-
tain signals as fast as 32 gigabits per second.
ese digital signals are operating in the fre-
quency band once considered microwave. As
a result, laminate considerations that once
applied only to RF and microwave are integral
to success with these products. Among these
are copper loss, loss in the laminates and uni-
formity of the glass weave, none of which were
issues as recently as 2000 with digital prod-
ucts. In fact, demands placed on laminate
suppliers by the digital world are more di -
cult than those placed by the RF/microwave
community.
e di erence between the RF and digital
world has blurred as a result of the speed
increases in digital electronics. RF and micro-
wave have usually involved signals in excess
of a few gigahertz while digital was con ned
to below 100 megahertz. As described, high
performance digital products now operate in
the same frequency spectrum as many micro-
(Photo courtesy of Robert Koxx / pixelio.de)
