23Fujikura Technical Review, 2006
1. Introduction
In the field of electronic circuitry, if necessary
wires were properly connected on a printed circuit
board and the wiring pattern had a sufficient current
capacity relative to the current, no trouble occurred
in the operation of the printed circuit board. Along
with the recent remarkable increase in the circuit
operation speed, however, the above-mentioned con-
ditions are not sufficient for the design of a printed
circuit board. One of the required items is the charac-
teristic impedance of a pattern.
So far, for a device using only a low-speed signal of
several MHz or less, only the characteristic imped-
ance of telecommunication cables connecting
between the devices should be considered. This was
because the wavelength of the signal to be used was
long enough compared to the size of the device, and
the circuit in the device could be regarded as a
lumped constant circuit. The cables connecting the
devices are not sufficiently short in relation to the
wavelength that is to be used, hence it should be
regarded as a distributed constant circuit. Therefore,
control of the characteristic impedance is required
for cables.
Recently, it is not uncommon to use electrical sig-
nals with the frequency of several 100 MHz to several
GHz in a device. If such a high-speed signal is trans-
mitted, a transmission line should be regarded as a
distributed constant circuit even if the transmission
line is short and intended for use only in the device.
Therefore, the control of the characteristic imped-
ance on the printed circuit board is required.
In light of the above situation, a flexible printed cir-
cuit (hereinafter referred to as FPC) is growing in
demand. This report describes the challenges in con-
trolling the characteristic impedance on FPC and the
present achievements.
2. Outline
2.1. Characteristic impedance
Characteristic impedance is determined by the
square root of the ratio between the induction ele-
ment and the capacity element of the signal line per
unit length.
If the output impedance of the signal output circuit,
the load impedance connected to the terminal portion
of the signal line, and the characteristic impedance of
the transmission line are consistent, no signal reflec-
tion occurs on the transmission line. The connection
of input/output impedance that is equal to the charac-
teristic impedance is equivalent to an infinitely contin-
ual transmission line having a uniform characteristic
impedance.
2.2. High-speed signal transmission line
The signals often used for a high-speed data trans-
mission are PECL, LVDS, CML, etc. For these sig-
nals, a driver for connecting the load of 50 Ω of
impedance is used and the output impedance is
adjusted to 50 Ω. On such an interface, the character-
A Study of Impedance Control in Flexible Printed
Circuit (FPC)
Tomonari Yokoyama
1
, Hirohito Watanabe
1
, Yukiharu Sakaguchi
1
,
Haruo Miyazawa
1
, Yoshiharu Unami
1
and Nobuyuki Sadakata
1
Ordinary, requirements for printed circuits are only to be connected correctly and to have
enough electrical current capacity. If signals used inside of equipments are very low frequency,
impedance control of circuit is not needed. Impedance control is needed only for long communi-
cation cables between equipments.
When electrical circuits with high frequency signal are operated, impedance control is need-
ed for not only communication cables but also printed circuits.
Recently it is not rare that the high frequency signal (several hundreds MHz to several GHz)
is used inside of equipment. In this case, the circuit should be considered as distributed con-
stant circuit even if the circuit is very short. Therefore impedance control of circuit is needed on
Printed Circuit Board.
Based on the background described above, we have tried to control characteristic impedance
of Flexible Printed Circuit (FPC) .
1 Circuit Technology Development Division, Electronic and Electrical
Development Center
istic impedance of the transmission line to be con-
nected needs to be controlled to 50 Ω
2.3. Differential impedance
In such a high-speed signal, differential input and
output are generally used to enhance resistance to
the common mode noise from the outside.
Input impedance at the termination of the transmis-
sion line is also defined by differential impedance,
which is 100 Ω. In this case, the characteristic imped-
ance of the transmission line is also defined by the
differential impedance, which is 100 Ω, thereby pre-
venting signal reflection between transmission lines.
Generally, differential impedance is not equal to
the sum of the characteristic impedances of two sig-
nal wirings. The larger the coupling coefficient
between two signal wires, the smaller the differential
impedance. If there is a large gap between two signal
wires so that it may be regarded as no connection of
two wires, the differential impedance becomes equal
to the sum of the characteristic impedances of two
signal wires.
3. Simulation
3.1. Modeling of transmission line
To control the characteristic impedance on the
wiring board, a co-planar line, microstrip line, or strip
line is generally used.
3.1.1. Co-planar line
A co-planar line has a structure of the type of trans-
mission line that is used for the cables such as a feed-
er line. It is formed by a parallel wiring so that the
gap between the circuit pattern transmitting a signal
and the circuit pattern of the ground becomes con-
stant. The weak points in using a co-planar line are
that the degree of freedom for wiring becomes low
because the impedance is controlled only by the
width and the gap, and the noise radiated from the
signal line becomes greater than in a microstrip line
or a strip line because of the line wiring at the
ground.
3.1.2. Microstrip wiring
On a microstrip line, signal patterns are arranged
to overlap a ground plane. A co-planar line can be
made up of a single-sided board, whereas a
microstrip line requires a board having at least two-
layers. On a microstrip line, the characteristic imped-
ance can be controlled by the distance between the
ground plane and the signal pattern and the wiring
width of the signal pattern.
This is a transmission line that is relatively easy to
connect, can reduce radiant noise, and can be used
most frequently on the printed circuit board.
3.1.3. Strip line
A strip line has a structure where signal patterns
are arranged so as to get caught in the ground plane.
Therefore, the board must have at least three layers.
Similar to a microstrip line, the characteristic imped-
ance can be controlled by the distance between a
ground conductor and a signal pattern and the wiring
width of the signal pattern. This is a structure that
can reduce the largest amount of radiant noise.
24
(a) Co-planar line
(b) Microstrip line
(c) Strip line
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