Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2000-10-12
2003-02-04
Martin, David (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S763000, C361S765000, C361S811000, C361S502000
Reexamination Certificate
active
06515868
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printed circuit board mounted on an electronic device, and more particularly to a printed circuit board in which undesired electromagnetic wave radiation (hereinafter referred to as electromagnetic wave radiation) is suppressed.
2. Description of the Related Art
In recent years, as the technology of high-higher speed and higher density electronic apparatus progresses, undesired electromagnetic wave radiation from the electronic apparatus causes problems such as malfunction of electronic devices disposed near the apparatus (for example, fluctuations in image on TV screen). Electronic device manufactures are developing technology to suppress such electromagnetic wave radiation.
The electromagnetic wave radiation results from high-frequency signals transmitted through a printed circuit board. Thus, in order to suppress the radiation of electromagnetic waves from an electronic device, it is effective to apply a countermeasure to a printed circuit board to be mounted on electronic device.
For this reason, various techniques have been developed to suppress electromagnetic wave radiation from printed circuit boards, and a need exists for a technique to suppress the electromagnetic wave radiation at a higher level.
Effective suppressing measures taken for a printed circuit board itself can eliminate, for example, the need to provide a shield plate for covering the printed circuit board, or the need to use an expensive shield cable for a cable connected to the printed circuit board, thereby allowing a significant reduction in cost of products.
As a technique for suppressing the electromagnetic wave radiation for a printed circuit board, Japanese Patent Laid-open No. 9-283974 proposes a low EMI multi-layered circuit board which suppresses strong electromagnetic wave radiation due to resonance of a power source layer and a ground layer in the board.
As shown in
FIG. 1
, the low EMI multi-layered circuit board comprises a board structure in which first dielectric layer
203
is sandwiched between power source layer
201
and first ground layer
202
to form capacitance C
1
, second dielectric layer
205
is sandwiched between power source layer
201
and second ground layer
204
to form capacitance C
2
, and resistor
206
is connected between first ground layer
202
and second ground layer
204
.
As to power source layer
201
and first ground layer
202
, a serial circuit including resistor
206
and a capacitor (capacitance C
2
) is connected between them. For power source noise at high frequency, they act as a structure in which only resistor
206
is connected between the two layers
201
,
202
.
Resistor
206
holds Q value (an index representing the ratio of stored energy to consumed energy) of resonance caused between power source layer
201
and first ground layer
202
at a low level, thereby making it possible to suppress strong electromagnetic wave radiation due to the resonance.
In addition, since the low EMI multi-layered circuit board is configured to suppress the electromagnetic wave radiation by means of the whole of the board, it has, for example, an advantage of easier mounting design around an LSI package.
The low EMI multi-layered circuit board, however, requires a high dielectric layer and a second ground layer in addition to a typical ground layer. Thus, it has problems that it is not obtained by easily changing board design based on normal rules and that the special board structure results in an increase in manufacturing cost.
As another technique for suppressing the electromagnetic wave radiation, Japanese Patent Publication No.7-46748 proposes a printed circuit board which suppresses strong electromagnetic wave radiation due to resonance of a power source layer and a ground layer in the board.
As shown in
FIG. 3
, the circuit board is configured such that a power source layer within printed circuit board
300
is divided into main power plane
301
and sub-power plane
304
, and power source terminals
305
of LSI
303
are electrically connected to sub-power plane
304
and capacitors
306
, and power source terminals
305
are electrically connected to ground systems
302
of main power plane
301
through ferrite bead
308
and capacitor
307
(for FB).
The configuration in which power source terminals
305
are electrically connected to main power plane
301
through a &pgr; type decoupling circuit comprising capacitor
306
, ferrite bead
308
and capacitor
307
(for FB) serves to prevent power source noise from leaking to main power plane
301
from LSI
303
, suppressing the electromagnetic wave radiation from main power plane
301
.
Printed circuit board
300
has an advantage of easy implementation in terms of design and manufacturing steps since it can be obtained by slightly changing a typical board structure.
However, when an LSI with a number of pins is mounted, a problem occurs that an increased size of sub power plane
304
may cause resonance between sub power plane
304
and the ground layer.
The printed circuit board also has a problem of higher manufacturing cost due to the use of the expensive ferrite bead.
As another technique for suppressing the electromagnetic wave radiation, the present inventors have proposed a printed circuit board which suppresses strong electromagnetic wave radiation caused by resonance of a power source layer and a ground layer in the printed circuit board.
As shown in
FIG. 2
, the printed circuit board is configured such that power source terminal
405
a
of IC
404
is electrically connected to common power source wiring
406
of the printed circuit board through a decoupling circuit comprising first capacitor
401
, power source wiring
402
and second capacitor
403
. The electrodes of first and second capacitors
401
,
403
on the ground side are electrically connected to via holes
407
a
,
407
b
(ground conductors), respectively.
Ground terminal
405
b
of IC
404
is electrically connected to via hole
407
c
(ground conductor).
In the printed circuit board, the characteristic impedance of the power source wiring is set to be sufficiently higher than the impedance in the first capacitor, and the length thereof is set to approximately one quarter the wavelength at the upper limit frequency in a frequency range in the electromagnetic wave radiation. Thus, the power source wiring serves as an inductor with high impedance within the frequency range in the electromagnetic wave radiation to suppress high frequency noise which leaks from the power source terminal to the common power source wiring, resulting in suppression of electromagnetic wave radiation due to resonance of the common power source wiring.
In this manner, since the printed circuit board includes the decoupling circuit formed of the inexpensive capacitors and the power source wiring, the problem of increased cost is not remarkable, and this technique allows low manufacturing cost while harmful electromagnetic wave radiation can be suppressed.
The printed circuit board, however, applies the decoupling circuit to all of the power source terminals in mounting an LSI, so that routing of signal wiring may not be easy in wiring design, in which case a problem occurs that a reduction in design cost cannot be achieved.
To solve the aforementioned problems, it is an object of the present invention to provide a printed circuit board at lower cost and produce higher radiation suppressing effect by improving the printed circuit board shown in
FIG. 27
to allow efficient arrangement of decoupling circuits for an LSI having a number of power source terminals.
SUMMARY OF THE INVENTION
To achieve the aforementioned object, a printed circuit board according to a first aspect of the present invention, including ground conductors and a power source conductor, on which an electronic component provided with two or more power source terminals is mounted, comprises: two or more first capacitors for electrically connecting the two or more power source terminals to the ground co
Harada Takashi
Kikukawa Takahiko
Kobayashi Hideaki
Oe Shuichi
Sasaki Hideki
Bui Hung
Martin David
NEC Corporation
Sughrue & Mion, PLLC
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