In recent years, a requirement for a digital electronic equipment to have a higher performance is boundless, and a semiconductor integrated circuit (IC/LSI) which is mounted on a printed circuit board and is used as a semiconductor device tends to be more highly integrated and use higher frequency. For this reason, the number of simultaneous switching buffers in the IC/LSI tends to increase, a larger electric current tends to be required, and the switching cycle also tends to become higher frequency. In order to stably operate the IC/LSI mounted on this printed circuit board, a charge supplying function which follows a high-speed operation of the IC/LSI needs to be provided on the printed circuit board. This charge supplying function for the IC/LSI is achieved by a capacitative element connected to the IC/LSI through a wiring or a planate conductor formed on the printed circuit board.
However, in a high-frequency band, impedance cannot be neglected which originates in parasitic components of the wiring and the planate conductor, and accordingly, potential fluctuations of a power source and a ground are induced by an electric current for supplying an electric charge to the IC/LSI. Furthermore, a cable is connected to a connector mounted on the printed circuit board. The cable connected to this printed circuit board works as an antenna, and results in contributing to emanation of an electromagnetic noise. For this reason, in order to reduce the electromagnetic radiation noise, it becomes important to make the potential fluctuations of the power source and the ground occurring with the high-speed operation of the IC/LSI mounted on the printed circuit board not to propagate to the position of the connector arranged on the printed circuit board.
On the other hand, the electromagnetic radiation noise generated in these electronic equipments can become not only a cause of inducing a malfunction of a self-circuit, but also a cause of inducing a malfunction of other electronic equipment. For this reason, in order to market an electronic product, the electromagnetic radiation noise must be suppressed within a regulation value concerning an electromagnetic radiation noise such as VCCI, CISPR and FCC in each country.
Then, PTL 1 describes a technology of reducing the electromagnetic radiation noise by separating the electrodes into a planate power source electrode to which a power source is connected and a power source electrode for the LSI which is connected to the semiconductor device, and reducing the amount of a noise current flowing from the power source electrode for the LSI to the power source electrode.
FIG. 8 is an exploded perspective view illustrating a conventional printed circuit board. In the conventional printed circuit board 101, one surface of a printed circuit board 102 is determined to be a first wiring layer 3, and the other surface is determined to be a fourth wiring layer 7. A second wiring layer 4 having a ground conductor plane 13 and a third wiring layer 6 having first and second power source conductor planes 11 and 12 are provided between these first wiring layer 3 and fourth wiring layer 7 through a not-shown dielectric layer.
A semiconductor device 8 is provided on the first wiring layer 3. In addition, a connector 16 to which a not-shown power source cable of a direct-current power source is connected is provided on the first wiring layer 3, and a terminal of the connector 16 is connected to the first power source conductor plane 11 and the ground conductor plane 13 through connecting conductors 17 and 18 of a via or the like. The first power source conductor plane 11 and the second power source conductor plane 12 are connected to each other by a connecting conductor 111 of the via or the like. The power source terminal 8a of the semiconductor device 8 and the second power source conductor plane 12 are connected to each other by a connecting conductor 21 of the via or the like, and the ground terminal 8b of the semiconductor device 8 and the ground conductor plane 13 are connected to each other by a connecting conductor 24 of the via or the like. By connecting the direct-current power source to the connector 16, the electric power is supplied to the semiconductor device 8 through the connecting conductor 17, the first power source conductor plane 11, the connecting conductor 111, the second power source conductor plane 12 and the connecting conductor 21.
In this conventional printed circuit board 101, the power source conductor plane is separated into the first power source conductor plane 11 and the second power source conductor plane 12, and the conductor planes are connected to each other by a connecting conductor 111 of the via or the like, thereby intending to reduce the amount of the noise current flowing into the first power source conductor plane 11, which has been generated in the semiconductor device 8 and has reached the second power source conductor plane 12. In addition, a planar capacitor 14 is formed by arranging the second power source conductor plane 12 and the ground conductor plane 13 so as to closely face each other. Thereby, the noise current which has been generated in the semiconductor device 8 and has reached the second power source conductor plane 12 returns to the semiconductor device 8 through a path formed by the planar capacitor 14 and the connecting conductor 24.
Furthermore, in the printed circuit board 101, a capacitor element 9 which is a bypass capacitor is mounted on the fourth wiring layer 7, and the capacitor element 9 is connected between the second power source conductor plane 12 and the ground conductor plane 13 through the connecting conductors 112 and 23 of the via or the like. Thereby, the remainder of the noise current except for a noise current which has not returned to the planar capacitor 14 returns to the semiconductor device 8 through a path formed by the connecting conductor 112, the capacitor element 9, the connecting conductor 23, the ground conductor plane 13 and the connecting conductor 24.
In other words, intention is made to reduce the electromagnetic radiation noise by increasing the total electrostatic capacitance by the planar capacitor 14 and the capacitor element 9, and returning the noise current which has been generated in the semiconductor device 8 to the semiconductor device 8 through the planar capacitor 14 and the capacitor element 9.
However, in recent years, the semiconductor device such as the IC/LSI progressively tends to use a higher frequency, a conventional configuration is insufficient to reduce the electromagnetic radiation noise originating in the noise current of high frequency generated in the semiconductor device, and has been required to be further improved.
In other words, in the conventional configuration illustrated in FIG. 8, the path of the noise current includes a path in which the noise current leaks to the first power source conductor plane 11 from a branch point of the second power source conductor plane 12 through the connecting conductor 111, in addition to a path in which the noise current reaches the semiconductor device 8 from the branch point through the planar capacitor 14 or the capacitor element 9. The noise current which has leaked to the first power source conductor plane 11 passes through a not-shown cable via the connector 16, and accordingly, becomes a cause of the electromagnetic radiation noise.
A method of enlarging the second power source conductor plane is considered in order to reduce the noise current which leaks to the first power source conductor plane and efficiently return the noise current to the semiconductor device by the planar capacitor, but there is a limit on the enlargement of the second power source conductor plane because the size of the printed circuit board is restricted.
In addition, there are an inductance component of the connecting conductor 112, an equivalent serial inductance (ESL) component of the capacitor element 9 and inductance components of the connecting conductors 23 and 24, in a path which reaches the semiconductor device 8 from the branch point through the capacitor element 9. For this reason, in order to reduce the amount of the noise current which flows out from the branch point to the first power source conductor plane 11, it is necessary to decrease the inductance components existing in the path which reaches the semiconductor device 8 from the branch point through the capacitor element 9.