JP2000101204A - Wiring board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce EMI on a floating surface conductive layer. SOLUTION: As a shield layer, a conductive layer 4 of a thick resistor on a surface is arranged and the conductive layer 4 is not connected to any pattern inside a wiring board, so that the distribution of electric fields inside the board can be made smooth and high frequency noises can be reduced. Besides, the frequency of propagation signals on a high-speed signal line is transmitted but the EMI electromagnetic wave of its higher harmonic component can be shielded. A floating surface conductive layer 3 is formed through a dielectric film on high-speed signal wiring 1 and when the thickness of the conductive layer 3 is defined as (t), resistivity is defined as ρ, transmissivity is defined as μ and the frequency of a clock signal is defined as (f), these values are selected so as to satisfy an expression.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wiring board provided with EMI measures.

[0002]

2. Description of the Related Art In recent years, the data handled by personal computers has become large, such as high-definition images, and it has been desired to improve the processing capability.
Therefore, the clock frequency of the CPU has been increased, and bus wiring, clock lines, data lines, and the like to peripheral ICs have been increased in speed and density, and EMI has become a problem.

As a means for reducing EMI, a method of shielding electromagnetic waves by applying a film such as a metal or a magnetic material on the surface of a circuit board is known, and the structure thereof will be described below.

As shown in FIG. 19, there is a structure in which after forming a photosensitive solder resist film on a printed circuit board, a shield conductive film is formed at a predetermined place (Japanese Patent Laid-Open No. 2-9).
8192). Further, as shown in FIG. 20, there is a printed wiring board in which a conductor such as a copper foil dropped to a power supply potential after the printed board is completed is bonded (Japanese Patent Laid-Open No. 2-249290). This is shown in FIG.
Rather than forming the shield film and the circuit board integrally as in 9, there is an advantage that the wiring can be easily arranged and corrected by bonding the conductor later. By arranging the conductor on the surface in both FIGS. 19 and 20,
Electric field shielding in all frequency bands from low frequency to high frequency is realized.

Further, as shown in FIG. 21, there is a structure in which a ferrite layer is formed on an insulating substrate on which a circuit is formed by a conductor to suppress radiation of electromagnetic wave noise (Japanese Patent Laid-Open No. 3-256598). This is a magnetic field shield that absorbs a magnetic field, unlike FIGS.

Further, as shown in FIG. 22, by covering the side surface around the substrate with a conductive film, electromagnetic wave radiation from the side surface can be reduced (Japanese Patent Laid-Open No. Hei 7-235776).
issue). This structure is effective in suppressing not only the electromagnetic field noise from the signal line but also the emission of high-frequency noise between the power supply and ground layers.

Since the conductor shield layer as shown in FIGS. 19 and 20 is close to the signal line layer, the characteristic impedance of the high-speed signal line must be designed in a strip line-like structure in consideration of the shield film. Although the strip line does not emit electromagnetic waves to the outside, it becomes a low impedance wiring, and the electric field concentrates in the dielectric, so that the electric field intensity distribution becomes steep and high frequency noise is likely to occur.

Further, as shown in FIG. 21, there is a structure that suppresses the emission of electromagnetic wave noise by forming a ferrite layer on an insulating substrate on which a circuit is formed by conductors (Japanese Patent Application Laid-Open No. 3-256598). Although this structure is not a shield of a conductor but a shield of a magnetic material and absorbs electromagnetic waves, the above problem does not occur because it is not a low impedance wiring like a strip line. However, ferrite is expensive, and there is a possibility that the waveform of the high-speed signal line will be deteriorated if the ferrite is in the immediate vicinity of the signal line.

Further, as shown in FIG. 22, by covering the side surface around the substrate with a conductive film, the electromagnetic wave radiation from the side surface can be reduced (Japanese Patent Laid-Open No. 7-235776).
issue). This structure is effective for suppressing radiation of high-frequency noise such as switching noise between power supply grounds as well as signal lines. However, by confining the electromagnetic waves in the metal housing as described above, irregular reflection of the electromagnetic waves in the printed circuit board occurs, and the effect may be radiated to the outside via a cable.

[0010]

Heretofore, it has been considered that a material having high conductivity and high magnetic permeability is effective as an electromagnetic wave shield. However, if a material with high conductivity such as copper, aluminum, or silver is used, the electric field generated from the signal lines in the wiring board is almost reflected into the circuit board, so that the electric field distribution in the board becomes sharp and high frequency noise is reduced. Can be large. Therefore, as a shield layer, although the problem is small as EMI, but as a propagation signal, an electromagnetic wave having a frequency f of a high-speed signal line is transmitted to the outside to some extent so that the electric field distribution inside the substrate is moderated. This is effective in reducing high-frequency noise in the substrate. In the EMI frequency band, a problem often arises of a harmonic component of the frequency of the signal line actually running in the substrate.

Generally, as a shielding effect of the conductive layer, it is said that when the film thickness is thinner than the skin, the electromagnetic wave passes, but when the film thickness is thicker than the skin, the electromagnetic wave does not pass.

The thickness of the skin is expressed as follows.

[0013]

(Equation 2) According to equation (1), the thickness of the skin becomes smaller as the resistivity becomes smaller and as the frequency becomes higher.
Therefore, if a conductive layer having a certain resistance and a thickness is disposed on the surface, a phenomenon occurs in which electromagnetic waves of a certain frequency or more are transmitted, but electromagnetic waves of a certain frequency or less are not transmitted. Therefore, with respect to a signal line of a desired frequency, the electromagnetic wave of the frequency of the signal line is transmitted to the outside to some extent from the surface conductive layer, but the unnecessary electromagnetic wave (EMI), which is its harmonic, is absorbed without being transmitted. If you select the rate and thickness, EMI
It is effective for reduction.

Conventionally, as a countermeasure against EMI, a method of reducing the high frequency noise by passing a resistor in series with a digital wave of a high frequency signal line to smooth the rising of the waveform has been adopted. Also in this regard, by forming a conductive layer having a resistance above the microstrip line, there is an effect that the waveform is smoothed and high-frequency noise is reduced.

[0015]

According to a first aspect of the present invention, there is provided a wiring board according to the present invention, wherein a floating surface conductive layer is formed on a high-speed signal wiring via a dielectric film, and the resistance of the surface conductive layer is reduced. By controlling the ratio ρ and the thickness t, electromagnetic waves are selectively shielded.

According to the second aspect of the present invention, a floating surface conductive layer is formed on a high-speed signal wiring via a dielectric film, and the thickness t of the conductive layer is a resistivity. Is ρ, permeability is μ, and clock frequency is f.

(Equation 3) Thus, the electromagnetic wave on the higher frequency side than the clock frequency f is shielded.

According to a third aspect of the present invention, in the wiring substrate, the surface conductive layer is disposed at a position at least twice the width of the signal line from immediately above the high-speed signal line.

According to a fourth aspect of the present invention, in the wiring substrate, the conductive layer is formed at a peripheral end of the wiring substrate.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to examples.

First, FIG. 1 will be described by using a cross-sectional view of a wiring board according to an embodiment of the present invention in comparison with a conventional wiring board. A simulation of the electric field distribution was performed by comparing the following three cases.

A. 1. In FIG. 1, when there is no conductive layer on the dielectric film above signal line 1, B. In FIG. 1, when a low-resistance conductive layer such as copper exists on the dielectric above the signal line 1 and drops to the ground potential, In FIG. 1, a case where a conductive layer having resistance is present on a dielectric above the signal line 1 and the potential is not connected anywhere.

In the case of A, FIG. 2 shows the electric field distribution of α-α ′ in which the end of the signal line is cut perpendicular to the layer, and FIG. 3 shows the β-β ′ in which the end of ground is cut perpendicular to the layer. 5 shows the electric field distribution of FIG. Similarly, in the case of B, FIG. 4 shows the electric field distribution of α-α ′ with the end of the signal line cut perpendicular to the layer, and FIG. 5 shows the β-β ′ with the end of ground cut perpendicular to the layer. 5 shows the electric field distribution of FIG. Finally, in the case of C, FIG. 6 shows the electric field distribution of α-α ′ with the end of the signal line cut perpendicular to the layer, and FIG. 7 shows the β-β ′ with the end of ground cut perpendicular to the layer. 4 shows an electric field distribution.

2 and 3 that the electric field distribution shows a steep change in the signal line layer 1 portion. Next, in FIG. 4, it was found that the presence of the conductor layer having a ground potential on the surface increased the electric field intensity in the dielectric 3 by nearly 10 times. Finally, since the surface has a conductive layer having a certain resistance which has not dropped to any potential, it can be seen from FIGS. 6 and 7 that the signal line portion and the ground portion have a gentle electric field distribution. Since the electric field distribution is gentle, even if noise is generated in the signal line portion or the ground plane, the influence on the surroundings is small.

Next, the conditions applied to the present invention when the resistivity of the conductive layer 3 in FIG. 1 is changed will be examined.

FIG. 8 shows the results of calculating the frequency dependence of the skin thickness in the case of copper, aluminum, brass, and titanium. From FIG. 8, copper or aluminum with low resistivity is
It can be seen that if the thickness is about 20 μm, it is possible to shield all the frequency bands above 20 MHz. However, titanium having a relatively high resistivity has a thickness of 2
At 0 μm, electromagnetic waves with a frequency of 20 MHz pass, but 50
It becomes possible to select not to pass a high frequency of 0 MHz or more.

FIG. 9 shows an experimental result of actually examining the shielding effect of the metal plate. It can be seen that the shielding effect does not change from tens of kHz to tens of MHz with a low-resistance metal plate such as aluminum, but the shielding effect decreases from several hundred kHz with a metal having resistance such as titanium. To explain these phenomena in a cross-sectional view of the wiring board, the electric field of the signal line passes through the conductive layer on the low frequency side as shown in FIG. 10, but the electric field of the signal line passes through the conductive layer on the high frequency side as shown in FIG. Shielded without passing.

In FIG. 12, when the clock frequency is a rectangular wave, the frequency f of the fundamental wave and the frequencies 3f, 5f of its harmonics are shown.
2 shows how the current density in the conductive layer attenuates when the current density J on the surface is 1.

The depth from the surface is calculated by using the expression of the thickness of the skin in the expression (1),

(Equation 4) Then, the current density of the fundamental wave is 0.37 <J <0.47, but the current density of the harmonic 3f is 0.17 <J <0.27, and the harmonic 5f
Has a current density of 0.11 <J <0.19, and the fundamental electromagnetic wave is 4
It can be seen that the leakage of the electromagnetic waves is reduced to 25% or less for the higher harmonics, though the light passes through the surface conductive layer around 0% and leaks to the outside. From these, the thickness t of the conductive layer is 3 times the thickness δ of the skin.
When the value is equal to or more than / 4 and equal to or less than 1, the electromagnetic wave having the frequency f of the signal line is transmitted to some extent, and a conductive layer that shields a higher frequency than that can be realized.

A rectangular wave signal was actually input to the microstrip line, Cu, Ti, and SUS metals were floated and placed on the wiring, and the magnetic field in the vicinity was measured. At this time, FIG. 13 shows a result of examining the shielding effect of each metal plate on the printed wiring boards having the same wiring length and different sizes (substrates A, B, and C; sizes A <B <C). Shown in

It can be seen that when there is no shield, radiation noise increases as the size of the substrate increases. This is because the near magnetic field is generated by resonance not only on the wiring but also on the power supply ground layer in the printed circuit board. Also, from FIG. 13, it was demonstrated that when Ti was placed on the microstrip line, the shielding effect was higher than that of Cu and SUS. This is because, as shown in FIG. 8, Ti has the largest skin thickness among Cu, Ti, and SUS and is close to the plate thickness t, so that the clock signal is transmitted, but its harmonic component is not transmitted, and This is because the electric field distribution in the wiring substrate becomes gentle because the reflection to Cu is smaller than Cu, and the radiation noise is reduced.

Therefore, according to the equation (2), the thickness t of the conductive layer and the resistivity ρ
Shows the relationship. As an example, FIG.
At the frequency of the signal line at 0 MHz and 1 GHz, the electromagnetic wave below the frequency of the signal line is transmitted, but the electromagnetic wave above the frequency of the signal line is not transmitted. The relationship of thickness is shown.
The range of expression (2) is shown by the hatched portion in FIG. From FIG. 14, it can be seen that, for example, when the frequency of the signal line is 60 MHz, and when the resistivity is 10 × 10 ⁻⁸ , the thickness may be about 15 μm to about 20 μm.

Next, in the sectional view of the circuit board of FIG.
FIG. 15 shows a simulation result of the maximum electric field intensity in cases A, B, and C when the thickness u of the dielectric 3 between the signal line and the external shield film is changed. In the cases A, B, and C, it is expected that the thicker the dielectric 3, the lower the maximum electric field intensity and the lower the high-frequency noise in the substrate. However, dielectric 3
It can be seen that the effect of reducing the maximum electric field intensity is small when the film thickness of is １ ／ or more of the thickness of the inner dielectric material 9.

In a case where high-frequency analog signals and digital signals are arranged close to each other, for example, when it is desired to suppress the deterioration of high-frequency waveforms, as shown in FIG. By arranging the EMI at a position at least two times the width of the signal line from immediately above, rather than directly above, the EMI can be shielded to some extent without affecting the electric field generated in the signal line.

FIG. 14 shows the relationship between the resistivity ρ and the thickness t. The desired resistivity and the desired thickness t of the conductive layer are obtained.
17 and 18, by forming a plurality of metals in layers as shown in FIGS. 17 and 18, a conductive layer is formed that shields electromagnetic waves of a certain frequency f or higher but transmits electromagnetic waves of a certain frequency or lower. .

Lastly, as shown in FIG. 19, in order to reduce high-frequency noise such as common mode noise between layers in the wiring board, the surface conductive layer is formed at the peripheral end of the wiring board for shielding. Can be.

[0036]

As described above, according to the present invention, it is possible to provide a circuit board in which the electric field strength inside the wiring board is small, high-frequency noise is hardly generated in the board, and EMI can be reduced.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a wiring board of the present invention.

FIG. 2 is an electric field distribution diagram at a signal line end (α-α ′) in the case where there is no external conductor layer in FIG.

FIG. 3 is an electric field distribution diagram at a ground end (β-β ′) in the case where there is no external conductor layer in FIG. 1;

FIG. 4 is a signal line end (α-α ′) in the case where the external conductor layer is a low resistance body and is connected to the ground potential in FIG.
FIG.

FIG. 5 is a diagram illustrating a ground end portion (β−) when the external conductor layer is connected to the ground potential with a low resistance body in FIG.
FIG. 6 is an electric field distribution diagram at β ′).

FIG. 6 shows a signal line end (α) in the case where the external conductor layer is a conductor having a low resistance and is not connected to any potential in FIG.
-Α ').

FIG. 7 is an electric field distribution diagram at a ground end (β-β ′) in the case where the outer conductor layer in FIG. 1 is a conductor having a low resistance and not connected to any potential.

FIG. 8 is a diagram showing the frequency dependence of the thickness of the skin.

FIG. 9 is a diagram illustrating frequency dependence of a shielding effect by a metal plate.

FIG. 10 is a diagram showing a state where an electric field of a low-frequency component of a signal line is transmitted through a conductive layer having a certain resistance.

FIG. 11 is a diagram showing a state where an electric field of a high frequency component of a signal line is shielded by a conductive layer having a certain resistance.

FIG. 12 is a diagram showing a decrease in current density in a conductive layer.

FIG. 13 is a diagram showing near-field radiation noise (actually measured values) on a microstrip line.

FIG. 14 is a diagram showing the relationship between the resistivity and the thickness of the conductive layer on the surface when shielding is performed for electromagnetic waves of a certain frequency f or more.

FIG. 15 is a diagram showing the relationship between the thickness of a dielectric between a signal line and an external shield film and the maximum electric field intensity.

FIG. 16 is a cross-sectional view illustrating an example of a wiring board of the present invention.

FIG. 17 is a cross-sectional view illustrating an example of the wiring board of the present invention.

FIG. 18 is a cross-sectional view illustrating an example of the wiring board of the present invention.

FIG. 19 is a cross-sectional view illustrating an example of the wiring board of the present invention.

FIG. 20 is a sectional view showing a conventional wiring board.

FIG. 21 is a sectional view showing a conventional wiring board.

FIG. 22 is a cross-sectional view showing a conventional wiring board.

FIG. 23 is a cross-sectional view showing a conventional wiring board.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 signal wiring 2 first power ground layer 3 surface dielectric layer 4 conductive layer 5 second power ground layer 6
Third power ground layer, 7: fourth power ground line (surface layer), 8: air, 9: inner dielectric layer, 10: signal line, 1
1. Electric lines of force

Claims (4)

[Claims] 1. A floating surface conductive layer is formed on a high-speed signal wiring via a dielectric film, and by controlling the resistivity ρ and the thickness t of the surface conductive layer, electromagnetic waves can be selectively generated. A wiring board characterized by being shielded. 2. The structure according to claim 1, wherein, with respect to the thickness t of the surface conductive layer, the resistivity is ρ, the magnetic permeability is μ, and the clock frequency is f. Wherein the electromagnetic wave on the higher frequency side than the clock frequency f is shielded. 3. The wiring board according to claim 1, wherein the conductive layer is disposed at a position separated from the position immediately above the high-speed signal line by at least twice the width of the signal line. 4. The wiring board according to claim 1, wherein said conductive layer is formed at a peripheral end of the wiring board.