Wiring substrate and process for producing the same

Details Wiring substrate and process for producing the same Wiring substrate and process for producing the same

2000-12-08

2002-11-05

Cuneo, Kamand (Department: 2827)

Electricity: conductors and insulators

Conduits, cables or conductors

Preformed panel circuit arrangement

C174S258000

Reexamination Certificate

active

06476330

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wiring substrate having a signal wiring suitable for transmitting high frequency digital signals, and a process for producing the same. Especially, the invention relates to a wiring substrate having a specific signal-wiring configuration for enabling high-speed transmission of high frequency digital signals in digital systems having components such as CPU and main memory, and a process for producing the same.
2. Prior Art
Conventional wiring substrates and processes for producing them will be reviewed hereinafter.
In digital systems, digital signals, in other words, pulse signals, are used. In terms of sine-wave frequency convolution, the pulse signals have higher harmonic components. For example, a pulse signal of 200 MHz has third order sine-wave harmonics and fifth order sine-wave harmonics, with relative amounts measured in energy being about 30% and 10%, respectively. Thus, at the pulse signal of 200 MHz, it is required to design a transmission line configuration taking account of the frequency component of 1 GHz sine wave.
FIG. 12
shows a wiring substrate for a digital system having integrated circuit chips, which was examined by the inventors of the present invention as a typical example of the transmission line design for high frequencies. A CPU
2
and a main memory
3
are mounted on a wiring substrate
1
. The CPU
2
and the main memory
3
are connected by a plurality of signal wirings
4
, the number of which is determined by the number of the bits of the signal, for transmitting the digital signals.
Recently, as the developments in the high-speed CPU
2
are being accelerated, those with the operation frequencies as high as 400 MHz to 733 MHz have been developed. However, the frequency range of the digital signals allowed on the current print wiring boards is 133 to 200 MHz. This does not permit the signal wiring
4
to accommodate the higher frequencies of the digital signals transmitted form the CPU
2
, resulting in a typical inconvenience that the data is not stored in the main memory
3
.
Thus, the example above will need a development of a transmission line suitable for a frequency range of 2 GHz to 3 GHz in terms of the sine-wave frequency convolution of the digital signals. This proves that the design for transmission lines of a print wiring board now has to deal with GHz frequency range in digital systems.
Furthermore, in comparison to a RF (Radio Frequency) system, a digital system has a number of signals, more than 64 bits for example, being transmitted in parallel. This creates more difficult issues for the high frequency digital systems than the RF systems.
SUMMARY OF THE INVENTION
According to the present invention, it is possible to provide a wiring substrate structure and a manufacturing method thereof for transmitting high frequency digital signals.
The understanding of the phenomena involved in the transmission of the high frequency signals through the wiring lines is a crucial ingredient of the present invention, and now will be described hereinafter, before the description of the realization of the invention.
The difficulties confronted in the design of the high frequency digital signals in the GHz range are categorized in three issues. The first is that it is difficult to keep the characteristic impedance constant among the large number of bit wirings running in parallel. As the characteristic impedance changes depending on the interference from the neighboring wirings, it is required to eliminate most of the neighboring interference (the interference from the neighboring wirings).
A few designs for the high frequency wiring structure have been proposed in Japanese Laid-Open Patent Publication No. 11-284126. The idea is to suppress the neighboring interference by making a signal wiring pair by pairing two signal wirings, and by making the distance between signal and ground shorter than the distance between neighboring signal wiring pairs, for increasing the coupling between the two signal wirings composing a signal wiring pair.
One important point which needs attention is that higher reproducibility of the configuration and the size of the signal wiring is needed to keep the characteristic impedance constant, creating a need for a production process with even higher accuracy. Specifically, for example, the distance between the neighboring signal wiring pairs and the distance between the two signal wirings composing a signal wiring pair should be controlled so that the two distances are kept constant respectively. The wiring structures disclosed in the above Japanese Laid-open Patent Publication, however, do not satisfy this need.
The second is that the electric current flows only near the surface of the signal wiring while high frequency signals being supplied to the signal wiring, resulting in the reduction of the effective cross section of the electric current, and thereby in an increase in the DC current resistance and a decrease in the signal amplitude. This is called skin effect.
The inventors of the present invention have found that the skin effect is one of the major contributors degrading the transmission characteristics of the conventional signal wirings. Thus, the skin effect will be reviewed hereinafter extensively.
FIGS. 13A
,
13
B and
FIGS. 14A
,
14
B are conceptual views for describing the skin effect occurring in the signal wirings. When DC current
6
flows through signal line
5
as shown in
FIG. 13A
, the DC current
6
flows with a same density all across the cross-section of the signal line
5
.
If a high frequency signal is supplied to the signal line
5
, the current flows in one direction, stops and then flows in opposite direction, and this sequence is repeated with a high speed. It is known that lines of magnetic force are generated around an electric current. Thus, while a high frequency signal is supplied, a cycle of generation and annihilation of the line of magnetic force is added to the sequence of the current flow comprising a flow in one direction, a stop of flow, and a flow in an opposite direction. The line of magnetic force can be perceived as having inertia in a similar manner as the mass has in dynamics. In other words, energies are required for generating line of magnetic force once the same is annihilated.
When the signal line
5
is considered as an assembly of a number of thin lines, the lines of magnetic force generated around the electric current elements
7
flowing through each thin line have the same direction. Thus, the lines of magnetic force generated around neighboring electric current elements collide each other and create a high-energy state.
Thus, following the principal of the nature, the electric current density becomes higher only near the surface of the signal line
5
, as shown in
FIG. 13B
, so that the energy of the electromagnetic system is minimized.
This happens, as the energy state is lower near the surface of the signal line
5
due to the suppressed mutual interference among the neighboring lines of magnetic force. This is the cause of the skin effect. However, having the skin with high electric current density all around the surface of signal line
5
is achieved under an assumption that a ground
9
is placed near the signal line
5
.
Under the assumption above, the electric current flows only near the surface all around the contour of the signal line. The skin depth &dgr;
s
is obtained by the following relation.
&dgr;
s
={square root over ( )}2/&ohgr;&mgr;
&ggr;
&sgr;  (1)
And the skin depth is in proportional to {square root over ( )}2&pgr;/&ohgr;.
In the equation above, &ohgr; denotes an angular frequency, &mgr;
&ggr;
the magnetic permeability of the signal line conductor, and &sgr; the electric conductivity of the signal line conductor. Assuming a Cu (Copper) wiring and 1 GHz sine wave, the skin depth is 2.2 &mgr;m. More than 60% of the electric current flows within this depth, and almost 100% of the electric current flows within the depth of 5 &mgr;m.
Now, t

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