JPH0555746A - High frequency copper clad laminated board and printed circuit board - Google Patents

Abstract

PURPOSE:To obtain a printed circuit board with less high frequency transmission loss. CONSTITUTION:A copper foil which is given a surface length coefficient (obtained by multiplying the surface length per unit straight distance on the surface with 100) by making smooth both front and rear surfaces through elimination of protruded and recessed areas) is used for a metal layer to be used for signal transmission. (c) and (d) have a surface length coefficient which is smaller than that of (a) and (b). Moreover, the surface length coefficient is in the following relation, (c)=(d), (a)=(b).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copper clad laminate excellent in high frequency characteristics and a printed wiring board.

[0002]

2. Description of the Related Art Various characteristics are required for using a circuit board at high frequencies. For the dielectric (insulating layer), t
It is necessary that an δ be low and be appropriate ε _r , and a polyimide resin, a modified polyimide resin or a fluororesin is used. For conductors, it is necessary to have high conductivity, rust resistance, and small surface roughness, usually copper,
Gold is used.

A skin effect is a phenomenon peculiar to high-frequency current. The skin effect is a phenomenon in which current concentrates on the surface layer of the conductor as the frequency increases. The current density decreases as it goes deeper from the surface, but the depth at which 1 / e (e is the natural logarithm) of the surface value is called the skin depth, which is a measure of the depth at which the current flows. The skin depth depends on the frequency, and becomes smaller as the frequency becomes higher.

The copper foil used for the circuit board is roughened on one side or both sides in order to improve the adhesiveness, and the surface of the copper foil is roughened by an acid treatment or the like when performing multi-layered bonding. ..
However, as the frequency increases, the electric current concentrates on the surface layer due to the skin effect, and the electric resistance (skin resistance) increases. As a result, not only the loss of current increases, but when the skin depth becomes smaller than the surface roughness of the conductor, the current flows through the uneven surface of the conductor, the transmission distance becomes longer, and the time and current required for signal transmission are increased. The loss will increase.

Compared with the transmission time and current loss of the surface layer for this high frequency, it can be obtained by lowering the permittivity of the dielectric of the circuit board, or by increasing the mounting density of electronic parts and shortening the line length. The effect that can be achieved is only about 10%. Therefore, a high-frequency metal foil-clad laminate in which the surface roughness of the conductor is reduced has been proposed (JP-A-60-2483).
44).

[0006]

However, it has been found that when the frequency becomes high, even if the surface roughness is made small, there is no effect. An object of the present invention is to reduce the loss due to the skin effect even in an ultra high frequency range such as used in satellite communication.

[0007]

According to the present invention, a metal layer for transmitting a high frequency signal has a surface length ratio of 150 on both front and back surfaces.
A high-frequency copper-clad laminate comprising the following copper foil. If the surface length ratio exceeds 150, the signal transmission time becomes long and the intended purpose cannot be achieved.

The surface length ratio (hereinafter referred to as RSI) is a number obtained by multiplying the surface length per unit linear distance of the surface by 100. If there is no unevenness on the surface, the RSI will be 100.
If the surface length is twice that straight distance, the RSI is 20.
It becomes 0.

The RSI is measured as follows. A curable resin is cast on the metal layer portion and polished to prepare a sample in which the cross section perpendicular to the surface direction of the metal layer can be clearly discriminated. About 1000 to 5000 of this using an electron microscope.
Shoot at double. The surface length of the cross-section of the metal layer in this photograph is measured using a quilvimeter (surface distance meter).
The unit distance (the linear distance in the longitudinal direction of the metal layer) is 5 times or more of the maximum surface roughness (Rmax) of the surface to be measured. Also, the measurement is performed at several different parts.

For copper foils having an RSI of 150 or less, a rolled copper foil which is not subjected to a roughening treatment, a rolled copper foil (dull foil) which is physically roughened in the rolling step, and a roughened surface is subjected to an electrical or chemical treatment. There is an electrolytic copper foil that is smooth and smooth. The rolled copper foil is usually used after being roughened by some method, but in the present invention, it is used without being roughened. It goes without saying that degreasing and anticorrosion treatment are required.

Among the rolled copper foils, the one having irregularities formed by using the irregularity roll in the rolling process, that is, the dull foil is the RSI of the rolled copper foil.
Is 150 or less and can be used. The electrolytic copper foil has a roughened surface on at least one side due to its manufacturing method. In an ordinary electrolytic copper foil, the RSI is around 200 for any of the thicknesses of 35 μm, 18 μm, and 13 μm. So after manufacturing,
The roughened surface is used by electrical or chemical treatment to smooth the surface.

In the present invention, none of the copper foils used are subjected to the acid treatment or reduction treatment which is usually carried out in the multilayer adhesion step. The reason is that in both cases, the roughening becomes very fine and the RSI becomes 150 or more.

Further, since the copper foil layer is usually processed into a circuit by etching, the side surface thereof is extremely finely roughened. In addition, since the copper foil is easily oxidized, an oxide is likely to be generated on the side surface to form unevenness having a large change. Therefore, after the circuit processing is performed on the copper foil layer, gold plating is performed. The thickness of gold plating is 2 of the skin depth at the operating frequency.
Double the amount. Gold plating reduces the conductor resistance, and since it does not oxidize, unevenness does not increase and the initial surface shape is retained.

The printed wiring board of the present invention is also used as a multilayer printed wiring board formed by laminating and integrating another printed wiring board or a metal foil via an adhesive resin film.
The laminated board or the printed wiring board according to the present invention cannot be adhered by a usual method because a smooth copper foil is used for the metal layer. Therefore, adhesive resin films are arranged above and below the copper foil layer used as the signal transmission layer so as to be integrated in the subsequent multilayer adhesion step. As the resin of the adhesive resin film, one similar to the resin component used for the substrate is used.

[0015]

In general, simply reducing the surface roughness does not always reduce the RSI. The reason will be described with reference to the drawings. It is assumed that the irregularities forming the surface roughness have an equilateral triangle shape (FIG. 1) or an arc shape (FIG. 2). The surface roughness is reduced while maintaining this shape. As is clear from the comparison between (a) and (b) in the figure, the surface length does not change. On the other hand, as is apparent from (c) and (d) of the figure, the apex angle of the rough surface is made large (60 to 120 degrees in FIG. 1) or the central angle of the arc is made small (see FIG. 2 to 180 degrees to 90
Degree) and the surface length becomes smaller. In the high frequency band where the skin depth is smaller than the surface roughness, the signal current flows along the unevenness of the surface. Therefore, for example, an electrolytic copper foil having a thickness of 35 μm has a roughened surface RSI of about 200, so that the transmission distance is about doubled, the time required for transmission is also doubled, and the loss is also doubled. Therefore, even if the surface roughness is reduced, the transmission loss is not reduced, but if the surface shape is made gentle, that is, RSI is reduced, the transmission loss is also reduced.

[0016]

Example 1 Rolled copper foil without surface roughening (thickness 35 μm, RSI = 1
15) and a polyimide resin-impregnated glass cloth prepreg were integrated via a polyimide resin film to obtain a four-layer printed wiring board substrate. The adhesive strength of the copper foil layer is 1.
The value was 0 kg / cm, which was practically acceptable. next,
Circuit processing was performed on this substrate to form an electric circuit pattern, which was kept at 60 ° C. and a humidity of 90% for 3000 hours. When the transmission time of the signal at 9 GHz was measured, it was 3% longer than the initial value.

Example 2 The printed wiring board obtained in Example 1 was subjected to gold plating with a thickness of about 5 μm on the externally exposed surface (the surface not in contact with the insulating layer) of the metal layer on which the electric circuit pattern was formed. The same test as in Example 1 was conducted on this circuit. 60
There was no change in the transmission time before and after 3000 hours at 90 ° C. and 90% humidity, which was almost the same as the initial value for the printed wiring board of Example 1.

Comparative Example Using a single-sided roughened rolled copper foil (thickness: 35 μm, RSI = 207), a four-layer printed wiring board substrate was prepared in the same manner as in Example 1 below. The adhesive strength of the copper foil layer is 1.2 kg / cm
Met. Circuit processing was performed on the copper foil of this laminated plate to form an electric circuit pattern, and the transmission time was measured. The transmission time was about 2.17 times that of Example 1.

[0019]

According to the present invention, it is possible to obtain a printed wiring board with a small signal transmission delay due to the skin effect peculiar to the high frequency band. Moreover, the signal transmission ability is not deteriorated by plating the circuit surface with gold.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a relationship between a surface length and a surface shape.

FIG. 2 is a diagram showing another example of the relationship between the surface length and the surface shape.

Claims (4)

[Claims] 1. A high-frequency copper-clad laminate in which metal layers for transmitting high-frequency signals are copper foils having a surface length ratio of 150 or less on both front and back surfaces. 2. A high-frequency printed wiring board obtained by circuit-processing the high-frequency copper-clad laminate according to claim 1. 3. A high-frequency printed wiring board obtained by circuit-processing the high-frequency copper-clad laminate according to claim 1, and then plating the metal surface with gold. 4. A multilayer printed wiring board obtained by laminating and integrating the printed wiring board according to claim 2 and another printed wiring board or a metal foil with an adhesive resin film interposed therebetween.