WO2016035876A1 - Copper foil, copper clad laminated plate, and substrate - Google Patents

Abstract

The present invention has a plurality of protrusions formed on a surface, wherein when a skin depth d (m) is equal to √ (1/(σ·μ·π·f)) (where σ is conductivity (S/m), μ is magnetic permeability (H/m), f is frequency included in an electrical signal (Hz)), f ≥ 5 GHz; and when h (μm) is taken as the height of a protrusion, and w (μm) is taken as the width of a protrusion at a position at a height of h/2, at least one protrusion for which h/d is ≥ 1 is formed for every 5-μm length, and in at least 80% of the protrusions having h/d ≥ 1, w is ≥ 0.1 μm, and the equation w/d ˂ -0.1 h/d + 1.4 is satisfied.

Description

Copper foil, copper clad laminate, and substrate

The present invention relates to a copper foil or the like having excellent adhesion to a resin base material and excellent high-frequency signal transmission characteristics.

In recent years, with the miniaturization and performance enhancement of electronic components, small and high density printed wiring boards are being used. Such a printed wiring board is manufactured from a copper clad laminate in which a circuit forming copper foil is disposed and integrated on the surface of an insulating resin base material. A circuit pattern is formed by applying a mask pattern to the copper foil and etching the copper-clad laminate.

The copper foil and the resin base material are integrated by heating and pressurization, but the adhesiveness of a predetermined level or more is required. As a method for ensuring such adhesion, a method of subjecting a copper foil to a predetermined roughening treatment is generally used (for example, Patent Documents 1 to 3).

Japanese Patent Laid-Open No. 7-231152 JP 2006-210689 A Japanese Patent No. 5204908

The method of Patent Document 1 defines the shape of a fine hump formed on the surface of a copper foil in order to ensure adhesion by an anchor effect on a resin base material.

In addition, Patent Document 2 performs a low-roughness treatment in order to improve the linearity of the bottom line of the copper foil, and in order to ensure adhesion, a heat-resistant / rust-proof layer, a chromate film layer, and a silane coupling. The agent layer is formed.

Further, Patent Document 3 specifies the surface roughness and the like of the copper foil, prevents the occurrence of poor insulation due to the copper particles remaining after etching, and ensures adhesion.

However, although Patent Documents 1 to 3 ensure adhesion, high frequency transmission characteristics are not always considered sufficiently. In copper foil for high-frequency signal transmission, coherence of transmission characteristics when using a copper-clad laminate is a major issue as well as ensuring adhesion to a resin substrate.

Here, it is said that the transmission characteristics deteriorate as the height of the roughened protrusion, that is, the degree of surface roughness is increased. In Patent Document 2, it is interpreted as an increase in transmission length due to surface roughening, but it is not quantitatively shown. In addition, there is no current state-of-the-art policy based on a specific mechanism that should be patented to achieve both the securing of adhesion by the anchor effect and the securing of transmission characteristics.

In order to ensure the adhesion between the copper foil and the resin base material, another problem may arise as a trade-off when the height of the roughening protrusion is increased. For example, if the protrusion is too long, an etching residue is likely to occur. In order to avoid this, it is assumed that a sufficient etching time is taken. Moreover, if the protrusion width is narrowed with the protrusion having a certain length, a problem of powder falling may occur. In order to avoid this, it is assumed that physical contact with the copper foil is reduced by handling or the like when manufacturing the copper foil. However, either workaround can lead to increased manufacturing costs and reduced quality.

The present invention has been made in view of such problems, and an object of the present invention is to provide a copper foil or the like that has excellent adhesion to a resin base material and excellent high-frequency transmission characteristics.

In order to achieve the above object, a first invention is a copper foil for transmitting a high-frequency electrical signal, wherein a plurality of protrusions are formed on the surface, and a skin depth d (m) = √ (1 / (σ · μ · π · f)) (where σ: conductivity (S / m), μ: permeability (H / m), f: frequency (Hz) included in the high-frequency electrical signal), f ≧ 5 GHz, 5 μm of the protrusion satisfying h / d ≧ 1 when the height of the protrusion is h (μm) and the width of the protrusion at the height of h / 2 is w (μm). The average number per length is 1 or more, and 80% or more of the protrusions satisfying h / d ≧ 1, w ≧ 0.1 μm and w / d <−0 A copper foil characterized by satisfying 1 h / d + 1.4.

The first invention is a copper foil for transmitting a high-frequency electric signal, wherein a plurality of protrusions are formed on the surface, and a skin depth d (m) = √ (1 / (σ · μ · π · f )) (Where σ: conductivity (S / m), μ: permeability (H / m), f: frequency (Hz) included in the high-frequency electrical signal), f ≧ 5 GHz, When the height of the protrusion is h (μm) and the width of the protrusion at the height of h / 2 is w (μm), the average per 5 μm length of the protrusion with h / d ≧ 2 80% or more of the protrusions having the number of one or more and h / d ≧ 2 satisfy w ≧ 0.1 μm and w / d <−0.1 h / d + 1. 4 is a copper foil characterized by satisfying 4.

H is preferably 0.4 μm or more.

W is desirably 0.2 μm or more.

More preferably, the frequency f included in the high-frequency electrical signal is 20 GHz or more.

More preferably, the ratio of the three-dimensional surface area by the optical interference microscope to the two-dimensional surface area of the roughened surface is less than three times.

According to the first invention, since the shape of the protrusion is standardized by the skin depth defined according to the transmission frequency, sufficient adhesion with the resin base material and high transmission characteristics suitable for the use conditions are obtained. Can be obtained. Specifically, if one or more protrusions satisfying h / d ≧ 1 are formed on average per 5 μm length, adhesion with the resin base material can be ensured. Further, since the number of protrusions satisfying w / d <−0.1 h / d + 1.4 is 80% or more, good electrical characteristics can be obtained even when the protrusion height is high.

Further, in order to ensure good adhesion, one or more protrusions satisfying h / d ≧ 2 are formed on average per 5 μm length, and w / d <−0.1 h / d + 1.4 is satisfied. If the protrusion is 80% or more, better electrical characteristics can be obtained while maintaining adhesion.

In particular, if h is 0.4 μm or more, adhesion to the resin base material can be obtained more reliably. Moreover, if w is 0.2 μm or more, occurrence of powder falling can be suppressed.

In addition, at 20 GHz or higher, the ratio of the three-dimensional surface area measured three-dimensionally with an optical interference microscope to the two-dimensional surface area of the roughened surface is less than three times, so that even better electrical characteristics can be obtained. it can.

In a second aspect, the copper foil according to the first aspect and a resin base material are laminated and bonded together, and the resin base material has a dielectric constant of 4 or less and a dielectric loss tangent tan δ of 0.006 or less. It is a copper clad laminated board characterized by being.

The resin substrate is made of a liquid crystal polymer, a fluororesin, a polyetherimide, a polyetheretherketone, a polyphenylene ether, a polycycloolefin, a bismaleimide resin, a low dielectric constant polyimide, or a mixture of these. It is desirable to be.

According to the second invention, a low-loss copper-clad laminate can be obtained efficiently. Usually, high frequency resins often have poor adhesion to copper foil. As such a high-frequency resin, a resin polymer or the like having a dielectric constant of 4 or less and a dielectric loss tangent tan δ of 0.006 or less is used. In this invention, a higher effect can be acquired by applying to such resin.

According to a third aspect of the present invention, a line is formed by patterning the copper foil on the copper clad laminate according to the second aspect of the invention, and the line has a wavelength defined by the frequency f of the high-frequency electrical signal. On the other hand, the substrate has a length of 10 effective wavelengths or more.

According to the third aspect of the invention, the transmission loss reduction effect can be obtained more efficiently. This is because a longer line has a larger loss value as an effect, and if the line length is too short, the effect is small.

According to the present invention, it is possible to provide a copper foil or the like that is excellent in adhesion to a resin base material and excellent in high-frequency transmission characteristics.

The figure which shows the board | substrate 1 (copper clad laminated board 2). The figure which shows the relationship between what normalized the surface roughness with skin depth, and permeation | transmission conductivity. The cross-sectional enlarged view of the copper foil 7. The figure which shows distribution of the apparent electrical conductivity with respect to height h and width w of the protrusion 9 normalized by the skin depth. FIGS. 5A and 5B are conceptual diagrams showing a current density distribution in the vicinity of the protrusions. FIG. 5A is a view showing a wide protrusion, and FIG. 5B is a view showing a narrow protrusion. The figure which shows distribution of the apparent electrical conductivity with respect to height h and width w of the protrusion 9 normalized by the skin depth. FIG. 7A is a diagram showing the scope of the present invention in the apparent conductivity distribution with respect to the height h and width w of the protrusion 9 normalized by the skin depth, and FIG. (B) is a figure shown about 80 GHz. FIGS. 8A and 8B are diagrams illustrating transmission characteristics with respect to frequency. FIG. 8A illustrates a case where the dielectric tangent of the resin base material tan δ = 0.01, and FIG. 8B illustrates a case where tan δ = 0.004. .

(Substrate 1)
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a substrate 1 according to the present invention. The substrate 1 is obtained by patterning a line 5 on a resin base material 3. The line 5 is formed by a copper foil 7. That is, the copper foil 7 and the resin base material 3 are bonded together, and the line 5 is formed by masking and etching. In addition, the copper-clad laminate 2 is obtained by bonding and integrating the copper foil 7 and the resin base material 3 before etching. As a method for forming the copper clad laminate 2 by laminating the resin base material 3 and the copper foil 7, a known method such as a hot press method, a continuous roll laminating method, a continuous belt pressing method, or the like can be used.

The copper foil 7 may be any of an electrolytic copper foil, an electrolytic copper alloy foil, a rolled copper foil, and a rolled copper alloy foil, and can be appropriately selected according to the use of the copper clad laminate 2 and the like. The details of the copper foil 7 will be described later.

The resin base material 3 desirably has a dielectric constant of 4 or less and a dielectric loss tangent tan δ of 0.006 or less. A liquid crystal polymer can be applied as such a material.

Here, the length of the line 5 has a side that can be expressed by a transmission frequency and a transmission wavelength in addition to the dimensional length. For example, in the case of the line 5 having a length of 100 mm, the effective wavelength (= the wavelength considering the wavelength shortening by the resin base material 3, converted to the wavelength of the line length) is about 1 wavelength at a frequency of 2 GHz. On the other hand, at a frequency of 20 GHz, the effective wavelength is on the order of about 10 wavelengths. Since the transmission loss becomes significant in a relatively long signal line of about 10 wavelength order or more with respect to the effective wavelength, it can be said that the application of the present invention is suitable. That is, the line 5 is preferably 10 or more effective wavelengths long with respect to the wavelength defined by the frequency f of the high-frequency electrical signal. In addition, the board | substrate 1 of this invention shall be used for transmission of the high frequency signal of 5 GHz or more. This is because the effect of the present invention cannot be sufficiently obtained when the frequency is lower than this.

(Copper foil)
Next, the copper foil 7 will be described in detail. FIG. 3 is an enlarged cross-sectional view of the resin contact surface of the copper foil 7 for high-frequency electrical signal transmission. The copper foil 7 has protrusions 9 formed on a copper base material 11. The copper foil for high-frequency circuits of the present invention is provided with roughened particles by burnt plating on the surface of the copper foil as a metal substrate (the surface roughness is not particularly limited, but Rz is preferably 5.0 μm or less). To form a roughened particle layer. The roughened particles are preferably made of copper. In the present invention, “protrusions” include those formed by roughened particles.

Although details are omitted, it is desirable to form a rust preventive layer made of a chromate film on the roughened particles. Furthermore, you may give a silane coupling process on a rust prevention layer. The silane coupling agent can be appropriately selected from epoxy, amino, methacrylic, vinyl, mercapto and the like depending on the target resin substrate 3. For the resin base material 3 used for the high frequency substrate 1, epoxy, amino, and vinyl coupling agents that are particularly excellent in compatibility can be selected.

Here, when a high-frequency current flows on the copper foil, the current distribution concentrates in the region of the skin depth d from the surface of the copper foil, and current loss occurs due to the conductor resistance at the concentrated portion. In particular, when a current flows through a roughened conductor having a protrusion formed on the surface, not a smooth conductor, current loss increases. This increase in current loss due to roughening can be replaced with an increase in loss due to a decrease in conductivity. That is, it is possible to evaluate the high-frequency characteristics in the roughened state as apparent conductivity. In the following description, the apparent conductivity is referred to as equivalent conductivity.

In the line 5 formed from the copper clad laminate 2, the transmission characteristics deteriorate due to a decrease in the equivalent conductivity of the copper foil 7. Such a decrease in equivalent electrical conductivity becomes significant due to the increase in frequency in addition to the increase in surface roughness. Such an increase in loss due to the roughened conductor has been modeled by Hammarsted et al. For a long time using the surface roughness as a parameter.

FIG. 2 is a diagram showing the relationship between the surface conductivity normalized by the skin depth and the transmission conductivity. For example, in a situation where the skin depth d of the smooth copper foil at 1 GHz is about 2.1 μm and the surface roughness (indicated by Rq) is sufficiently smaller than this, for example, 0.4 μm, as shown by the dotted line O, There is almost no effect on the decrease in equivalent conductivity. In the case where the surface roughness Rq is on the same order or larger than the skin depth d, the equivalent conductivity is significantly reduced.

On the other hand, when the skin depth d, which is defined by the frequency as the frequency increases, is on the same order or smaller than the surface roughness Rq, the equivalent conductivity is also reduced. For example, the skin depth of about 2.1 μm at 1 GHz is about 0.9 μm at 5 GHz, and the skin depth d is about 0.5 μm at 20 GHz, and the surface has almost no influence like the dotted line O at 1 GHz. Even in the roughness, it becomes like a dotted line P at 5 GHz, and like a dotted line Q at 20 GHz, and the reduction in equivalent conductivity becomes remarkable due to the increase in frequency. Thus, for example, even in the case of a roughened conductor having a surface roughness that hardly affects 1 GHz, the equivalent conductivity is significantly reduced by increasing the frequency of use such as 5 GHz and 20 GHz. The deterioration of the transmission characteristics of the signal line becomes a problem.

As a result of intensive studies, the inventors have normalized the surface protrusion shape that affects the equivalent conductivity by the skin depth d and investigated the distribution of the equivalent conductivity. I found that there are areas that change. The skin depth d (m) is √ (1 / (σ · μ · π · f)) (where σ: conductivity (S / m), μ: permeability (H / m), f: (Frequency (Hz)). In the copper foil, conductivity σ = 5.82 × 10 ⁷ and permeability μ = 4π × 10 ⁻⁷ . Hereinafter, the relationship between the protrusion shape and the equivalent conductivity will be described.

(Relationship between protrusion shape and equivalent conductivity)
As shown in FIG. 3, the height of the protrusion 9 on the copper foil in the present invention is h, and the width of the protrusion 9 at a height of h / 2 is w. More specifically, in the cross-section in the width direction of the copper foil measured by HR-SEM, the vertical direction from the boundary line between the base material 11 (part corresponding to the untreated copper foil) and the protrusion 9 toward the top of the protrusion 9 A line is drawn, and the length h of this line is defined as the protrusion height. Further, w is a protrusion width at a position half the protrusion height h. The equivalent conductivity was calculated for the protrusion shape thus defined.

The following describes the calculation model. It is possible to calculate the above-mentioned equivalent conductivity by irradiating a plane wave of high-frequency electromagnetic waves perpendicularly to a copper foil having an arbitrary rough shape and observing the reflection characteristics. In the calculation model, the protrusion shape is a simple conical shape. In addition, the calculation was performed using a model in which the same protrusions were periodically spread on the surface of the copper foil. The surface roughness Rz that is conventionally considered to contribute to transmission characteristics can be considered as the protrusion height h in this model. Similarly, the width w of the conical protrusion at the height of h / 2 was assumed as a parameter, and the equivalent conductivity was calculated for any combination of the protrusion height h and the protrusion width w.

Here, when the material is pure copper, the skin depth d is uniquely determined by the frequency used. As described above, in the present invention, the dimension parameters h and w are defined by the size normalized by the skin depth d.

FIG. 4 is a diagram showing the calculation result of the equivalent conductivity in each protrusion shape by normalizing each protrusion height h and protrusion width w by the skin depth. Specifically, FIG. 4 is a diagram showing a result of calculating an equivalent conductivity distribution with h / d and w / d normalized by the skin depth d taken on the horizontal axis and the vertical axis, respectively. In FIG. 4, the equivalent conductivity decreases from the lower left region to the upper right region.

It can be derived from the conventional model that the equivalent conductivity varies with h / d which is the vertical axis. That is, if the protrusion height h is small, there is no significant decrease in equivalent conductivity, and it is considered acceptable as transmission characteristics as a copper foil. Conventionally, it has been known that the smaller the surface roughness, the better the transmission characteristics. In order to ensure adhesion, increasing the height of the protrusion as a roughened surface to a depth greater than the skin depth is the transmission characteristic. In order to obtain particularly good transmission characteristics in the past, the protrusion height higher than the skin depth, that is, the characteristic starts to deteriorate in the region of h / d ≧ 1, and further, the protrusion height such as twice that height, That is, it was difficult to adopt the h / d ≧ 2 region.

On the other hand, the phenomenon in which the equivalent conductivity rapidly changes depending on the horizontal axis w / d is an event newly discovered in the study according to the present invention. Specifically, when w / d is 1 or less, the equivalent conductivity is improved by a non-linear change. Further, by setting w / d to 0.5 or less, the above-described protrusion height h is reduced to the skin depth. The same improvement can be seen as less than twice the height. The effect of improving the equivalent conductivity due to this rapid change is particularly remarkable when the protrusion height is high (surface roughness is rough), such as h / d ≧ 1. That is, it can be said that it is particularly effective when the projection height h needs to be sufficiently large in order to secure the adhesion with the resin substrate 3.

Such a phenomenon that the equivalent conductivity suddenly changes due to the change in the protrusion width normalized by the skin depth d is considered to be based on the following principle.

(Protrusion width and current density)
FIG. 5 is a conceptual diagram showing a high frequency conduction current density on a copper foil cross section when a high frequency electric field is applied in the horizontal direction on the paper surface in electromagnetic field analysis. The dotted line in the figure is an equicurrent density line. FIG. 5A shows a case where the standardized width is a relatively thick protrusion, and FIG. 5B shows a case where the standardized width is relatively narrow. In FIG. 5A, point A is a point where the current density is low, and point B is a point where the current density is high. In FIG. 5B, point E is a point where the current density is low, and point F is a point where the current density is high.

Usually, the skin effect becomes remarkable as the conduction current increases in frequency, and the current is interpreted to flow more concentrated on the copper foil surface. Such a phenomenon is based on the assumption that the copper foil has a smooth structure, and the current density in the case of having a protrusion shape on the surface as in the present invention is compared with that in the case where the copper foil is smooth. It will be very special. Specifically, in both FIG. 5A and FIG. 5B, it is confirmed that the current hardly flows on the front end side of the protrusion, which can be said to be the surface side (point A, point E). What is happening at the tip is thought to be cancellation of the conduction current, which is a phenomenon that exhibits the characteristic characteristics of the present invention.

This phenomenon is due to the following reasons. If most of the current is concentrated within the skin depth of the copper foil surface and the concentrated portions do not interfere with each other, a conduction current is generated. On the other hand, when the portions below the skin depth interfere with each other, such as the tip of the protrusion, the current flows in the opposite direction at the interference portion, cancels out, and the conduction current does not flow. For example, if the current flowing toward the tip of the protrusion interferes with the current flowing from the tip to the base side, the current is canceled out, and the conductive current does not flow, so that it does not become a current loss source. This is a phenomenon newly studied in the present invention.

In more detail, the difference between FIG. 5 (a) and FIG. 5 (b) will be described. In the case of FIG. 5B, the width of the protrusion is narrower than that in FIG. 5A, and the current density in the protrusion is relatively small. For example, in the case of FIG. 5A, the protrusion surface position (point D) having a current density equivalent to the current density of the protrusion inner center point (point C) connecting the protrusion base is 1/2 of the protrusion height h. Shift to the tip side of the protrusion rather than the height. That is, the current density at the surface of the center portion of the protrusion height is higher than the current density at the center of the protrusion base.

On the other hand, in the case of FIG. 5B, the protrusion surface position (point I) having a current density equivalent to the current density of the protrusion inner center point (point G) connecting the protrusion base is 1 / of the protrusion height h. It shifts to the protrusion base side rather than the height of 2. That is, the current density at the surface of the central portion of the protrusion height is lower than the current density at the center of the protrusion base. That is, in FIG. 5B, the current flowing inside the protrusion is small, and the current flowing on the base side of the protrusion tends to increase. In this way, by selecting a projection structure that reduces the amount of current flowing inside the projection (especially the tip side from the projection height h / 2) relative to the current flowing near the projection base, current loss due to the projection is reduced. It is thought that this can be reduced.

(Projection shape design)
Next, a particularly preferable protrusion shape in the present invention will be described. FIG. 6 is a view similar to FIG. A straight line J in FIG. 6 is a straight line with h / d = 1. As described above, sufficient equivalent conductivity can be secured in a region where the projection height is low, such as h / d <1. Therefore, the present invention is particularly effective when the projection height is high (surface roughness is rough), such as h / d ≧ 1. A straight line K in FIG. 6 is a straight line of w / d = −0.1 h / d + 1.4. As shown in FIG. 6, in the region where w / d <−0.1h / d + 1.4 (that is, the region on the left side of the straight line K), the ideal state (equivalent conductivity in the smooth state) is obtained at any projection height. ) Equivalent electrical conductivity of about 50% or more. Here, the straight line K is a linear function that substantially matches the tangent at 5 to 7 so that at least 50% of the ideal conductivity can be secured in the region where h / d is 7 or less, for example. Yes. In FIG. 6, it can be said that the region formed by the straight lines K and J is a desirable range from the viewpoint of transmission characteristics and adhesion.

In particular, when it is necessary to secure adhesion, h needs to be increased, so loss due to copper foil is a problem even at a signal frequency of about 5 GHz. However, in the present invention, it is possible to reduce the transmission loss of the copper foil by designing the protrusion shape in the region where w / d <−0.1 h / d + 1.4. In order to secure adhesion, h is preferably 0.4 μm or more.

As described above, it has been known that copper foil having a small protrusion height has no major problem in terms of transmission loss. In particular, by using a chemical bond such as a silane coupling material, the low-roughened copper foil can also improve the adhesion strength to the resin even in a situation where the protrusion height is small.

On the other hand, for materials that are difficult to chemically bond, such as liquid crystal polymers, it is necessary to ensure adhesion with a physical shape. That is, there may be a substrate situation in which the height of the protrusions must be increased.

There are cases where the protrusion height h is 1 μm or more with respect to a copper foil for a general liquid crystal polymer. Even when such collaterality of adhesion is particularly necessary, it is necessary to reduce copper foil loss. For example, in the case of 5 GHz, the skin depth d is about 0.9 μm. Even in this case, the present invention satisfies the relationship of h / d ≧ 1 and satisfies the relationship of w / d <−0.1 h / d + 1.4, even when a signal frequency of 5 GHz is transmitted to the copper foil. Loss can be reduced.

Also, it is assumed that the transmission frequency exceeds several GHz order. For example, characteristics of about 20 GHz or more are required for high-speed digital signal transmission exceeding 10 Gbps, and characteristics up to about 80 GHz are required for millimeter wave radar. In this way, when the frequency is in the quasi-millimeter wave or millimeter wave band such as 20 GHz or 80 GHz, the skin depth d is reduced, so that h / d is further increased. For this reason, equivalent electrical conductivity falls. Therefore, when the height of the protrusion for securing the adhesion force is secured, there is a possibility that even if there is no problem at several GHz, it becomes impossible to disregard as a cause of transmission loss with the increase of the frequency exceeding 20 GHz. However, in the present invention, even if the protrusion height is twice or more of the skin depth, the protrusion width is designed so that w / d <−0.1 h / d + 1.4 according to the operating frequency. It becomes possible to reduce transmission loss.

On the other hand, if the protrusion width is too narrow, there may be a problem in quality. For example, if the protrusion width becomes extremely narrow, there is a concern that powder may fall off during the process. For this reason, it is necessary to select a projection width that does not cause powder falling. Specifically, in a protrusion with w smaller than 0.1 μm, the possibility of avoiding powder falling increases, so w is desirably 0.1 μm or more, and more desirably, w is 0.2 μm or more.

FIG. 7 is a diagram showing a region that is particularly effective in the present invention when w is 0.1 μm or more. In FIG. 7A, straight lines K and J are the same as those in FIG. The straight line L in FIG. 7A is a case where w = 0.1 μm, and w / d (= about 0.2) normalized by the skin depth d (about 0.47 μm) for a high frequency signal of 20 GHz. ).

Similarly, in FIG. 7B, straight lines K and J are the same as those in FIG. A straight line M in FIG. 7B is a case where w = 0.1 μm, and w / d (= about 0.4) normalized by the skin depth d (about 0.23 μm) for a high-frequency signal of 80 GHz. ).

In the present invention, it is desirable that h / d ≧ 1, w ≧ 0.1, and w / d <−0.1 h / d + 1.4. Therefore, as a copper foil used for a high frequency signal of 20 GHz, it can be said that a region surrounded by straight lines K, L, and J in FIG. 7A is a desirable range including the copper foil quality. Moreover, as copper foil used for a high frequency signal of 80 GHz, it can be said that the area surrounded by the straight lines K, L, and M in FIG. 7B is a desirable range including the copper foil quality.

As described above, h ≧ 0.4 μm is desirable from the viewpoint of adhesion, but if h can be compatible with transmission characteristics, a larger h can be taken as long as etching residue does not become a problem. preferable.

By setting the protrusion shape in the region shown in FIG. 7, it is possible to obtain a substrate having excellent adhesion to the copper foil 7 and the resin base material 3 and excellent transmission characteristics. In addition, there is a concern that an extremely high protrusion height h may lead to an etching residue. For this reason, the protrusion height h needs to be set to such an extent that no etching residue is generated. As described above, in the present invention, in consideration of the range of the substantial protrusion height h and protrusion width w that can be selected in terms of quality, the transmission characteristic of the copper foil 7 according to the operating frequency is taken into consideration, and the selection of the protrusion shape parameters Can do the design. Note that, from the viewpoint of reducing etching residue, it is assumed that the protrusion extends in a direction substantially perpendicular to the copper foil surface.

Also, the present invention does not need to be particularly uniform with respect to the uniformity of the protrusion shape and height, and it is not necessary for all protrusions to be within this region. For example, in any cross section, the average number of protrusions having a height satisfying h / d ≧ 1 (more desirably h / d ≧ 2) per 1 μm is 1 or more, and 80 of these protrusions. % Or more so as to satisfy w ≧ 0.1 μm and w / d <−0.1 h / d + 1.4, the effect of the present invention can be obtained.

However, in the high frequency region of 20 GHz or more, in order to further improve the transmission characteristics, the ratio of the three-dimensional surface area by the optical interference microscope to the two-dimensional surface area of the roughened surface is less than three times, so that the transmission characteristics can be more reliably improved. Can be increased.

Note that the width and height of the protrusion can be obtained by observing with a HR-SEM (high resolution scanning electron microscope) at a measurement magnification of 3000 times or more (for example, 10,000 times). Further, the fact that one or more protrusions having a height satisfying h / d ≧ 1 exist on average per 5 μm is determined by the average number when observing at least 20 arbitrary portions. Further, the width and height of the protrusion may be obtained by observing by a method other than HR-SEM.

(effect)
Next, the effect of the present invention in the substrate using the copper foil described above will be described. As shown in FIG. 1, the board | substrate 1 bonds the copper foil 7 to the resin base material 3, forms the copper clad laminated board 2, and also forms the track | line 5 by pattern processing. It is known that the transmission loss of the line 5 is expressed as the sum of dB of the conductor loss due to the copper foil 7 and the dielectric loss in the substrate 1 (resin base material 3). Therefore, ensuring the transmission characteristics as the substrate 1 requires not only the copper foil 7 but also the characteristics of the resin base material 3.

For example, although FR4 is used in most electronic devices as a general general-purpose resin base material, the dielectric loss tangent tan δ cannot be said to be sufficiently good. On the other hand, low loss resin base materials having a dielectric constant of 4 or less and tan δ of less than 0.006 are provided by various companies. In order to secure transmission characteristics as the substrate 1, such low loss resin bases are provided. A material is preferred. As such a low-loss base material, the resin base material is any one of a liquid crystal polymer, a fluororesin, a polyetherimide, a polyetheretherketone, a polyphenylene ether, a polycycloolefin, a bismaleimide resin, and a low dielectric constant polyimide. Or a mixture of these, and specific examples include Megtron 6, BT resin, and the like.

FIG. 8 shows an example of analyzing the transmission characteristics when the protrusion shape of the resin base material and the copper foil is changed. FIG. 8A shows an example using a resin base material having a dielectric constant of 3.7 and tan δ of 0.01, and FIG. 8B shows a dielectric constant of 3.7 and tan δ of 0.004. The case where the resin base material which is is shown is shown. The line length is 100 mm in all cases. The line S in the figure is for h / d = 4.3 and w / d = 1.6, and the line T is for h / d = 3.2 and w / d = 1.6. Line R is the case where h / d = 3.2 and w / d = 0.4. That is, only the line R satisfies w / d <−0.1 h / d + 1.4.

Comparing the lines S, T, R, it can be seen that the loss of the line R is small. Further, when considering the improvement effect from the line T to the line R as a percentage, the example of FIG. 8A is about 10%, whereas the example of FIG. 8B is about 20%. It will be about. That is, in combination with a low-loss resin base material, it can be said that the contribution of improvement due to the protrusion shape of the copper foil is large. Furthermore, the effect obtained by the present invention is further increased by further increasing the operating frequency and further extending the line length.

In addition, although the example of the simple microstrip line was shown as the track | line 5, it is not restricted to this. As the type of the line 5, the same effect is exhibited in the triplate line and the differential line. The pattern can be applied not only to straight lines but also to various shapes of lines including bending, branching, filters, antennas and the like. In any case, as described above, it is suitable for a portion that transmits at least a high-frequency signal for a long section having an effective wavelength of about 10 wavelengths or more.

(Preferable high frequency application to which the present invention is applicable)
High frequency applications suitable for implementing the present invention can be broadly classified into high frequency analog signal transmission applications and high speed digital signal transmission applications. Regarding high-frequency analog signal transmission, for example, in application to radio equipment products, the upper limit frequency that can be used depending on the application is determined by the laws and regulations relating to radio waves in each country. The upper limit frequency determined by the application and transmitting the line is considered as the frequency to be secured on the board.

On the other hand, regarding the transmission of high-speed digital signals, there are limits to what should be considered from the viewpoint of signal quality up to the highest possible frequency components, and the requirements of the high-frequency characteristics to be ensured depend on the application. Actually, there is a case where a high-frequency collateral design is performed up to a frequency including the third and fifth harmonics of the clock fundamental wave. In order to realize high-frequency characteristics, the present invention considers that a frequency up to at least a third harmonic of the clock fundamental wave should be secured as a frequency included in the high-frequency electric signal. Furthermore, in the transmission of high-speed digital signals exceeding 10 Gbps, the frequency that can be expressed as f = 0.35 / t depending on the rise time (10% -90%) time t (seconds) of the digital signal input to the transmission line on the substrate. In some cases, high-frequency collateral design is performed. Accordingly, in the present invention, the frequency that is at least the third harmonic of the clock fundamental wave when the transmission line is actually used, or f = 0.35 / t determined from the rise time, is the frequency to be secured in the high-frequency electrical signal. Think. In the present invention, it is considered that good signal transmission can be realized by providing a copper foil having suitable characteristics at these frequencies.

Specifically, the former is suitable for high-frequency applications such as millimeter-wave communication and millimeter-wave radar and large-scale applications such as base stations, and the latter is suitable for signal transmission on workstations and high-speed transmission on server backplanes. Application to the above is preferable.

Hereinafter, the results of evaluating transmission loss and adhesion to various roughened copper foils will be described.

<Examples 1 to 5, Comparative Examples 1 to 3>
A smooth untreated copper foil having a surface roughness Rz of about 0.5 μm and a thickness of 18 μm is prepared as a metal substrate, and the untreated copper foil is subjected to burnt plating to form a roughened particle layer (protrusion). did. Burn plating is a method in which a fine projection group of granular copper is adhered by performing electrolysis near a limit current density using a copper foil as a cathode in an acidic copper electrolytic bath. The solutions used for the burnt plating are shown in Table 1.

In Table 1, the solution A can be uniformly roughened, the solution B has a roughened particle shape that becomes round and thick, and the solution C has a roughened particle shape that becomes thin. When the coarse particle height is high (low frequency), the solution C is selected to reduce the thickness of the coarse particle, and when the coarse particle height is low (high frequency), a certain amount Since it is necessary to ensure the thickness of the roughened particles, it is desirable to select the solution depending on the height of the roughening, such as selecting the solution B.

Using the above metal base material, burnt plating was performed. Table 2 shows the conditions for the burnt plating.

Furthermore, capsule plating was performed on the roughened particle layer by burn plating. Capsule plating covers the fine projections of granular copper applied by burn plating with a thin layer of ordinary copper plating (so-called “capsule layer”), and the fine projections of granular copper are coated on the surface of the copper foil. It is to be fixed. The conditions for capsule plating are as follows.

Sulfuric acid concentration 80 ~ 120g / L
Copper sulfate (as Cu concentration) 40-60g-Cu / L
Bath temperature 45-60 ° C
Current density 10 ~ 20A / dm ^{2 by} DC rectification

After performing the capsule plating, in Examples 1 to 3 and Comparative Example 1, burn plating and capsule plating were performed again.

In addition, after the capsule plating, both surfaces of the copper foil were subjected to rust prevention treatment with a known chromate treatment solution (corresponding to 3.0 g / L in CrO ₃ concentration).

<Section observation>
Each copper foil thus produced was subjected to cross-sectional processing in the width direction using ion milling (IM4000 manufactured by Hitachi High-Tech), and an acceleration voltage of 3 kV (secondary) using HR-SEM (SU8020 manufactured by Hitachi High-Tech). An electron image and a low-angle reflected electron image) were subjected to cross-sectional observation at a magnification of 20,000 times, and the height and width of roughened particles in a 5 μm range of arbitrary 20 copper foil cross sections were measured.

In addition, said cross-sectional observation can also be performed about the circuit board produced by pattern-processing the copper foil of the copper clad laminated board which bonded copper foil and the resin base material together. In this case, the cross section is processed in the longitudinal direction of the circuit pattern (line) so that the interface between the copper foil and the resin base material can be observed, and any 20 sections of the copper foil in the vicinity of the center of the circuit pattern (line). The height and width of roughened particles in the 5 μm range are measured.

<Surface area ratio measurement>
Using the three-dimensional white-light interference microscope (BRUKER Wyko Control GT-K), the ratio of the three-dimensional surface area to the two-dimensional surface area was measured (measurement conditions were a measurement magnification of 10 times, using a high-resolution CCD camera, After measurement, it was digitized without applying a special filter), and less than 3 was “◯”, 3 or more and less than 4.5 was “Δ”, and more was “x”.

<Evaluation of transmission characteristics>
The produced copper foil was laminated on a resin substrate by a hot press method, and a microstrip line as shown in FIG. 1 was produced as a signal line for evaluating transmission characteristics by etching. As the resin base material, polyphenylene ether resin (product name: Megtron 6 manufactured by Panasonic Corporation: dielectric constant 3.7, dielectric loss tangent tan δ 0.002) was used. About this microstrip line, the transmission loss with respect to the high frequency signal up to 40 GHz was measured with the network analyzer. The characteristic impedance was 50Ω.

The transmission characteristics are evaluated as follows: transmission loss is −0.7 dB / 100 mm @ 5 GHz or less, −1.8 dB / 100 mm @ 15 GHz or less, −4.7 dB / 100 mm @ 40 GHz or less ○, −0.7 dB / Those exceeding 100 mm @ 5 GHz, those exceeding −1.8 dB / @ 15 GHz, and those exceeding −4.7 dB / 100 mm @ 40 GHz were evaluated as x. Note that the above boundary value is the transmission loss of the untreated copper at each frequency and the equivalent conductivity of 75% due to the roughening treatment with respect to the conductivity of copper in the ideal state (untreated copper). It calculated from the sum of dB of loss.

<Peel test>
The produced copper foil was laminated | stacked on the resin base material (Megtron 6 by Panasonic Corporation) with the hot press system, and the copper clad laminated board was produced. The copper foil portion of this copper clad laminate was masked with a 10 mm wide tape, and after copper chloride etching, the tape was removed to produce a 10 mm wide circuit wiring. Using a Tensilon tester manufactured by Toyo Seiki Seisakusho Co., Ltd., the circuit wiring was peeled off at a speed of 50 mm / min in the 90-degree direction to determine peel strength. Peel strength criterion 0.5 kN / m or more was evaluated as ◯, and less than it was evaluated as ×. The results are shown in Table 3.

In cross-sectional observation, in Examples 1 to 5 and Comparative Examples 1 and 3, on average, one or more protrusions satisfying h / d ≧ 1 (hereinafter referred to as Condition A) were present per 5 μm. In Example 2, the protrusion height was low, and the average number of protrusions satisfying the condition A was less than 1 per 5 μm at any frequency.

In addition, among the protrusions satisfying the condition A, the ratio of the protrusions satisfying the condition of w / d <−0.1 h / d + 1.4 (hereinafter referred to as condition B) was evaluated. Here, as described above, even if the protrusion shape is the same (h and w), d varies depending on the use condition (frequency), so the above condition is determined for each frequency.

Specifically, in Example 1, at 5 GHz, among the protrusions satisfying the condition A, the protrusion satisfying the condition B was 80% or more, but at 15 GHz or more, it was less than 80%. Similarly, in Examples 2 and 3, the protrusion satisfying the condition B was 80% or more up to 15 GHz, but it was less than 80% at 30 GHz or more. In Examples 4 and 5, the protrusions satisfying the condition B among the protrusions satisfying the condition A were 80% or more at all frequencies.

In Comparative Example 2, as described above, the protrusion height is low, and there are less than one protrusion satisfying the condition A, and the condition B cannot be satisfied. In Comparative Examples 1 and 3, the condition A was satisfied by repeating the burn plating and the capsule plating a plurality of times. However, in Comparative Example 1, w / d was large, and the protrusion satisfying the condition B was less than 80%. It was. In Comparative Example 3, w / d was small, and the number of protrusions satisfying w ≧ 0.1 was less than 80%.

As a result of the transmission loss evaluation, in Examples 1 to 5, the evaluation of the transmission loss was evaluated as good when 80% or more satisfied the projection shape condition B at each frequency. Examples 1 to 5 all satisfied the desired peel strength.

On the other hand, Comparative Example 2 was satisfied with the transmission loss because the protrusion height was low, but the peel strength was insufficient. In Comparative Example 1, since the protrusion height was sufficient, the peel strength was satisfactory, but since w / d was out of the standard, the transmission loss was x at all frequencies. Further, in Comparative Example 3, the protrusion height was sufficient, but w / d was very small, so that the peel strength was insufficient, or rough powder was generated.

As described above, according to the present invention, it is possible to satisfy both securing of adhesion with a resin base material and securing of transmission characteristics, which have been difficult to achieve in the past. In particular, it is possible to provide a copper foil and a high-frequency signal transmission method suitable for applications for high-frequency signal transmission of 5 GHz or more, and further 20 GHz or more.

More specifically, when the height normalized by the skin depth d defined by the transmission frequency is set to h / d, the height of the resin base material is set to h / d ≧ 1. Adhesion can be ensured. Furthermore, if h is 0.4 um or more, the adhesion to the resin substrate can be more reliably ensured.

Also, since the protrusion width satisfies w / d <−0.1 h / d + 1.4, the amount of current flowing in the vicinity of the protrusion tip can be reduced relative to the vicinity of the protrusion base. For this reason, it becomes possible to reduce the conductor loss in a protrusion structure. As a result, it is possible to provide a copper foil with good signal line transmission characteristics at the used frequency. In particular, at a high frequency such as 20 GHz or higher, the height of the protrusion necessary for ensuring adhesion can be greater than or equal to the skin depth d order, which may cause an increase in transmission loss. This is effective in a situation where it is difficult to achieve both adhesion and transmission characteristics.

Also, by setting the protrusion width to 0.1 um or more, which is the minimum width required for quality, it is possible to avoid quality deterioration such as powder falling. Further, although the adhesion is improved by increasing the protrusions, there is a trade-off relationship that etching residue is likely to occur. However, the present invention includes the protrusions for each frequency as shown in FIG. The optimum design of the shape becomes possible.

Further, by bonding the copper foil of the present invention to a resin base material that is generally considered to have a low loss, such as a dielectric constant of 4 or less and tan δ of 0.006 or less, the ratio of the characteristic improvement contribution becomes more remarkable. Therefore, it is suitable for application to a high-frequency low-loss substrate. Also, for substrates with low loss, such as liquid crystal polymers, which are difficult to ensure chemical adhesion, as described above, with sufficient protrusion height to ensure adhesion and compatibility with transmission characteristics Can be achieved.

In addition, by applying the copper foil of the present invention to a substrate having a long line pattern that effectively becomes 10 wavelengths or more specified by the transmission frequency, the effect of improving the transmission characteristics becomes remarkable, and at a higher frequency. It can contribute to securing characteristics in large applications.

The embodiment of the present invention has been described above with reference to the accompanying drawings, but the technical scope of the present invention is not affected by the above-described embodiment. It is obvious for those skilled in the art that various modifications or modifications can be conceived within the scope of the technical idea described in the claims, and these are naturally within the technical scope of the present invention. It is understood that it belongs.

DESCRIPTION OF SYMBOLS 1 ......... Substrate 2 ...... Copper-clad laminate 3 ......... Resin base material 5 ......... Line 7 ......... Copper foil 9 ...... Protrusion 11 ...... Base material

Claims (9)

1. A copper foil for high-frequency electrical signal transmission,
   Having a plurality of protrusions on the surface,
   Skin depth d (m) = √ (1 / (σ · μ · π · f)) (where σ: conductivity (S / m), μ: permeability (H / m), f: the high-frequency electricity In the case of frequency (Hz) included in the signal,
   f ≧ 5 GHz,
   When the height of the protrusion is h (μm) and the width of the protrusion at the height of h / 2 is w (μm),
   The average number of the protrusions with h / d ≧ 1 per 5 μm length is 1 or more, and
   80% or more of the protrusions satisfying h / d ≧ 1 satisfy w ≧ 0.1 μm and satisfy w / d <−0.1 h / d + 1.4 .
2. A copper foil for high-frequency electrical signal transmission,
   Having a plurality of protrusions on the surface,
   Skin depth d (m) = √ (1 / (σ · μ · π · f)) (where σ: conductivity (S / m), μ: permeability (H / m), f: the high-frequency electricity In the case of frequency (Hz) included in the signal,
   f ≧ 5 GHz,
   When the height of the protrusion is h (μm) and the width of the protrusion at the height of h / 2 is w (μm),
   The average number of protrusions per 5 μm length of h / d ≧ 2 is 1 or more, and
   80% or more of the protrusions satisfying h / d ≧ 2 satisfy w ≧ 0.1 μm and satisfy w / d <−0.1 h / d + 1.4 .
3. 3. The copper foil according to claim 1, wherein h is 0.4 μm or more.
4. The copper foil according to claim 1, wherein w is 0.2 μm or more.
5. The copper foil according to claim 1 or 2, wherein the frequency f included in the high-frequency electric signal is 20 GHz or more.
6. The copper foil according to claim 5, wherein the ratio of the three-dimensional surface area by the optical interference microscope to the two-dimensional surface area of the roughened surface is less than three times.
7. The copper foil according to claim 1 or claim 2 and a resin base material are laminated and adhered,
   The resin base material has a dielectric constant of 4 or less and a dielectric loss tangent tan δ of 0.006 or less.
8. The resin substrate is made of any one of a liquid crystal polymer, a fluororesin, a polyetherimide, a polyetheretherketone, a polyphenylene ether, a polycycloolefin, a bismaleimide resin, a low dielectric constant polyimide, or a mixed resin thereof. The copper clad laminate according to claim 7.
9. The copper-clad laminate according to claim 7, wherein the copper foil is patterned to form a line,
   The substrate, wherein the line has a length of 10 effective wavelengths or more with respect to a wavelength defined by a frequency f of a high-frequency electric signal.

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