JP2714985B2 - Multilayer film for additive wiring board - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a multilayer film used when manufacturing a wiring board by an additive method, and more specifically, a circuit is formed by electroless plating. The present invention relates to a multilayer film including an organic layer (hereinafter, referred to as an additive layer) and an organic layer (hereinafter, referred to as an insulating layer) having an insulating function with respect to an inner layer circuit.

[Conventional technology]

2. Description of the Related Art Conventionally, a wiring board obtained by an additive method is obtained by laminating a plurality of conductive circuits via an insulating layer, and has been variously used as components of electric products and the like. This wiring board is manufactured by, for example, the insulating substrate 1 shown in FIG. 3 (A).
As shown in FIG. 3 (B), an insulating layer 4 is disposed on both sides of a printed circuit board 3 provided with copper patterns 2 on both sides by a screen printing method, and then as shown in FIG. 3 (C). Next, an additive layer 5 is provided on the surface of the insulating layer 4 by a screen printing method, and the surface of the additive layer 5 is hydrophilized (roughened) with a chromic acid mixed solution (CrO ₃ + H ₂ SO ₄ ). A catalyst such as palladium is applied to perform a surface activation treatment (excluding the case where a plating catalyst is previously mixed), and a non-circuit-forming portion of this surface is coated with a photosensitive lacquer (photoresist) by a photographic method or screen printing. (Formation of a resist film), and then electroless plating is performed on the circuit forming portion. The printed circuit board 3 shown in FIG.
Is generally made of a normal copper-clad laminate by a subtractive method.

However, manufacturing a wiring board in this way has problems such as a complicated manufacturing configuration.

[Problems to be solved by the invention]

The present invention has been made in order to simplify a manufacturing process of a wiring board, and is a multilayer film used by being laminated on a printed circuit board or the like as shown in FIG. The purpose is to provide.

[Means for solving the problem]

For this reason, the present invention comprises an additive layer and an insulating layer, wherein the additive layer is composed of a synthetic rubber R and an epoxy resin E, and their mixing ratio R / E (volume ratio)
Is in the range of 0.5 ≦ R / E ≦ 2.5, and the insulating layer is composed of synthetic rubber R and epoxy resin E, and the compounding ratio R / E (volume ratio) thereof is 0.1 ≦ R / E ≦ 0.4. The present invention provides a multi-layer film for an additive method wiring board, which is characterized by being within the range.

 Hereinafter, this means will be described in detail with reference to the drawings.

FIG. 1 is an explanatory sectional view of an example of the multilayer film for an additive method wiring board of the present invention.

In FIG. 1, the multilayer film M includes an insulating layer 4 and an additive 5.

The additive layer 5 is a compound comprising a synthetic rubber R and an epoxy resin E. As the synthetic rubber R, styrene-butadiene rubber, butadiene rubber, acrylonitrile-butadiene rubber (NBR), acrylonitrile-butadiene-isoprene rubber (NBIR), chloroprene rubber, isoprene rubber, butyl rubber and the like are blended by one or more kinds. Can be used. Among them, NBR is particularly preferred. As NBR, acrylonitrile content 20-50%,
A Mooney viscosity (ML _{1 + 4} , 100 ° C.) in the range of 25 to 80 is preferably used. As the epoxy resin E, a bisphenol / epichlorohydrin type, a novolak type or an alicyclic epoxy resin can be used. When imparting flame retardancy, a Br-epoxy resin may be used. These epoxy resins can be used alone or in combination of two or more.

The compounding ratio R / E (volume ratio) of the synthetic rubber R and the epoxy resin E in the additive layer 5 is in the range of 0.5 ≦ R / E ≦ 2.5. If the R / E is less than 0.5, the amount of the rubber which is a roughening elution component is small, so that an appropriate roughened surface cannot be obtained on the additive layer 5, and as a result, the plating deposition property deteriorates. On the other hand, if R / E exceeds 2.5, the volume occupancy of the rubber component becomes too high, and the surface to be plated does not stop at roughening, but the unevenness of the surface to be plated becomes extremely rough. Swelling and peeling may occur.

The insulating layer 4 is made of the same type of synthetic rubber R and the same type of epoxy resin E as in the additive layer 5. The components of the insulating layer 4 and the additive layer 5 are of the same type for the following reason. That is, since the insulating layer 4 and the additive layer 5 are in contact with each other at the same interface, when heating is performed during processing (particularly in a curing step), an interaction in which components enter each other through this interface occurs between the layers, and the components move. . In this case, the thinner the thin films, the greater the degree of influence of mutual movement, and if the components of each layer are different, the influence of the insulating layer 4 on the additive layer 5 increases,
This makes morphology control, which is a basic technique in the additive process, very difficult. Therefore, the constituent components of the insulating layer 4 and the additive layer 5 are the same,
The degree of influence between the layers, particularly from the insulating layer 4 to the additive layer 5
The morphology of the additive layer 5 is basically kept unchanged by minimizing the degree of influence on the additive layer 5. Here, morphology means that when synthetic rubber and epoxy resin are mixed, microphase separation occurs due to the difference in SP value, but each compatible state is obtained depending on the type of material, molecular weight, mixing ratio, etc. Can be This compatibility state is referred to.

The mixing ratio R / E (volume ratio) of the synthetic rubber R and the epoxy resin E in the insulating layer 4 is in the range of 0.1 ≦ R / E ≦ 0.4. When R / E is less than 0.1, the fluidity becomes large when laminating the insulating layer 4 on the inner layer circuit board, and further, the resin flows when the insulating layer 4 is cured by heating, so that it is difficult to maintain the thickness between layers. On the other hand, R /
When E exceeds 0.4, the fluidity becomes insufficient when laminating on the inner layer circuit board. In particular, when the inner layer circuit board has a fine wire circuit, the flow between the circuits becomes insufficient.

As a curing agent for the epoxy resin E, for example, an aliphatic amine, an aromatic amine, an acid anhydride, a guanidine compound, an imidazole compound and the like can be used, and further, a suitable curing accelerator can be used.

The synthetic rubber R may be vulcanized, and in the case of vulcanization, sulfur or a vulcanization accelerator may be used in combination, or resin vulcanization, peroxide vulcanization, or the like may be performed.

Additive layer 5 and insulating layer 4 may each contain a filler. Examples of the filler include carbonates such as calcium carbonate and magnesium carbonate, silicates such as calcium silicate and magnesium silicate, and metal oxides such as zinc oxide and titanium oxide.

When such a filler is contained in the additive layer 5, the hardness of the additive layer 5 is improved, and the roughening and elution properties of the surface are moderate. In addition, when contained in the insulating layer 4, the electrical insulation of the insulating layer 4 is improved, and the workability of the through hole is improved. Further, the filler plays a role of a flow control aid or the like.

In each of the additive layer 5 and the insulating layer 4, the epoxy resin E forms a continuous layer to form a sea, and the synthetic rubber R forms a dispersion layer to form an island.

The insulating layer 4 is in an uncured state in order to improve the adhesion with the copper pattern of the inner circuit board when being laminated on the inner circuit board, but the additive layer 5 may be uncured or cured. When laminating the insulating layer 4 on the inner circuit board (120 ° C ma
The viscosity of x) is preferably in the range of 10 ³ to 10 ⁵ poise. If it is less than 10 ³ poise, the fluidity becomes excessive, and it becomes difficult to maintain the thickness as an insulating layer. With more than 10 ⁵ poise, the flow between the circuits is insufficient.

To prepare the multilayer film M in which the additive layer 5 is not cured, the insulating layer 4 is applied to the surface of the release film, and the additive layer 5 is applied to the surface of another release film. 4 and the surface of the additive layer 5 may be matched. Examples of the release film used in this case include a polyethylene terephthalate film (PET), a silicon-treated polyester film,
Any material may be used as long as it has release properties, such as release paper or a metal foil such as an aluminum foil that has been release-treated with wax or the like.

In order to produce the multilayer film M when the additive layer 5 is cured, the insulating layer 4 may be applied to the previously cured additive layer. When the additive layer 5 is cured, the components of the insulating layer 4 are prevented from diffusing into the additive layer 5 as compared with the case where the additive layer 5 is not cured, so that the additive layer 5 is roughened during electroless plating. Becomes easier.

Further, as shown in FIG. 2, an adhesive layer 6 may be laminated on the insulating layer 4 in order to further enhance the adhesiveness with the inner circuit board. The lamination of the adhesive layer 6 is performed by applying an adhesive to the insulating layer 4. The surface of the adhesive layer 6 may be protected by covering it with the release film described above. Examples of the adhesive include an epoxy-based adhesive. The viscosity at the time of laminating the adhesive layer 6 (120 ° C. max) also
As in the case of the insulating layer 4, it is preferably in the range of 10 ³ to 10 ⁵ poise.

The multilayer film of the present invention thus obtained is laminated on a printed circuit board or the like as shown in FIG. 3 (A), and the surface of the additive layer 5 is activated by a conventional method (in the case of containing a catalyst). The activation process is omitted), and electroless plating can be performed.

 Examples and comparative examples are shown below.

Example 1 Additive layer NBR 100 parts by weight (nitrile content 41%, specific gravity 1.00) Epoxy resin 56 parts by weight (cresol novolac type, epoxy equivalent 220, specific gravity 1.22) Imidazole compound 7 parts by weight Peroxide 2 parts by weight Calcium carbonate 140 parts by weight R / E 2.20 Make a 38% solids MEK solution (methyl ethyl ketone solution) with the above composition and dry film on PET film 35
to obtain an uncured film (additive layer film).

Insulation layer Epoxy resin 50 parts by weight (bisphenol / epichlorohydrin type, epoxy equivalent 500, specific gravity 1.19) Epoxy resin 50 parts by weight (cresol novolak type, epoxy equivalent 220, specific gravity 1.22) NBR 20 parts by weight (nitrile content 41%, specific gravity 1.00) 10 parts by weight of imidazole compound 1 part by weight of peroxide 1 part by weight of clay 60 parts by weight of R / E 0.24 A MEK solution having a solid content of 53% of the above formulation is prepared, and coated on a release film (PET) so as to have a dry film thickness of 100 μm.
An uncured film was produced (insulating layer film).

These additive layer film and insulating layer film were bonded together at 80 ° C. using a roll laminator to obtain a multilayer film.

This multi-layer film is line and space (L / S) 0.5
When vacuum lamination (120 ° C, 40 Torr) was applied to a circuit board having a mm / 0.5 mm comb-tooth circuit (1 oz electrolytic copper) so that the insulating layer was in contact with the circuit board, and the flowability between the circuits was confirmed. , A good thing was obtained with a void dress.

Further, this multilayer film-lamination substrate was cured by heating in an electric oven at 150 ° C. × 1 hour, and subjected to a roughening treatment with a chromic acid mixed solution according to a conventional method of an additive method, and the surface roughened state was measured by a scanning electron microscope. As a result, it was confirmed that the surface was appropriately roughened (depth: 10 to 15 μ, size of unevenness: 20 μ or less). That is, the surface roughness was good.

Next, after performing catalyst addition and activation treatment, copper was deposited to a predetermined thickness by electroless plating, an annealing treatment was performed, and the properties were evaluated.The peel strength of the plated copper was 1.8 kg / cm. The solder heat resistance at 260 ° C was 180 seconds or more, which was a good result. Further, the flowability of the insulating layer was good.

Example 2 Additive layer NBR 80 parts by weight (nitrile content 33%, specific gravity 0.98) NBR 20 parts by weight (carboxyl group-containing, nitrile content 27%, specific gravity 0.98) Epoxy resin 20 parts by weight (cresol novolac type, epoxy equivalent 220, specific gravity) 1.22) Epoxy resin 30 parts by weight (bisphenol / epichlorohydrin type, epoxy equivalent 450, specific gravity 1.19) Imidazole compound 5 parts by weight Peroxide 2 parts by weight Calcium carbonate 100 parts by weight Plating catalyst 0.03 parts by weight (palladium chloride) R / E 2.45 A MEK solution (methyl ethyl ketone solution) with a solid content of 33% was made and dried on a release film (PET).
to obtain an uncured film (additive layer film).

Insulation layer Epoxy resin 100 parts by weight (cresol novolak type, epoxy equivalent 200, specific gravity 1.22) NBR 20 parts by weight (nitrile content 41%, specific gravity 1.00) Imidazole compound 8 parts by weight Peroxide 1 part by weight Mica 35 parts by weight Plating catalyst 0.03 parts by weight (Palladium chloride) R / E 0.24 A 50% solids MEK solution with the above formulation was prepared and coated on a release film (PET) to a dry film thickness of 100μ to produce an uncured film (insulating film). .

These additive layer film and insulating layer film were bonded together at 80 ° C. using a roll laminator to obtain a multilayer film.

This multi-layer film is line and space (L / S) 0.5
When vacuum lamination (120 ° C, 40 Torr) was applied to a circuit board having a mm / 0.5 mm comb-tooth circuit (1 oz electrolytic copper) so that the insulating layer was in contact with the circuit board, and the flowability between the circuits was confirmed. , A good thing was obtained with a void dress.

Further, this multilayer film-lamination substrate was cured by heating in an electric oven at 150 ° C. × 1 hour, and subjected to a roughening treatment with a chromic acid mixed solution according to a conventional method of an additive method, and the surface roughened state was measured by a scanning electron microscope. As a result, it was confirmed that appropriate roughening (depth: 10 to 15 μ, size of unevenness: 15 μ or less) was performed. That is, the surface roughness was good.

Next, after performing catalyst addition and activation treatment, copper was deposited to a predetermined thickness by electroless plating, an annealing treatment was performed, and the properties were evaluated.The peel strength of the plated copper was 2.0 kg / cm. The solder heat resistance at 260 ° C was 180 seconds or more, which was a good result. Further, the flowability of the insulating layer was good.

Comparative Example 1 Additive layer NBIR 100 parts by weight (nitrile content 35%, specific gravity 0.90) Phenol resin 54 parts by weight (resole type, specific gravity 1.25) Peroxide 7 parts by weight Calcium carbonate 23 parts by weight (R / E) (2.36) A MEK solution (methyl ethyl Kent solution) with a solid content of 33% was made and dried on a release film (PET).
to obtain an uncured film (additive layer film). The above (R / E) is N
This is the mixing ratio of BIR and phenol resin.

Insulation layer Epoxy resin 100 parts by weight (cresol novolak type, epoxy equivalent 220, specific gravity 1.22) NBR 20 parts by weight (nitrile content 41%, specific gravity 1.00) Imidazole compound 10 parts by weight Peroxide 1 part by weight 50 parts by weight R / E 0.24 An MEK solution having a solid content of 50% was prepared and coated on a release film (PET) so as to have a dry film thickness of 100 μ to prepare an uncured film (insulating layer film).

These additive layer film and insulating layer film were bonded together at 80 ° C. using a roll laminator to obtain a multilayer film.

This multi-layer film is line and space (L / S) 0.5
When vacuum lamination (12 ° C, 40 Torr) was applied to a circuit board having a mm / 0.5 mm comb-tooth circuit (1 oz electrolytic copper) so that the insulating layer was in contact with the circuit board, and the flowability between the circuits was confirmed. , A good thing was obtained with a void dress.

Further, this multilayer film-lamination substrate was cured by heating in an electric oven at 150 ° C. × 1 hour, and subjected to a roughening treatment with a chromic acid mixed solution according to a conventional method of an additive method, and the surface roughened state was measured by a scanning electron microscope. As a result, it was confirmed that the surface was appropriately roughened. That is, the surface roughness was good.

Next, after performing catalyst addition and activation treatment, copper was deposited to a predetermined thickness by electroless plating, an annealing treatment was performed, and the properties were evaluated.The peeling strength of the plated copper was 1.4 kg / cm. Solder heat resistance at 260 ° C was not a good result in 60 seconds. The flow properties of the insulating layer were good.

Comparative Example 2 Additive layer NBR 100 parts by weight (nitrile content 41%, specific gravity 1.00) Epoxy resin 267 parts by weight Cresol novolac type, epoxy equivalent 220, specific gravity
1.22) Imidazole compound 25 parts by weight Peroxide 2 parts by weight Calcium carbonate 330 parts by weight R / E 0.45 A 30% solids MEK solution (methyl ethyl ketone solution) with the above composition is prepared and dried on a release film (PET) 35
to obtain an uncured film (additive layer film).

Insulation layer Epoxy resin 50 parts by weight (bisphenol / epichlorohydrin type, epoxy equivalent 500, specific gravity 1.19) Epoxy resin 50 parts by weight (cresol novolak type, epoxy equivalent 220, specific gravity 1.22) NBR 20 parts by weight (nitrile content 41%, specific gravity 1.00) Imidazole compound 10 parts by weight Peroxide 1 part by weight Clay 60 parts by weight R / E 0.24 Make a MEK solution with a solid content of 53% of the above composition, coat it on a release film (PET) to a dry film thickness of 100 μm, and uncured A film was produced (insulating layer film).

These additive layer film and insulating layer film were bonded together at 80 ° C. using a roll laminator to obtain a multilayer film.

This multi-layer film is line and space (L / S) 0.5
When vacuum lamination (120 ° C, 40 Torr) was applied to a circuit board having a mm / 0.5 mm comb-tooth circuit (1 oz electrolytic copper) so that the insulating layer was in contact with the circuit board, and the flowability between the circuits was confirmed. , A good thing was obtained with a void dress.

Further, this multilayer film-lamination substrate was cured by heating in an electric oven at 150 ° C. × 1 hour, and subjected to a roughening treatment with a chromic acid mixed solution according to a conventional method of an additive method, and the surface roughened state was measured by a scanning electron microscope. As a result, the surface roughening state was poor (roughening hardly occurred).
No plating was deposited.

 The flow properties of the insulating layer were good.

Comparative Example 3 Additive layer NBR 100 parts by weight (nitrile content 41%, specific gravity 1.00) Epoxy resin 45 parts by weight (cresol novolak type, epoxy equivalent 220, specific gravity 1.22) Imidazole compound 7 parts by weight Peroxide 2 parts by weight Calcium carbonate 130 parts by weight R / E 2.71 Make a 35% solids MEK solution (Methyl Ethyl Ketone Solution) with the above composition and dry film thickness on PET film (PET) 35
to obtain an uncured film (additive layer film).

Insulation layer Epoxy resin 50 parts by weight (bisphenol / epichlorohydrin type, epoxy equivalent 500, specific gravity 1.19) Epoxy resin 50 parts by weight (cresol novolak type, epoxy equivalent 220, specific gravity 1.22) NBR 20 parts by weight (nitrile content 41%, specific gravity 1.00) Imidazole compound 10 parts by weight Peroxide 1 part by weight Clay 60 parts by weight R / E 0.24 Make a MEK solution with a solid content of 53% of the above composition, coat it on a release film (PET) to a dry film thickness of 100 μm, and uncured A film was produced (insulating layer film).

These additive layer film and insulating layer film were bonded together at 80 ° C. using a roll laminator to obtain a multilayer film.

This multi-layer film is line and space (L / S) 0.5
When vacuum lamination (120 ° C, 40 Torr) was applied to a circuit board having a mm / 0.5 mm comb-tooth circuit (1 oz electrolytic copper) so that the insulating layer was in contact with the circuit board, and the flowability between the circuits was confirmed. , A good thing was obtained with a void dress.

Further, this multilayer film-lamination substrate was cured by heating in an electric oven at 150 ° C. × 1 hour, and subjected to a roughening treatment with a chromic acid mixed solution according to a conventional method of an additive method, and the surface roughened state was measured by a scanning electron microscope. As a result, it was found that the roughened surface was poor (too roughened), and blisters were formed on the plating surface. The flow properties of the insulating layer were good.

Comparative Example 4 Additive layer NBR 100 parts by weight (nitrile content 41%, specific gravity 1.00) Epoxy resin 68 parts by weight (cresol novolak type, epoxy equivalent 220, specific gravity 1.22) Imidazole compound 8 parts by weight Peroxide 2 parts by weight Calcium carbonate 150 parts by weight R / E 1.79 Make a 35% solids MEK solution (methyl ethyl ketone solution) with the above composition and dry film thickness of 35% on release film (PET).
to obtain an uncured film (additive layer film).

Insulation layer Epoxy resin 100 parts by weight (cresol novolak type, epoxy equivalent 220, specific gravity 1.22) NBR 4 parts by weight (nitrile content 41%, specific gravity 1.00) Imidazole compound 7 parts by weight Peroxide 0.2 parts by weight Clay 60 parts by weight R / E 0.05 above An MEK solution having a solid content of 70% was prepared and coated on a release film (PET) so as to have a dry film thickness of 100 μ to prepare an uncured film (insulating layer film).

These additive layer film and insulating layer film were bonded together at 80 ° C. using a roll laminator to obtain a multilayer film.

This multi-layer film is line and space (L / S) 0.5
When vacuum lamination (120 ° C, 40 Torr) was applied to a circuit board having a mm / 0.5 mm comb-tooth circuit (1 oz electrolytic copper) so that the insulating layer was in contact with the circuit board, and the flowability between the circuits was confirmed. , A good thing was obtained with a void dress.

Further, this multilayer film-lamination substrate was cured by heating in an electric oven at 150 ° C. × 1 hour, and subjected to a roughening treatment with a chromic acid mixed solution according to a conventional method of an additive method, and the surface roughened state was measured by a scanning electron microscope. As a result, it was confirmed that the surface was appropriately roughened. That is, the surface roughness was good.

Next, after performing a catalyst addition and activation treatment, copper was deposited to a predetermined thickness by electroless plating, an annealing treatment was performed, and a characteristic evaluation was performed.The peeling strength of the plated copper was 1.5 kg / cm, The solder heat resistance at 260 ° C was 180 seconds or more, which was a good result. However, the insulating layer flowed too much, the thickness retention and uniformity were poor, and the flowability of the insulating layer was poor.

Comparative Example 5 Additive layer NBIR 100 parts by weight (nitrile content 35%, specific gravity 0.90) Epoxy resin 100 parts by weight (cresol novolac type, epoxy equivalent 220, specific gravity 1.22) Imidazole compound 10 parts by weight Peroxide 2 parts by weight Calcium carbonate 200 parts by weight R / E 1.22 Make a 33% solids MEK solution (methyl ethyl ketone solution) with the above composition and dry film thickness to release film (PET) 35
to obtain an uncured film (additive layer film).

Insulation layer Epoxy resin 50 parts by weight (bisphenol / epichlorohydrin type, epoxy equivalent 500, specific gravity 1.19) Epoxy resin 50 parts by weight (cresol novolak type, epoxy equivalent 220, specific gravity 1.22) NBR 37 parts by weight (nitrile content 41%, specific gravity 1.00) Imidazole compound 15 parts by weight Peroxide 1 part by weight Clay 40 parts by weight R / E 0.45 Make a 47% solids MEK solution with the above composition, coat it on a release film (PET) to a dry film thickness of 100μ, and uncured A film was produced (insulating layer film).

These additive layer film and insulating layer film were bonded together at 80 ° C. using a roll laminator to obtain a multilayer film.

This multi-layer film is line and space (L / S) 0.5
When vacuum lamination (120 ° C, 40 Torr) was applied to a circuit board having a mm / 0.5 mm comb-tooth circuit (1 oz electrolytic copper) so that the insulating layer was in contact with the circuit board, and the flowability between the circuits was confirmed. In addition, the flowability of the insulating layer was poor, and the flow between the circuits was insufficient.

〔The invention's effect〕

As described above, according to the present invention, it is possible to obtain a multilayer film having a uniform thickness and a stable quality without warping without generation of voids or pinholes. Further, in manufacturing a wiring board, the multilayer film may be laminated on a printed circuit board as shown in FIG. 3 (A), so that the manufacturing process of the wiring board can be simplified as compared with the related art. A highly reliable wiring board can be obtained.

[Brief description of the drawings]

FIGS. 1 and 2 are cross-sectional explanatory views of an example of the multilayer film for an additive method wiring board of the present invention, and FIGS.
(C) is an explanatory view showing a conventional process of manufacturing a wiring board by the additive method; DESCRIPTION OF SYMBOLS 1 ... Insulating board, 2 ... Copper pattern, 3 ... Printed circuit board, 4 ... Insulating layer, 5 ... Additive layer, 6 ... Adhesive layer.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Takahashi 1500 Oji Ogawa, Shimodate City, Ibaraki Pref.Hitachi Chemical Industry Co., Ltd. Ninomiya factory

Claims (1)

(57) [Claims] 1. An additive layer comprising an additive layer and an insulating layer, wherein said additive layer comprises a synthetic rubber R and an epoxy resin E, and a compounding ratio R / E (volume ratio) thereof is 0.5 ≦ R / E.
≦ 2.5, and the insulating layer is made of synthetic rubber R
And epoxy resin E.
A multilayer film for an additive method wiring board, characterized in that R / E (volume ratio) is in the range of 0.1 ≦ R / E ≦ 0.4.