JP2001036203A - Flexible printed wiring board and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely stick conductor foil, such as the copper foil, etc., and a film-like insulating layer to each other in one body by thermal fusion in a flexible printed wiring board using a thermoplastic insulating layer so that the foil may have a soldering heat resistance. SOLUTION: A flexible printed wiring board is provided with a film-like insulator which contains a styrene-based resin composition having a syndiotactic structure and a thermoplastic resin which is compatible with the resin composition as main ingredients. The content of the styrenebased resin composition is adjusted to >=35 wt.% and the thermoplastic resin has a peak crystal melting point of >=260 deg.C when the melting point is measured after the temperature of the resin is raised for differential scanning colorimetry and meets an inequality, [(ΔHm-ΔHc)/ΔHm]<=0.6, expressing the relation between the quantity of crystal melting heat ΔHm and the quantity of crystallizing heat ΔHc which occurs due to the crystallization during a temperature rise. In addition, conductor foil is melt-stuck to one or both surfaces of the film-like insulator and a conductive circuit is formed of the conductor foil.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a flexible printed wiring board, and more particularly to a flexible printed wiring board having an insulating layer formed of a thermoplastic resin and a method of manufacturing the same.

[0002]

2. Description of the Related Art In order to meet the recent demand for smaller and lighter electronic devices, compared to a rigid substrate using a prepreg in which glass cloth is impregnated with an epoxy resin, it is lighter, occupies less space, and has free three-dimensional wiring and wiring. Flexible printed circuit boards (hereinafter, abbreviated as FPC boards) that can simplify the above have been widely used.

As an insulating material of the FPC board, a polyester resin or a polyimide resin is generally used. However, the polyester resin has poor solder heat resistance (about 260 ° C.), and the use of the FPC board using the same is narrow. Is limited to FPC boards using a polyimide resin are roughly classified into a three-layer type in which a polyimide film is bonded to a copper foil using an adhesive such as an epoxy resin, and a two-layer type in which no adhesive is used. Problems common to the three-layer type and the two-layer type include poor chemical resistance due to the material properties of polyimide (weak against strong bases), and high water absorption and difficulty in dimensional stability. . Also, as a problem of the three-layer type, since various properties such as heat resistance, chemical resistance and electric properties are influenced by the properties of the adhesive, the excellent properties inherent in the polyimide resin cannot be sufficiently utilized. There is.

[0004] One of the two-layer type manufacturing methods is disclosed in Japanese Patent Publication No. 2724026 by directly casting and coating a polyimide solution or a polyamic acid solution which is a precursor of polyimide on a metal foil. It is described that the entire insulating layer of the substrate is formed of polyimide. In addition, the above-mentioned publication describes a method of forming a polyimide layer having adhesiveness on one or both sides of a polyimide film, laminating a metal foil on the polyimide layer, and manufacturing an FPC board in which the insulating layer is entirely polyimide by heating and pressing. I have.
As another method of manufacturing the two-layer type, a method of pressing a thermoplastic polyimide film with a copper foil at a temperature of about 300 ° C. is known.

[0005] Problems of the two-layer type FPC board include a quality problem that a dent is easily formed in a copper foil in an etching process, and a problem of high cost due to a trouble of manufacturing a copper clad board. Compared to the type, the characteristics as a substrate are good, but they are not very popular at present.

[0006]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned problems of the conventional polyimide substrate material and to provide a flexible printed wiring board (FPC) using a thermoplastic insulating layer having solder heat resistance. (2) An object of the present invention is to provide an FPC board which is securely bonded and integrated to a conductor foil such as a copper foil by heat fusion at a relatively low temperature.

[0007]

In order to solve the above-mentioned problems, the present invention provides a styrene resin composition having a syndiotactic structure and a thermoplastic resin compatible with the styrene resin composition. The relationship between the heat of crystal fusion ΔHm measured when the temperature is raised by differential scanning calorimetry and the heat of crystallization ΔHc generated by crystallization during the temperature rise is as follows: formula
(I) A film-shaped insulator is provided so as to satisfy the relationship shown in (I), a conductor foil is laminated on one or both sides of the film-shaped insulator, and the conductor foil is heat-fused to form a conductive circuit on the conductor foil. Thus, a flexible printed circuit board was formed.

Formula (I): [(ΔHm−ΔHc) / ΔHm] ≦ 0.6 In the invention according to the production method of the present application, in order to solve the above-mentioned problem, a styrene-based resin having a syndiotactic structure A composition and a film-shaped insulator mainly containing a thermoplastic resin compatible with the styrene-based resin composition and having a content of the styrene-based resin composition of 35% by weight or more, which is measured by differential scanning calorimetry. The crystal melting peak temperature measured when the temperature is raised is 260 ° C. or higher, and the heat of crystal fusion ΔHm and the heat of crystallization ΔH generated by crystallization during the temperature rise.
Forming and processing a film-like insulator so that the relationship with c satisfies the relationship represented by the following formula (I), and laminating a conductor foil on one or both sides of the film-like insulator, the thermoplastic resin composition After heat-fusing so as to satisfy the relationship represented by the following formula (II), a method was adopted in which the conductive foil was etched to form a conductive circuit.

Formula (I): [(ΔHm−ΔHc) / ΔHm] ≦ 0.6 Formula (II): [(ΔHm−ΔHc) / ΔHm] ≧ 0.7 The flexible of the present invention configured as described above The printed wiring board has an insulating layer in which a predetermined amount of a styrene-based resin composition having a syndiotactic structure and a thermoplastic resin compatible with the styrene-based resin composition are blended. Due to its excellent properties, it has heat sealability and solder heat resistance, and has the flexibility, mechanical strength and electrical insulation normally required for FPC.

In a film-like insulator made of such a thermoplastic resin composition, the relationship between the heat of crystal fusion ΔHm and the heat of crystallization ΔHc generated by crystallization during heating is expressed by the above formula.
Since the relationship represented by (I) is satisfied, the progress of crystallization by heating is adjusted to an appropriate range.For example, it is possible to exhibit thermal fusion at a relatively low temperature of 190 ° C. or less. it can.

In the method of manufacturing an FPC according to the present invention, a conductor foil is laminated on one or both sides of a film-like insulator made of the above-mentioned thermoplastic resin composition, and the thermoplastic resin composition has the formula (II)
Is heat-fused under heating and pressurizing conditions so as to satisfy the relationship shown by.

In the thermoplastic resin composition after heat fusion, since the crystallinity of the styrenic resin composition having a syndiotactic structure is appropriately advanced, an insulating material having solder heat resistance to withstand 260 ° C. is ensured. It becomes a layer and has a high adhesive strength with the conductor foil. Thereafter, the conductive circuit formed by etching the conductor foil is firmly adhered to the insulating layer and hardly peeled off. When a conductor foil whose surface is roughened is used as the conductor foil, the adhesive strength is further increased.

In addition, since the film-like insulator and the conductive foil are bonded by heat without interposing an adhesive such as epoxy resin between the layers, the FPC has various properties such as heat resistance, chemical resistance, and electrical properties. Without being governed by the properties of the adhesive
The excellent properties of the insulating layer are fully utilized. Also, FP
Since there is no step of applying an adhesive or the like in the manufacturing process of C, an FPC manufacturing method with high manufacturing efficiency can be obtained.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments relating to a structure of a flexible printed wiring board of the present invention and a method of manufacturing the same will be described.
This will be described below with reference to the accompanying drawings. As shown in FIG. 1E, in the flexible printed wiring board according to the present invention, the copper foil 2 satisfies the physical properties described below by superposing the copper foil 2 on both surfaces of a film-like insulator 1 made of a predetermined thermoplastic resin composition. In this manner, the copper foil 2 is etched to form a conductive circuit.

In order to manufacture such a flexible printed circuit board, first, as shown in FIG. 1A, a styrene resin having a syndiotactic structure and a thermoplastic resin compatible with the styrene resin are used. To prepare a thermoplastic film-like insulator 1 having the predetermined crystallinity represented by the above formula (I). Then, as shown in FIG. 1B, the copper foils 2 are stacked on both sides and hot-pressed using a vacuum hot press machine to produce a double-sided copper-clad laminate 5. A hole 3 for forming a through hole is formed at a key point of the double-sided copper clad board by a laser or a drill (FIG. 1 (c)), and this is plated with copper (FIG. 1 (d)). Means of connection. Next, a conductive circuit is formed by a subtractive method (FIG. 1E).

When a conductive paste is used for interlayer connection, as shown in FIG.
2. Holes 3 for forming holes are formed at important points by laser or drill and filled with conductive paste 4 (FIG. 2).
(C)) After drying, the copper foils 2 are stacked on both sides and hot-pressed using a vacuum hot press machine to produce a double-sided copper-clad laminate 5 (FIG. 2 (d)). Next, a conductive circuit is formed by a subtractive method (FIG. 2E).

In the present invention, the styrene-based resin having a syndiotactic structure, which is the first component constituting the film-like insulator, has a main stereochemical structure of a syndiotactic structure and is formed from carbon-carbon bonds. It is a heat-resistant crystalline polymer having a structure in which phenyl groups, which are side chains, are alternately located in opposite directions to the main chain.

The thermoplastic resin compatible with the styrene resin composition as the second component constituting the film-like insulator can be arbitrarily selected from conventionally known thermoplastic resins. Modified polyphenylene ether resin can be suitably used.

The film-like insulator used in the present invention is mainly composed of a composition obtained by blending the above two kinds of heat-resistant resins at a predetermined ratio. The compounding ratio of the two types of heat-resistant resin is such that the styrene-based resin is compounded in a large amount exceeding 70% by weight, or the compounding ratio of the modified polyphenylene ether resin is a small amount of less than 30% by weight.
The crystallization rate of the composition increases, and the heat-fusibility with the conductive foil decreases. If the content of the styrene resin is less than 35% by weight or the content of the modified polyphenylene ether resin exceeds 65% by weight, the crystallinity of the composition becomes low. It is not preferable because the heat resistance decreases.

The rubber-like elastic material as the third component constituting the film-like insulator includes styrene-butadiene block copolymer (SBR), hydrogenated styrene-butadiene block copolymer (SEB), and styrene-butadiene. -Styrene block copolymer (SBS), hydrogenated styrene-butadiene-styrene block copolymer (SEB)
S) and the like, but are not limited thereto. In the present invention, in the rubber-like elastic body, SE
BS is preferably used. The rubber-like elastic body is preferably contained in the range of 10 to 20% by weight of the film-like insulator, and if less than 10% by weight, the effect of improving strength is small, and
If the amount exceeds the weight percentage, heat resistance tends to decrease.

The thermal characteristics of the film-like insulator, which is an important control factor in the present invention, are as follows: The relationship between the heat of crystal fusion ΔHm and the heat of crystallization ΔHc generated by crystallization during heating is expressed by the following formula (I). That is, the relationship represented by is satisfied. Formula (I): [(ΔHm−ΔHc) / ΔHm] ≦ 0.6 The thermal characteristics are based on JIS K 7121 and JIS K71.
DSC when the temperature is raised by differential scanning calorimetry according to No. 22
Measured values of two transition heats appearing in the curve, heat of crystal fusion ΔH
It is calculated from the value of m (J / g) and the value of the heat of crystallization ΔHc (J / g) by the above equation.

The value of the above formula (I) also depends on the type and molecular weight of the raw material polymer and the compounding ratio of the composition, but greatly affects the conditions for forming and processing the film-shaped insulator. That is, when the film is formed into a film, the raw material polymer is melted and then cooled immediately, whereby the value of the above formula can be reduced. Further, these numerical values can be controlled by adjusting the heat history in each step. The heat history here refers to the temperature of the film-shaped insulator and the time during which the temperature has been reached, and the higher the temperature, the larger the numerical value tends to be. The relationship represented by the above formula (I) indicates that in the process of manufacturing the FPC, the FP obtained by heat-sealing the conductor foil to at least one surface of the film-shaped insulator is used.
It is based on the measurement before the heat fusion process about the raw material plate for C.

If the value represented by the above formula (I) exceeds 0.6 before heat fusion, it becomes difficult to perform heat fusion at a low temperature of 190 ° C. or less because the crystallinity is already high. In this case, it is necessary to perform heat fusion with the conductor foil at a high temperature, which is not preferable in terms of manufacturing efficiency. Then, the thermal characteristics of the film-like insulator after heat-sealing with the conductor foil must satisfy the relationship of the following formula (II). Formula (II): [(ΔHm−ΔHc) / ΔHm] ≧ 0.7 Because when the value of the above formula (II) is lower than 0.7, the crystallization of the insulating layer is insufficient, and This is because heat resistance (usually 260 ° C.) cannot be maintained.

The film-like insulator used in the present invention usually has a thickness of 25 to 300 μm, and its production method is, for example, a known film-forming method such as an extrusion casting method using a T-die or a calendering method. What is necessary is just to employ | adopt and it is not necessary to employ | adopt a manufacturing method especially limited. In addition, it is preferable to employ the extrusion casting method using a T-die from the viewpoint of film forming property and stable productivity. The molding temperature of the extrusion casting method is appropriately adjusted depending on the flow characteristics and film forming characteristics of the composition, but is generally from the melting point of the composition to 300 ° C. or less.

The resin composition constituting the film-like insulator used in the present invention contains a resin composition which does not impair the effects of the present invention.
Other resins and other additives may be blended, and specific examples thereof include a heat stabilizer, an ultraviolet absorber, a light stabilizer, a coloring agent, a lubricant, a flame retardant, and an inorganic filler. Further, the surface of the film-shaped insulator may be subjected to embossing or corona treatment for improving the handling properties and the like.

Examples of the conductive foil used in the present invention include metal foils having a thickness of about 8 to 70 μm, such as copper, gold, silver, aluminum, nickel, and tin. Among them, the metal foil to be applied is particularly preferably a copper foil whose surface has been subjected to a chemical conversion treatment such as a black oxidation treatment. The conductor foil is
In order to enhance the bonding effect, it is preferable to use a material whose contact surface (overlapping surface) with the film-shaped insulator is chemically or mechanically roughened in advance. Specific examples of the conductor foil subjected to the surface roughening treatment include a roughened copper foil that has been electrochemically treated when producing an electrolytic copper foil.

When the conductor foil is superimposed on one or both surfaces of the film-shaped insulator and heat-sealed under heating and pressing conditions, for example, a hot pressing method or a heat laminating roll method, a method combining these, or other well-known methods Can be employed.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the production methods of Production Examples 1 to 4 of the film insulator satisfying the conditions of the film insulation of the present invention, the production methods of Reference Examples 1 and 2, and the physical properties thereof will be described below. .

[Production Example 1 of Film Insulator] Styrene resin having a syndiotactic structure (Zarek, manufactured by Idemitsu Kosan Co., Ltd.)
Abbreviated as SPS. ) 60% by weight and 40% by weight of a modified polyphenylene ether resin (manufactured by Mitsubishi Engineering-Plastics Corporation: Iupiace) (abbreviated as modified PPE in the following text or in Tables 1 and 2) were dry-blended. This mixed composition was extruded to a thickness of 25 μm.
Was manufactured.

[Production Example 2 of Film Insulator] Production Example 1
In, the compounding ratio of the mixed composition is SPS 40% by weight,
A film-like insulator was produced in the same manner except that the modified PPE was 60% by weight.

[Production Example 3 of Film Insulator] Production Example 1
In, the mixture ratio of the mixed composition is SPS 30% by weight,
A film-like insulator was produced in the same manner except that the modified PPE was 70% by weight.

[Production Example 4 of Film Insulator] Production Example 1
In, the compounding ratio of the mixed composition is SPS 40% by weight,
A film-like insulator was produced in the same manner except that the modified PPE was 45% by weight and the SEBS was 15% by weight.

[Reference Examples 1 and 2 of Film Insulator] Except that in Preparation Example 1, the mixture ratio of the mixed composition was 100% by weight of SPS (Reference Example 1) or 100% by weight of modified PPE (Reference Example 2). Manufactured the respective film-shaped insulators in the same manner.

In order to examine the physical properties of the film-shaped insulator obtained in the above Production Examples and Reference Examples, the following (1) and (2)
The following items were measured or calculated values were calculated from the measured values.
These results are summarized in Table 1.

(1) Glass transition temperature (° C.), crystallization temperature (° C.), crystal melting peak temperature (° C.) Using a 10 mg sample according to JIS K7121 and heating using Perkin Elmer: DSC-7 Speed 1
Each of the above temperatures when the temperature was raised at 0 ° C./min was determined from a thermogram.

(2) (ΔHm−ΔHc) / ΔHm According to JIS K7122, 10 mg of a sample was used, and the heating rate was set to 1 using DSC-7 manufactured by PerkinElmer.
The heat of crystal fusion ΔHm (J / g) and the heat of crystallization ΔHc (J / g) were determined from the thermogram when the temperature was raised at 0 ° C./min, and the value of the above equation was calculated.

[0037]

[Table 1]

Example 1 Thickness 25 obtained in Production Example 1
Electrochemically roughened electrolytic copper foil having a thickness of 12 μm is superposed on both sides of a film-shaped insulator of μm, and 760 mmHg in a vacuum atmosphere, a press temperature of 180 ° C., a press pressure of 30 kg / cm ² , and a press time Heat bonding was performed for 20 minutes to produce a double-sided copper-clad laminate. (2) (ΔHm−ΔH)
The measurement test of c) / ΔHm was performed in the same manner as described above, and the formula values are shown in Table 2. Further, for the obtained double-sided copper-clad laminate, the adhesive strength by the method (3) described later,
The bending strength was examined by the method (4), and the results are shown in Table 2. Next, a circuit pattern was formed on the obtained double-sided copper-clad laminate by a subtractive method, and an FPC in which a conductive circuit was formed by etching was manufactured. The solder heat resistance of the obtained FPC was examined by the following test method (5).
The results are shown in Table 2. Also, the presence or absence of delamination of the obtained FPC was examined by the following method (6), and the results are also shown in Table 2.

(3) Adhesive Strength The peel strength of the copper foil of the FPC was measured in accordance with the normal peel strength of JIS C6481, and the average value was shown in kgf / cm.

(4) Folding strength According to the bending strength test of JIS C6481, FPC
Was performed, and the number of times until the FPC was broken was shown.

(5) Solder heat resistance According to the normal solder heat resistance of JIS C6481, 2
After the test piece was floated on a solder bath at 60 ° C. for 10 seconds while the copper foil side of the FPC plate was in contact with the solder bath, the test piece was taken out of the bath, allowed to cool to room temperature, and visually inspected for swelling or peeling. , The quality was evaluated.

(6) The FPC is embedded in epoxy resin, a sample for cross-section observation is prepared with a precision cutting machine, and the cut surface is observed with a scanning electron microscope (SEM). The presence or absence of delamination with the conductive circuit was evaluated.

[0043]

[Table 2]

Example 2 In Example 1, the production temperature was changed to 190 ° C. and the press time was changed to 20 minutes in the production of the double-sided copper-clad laminate using Production Example 2 as the film-like insulator. Except for the above, an FPC was manufactured in the same manner as in Example 1, and the evaluations of Tests (3) to (6) are also shown in Table 2.

Comparative Example 1 An FPC was produced in the same manner as in Example 2 except that the press temperature and the press time for producing a double-sided copper-clad laminate were changed to 180 ° C. and the press time was changed to 10 minutes. Table 2 also shows the evaluations of Tests (3) to (6).

[Comparative Example 2] In Example 1, the production temperature was changed to 220 ° C. and the press time was changed to 20 minutes in the production of the double-sided copper-clad laminate using Production Example 3 as the film-like insulator. Except for the above, an FPC was manufactured in the same manner as in Example 1, and the evaluations of Tests (3) to (6) are also shown in Table 2.

Example 3 In Example 1, the production temperature was changed to 190 ° C. and the press time was changed to 20 minutes in the production of the double-sided copper-clad laminate using Production Example 4 as the film-like insulator. Except for the above, an FPC was manufactured in the same manner as in Example 1, and the evaluations of Tests (3) to (6) are also shown in Table 2.

As is clear from the results in Table 2, the adhesive strength of the double-sided copper-clad laminate of Example 1 was 1.3 kgf / 10
cm, and the results of the solder heat resistance test showed that no swelling or peeling was observed on the substrate, and no delamination was observed by SEM observation of the FPC after forming the conductive circuit.

The adhesive strength of the double-sided copper-clad laminate of Example 2 was a good value of 1.2 kgf / 10 cm, the result of the solder heat resistance test was good, and the FPC after the conductive circuit was formed by etching was used. No delamination was observed at all by SEM observation. On the other hand, Comparative Example 1
In the SEM observation with respect to FPC, the adhesion between the layers was good because of the adhesion, but the solder heat resistance was such that swelling and peeling were observed on the substrate, which was a result of failure.

In the FPC of Comparative Example 2, the adhesive strength of the double-sided copper-clad laminate was a poor value of 0.1 kgf / 10 cm, and the copper foil of the circuit portion was peeled off after the formation of the conductive circuit by etching. Also, from the results of Example 3, it can be seen that the addition of 15% by weight of the rubber-like elastic body significantly improved the bending strength of the copper clad board.

[0051]

As described above, the flexible printed wiring board of the present invention comprises a styrene resin composition having a syndiotactic structure and a thermoplastic resin compatible with the styrene resin composition. By providing a film-like insulator made of a thermoplastic resin composition having a predetermined thermal property satisfying a predetermined relationship as an insulating layer, and providing a conductive foil having a conductive circuit formed thereon by superimposing the film-shaped insulator on the insulating layer. It has the required flexibility, mechanical strength and sufficient solder heat resistance, and a conductor foil such as a copper foil and an insulating layer are laminated and integrated by relatively low-temperature heat fusion. It has the advantage of having adhesive strength.

Further, the invention relating to the method for manufacturing a flexible printed wiring board is a method for efficiently manufacturing a flexible printed wiring board having the above-mentioned advantages because steps such as application of an adhesive are omitted.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a manufacturing process of a flexible printed wiring board.

FIG. 2 is a schematic view showing a manufacturing process of a flexible printed wiring board.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Film-shaped insulator 2 Copper foil 3 Hole 4 Copper plating 5 Double-sided copper-clad laminate

Claims (6)

[Claims] 1. A styrenic resin composition having a syndiotactic structure, and a thermoplastic resin compatible with the styrenic resin composition as a main component, and the content of the styrenic resin composition is 35% by weight or more. Having a crystal melting peak temperature of 260 ° C. or higher when measured by differential scanning calorimetry and a crystal melting heat ΔH.
A film-like insulator made of a thermoplastic resin composition, wherein the relationship between m and the heat of crystallization ΔHc generated by crystallization during the temperature rise satisfies the relationship represented by the following formula (I), is provided. A flexible printed wiring board obtained by heat-sealing a conductor foil on one or both surfaces of the above and forming a conductive circuit on the conductor foil. Formula (I): [(ΔHm−ΔHc) / ΔHm] ≦ 0.6 2. The flexible printed circuit board according to claim 1, wherein the conductive foil is a surface-roughened conductive foil. 3. A styrene resin composition having a syndiotactic structure and a thermoplastic resin compatible with the styrene resin composition as a main component, and the content of the styrene resin composition is 35% by weight or more. Having a crystal melting peak temperature of 260 ° C. or higher when measured by differential scanning calorimetry and a crystal melting heat ΔH.
m is formed into a film-like insulator so that the relationship between m and the heat of crystallization ΔHc generated by crystallization during temperature rise satisfies the relationship represented by the following formula (I). After the conductor foil is laminated on both sides and the thermoplastic resin composition is heat-sealed so as to satisfy a relationship represented by the following formula (II), the conductive foil is etched to form a conductive circuit. A method for manufacturing a flexible printed wiring board. Formula (I): [(ΔHm−ΔHc) / ΔHm] ≦ 0.6 Formula (II): [(ΔHm−ΔHc) / ΔHm] ≧ 0.7 4. The method for manufacturing a flexible printed circuit board according to claim 3, wherein the conductor foil to be laminated on one or both surfaces of the film-shaped insulator is a conductor foil having a roughened surface. 5. The flexible printed circuit board according to claim 1, wherein the thermoplastic resin compatible with the styrene resin composition is a modified polyphenylene ether resin. 6. The film-like insulator is made of one rubber-like elastic body.
6. The flexible printed circuit board according to claim 1, wherein the flexible printed circuit board is contained in a range of 0 to 20% by weight.