Paper making and fiber liberation – Processes and products – Multi-layer waterlaid webs or sheets
Reexamination Certificate
1999-11-17
2003-05-06
Yao, Sam Chuan (Department: 1733)
Paper making and fiber liberation
Processes and products
Multi-layer waterlaid webs or sheets
C162S132000, C162S135000, C162S146000, C162S206000, C162S225000, C156S062200
Reexamination Certificate
active
06558512
ABSTRACT:
DETAILED DESCRIPTION OF THE INVENTION
1. Technical Field to which the Invention Belongs
The present invention relates to a base material for a laminate produced by laminating non-woven fabric sheets mainly comprising organic fibers, and a process for producing it. The present invention further relates to a prepreg and a laminate obtained from the base material for a laminate.
2. Prior Art
Recently, a smaller size and a higher function are required of electronic equipments and, accordingly, printed boards of a high wiring density are now the mainstream, components to be mounted are being changed from those of insertion type to those of surface attachment type, and the mounting method is now mainly the surface mounting method.
In this method, the connection reliability between components such as chips to be mounted on the surface and a printed board is an important problem. Namely, the coefficients of thermal expansion of both of them must be made as close to each other as possible.
The coefficient of thermal expansion of chips of thin surface mounting type is 5×10
−6
/° C., while that of a base material obtained by impregnating a non-woven glass fabric with an epoxy resin is about three times as high as this value.
Further, the dielectric constant is also to be taken into consideration. The dielectric constant of ordinary FR-4 is about 4.7 to 5.1. Such a relatively high dielectric constant causes the reduction of the velocity of propagation of the electric pulse of an adjacent signal circuit to cause a serious signal delay. Because the delay by the signal propagation in the printed boards will be very serious in future, a base material for a laminate of a low dielectric constant will be necessitated. FR-4 is a copper-clad laminate for printed board obtained by impregnating a glass fabric base material with an epoxy resin and laminating them NEMA (National Electrical Manufacturers association) standards.
Under these circumstances, investigations are made on a laminate produced by using a non-woven fabric of para-aramid fibers having a negative coefficient of heat expansion and a low dielectric constant to be used as a laminate using a base material for a printed board.
A typical example of such a base material is disclosed in Japanese Patent Publication for Opposition Purpose (hereinafter referred to as “J. P. KOKOKU”) No. Hei 5-65640 (U.S. Pat. Nos. 4,698,267 and 4,729,921). The publication discloses a base material produced by mixing p-phenylene terephthalamide fiber flocks with m-phenylene isophthalamide fibrids, making sheets from the obtained mixture and then compressing the sheets under heating.
A printed board produced by impregnating the base materials obtained as described above, with an epoxy resin and laminating them has a coefficient of heat expansion far lower than that of a laminate produced by using an ordinary non-woven glass fabric as the base material, because the base material having a negative coefficient of heat expansion is impregnated with the epoxy resin having a positive coefficient of heat expansion, and accordingly, they compensate the coefficients thereof each other. Therefore, such a printed board is suitable for boarding very small chips having many pins such as CSP (stand for “chip size pakage”), and also for portable equipments required to be small and light in weight.
However, it is demanded to further reduce the size and weight of the electronic appliances and also to further improve the function thereof and, accordingly, the semiconductor chips to be mounted are demanded to be smaller, and the increase in number of the pins is also demanded. As a matter of course, it will be indispensable for the printed boards, on which they are to be mounted, that they are usable for a higher density wiring. Further, it will be demanded that the printed board itself is thinned and reduced in weight for the purpose of reducing the weight of the electronic appliances.
A laminate produced by impregnating the above-described base material with an epoxy resin and pressing the base material has a low dielectric constant and a coefficient of linear expansion close to that of bare chips and, accordingly, it is supposed that an excellent performance can be obtained by using it for interposers such as CSP. However, the thickness of the laminate must be further reduced in this case.
To obtain the base material for a printed board and a laminate satisfying the above-described requirements, the investigation of only the fibrous materials is no more sufficient. Namely, it is demanded that the orientation of the fibers in the non-woven fabric base material in the machine direction is only slight and that the uniformity of the basis mass (mass per 1 m
2
) and the fiber orientation is high.
To make the laminates thin, it is also necessary to make the base material thin as a matter of course. In such a case, a high tensile strength, particularly, a high tensile strength when immersed in a solvent such as acetone or MEK, is required of the base material. Further, it is important to suitably adjust the density of the base material.
Reasons why the above-described, three conditions are required of the base material will be described below.
In the high-density wiring, the copper foil patterns are extremely narrow, and the copper foil is now being made thinner for reducing the weight of the printed board. Therefore, the wires are easily broken by the warp and torsion of the printed board. In addition, because the distances between the copper foil patterns are narrowed, the precision of the patterns must be more improved.
However, when the fibers of the base material are strongly oriented in the machine direction (hereinafter referred to as “MD”) of the sheet, the effect of the aramid fibers which control the heat expansion of the printed board by its negative heat expansion is not exhibited in the cross direction of the base, and the heat expansion in a direction (cross direction; hereinafter referred to as “CD”) rectangular to the flow of the sheet becomes high. When the fiber orientation is irregular, the warp and torsion of the printed board easily occur.
When the distribution of the basis mass of the base material is wide, the distribution in the density after the heat treatment under pressure is also wide, and the epoxy resin content is not uniform and it varies depending on the sites. On the other hand, in the printed board produced from the aramid fiber base material, the thermal expansion is kept low by the combination of the material having a negative coefficient of heat expansion and the material having a positive coefficient of heat expansion. As a result, when the epoxy resin content varies, the coefficient of heat expansion of the printed board varies to increase the distribution in the dimension caused by the heat in the step of producing the printed board or, in other words, to lower the accuracy of the copper foil pattern.
In the production of the laminate, the base material must be impregnated with an epoxy resin and then dried to obtain a prepreg coated with the semi-cured resin. In this step, the base material is subjected to a high tension because it is immersed in a solvent for the epoxy resin, such as acetone or MEK, and then the excessive resin is scraped off. Thus, the base material must have a strength resistant to such a high tension while it is immersed in the solvent. Hereinafter, this strength is referred to as “solvent-resistant strength”. When the base material is thin, its strength is reduced as a matter of course, and the production of the base material durable in the prepreg-producing step is thus difficult.
In addition, when the base material has a low strength, a sufficient tension cannot be applied to it, because it is easily broken even when it has a certain thickness. In such a case, the production velocity is lowered to reduce the production efficiency.
When the density of the base material is unsuitable, the following defects are caused. When the density is excessively high, the impregnation with epoxy resin is insufficient in t
Adachi Takekazu
Hiraoka Hirokazu
Ichioka Isao
Kato Yoshihisa
Murata Mamoru
Oji Paper Co. Ltd.
Yao Sam Chuan
LandOfFree
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