Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2002-01-04
2003-09-16
Dawson, Robert (Department: 1712)
Metal working
Method of mechanical manufacture
Electrical device making
C029S830000, C523S440000, C523S466000
Reexamination Certificate
active
06618933
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a liquid thermosetting insulating resin composition which is particularly useful for permanently filling such holes as via holes and through holes in such printed circuit boards as a multilayer board and a double-sided board. This invention further relates to a method for permanently filling such holes as via holes and through holes in printed circuit boards by the use of the composition mentioned above.
2. Description of the Prior Art
As a composition for permanently filling such holes in printed circuit boards, epoxy resin compositions of the thermosetting type and the UV/heat-curable type have been heretofore used. Such epoxy resin compositions have been finding utility in a wide variety of fields covering electrical insulating materials, FRP (fiber-reinforced plastics) and other similar composite materials, coating materials, and adhesive agents because their hardened products excel in mechanical, electrical, and chemical properties, and exhibit good adhesiveness as well.
The thermosetting type epoxy resin composition uses a primary or secondary aromatic amine or acid anhydride as a curing agent and a tertiary amine or imidazole as a catalyst and its hardened product can be obtained by heating it.
When an aromatic amine is used, however, the resin composition shrinks largely during the course of thermal curing and the hardened filler eventually formed of the resin composition in a through hole gives birth to a gap between itself and the wall of the through hole or gives rise to voids (empty cavities) in the hardened filler filling the hole. When the thermosetting type epoxy resin composition employs other curing systems, it suffers from the curing reaction thereof and terminates instantaneously on account of a chain reaction, such that it is difficult to control. The resultant hardened filler has a high hardness such that it may not be easily polished or cut to a flat surface.
In the case of the thermosetting type epoxy resin composition containing a solvent, since the solvent vaporizes while the resin composition is being thermally cured, the hardened filler formed of the resin composition in the through hole has the problem of caving in or cissing and voids. The similar problem will arise in the UV/heat-curable type epoxy resin composition so long as it contains a solvent.
On the contrary, an attempt to decrease the content of the diluting solvent in the composition for permanently filling the holes for the purpose of suppressing the occurrence of voids or the like due to the vaporization of the solvent in the composition or further to incorporate a filler into the composition in a higher amount for the purpose of suppressing the voluminal expansion of the hardened product is made. According to such a method, although the thermal expansion of the resin can be suppressed owing to the incorporation of the filler into the composition in a higher amount, it has another problem of decrease in its property of filling in the through holes etc. due to the change in viscosity or thixotropy of the composition.
On the other hand, the UV/heat-curable type epoxy resin composition is hardened through the precure of the composition by the radical polymerization reaction of the double bond of a relevant photosensitive compound and subsequently the thermal curing of the epoxy resin during the course of heating to obtain a hardened product thereof. In the precuring step thereof by being exposed to an ultraviolet light, however, since the radical polymerization of the double bond of the photosensitive compound included in the composition such as, for example, acrylate proceeds more quickly in the surface part than in the inner part of the composition, the degree with which the composition is photocured varies in the surface part and the inner part thereof and the shrinkage which the composition sustains during the course of thermal post-curing tends to be large. Further, the resultant hardened product possesses high hygroscopicity and suffers from the drawback of failing to acquire electrical insulation properties and PCT (pressure cooker treatment) resistance sufficiently.
As respects the curing system which combines an epoxy resin with a phenolic resin, though having no bearing on a composition intended for permanently filling holes in printed circuit boards, published Japanese Patent Application, KOKAI (Early Publication) No. 8-157,561 discloses a semiconductor sealing epoxy resin composition characterized by containing a solid epoxy resin of a specific structure including at least one hydrocarbon group in a benzene ring in combination with an imidazole compound of a specific structure. This published specification discloses a composition example using a solid epoxy resin and a solid phenolic resin. The sealing resin which is obtained from this composition exhibits satisfactory characteristics and physical properties after it has been hardened, which will satisfy the properties required for the composition to be used for filling holes. The composition nevertheless is deficient in the ability to be coated as by screen printing or roll coating (the ability to fill the holes) because the epoxy resin and the phenolic resin which are raw materials are both in a powdery state. Further, since this composition is formulated to be in a liquid state by using a diluting agent such as a solvent, the possibilities of a voluminal decrease thereof due to the vaporization of the diluting agent and an adverse effect of the diluting agent on the natural environment are considered. This composition, therefore, has not yet found actual adoption as a filling material for through holes etc. in printed circuit boards due to its poor workability and printability.
SUMMARY OF THE INVENTION
The primary object of the present invention, therefore, is to provide a liquid thermosetting insulating resin composition which can establish incorporation of a filler therein in a high amount without impairing its ability to fill the holes (the ability to plug the holes) such as through holes and which shrinks only sparingly during the course of thermal curing and ultimately forms a hardened product excelling in thermal stability, resistance to heat and resistance to humidity, producing virtually no voluminal expansion even under the conditions of high temperature and high humidity, and excelling in PCT resistance.
A further object of the present invention is to provide a liquid thermosetting insulating resin composition which allows easy filling of holes in printed circuit boards by the conventional technique such as, for example, the screen printing process or roll coating process, enables the reaction thereof proceeding during the course of thermal curing to be arbitrarily controlled, can be precured and forms after the precure a hardened filler capable of being easily deprived of an unnecessary part by physical polishing, and proves particularly suitable as a composition for permanently filling holes in printed circuit boards.
Another object of the present invention is to provide a method for permanently filling holes in a printed circuit board, which method enables the holes in the printed circuit board to be filled with high operational and productional efficiency and produces in the holes hardened fillers excellent in both characteristics and physical properties.
To accomplish the objects mentioned above, the present invention provides a liquid thermosetting insulating resin composition for filling a via hole and/or a through hole in a printed circuit board. One embodiment of the composition is characterized by comprising (A) an epoxy resin which is in a liquid state at room temperature, (B) a phenolic resin which is in a liquid state at room temperature, (C) a curing catalyst, and (D) a filler, wherein the filler (D) contains a spherical filler and a ground filler. Another embodiment of the composition is characterized by comprising (A) an epoxy resin which is in a liquid state at room temperature, preferably at least two kinds of epoxy r
Yamamoto, née Takahashi Rieko
Yoda Kyoichi
Aylward D.
Dawson Robert
Rader & Fishman & Grauer, PLLC
Taiyo Ink Manufacturing Co., Ltd.
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