Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
1998-09-23
2001-01-16
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S901000, C174S258000
Reexamination Certificate
active
06174589
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a printed circuit board excellent in suppressing ion migration causing a dielectric breakdown, and a method for producing the same.
BACKGROUND OF THE INVENTION
As the miniaturization, the light weight and the high performance of electronic equipment have been successfully pursued in recent years, the miniaturization, the light weight, the high speed signal processing and the implementation at a high density of a printed circuit board are in increasing demand. The printed circuit board technique responds to such a demand by accelerating the formation of a larger number of layers, smaller via holes and finer circuitry. In particular, with the development of the finer circuitry, the gap between lines and via holes is reduced, so that dielectric breakdown can be caused readily due to ion migration. The ion migration is an electrolytic function, referring to a phenomenon in which an electrolyte between metals to which a voltage is applied, the metal at the anode is eluted, moves to the cathode and is precipitated. Since this phenomenon is based on an electrochemical function, it is required that ions flow between the metals. The ion migration in the printed circuit board is generally effected in the following manner: a path of water is generated due to the absorption of moisture at the interface between an impregnated resin and a base material, in foreign fibers attached to a circuit pattern, in cracks in an insulating layer, or the like, and an ion current flows in the path.
A number of methods for preventing the ion migration in the printed circuit board were conventionally proposed, such as a method of making a metal into an alloy in order to suppress the ionization of the metal and a method of adding a reducing agent to the board. Another method of adding a chelating agent to the board in order to capture eluted metal ions was proposed.
However, the aforementioned methods are not necessarily satisfactory as a measure to counter the ion migration in view of the cost and the performance. For example, the cost of producing an alloy is high because expensive palladium is used. Furthermore, for the reducing agent, an aldehyde, hydroquinon, hydrazine or the like is often used. Disadvantageously, they are chemically unstable and are readily degraded by heat or light. Similarly, the chelating agent is likely to be degraded or deactivated by heat.
Furthermore, in addition to the formation of a finer circuitry, the formation of the printed circuit board of a larger number of layers is accelerated. For this reason, more and more boards are formed of an inexpensive and light-weight resin or an inexpensive nonwoven fabric. The resin board generally has a relatively high hygroscopicity, and thus the ion migration is likely to be caused. Furthermore, when a glass or a nonwoven fabric such as aramid is used as the base material, an impregnated resin is unlikely to exist between the base material and a metal foil. This means that a base material fabric not coated with a resin, or a foreign fabric that easily absorbs moisture such as cellulose or the like, is directly in contact with the metal foil. Thus, a path of water is generated due to the absorption of moisture by the board, and then the ion migration is caused. Thus, a printed circuit board having improved resistance to ion migration is significantly desired.
SUMMARY OF THE INVENTION
A printed circuit board of the present invention includes insulating layers formed by impregnating a base material (substrate) with a resin and a metal foil pattern formed on a desired layer of the insulating layers. Ions for forming a hardly soluble metal salt by combining with metal ions free from a portion of the board are present in the insulating layer or on a surface of the metal foil pattern.
In one embodiment, the ions to be combined with the free metal ions are sulfide ions.
In another embodiment, the sulfide ions are supplied from at least one selected from the group consisting of ammonium sulfide, potassium sulfide and sodium sulfide.
In still another embodiment, a solubility product of the hardly soluble metal salt in water (at 25° C.) is in the range from 10
−40
to 10
−10
.
In yet another embodiment, an addition amount of a sulfide compound for supplying ions to be combined with the free metal ions is in the range from 0.001 wt % to 0.1 wt % on the basis of the total weight of the board.
In another embodiment, the metal foil pattern is formed of at least one metal selected from the group consisting of copper, silver and nickel.
In still another embodiment, the base material is at least one selected from the group consisting of a glass woven fabric, a glass nonwoven fabric, an aramid woven fabric, an aramid nonwoven fabric and an aramid film.
In yet another embodiment, the resin to be impregnated is at least one resin selected from the group consisting of an epoxy resin, a cyanate ester resin, a bismaleimide triazine resin, a polyimide resin, and a polyphenylene oxide (PPO) resin.
According to another aspect of the printed circuit board of the present invention including insulating layers formed by impregnating a base material with a resin and a metal foil pattern formed on a desired layer of the insulating layers, a sulfur-containing compound for reacting with metal ions free from a portion of the board is present in the insulating layer or on a surface of the metal foil pattern.
In one embodiment, the sulfur-containing compound to react with the free metal ions is thiourea.
In another embodiment, an addition amount of the sulfur-containing compound for reacting with the free metal ions is in the range from 10 ppm to 10000 ppm.
According to another aspect of the present invention, a method for producing a printed circuit board includes the steps of, impregnating a base material with a resin varnish containing ions for forming a hardly soluble metal salt by combining with metal ions free from a portion of a board and drying so as to form a prepreg; laminating metal foils on the prepreg to be bonded under thermocompression; and forming a circuit pattern on the metal foil.
In one embodiment, the ions to be combined with the free metal ions are sulfide ions.
In another embodiment, the sulfide ions are supplied from at least one selected from the group consisting of ammonium sulfide, potassium sulfide and sodium sulfide.
In still another embodiment, a solubility product of the hardly soluble metal salt in water (at 25° C.) is in the range from 10
−40
to 10
−10
.
According to another aspect of the present invention, a method for producing a printed circuit board includes the steps of: immersing a base material into a solution containing ions for forming a hardly soluble metal salt by combining with metal ions free from a portion of a board and drying;
impregnating the base material with a resin varnish and drying so as to form a prepreg; laminating metal foils on the prepreg to be bonded under thermocompression; and forming a circuit pattern on the metal foil.
In one embodiment, the ions to be combined with the free metal ions are sulfide ions.
In another embodiment, the sulfide ions are supplied from at least one selected from the group consisting of ammonium sulfide, potassium sulfide and sodium sulfide.
In still another embodiment, a solubility product of the hardly soluble metal salt in water (at 25° C.) is in the range from 10
−40
to 10
−10
.
In yet another embodiment, the metal is at least one metal selected from the group consisting of copper, silver and nickel.
In another embodiment, the base material is at least one selected from the group consisting of a glass woven fabric, a glass nonwoven fabric, an aramid woven fabric, an aramid nonwoven fabric and an aramid film.
In still another embodiment, the resin to be impregnated is at least one resin selected from the group consisting of an epoxy resin, a cyanate ester resin, a bismaleimide triazine resin, a polyimide resin, and a polyphenylene oxide (PPO) resin.
According to another aspect of the presen
Kawakita Yoshihiro
Nakatani Seiichi
Tanahashi Masakazu
Lam Cathy
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
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