Etching a substrate: processes – Forming or treating electrical conductor article – Forming or treating of groove or through hole
Reexamination Certificate
1999-05-28
2001-08-28
Gulakowski, Randy (Department: 1746)
Etching a substrate: processes
Forming or treating electrical conductor article
Forming or treating of groove or through hole
C216S018000, C216S065000, C216S078000, C219S121710, C029S846000, C029S852000, C427S097100
Reexamination Certificate
active
06280641
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a printed wiring board having a micro-via hole. More specifically, it relates to a printed wiring board having a structure in which a micro-via hole is formed by removing part of surface of a copper foil positioned below an outermost layer, i.e., a surface layer, so as not to penetrate through the copper foil in the bottom, and the surface layer and the copper foil are electrically connected with a plating or an electrically conductive coating composition forming a wall surface of the micro-via hole, the printed wiring board being highly reliable with regard to conduction and being mainly for use for a small-sized semiconductor plastic package.
PRIOR ART OF THE INVENTION
In a high-density multi-layered printed wiring board for use in a semiconductor plastic package, conventionally, a via hole is made directly with a mechanical drill, or it is made by removing a hole-forming portion of a surface-layer copper foil by etching and then making a hole in an insulation layer with a carbon dioxide gas laser. When a mechanical drill is used, it is difficult to make a via hole discontinued in a middle of an intended inner-layer copper foil in its thickness direction if the inner-layer copper foil has a small thickness or if a multi-layered board varies in thickness. In some cases, the via hole goes through the intended inner-layer copper foil and reaches a copper foil layer below the inner-layer copper foil, which causes a failure in function. When a carbon dioxide gas laser is used, a resin layer having approximately 1 &mgr;m remains on a wall surface of a micro-via hole and on a copper foil surface forming a bottom of the micro-via hole, and it is therefore required to carry out desmear treatment before copper-plating. When a micro-via hole has a small diameter or when the desmear treatment is insufficient due to insufficient contact of a desmearing liquid to inner surfaces of a micro-via hole, the adhesion of a copper plating to a micro-via hole bottom is defective, which results in poor reliability in conduction. That is, a conduction failure often takes place since the conduction between a surface-layer copper foil and a copper foil in a bottom or a wall surface of a micro-via hole is poor. Moreover, there is another problem that the above desmear treatment requires a time period 2 to 3 times the time period of desmear treatment for a general through hole, which causes poor workability.
Further, when a copper foil surface is directly irradiated with a carbon dioxide gas laser, laser beam is reflected and no hole is made in the copper foil. Conventionally, therefore, a predetermined size of a hole-making portion of a surface-layer copper foil is removed by etching, and an exposed resin portion is irradiated with a low-output carbon dioxide gas laser to make a via hole. When a hole having a small diameter of 100 &mgr;m or less is made in a copper foil by etching, there is caused a problem that the hole has poor diameter accuracy if the copper foil has a large thickness.
Furthermore, when a copper foil is present as an inner layer, even if a hole is made in a surface-layer copper foil with low energy after the concerned portion of the surface-layer copper foil is removed by etching, no hole can be made in the inner-layer copper foil, or the intended micro-via hole having a copper layer on its wall surface cannot be formed.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a printed wiring board having a highly reliable micro-via hole electrically connecting a copper foil on an outermost layer to a copper foil positioned in the bottom of the micro-via hole, and a method of forming the above micro-via hole.
It is another object of the present invention to provide a printed wiring board having a highly reliable micro-via hole electrically connecting a copper foil on an outermost layer to a copper foil positioned in the bottom of the micro-via hole through a copper foil exposed on a wall surface of the micro-via hole, and a method of forming the above micro-via hole.
It is still further another object of the present invention to provide a printed wiring board having a small-diameter micro-via hole formed without any desmear treatment and a method of forming the micro-via hole having the above characteristic.
It is yet another object of the present invention to provide a printed wiring board having a micro-via hole that can be formed at a high rate and a method of forming the micro-via hole having the above characteristic.
According to the present invention, the above objects of the present invention are achieved by the following (1) to (8).
(1) A printed wiring board having a micro-via hole electrically connecting a first copper foil as an outermost layer to a copper foil positioned below the first copper foil, the micro-via hole electrically connecting the outermost layer to the copper foil in a bottom of the micro-via hole with a plating or an electrically conductive coating composition, the micro-via hole not penetrating through the copper foil in the bottom of the micro-via hole when formed by removing part of the copper foil in a bottom portion of the micro-via hole with a carbon dioxide gas laser.
(2) A method of making a micro-via hole for electrically connecting a first copper foil as an outermost layer of a printed wiring board to a copper foil in a bottom of the micro-via hole with a carbon dioxide gas laser, the method comprising providing a coating or sheet of an organic substance containing 3 to 97% by volume of at least one selected from a metal compound powder, a carbon powder or a metal powder having a melting point of at least 900° C. and a bond energy of at least 300 kJ/mol on a copper foil as an outermost layer of a copper-clad laminate having at least two copper layers, irradiating the coating or sheet with a carbon dioxide gas laser at an output of 20 to 60 mJ/pulse, thereby removing a micro-via-hole-forming portion of at least the copper foil as the outermost layer, then irradiating micro-via-hole-forming portions of the remaining layers with a carbon dioxide gas laser at an output of 5 to 35 mJ/pulse to make a micro-via hole which does not penetrate through the copper foil in a bottom of the micro-via hole, and electrically connecting the copper foil as the outermost layer and the copper foil in the bottom of the micro-via hole with a metal plating or an electrically conductive coating composition.
(3) A method according to the above (2), wherein, after the removal of the micro-via-hole-forming portion of the copper foil as the outermost layer with the carbon dioxide gas laser, the copper foil positioned below the first copper foil is removed partly in the thickness direction thereof with the carbon dioxide gas laser at an output of 20 to 35 mJ/pulse.
(4) A method according to the above (2), wherein the first copper foil as the outermost layer is surface-etched to decrease the first copper foil partly in the thickness direction of the first copper foil and at the same time to remove a copper foil burr.
(5) A method according to the above (2), wherein part of the copper foil surface in the bottom of the micro-via hole is removed and then an inside of the micro-via hole is treated in a gaseous phase to remove remaining resin layer, followed by wetting treatment.
(6) A method according to the above (5), wherein the remaining resin is removed by the treatment in a gaseous phase and the wetting treatment.
(7) A method according to the above (5), wherein the treatment in a gaseous phase is plasma treatment or low-pressure ultraviolet light treatment.
(8) A method according to the above (2), wherein the first copper foil and the copper foil layer in the bottom of the micro-via hole are electrically connected by filling at least 80% by volume of the micro-via hole with a copper plating.
REFERENCES:
patent: 4258468 (1981-03-01), Balde
patent: 4717639 (1988-01-01), Dubin et al.
patent: 5063280 (1991-11-01), Inagawa et al.
patent: 5073687 (1991-12-01), Inagawa et al.
patent: 5121298 (1992-06-
Gaku Morio
Ikeguchi Nobuyuki
Tanaka Yasuo
Chaudhry Saeed
Gulakowski Randy
Mitsubishi Gas Chemical Company Inc.
Wenderoth , Lind & Ponack, L.L.P.
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