Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-03-30
2001-06-05
Huson, Gregory L. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S830000, C029S846000, C427S097100
Reexamination Certificate
active
06240636
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of making multi-layer printed wiring boards, more particularly, to a method wherein a via hole is easily formed in the multi-layer printed wiring board using a laser, while improving the adhesion between an outer wiring made from plated copper (a copper layer) and an insulating resin (a thermosetting resin layer) which is present between the outer wiring and the inner wiring.
BACKGROUND OF THE INVENTION
As electronic equipment becomes smaller and lighter with higher performance, it is necessary to reduce the width of wiring lines and the diameter of the via holes which connect layers in multi-layer printed wiring boards. It is extremely difficult to use mechanical drilling to form holes below about 200 &mgr;m diameter, and lasers have been used to make such small holes.
A carbon dioxide laser can form holes at high speed in organic substances, such as epoxy resin and polyimide resin. Such lasers have been widely used in making printed wiring boards. Forming holes in thick copper foil is difficult, however, because the copper foil reflects the laser beam. To solve this problem, as disclosed in Japanese Patent Publication No. 4-3676, a hole is etched through the copper foil having the same diameter as the via hole to be made. Then, the laser beam is used to form the hole through the organic substrate, with the beam diameter being larger than that of the via hole.
In the method of Japanese Patent Publication No. 4-3676, since it is necessary that a hole having the same diameter is etched through the copper foil, followed by irradiating with the laser beam to form the via hole, etching must be done twice, i.e., first, for forming the via hole in the copper foil and, second, for forming the wiring pattern. The repeated etching in such a process results in a serious loss of productivity. This may be compared with the use of conventional mechanical drilling wherein the etching is done only once, that is, for forming the wiring pattern. Furthermore, it is not easy to etch a hole in an outer wiring so as to align the hole with inner pad, because highly accurate alignment is required.
In another process, the faces of an inner insulating layer board which have inner wiring patterns are coated with an insulating resin, and holes are formed in the resin by the laser beam, and then the resin surfaces are directly plated with copper to form an outer copper layer. Only a single layer of copper is deposited. In this process, however, it is necessary to roughen the surfaces of the insulating resin layers formed by applying the insulating resin to obtain acceptable adhesive strength between the plated copper and the insulating resin layer. Roughening of the insulating resin surfaces often cannot provide sufficient adhesive strength between the copper layer and the insulating resin layer.
The present invention solves the above problems of the prior art, and provides a process for making multi-layer printed wiring boards, in which via holes are easily formed with a laser, and adhesion between the outer wiring pattern formed from the plated copper layer and the insulating resin layer between the outer wiring pattern and the inner wiring pattern is improved.
SUMMARY OF THE INVENTION
The inventors earnestly studied the above problems associated with the prior art and have found that a hole can be formed quickly in the outer copper layer and the insulating resin layer without damaging the inner wiring pattern by the method of making a multi-layer printed wiring board in which an inner core comprising an inner wiring pattern and an outer layer of copper foil are laminated with an organic insulating resin layer between the inner wiring pattern and the outer layer of copper foil to form a multi-layer board (a), a via hole is formed on the multi-layer board (a) by irradiating with the laser beam, and the outer layer of copper foil and the inner circuit pattern are electrically connected to each other by depositing copper on the multi-layer board (b) having the via hole, wherein the thickness of said outer layer of copper foil is no more than a specific thickness and said via hole is formed in both said outer layer of copper foil and said insulating resin layer simultaneously by irradiating with a laser beam, preferably a carbon dioxide laser. The present invention has been made on the basis of the findings.
That is, the method for making a multi-layer printed wiring board of the present invention is one in which an inner core comprising an inner wiring pattern and an outer layer of copper foil are laminated with an insulating resin layer between the inner wiring pattern and the outer layer of copper foil to form a multi-layer board (a), a via hole is formed on the multi-layer board (a) by irradiating with a laser beam, and the outer copper foil and the inner wiring pattern are electrically connected to each other by depositing copper on the multi-layer board (b) having the via hole, wherein the thickness of said outer copper foil is no more than one-fifth of the thickness of said inner wiring pattern, but will not exceed 7 &mgr;m, preferably 4 &mgr;m, and said via hole is formed in both said outer copper foil and said insulating resin layer simultaneously by irradiating with a laser beam, preferably a carbon dioxide laser.
REFERENCES:
patent: 5617629 (1997-04-01), Ekström
patent: 5690837 (1997-11-01), Nakaso et al.
patent: 0 342 669 A2 (1989-11-01), None
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“Structuring Of Laminated Cu-Polyimide Films Via Laser Ablation,”IBM Technical Disclosure Bulletin,vol. 37, No. 8, p. 481 (Aug. 1994).
Kestenbaum, Ami, et al., “Laser Drilling Of Microvias In Epoxy-Glass Printed Circuit Boards,”IEEE Transactions on Components, Hybirds, and Manufacturing Technology,vol. 13, No. 4, pp. 1055-1062 (Dec. 1990).
Pargellis, A. N., et al., “Formation Of Microvias In Epoxy-Glass Composites By Laser Ablation,”Optics&Laser Technology,vol. 22, No. 3, pp. 205-207 (Jun. 1990).
Asai Tsutomu
Kuwako Fujio
Obata Shin-ichi
Chang Rick Kiltae
Huson Gregory L.
Jenkens & Gilchrist
Mitsui Mining & Smelting Co. Ltd.
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