Electric heating – Metal heating – By arc
Reexamination Certificate
2000-04-14
2002-04-16
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06373026
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a machining method for a wiring board and a machining apparatus for a wiring board employing a laser beam for machining such as drilling for a through-hole, an inner via hole, and a blind via hole, grooving, and cutting for an outside shape of the wiring board referred to as so-called printed board, and more particularly to a machining method for a wiring board and a machining apparatus for a wiring board in which a fine conduction hole can rapidly and accurately be formed, and a carbonic acid gas laser oscillator to generate a pulsed laser beam most suitable for the above machining.
2. Description of the Prior Art
A printed board includes a plurality of insulating base materials with conductor layers, stacked and joined in a multi-layer fashion. Among the conductor layers applied onto the insulating base materials, the optional conductor layers in a vertical direction are electrically connected through conduction holes referred to as a through-hole, an inner via hole, and a blind via hole.
FIG. 33
is a sectional view of such a conventional multi-layer printed board. In the drawing, reference numeral
51
means a printed board,
52
to
56
are conductor layers,
57
is metallic deposits,
61
to
64
are insulating base materials, and
65
to
68
are conduction holes. In the five-layer printed board
51
including the conductor layers
52
to
56
, the insulating base materials
61
and
63
with both sides coated with copper foil and the conductor layer
56
including copper foil are stacked and joined by using the insulating base materials
62
and
64
referred to as prepreg, and the conduction holes
65
to
68
are provided to pass through the conductor layers
52
to
56
.
As shown in
FIG. 33
, the conduction hole
65
is mounted for conduction between the conductor layer
52
and the conductor layer
53
in the insulating base material
61
, and the conduction hole
66
, referred to as blind via hole (BVH), is mounted for conduction between the conductor layer
52
in the insulating base material
61
and the conductor layer
54
in the insulating base material
63
. The conduction hole
67
, referred to as inner via hole (IVH), is mounted for conduction between the conductor layer
54
and the conductor layer
55
in the insulating base material
63
. The conduction hole
68
, referred to as through-hole (TH), is mounted for conduction between the conductor layer
52
in the insulating base material
61
and the conductor layer
56
stacked and joined through the insulating base material
64
.
The conduction holes
65
to
68
shown in
FIG. 33
are holes machined by a drill. Further, after drilling, the conduction holes are plated through the metallic deposits
57
, and the conductor layers are electrically connected.
In the prior art, a machining method for the conduction hole includes, for example, drill machining using a rotary milling cutter. Further, a machining method for grooving or cutting for an outside shape includes, for example, router machining using a rotary milling cutter. On the other hand, in recent years, higher density wiring has been desired for higher performance of an electronic device. A more multi-layered and smaller printed board is required to meet the above requirement. Further, it is essential to provide a finer hole diameter of the conduction hole for this purpose. With the current state of the art, the conduction hole is generally provided in the printed board by the mechanical method using the drill. However, the method has drawbacks in that the finer hole diameter is limited, for example because drilling a hole diameter of &phgr;0.2 mm or less is very difficult to cause heavy wear of the drill such as breakage, resulting in poor productivity due to the long time required for replacement of the drill. Further, it is difficult to simultaneously machine adjacent positions, thereby requiring a considerable machining time. In addition, the insulating base material has a thickness of 0.1 mm or less because of the smaller printed board. Since it is difficult to control a hole depth in the drill machining with accuracy of 0.1 mm or less, it is difficult to form a blind via hole in such a thin-walled insulating base material. Further, in order to realize cost reduction by the smaller printed board and an increase in yield, the grooving and the cutting for the outside shape require an accurate depth control in the grooving, a narrower cutting width, and cutting after parts are packaged. However, the mechanical methods such as router machining are unpractical since the above limitation is similarly imposed thereon.
Instead of the machining methods for the printed board including the above mechanical methods, attention has been given to methods, which have partially been put to practical use, employing a laser beam such as an eximer laser or carbonic acid gas laser, disclosed in IBM Journal of Research and Development, Vol. 126, No. 3, pp. 306-317 (1982), and Japanese Patent Publication (Kokoku) No. 4-3676. These laser beam machining methods utilize a difference in absorption coefficient of light energy such as the eximer laser or the carbonic acid gas laser between resin or glass fiber serving as the insulating base material forming the printed board, and copper serving as the conductor layer. For example, since almost the entire laser beam emitted from the above laser can be reflected at the copper, a copper foil removed portion having a required diameter is formed in top copper foil through etching and so forth, and the copper foil removed portion may be irradiated with the laser beam. It is thereby possible to selectively decompose and remove the resin and the glass so as to form a fine through-hole and a fine inner via hole in a short time. If internal-layer copper foil is previously stacked in a machined portion, the decomposition and the removal of the insulating base material are terminated at the internal-layer copper foil. It is thereby possible to form a blind via hole which can surely be terminated at bottom copper foil. There is an advantage of no wear of the tool such as breakage because the laser beam machining methods are contactless machining methods.
The above laser beam machining methods employ a pulse laser such as the eximer laser and a TEA-carbonic acid gas laser, with an extremely narrow pulse width of 1 &mgr;s or less. The pulse laser can finely divide into chips (1) a single base material made of high polymeric material such as polyimide, or epoxy, (2) a composite material reinforced by aramid fiber or the like, containing the polyimide, the epoxy, and so forth, and (3) an inorganic material such as glass. It is thereby possible to rapidly and accurately form a good machined hole with a smooth machined portion and less altered layer in a printed board in which as the insulating base material is used composite material dispersed in the polyimide, the epoxy, and so forth.
The conventional laser beam machining method for the wiring board has the above structure. The eximer laser or the TEA-carbonic acid gas laser is used to provide the through-hole and the inner via hole in the most commonly used printed board having the insulating base material made of glass cloth containing the glass fiber and the resin, such as a glass epoxy printed board referred to as FR-4 made of the glass cloth and epoxy resin. In this case, there are problems in that metallic deposit for conduction can not easily be coated on a hole inner wall due to the extremely rough inner wall of the hole, and reliability of the metallic deposit can not be ensured. The problems are generated because the insulating material of the printed board is not only the composite material made of organic material and inorganic material but also heterogeneous material in which the organic material and the inorganic material are contained in clusters to some extent.
Further, there is another problem in that a uniform machined hole can not be provided due to differences in, for example,
Fukushima Tsukasa
Kaneko Masayuki
Kurosawa Miki
Mizuno Masanori
Moriyasu Masaharu
Heinrich Samuel M.
Mitsubishi Denki & Kabushiki Kaisha
Sughrue & Mion, PLLC
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