Electric heating – Metal heating – By arc
Reexamination Certificate
2000-06-22
2002-09-17
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121730, C359S569000
Reexamination Certificate
active
06452132
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a laser hole boring (drilling) apparatus for printed circuit boards or packages. The printed circuit board of electronic apparatuses is equipped with tiny electronics devices, e.g., ICs, LSIs, capacitors, resistors or so. For mounting the electronic devices, plenty of pin holes are bored through the printed circuit board. At present, the holes of the printed circuit board are bored one by one by a mechanical drilling apparatus which repeats the operation of moving a rotating microdrill to a determined spot, lowering the microdrill onto the spot of the object printed circuit board, boring a hole and raising the microdrill.
The mechanical boring is endowed with flexibility. The mechanical processing is preferable for the purpose of perforating holes of a small number of products having various dispositions of holes. The prevalent boring method is still the mechanical boring. The mechanical processing has a drawback of low drilling speed, because the microdrill bores holes one by one.
This application claims the priority of Japanese Patent Application No. 11-177376 (177376/1999) filed Jun. 23, 1999 and No.2000-154514 (154514/2000) filed May 25, 2000 which are incorporated herein by reference.
2. Description of Related Art
In the description, the word “ray” means an individual line representing an optical path of light. A beam is an assembly of rays which starts from a point, travels along different paths and converges at another point. The beam is a collective concept. The ray is an individual concept. But a beam can be drawn by a line. The light packets which have different starting points or different converging points cannot be deemed as a beam. The beam should be discriminated from the ray. Raising mounted device density and multilayered wiring of recent printed circuit boards requires to bore smaller diameter holes. Packages for semiconductor devices require plenty of microholes boring. For example, less than 0.15 mm of a hole diameter will be required in near future. It is difficult to bore such a small hole by the mechanical drilling. The small hole diameter requires a smaller drill diameter. Such a small diameter deprives the drill of the strength of maintaining itself as a tool. A promising candidate of perforating such small holes is an optical boring by laser beams. This invention aims at providing a high speed laser boring apparatus which is preferable f or mass production of a small number of types of printed circuit boards.
A laser beam scanning boring method using a laser, galvanomirrors and lenses has been proposed as a new boring method which will be faster than the present mechanical boring method. This method uses a pulse laser beam of a high power laser, for example, a CO
2
laser. The laser beam scanning boring apparatus perforates holes one by one by the steps of swaying galvanomirrors by a unit angle, reflecting a pulse laser beam by the galvanomirrors, converging the beam on a point of a printed circuit board by a lens and burning out a hole through the board by heat in an instant. This method scans on an object printed circuit board with a pulse laser beam by deflecting the laser beam in two directions perpendicular with each other by two galvanomirrors which sway in definite amplitudes with predetermined speeds around different axes vertical to each other. Since the laser generates a pulsed beam, the pulsed laser beam perforates small holes at discrete, predetermined spots despite the continual oscillation of the galvanomirrors. The laser beam scanning boring method can perforate smaller holes at a faster rate than the mechanical drilling, since a small pulse beam converged by the lens burns small areas of the board. The laser beam scanning method is a promising, novel boring method which will be realized in near future. However, the Inventor of the present invention is aware that this novel laser beam scanning method would have some problems.
A first problem is the converging lens. A conventional and traditional lens has an action of converging rays of a beam together on a spot of an image plane distanced by a focal length from the lens. A perpendicularly incident beam is converged onto the center of the image plane. Another beam slanting at an angle &thgr; to the optical axis is converged on a spot distanced by f tan &thgr; from the center of the image plane. The distance of an image spot from the center on the image plane is here defined as a “height” of the image spot. Since the height of the spot image of a &thgr; inclining beam is f tan &thgr;, the conventional, traditional lens can be called here an “f tan &thgr;” lens.
When the galvanomirrors scan a single laser beam in the x-direction and in the y-direction at a constant rate, the velocity of the swaying angle &thgr; is constant. The interval between the neighboring holes on the board should be constant. The conventional f tan &thgr; lens cannot satisfy such a linear relation h=f&thgr; between the angle &thgr; and the spot height h. The beam of an incidence angle &thgr; should make an image at a spot of a height of f&thgr; by a lens for boring holes at a constant interval. Instead of conventional traditional lenses, the laser beam scanning method requires a special lens which makes &thgr; inclining rays converged on a spot image of an f&thgr; height (h=f&thgr;) on the image plane. Such a lens is called an “f&thgr;” (f-theta) lens. The f&thgr; lens is different from the conventional lenses. The f&thgr; lens which is realized by an assembly of plurality of lenses requires a special design for giving the f&thgr; property.
Another problem of the laser beam scanning method is the orthogonality of the beam to the board. The scanned beam should always be vertical to the board for boring holes perpendicular to the board. Although the incident beams are slanting to the lens axis and pass the lens at non-central point, the beams must construct a outgoing, convergent beam perpendicular to the board. This is a very difficult property for a conventional lens to realize. The property that slantingly-incident beams are converted into a vertical converging beam by a lens is called “telecentricity” or “telecentric property”. The laser beam scanning boring method by the galvanomirrors requires the “f&thgr; property” and the “telecentricity” for the converging lens.
It is expected that the galvanomirror laser beam scanning boring method by the galvanomirrors and the f&thgr; lens would enable the hole boring processing on printed circuit boards or on IC packages to perforate up to 500 holes per a second.
FIG. 1
shows a schematic view of the laser beam scanning boring apparatus which will be applied to an actual processing in near future. A laser beam
1
is once reflected by an x-axis scanning galvanomirror
2
. The laser beam is again reflected by a y-axis scanning galvanomirror
3
. The x-axis scanning galvanomirror
2
sways along the x-axis. The reflected beam sways right and left in the x-direction in a definite amplitude with determined timing. The y-axis scanning galvanomirror
3
sways along the y-axis. The reflected beam sways up and down in the y-direction in a definite amplitude with determined timing. The timing of moving in the x- and y-directions depends upon the mode of the scanning.
The twice reflected beam scans horizontally and vertically on a lens
4
. The lens
4
converges the beam on a printed circuit board
5
. Since the beam runs both in the x-direction and in the y-direction and the laser emits a pulsed beam, the apparatus can perforate a lot of holes
6
aligning their locations in the x-direction and in the y-direction at an ultrahigh speed. The laser beam scanning boring method will make big progress over the current mechanical drilling in speed in near future. Practical models have been produced for galvanomirrors of small inertia and galvanometers for oscillating the mirror at a high speed. The far-infrared lenses have been designed and prepared for CO
2
lasers.
However, a request for perf
Evans Geoffrey S.
Sumitomo Electric Industries Ltd.
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