Electric heating – Metal heating – By arc
Reexamination Certificate
2000-09-08
2003-02-18
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121700
Reexamination Certificate
active
06521866
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a laser machining method and a laser machine, and particularly relates to a laser machining method and a laser machine suitable for machining a printed board.
BACKGROUND ART
When a blind hole (hereinafter simply referred to as “hole”) for making a connection between layers is machined by a laser beam in a built-up type printed board, a conformal mask method or a direct method is adopted. In the case of the conformal mask method, an insulating layer is irradiated with a laser beam through an etching window which is formed by removing an outer-layer copper foil by etching in advance. On the other hand, in the case of the direct method, an insulating layer having no outer-layer copper foil is irradiated with a laser beam directly. Thus, the insulating layer formed of resin containing glass reinforced fiber or filler is removed by the laser energy. In some laser machines, a laser beam outputted from a laser oscillator is supplied to a plurality of machining heads so that the machining speed is enhanced. Such a laser machine will be described with reference to FIG.
10
.
FIG. 10
is a configuration view of a background-art laser machine. A laser oscillator
1
outputs a pulsed laser beam
2
. A half mirror
3
transmits about 50% of the laser beam
2
incident thereto and reflects the rest of the laser beam
2
. Hereinafter, the laser beam
2
transmitted through the half mirror
3
will be referred to as a transmitted beam
2
a
, and the laser beam
2
reflected on the half mirror
3
will be referred to as a reflected beam
2
b
. The reflecting surfaces of total reflection corner mirrors
4
a
to
4
c
are fixed. As indicated by the arrows in
FIG. 10
, galvanomirrors
5
a
to
5
d
rotate desirably around the rotation axes thereof so that the reflecting surfaces thereof can be positioned at desired angles respectively. Condensing lenses (f&thgr; lenses)
6
a
and
6
b
are held by a first machining head
7
a
and a second machining head
7
b
respectively. A printed board
8
is fixed to an X-Y table
9
. A scan area
10
a
of the galvanomirrors
5
a
and
5
b
and a scan area
10
b
of the galvanomirrors
5
c
and
5
d
measure about 50 mm by 50 mm respectively.
Next, the operation of the background-art laser machine will be described.
The laser beam
2
outputted from the laser oscillator
1
is split into the transmitted beam
2
a
and the reflected beam
2
b
by the half mirror
3
. The transmitted beam
2
a
is reflected by the total reflection corner mirrors
4
a
and
4
b
to be made incident onto the galvanomirror
5
a
, passed through an optical path defined by the galvanomirrors
5
a
and
5
b
, and condensed by the condensing lens
6
a
so as to machine a hole in the scan area
10
a
. The reflected beam
2
b
is reflected by the total reflection corner mirrors
4
c
to be made incident onto the galvanomirror
5
c
, passed through an optical path defined by the galvanomirrors
5
c
and
5
d
, and condensed by the condensing lens
6
b
so as to machine a hole in the scan area
10
b
. Then, the galvanomirrors
5
a
to
5
d
are operated so that the machining head
7
a
machines the hole in the scan area
10
a
and the machining head
7
b
machines the hole in the scan area
10
b
, sequentially. After the holes in the scan areas
10
a
and
10
b
have been machined down, the X-Y table
9
is moved so that machining in the next scan areas
11
a
and
11
b
is performed. Incidentally, a distance L between the machining head
7
a
and the machining head
7
b
is designed to be adjustable. The distance L is adjusted in advance so that the scan area
10
a
and the scan area
10
b
are not put on each other and the number of times to move the X-Y table
9
is minimized.
Incidentally, in order to machine a hole, a plurality of pulsed laser beams
2
(hereinafter, a pulse of laser beam will be referred to as a “laser pulse”) are often radiated. A machining method in which a plurality of laser pulses are radiated continuously for one hole and the next hole is machined after the preceding hole has been machined down, is called “burst machining”. A machining method in which a plurality of holes are grouped into one set, every hole in one set is irradiated with one laser pulse, and this operation is repeated till the holes in the one set have been machined down, is called “cycle machining”.
FIG. 11
is a timing chart of respective portions in the cycle machining; (a) designates a start signal for the laser oscillator
1
; (b) designates the magnitude of energy of the laser beam
2
; (c) designates the magnitude of energy of the transmitted beam
2
a
; (d) designates a positioning signal for the galvanomirrors
5
a
and
5
b
; (e) designates the magnitude of energy of the reflected beam
2
b
; and (f) designates a positioning signal for the galvanomirrors
5
c
and
5
d.
When the start signal is turned ON (at time T
0
), the radiation of the laser beam
2
is started after a delay period T
DL
of several &mgr;s has passed (at time T
1
, in this case, T
DL
). The magnitude of the energy increases gradually and reaches substantially a peak value W
P
after a rising period T
R
has passed (at time T
2
). When the start signal is turned OFF after a pulse period T
P
has passed since the time T
0
(at time T
3
), the energy decreases gradually and reaches 0 after a falling period T
D
has passed (at T
4
). Then, the galvanomirrors
5
a
to
5
d
are operated during a period T
G
after the time T
5
so as to be positioned in the next machining positions. After the positioning is completed (at time T
6
), the start signal is turned ON again (at time T
7
). The above-mentioned operation is repeated hereafter. In this case, since the transmitted beam
2
a
and the reflected beam
2
b
are obtained by splitting the laser beam
2
, each of the beams
2
a
and
2
b
has energy the peak value of which is W
P
/2. Incidentally, if the time T
5
is set to be simultaneous with the time T
4
, and if the time T
7
is set to be simultaneous with the time T
6
, the machining speed can be accelerated.
The laser pulse period during which the laser oscillator can oscillate is 0.33 ms (frequency: 3 kHz), and the pulse period T
P
is several tens of &mgr;s. On the other hand, the period T
G
required for positioning the galvanomirrors
5
c
and
5
d
is about 2 ms, and the period required for positioning the table is about 200 ms. Therefore, burst machining can accelerate increase the machining speed in comparison with cycle machining.
However, in the case where burst machining is performed by the conformal mask method, if the pulse period is set to be not longer than 2 ms, decomposed flying matters generated by a laser pulse previously radiated remain inside and near the hole. Then, the remaining decomposed flying matters absorb the energy of a succeeding laser pulse so as to be high-temperature plasma. Thus, the resin in the flank of the hole is hollowed so that the diameter of the intermediate portion of the hole in the direction of depth is expanded to be larger than the diameter of the upper or lower portion. Thus, the hole is formed into a so-called barrel-like hole, so that the quality of the hole deteriorates.
Moreover, in the case where burst machining is performed by the direct machining method, if the insulating material is of FR-4 which contains glass reinforced fibers, only the resin is hollowed due to the difference in decomposition energy between the resin and the glass (resin:glass=1:3 to 4). Thus, the glass fibers project over the flank of the hole so that the quality of the hole deteriorates.
Further, even in a laser of RF excitation which rises quickly, the period T
R
to reach the peak value W
P
is about 15 &mgr;s as shown in FIG.
11
. Thus, it is impossible to obtain the peak value W
P
in a range where the pulse width is not longer than 15 &mgr;s.
In addition, since the falling period T
D
after the start signal is turned OFF is in a range of from 30 &mgr;s to 50 &mgr;s, the real pulse width becomes longer than the set pulse widt
Arai Kunio
Watanabe Humio
Evans Geoffrey S.
Hitachi Via Mechanics Ltd.
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