Electric heating – Metal heating – By arc
Reexamination Certificate
2001-10-05
2003-03-11
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121610, C219S121700, C219S121760
Reexamination Certificate
active
06531677
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates to an improved method and apparatus for drilling printed wiring boards with laser lights. More particularly, it relates to drilling blind via holes to connect between upper conductive layers and lower conductive layers.
BACKGROUND OF THE INVENTION
FIG. 17
shows a diagram of a conventional optical system for drilling with laser light. In this laser drilling system, a laser beam
2
emitted from a laser head
1
is collimated and magnified or minified with a collimator
3
, then shaped into a suitable diameter for drilling with an aperture
4
. The shaped laser beam
2
is reflected by a corner mirror
5
and a mirror
14
in a machining head Z, then reflected by a pair of galvanometer mirrors
15
a
,
15
b
to a f-&thgr; lens
16
. The laser beam
2
is positioned with the galvanometer mirrors
15
a
,
15
b
and incident vertically on a machining surface through the f-&thgr; lens. The machining is conducted on every machining area
18
defined by the size of f-&thgr; lens
16
and the area is moved by a X-Y table (not shown) from
18
1
to
18
N
sequentially.
FIG. 18
a
shows effects of the collimator
3
and aperture
4
. The graphs on the lower part of this figure show distributional relationships between laser light energy (ordinate) and radial positions in laser beam (abscissa). Since the spatial energy distribution at the output window of the laser head
1
is a gaussian distribution in general, the spatial energy distribution of the laser beam passed through the collimator
3
is also a gaussian distribution. The size of the laser beam can be varied by magnifications (magnifying ratios or minifying ratios) of the collimator
3
. That is, when the magnification is low, the diameter of the laser beam becomes small and the spatial energy distribution shows a high energy (or power) density profile of “a′-distribution (dotted line)” shown in
FIG. 18
a,
and when the magnification is high, the diameter of the laser beam becomes large and the spatial energy distribution shows a low energy (or power) density profile of “b′-distribution (dotted line)” shown in
FIG. 18
a.
In particular, if the diameter of the aperture
4
is larger, the bottom of a machined via hole (i.e. the surface of an inner conductive layer) may be damaged because the energy is concentrated at the center. Therefore, an “A′-distribution (solid line)” or a “B′-distribution (solid line)” is made to avoid the damages by cutting out a central part of the beam, witch is a relatively homogeneous part of energy distribution, with the appropriate aperture
4
. Hereafter, a full spatial energy distribution got by removing the aperture
4
from the optical path is called “C′-distribution”.
FIG. 18
b,
on the other hand, shows a spatial energy distribution when a beam homogenizer
30
is used in the optical path. The spatial energy distribution is shaped like a rectangle with the beam homogenizer
30
, minified or magnified by a collimator
3
(“a-distribution (dotted line)” or “b-distribution (dotted line)” in
FIG. 18
b
), then cut out with a aperture
4
, made highly homogenized (“A-distribution (solid line)” or “B-distribution (solid line)” in
FIG. 18
b
). Hereafter, these rectangle-like shaped distributions are called “top-hat shaped” distributions and a full spatial energy distribution with the beam homogenizer
30
, obtained by removing the aperture
4
from the optical path, is called “C-distribution”. Various commercial optical products can be used for the beam homogenizer
30
, such as an aspheric lens system or a diffractive optical system.
Typical structures of printed wiring boards are an “glass-containing substrate” (FR-4) which is a substrate laminated alternately with a layer or layers of conductor and a layer or layers of resin containing glass-fibers and whose surface layer is a conductive layer, an “RCC substrate” which is a substrate laminated alternately with a layer or layers of conductor and a layer or layers of resin and whose surface layer is a conductive layer, and a “resin-direct substrate” whose conductive layer is coated with a resin layer. Epoxy or polyimide is mainly used as the resin. Instead of glass-fibers, ceramic materials are sometimes used to reinforce the resin layer.
The following drilling methods with CO
2
laser having a wavelength of 10.6 &mgr;m are well known. A method of forming a blind via hole in the resin layer of a resin-direct substrate, called “CO
2
resin direct method”, was disclosed in “GENERATING SMALL HOLES FOR IBMs NEW LSI PACKAGE DESIGN” in IPC Technical Review, Pages 12-15, April 1982 and has been put to practical use. A method of forming a blind via hole in the resin layer of a glass-containing substrate with CO
2
laser after forming a window previously by chemical etching or drilling was disclosed in Japanese Publication No. 58-64097 JP A1 and U.S. Pat. No. 5,010,232.
Moreover, a method for drilling through via holes or blind via holes in a substrate laminated alternately with multiple conductive layers and multiple resin layers was disclosed in Japanese Publication No. 01-266983 JP A1. That is a process of forming a window in a conductive layer by circular processing (in another word, “trepanning”) with a ultraviolet (“UV”) laser light, which can effectively remove metals, and of drilling a resin layer with a CO
2
laser light, repeatedly.
However, it is known that a thin residual resin layer (called “smear”) having thicknesses (t
c
) in the range of 0.2~3 &mgr;m remains on the bottom of a via hole, in other words, just upon the conductive layer after the CO
2
laser drilling. Furthermore, we found that the thickness of t
c
cannot be varied even if the energy densities or the numbers of shots of the CO
2
laser pulses are variously changed.
The following is our speculation about a cause of the remaining. The CO
2
laser drilling is a method utilizing thermal decomposition of resin layer at a temperature increased with the absorption of the infrared laser light. Hence, since the thermal conductivity of the (inner) conductive layer, for example, copper, is 1000 times higher than that of the resin layer, the thermal energy begins to flow into the inner conductive layer when the resin layer becomes thin. Therefore, the temperature of the resin layer cannot go up to the decomposition temperature in a thin residual layer and the residual layer of thickness 0.2~3 &mgr;m remains consequently.
When the layer remains, a chemical desmear process is inevitable to remove the residual layer, which comprises steps for conditioning, washing, boiling, cooling, washing, swelling, washing, desmearing by oxidization, washing, neutralizing, washing, drying, etc. In this chemical desmear process, the wettability in the holes is low at diameters of the via holes less than 100 &mgr;m, that is, it is difficult for the desmear fluid to enter the via holes deeply, and therefore the reliability of the process decreases. Moreover, there is a problem that diameters of the via holes by drilling with CO
2
laser become 10 &mgr;m larger in maximum usually, because the sides of the via holes are also cut 3~5 &mgr;m by the desmear fluid though the purpose of the desmear process is to remove the residual layer on the bottoms.
On the other hand, A method of forming a blind via hole in the resin layer of a resin-direct substrate with a UV laser, called “UV resin direct method”, was disclosed in “Excimer Lasers: An emerging technology in the electronics industry” in IPC Technical Review, Pages 16-20, November 1987 and has been put to practical use. A method of forming a via hole in a substrate laminated with conductive layers and resin layers only with a UV laser was disclosed in U.S. Pat. No. 5,593,606.
The residual layer does not exist on the bottoms of via holes with the UV laser, which is different from the CO
2
laser method. However, if we use enough energy to obtain a practical processing speed, the surface of the conductive layer is to be also shaved by the excessive energy, and the surface roughnes
Arai Kunio
Ishii Kazuhisa
Kita Yasuhiko
Crowell & Moring LLP
Evans Geoffrey S.
Hitachi Via Mechanics Ltd.
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