U.S. patent application number 13/226029 was filed with the patent office on 2011-12-29 for product having through-hole and laser processing method.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Masaki Hashida, Hidehiko MISHIMA, Yasuhiro Okuda, Shuji Sakabe, Seiji Shimizu.
Application Number | 20110318530 13/226029 |
Document ID | / |
Family ID | 38723267 |
Filed Date | 2011-12-29 |
United States Patent
Application |
20110318530 |
Kind Code |
A1 |
MISHIMA; Hidehiko ; et
al. |
December 29, 2011 |
PRODUCT HAVING THROUGH-HOLE AND LASER PROCESSING METHOD
Abstract
A processing method of forming a through-hole in a workpiece by
means of a pulsed laser beam includes the steps of providing a
removable sacrifice layer on the workpiece, forming a through-hole
in the workpiece by the laser beam in a state where the sacrifice
layer is provided, and removing the sacrifice layer from the
workpiece after the step of forming the through-hole.
Inventors: |
MISHIMA; Hidehiko; (Osaka,
JP) ; Okuda; Yasuhiro; (Osaka, JP) ; Sakabe;
Shuji; (Uji-shi, JP) ; Hashida; Masaki;
(Uji-shi, JP) ; Shimizu; Seiji; (Uji-shi,
JP) |
Assignee: |
Sumitomo Electric Industries,
Ltd.
Osaka
JP
|
Family ID: |
38723267 |
Appl. No.: |
13/226029 |
Filed: |
September 6, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12301666 |
Nov 20, 2008 |
|
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PCT/JP2007/060148 |
May 17, 2007 |
|
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13226029 |
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Current U.S.
Class: |
428/131 |
Current CPC
Class: |
H05K 2203/1383 20130101;
B23K 2103/42 20180801; B23K 26/40 20130101; Y10T 428/24273
20150115; B23K 2103/50 20180801; B23K 26/382 20151001; B23K 26/18
20130101; H05K 3/0032 20130101; B23K 2103/14 20180801; B23K 26/009
20130101 |
Class at
Publication: |
428/131 |
International
Class: |
B32B 3/10 20060101
B32B003/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 20, 2006 |
JP |
2006-140967 |
Claims
1-4. (canceled)
5. A product having a through-hole made by a pulsed laser, wherein
(.theta..times.d.sup.0.68)/.phi..ltoreq.4.0 is satisfied where
.phi. (.mu.m) denotes a diameter of said pulsed laser, .theta.
(.degree.) denotes a taper angle of said through-hole and d (.mu.m)
denotes a thickness of said product.
6. A product having a through-hole made by a pulsed laser, wherein
said through-hole is straight-like in shape.
7. A product having a through-hole made by a pulsed laser, wherein
a wall surface of said through-hole has no curved portion that is
inwardly convex at one end of said through-hole such that the
diameter of said through-hole expands as approaching an opening at
said one end.
8. The product having the through-hole according to claim 7,
wherein no protruded portion is present at a peripheral edge of an
opening of said through-hole.
9. The product having the through-hole according to claim 7,
wherein both surfaces where said through-hole opens have no
attachment of scattered fragments due to laser ablation.
Description
RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. application Ser.
No. 12/301,666, filed on Nov. 20, 2008, which is the U.S. National
Phase under 35 U.S.C. .sctn.371 of International Application No.
PCT/JP2007/060148, filed on May 17, 2007, which in turn claims the
benefit of Japanese Application No. 2006-140967, filed on May 20,
2006, the disclosures of which Applications are incorporated by
reference herein.
TECHNICAL FIELD
[0002] The present invention relates to a product having a
through-hole processed by a pulsed laser, and to a laser processing
method.
BACKGROUND ART
[0003] In a wiring board such as a multilayered high-density wiring
board, through-holes have conventionally been formed with a
mechanical process using a drill or the like. However, the
mechanical processing is not readily applicable under the recent
circumstances in which the wiring density is increased, the
diameter of though-hole is made smaller, and the pitch between
through-holes is also made smaller in the wiring board. For the
purposes of solving these problems and further improving the
processing efficiency, there is a tendency to employ laser
processing with a laser beam. In the case that a pulsed laser is
used to form a through-hole, however, the through-hole has a
tapered shape as shown in FIG. 10 (a), (b), causing a problem of
defective plating for example. Furthermore, scattered fragments,
burrs and the like are generated as shown in FIG. 11 (a), (b),
causing a problem of increase in number of processes for rework,
for example. In order to solve these problems, it is desired to
develop a high-precision technique for forming through-holes by
means of a pulsed laser. As for FIG. 10 (a), (b) and FIG. 11 (a),
(b), a detailed description will be given in connection with
Examples.
[0004] In order to meet the requirements as described above, there
have been proposed laser processing methods improved in accuracy.
For example, there is a proposed method in which a laser beam is
applied to each of the surfaces of a workpiece in a process of
forming a through-hole, so as to make the through-hole
straight-like (approach to a right circular cylinder, or reduction
in taper angle) (Patent Document 1). With this method, the
diameters on the front and back sides of through-hole can be made
almost equal to each other. Thus, it becomes possible to reduce the
taper angle of through-hole and then to form a straight-like
through-hole.
[0005] There is another proposed processing method that utilizes
coherent laser light reflected from a workpiece in a laser ablation
process so as to make it possible to readily form a through-hole
having a reduced difference between the diameters on its front and
back sides (Patent Document 2). With this method, the reflected
light can increase the energy density of light used for processing
and change the shape of through-hole, whereby making it possible to
arrange through-holes at a high density.
Patent Document 1: WO99/59761
Patent Document 2: Japanese Patent Laying-Open No. 2000-77824
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0006] According to the method disclosed in Patent Document 1 as
described above, (1) the application of beams to both of the front
and back surfaces requires enormous efforts because of difficulty
in positional matching between the front and the back surfaces; (2)
the tapered shape peculiar to the laser processing still remains
whereby causing formation of a through-hole having
non-straight-like sectional shape and a reduced diameter in its
middle portion; and (3) burrs are generated due to the laser
processing, whereby necessitating a process for removing the
burrs.
[0007] Further, according to the method disclosed in Patent
Document 2, (1) the use of a photomask complicates the process of
forming a hole; and (2) the process control is difficult because
intensity of the reflected beam varies depending on the material of
workpiece and some materials are not reflective. Further, as
described above, (3) burrs are generated due to the laser
processing, whereby necessitating a process for removing the
burrs.
[0008] An object of the present invention is to provide a
simplified laser processing method that can reduce the taper angle
and can prevent generation of burrs and attachment of scattered
fragments, and then provide a product having a through-hole formed
by using the laser processing method.
Means for Solving the Problems
[0009] A laser processing method of the present invention refers to
a processing method for forming a through-hole in a workpiece by
means of a pulsed laser beam. The processing method includes the
steps of providing a removable sacrifice layer on the workpiece,
forming a through-hole in the workpiece by the laser beam in a
state where the sacrifice layer is provided, and removing the
sacrifice layer from the workpiece after the step of forming the
through-hole.
[0010] By providing the sacrifice layer before the laser processing
and removing the same after the laser processing as described
above, it becomes possible to easily reduce the taper angle of
through-hole due to the laser processing. Therefore, it becomes
possible to open a substantially straight-like through-hole
(taperless). Further, it also becomes possible to completely remove
attached scattered fragments as well as protruded portions such as
burrs that are always generated in the laser processing. The
sacrifice layer may be made of the same material as or a material
different from the material of workpiece.
[0011] The material of workpiece can be metal or organic polymer
material, or titanium or fluorine compound, and may have a porous
structure. The reason of this is that the above-described
manufacturing method can be used to relatively easily open a
straight-like through-hole in a workpiece of any of the
above-mentioned materials such as the fluorine compound having the
porous structure (in which it is usually difficult to form a
straight-like through-hole).
[0012] The ablation threshold value of the sacrifice layer may be
selected to be not less than that of the workpiece. In virtue of
this feature, it is possible to ensure the effects achieved by
providing the sacrifice layer. If the ablation threshold value of
the sacrifice layer is smaller than that of the workpiece, the
preferable effects due to provision of the sacrifice layer is
reduced because a large hole is formed in the sacrifice layer.
[0013] Further, the sacrifice layer may include a plurality of
layers. For example, in the case that the ablation threshold value
of workpiece is considerably large and the thickness of the film
cannot be increased, it is possible to use a combined structure in
which a sacrifice layer of the same material as that of the
workpiece is provided as a top layer and a material having a
relatively smaller ablation threshold value is provided as an
underlying layer. Furthermore, the material for the sacrifice layer
may be selected from a wider variety of materials depending on
other conditions.
[0014] It is preferable to carry out the processing so as to
satisfy a relation: (.theta..times.d.sup.0.68)/.phi..ltoreq.4.0
where .phi. (.mu.m) denotes the diameter of laser beam, .theta.
(.degree.) denotes the taper angle of through-hole and d (.mu.m)
denotes the thickness of workpiece. The taper angle of through-hole
depends on the thickness of workpiece and the diameter of laser
beam. Therefore, the processing conditions may be set to satisfy
the above-described relation so that a straight-like through-hole
can be obtained. Laser beam diameter .phi. refers to the diameter
at the front surface of the base film after the sacrifice layer is
removed. Here, the above-described expression is derived from
experimental data, and details thereof will be described in
connection with Example 2.
[0015] A product having a through-hole according to the present
invention refers to a product having a through-hole formed by a
pulsed laser. This product has a feature that
(.theta..times.d.sup.0.68)/.phi..ltoreq.4.0 is satisfied where
.phi. (.mu.m) denotes the diameter of pulsed laser, .theta.
(.degree.) denotes the taper angle of through-hole and d (.mu.m)
denotes the thickness of product. Here, the taper angle refers to
an average taper angle determined form diameters at the front and
rear surfaces of the hole on a supposition that there is a common
axis line, namely axisymmetry. By virtue of this feature, in a
process of forming an electrically conductive portion in a
thickness direction of a multilayer board that is recently required
to have a higher wiring density, for example, it becomes possible
to prevent defective plating on the through-hole wall surface and
form the conductive portion for highly reliable electrical
connection.
[0016] Another product having a through-hole according to the
present invention also refers to a product having a through-hole
formed by a pulsed laser. This product has a feature that the
through-hole is straight-like in shape. Here, the straight-like
through-hole refers to a right-cylindrical through-hole. More
specifically, the straight-like through-hole refers to a
through-hole in the shape of a right cylinder or a through-hole
whose wall surface does not include a curved portion in which the
wall has an inwardly convex surface such that the diameter at one
end of the through-hole becomes larger, as described hereinlater.
In general, the wall surface of through-hole formed by a pulsed
laser is curved to be inwardly convex in a longitudinal cross
section, causing a problem such as defective plating in producing a
wiring board, which is one factor of deterioration in reliability.
In contrast, the straight-like through-hole as described above can
prevent defective plating of a wiring board or the like and can
ensure an electrically conductive portion for highly reliable
electrical connection.
[0017] A still another product having a through-hole according to
the present invention also refers to a product having a
through-hole formed by a pulsed laser. This product has a feature
that the wall surface of through-hole does not include a curved
portion in which the wall has an inwardly convex surface such that
the diameter at one end of the through-hole becomes larger. By
virtue of this structure, it becomes possible to produce a highly
reliable wiring board or the like.
[0018] All of the above-described products having their respective
through-holes can be produced without protrusions at peripheries of
openings of the through-holes. By virtue of this feature, it
becomes easy to carry out a post process in fabrication of an
electronic device on a multilayer wiring board or the like, for
example. Here, the protrusions are mainly formed by burrs.
[0019] Moreover, it is also possible to avoid attachment of
scattered fragments due to laser ablation on both surfaces where
the through-hole is open. With this feature, it becomes possible to
improve the reliability of the multilayer wiring board or the like,
for example.
Effects of the Invention
[0020] The present invention can provide a product having a
straight-like through-hole and a laser processing method that can
reduce a taper angle and avoid burrs and attachment of scattered
fragments on the product. Therefore, the invention can prevent
defective plating on a through-hole wall surface in a multilayer
board and can contribute to supply of highly reliable multilayer
boards and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a diagram showing a spatial distribution of energy
density in a laser beam cross section.
[0022] FIG. 2 is a schematic diagram qualitatively showing a shape
of an opening formed by a laser beam.
[0023] FIG. 3 is a diagram showing a simulation result illustrating
the shape of the opening that changes depending on increase in
number of pulsed laser shots.
[0024] FIG. 4 illustrates a definition of a taper angle.
[0025] FIG. 5 is a diagram showing a state of a through-hole opened
by a pulsed laser with a sacrifice layer being provided, in
manufacturing a product having the through-hole in Example A of the
present invention, where (a) shows an SEM image of a through-hole
cross section and (b) shows a schematic diagram thereof.
[0026] FIG. 6 is a diagram showing a state of the through-hole in
which the sacrifice layer in the state of FIG. 5 is removed, where
(a) shows an SEM image of the through-hole cross section and (b)
shows a schematic diagram thereof.
[0027] FIG. 7 is a diagram showing a state of a through-hole opened
by a pulsed laser with a sacrifice layer being provided, in
manufacturing a product having the through-hole in Example B of the
present invention, where (a) shows an SEM image of a through-hole
cross section and (b) shows a schematic diagram thereof.
[0028] FIG. 8 is a diagram showing a state of the through-hole in
which the sacrifice layer in the state of FIG. 7 is removed, where
(a) shows an SEM image of the through-hole cross section and (b)
shows a schematic diagram thereof.
[0029] FIG. 9 is a diagram showing a front surface of the product
having the through-hole in Example A of the present invention,
where (a) shows an SEM image of a through-hole cross section and
(b) shows a schematic diagram thereof.
[0030] FIG. 10 is a diagram showing a longitudinal cross section of
a product having a through-hole formed by a titanium-sapphire laser
(conventional example), where (a) shows an SEM image of the cross
section and (b) shows a schematic illustration thereof.
[0031] FIG. 11 is a diagram showing a front surface of the product
having the through-hole in FIG. 10, where (a) shows an SEM image of
a cross section and (b) shows a schematic illustration thereof.
[0032] FIG. 12 is a diagram showing the dependency of the taper
angle on the laser beam diameter.
[0033] FIG. 13 is a diagram showing the dependency of the taper
angle on the base film thickness.
[0034] FIG. 14 is a diagram showing a sacrifice layer etc. of
Example C in Example 3 of the present invention.
[0035] FIG. 15 is a diagram showing a sacrifice layer etc. of
Example D in Example 3 of the present invention.
[0036] FIG. 16 is a diagram showing a base film of a comparative
example in Example 3.
[0037] FIG. 17 is a diagram showing a processed diameter at a front
surface of a base film in Example D of the present invention.
[0038] FIG. 18 is a diagram showing a processed diameter at a rear
surface of the base film in Example D of the present invention.
[0039] FIG. 19 is a diagram showing a processed diameter at a front
surface of a base film in a comparative example.
[0040] FIG. 20 is a diagram showing a processed diameter at a rear
surface of the base film in the comparative example.
DESCRIPTION OF THE REFERENCE SIGNS
[0041] 1 workpiece, 1a sacrifice layer, 1b base film, 5
through-hole, 10 product having a through-hole, W through-hole wall
surface, Wa through-hole portion (wall) where the diameter expands
as approaching the front side, Wb straight-like through-hole
portion (wall), Ws curved portion (wall), Da diameter (larger
diameter) at a front surface, Db diameter (smaller diameter) at a
rear surface, d base film (product) thickness, t sacrifice layer
thickness, 105 through-hole, 125 burr, 126 scattered fragment.
BEST MODES FOR CARRYING OUT THE INVENTION
[0042] Embodiments of the present invention will now be described
with reference to the drawings.
Principle of the Present Invention
[0043] The laser fluence (energy density) of a pulsed laser beam
shows a spatial distribution in which it is high at a central
portion and low at a peripheral portion, or generally shows a
Gaussian distribution, as shown in FIG. 1. Therefore, supposing
that O represents the center position of the beam and A.sub.1 and
A.sub.2 represent positions where the laser fluence is equal to the
threshold value of the ablation fluence, there is the relation
OA.sub.1=OA.sub.2 and the distribution has axisymmetry. A hole is
dug by the ablation in the central side of the distribution where
the laser fluence is not less than the threshold value at which a
workpiece is ablated. The energy density is higher as the position
is closer to the center. Thus, there is a tendency that a portion
closer to the center is dug deeper. Accordingly, the wall surface
of the hole is inclined as shown in FIG. 2. FIG. 2 is a cross
section showing a workpiece 1 immediately after a hole 5 is opened.
After hole 5 is made through from the front surface to the rear
surface of workpiece 1, even the peripheral edge of the laser beam
having a lower fluence causes ablation as long as the fluence is
equal to or larger than the ablation threshold value. Therefore, it
is considered that, as the number of pulse shots increases, the
hole is further dug gradually and finally the taper angle becomes
0.degree. C. (straight). As a matter of fact, however, this does
not occur. Specifically, the taper angle does not approach zero
even if the number of pulse shots is increased, and a certain taper
angle remains.
[0044] Considering these phenomena, the present inventors reached
an idea that "while the phenomena cause an inclined surface to be
formed by initial shots, the laser fluence used for the ablation is
lower at the inclined surface as compared with the case where the
laser beam is applied to a flat surface; i.e., the amount of energy
absorbed at the inclined surface is smaller and thus the energy
applied to the ablation decreases". According to this idea, the
laser fluence used for the ablation decreases at the material of
the inclined portion. At the inclined portion, a region absorbing
energy less than the ablation threshold value somewhat extends
toward the center. Then, in the region absorbing energy less than
the ablation threshold value, the hole is not dug even if the
number of pulse shots is increased.
[0045] Regarding the case where pulsed laser processing by means of
a titanium-sapphire laser is performed on a workpiece of
fluorocarbon resin (porous structure), the above-described idea was
verified by calculations (simulations) and experiments. FIG. 3
shows the results of calculations based on the above-described
idea, illustrating the shape of a hole that is dug down during
irradiation of a first shot to a fourth shot of a pulsed laser
beam. A wall surface having a gentle slope is formed in a region
located somewhat closer to the center relative to another region
that is near a peripheral edge of a laser beam cross section where
the fluence is less than the ablation threshold value. In a region
indicated by a circle S, there is formed the wall surface with the
gentle slope. The wall surface with the gentle slope is little
ablated by the first to fourth shots, so that the gentle slope is
held as it is. Thus, near the tail of the fluence distribution,
there is a portion where positions of the walls formed by the first
to fourth shots respectively are unchanged and overlap each other.
The overlapped portion little changes and remains even if the
number of shots is increased after the hole is opened through the
workpiece. In summary, the following phenomenon could be confirmed
through the calculations (simulations). There is caused the
peripheral portion of hole where the taper angle of a sectional
shape of the hole is large. This portion of a large taper angle
corresponds to a through-hole wall portion Wa described in the
following in connection with the results of experiments (FIG. 10
(a), (b)).
[0046] A hole was actually formed by means of a titanium-sapphire
laser (conventional example), and a cross-sectional shape thereof
was observed with an SEM (Scanning Electron Microscope). FIG. 10
(a) is an SEM image of the conventional example, and FIG. 10 (b) is
a schematic illustration thereof. In a through-hole 105 of a
workpiece 101, a hole diameter Da at the front surface that has
been irradiated with the laser beam is considerably larger than a
hole diameter Db at the rear surface. At a wall surface Wa on the
front side of through-hole 105, the diameter increases as
approaching the front surface. At a wall surface Wb on the rear
side of the through-hole, the diameter is substantially unchanged
continuously and thus the shape is straight-like. There is a curved
portion Ws that is inwardly convex and overlaps wall surfaces Wa
and Wb. The portion of wall surface Wa corresponds to the wall
surface portion having a gentle slope in region S of FIG. 3, and
expands like the opening end of a trumpet as approaching the front
surface.
[0047] Regarding formation of a through-hole in a workpiece by
using a pulsed laser beam, the present inventors come up with a
method according to which a removable sacrifice layer is provided
on a workpiece, a through-hole is formed in the workpiece by the
laser beam in the state where the sacrifice layer is provided, and
then the sacrifice layer is removed from the workpiece after the
thorough hole is formed. As described above, the sacrifice layer is
provided before the laser processing and is removed after the laser
processing, and accordingly the portion of Wa and further the
portion of Ws can be mainly included in the sacrifice layer and
then can be removed. The magnitude of thickness t of the sacrifice
layer can be appropriately selected depending on the required
dimensional accuracy of the through-hole. Consequently, it becomes
possible to form a substantially straight-like (taperless)
through-hole. Further, since the sacrifice layer is removed after
the laser processing, it is possible to completely remove scattered
fragments attached to the surface of the sacrifice layer as well as
burrs or the like protruding from the edge of the opening.
[0048] As for a method of providing a sacrifice layer on a surface
of a subject material (workpiece), any method may be used as long
as no interspace is caused when a laser beam is applied. For
example, on a workpiece of a fluorocarbon resin (porous structure),
a sacrifice layer of the same material can be placed by fusion
bonding (bonding surfaces are fused and then cooled so as to bond
together). In the case of placing a sacrifice layer of (a Ti thin
film+a fluorocarbon resin layer) on a workpiece of a metal such as
Ti, the placing can be done with electrostatic force. Specifically,
in the case of an ultrathin sheet, electrostatic force is generated
therein and thus it is possible to simply put the sacrifice layer
on the workpiece so as to adhere to each other. Alternatively, the
sacrifice layer may be attached to the workpiece with an adhesive
for example.
[0049] The above-described method may be applied to a workpiece to
fabricate a product having a through-hole as described below. In a
product having a through-hole of the present invention, the
through-hole is provided by means of a pulsed laser, and taper
angle .theta. of the through-hole is reduced to form a
straight-like shape. Here, taper angle .theta. is defined as taper
angle .theta.=Arctan {(0.5Da-0.5Db)/d} under the condition that
there is the axisymmetry as described above. Da and Db represent
diameters of openings at the front and the rear surfaces,
respectively. Further, d represents the thickness of product 10,
base film 1b or workpiece 101. Each of opening diameters Da and Db
is an average value obtained from at least three times
measurements. Thickness d is also similarly measured. Since taper
angle .theta. of the through-hole of the present invention is
small, substantially the same result can be obtained even if radian
is used as the unit of angle so as to use approximation of taper
angle .theta. (radian)=(0.5Da-0.5Db)/d. Accordingly, in a process
of forming an electrically conductive portion in a thickness
direction of a multilayer board whose wiring density has been
increasing recently, it becomes possible to prevent defective
plating on the through-hole wall surface and form the electrically
conductive portion for highly reliable electrical connection.
[0050] With use of the above-described fabricating method, another
product having a through-hole according to the present invention
can have a straight-like through-hole. The definition of
straight-like is the one as described above. In still another
product having a through-hole according to the present invention,
the through-hole wall surface does not includes a curved portion
that is inwardly convex at one end of the through-hole such that
the diameter of the through-hole increases as approaching the
opening at that end. This through-hole specifically refers to a
through-hole formed by a pulsed layer without including curved
portion Ws in FIG. 10.
[0051] The product having the through-hole formed by the pulsed
laser as described above can have a straight-like through-hole.
Therefore, in a process of forming an electrically conductive
portion in a thickness direction of a multilayer board whose wiring
density has been increasing recently, for example, it becomes
possible to prevent defective plating on the through-hole wall
surface and obtain the electrically conductive portion for highly
reliable electrical connection. It is possible to achieve the
structure that does not include protruded portions (such as burrs)
at the edge of the through-hole opening and does not include
attachment of scattered fragments due to laser aberration.
EXAMPLES
Example 1
1. Shape of Through-Hole
[0052] A hole was formed by a pulsed laser in workpiece 1 including
base film 1b provided with sacrifice layer 1a. The thickness of
fluorocarbon resin of base film 1b was 150 .mu.m and the thickness
of fluorocarbon resin layer of sacrifice layer 1a was 30 .mu.m.
While base film 1b and sacrifice layer 1a were made of the same
material in this Example, they may be made of different materials
respectively as described above.
[0053] FIGS. 5 and 6 illustrate a process of forming a through-hole
of Example A of the present invention. FIG. 5 (a), (b) show a state
where through-hole 5 is formed by a pulsed laser in workpiece 1 in
which sacrifice layer 1a of fluorocarbon resin of 30 .mu.m
thickness is provided on base film 1b of fluorocarbon resin of 150
.mu.m thickness. FIG. 5 (a) shows an image of an SEM cross section,
and FIG. 5 (b) shows a schematic illustration thereof. According to
these diagrams, the portion of wall surface Wa where the diameter
expands as approaching the front surface is included in sacrifice
layer 1a of thickness t and is removed afterwards. FIG. 6 (a) shows
an SEM image of base film 1b or product 10 after sacrifice layer 1a
is removed, and FIG. 6 (b) shows a schematic illustration thereof.
Product 10 having the through-hole or base film 1b thus includes
the through-hole formed with straight-like wall surface Wb. While
the taper angle in the state of FIG. 5 (a), (b) was 5.1.degree.,
the taper angle is reduced to 3.0.degree. in the state of FIG. 6
(a), (b).
[0054] With product 10 including the straight-like through-hole, in
a process of forming an electrically conductive portion in a
thickness direction of a multilayer board whose wiring density has
been increasing recently, it is possible to prevent defective
plating on the through-hole wall surface and form the electrically
conductive portion for highly reliable electrical connection.
Further, it is also possible to provide the product not including
burrs and scattered fragments, as described in detail
hereinlater.
[0055] FIGS. 7 and 8 illustrate a process of forming a through-hole
of Example B of the present invention. Example B of the present
invention is basically the same as Example A of the present
invention. Specifically, FIGS. 7 (a) and (b) are each a diagram
showing a state where through-hole 5 is formed by means of a pulsed
laser in workpiece 1 in which sacrifice layer 1a of fluorocarbon
resin of 30 .mu.m thickness is provided on base film 1b of
fluorocarbon resin of 150 .mu.m thickness. FIG. 7 (a) is an SEM
cross-sectional image, and FIG. 7 (b) is a schematic illustration
thereof. According to FIG. 7 (a), the portion of wall surface Wa
where the diameter expands as approaching the front surface is
included in sacrifice layer 1a of thickness t and is removed after
the through-hole is formed. FIG. 8 (a) shows an SEM image of base
film 1b or product 10 having the through-hole after sacrifice layer
1a is removed, and FIG. 8 (b) is a schematic illustration thereof.
Base film 1b or product 10 having the through-hole is thus formed
by straight-like wall surface Wb. While the taper angle in the
state of FIG. 7 (a), (b) was 5.1.degree., the taper angle in the
state of FIG. 8 (a), (b) was 3.0.degree.. This product having the
through-hole provides the advantages as described above.
[0056] In contrast, in the conventional example as shown in FIG. 10
(a), (b), there is wall surface Wa having a large taper angle in
the portion corresponding to the periphery of the laser beam and
located on the front side of through-hole 105. Further, there is
curved portion Ws that is inwardly convex between straight-like
portions Wb and Wa and overlapping these portions. Wall surface Wa
having a large taper angle and corresponding to the periphery of
the laser beam as described above corresponds to the portion of the
wall surface where the hole is not further dug and the wall
portions overlap each other while the first to fourth shots are
applied as shown in FIG. 3.
[0057] Because of the presence of portion Wa having a large taper
angle, straight-like portion Wb and curved portion Ws overlapping
both of the other portions, defectiveness is caused in plating for
forming an electrically conductive portion and thus the reliability
of the wiring board is deteriorated. Examples A and B of the
present invention cause no defectiveness in plating since portion
Wa of a large taper angle is completely removed from the
through-hole as shown in FIG. 6 (a), (b) and FIG. 8 (a), (b).
2. Burrs and Scattered Fragments
[0058] FIGS. 9 (a) and (b) each illustrate the front surface of the
product having the through-hole of Example A of the present
invention (the front surface of FIG. 6 (a), (b)). In contrast,
FIGS. 11 (a) and (b) each illustrate the front surface of the
product having the through-hole in FIG. 10 (a), (b). In each of
FIGS. 11 and 9, (a) shows an SEM image of a through-hole cross
section, and (b) shows a schematic illustration thereof. As shown
in FIG. 11 (a), (b), burrs 125 are formed on the peripheral edge of
the opening at the front surface of through-hole 105, and scattered
fragments 126 are also attached thereon. In contrast, in Example A
of the present invention, there is no scattered fragments or burrs
on the front surface of base film 1b or product 10 having the
through-hole after the sacrifice layer is removed.
Example 2
Relation Between Taper Angle, Laser Diameter and Thickness of
Workpiece
[0059] In the case that a through-hole is formed with a sacrifice
layer provided, the taper angle is strongly influenced by laser
beam diameter .phi. and thickness d of the base film (subject
material) (although the taper angle even in the case of forming a
through-hole with no sacrifice layer is also influenced by
above-described .phi. and d, it is influenced in different
manners). FIG. 12 is a diagram showing a dependency of taper angle
.theta. (.degree.) on laser diameter .theta. (.mu.m), and it is
seen that the taper angle (.degree.) is proportional to the laser
diameter .phi. (.mu.m). FIG. 13 is a diagram showing a dependency
of taper angle .theta. (.degree.) on base film thickness d (.mu.m),
and it is seen that the taper angle (.degree.) is proportional to
the base film thickness {d (.mu.m)}.sup.-0.68.
[0060] Expression (1)=(.theta..times.d.sup.0.68)/.phi. is provided
here. Then, expression (1) can be regarded as the one indicating
the magnitude of taper angle .theta. considering base film
thickness d and laser diameter .phi. (i.e., corrected with base
film thickness d and laser diameter .phi.). Experimental data
corresponding to the plots in above-described FIGS. 12 and 13 are
summarized in Table 1 and Table 2. Table 1 shows the results
regarding the process of forming a through-hole in a base film
(workpiece or subject material) of PTFE (polytetrafluoroethylene),
and Table 2 shows the results regarding the process of forming a
through-hole in a base film of Ti.
TABLE-US-00001 TABLE 1 (workpiece: PTFE) sacrifice .phi.: laser
layer d: thickness beam .theta.: taper value of present/ of base
film diameter angle expression absent [.mu.m] [.mu.m] [.degree.]
(1) Invention's present: 180 20 0.5-2.5 0.85-4.27 Example 1 PTFE
Invention's present: 60 20 1.9-3.1 1.54-2.51 Example 2 PTFE
Invention's present: 120 20 1.4-1.8 1.82-2.33 Example 3 PTFE
Invention's present: 180 30 1.5-3.5 1.71-3.99 Example 4 PTFE
Invention's present: 180 50 3.5-6.0 2.39-4.10 Example 5 PTFE
Comparative absent 30 20 8-13 4.04-5.05 Example 1 Comparative
absent 120 20 3.6-5.7 4.67-7.40 Example 2 Comparative absent 180 20
2.5-3.7 4.27-6.32 Example 3 Comparative absent 240 20 2.2-2.6
4.57-5.40 Example 4 note 1: expression (1) = (.theta. .times.
d.sup.0.68)/.phi.
TABLE-US-00002 TABLE 2 (workpiece: Ti) sacrifice .phi.: laser layer
d: thickness beam .theta.: taper value of present/ of base film
diameter angle expression absent [.mu.m] [.mu.m] [.degree.] (1)
Invention's present: 20 28.5 5-21 1.35-5.65 Example 6 one Ti layer
Invention's present: 20 28.5 4-12 1.08-3.23 Example 7 two Ti layers
Comparative absent 20 26.7 21-55 6.03-15.80 Example 5 note 1:
expression (1) = (.theta. .times. d.sup.0.68)/.phi.
[0061] In each of Invention's Examples 1 to 5 shown in Table 1,
PTFE that is the same material as that of the base film is used for
the sacrifice layer for the following reasons. It is considered
that it is natural to use the sacrifice layer of the same material
as that of the base film to be processed. In addition, even if it
is attempted to select another material, it is difficult to find a
material having a higher ablation threshold value than the very
high ablation threshold value of the PTFE of the workpiece. In
Table 2, there are shown the case where one sacrifice layer of Ti
that is the same material as the material Ti of the workpiece is
provided (Invention's Example 6) and the case where two sacrifice
layers are provided (Invention's Example 7). The fluorocarbon resin
has an ablation threshold value of 0.44 J/cm.sup.2 and Ti has an
ablation threshold value of 0.05 J/cm.sup.2.
[0062] Referring to the values of expression (1) in Table 1 and
Table 2, in the case that the value of expression (1) is not more
than 4.0, taper angle .theta. itself is small, and thus it can be
said that the processed product has a taperless through-hole or
straight-like through-hole. In some embodiments of the present
invention, the value of not more than 4.0 of expression (1) is
derived from FIGS. 12 and 13 and the data in Table 1 and Table
2.
Example 3
Formation of Through-Hole in Ti Film
[0063] In Example 3 of the present invention, laser processing was
performed to provide a through-hole in a Ti base film of 20 .mu.m
thickness. FIGS. 14 and 15 show respective structures each
including sacrifice layer 1a and base film 1b in Example C and
Example D of the present invention. In Example C of the present
invention, PTFE of 60 .mu.m thickness was used for sacrifice layer
1a. In Example D thereof, a combination of Ti of 5 .mu.m thickness
and PTFE of 60 .mu.m thickness was used for sacrifice layer 1a. In
consideration of the fact that the sacrifice layer of a porous PTFE
causes scattering and transmission of incident light and thus the
effect of the sacrifice layer is lessened, Ti (5 .mu.m) was used as
a shield for preventing transmission of the light. Then, such a Ti
base film with no sacrifice layer as shown in FIG. 16 is used as a
comparative example. Table 3 and Table 4 show laser parameters and
the like for Examples C and D of the present invention and the
comparative example.
TABLE-US-00003 TABLE 3 Laser Parameters and Material (Invention's
Examples C, D) laser wavelength 800 nm laser repetition frequency
10 Hz laser pulse width 120 fs laser energy 54-0.35 .mu.J workpiece
(base film) Ti (99.5%), thickness: 20 .mu.m, purchased from Nilaco
Corporation (model number: Ti-453212)
TABLE-US-00004 TABLE 4 Laser Parameters and Material (Comparative
Example) laser wavelength 800 nm laser repetition frequency 10 Hz
laser pulse width 160 fs laser energy 31-2.65 .mu.J workpiece (base
film) Ti (99.5%), thickness: 20 .mu.m, purchased from Nilaco
Corporation (model number: Ti-453212)
[0064] Table 5 shows the result for the through-hole in Example C
of the present invention in the case that the laser energy is 30
.mu.J or 10 .mu.J. Table 6 shows the result for the through-hole in
Example D of the present invention in the case that the laser
energy is 15 .mu.J, 10 .mu.J or 8 .mu.J. It is seen from Table 5
and Table 6 that there is a tendency that the taper angle is
smaller as the laser energy is smaller, and the effect of providing
two layers (Ti/PTFE) as the sacrifice layer can be confirmed to
some degree.
TABLE-US-00005 TABLE 5 (Invention's Example C) sacrifice layer
energy (.mu.J) 30 10 PTFE process diameter (.mu.m) 37-49 22-26
(PTFE front surface) taper angle (.degree.) 3-9 2-5 workpiece
energy (.mu.J) 30 10 Ti process diameter (.mu.m) 19-25 15-19 (20
.mu.m, Ti front surface) process diameter (.mu.m) 15-20 10-12 (20
.mu.m, Ti rear surface) taper angle (.degree.) 6-21 5-11
TABLE-US-00006 TABLE 6 (Invention's Example D) sacrifice energy
(.mu.J) 15 10 8 layer process diameter (.mu.m) 29-34 22-26 20-22
PTFE (PTFE front surface) taper angle (.degree.) -- -- -- workpiece
energy (.mu.J) 15 10 8 Ti process diameter (.mu.m) 16-20 9-15 7-10
(20 .mu.m, Ti front surface) process diameter (um) 10-13 4-7 4 (20
.mu.m, Ti rear surface) taper angle (.degree.) 7-11 5-12 4-8
TABLE-US-00007 TABLE 7 (Comparative Example) energy (.mu.J) 31 23
process diameter (.mu.m) 80 (max)-77 (min) 44-42 (20 .mu.m, Ti
front surface) process diameter (.mu.m) 24-19 26-21 (20 .mu.m, Ti
rear surface) taper angle (.degree.) 55-53 29-23
TABLE-US-00008 TABLE 8 (Comparative Example) energy (.mu.J) 10 5
2.65 process diameter (.mu.m) 40-38 36-34 34-33 (20 .mu.m, Ti front
surface) process diameter (.mu.m) 21-19 19-15 18-15 (20 .mu.m, Ti
rear surface) taper angle (.degree.) 22-26 26-22 24-21
[0065] On the other hand, Table 7 and Table 8 show the results for
the through-holes in the comparative examples. For example, when
respective results concerning the laser energy of 10 .mu.J are
compared with each other, it is seen that the taper angles of
Examples C and D of the present invention are particularly reduced
and improved to be as small as 1/2 to 1/4 compared with those of
the comparative examples. When respective results concerning the
laser energy of approximately 30 .mu.J are compared with each
other, the taper angle of Example C of the present invention is 6
to 21.degree. as shown in Table 5, while the taper angle of the
comparative example of Table 7 is 55 to 53.degree.. Thus, it is
possible to confirm the remarkable effect of improvement in Example
C of the present invention. Further, the taper angle in Table 8 is
26 to 22.degree. for the laser energy of 10 .mu.J to 5 .mu.J. In
Example D in Table 6, the taper angle is 8 to 4.degree. for the
laser energy of 8 P. Therefore, it can be said that dramatic
improvements are achieved in forming a taperless through-hole in
any of Examples of the present invention.
[0066] FIGS. 17 and 18 show respective process diameters at the
front and rear surfaces of the base film (Ti) regarding the laser
energy of 15 .mu.J in Example D of the present invention. FIGS. 19
and 20 show respective process diameters at the front and rear
surface of the base film (Ti) regarding the laser energy of 5 .mu.J
in the comparative example. As seen from Example 1 as well, in the
case that the sacrifice layer is not used, burrs are formed and
scattered fragments are also found near the peripheral edge of the
hole at the front surface of the base film. In terms of the
circularity of hole as well, it is seen that the comparative
examples are inferior to Example D of the present invention.
[0067] While embodiments and examples of the present invention have
been explained above, the embodiments and examples of the present
invention disclosed above are provided merely by way of
illustration and example, and the scope of the present invention is
not limited to these embodiments of the invention. The present
invention includes, in its technical scope, all of laser processing
methods according to which a portion influenced by the inclined
surface generated in the process of applying initial pulsed laser
shots still remains in the subsequent process of applying shots,
and is included in the sacrifice layer even to a smaller extent.
The scope of the present invention is defined by claims and
includes all modifications in meaning equivalent to and in the
scope of the claims.
INDUSTRIAL APPLICABILITY
[0068] The present invention can provide a product having a
straight-like through-hole and a laser processing method that can
reduce a taper angle in the through-hole and avoid burr and
attachment of scattered fragments on the product. Therefore, the
invention can prevent defective plating on a through-hole wall
surface in a multilayer board and contribute to supply of highly
reliable multilayer board or the like.
* * * * *