Patent Grant
8268182
This application is the U.S. National Phase under 35 U.S.C. §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.
The present invention relates to a product having a through-hole processed by a pulsed laser, and to a laser processing method.
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
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
It is preferable to carry out the processing so as to satisfy a relation: (θ×d0.68)/φ≦4.0 where φ (μm) denotes the diameter of laser beam, θ (°) denotes the taper angle of through-hole and d (μm) 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 φ 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.
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 (0×d0.68)/φ≦4.0 is satisfied where φ (μm) denotes the diameter of pulsed laser, θ (°) denotes the taper angle of through-hole and d (μm) 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.
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.
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.
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.
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.
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.
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.
Embodiments of the present invention will now be described with reference to the drawings.
(Principle of the Present Invention)
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
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.
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.
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).
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.
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.
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 θ of the through-hole is reduced to form a straight-like shape. Here, taper angle θ is defined as taper angle θ=Arctan {(0.5 Da−0.5 Db)/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 θ 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 θ (radian)=(0.5 Da−0.5 Db)/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.
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
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.
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 μm and the thickness of fluorocarbon resin layer of sacrifice layer 1a was 30 μm. 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.
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.
In contrast, in the conventional example as shown in
In the case that a through-hole is formed with a sacrifice layer provided, the taper angle is strongly influenced by laser beam diameter φ 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 φ and d, it is influenced in different manners).
Expression (1)=(θ×d0.68)/φ is provided here. Then, expression (1) can be regarded as the one indicating the magnitude of taper angle θ considering base film thickness d and laser diameter φ (i.e., corrected with base film thickness d and laser diameter φ). Experimental data corresponding to the plots in above-described
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/cm2 and Ti has an ablation threshold value of 0.05 J/cm2.
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 θ 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
In Example 3 of the present invention, laser processing was performed to provide a through-hole in a Ti base film of 20 μm thickness.
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 μJ or 10 μJ. 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 μJ, 10 μJ or 8 μJ. 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.
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 μJ 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 ½ to ¼ compared with those of the comparative examples. When respective results concerning the laser energy of approximately 30 μJ are compared with each other, the taper angle of Example C of the present invention is 6 to 21° as shown in Table 5, while the taper angle of the comparative example of Table 7 is 55 to 53°. 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° for the laser energy of 10 μJ to 5 μJ. In Example D in Table 6, the taper angle is 8 to 4° for the laser energy of 8 μJ. Therefore, it can be said that dramatic improvements are achieved in forming a taperless through-hole in any of Examples of the present invention.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-140967 | May 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/060148 | 5/17/2007 | WO | 00 | 11/20/2008 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2007/135955 | 11/29/2007 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5208604 | Watanabe et al. | May 1993 | A |
| 5208980 | Hayes | May 1993 | A |
| 5609746 | Farrar et al. | Mar 1997 | A |
| 5646657 | Aoki | Jul 1997 | A |
| 6063476 | Kinoshita | May 2000 | A |
| 6359255 | Yamamoto et al. | Mar 2002 | B1 |
| 20010008234 | Hasegawa et al. | Jul 2001 | A1 |
| 20050242059 | Brennen et al. | Nov 2005 | A1 |
| 20080189943 | Hirose et al. | Aug 2008 | A1 |
| Number | Date | Country |
|---|---|---|
| 0 706 309 | Apr 1996 | EP |
| 0 867 294 | Sep 1998 | EP |
| 0 968 824 | Jan 2000 | EP |
| 1-262087 | Oct 1989 | JP |
| 2-187345 | Jul 1990 | JP |
| 10-146981 | Jun 1998 | JP |
| 2000-077824 | Mar 2000 | JP |
| 2005-211962 | Aug 2005 | JP |
| WO 9959761 | Nov 1999 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20090151996 A1 | Jun 2009 | US |
