JP4040783B2 - Blind hole forming method for printed circuit board - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming a blind hole in a printed circuit board when a blind hole for connecting an outer layer copper foil and an inner layer copper foil is processed by a laser.
[0002]
[Prior art]
In a multilayer printed circuit board, plating is performed after forming a hole (hereinafter referred to as a blind hole) from the outer layer copper foil (upper layer copper foil) to the inner layer copper foil (lower layer copper foil), and the outer layer copper foil is passed through the plating layer. And connect the inner layer copper foil. In order to securely connect the outer layer copper foil and the inner layer copper foil, it is necessary to sufficiently put the plating solution into the blind hole. In addition, after forming the plating layer, it is necessary to reliably fill the interior of the blind hole with resin. For this reason, it is required to form the blind hole in a so-called mortar shape.
[0003]
Conventionally, a portion of the outer layer copper foil in which the blind hole is to be processed is previously removed by etching to form a window, and a laser (for example, a CO ₂ laser) is irradiated through the window to thereby form an inner layer copper. A conformal mask method has been adopted in which an insulator up to the foil, that is, a resin such as epoxy or polyimide is removed. However, in order to form a window in the outer layer copper foil, it is not only necessary to produce a photomask, but also a lot of resist application, photomask alignment, exposure, development, baking, etching, cleaning, etc. A processing step is required and is expensive.
[0004]
Therefore, according to Japanese Patent Application No. 10-325167, the applicant of the present application forms irregularities on the surface of the outer layer copper foil in advance, and irradiates the laser light of the CO ₂ laser from above the outer layer copper foil to form the blind hole. Proposed to process. In this method, after laser processing, the thickness of the outer layer copper foil is reduced by etching, and then the resin surface inside the blind hole is finished flat by desmearing, and then a plating layer is formed.
[0005]
FIG. 6 is a cross-sectional view of a blind hole when a printed board having a thick outer layer copper foil is drilled by the method proposed in Japanese Patent Application No. 10-325167. The blind hole 1 is composed of a hole 1 a opened in the outer layer copper foil 2 and a cavity 1 b extending from the hole 1 a of the resin 3 to the inner layer copper foil 4. The energy required for removing the outer layer copper foil 2 corresponding to the hole 1a is several times larger than the energy required for removing the resin 3 corresponding to the cavity 1b. For this reason, when the plate | board thickness of the outer layer copper foil 2 is thick, the resin 3 inside the outer layer copper foil 2 is rubbed, and the diameter of the cavity part 1b may become larger than the diameter of the hole 1a. Further, the bank 5 may be formed by the scattered copper adhering around the entrance of the hole 1a. Even in such a case, as shown in FIG. 7, the copper scattered matter formed around the entrance of the hole 1 a is removed and the diameter of the hole 1 a is widened by etching. Resin 3 is exposed on the surface. Then, the resin 3 exposed on the surface is removed together with the resin inside the blind hole 1 by a desmear process. And according to this method, since it is not necessary to form a window in the outer layer copper foil 2, the blind hole processing step can be greatly reduced.
[0006]
[Problems to be solved by the invention]
However, the desmear treatment not only removes the resin residue remaining in the blind hole 1 but also removes the resin surface by about 3 to 5 μm. For this reason, as shown in FIG. 8, the resin 3 on the back side portion of the outer layer copper foil 2 in the vicinity of the hole 1a is removed more than necessary, and the outer layer copper foil 2 may protrude into the upper part of the cavity portion 1b again in an eaves shape. It was. When plating was performed in this state, the shape of the blind hole after the formation of the plating layer 6 became a bottle shape squeezed at the entrance as shown in FIG.
[0007]
An object of the present invention is to solve the above-described problems in the prior art and provide a method for forming a blind hole in a printed circuit board that can make a cross-sectional shape of a blind hole formed by a laser into a mortar shape.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the invention of claim 1 is directed to a blind hole forming method for a printed circuit board in which a blind hole for connecting an outer layer copper foil and an inner layer copper foil is formed. Are formed in the order of the step of drilling with a laser, the step of desmearing the processed hole, the step of etching the outer layer copper foil and the inner layer copper foil, and in the step of etching, the hole formed in the outer layer copper foil The rigid copper foil peeling portion, the outer layer copper foil surface, and the inner layer copper foil surface are etched until the diameter reaches a hole diameter capable of removing the copper foil peeling portion .
[0009]
The invention of claim 2 is characterized in that, in claim 1, the laser for drilling is a CO ₂ laser.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described based on the illustrated embodiments.
1 to 5 are cross-sectional views of a printed circuit board at a portion where a blind hole is formed in each step according to the processing procedure of the blind hole forming method of the present invention. FIG. 1 shows a state before processing, and the outer layer copper foil 2 has a plate thickness Tt of 4 to 12 μm, the surface is roughened by roughening treatment, and the back surface is 1 to 7 μm due to deposited plating. The overall plate thickness TT is about 7-22 μm which is 2-10 μm thicker than the plate thickness Tt. The front and back surfaces of the inner layer copper foil 4 also have irregularities similar to those of the outer layer copper foil 2, and the overall plate thickness TB is about 7 to 25 μm, which is about 1 to 3 μm thicker than the plate thickness TT.
[0012]
FIG. 2 shows immediately after laser processing. Here, the outer layer copper foil 2 is removed by using the CO ₂ laser three times with the laser light, that is, the first irradiation energy is increased, and the resin 3 is applied with the irradiation energy being decreased in the second and third times. By removing, a blind hole is formed. In the case of illustration, since the first irradiation energy was large, the bank 5 was formed in the outer edge part of the hole 1a by adhesion of copper. In addition, since the irradiation energy was large in the second and third times, the maximum diameter DD of the cavity 1b was larger than the diameter DC of the hole 1a, and the resin 3 was layered on the back side portion of the outer copper foil 2 ( Hereinafter, the resin 3 having a thickness of about 1 μm or the residue 8 of the resin 3 remains on the surface of the inner layer copper foil 4. Further, when the thickness of the outer layer copper foil 2 is thick, if the laser energy is increased in order to remove the outer layer copper foil 2 in the hole 1a, the outer layer around the hole 1a is increased due to the local temperature rise of the outer layer copper foil 2. On the back surface of the copper foil 2, a copper foil peeling portion 7 having a length LH (about 1 to 3 μm) may occur.
[0013]
FIG. 3 shows the state after desmear processing. Resin 3 or residue 8 adhered to the surface of inner layer copper foil 4 shown in FIG. 2 is removed. Moreover, 3-5 micrometers of resin 3 surfaces of the cavity part 1b are removed with respect to the shape before desmear shown with a dashed-two dotted line. In addition, the layered resin 3a is removed, the outer diameter of the upper end of the hollow portion is expanded, and the outer edge portion 9 of the hole 1a of the outer layer copper foil 2 is projected in an eave shape by Lk (1 to 5 μm). Note that the thickness of the outer layer copper foil 2 and the inner layer copper foil 4 and the diameter of the hole 1a do not change before and after the desmear treatment.
[0014]
FIG. 4 shows the progress of the etching process, and FIG. 5 shows the state after the etching process. By the etching process, the dimensions of the respective parts are changed in the order of the dotted line, the one-dot chain line and the two-dot chain line in FIG. Finally, the surface of the outer layer copper foil 2 is removed until the finished plate thickness Tte is about 2 to 6 μm, which is about ½ of the original plate thickness Tt, and the bank 5 becomes almost flat. Further, the surface of the inner layer copper foil 4 is also removed at the same time, and the finished plate thickness TBe becomes about 3 to 9 μm. Normally, the depth at which the surface of the inner layer copper foil 4 is removed is shallower than the depth at which the surface of the outer layer copper foil 2 is removed. Further, the outer edge portion 9 is etched from the front surface, the side surface, and the back surface by the etching solution, and almost all of the outer edge portion 9 is removed. It is more effective to select the etching conditions such that the finished diameter DF of the outer layer copper foil 2 is equal to or larger than the finished diameter Df of the resin 3.
[0015]
FIG. 5 shows the state after the plating process, and the blind hole 1 after the plating is formed in a mortar shape.
[0016]
Next, an actual processing example will be described.
[0017]
When the thickness Tt of the outer layer copper foil 2 is 5 μm, the first CO ₂ laser is irradiated with one pulse with an energy density of 30 J / cm ² and a pulse width of 12 μs, and the second and third times with a pulse width of 3 μs. Each was irradiated with one pulse (ie, 1/4 of the first irradiation energy). After that, when the outer layer copper foil 2 was etched by 3 μm and the plate thickness Tte of the outer layer copper foil 2 was about 2 μm, not only the outer edge portion 9 could be removed, but also the lower side of the outer layer copper foil 2 The finished diameter DF is finished to about +2 to +4 μm with respect to the finished diameter Df of the resin 3, and the finished diameter on the upper side of the outer layer copper foil 2 is about +2 to +4 μm with respect to the finished diameter DF on the lower side of the outer layer copper foil 2. I was able to.
[0018]
Moreover, when the plate thickness Tt of the outer layer copper foil 2 is 8 μm, the copper foil is peeled off simultaneously with the bank 6 by increasing the etching amount and performing the etching process until the plate thickness Tte of the outer layer copper foil 2 becomes 2 to 3 μm. Part 7 could also be removed. That is, even if the peeling length LH between the outer layer copper foil 2 and the resin 3 is 3 μm, an etching solution enters from the peeled portion, and the finished diameter DF on the lower side of the outer layer copper foil 2 is etched by 4 to 6 μm. The portion peeled from the resin 3 of the outer layer copper foil 2 could be removed. As a result, it was possible to prevent the peeling between the outer layer copper foil 2 and the resin 3 after plating.
[0019]
As described above, according to the present invention, the blind hole can be processed by irradiating the laser light of the CO ₂ laser from the surface of the outer layer copper foil, and the shape of the blind hole can be made into a mortar shape. The quality of the hole, that is, the printed circuit board is improved, and the processing steps can be greatly reduced.
[0020]
In the above description, the case of the CO ₂ laser has been described. However, the present invention can also be applied to the case of processing with another excimer laser or UV laser.
[0021]
Moreover, although the unevenness | corrugation was provided in the surface of the outer layer copper foil, it is applicable also when the surface is flat.
[0022]
Furthermore, in the above embodiment, the thickness TB is 3 μm thicker than the thickness Tt in order to prevent the inner layer copper foil from becoming too thin by the etching process, but when the etching process can be reliably managed, The plate thickness TB and the plate thickness Tt may be the same.
[0023]
【The invention's effect】
As described above, according to the present invention, the cross-sectional shape of the blind hole formed by the laser can be formed in a mortar shape, and the quality of the printed circuit board can be improved.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a printed circuit board before processing according to the present invention.
FIG. 2 is a cross-sectional view of a printed circuit board immediately after laser processing according to the present invention.
FIG. 3 is a cross-sectional view of a printed circuit board after desmear processing according to the present invention.
FIG. 4 is a cross-sectional view of a printed circuit board showing the progress of an etching process according to the present invention.
FIG. 5 is a cross-sectional view of a printed circuit board after an etching process according to the present invention.
FIG. 6 is an explanatory diagram of a conventional technique.
FIG. 7 is an explanatory diagram of a conventional technique.
FIG. 8 is an explanatory diagram of the prior art.
FIG. 9 is an explanatory diagram of a prior art.
[Explanation of symbols]
1 Blind hole 2 Outer layer copper foil 3 Resin 4 Inner layer copper foil

Claims (2)

In the printed circuit board blind hole forming method for forming the blind hole connecting the outer layer copper foil and the inner layer copper foil,
A step of drilling an insulator to the outer layer copper foil and the inner layer copper foil with a laser;
A process of desmearing the machined holes,
The outer layer copper foil and the inner layer copper foil are formed in the order of the etching process. In the etching process, the copper foil is formed until the hole diameter formed in the outer layer copper foil becomes a hole diameter capable of removing the copper foil peeling portion. A method for forming a blind hole in a printed circuit board , comprising etching the peeling portion, the outer layer copper foil surface, and the inner layer copper foil surface . _{2. The} method for forming a blind hole in a printed circuit board according to claim 1, wherein the laser for drilling is a CO2 laser.

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