JP4160439B2 - Laser drilling method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a laser drilling method for a printed circuit board.
[0002]
[Prior art]
The mounting wiring density of printed circuit boards (hereinafter referred to as “printed circuit boards”) used for high-density mounting of semiconductor ICs in cellular phones and the like has increased rapidly year by year, and the hole diameter required for printed circuit boards is now less than 100 μmφ. It is coming.
[0003]
A laser processing method using a carbon dioxide laser has been adopted as a method for making such a small-diameter hole, but because the reflectance of the outer metal layer with respect to the wavelength of the carbon dioxide laser (9.3 to 10.6 μm) is high, A so-called conformal mask method is used in which a window is formed by partial removal by other methods (drilling, chemical etching, etc.) in advance, and the insulating resin layer is directly irradiated with a laser to process it.
[0004]
However, when opening a window in both metal layers of a substrate having a metal layer on the front side and the back side of the insulating resin layer and processing a through hole in the insulating resin layer by irradiating laser light from the front side, it is provided on the front side and the back side. There is a problem that the position of the window cannot be accurately matched, and even if the laser beam is irradiated as it is, an opening defect occurs as shown in FIG.
[0005]
3A is a cross-sectional view before irradiation with laser light, and FIG. 3B is a cross-sectional view after irradiation with laser light. 1 is a front side metal layer (copper foil), 2 is a back side metal layer (copper foil), 3 is an insulating resin layer, 4a is a front side window, and 4b is a back side window. Thus, if there is a displacement between the front and back windows 4a and 4b, as shown in (b), a part of the hole after laser irradiation, or in the worst case, the whole is covered with the back metal layer 2. As a result, the opening becomes defective.
[0006]
In order to improve such an opening defect due to the window misalignment, a method of eliminating the opening defect by making the front-side window into which the laser beam is incident wider than the back-side window is generally used (see, for example, Patent Document 1). ).
[Patent Document 1]
JP 2000-277911 (paragraph 29, FIG. 1)
[0007]
[Problems to be solved by the invention]
FIG. 2 shows an example in which the front side window is widened. (A) is a cross-sectional view before irradiating a laser beam, (b) is after irradiating the laser beam, and (c) is a cross-sectional view after the plating film is formed. 1 is a front side metal layer (copper foil), 2 is a back side metal layer (copper foil), 3 is an insulating resin layer, 4a is a front side window, 4b is a back side window, and 5 is a plating layer. As can be seen from (b) and (c), when the front window 4a is made wide in this way, the back metal layer 2 is exposed and the plating layer 5 can be widely contacted, which is advantageous for electrical conduction. The diameter of the hole is matched with the diameter of the wide front side window 4a and becomes wide. Accordingly, the wiring width of the printed circuit board and the diameter of the electrode pad must be designed in accordance with the diameter of the wide front side window 4a, which hinders improvement in mounting density.
[0008]
An object of the present invention is to solve the above-described problems of the prior art.
[0009]
[Means for Solving the Problems]
In a laser drilling method in which a window is opened in both metal layers of a substrate having a metal layer on the front side and the back side of the insulating resin layer, and a through hole is formed in the insulating resin layer by irradiating laser light from the front side, the metal layer on the back side It is preferable to use a laser drilling method characterized in that the aperture of the window to be opened is made larger by a manufacturing tolerance than the aperture of the front metal layer.
[0010]
In contrast to the conventional method, by widening the window on the back side, the hole diameter to be formed can be reduced, and the mounting density of the printed circuit board can be improved.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a laser drilling method according to the present invention. (A) is a cross-sectional view before irradiating a laser beam, (b) is after irradiating the laser beam, and (c) is a cross-sectional view after the plating film is formed. 1 is a front metal layer (copper foil, etc.), 2 is a back metal layer (copper foil, etc.), 3 is an insulating resin layer (epoxy resin with glass fiber, etc.), 4a is a front side window, 4b is a back side window, 5 is a plating layer It is.
[0012]
The general thickness of the printed circuit board is 18 μm for the copper foil and 100 μm for the insulating resin layer. As the laser light, carbon dioxide laser light (9.3 to 10.6 μm) was used, but it can be similarly applied to laser light having a wavelength such as visible light where the metal layer cannot be ablated.
[0013]
The diameter of the back side window 4b is set to be larger than the diameter of the front side window 4a by a deviation (manufacturing tolerance) of the center position of the front and back windows. In this example, since the manufacturing tolerance of the window is about 60 μm, the diameter of the back window 4b is set to 125 μm with respect to the diameter of the front window 4a of 60 μm. Thereby, even if the laser beam is irradiated from the front side, a through hole having a small diameter of the front side window 4 a is formed, and the hole is not blocked by the back side metal layer 2.
[0014]
Further, as can be seen from (c), when the plating layer 5 is formed in the subsequent plating step, a step corresponding to the thickness of the metal layer 2 occurs on the back side, but the processed hole diameter (= front side window) The diameter of 4a is processed almost as designed. This is because the hole diameter can be reduced, so that the wiring density of the printed circuit board can be improved.
[0015]
Here, when it is desired to reduce the level difference corresponding to the thickness of the metal layer 2 that can be formed on the back side, it is preferable to perform an etching process before the plating process and then perform the plating after making the metal layer slightly thinner.
[0016]
【The invention's effect】
In the laser drilling method according to the present invention, a window is formed in both side metal layers of a substrate having a metal layer on the front side and back side of the insulating resin layer, and a through hole is formed in the insulating resin layer by irradiating laser light from the front side. Since the hole diameter can be reduced, the mounting density of the printed circuit board can be improved.
[Brief description of the drawings]
FIG. 1 is a sectional view showing a laser drilling method according to the present invention.
FIG. 2 is a sectional view showing a conventional general laser drilling method.
FIG. 3 is a cross-sectional view showing a defective opening due to a positional shift between front and back windows.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Front side metal layer 2 ... Back side metal layer 3 ... Insulating resin layer 4a ... Front side window 4b ... Back side window 5 ... Plating layer

Claims (1)

In the laser drilling method in which a window is opened on both metal layers of a substrate having a metal layer on the front side and the back side of the insulating resin layer, and through holes are formed in the insulating resin layer by irradiating laser light from the front side, A laser drilling method characterized in that a diameter of a window to be opened is made larger by a manufacturing tolerance than a diameter to be opened in a front metal layer.

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