JP2021089926A - Laser processing method - Google Patents

Abstract

To suppress variations in hole shape to make a small-diameter hole in which the processing quality is ensured when a CO2 laser beam is used to make a hole in a printed wiring board in which a thin conductor is used as a surface layer and an insulating layer is laminated under the surface layer.SOLUTION: A laser processing method of irradiating a drilling position of a printed wiring board in which a conductor is used as a surface layer and an insulating layer is laminated under the surface layer with a CO2 laser beam includes a first step of irradiating the drilling position of the surface layer with a laser beam to form an oxide film of the conductor on the surface layer, a second step of irradiating the drilling position with the laser beam to remove the surface layer after the first step, and a third step of irradiating the drilling position with the laser beam to remove the insulating layer after the second step.SELECTED DRAWING: Figure 1

Description

The present invention relates to a laser processing method for drilling or the like in a workpiece such as a printed wiring board using laser light.

As a circuit formation method using a printed wiring board, there is a construction method called MSAP (Modified Semi Additive Process) that uses a thin copper foil with a thickness of 3 um or less on the surface layer and a printed wiring board on which an insulating layer is laminated. .. When drilling a hole with a small diameter of 30 um or less in such a printed wiring board using CO2 laser light, it is necessary to reduce the energy of the laser light.
However, the energy for decomposing the copper foil is insufficient with the laser beam with reduced energy, and melting and evaporation become unstable, causing variations in the copper foil opening diameter dimension of the hole and the hole shape such as a perfect circle, and processing. Quality cannot be ensured.

On the other hand, as disclosed in Patent Document 1, for example, the CO2 laser beam of the copper foil is subjected to a chemical surface treatment such as a blackening treatment in which the copper foil is oxidized using a chemical treatment liquid containing an oxidizing agent. It is possible to increase the absorption rate, but if the thickness of the copper foil is thin, there is a problem that the copper foil disappears, and this method cannot be applied.

Japanese Patent Application Laid-Open No. 2006-339259

Therefore, in the present invention, when a thin conductor is used as a surface layer and a CO2 laser beam is used to make a hole in a printed wiring board in which an insulating layer is laminated under the surface layer, variation in hole shape is suppressed to ensure processing quality. The purpose is to make a small diameter hole.

In order to solve the above problems, a typical laser processing method among the inventions disclosed in the present application irradiates a CO2 laser beam at a drilling position of a printed wiring board in which a conductor is used as a surface layer and an insulating layer is laminated under the conductor. In the laser processing method, the first step of irradiating the drilling position of the surface layer with the laser beam to form an oxide film of the conductor on the surface layer.
After the first step, the laser beam is irradiated to the drilling position to remove the surface layer, and after the second step, the laser beam is irradiated to the drilling position. It is characterized by having a third step of removing the insulating layer.

The typical features of the invention disclosed in the present application are as described above, but the features not described here are applied to the examples described below, and are also included in the claims. As shown.

According to the present invention, when a thin conductor is used as a surface layer and a CO2 laser beam is used to make a hole in a printed wiring board in which an insulating layer is laminated under the surface layer, variation in hole shape is suppressed to ensure processing quality. It is possible to make a small diameter hole.

It is sectional drawing of the printed wiring board processed by one Example of this invention. It is a figure for demonstrating the 1st step in one Example of this invention. It is a figure for demonstrating the 2nd step in one Example of this invention. It is a figure for demonstrating the 3rd step in one Example of this invention. It is the figure which compared the case by this Example and the case by a prior art about the copper foil opening diameter. It is the figure which compared the case by this Example and the case by a prior art about the standard deviation value of a copper foil opening diameter.

An embodiment will be described in the present invention.
FIG. 1 is a cross-sectional view of a printed wiring board processed according to this embodiment. This printed wiring board is an example of the printed wiring board used in the MSAP method.
The printed wiring board 1 has a structure in which a copper foil 2 as a front conductor, a copper foil 3 as a back conductor, an insulating layer 4 made of an epoxy resin containing glass fiber and the like, and a copper foil 5 as an inner conductor are laminated. ..
In the general MSAP method, the thickness of the copper foils 2 and 3 is 1.5 to 2 μm, and the thickness of the insulating layer 4 is 20 to 25 μm.

In this embodiment, the printed wiring board 1 is irradiated with the laser beam 6 to make a blind hole. The laser beam 6 here is a CO2 laser having a wavelength of 9.3 to 10.6 μm. 2 to 4 are views showing a drilling process.
First, as the first step, the laser beam 6 is irradiated to the range of the diameter D1 suitable for opening the target processing diameter at the drilling position of the printed wiring board 1. The state after performing this first step is shown in FIG.
Here, the surface temperature of the copper foil 1 is about 400 ° C. to 500 ° C., and the energy of the laser beam 6 is applied so that the copper oxide film 7 made _{of Cu 2 O is formed on the surface without melting the copper foil 2.} It will be adjusted.

Next, as a second step, the laser beam 6 is irradiated again to the range of the diameter D2, which is the target processing diameter of the drilling position of the printed wiring board 1, and the copper foil 2 at this portion is removed. The state after performing this second step is shown in FIG.
In the previous stage, since the copper oxide film 7 has already been formed on the surface of the copper foil 2, the absorption rate of the CO2 laser can be increased. Here, at the portion where the copper oxide film 7 of the copper foil 2 is formed, The central part of the hole has the evaporation temperature of the copper foil of 2567 ° C or higher so that the copper foil 2 has a hole with a predetermined processing shape, and the outer peripheral part of the hole has the melting temperature of the copper foil of 1084.4 ° C or higher and less than 2567 ° C. The energy of the laser beam 6 is adjusted so as to be.
Normally, the upper part of the insulating layer 4 under the copper foil 2 is also removed to form the opening hole 8. The diameter D2 is smaller than the diameter D1.

Next, as a third step, the position of the opening hole 8 is irradiated with the laser beam 6 again to remove the insulating layer 4 at this portion.
In the previous stage, a hole has already been made in the upper part of the copper foil 2 and the insulating layer 4 below it, and here, the insulating layer 4 on the copper foil 5 which is the inner metal layer reaches the evaporation temperature. The energy of the laser beam 6 is adjusted so that the insulating layer 4 can be removed.
The state after performing this third step is shown in FIG. 4, and a blind hole 9 is formed at the drilling position. Note that d1 is the final opening diameter of the copper foil 2, d2 is the hole bottom diameter, and normally, the hole bottom diameter d2 is smaller than the opening diameter d1.

The above processing step is performed using, for example, a laser processing apparatus as disclosed in FIG. 1 of JP-A-2017-51987.
The laser processing device used here is a crystal such as germanium that oscillates a laser beam to a laser oscillator based on the completion of a positioning operation by a galvano scanner for changing the laser irradiation position to generate a diffraction grating by ultrasonic waves. The AOM control unit using an acousto-optic element (hereinafter abbreviated as AOM) deflects the laser beam output from the laser oscillator in the machining direction or the non-machining direction, and the laser beam deflected in the machining direction is placed on the table. It irradiates the printed wiring board.

In the first to third steps, the energy of the laser beam 6 is adjusted, and this adjustment can be performed by the AOM based on the control information registered in the AOM control unit.
When the above steps 1 to 3 are applied to a plurality of drilling positions of the printed wiring board 1, for example, the first step, the second step, and then the third step are performed in order for all the drilling positions. You can do it in.

FIG. 5 is a diagram showing a comparison between the case of the present embodiment and the case of the conventional technique of sequentially irradiating the copper foil and the insulating layer with laser light without forming the oxide film 7 in advance with respect to the above-mentioned opening diameter d1.
Further, FIG. 6 is a diagram showing a comparison of the standard deviation value (variation of the hole diameter) of the opening diameter d1 between the case according to the present embodiment and the case according to the conventional technique.

According to the prior art, the heat diffusion generated by the laser beam absorbed by the copper foil 2 tends to be large, and the molten portion of the copper foil 2 at the outer peripheral portion of the hole tends to be large. This melted portion is blown off by the ejection of the vaporized decomposition gas of the insulating layer 4 in the subsequent step of irradiating the insulating layer 4 with the laser beam. Therefore, if the range of the molten portion is large, the copper foil opening diameter d1 becomes large, and the variation in the copper foil opening diameter dimension and the hole shape such as a perfect circle becomes large.
In the case of this embodiment, by forming the oxide film 7 on the surface of the copper foil 2 at the drilling position in advance, the absorption of heat generated by the laser beam is selectively promoted in the vertical direction, so that the outer peripheral portion of the hole is formed. The expansion of the molten portion of the copper foil 2 can be suppressed, the copper foil opening diameter d1 can be reduced, and the variation in the hole shape can be reduced.

In this embodiment, as described above, the drilling can be performed only by laser irradiation without performing the troublesome chemical treatment, and the copper foil 2 is oxidized without using the chemical treatment liquid, so that copper is used. Even if the foil 2 is thin, it is possible to make a hole having a small diameter while suppressing variation in the hole shape and ensuring processing quality.

Although the present invention has been specifically described above based on the embodiments, it goes without saying that the present invention is not limited to the above-described embodiments and can be variously modified without departing from the gist thereof. , Various variants are included.
For example, in the above examples, copper foil is used as the front conductor layer, the back conductor layer, and the inner conductor layer of the printed wiring board 1, but other conductors may be used.

1: Printed wiring board 2, 3, 5: Copper foil 4: Insulation layer 6: Laser beam 7: Oxidation film
8: Opening hole 9: Blind hole

Claims (4)

In a laser processing method in which a conductor is used as a surface layer and a CO2 laser beam is applied to a hole in a printed wiring board on which an insulating layer is laminated.
The first step of irradiating the drilling position of the surface layer with the laser beam to form an oxide film of the conductor on the surface layer.
After the first step, a second step of irradiating the drilling position with the laser beam to remove the surface layer,
A laser processing method comprising, after the second step, a third step of irradiating the drilling position with the laser beam to remove the insulating layer. The laser processing method according to claim 1, wherein the conductor is a copper foil and the oxide film is a copper oxide film made _{of Cu 2 O.} The laser processing method according to claim 1 or 2, wherein the printed wiring board is used in a circuit forming method by MSAP (Modified Semi Additive Process). In the laser processing method according to any one of claims 1 to 3, when there are a plurality of drilling positions of the printed wiring board, the first step, then the second step, and then the next step are performed for all the drilling positions. A laser processing method characterized in that the third step is performed in order.

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