JP2000202664A - Lasder drilling method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser drilling method that dispenses with a complicated post-processing such as a wet type post-processing. SOLUTION: In the hole formed on a workpiece 17 by the irradiation of a laser beam from a first laser generator 11, a second pulsed laser is used which has a pulse width of 50 (nsec) or less from a second laser generator 12, removing machining residue by emitting a laser beam, whose diameter is larger than that of the hole, at an irradiation energy density of 100 (mJ/cm2) or above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a drilling method for drilling a resin layer such as a printed circuit board using a laser beam.

[0002]

2. Description of the Related Art In response to the demand for higher density of printed circuit boards in accordance with the miniaturization and high-density mounting of electronic devices, recently, multilayer printed circuit boards in which a plurality of printed circuit boards are stacked have appeared. In such a multilayer printed circuit board, it is necessary to electrically connect conductive layers (typically, copper patterns) between printed circuit boards stacked one above another. Such a connection forms a hole called a via hole that reaches the underlying conductive layer in the insulating resin layer (polymer such as polyimide or epoxy resin) of the printed circuit board,
This is realized by plating the inside of the hole.

Recently, laser light has begun to be used for such drilling. A laser drilling apparatus using a laser beam is superior to a mechanical processing using a mechanical fine drill in that the processing speed and the diameter of a hole can be reduced. As a laser beam, a CO ₂ laser or YAG (yttrium aluminum garnet) is used because the price and running cost of a laser oscillator are low.
Lasers are commonly used.

[0004]

However, a drilling method using a CO ₂ laser or a YAG laser is inexpensive and can be processed at high speed, but it is intended to expose the bottom of the hole formed in the resin layer, that is, to expose the hole. A part of or the entire surface of the conductive layer to be formed (thickness 1 in epoxy resin and polyimide)
There is a problem that a processing residue remains. This processing residue cannot be completely removed even if the same laser beam is further irradiated thereafter. This is because even if laser light is further applied to evaporate the processing residue, the surrounding polymer elutes at this time (the conductor layer is often made of copper and heat diffusion is fast), so new processing is performed. Probably because of the formation of remnants.

[0005] Therefore, these laser drilling methods have a problem that a wet post-treatment with a strong oxidizing agent (for example, potassium dichromate) or the like is required after laser processing. This post-processing is called desmear.

However, the wet post-treatment with the chemical may damage the resin layer constituting the printed circuit board. In addition, in this wet post-treatment method, when performing the treatment, the concentration of the chemical decreases due to the mixing of the removed processing residues, so when the concentration falls below a certain concentration, the chemical is replaced or replaced. It has to be regenerated, causing cost and waste disposal problems.

An object of the present invention is to provide a laser drilling method which does not require a complicated post-processing step such as a wet post-processing.

[0008]

According to a first aspect of the present invention, there is provided a laser drilling method for irradiating a resin layer on a metal film with a first laser beam to form a hole.
A pulse width of 50 is applied to the hole formed by the laser light irradiation.
(Nsec) There is provided a laser drilling method characterized by removing a processing residue by irradiating a second pulse laser described below.

According to a second aspect of the present invention, there is provided a laser drilling method for irradiating a resin layer on a metal film with a first laser beam to form a hole. A laser drilling method, comprising irradiating the formed hole with a second pulse laser having a pulse width of 50 (nsec) or less to remove processing residues. The present invention provides a laser drilling method for forming a hole by irradiating a resin layer on a metal film with a first laser beam, wherein the hole formed by the irradiation of the first laser beam has a pulse width of 50 ( nse
c) Laser drilling characterized by irradiating a laser beam having an irradiation energy density of 100 (mJ / cm ² ) or more and a diameter larger than the diameter of the hole by using the following second pulse laser to remove processing residues. A method is provided.

In the first embodiment, the hole may be irradiated with the second pulse laser via an optical system for making the energy density distribution uniform.

In the first embodiment, the hole may be irradiated with the second pulse laser via an optical system that changes the energy density distribution into a pseudo Gaussian distribution.

In the first embodiment, the plurality of holes can be simultaneously irradiated with the beam diameter of the second pulse laser so as to cover a plurality of adjacent holes.

[0013]

Embodiments of the present invention will be described below. The present invention is applied to a hole formed by laser processing, and performs desmearing using a laser. Hereinafter, this is called dry desmear. The present inventors considered the energy per area (fluence) and the irradiation method for dry desmear.

First, a description will be given of a first test result concerning energy required for performing dry desmear. The materials used in this test are epoxy (including glass epoxy) and polyimide, and are the most common materials used for printed circuit boards.
The laser used is a second harmonic pulse laser manufactured by New Wave Research, Quantronix, Quantel, and Spectra-Physi.
It is a pulse laser manufactured by cs. The specifications of these pulsed lasers are as shown in Table 1 below.

[0015]

[Table 1]

In Table 1, the required number of shots indicates the number of shots of the pulse laser required to remove the processing residue. From the results shown in Table 1, it is apparent that this is useful as one factor for performing dry desmear with the number of shots in which the pulse laser width is small.

Next, RCC (Reinforced l)
Table 2 shows the results of a second test on an epoxy-based material used for a reinforced type circuit board called an "amplified Copper Circuit".

[0018]

[Table 2]

The laser used is New Wave Rese.
Arch is a second harmonic pulse laser. As a factor required for dry desmear, the pulse width of a pulse laser becomes an issue for an epoxy-based material called RCC, which is different from the general epoxy-based and polyimide-based materials used in the first test. The fluence as the test condition is about 260 (mJ / cm ² ) (1 mJ / pulss).
e) was unified. The quality of the desmear is determined by the pulse width,
Irradiation fluence (energy per unit area) is an important factor. That is, the energy required for drilling has a certain threshold value, and has a limit value such that when the irradiation is performed with an energy less than a certain value, no processing is performed at all. In the material called RCC, it is about 100 (mJ). In actual processing, in addition to this value, it is determined in consideration of the degree of safety for laser output stability and workability.

In any of the first and second tests, if the irradiation energy is too high, not only the processing residue on the bottom surface of the hole of the material to be processed, but also the surrounding area is damaged by the laser beam. In order to remove the processing residue on the bottom of the hole, the beam diameter of the pulse laser is actually made larger than the size of the hole for irradiation. In addition, it is preferable to irradiate a slightly larger pulse laser beam in consideration of positioning accuracy at the hole position and variation in the hole size.

From the results of Tables 1 and 2, it was clarified that the processing characteristics differed greatly depending on the pulse width of the pulse laser. However, when the pulse width was long, the rate of heat conduction to the periphery was high. The degree of thermal damage to the surroundings increases.

Regarding the difference in workability depending on the material to be processed, in addition to the above-mentioned RCC, a typical example is an epoxy resin (with an adhesive) as a typical example.
In a test using a pulse laser manufactured by Rch or Quantel, the fluence that can be removed by one pulse is 7
00 (mJ / cm ² ).

In some cases, a peak value (a value obtained by dividing energy by a pulse width) is evaluated as a characteristic of a laser.
In actual processing, it is important how much energy density is applied to the material to be processed, and the peak value is a meaningless value.

Therefore, an important factor in evaluating dry desmear is, in particular, pulse width and then energy density. The actual irradiation conditions in the above-mentioned test were such that an image forming optical system for temporarily blocking light from each laser with a mask and transferring an image of the mask to a processing surface was used. The used mask is φ3.5
(Mm), using a lens with a focal length f = 100 (mm), and reducing the beam size on the processed surface to φ0.7 at a reduction ratio of 5.
(Mm). The energy density of the processed surface was adjusted with an attenuator (attenuation plate) installed before the mask.

In Table 1, Quant was used to remove machining residues on the bottom surface of the hole generated by laser drilling.
ronix Nd: YAG second harmonic pulse laser (beam size φ0.5 mm, pulse width about 200 ns
Using c), a fluence of about 800 (mJ / cm ² ) is applied to a processed conformal substrate having a hole diameter of about 150 (μm).
Was irradiated, the number of shots required to completely remove the processing residue was about 700 shots.

On the other hand, New Wave Response
Using a Nd: YAG second harmonic laser (beam size φ0.35 mm, pulse width of about 7 nsec) manufactured by Arch Inc. with a fluence of 650 (mJ / cm ² ), 1 to 2 shots were obtained. Removal was possible with a small number of shots.

In Table 2, a hole (approximately 12 mm in diameter) was formed using a different material (RCC) from the case of Table 1 by using a laser.
0 μm), and the above New Wave having a pulse width of 150 (nsec) is used.
Using a Nd: YAG second harmonic pulse laser manufactured by Research, the fluence was about 260 (mJ / c).
m ² ), and the beam size was φ0.35 (mm). Processing required 8 shots to remove the processing residue on the bottom of the hole, and the resin layer on the surface was discolored or damaged.

However, using the above-mentioned pulse laser having a pulse width of 5 (nsec), the fluence is about 260 (mJ / c).
When the same processing was performed at m ² ), processing residues were removed in one shot, and no particular damage or discoloration was observed on the surface resin layer.

In addition, depending on the material to be processed, when a processing residue is removed by using a pulse laser having a pulse width of 200 (nsec), a peripheral resin layer melts out.
Conversely, a phenomenon was observed in which it adhered to the bottom of the hole and the processing residue could not be removed.

According to the test results, it is possible to remove processing residues even with a pulse laser having a long pulse width.
A large amount of irradiation from 0 to 800 shots is required. On the other hand, by using a pulse laser having a short pulse width, the number of shots can be reduced from one to two with the same energy density (fluence) without damaging the hole bottom surface and the resin layer surface or the conductive layer surface due to copper. It was confirmed that desmear was possible with a small number of shots.

In view of the above, preferably, the pulse width is 1
-50 (nsec), irradiation energy density 100 (mJ)
/ Cm ² ) to 1 (J / cm ² ) using a pulsed laser.
In addition, by irradiating the laser beam with the diameter slightly larger than the diameter of the hole, it is possible to obtain a good effect of removing processing residues with a small number of shots.

On the other hand, a pulse laser irradiation method includes a method of irradiating a laser beam emitted from a laser oscillator onto a processing surface as it is, and a method of beam shaping, that is, the energy of a laser beam through an optical element called a homogenizing element or a homogenizer. There is a method of irradiating an object to be processed after making the density distribution uniform. As an example of the homogenizing element, there is known a kaleidoscope for making a laser beam pass through a rectangular inner mirror to make the laser beam uniform (for example, Japanese Patent Application No. Hei 9-138165). Further, a cone type lens that can obtain a pseudo Gaussian profile through a cone (conical) lens is also effective. Further, a biprism method, a fiber transmission method, a diffractive optics method, or the like may be employed.

In the case where a pulse laser having a large energy is used in adopting the above method, the method of shielding a part of the laser beam with the mask as described above wastes the energy of the laser beam. Therefore, it is not practical, and it is desirable to effectively use the energy of the laser beam. This is important from the following viewpoints.

With the recent progress of downsizing of electronic devices and printed wiring boards, high density and high multilayer structure are being rapidly promoted. In this sense, a plurality of holes that have been laser-processed at high density can be simultaneously irradiated to improve the processing speed. This can be easily realized by making the laser beam have a beam diameter capable of covering a plurality of holes without being confined by means such as a mask.

Next, regarding the wavelength of the laser, Table 1
In this test, 532 (nm) and 1064
(Nm) laser light is used. But what matters is not the wavelength dependence. Therefore, regardless of the wavelength of the laser light, the present invention can be applied to YAG 4ω, 5ω, picosecond or femtosecond laser, excimer, YAG fundamental wave 3ω, and other lasers.

As an application field of the present invention, the present invention is also effective for peeling a thin metal film or an organic film such as paint applied to the surface of a material to be processed. Further, it is possible to remove dust attached to the surface of the material to be processed, for example, to perform processing in a vacuum chamber, and to perform cleaning by applying a laser beam on the spot without breaking vacuum.

Next, a schematic configuration of a laser drilling apparatus to which the present invention is applied will be described with reference to FIG.
The laser drilling apparatus includes first and second laser oscillators 11 and 12, and first and second biaxial scan mirrors 13 corresponding to the first and second laser oscillators 11 and 12, respectively. 14 and the first and second processed lenses 1
XY stage 1 on which workpieces 5 and 16 are placed
Eight. The first laser oscillating unit 11, the first biaxial scan mirror 13, and the first processing lens 15 are for performing drilling, and the second laser oscillating unit 12, the second biaxial scanning The mirror 14 and the second processing lens 16 are for performing dry desmear. If the beam diameter is sufficiently large, no scan mirror is required.

The first laser oscillating section 11 includes a carbon dioxide gas laser (TEA: CO ₂ gas laser), a Nd; YAG laser, or a Nd; YLF (lithium yttrium
A laser oscillator such as a (fluoride) laser is used.

Also, as the second laser oscillation section 12,
A laser oscillator such as a CO ₂ gas laser, Nd; YAG laser, or Nd; YLF laser similar to the above can be used, but any pulse laser having the above pulse width and fluence may be used. Not restricted.

Further, as the first laser oscillating unit 11, CO
_{When a two-} gas laser is used, the first processing lens is made of ZnS, so that Nd; YAG or Nd;
YLF laser light can be simultaneously introduced. That is, the first processed lens can be shared without the second processed lens.

The two-axis scan mirror is realized by combining two mirrors that can rotate in different directions from each other, and converts the laser light from the first and second laser oscillation units 11 and 12 into X-axis light.
-Scan within the Y plane. The processing lens is designed such that the laser light is focused on and vertically incident on the workpiece 17 irrespective of the incident angle of the laser light incident from the biaxial scan mirror. Therefore, by combining the two-axis scan mirror and the processing lens, a hole is formed at a predetermined position on the workpiece 17 without moving the workpiece 17 using the XY stage 18, and further desmearing is performed. It can be carried out. Further, within the predetermined range, there is no need to move the workpiece 17 using the XY stage 18 even when forming a plurality of holes.

In the above-described apparatus, the two two-axis scan mirrors can be independently controlled, so that the laser beams from the first and second laser oscillators 11 and 12 can be applied to different positions. . Therefore, when a plurality of holes are formed in a predetermined range, the first laser oscillation unit 1
Forming a hole in the resin layer with the laser light from step 1;
And the step of removing the processing residue remaining on the bottom surface of the hole with the laser light from the laser oscillation unit 12 (for different holes). As described above, by irradiating a plurality of holes with a laser beam having a diameter that covers the holes, the processing time can be greatly reduced as compared with the case of sequentially processing.

In the above apparatus, the XY stage 1
8 is less necessary (when processing is performed over a wide range, the processing is performed for each predetermined range.
Although the movement by the stage 18 is necessary, it is not necessary at all within the predetermined range), so that high-speed processing can be performed. More specifically, the movement time interval of the workpiece by the XY stage is:
Although it is only about 10 Hz, the movement time interval of the focal position by the two-axis scan mirror can be realized at 100 Hz or more, and the movement time between the processing points of the laser beam is so short as to be insignificant.

[0044]

According to the present invention, the removal of processing residues generated on the bottom surface of a hole formed by laser drilling on a printed circuit board can be performed by a dry process without using a wet post-treatment with a conventional chemical. Can be easily performed,
Labor saving, cost reduction, and prevention of environmental pollution due to waste liquid can be achieved when forming holes.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a laser drilling apparatus to which the present invention is applied.

[Explanation of symbols]

 11, 12 Laser oscillation unit 13, 14 Biaxial scan mirror 15, 16 Processing lens 17 Workpiece 18 XY stage

Claims (5)

[Claims] 1. A laser drilling method for irradiating a resin layer on a metal film with a first laser beam to form a hole,
A laser drilling method, comprising: irradiating a hole formed by the irradiation of the first laser light with a second pulse laser having a pulse width of 50 (nsec) or less to remove processing residues. 2. A laser drilling method for irradiating a resin layer on a metal film with a first laser beam to form a hole,
A second pulse laser having a pulse width of 50 (nsec) or less is used for a hole formed by the irradiation of the first laser beam, and a laser having an irradiation energy density of 100 (mJ / cm ² ) or more and larger than the diameter of the hole. A laser drilling method characterized by removing a processing residue by irradiating a beam. 3. The laser drilling method according to claim 2, wherein the second pulse laser is applied to the hole via an optical system that makes the energy density distribution uniform. Method. 4. The laser drilling method according to claim 2, wherein the second pulse laser is irradiated to the hole via an optical system that changes the energy density distribution to a pseudo-Gaussian distribution. Drilling method. 5. The laser drilling method according to claim 2, wherein a beam diameter of the second pulse laser is set to a size to cover a plurality of adjacent holes. A laser drilling method characterized by simultaneously irradiating a laser beam.