HK1060085B - Laser drilling method - Google Patents

Description

Laser drilling method

Technical Field

The present invention relates to a laser drilling method for drilling a resin layer constituting a printed circuit board with a laser beam.

Background

With the miniaturization and high-density mounting of electronic devices, the density of circuits on printed circuit boards is increasingly required. In response to the above-described demand, in recent years, a multilayer printed circuit board formed by laminating a plurality of printed circuit boards has been provided. Among the above-described multilayer printed circuit boards, the conductive layers need to be electrically connected to each other between the printed circuit boards stacked one on top of another. The conductive layer is typically implemented using copper patterns (patterns). The connection as described above is realized by forming a hole called a via hole reaching the lower conductive layer on the insulating resin layer of the printed circuit board and plating the inside of the formed hole. In general, a polymer material such as polyimide or epoxy resin is used for the insulating resin layer.

Recently, the use of a pulse-like laser beam has been started in the drilling process as described above. A laser drilling apparatus using a laser beam is advantageous in terms of processing speed, reduction in hole diameter, and the like, as compared with machining using a mechanical micro drill. The laser beam generating device generally employs an excimer laser (excimer laser) or a carbon dioxide laser (CO)₂laser), and yttrium aluminum garnet laser yag (yttrium aluminum garnet) and the like.

As for excimer lasers, generation devices using ultraviolet laser beams are the mainstream. The oscillation wavelength of the excimer laser can be changed by changing the gas type to KrF, ArF, XeF, XeCl, or the like. However, the above-mentioned lasers are all gas lasers, and the oscillation time, i.e., the pulse width, is generally about 10 to 20(nsec), and is very short. Therefore, when the processing target object is irradiated with the pulse-shaped laser beam, the energy of the laser beam is absorbed in the vicinity of the surface of the processing target object. This means that, for some thick processing object, the excimer laser has a lower drilling capability than a carbon dioxide laser or a YAG laser that uses thermal processing.

Therefore, compared with a carbon dioxide laser, a Nd: YAG (neodymium-doped yttrium aluminum garnet) laser, an excimer laser, and a Nd: YLF (neodymium-doped yttrium lithium fluoride) laser, a Nd: YAG (neodymium-doped yttrium aluminum garnet) laser, a Nd: YVO₄The processing speed per 1 pulse of the 3-, 4-and 5-fold waves (third, fourth and fifth harmonics) of the (neodymium-doped yttrium vanadate) laser becomes low. Therefore, such an ultraviolet laser using a laser beam generator has a disadvantage of low throughput, particularly processing speed, because it depends on the wavelength of the ultraviolet laser beam.

Disclosure of Invention

The invention aims to provide a method for improving the processing speed by setting the pulse width of a pulse laser beam appropriately when drilling a printed circuit board by using an ultraviolet pulse laser.

The present invention is a drilling method for drilling a hole in a printed circuit board by irradiating the printed circuit board with an ultraviolet laser beam, wherein the ultraviolet laser beam having a pulse width of 100 to 300(nsec) is irradiated to a resin layer of the printed circuit board.

In particular, in the present invention, a Nd: YLF pulse laser or a Nd: YAG pulse laser is used as a source of the ultraviolet laser beam.

Drawings

Fig. 1 is a diagram showing a system configuration in which a laser drilling apparatus of the present invention is used.

Detailed Description

Hereinafter, embodiments of the present invention will be described. The present invention is directed to improving the processing speed of drilling using an ultraviolet laser beam, and particularly to a pulse width due to the configuration of a laser oscillator. That is, the pulse width is adjusted by adjusting the interval between a pair of mirrors constituting the oscillator in the laser oscillator.

Here, the drilling processability of both the XeF laser (wavelength of 351nm) and the Nd: YLF pulse laser, which use excimer lasers having different pulse widths, i.e., the excimer laser, were compared. That is, the XeF laser has a pulse width of about 20(nsec), while the Nd: YLF pulse laser has a third harmonic pulse width of about 170 (nsec).

Fig. 1 shows a system for performing drilling. The laser oscillator 11 uses a XeF laser or a Nd: YLF pulse laser. Particularly, the third harmonic is used in the case of an Nd: YLF pulse laser. The laser beam generated by the laser oscillator 11 is irradiated onto the mask 15 through the mirror 12, the attenuator 13, and the mirror 14. An image determined by the opening of the mask 15 is reduced and projected onto an object 18 to be processed via a mirror 16 by a processing lens 17 called an f θ lens. The object 18 to be processed is placed on an X-Y stage 19. The X-Y stage 19 can move the stage on which the object 18 to be processed is mounted in the X-axis direction and the Y-axis direction. As a result, the laser beam can be used to drill a hole in any position of the object 18.

The attenuator 13 is provided to adjust the energy of the XeF laser that outputs a large amount of light. Therefore, YLF pulsed lasers can be eliminated when used. In this way, the influence (energy density) of the photon beam on the object 18 is adjusted to be almost the same regardless of whether the XeF laser or the Nd: YLF pulse laser is used.

A polyimide film (polyimide film: 100 μm in thickness) was used as the object 18. The 2 lasers are not different in wavelength, and the difference between the two lasers is only in pulse width. Using the third harmonic of XeF laser and Nd: YLF pulse laser, both of which are used at 10 (J/cm) on the surface of the object to be processed²) The processing test was conducted with the irradiation energy of (2). As a result, the processing depth per 1 pulse was about 2(μm) in the case of the XeF laser and about 7(μm) in the case of the Nd: YLF pulse laser.

From the above results, it can be judged that the third harmonic drilling capability of the Nd: YLF pulse laser is strong. The reason for this is that when the pulse width is small, the irradiation with the laser beam is completed in a short time, the energy of the laser beam is accumulated near the surface of the object to be processed, and the penetration of the energy into the object to be processed is low. On the other hand, if the pulse width of the laser beam is wide, the thermal diffusion to the object to be processed is high, and therefore the thermal etching reaction and the thermal deterioration of the irradiated portion are accelerated, and as a result, it is estimated that the drilling speed is increased.

This point is also applicable to the fundamental wave and second to fifth harmonics of Nd: YLF pulse laser or Nd: YAG pulse laser. For example, the drilling speed is improved even when the pulse width is 150(nsec) at the third harmonic of the Nd: YAG pulse laser. In addition, the range of the ideal pulse width is confirmed to be 100 to 300 (nsec).

As described above, according to the present invention, when drilling a printed circuit board using an ultraviolet pulse laser, particularly, a Nd: YLF pulse laser or a Nd: YAG pulse laser, the machining speed (drilling speed) can be increased by appropriately setting the pulse width of the pulse laser.

Industrial applicability of the invention

The present invention can be considered to be applied to drilling a resin layer of a printed circuit board, perforating a polymer material, and the like.

Claims (5)

1. A laser drilling method is characterized in that: when a printed circuit board is drilled using a laser beam, the resin layer of the printed circuit board is irradiated with a laser beam having a pulse width of 100 to 300nsec using a Nd: YLF pulse laser or a Nd: YAG pulse laser.

2. The laser drilling processing method according to claim 1, characterized in that: the printed circuit board has a conductive layer on a lower layer of the resin layer, and the drilling process is to form a hole reaching the conductive layer.

3. The laser drilling processing method according to claim 1 or 2, characterized in that: the pulse laser beam is any one of a fundamental wave and second to fifth harmonics of the Nd: YLF pulse laser or the Nd: YAG pulse laser.

4. A laser drilling device for drilling holes on a resin substrate by irradiating laser, characterized in that: a device for generating a harmonic of Nd: YLF pulse laser or Nd: YAG pulse laser as the laser generating device; the pulse width of the pulse laser is set to 100 to 300 nsec.

5. The laser drilling processing apparatus according to claim 4, characterized in that: a mask is disposed on an optical path of the laser beam, and an f [ theta ] lens for reducing and projecting an image of the mask onto the resin substrate is disposed on the optical path between the mask and the resin substrate.