1 Application of laser beam

The high-density PCB board is a multi-layer structure, which is separated by insulating resin mixed with glass fiber materials, and a conductive layer of copper foil is inserted between them. Then it is laminated and bonded. Figure 1 shows a section of a 4-layer board. The principle of laser processing is to use laser beams to focus on the surface of the PCB to instantly melt and vaporize the material to form small holes. Since copper and resin are two different materials, the melting temperature of copper foil is 1084°C, while the melting temperature of insulating resin is only 200-300°C. Therefore, it is necessary to reasonably select and accurately control parameters such as beam wavelength, mode, diameter, and pulse when laser drilling is applied.

1.1 The influence of beam wavelength and mode on processing

What are the applications of laser processing in high-density PCB manufacturing

Figure 1 Cross-sectional view of 4-layer PCB

It can be seen from Figure 1 that the laser is first to process the copper foil when perforating, and the absorption rate of the copper to the laser increases with the increase of the wavelength. The YAG/UV laser absorption rate of 351 to 355 m is as high as 70%. YAG/UV laser or conformal mask method can be used to perforate ordinary printed boards. In order to increase the integration of high-density PCB, each layer of copper foil is only 18μm, and the resin substrate under the copper foil has a high absorption rate of carbon dioxide laser (about 82%), which provides conditions for the application of carbon dioxide laser perforation. Because the photoelectric conversion rate and processing efficiency of carbon dioxide laser is much higher than that of YAG/UV laser, as long as there is enough beam energy and the copper foil is processed to increase its absorption rate of the laser, the carbon dioxide laser can be used to directly open the PCB .

The transverse mode mode of the laser beam has a great influence on the divergence angle and energy output of the laser. In order to obtain sufficient beam energy, it is necessary to have a good beam output mode. The ideal state is to form a low-order Gaussian mode output as shown in Figure 2. In this way, a high energy density can be obtained, which provides a prerequisite for the beam to be well focused on the lens.

What are the applications of laser processing in high-density PCB manufacturing

Figure 2 Low-cost Gaussian mode energy distribution

The low-order mode can be obtained by modifying the parameters of the resonator or installing a diaphragm. Although the installation of the diaphragm reduces the output of the beam energy, it can limit the high-order mode laser to participate in the perforation and help improve the roundness of the small hole. .

1.2 Obtaining micropores

After the wavelength and mode of the beam are selected, in order to obtain an ideal hole on the PCB, the diameter of the spot must be controlled. Only if the diameter of the spot is small enough, the energy can concentrate on ablating the plate. There are many ways to adjust the spot diameter, mainly through spherical lens focusing. When the Gaussian mode beam enters the lens, the spot diameter on the back focal plane of the lens can be approximately calculated with the following formula:

D≈λF/(πd)

In the formula: F is the focal length; d is the spot radius of the Gaussian beam projected by a person on the lens surface; λ is the laser wavelength.

It can be seen from the formula that the larger the incident diameter, the smaller the focused spot. When other conditions are confirmed, shortening the focal length is conducive to reducing the beam diameter. However, after F is shortened, the distance between the lens and the workpiece is also reduced. The slag may splash on the surface of the lens during drilling, which will affect the drilling effect and the life of the lens. In this case, an auxiliary device can be installed on the side of the lens and gas is used. Perform purge.

1.3 The influence of beam pulse

A multi-pulse laser is used for drilling, and the power density of the pulsed laser must at least reach the evaporation temperature of the copper foil. Because the energy of the single-pulse laser has been weakened after burning through the copper foil, the underlying substrate cannot be effectively ablated, and the situation shown in Fig. 3a will be formed, so that the via hole cannot be formed. However, the energy of the beam should not be too high when punching, and the energy is too high. After the copper foil is penetrated, the ablation of the substrate will be too large, resulting in the situation shown in Figure 3b, which is not conducive to the post-processing of the circuit board. It is most ideal to form the micro-holes with a slightly tapered hole pattern as shown in Fig. 3c. This hole pattern can provide convenience for the subsequent copper-plating process.

What are the applications of laser processing in high-density PCB manufacturing

Figure 3 Hole types processed by different energy lasers

In order to achieve the hole pattern shown in Figure 3c, a pulsed laser waveform with a front peak can be used (Figure 4). The higher pulse energy at the front end can ablate the copper foil, and the multiple pulses with lower energy at the back end can ablate the insulating substrate and Make the hole deepen until the lower copper foil.

What are the applications of laser processing in high-density PCB manufacturing

Figure 4 Pulse laser waveform

2 Laser beam effect

Because the material properties of the copper foil and the substrate are very different, the laser beam and the circuit board material interact to produce a variety of effects, which have an important impact on the aperture, depth, and hole type of the micropores.

2.1 Reflection and absorption of laser

The interaction between the laser and the PCB first starts from the incident laser being reflected and absorbed by the copper foil on the surface. Because the copper foil has a very low absorption rate of infrared wavelength carbon dioxide laser, it is difficult to process and the efficiency is extremely low. The absorbed part of the light energy will increase the free electron kinetic energy of the copper foil material, and most of it will be converted into the heat energy of the copper foil through the interaction of electrons and crystal lattices or ions. This shows that while improving the beam quality, it is necessary to carry out pre-treatment on the surface of the copper foil. The surface of the copper foil can be coated with materials that increase light absorption to increase its absorption rate of laser light.

2.2 The role of beam effect

During laser processing, the light beam radiates the copper foil material, and the copper foil is heated to vaporization, and the steam temperature is high, which is easy to break down and ionize, that is, photo-induced plasma is generated by light excitation. The photo-induced plasma is generally a plasma of material vapor. If the energy transmitted to the workpiece by the plasma is greater than the loss of light energy received by the workpiece caused by the absorption of the plasma. The plasma instead enhances the absorption of laser energy by the workpiece. Otherwise, the plasma blocks the laser and weakens the absorption of the laser by the workpiece. For carbon dioxide lasers, photo-induced plasma can increase the absorption rate of copper foil. However, too much plasma will cause the beam to be refracted when passing through, which will affect the positioning accuracy of the hole. Generally, the laser power density is controlled to an appropriate value below 107 W/cm2, which can better control the plasma.

The pinhole effect plays an extremely important role in enhancing the absorption of light energy in the laser drilling process. The laser continues to ablate the substrate after burning through the copper foil. The substrate can absorb a large amount of light energy, violently vaporize and expand, and the pressure generated can be The molten material is thrown out to form small holes. The small hole is also filled with photo-induced plasma, and the laser energy entering the small hole can be almost completely absorbed by the multiple reflections of the hole wall and the action of the plasma (Figure 5). Due to plasma absorption, the laser power density passing through the small hole to the bottom of the small hole will decrease, and the laser power density at the bottom of the small hole is essential to generate a certain vaporization pressure to maintain a certain depth of the small hole, which determines The penetration depth of the machining process.

What are the applications of laser processing in high-density PCB manufacturing

Figure 5 Laser refraction in the hole

3 Conclusion

The application of laser processing technology can greatly improve the drilling efficiency of high-density PCB micro-holes. Experiments show that: ①Combined with numerical control technology, more than 30,000 micro-holes can be processed per minute on the printed board, and the aperture is between 75 and 100; ② The application of UV laser can further make the aperture less than 50μm or smaller, which creates conditions for further expanding the use space of PCB boards.