PCB Technical - Laser processing technology in PCB manufacturing

Portable multifunctional electronic products have very high requirements on printed circuit boards (PCB). Closely interconnect many components in a limited area and stabilize the circuit. Circuit boards are becoming denser and denser. For example, the opening and line width are further reduced, the distance and accuracy between each other are continuously improved, and the diameter-to-depth ratio is continuously improved. The number of circuit layers can reach more than 10 layers. The number of micropores in the same layer is more than 50,000, but the spacing should be as small as 0.05 mm, and the pore diameter should be less than 150 μm. When mechanically drilling such printed circuit boards, it is difficult to overcome the problems of drilling materials, cooling, chip removal, and processing positioning. The application of laser processing can better meet the quality requirements.

1. Application of laser beam

The high-density PCB manufactured by iPCB is a multi-layer structure separated by insulating resin mixed with glass fiber material, and a conductive layer of copper foil is inserted in the middle. Then they are laminated and combined. The principle of laser processing is to use a laser beam to focus on the surface of the PCB to instantly melt and vaporize the material to form a small hole. 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, when laser drilling is used, parameters such as beam wavelength, mode, diameter, and pulse must be reasonably selected and controlled.

1.1 Influence of beam wavelength and mode on machining

When drilling holes, the laser first processes the copper foil, and the absorption of the laser by the copper increases with the increase of the wavelength. For example, the absorption rate of carbon dioxide laser with a wavelength of 9.4 to 10.6 μm is as high as 351 to 355 m, while the absorption rate of YAG/UV laser is as high as 70%, compared to only 0.15%. You can use YAG/UV laser or conformal mask method to drill holes on conventional printed boards. In order to improve 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 is a condition for application. Provide carbon dioxide laser drilling. The photoelectric conversion rate and processing efficiency of carbon dioxide lasers are much higher than that of YAG/UV lasers, so as long as there is enough beam energy and the copper foil is processed to increase the laser absorption rate, the carbon dioxide laser PCB can be used to open directly.
The transverse mode of the laser beam has a great influence on the divergence angle and energy output of the laser. To obtain sufficient beam energy, it is first necessary to establish a suitable beam output mode. The ideal state is to form a lower Gaussian mode output. This allows a very high energy density. This provides a prerequisite for the beam to be well focused on the lens. The low-order mode can be obtained by changing the resonator parameters or installing a diaphragm. Installing a diaphragm will reduce the beam energy output, but it can limit and assist high-mode lasers to participate in drilling. Improve the roundness of small holes.

1.2 Influence of beam pulse

A multi-pulse laser is used for drilling, and the output density of the pulsed laser must at least reach the evaporation temperature of the copper foil. After the copper foil is burned, the energy of the single pulse laser is weakened, and the underlying substrate cannot be effectively ablated, and the through hole cannot be formed. However, if the energy is too high, it is necessary to ensure that the energy of the beam is not too high when drilling. After the copper foil penetrates, the circuit board is excessively ablated and cannot be used for post-processing of the circuit board. A slightly tapered hole pattern formed by microholes is ideal, and this hole pattern is useful for subsequent copper coating processes.

2. Laser beam effect

Due to the large difference between the material properties of the copper foil and the substrate, the interaction between the laser beam and the circuit board material produces different effects, which have a significant impact on the aperture, depth and hole type of the micropores.

2.1 Laser reflection and absorption

The interaction between the laser and the PCB begins when the incident laser is first reflected and absorbed by the copper foil on the surface. Copper foil has a low absorption rate of infrared wavelength carbon dioxide laser, which is difficult to process and has high efficiency. very low. The absorbed light energy increases the free electron kinetic energy of the copper foil material, most of which is 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 also necessary to pre-treat the surface of the copper foil. A layer of material that enhances light absorption can be coated on the surface of the copper foil to increase the absorption rate of the laser.

2.2 Effect of beam effect

In laser processing, light irradiates the copper foil material, and the copper foil is heated and vaporized. Therefore, the steam temperature is high, which is prone to decomposition and ionization, and photo-induced plasma is generated by light excitation. .. Photo-induced plasma is generally a substance vapor plasma. When the energy delivered by the plasma to the workpiece is greater than the light energy lost by the workpiece due to the absorption of the plasma. On the contrary, the plasma enhances the absorption of laser energy by the workpiece. Otherwise, the plasma will block the laser and reduce the absorption of the laser by the workpiece. In the case of carbon dioxide lasers, photo-induced plasma can increase the absorption of the copper foil. However, too much plasma will be refracted when the beam passes, thereby affecting the accuracy of hole positioning. Generally speaking, the laser power density is controlled at a reasonable value below 107W/cm2, so that the plasma can be better controlled.
The pinhole effect plays a very important role in increasing the absorption of light energy during laser drilling. Even if the copper foil is burned, the laser will continue to ablate the substrate. The substrate absorbs a large amount of light energy, violently evaporates and expands., The pressure generated is: Throw the molten material out to form a small hole. The small hole is also filled with photo-induced plasma, and the laser energy entering the small hole is almost completely absorbed by the multiple reflections of the hole wall and the action of the plasma. Plasma absorption reduces the power density of the laser that passes through the pit to the bottom of the pit. The laser power density at the bottom of the pit is essential for generating a specific vaporization pressure to maintain a specific depth. . A small hole that determines the depth of penetration during processing.

3. Conclusion

Through the application of laser processing technology, the drilling efficiency of high-density PCB micro-holes can be greatly improved. Experiments show (1) Combining numerical control technology, printed circuit boards and openings can process more than 30,000 micro-holes per minute. Between 75 and 100. (2) Through the application of ultraviolet laser, the opening can be further reduced to below 50μm, creating conditions for further expanding the use space of the PCB board.