Drilling process of PCB hybrid laser
In commercial production, there are two laser technologies that can be used for laser drilling. The wavelength of CO2 laser is in the far infrared band, and the wavelength of UV laser is in the ultraviolet band. CO2 laser is widely used in the fabrication of industrial micro via of printed circuit board. The diameter of micro via is required to be greater than 100 μ m (Raman, 2001). For the production of these large aperture holes, CO2 laser has a very high productivity, because the time required for making large aperture holes is very short. UV laser technology is widely used in the fabrication of micro holes with diameter less than 100 μ M. with the use of micro circuit diagram, the aperture can be even less than 50 μ M. The output of ultra violet laser is very high when the diameter of the hole is less than 80 μ M. Therefore, in order to meet the increasing demand of microporous production, many manufacturers have begun to introduce double head laser drilling system. -The following are the three main types of dual head laser drilling systems used in today's market:
1) double head ultraviolet drilling system;
2) double head CO2 laser drilling system;
3) rod laser drilling system (CO2 and UV).
All these types of drilling systems have their own advantages and disadvantages. The laser drilling system can be simply divided into two types: single wavelength system with double drills and dual wavelength system with double drills. Regardless of the type, there are two main parts that affect the ability to drill:
1) laser energy / pulse energy;
2) beam positioning system.
The energy of laser pulse and beam transfer efficiency determine the drilling time. The drilling time refers to the time when the laser drilling machine drills a micro through hole. The beam positioning system determines the speed of moving between the two holes. These factors together determine the speed of the laser drilling machine to make the micro through hole with given requirements. The double head UV laser system is most suitable for drilling holes with the integrated circuit size less than 90 μ m, and its aspect ratio is also very high.
The double head CO2 laser system uses Q-switched RF excited CO2 laser. The main advantages of this system are high repetition rate (up to 100kHz), short drilling time and wide operation area. It only needs a few shots to drill a blind hole, but its drilling quality will be relatively low.
The most commonly used dual head laser drilling system is the hybrid laser drilling system, which consists of an ultraviolet laser head and a CO2 laser head. The hybrid laser drilling method can be used to drill copper and dielectric simultaneously. That is to say, copper is drilled with ultraviolet light to generate the size and shape of the required hole, and then the uncovered dielectric is drilled with CO2 laser. The drilling process is completed by drilling 2in x 2in block, which is called domain.
CO2 laser can effectively remove the dielectric, even the inhomogeneous glass reinforced dielectric. However, a single CO2 laser can not make small holes (less than 75 μ m) and remove copper, with a few exceptions, that is, it can remove the thin copper foil under 5 μ m after pretreatment (lustino, 2002). UV laser can make very small holes and remove all the common copper streets (3 - 36 μ m, 1 oz, even electroplated copper foil). The UV laser can also remove the dielectric material alone, but at a slower speed. Moreover, for non-uniform materials, such as reinforced glass FR-4, the effect is usually not good. This is because only when the energy density is increased to a certain extent, can the glass be removed, which will also destroy the inner layer of the pad. Because the clubbing laser system includes UV laser and CO2 laser, it can achieve the best in two fields. All copper foil and holes can be completed by UV laser, and the dielectric can be drilled quickly by CO2 laser. Figure 10-14 shows the structure diagram of the double head laser drilling system with programmable drilling distance. The spacing between two drills can be adjusted according to the layout of components, which ensures the maximum laser drilling capacity.
At present, the spacing between the two drills in most double head laser drilling systems is fixed, and step repeat beam positioning technology is also available. Step by step, the advantage of the repeater laser remote controller is that it has a large adjustment range (up to (50 x 50) μ m). The disadvantage is that the laser remote controller must move step by step in a fixed field, and the distance between the two bits is fixed. The distance between two bits of a typical double head laser remote adjuster is fixed (about 150 μ m). For different panel sizes, a fixed distance drill cannot be operated in the best configuration as a programmable spacing drill.
Now, the double head laser drilling system has different specifications of performance, which can be used not only for small PCB manufacturers, but also for large-scale PCB manufacturers.
Because of its high dielectric constant, ceramic alumina is used to make printed circuit board. However, it is difficult to complete the drilling process with standard tools because of its fragility, wiring and assembly, because the mechanical pressure must be minimized at this time, which is a good thing for laser drilling. Rangel et al. (1997) demonstrated that qnd: YAG laser can be used to drill holes for alumina substrates and alumina substrates covered with gold and anchors. Using the laser with short pulse, low energy and high peak power can help to avoid the damage of mechanical pressure on the sample, and can produce a high-quality through-hole with the aperture less than 100 μ M. This technique has been successfully applied to low noise microwave amplifiers in the frequency range of 8 - 18GHz (Betancourt et al., 1996)