Two kinds of laser technologies are commercially available as laser drilling systems; CO2 laser with wavelength in the far infra-red region of the spectrum, and UV laser with wavelength in the ultraviolet region of the spectrum. The CO2 lasers are widely used for microvia formation in the PCB industry wherein the microvia design calls for larger vias, 100 mm in diameter (Raman, 2001). The CO2 lasers have high productivity at these large diameter vias. The high productivity is due to the fact that the CO2 lasers can ‘punch’ large vias with very small drill times. The UV laser is widely used when the microvia design calls for < 100 mm via diameters, with the roadmap shrinking to even smaller vias of < 50 mm diameter. The UV laser technology delivers very high productivity at
< 80 mm vias. Therefore, given the everincreasing demand to improve productivity of microvia
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formation, many manufacturers have started introducing dual head laser drilling systems. The following are the three major types of dual head laser drills available in the market today:
a Dual head UV laser system;
a Dual head CO2 laser system; and a Hybrid laser system (UV and CO2).
All the major types of drill systems have their own inherent advantages and disadvantages. The laser drills can be simplified into two categories: dual head single wavelength system and dual head dual wavelength system. Irrespective of the categories, each laser drill has two main components that affect the productivity of the drilling system:
a Laser power/pulse energy; and a Beam positioning system.
The laser pulse energy and the efficiency of the beam delivery optics determine the drilling time for a via. The drilling time is defined as the time it takes the laser drill to drill one microvia. The beam positioning system determines how fast you can move between the vias. The combined effect of these factors determines how fast a laser drill can produce microvias for a given application.
Thedual UV wavelength laser systems are best suited for integrated circuit packaging applications for drilling < 90 mm blind vias as well as high aspect ratio through vias.
The dual CO2 laser system makes use of Q-switched radio frequency excited CO2 laser. The main advantage of this system is the high repetition rate (upto 100 kHz), very short drill time, and wide process window. It only requires a few shots to drill a blind via which may result in a poor via quality.
The most popular dual head laser drilling system is the hybrid laser system which consists of an ultraviolet laser (UV) laser and a CO2 laser. The integrated approach of the hybrid laser drilling solution allows the copper and dielectric to be processed in parallel. This means that while the UV laser removes copper, creating the via size and shape desired, the CO2 laser follows behind, removing the dielectric that is uncovered. The drilling routine is carried out in 2" ¥ 2" blocks called fields.
CO2 lasers efficiently remove dielectrics, even non-homogeneous, glass-reinforced dielectrics.
However, the CO2 laser alone cannot create small vias (say below 75 mm) and cannot remove copper, apart from the limited success it has achieved in removing pre-treated thin foils 5 mm and below (Justino, 2002). The UV laser can be used to create very small vias and remove all common copper foils (from 3 mm upto 36 mm, 1 oz., and even plated foils). UV lasers alone can also remove dielectrics but the material removal rate is slow. Moreover, the results are generally poor for non- homogeneous materials such as glass-reinforced FR4s because the glass can only be removed if the energy density is increased to levels that can damage the inner layer stop pad. Since hybrid systems include both a UV laser and a CO2 laser, these systems offer the best of both worlds. All copper foils and small vias can be achieved with the UV laser and all common dielectrics can be drilled fast with the CO2 laser.
Figure10.14 shows the architecture of a dual head laser drill with programmable spacing between the heads. The pitch between the heads is automatically set depending upon the layout of the part.
This ensures that the laser drill is performing at its maximum throughput.
Fig. 10.14 Arrangement of a dual head laser drill (redrawn after Raman, 2001)
Most of today’s dual head laser drilling systems have fixed spacing between the heads, together with a step-and-repeat beam positioning technology. The inherent advantage of the step-and-repeat beam positioner allows bigger field sizes (up to 50 ¥ 50 mm). Its inherent disadvantage is that the beam positioner has to move in fixed field steps as well, with a fixed spacing between the heads.
The typical dual head beam positioner has a fixed amount of spacing between the two heads (about 150mm). For varying panel sizes, fixed head spacing cannot perform at its optimum efficiency as compared to the programmable spacing of head.
Hybrid laser drilling systems are today available with a variety of standard options and features that cater to small PCB shops as well as high volume manufacturing houses.
Ceramic alumina is used in printed circuit fabrication because of its high dielectric constant.
However, due to its brittleness, manufacturing processes such as hole drilling needed to attach wiring and trimming, become difficult with standard tools. This then becomes a good case for laser processing, since mechanical stresses have to be reduced to a minimum. Rangel, et al. (1997) demonstrated the drilling of perforations in alumina substrates and in gold and chromium-covered alumina substrates, by using laser ablation with a Q-switched Nd: YAG laser. Using a short pulse, low energy, high peak power laser helps to avoid the induction of mechanical stresses that can break up the sample, and to make fine structure perforations of 100 mm diameter or less. The technique was successfully applied in the production of a low noise microwave amplifier in the 8-18 GHz frequency range (Betancourt, et al., 1996).
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