A laser is a powerful beam of light that is excited when a "ray" is stimulated by an external stimulus that increases its energy. Infrared and visible light have thermal energy, while ultraviolet light has optical energy. When this type of light hits the surface of a workpiece, three phenomena occur: reflection, absorption and penetration.
The main function of laser drilling is to be able to quickly remove the substrate material to be processed, mainly by photothermal ablation and photochemical ablation or so-called excision.
- Photo-thermal ablation: The principle of hole formation in which the material to be processed absorbs high energy laser light, heats up to melting in a very short time, and is evaporated. This process method in the substrate material is subjected to high energy, in the hole formed by the wall of the blackened carbonized residue, the hole must be cleaned up before.
- Photochemical ablation: refers to the ultraviolet region has a high photon energy (more than 2 eV electron volt), laser wavelength of more than 400 nanometers of high-energy photons play a role in the results. This high-energy photons can destroy the long molecular chain of organic materials, become smaller particles, and its energy is greater than the original molecules, the extreme force from which to escape, in the case of external pinch suction, so that the substrate material is quickly removed and the formation of microporous. This type of process does not contain thermal burning and does not produce carbonization. Therefore, it is very easy to clean up before poration. These are the basic principles of laser hole formation. Currently the most commonly used two types of laser drilling: printed circuit board drilling with lasers are mainly RF-excited CO2 gas lasers and UV solid-state Nd: YAG lasers.
- On the substrate absorbance: the success rate of the laser has a direct relationship with the absorbance of the substrate material. Printed circuit boards are made of copper foil and glass cloth and resin combination, the absorbance of these three materials is also different due to different wavelengths, but the copper foil and glass cloth in the ultraviolet 0.3mμ below the region of the absorption rate is higher, but into the visible light and IR after a substantial decline. Organic resin materials, on the other hand, can maintain a fairly high absorption rate in all three spectral bands. This is the characteristic that resin materials have and is the basis for the popularity of the laser drilling process.
What kinds of laser drilling are available in PCB factories?
A laser is a powerful beam of light that is excited when "rays" are stimulated by an external stimulus that increases its energy, with infrared and visible light having thermal energy and ultraviolet light having optical energy. When this type of light hits the surface of a workpiece, three phenomena occur: reflection, absorption and penetration. The main function of laser drilling is to be able to quickly remove the substrate material to be processed, which is mainly by photothermal ablation and photochemical ablation or so-called excision.
Two laser technologies are used for laser drilling in commercial PCB production: CO2 lasers with wavelengths in the far-infrared band, and UV lasers with wavelengths in the ultraviolet band.CO2 lasers are widely used in the production of industrial micropass holes in printed circuit boards, which are required to have diameters of greater than 100 μm (Raman, 2001). For the fabrication of these large aperture holes, CO2 lasers are highly productive due to the very short punching time required for the fabrication of large apertures with CO2 lasers. UV laser technology is widely used in the fabrication of microvias with diameters of less than 100 μm, and even less than 50 μm with the use of microfabricated wiring diagrams. UV laser technology is very productive in the production of holes with a diameter of less than 80 μm. Therefore, to meet the increasing demand for microvia productivity, many PCB manufacturers have begun to introduce dual head laser drilling systems.
The following are the three main types of dual head laser drilling systems available in the market today:
- Dual-head UV laser drilling systems
- Dual head CO2 laser drilling systems; and
- Stick laser drilling systems (CO2 and UV)
All these types of drilling systems have their own advantages and disadvantages. Laser drilling systems can simply be divided into two types, dual drill single wavelength systems and dual drill dual wavelength systems.
Regardless of the type, there are two main components that affect the ability to drill holes:
- The laser energy/pulse energy
- The beam positioning system
The energy of the laser pulse and the efficiency of the beam delivery determines the drilling time, the drilling time is the time it takes the laser drill to drill a micropass hole, and the beam positioning system determines the speed at which it can move between two holes. Together, these factors determine the speed at which the laser drilling machine can produce the microvias required for a given requirement. Dual-head UV laser systems are best suited for drilling holes smaller than 90μm in integrated circuits with high aspect ratios.
The dual-head CO2 laser system uses a Q-modulated RF-excited CO2 laser. The main advantages of this system are the high repeatability (up to 100 kHz), short drilling times, and the wide operating surface, which allows a blind hole to be drilled with only a few passes, but the quality of the drilled holes can be low.
The most common dual-head laser drilling system is the hybrid laser drilling system, which consists of a UV laser head and a CO2 laser head. This combined hybrid laser drilling method allows for the simultaneous drilling of copper and dielectrics. The copper is drilled with the UV laser to create the desired hole size and shape, and the CO2 laser is used to drill the uncovered dielectric immediately afterward. The drilling process is accomplished by drilling a 2in X 2in block called a field.
The CO2 laser effectively removes dielectrics, even non-uniform glass reinforced dielectrics. However, a single CO2 laser cannot make small holes (less than 75 μm) and remove copper, with the few exceptions that it can remove pre-treated thin copper foils of less than 5 μm (lustino, 2002). The UV laser is capable of making very small holes and removing all common copper streets (3 - 36 μm, 1oz, even plated copper foils). The UV laser can also remove dielectric materials alone, but at a slower rate. Moreover, for non-uniform materials, e.g. reinforced glass FR-4, the results are usually poor. This is because the glass can only be removed if the energy density is increased to a certain level, which also destroys the inner pads. Since the stick laser system consists of a UV laser and a CO 2 laser, it is optimal in both areas, with the UV laser all copper foils and small holes can be done, and with the CO 2 laser the dielectrics can be drilled quickly. The figure gives an illustration of the structure of a dual-head laser drilling system with programmable drilling spacing. The spacing between the two drills can be adjusted by itself according to the layout of the components, which ensures maximum laser drilling capability.
Nowadays, the spacing between the two drills is fixed in most dual-head laser drilling systems with step-and-repeat beam positioning technology. The advantage of the step-and-repeat laser remote controller itself is the large adjustment range of the domain (up to (50 X 50) μm). The disadvantage is that the laser teleconverter must be stepped over a fixed field and the spacing between the two drills is fixed. The distance between the two drills of a typical dual-head laser remote regulator is fixed (approximately 150 μm). For different panel sizes, fixed distance drills cannot be optimally configured to complete the operation as well as programmable spacing drills.
Today's dual-head laser drilling systems are available in a wide range of sizes and performances for both small-scale PCB manufacturers as well as for high-volume PCB manufacturers.
Ceramic aluminum oxide is used in the manufacture of printed circuit boards because of its high dielectric constant. However, due to its fragility, the drilling process required for wiring and assembly is difficult with standard tools, as mechanical stress must be minimized, which is a good thing for laser drilling.Rangel et al. (1997) demonstrated that for alumina substrates, as well as for alumina substrates coated with gold and anchors, it is possible to drill using a tuned QNd:YAG laser. The use of a short-pulse, low-energy, high-peak-power laser helped to avoid damage to the sample by mechanical stress and produced high-quality through-holes with diameters of less than 100 μm. This technology is successfully used in low noise microwave amplifiers in the frequency range of 8 - 18 GHz.
Nd:YAG laser technology has been used to process both blind and through holes in a wide range of materials. Among these is the drilling of pilot holes in polyimide copper-clad laminates with a minimum hole diameter of 25 microns. Analyzing the production cost, the most economical diameter used is 25-125 microns. The drilling speed is 10,000 holes/min. Direct laser punching process can be used, hole diameter of up to 50 microns. The inner surface of the molded holes is clean and free of carbonization and can be easily plated. The same can also be in the PTFE copper-clad laminate drilling through holes, the smallest hole diameter of 25 microns, the most economical diameter used for 25-125 microns. Drilling speed is 4500 holes/min. No pre-etching of windows is required. The resulting holes are clean and do not require additional special processing requirements.
