Ultraviolet Laser Cutting
The rapid development of the electronics industry, precision instrument-making, the development of new materials with special properties, for example, high heat resistance and durability, requires new micro-processing methods that provide high accuracy and efficiency. Laser cutting has advantages over mechanical cutting methods: it is contactless and does not create mechanical stresses that can damage the product.
Conventionally, in industry, laser sources with radiation of near-IR (fibre lasers, 1064 nm) and far-IR ranges (CO2 lasers, 10.6 μm) are used. The choice of the type of source depends on the absorption coefficient of the radiation of a material at a specific wavelength: for metals, these are fibre lasers with 1064 nm, non-metals – CO2 lasers with 10.6 μm.
But the processing of special materials such as sapphire, various glasses, polymer films, printed circuit boards, semiconductor materials, etc. has higher requirements that cannot be provided by the aforementioned laser sources. In the production of such products cutting is the main operation.
The use of laser cutting systems with ultraviolet radiation has opened up new possibilities for processing special materials. To obtain ultraviolet, a more complex laser configuration is needed. To generate the third harmonic (355 nm) in a solid-state diode-pumped laser, nonlinear optical phenomena are used. The generation is realized according to the following scheme: the main wavelength of 1064 nm excites the second harmonic (532 nm) on the first nonlinear crystal of potassium titanyl phosphate (KTiOPO4). Then the radiation of the main and second harmonics is mixed on the second nonlinear crystal, as a result of which the third harmonic with a wavelength of 355 nm is emitted at the output (Fig. 1).
UV-range laser radiation (355 nm) has a high quanta energy and a smaller spot diameter in the focusing region due to a shorter wavelength compared to fibre- (1064 nm) and CO2 lasers (10.6 μm). The radiation wavelengths of various lasers are shown in Fig. 2
The minimum possible focus spot diameter corresponds to the wavelength of the laser radiation. Accordingly, for a UV-laser with 355 nm, it is 30 times smaller than for an IR with a wavelength of 10.06 μm, and is several micrometres.
The shift of the wavelengths in the UV region increases the quantum energy, which increases the energy intensity of the laser radiation flux. Reducing the wavelength of laser radiation reduces the reflection coefficient of materials and increases the amount of energy absorbed by the material.
The energy absorbed by the material is consumed either on vibrational or electronic excitation, or on a photochemical reaction. During vibration excitation, the absorbed photon energy causes molecular oscillations in the material: stretching, bending, or rotating atomic bonds. If the incident photon has a sufficiently high energy, then it can cause electronic excitation. An excited electron can consume energy on the emission of a photon or on vibrational excitation. The emission of a photon does not change the material or its properties, but vibrational excitation, with a significant release of heat, can change the properties of the material. A photochemical reaction occurs if the energy of an absorbed photon corresponds to the energy of the beginning of a chemical reaction. The onset of the photochemical reactions requires high photon energy .
The photons of the incident radiation are absorbed by electrons in a thin surface layer of the material corresponding to the depth of optical penetration. The released energy is converted into heat and transferred to the chains of molecules. When the evaporation temperature of the material begins, ablation begins. The propagation of the heat front occurs from the surface layer deep into the material. The depth of thermal diffusion is a function of the diffusion coefficient of a material and the duration of a laser pulse.
The interaction of UV radiation with dielectric materials, such as polymers, in a substance occurs once burst of intramolecular bonds and electronic excitation of substance molecules. There is a photochemical mechanism of interaction, the so-called «cold» ablation. With a high-power density of laser radiation, conditions are created under which the quanta energy is sufficient both to break bonds between polymer chains, inside chains between individual molecules, and to break chemical bonds inside molecules (for example, C–C or C-H in polymers) . Cold ablation is characterized by a small heat-affected zone, only a few micrometres.
Printed circuit board cutting
A printed circuit board is a copper-laminated non-conductive plate on which electrically conductive circuits are formed to power the electronic components installed on it. There are several types of printed circuit boards: single-layer, double-layer, multi-layer and flexible printed circuit boards. They differ in the number of conductive layers and the type of insulating material. The materials mainly used for printed circuit boards are FR‑1, FR‑2, FR‑4, CEM‑3.
The technology of using ultraviolet laser is versatile: it is suitable for cutting, piercing holes and structuring both rigid and flexible printed circuit boards. In addition to a short wavelength and a smaller heat-affected zone, the high energy of ultraviolet photons allows you to work with a wide range of materials for printed circuit boards: from standard materials such as FR‑4, and similar resin substrates, ceramics to high-frequency ceramic composites and materials flexible printed circuit boards, including polyamide. Fig. 3 shows the absorption spectrum of materials used for printed circuit boards. Ultraviolet lasers have very high absorption rates for resin and copper, and also, a fairly high absorption in glass. Higher absorption rates among these materials of this group show only excimer lasers (248 nm), which, due to their high cost and complexity of maintenance, are rarely used for such purposes. A variety of materials for processing allows the use of UV lasers for a wide range of applications on printed circuit boards: from creating circuit loops to performing complex processes, such as creating pockets for embedding chips.
UV lasers in the printed circuit industry are used, in particular, to isolate finished devices from a printed circuit board panel. The usual method is to use a mechanical milling bit. However, manufacturers seek to increase the process throughput and reduce the cost of consumables.
UV radiation due to «cold ablation» minimizes the appearance of burrs, charring and other negative effects of thermal stress, usually occurring when exposed to lasers with higher power.
CO2 lasers are also suitable for some of the separation work, but they severely char cut surfaces. For many applications, the presence of carbonation is unacceptable: carbon products can be conductive and absorb moisture, which will lead to failure of the device. Furthermore, carbon compounds are aromatic, which is unacceptable for applications in which the product will be close to the user's face, such as mobile phones, headsets, and the like. The examples of cutting printed circuit boards with a UV laser are shown in Figure 5a, b. These examples are made on Poplar‑355-12, an ultraviolet laser with a nanosecond pulse duration (Huaray) .
Another application of the UV laser in the printed circuit industry is perforation. The performance of this process depends on the properties of the material. To maximize productivity, perforation is performed in two trips: ablation of the copper layer at high energy densities of laser radiation (about 4 J / cm2), then perforation of a non-conductive substrate with radiation with a lower energy density (100 mJ / cm2). This sequence provides the smallest heat-affected zone when exposed to the substrate material, thus allowing to obtain the best quality holes: 30 microns in diameter, the perforation rate can reach 250 holes per second. In addition, during this exposure, the adhesive properties of the copper surface are improved as a result of the structured laser, which has a good effect on the subsequent application of the coating. CO2 lasers, with the usual beam diameter of 70 μm and a large heat-affected zone, do not allow to achieve similar results.
The small spot size of a UV laser opens up new possibilities for laser structuring: direct ablation of varnishes and photoresists (direct photolithography with a UV laser) is an alternative to traditional photolithography. The minimum width of lines achieved by this method is 30 µm .
UV-laser systems work directly from the CAD data and exclude any mediation in the process of creating the board. This allows UV systems to work with high resolution and high repeat positioning accuracy. UV-lasers are ideal for HDI applications by using software (High-Density Interconnect). It is possible to cut complex products consisting of several layers of different materials (e. g., FR‑4 / polyimide / epoxy resin) with high speed and the absence of delamination.
The trend towards miniaturization in electronic devices has led to the use of flexible printed circuit boards. Flexible printed circuit boards are one or more layers of a dielectric with an electronic circuit formed on it. The fabrication of printed circuit boards from flexible materials expands the possibilities of their use due to the design flexibility, lower weight, greater layout density, resistance to dynamic and thermal loads and makes them the most optimal, having no alternatives to the interconnect method in electronics. The use of such a material imposes restrictions on its processing methods: cutting by a mechanical method has a too wide cut and creates high mechanical stresses that are unacceptable for circuits with a complex topology, laser cutting with a CO2 laser has a large heat-affected zone.
The absorption of laser radiation by dielectrics is due to the presence of vibrational degrees of freedom of the crystal lattice, intermolecular vibrations, impurities, defects, etc. The absorption coefficient depends on the radiation wavelength and is most important in the UV and IR radiation ranges. Dielectrics also have high absorption coefficients at a wavelength of λ = 10.6 μm of CO2-laser radiation, but IR lasers remove material by intense local heating, with carbonation products, charred edges and high thermal stresses. Furthermore, the CO2 laser radiation is completely reflected by metals, which makes it impossible to cut flexible printed circuit boards with metal chains. Laser cutting with a UV laser allows you to cut flexible printed circuit boards with high quality. Fig. 6 shows the results of cutting printed circuit boards with a CO2 laser and a UV laser. When cutting with a CO2 laser, traces of carbonization of the material are present, and with UV cutting, a clean, even edge of the cut.
Another application that testifies to the universality of ultraviolet lasers is engraving in depth. It allows you to create pockets for embedding chips and treads on printed circuit board substrates. An example of engraving in depth is shown in Fig. 7. Controlled deep engraving can be automated using software.
Precision cutting and perforation of metal foil
The third-harmonic wavelength of 355 nm of a diode-pumped solid-state laser is well absorbed by metals (Cu, Ni, Au, Ag). The absorption spectrum is shown in Fig. 8. The UV laser allows for precision metal processing. In fig. 9 shows an example of cutting with a diode-pumped solid-state laser with a wavelength of 355 nm of a 200-μm-thick stainless-steel foil and piercing a hole in a 50-μm-thick foil .
Cutting fragile materials
Laser glass processing is widely used in the production of consumer electronics. The motivating factors for this are cost reduction due to an increase in the yield of products and an improvement in the quality of the finished device. Lasers are used to process various types of glass: from inexpensive sodium-calcium to a variety of high-quality borosilicate glasses. The requirements for this process are to reduce the volume of the molten material and prevent the formation of microcracks. UV laser due to the small spot diameter and a small zone of thermal influence reduces the thermal load on the glass. Control of the formation of microcracks can be performed by setting the pulse duration. Also, the UV laser allows you to cut other fragile materials with a high melting point, such as ceramics, sapphire and aluminium oxide.
Cutting materials with a layered structure: mica
A UV-laser can cut mica with no delamination of material. Fig. 11 shows mica samples cut with fibre, CO2 and UV lasers. As one can see, the best result is achieved with the ultraviolet.
The systems for laser processing based on UV laser sources with a wavelength of 355 nm have higher accuracy, speed and better cutting quality, they are an ideal tool for micromachining and are most often used in the production of printed circuit boards and electronics, precision cutting of metals and brittle materials. They are suitable for industrial and scientific applications, for example, for experiments in atomic and molecular spectroscopy and chemical dynamics. The use of UV lasers suggests the emergence of new applications in nanotechnology, materials science, biology, chemistry, plasma physics and many other fields. Diode-pumped solid-state lasers with 355 nm radiation have low operating costs, high reliability and long service life.