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Metal Parts Products —Stamping & CNC Machining Manufacturers

Laser machining micro holes in a circuit board composite

When the circuit board composite material is processed into micro holes with a diameter of less than 0.2 mm, mechanical drilling is used, and the tool wear is accelerated, easy to break, and the cost is increased. The laser beam can reduce the spot diameter to the micron level, making it an ideal tool for processing microvias. Laser drilling is a contactless drilling technique that focuses the laser beam into a very small spot. The energy of the spot is melted or the gasified material forms micropores, which has the characteristics of high drilling speed, high efficiency, no tool loss, and high quality of processed surface. Particularly suitable for micro-hole drilling of composite materials. In particular, a large number of high-density group hole processing is performed on various materials such as hard, brittle, and soft.

The use of laser processing of composite materials is subject to complex physical and chemical changes. There are two main mechanisms for removing material:

1. Thermal processing mechanism: The laser heats the material to melt and vaporize the material;
2. Photochemical mechanism: Laser energy is used directly to overcome the chemical bonds between the molecules of a material, breaking the material into tiny gaseous molecules or atoms.

The key to drilling fiber reinforced composites is to choose the right laser source, mainly based on the characteristics of the material being processed. Such as the absorption of specific wavelength light, melting and gasification temperature, thermal conductivity and the like. Commonly used laser sources are CO2 laser, KrF excimer laser and Nd:YA G laser.

I. CO2 laser processing

The CO2 laser has a wavelength range of 9.3 to 10.6 μm and belongs to the infrared laser. The material to be cut is a thermal processing mechanism. When CO2 laser drills resin-based fiber reinforced composite materials, the influence of laser power and processing time on processing quality is relatively large. Setting the appropriate laser power and processing time can significantly improve the quality of the process. Aoyama et al. used a CO2 continuous laser with a wavelength of 10.6 μm and a maximum output of 25 OW to drill micropores with a diameter of 0.3 mm on a glass fiber/epoxy composite. It was found that when the laser power was 35W, the processing time was OAS, and the auxiliary gas was air, the epoxy resin on the surface of the hole wall showed almost no thermal damage; When the laser power is 75 W, the processing time is 0.1 s, and the assist gas is nitrogen, a black substance appears on the surface of the pore wall. This is because the laser energy continuously illuminates the resin, so that the temperature of the resin is not cooled, and when it accumulates to a certain extent, the resin is thermally damaged. Hirogaki et al. used a CO2 pulsed laser with a wavelength of 10.6 μm and a maximum output of 100 W to drill glass/epoxy and aramid fiber/epoxy composites. It was found that if the irradiation time is less than 5 ms, the epoxy resin hardly causes thermal damage. This is because reducing the irradiation time of the laser pulse can reduce the energy absorbed by the material. Moreover, the time interval between the pulses gives the material some cooling, so the thermal damage of the resin is further reduced.

II. KrF excimer laser processing
The KrF excimer laser has a wavelength of 248 nm and belongs to the ultraviolet laser. The material is a photochemical mechanism. High-energy ultraviolet photons can split the material directly into atoms for the purpose of cutting off the material. KrF excimer laser can significantly reduce laser processing thermal damage. Zheng et al. used a KrF laser with a wavelength of 248 nm, a pulse width of 20 ns, and an energy density of 400 nd/cm 2 to drill a glass fiber/epoxy composite. Not only does the black material appear on the wall of the hole, but the depth of the hole can be accurately controlled, and the drilling depth per pulse is 0.12 μm.

However, the KrF excimer laser may have a taper when drilling holes, which is due to the diffraction effect caused by the diffraction effect of the beam at the edge of the processed shape, which reduces the density of the energy and the etching rate; Another reason may be caused by the spherical deviation of the uncorrected prism. As the energy density increases, the taper gradually decreases, and even a negative taper occurs. This may be due to the fact that the beam energy density is greater than the critical energy at which diffraction occurs at the boundary and the defocusing causes the beam diameter to become larger.

III. Nd: YAG laser processing
Nd: YAG lasers are commonly used at wavelengths of 1.06 μm and 355 nm. They belong to infrared laser and ultraviolet laser respectively, and the two wavelengths correspond to the thermal processing mechanism and photochemical mechanism respectively. Laser power and pulse frequency have a major impact on thermal damage during Nd: AG laser drilling. Yang et al. used a Nd:YAG laser with a wavelength of 355 nm and an average power of 12 W to drill a 1.6 mm thick glass/epoxy composite. It was found that at a given pulse frequency, the higher the power, the higher the processing temperature. The coking of the epoxy resin and the melting of the glass fiber are accelerated, and the equivalent width of the thermal damage increases as the average laser power increases. At a given laser power, the equivalent width of thermal damage is greatest at a pulse frequency of 7 kHz; When it is less than 7KHz, it increases with the increase of frequency; Above 7 kHz, the width of the thermal damage decreases. This is because the higher the frequency, the shorter the time interval between laser pulses and the shorter the cooling time of the machined surface. When the frequency exceeds 7KHz, the higher the pulse frequency, the longer the pulse duration, the smaller the peak power of the laser pulse, the lower the temperature of the machined surface, and the equivalent width of the thermal damage is reduced. With Nd:YAG laser drilling with a wavelength of 355 nm, a power of 0.3 W, and a pulse frequency of 1 kHz, there was almost no thermal damage on the surface of the hole wall.

Due to the type of composite reinforcing fibers and the direction of the fibers of each layer. During the Nd:YAG laser drilling process, the accuracy of the hole decreases, the discontinuity of the hole at the interface between the layers, and the fiber expansion occur. Rodden et al. used a Nd:YAG laser with a wavelength of 1064 nm and a pulse width of 0.1 ms to drill a 2 mm thick carbon fiber/epoxy composite laminate; It was found that the shape of the hole changed from a circle to an ellipse and the shape of the hole at the interface between the layers was discontinuous. The former is because the heat transfer coefficient of the carbon fiber is much larger than the heat transfer coefficient of the epoxy resin, and the heat is first conducted along the direction of the carbon fiber, causing the hole to be stretched along the direction of the carbon fiber; The latter is because the carbon fiber directions of each layer are different, resulting in discontinuities in the pore shape between the layers. Cheng et al. used a Nd:YAG pulsed laser with a wavelength of 1.06 μm and a maximum average output energy of 135 W and a pulse duration of 0.5 to 5 ms to drill a carbon fiber/PEEK composite material of about 1 mm thick; It was found that the carbon fibers around the holes exhibited a radial expansion of up to 50% at the ends. The irreversible change of the partially filled structure due to the intense thermal expansion of the fiber and the rapid pressurization of the micropores in the fiber structure reinforce this effect.
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