With the rapid development of microelectronics technology, the wide application of large-scale and super large-scale integrated circuits and the progress of micro-assembly technology, the manufacturing of printed circuit board is developing towards lamination and multi-function, and the graphic wires of printed circuits are thin, microporous and narrow spacing, The mechanical drilling technology used in processing can not meet the requirements, and a new micro-hole processing method, namely laser drilling technology, has developed rapidly.
1. The Principle of Laser Hole Forming
A laser is a powerful beam when the “ray” is stimulated by external stimulation to increase its energy. Among them, infrared light and visible light have thermal energy, and ultraviolet light has optical energy. When this type of light strikes the surface of a workpiece, three phenomena occur reflection, absorption, and penetration.
The laser light spot hits the substrate through the optical component, and its composition has various modes, and there are three kinds of reactions with the illuminated spot.
The main function of laser drilling is to quickly remove the substrate material to be processed. It mainly relies on photothermal ablation and photochemical ablation or so-called cutting.
(1) Photothermal Ablation:
It refers to the pore-forming principle in which the processed material absorbs high-energy laser light, is heated to melt and evaporated in a very short time. In this process, under the action of high energy on the substrate material, there is a blackened carbonization residue on the formed hole wall, which must be cleaned before the hole is formed.
(2) Photochemical Ablation:
It refers to the effect of high photon energy (over 2 eV electron volts) in the ultraviolet region and high energy photons with a laser wavelength exceeding 400 nanometers. And this high-energy photon can destroy the long molecular chain of organic materials and become smaller particles, and its energy is greater than that of the original molecule, and it tries to escape from it. Under the condition of external force, the substrate material is quickly removed and Micropores are formed. Therefore, this type of process method does not contain thermal burning, so it will not produce carbonization. Therefore, cleaning before pore formation is very simple.
The above is the basic principle of laser drilling. At present, there are two most commonly used laser drilling methods: the lasers used for drilling printed circuit boards mainly include RF-excited CO2 gas lasers and UV solid-state Nd:YAG lasers.
(3) Regarding The Absorbance of The Substrate:
The success rate of the laser is directly related to the absorbance of the substrate material. The printed circuit board is composed of copper foil, glass cloth and resin. The absorbance of these three materials is also different due to different wavelengths, but copper foil and glass cloth have higher absorption rates in the area below 0.3mμ of ultraviolet light, but it fell sharply after entering visible light and IR. Organic resin materials can maintain a relatively high absorption rate in the three-segment spectrum. This is the characteristic of resin materials and is the basis for the popularity of the laser drilling process.
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