Introduction of laser drilling process for HDI products and solutions to common problems

- Oct 17, 2018-

The principle of laser hole formation

Laser is a powerful beam of light that is excited when the "ray" is stimulated by external stimuli. Infrared light and visible light have thermal energy, while ultraviolet light has optical energy.This type of light to the surface of the workpiece happens three phenomenon namely reflection, absorption and penetration. 

There are various patterns of laser light points which are struck on the substrate by the optical other, and there are three kinds of reactions with the illuminated point. 

The main role of laser drilling is to be able to quickly remove the substrate material to be processed. 

(1) thermal ablation: refers to the processed material absorption of the high-energy laser, heated to melt in a very short time and the pore forming principle of evaporated.This process under the action of substrate materials by high energy, in the formation of the hole wall have blackened coking residue, Kong Hua before must be cleaned up. 

(2) photochemical ablation: refers to the ultraviolet area of specular energy electron volts (more than 2 ev), more than 400 nm laser wavelength of high-energy photons work results.This high-energy photon can destroy the long molecular chain of organic materials and become a smaller particle, and its energy is greater than the original molecule, so as to escape from it. Under the pinch and suction of external forces, the substrate material is quickly removed to form micropores.Types of process method, therefore, does not contain thermal burn, also won't produce coking phenomenon.So, Kong Hua before cleaning is very simple. 

  

Above is the basic principle of laser into a hole.Currently there are two kinds of the most commonly used method of laser drilling: PCB drilling with laser mainly include RF excited CO2 gas laser and UV solid Nd: YAG laser. 

(3) on the substrate absorbance: laser of high and low success rate and the absorbance of substrate materials have a direct relationship.Printed circuit board is made up by copper foil and glass cloth and resin, the absorbance of the three materials are different because of different wavelengths in the uv 0 but the copper foil and glass cloth.3 m mu area below the absorption rate is higher, but after getting into visible light and IR drop dramatically.Organic resin material in three sections of the spectrum, can maintain a fairly high absorption rate.This is the characteristics of the resin materials, is the foundation of the laser drilling process is popular. 

The CO2 laser hole of different process methods

CO2 laser into the hole drilling method mainly include direct pore forming method and conformal mask into two hole method.So-called pore forming process is a laser beam directly by the equipment control system to the diameter of the beam modulation and the processing of printed circuit boards the same hole diameter, in the absence of copper foil directly on dielectric surface using the pore forming process.The method of coating mask is to apply a special mask on the surface of the printing plate.Then use these holes is greater than the diameter of laser beam, removal of exposed dielectric layer of resin.Are respectively introduced as follows:

(1) copper Windows open method:

First of all, a layer of RCC layer board after pressure (resin coated copper foil) made by photochemical methods window, then etching with resin, then USES the laser burn window substrate material is the formation of micro blind hole:

When the beam aperture to two groups by the enhanced micro reflections of the galvanometer scanning mirror, and a vertical for theta lens (F) and reach the district of the mesa can be excited, and then burn into the blind holes one by one.

In a square inch of the small batch Canon of fast electron beam after the positioning, blind hole of 0.15 mm can even play 3 guns into the hole.The first shot of the pulse width of about 15 mu s, provide energy to achieve the purpose of pore forming at this time.Gun used to again after cleaning eye wall residue and correction of holes in the bottom of the bore. 

Laser energy control good 0.15 mm micro blind hole of SEM cross-section map and 45 degrees, this kind of the pore forming technology of the open window, when little cushion (target) and need big typography or second-order blind holes, the alignment is more difficult.