PCB Technical - Different process methods of CO2 laser hole forming on HDI circuit board

Different process methods of CO2 laser hole forming on HDI circuit board

There are two main methods for CO2 laser drilling: direct hole forming method and shape mask hole forming method.

The so-called direct hole forming process is to adjust the diameter of the laser beam to the same diameter as the hole on the printed circuit board through the main control system of the equipment, and directly process the hole on the surface of the insulating medium without copper foil.

The coating process is to coat the surface of the printed circuit board with a special mask. The traditional process is to remove the coating window formed on the surface of the copper foil on the surface of the hole by exposure/development/etching process, and then irradiate the hole with a laser beam larger than the aperture to remove Exposed dielectric layer resin.

Their description is as follows:

(1) The diameter of the previous process is the same as the diameter of the copper window. If the operation is not careful, the position of the open window will deviate, which will cause the problem of misalignment with the center of the chassis. The deviation of the copper window may be due to the increase and shrinkage of the substrate material and the deformation of the film used for image transmission. Therefore, the process of opening a large copper window is to expand the diameter of the copper window to about zero. 05mm (usually determined according to the size of the aperture, when the aperture is 0.15mm, the diameter of the bottom pad should be about 0.25mm, and the diameter of the large window is 0.30mm). 15 mm, the diameter of the bottom pad should be about 0. 25 mm, the diameter of the large window is 0.30 mm), and then use laser drilling to burn off the micro blind holes on the chassis. Its main feature is the large degree of freedom, and the hole can be formed according to the insole pad procedure during laser drilling. This can effectively avoid the deviation caused by the copper window diameter and the hole diameter being the same, so that the laser spot cannot be aligned with the window, so that many incomplete half holes or residual holes appear on the surface of the large plate.

(2) Copper window method: First, a layer of RCC (resin-coated copper foil) is formed on the glass window by photochemical method, and then the resin is etched and exposed, and then the laser is fired into a micro blind hole. When the beam is strengthened, it passes through the aperture Achieve two sets of electrometer micro-moving mirrors, and then reach the excitation tube area through vertical alignment (Fθ lens), and then burn tiny blind holes one by one. After the electron fast beam is positioned within the area of a 1-inch square tube, the blind hole is 0.15 mm and can be shot into the hole three times in succession. The pulse width of the first gun is about 15μ, and provides energy to achieve the purpose of making holes. The latter gun is used to clean up residues and calibration holes at the bottom of the well wall. The SEM cross-section is 0.15 mm. The blind micro-hole has good laser energy control ability. When the chassis (target disk) is small, the chassis (target disk) needs large layout or second-order blind holes, it is difficult to complete the 45° picture achieve.

(3) In the process of directly forming holes on the resin surface, a variety of laser drilling methods are used for laser drilling.
A. Coat the upper layer of the substrate with resin copper foil on the inner laminate, and then etch all the copper foil to make the CO2 laser directly form holes on the exposed resin surface, and then perform hole treatment according to the plating hole process.
B. The substrate is made of FR-4 prepreg and copper foil instead of resin-coated copper foil.
C. Preparation of photosensitive resin coated copper foil
D. Use dry film as the dielectric layer, and press and paste the copper foil.
E. The process of coating other types of warm film and copper foil is made by coating other types of warm film and copper foil.

(4)Using the ultra-thin copper foil direct ablation process, the resin copper foil is pressed on both sides of the core board, and after coating the resin copper foil, the thickness of the copper foil is reduced to 5 microns by the "semi-etching method", and then black Oxidation treatment uses CO2 laser to form holes. The basic principle is that the surface of the copper foil after oxidation treatment absorbs light intensity. Under the premise of increasing the energy of the CO2 laser beam, holes can be formed directly on the surface of ultra-thin copper foil and resin.
But the most difficult thing is how to ensure that the "semi-erosion method" can obtain a uniform copper layer thickness, so special attention must be paid. Of course, we can use the back copper tear material (UTC), which is about 5 microns equivalent to a book of copper foil.

For this type of sheet processing, the following aspects are mainly adopted: This is mainly to put forward strict quality and technical indicators for the material supplier, and it is necessary to ensure that the difference in the thickness of the dielectric layer is between 51μm.

Because only by ensuring the uniformity of the dielectric thickness of the resin copper foil substrate, can the accuracy of the hole pattern and the cleanliness of the hole bottom be guaranteed under the same laser energy.

At the same time, in the subsequent process, the best decontamination process conditions must be used to ensure that the bottom of the blind hole is clean and free of residue, which has a good impact on the quality of the blind hole electroless plating and electroplating.

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