Laser Drilling Process Introduction And Common Solution of HDI Products Problem (2)

2. Different Process Methods of CO2 Laser Hole Forming

The drilling methods of CO2 laser hole forming mainly include direct hole forming method and conformal mask hole forming method. The so-called direct hole forming method is to modulate the diameter of the laser beam through the main control system of the equipment to be the same as the diameter of the hole on the printed circuit board to be processed, and directly perform hole forming on the surface of the insulating medium without copper foil. The conformal mask process method is to coat a special mask on the surface of the printed board, and use the conventional process method to remove the conformal window formed by the copper foil surface on the hole surface through the exposure/development/etching process. These holes are then irradiated with a laser beam larger than the aperture, excising the exposed dielectric layer resin. They are introduced as follows:

(1) Method of Opening Copper Windows:
First, a layer of RCC (resin-coated copper foil) is laminated on the inner layer board to form a window by photochemical method, and then the resin is exposed by etching, and then the substrate material in the window is burned by laser to form a micro-blind hole:

When the beam is enhanced, it reaches two groups of galvanometer-type micro-moving reflective scanning mirrors through the aperture, and is vertically aligned (Fθ lens) to reach the tube area of ​​the table that can be stimulated, and then fired micro blind holes one by one.
After the positioning of the electron fast beam in the small tube area of ​​1 inch square, the blind hole of 0.15mm can be shot three times in a row to form a hole. The pulse width of the first gun is about 15 μs, and energy is provided at this time to achieve the purpose of forming holes. Then the gun is used to clean the residue on the bottom of the hole wall and correct the hole.The SEM cross-section of the 0.15mm micro-blind hole with good laser energy control and the full picture of 45 degrees. This kind of hole-forming process method of opening the window requires large typesetting or second-order blind hole when the bottom pad (target plate) is not large. , its alignment is more difficult.

(2) The Process Method of Opening The Large Window:

The diameter of the hole formed by the former process method is the same as that of the opened copper window. If the operation is a little careless, the position of the opened window will be deviated, resulting in problem in the position of the blind hole formed by the hole and the misalignment with the center of the bottom pad. The reason for the deviation of the copper window may be related to the expansion and contraction of the substrate material and the deformation of the negative film used for image transfer. Therefore, the process method of opening a large copper window is to expand the diameter of the copper window to be about 0.05mm larger than that of the bottom pad (usually determined according to the size of the aperture, when the aperture is 0.15mm, the diameter of the bottom pad should be 0.25mm). The diameter of the large window is 0.30mm) and then laser drilling is performed to burn out the micro-blind holes whose positions are precisely aligned with the bottom pad. Its main feature is that it has a large degree of freedom of choice. When laser drilling, you can choose to make holes according to the program of the inner bottom pad. This effectively avoids the fact that the laser spot cannot be aligned with the window due to the offset caused by the copper window diameter and the hole-forming diameter so that there will be many incomplete half holes or residual holes on the surface of the large-scale panel.

(3) Direct Pore Forming Process on Resin Surface

There are several types of laser drilling methods using laser drilling:

A. The substrate is laminated with resin-coated copper foil on the inner layer board, and then all the copper foil is etched away, and CO2 laser can be used to directly form holes on the exposed resin surface, and then continue to carry out hole processing according to the plated-through hole process.

B. The substrate is a similar manufacturing process using FR-4 prepreg and copper foil instead of resin-coated copper foil.

C. The process method of coating photosensitive resin followed by laminating copper foil is made.

D. It is made by pressing dry film as dielectric layer and copper foil.

E. It is made by coating other types of warm film and copper foil lamination.

(4) The Process Method of Direct Ablation Using Ultra-Thin Copper Foil:

After the two sides of the inner core board are pressed and coated with resin copper foil, the “half etching method” can be used to reduce the thickness of the copper foil from 17 m to 5 microns after etching, and then conduct black oxidation treatment, and CO2 laser can be used to form holes.
The basic principle is that the oxidized black surface will strongly absorb light, and under the premise of increasing the beam energy of the CO2 laser, holes can be formed directly on the surface of the ultra-thin copper foil and resin. But the most difficult thing is how to ensure that the “half etch method” can obtain a copper layer of uniform thickness, so special attention should be paid to the production. Of course, the copper-backed tearable material (UTC) can be used, and the copper foil is quite thin about 5 microns.
According to this type of plate processing, the following aspects are mainly adopted in the process:
This mainly puts forward strict quality and technical indicators for material suppliers to ensure that the difference in the thickness of the dielectric layer is between 510μm. Because only by ensuring the uniformity of the medium thickness of the resin-coated copper foil substrate, under the action of the same laser energy, the accuracy of the hole pattern and the cleanness of the bottom of the hole can be ensured. At the same time, it is also necessary to use the best decontamination process conditions in the subsequent process to ensure that the bottom of the blind hole is clean and free of residues after laser hole formation. It will have a good effect on the quality of blind hole electroless plating and electroplating layer.

3. Nd: YAG Laser Drilling Process Method

Nd: YAG is neodymium and yttrium aluminum garnet. UV lasers coexcited by two solid-state crystals. The laser beam excited by diode pulses is used more recently, and it can be made into an effective laser sealing system without water cooling. This laser has a wavelength of 355 nanometers (nm) for the third harmonic and 266 nanometers (nm) for the fourth harmonic, and the wavelength is modulated by an optical crystal.

The biggest feature of this type of laser drilling is that it belongs to the ultraviolet (UV) spectral region, and the copper foil and glass fiber composed of copper clad laminates have strong absorbance in the ultraviolet region. The light spot of this kind of laser is small and the energy is high, so it can penetrate the copper foil and glass cloth strongly to form holes directly. Due to the small heat of the previous type of laser, it will not generate carbon residue like CO2 laser drilling, which provides a good surface for the subsequent process of the hole wall.

Nd: YAG laser technology processes blind holes and through holes in many materials. Among them, through-holes are drilled on the polyimide copper-clad laminate, and the minimum diameter is 25 microns. From the manufacturing cost analysis, the most economical diameter used is 25125 microns. The drilling speed is 10000 holes/min. The direct laser punching process method can be used, and the maximum aperture is 50 microns. The inner surface of the formed hole is clean and free of carbonization, and it is easy to be electroplated. Via holes can also be drilled in PTFE copper clad laminates, with a minimum diameter of 25 microns, and the most economical diameter used is 25125 microns. The drilling speed is 4500 holes/min. No pre-etched windows are required. The resulting holes are very clean and do not require additional special processing requirements. There are other material forming hole processing and so on. The following process methods can be used in specific processing:

According to the speed of the two types of laser drilling, two combined process methods are adopted. The basic operation method is to first use YAG to ablate the copper foil on the upper surface of the hole, and then use a CO2 laser that is faster than YAG to directly ablate the resin to form a hole.