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 circuit 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.

Principle of laser drilling

Laser is a powerful beam excited when the "Ray" is stimulated by external stimuli to increase energy. Infrared light and visible light have thermal energy, and ultraviolet light has optical energy. When this type of light strikes the surface of the workpiece, three phenomena occur, namely reflection, absorption and penetration.

The laser spot hit on the substrate through another optical piece has a variety of modes, and three reactions will occur with the illuminated point.

The main function of laser drilling is to quickly remove the substrate material to be processed. It mainly depends on photothermal ablation and photochemical ablation, or resection.

(1) Photothermal ablation: refers to the principle that the processed material absorbs high-energy laser and 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 are black carbonization residues on the formed hole wall, which must be cleaned before porosity.

(2) Photochemical ablation: refers to the action of high-energy photons with high photon energy (more than 2eV electron volts) in the ultraviolet region and laser wavelength of more than 400 nm. This high-energy photon can destroy the long molecular chain of organic materials and become smaller particles, and its energy is greater than the original molecules, trying to escape from it. Under the pinch and absorption of external force, the substrate material is quickly removed to form micropores. Therefore, this type of process method does not contain hot burning, so it will not produce carbonization. Therefore, cleaning before porosity 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 for PCB drilling mainly include RF excited CO2 gas laser and UV solid-state Nd: YAG laser.

(3) About the absorbance of substrate: the success rate of laser is directly related to the absorbance of substrate material. The printed circuit board is composed of copper foil, glass cloth and resin. The absorbance of the three materials is also different due to different wavelengths, but the ultraviolet light of copper foil and glass cloth is 0.3m μ The absorptivity of the following areas is high, but it falls sharply after entering visible light and IR. Organic resin materials can maintain quite high absorptivity in all three spectra. This is the characteristic of resin material and the popular basis of laser drilling technology.

II. Different process methods of CO2 laser drilling

There are two main drilling methods of CO2 laser drilling: direct drilling method and conformal mask drilling method. The so-called direct hole forming process method is to modulate the laser beam through the equipment main control system to the same diameter as the hole on the processed printed circuit board, and directly carry out hole forming processing on the insulating medium surface without copper foil. The forming 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 forming window formed by the copper foil surface on the hole surface through the exposure / development / etching process. Then, these holes are irradiated with a laser beam larger than the aperture to cut off the exposed dielectric layer resin. Here are the following:

(1) Copper window method:

Firstly, a layer of RCC (resin coated copper foil) is re pressed on the inner plate to make the window by photochemical method, then the resin is etched to expose the resin, and then the substrate material in the window is burned by laser to form micro blind holes:

When the beam is enhanced, it reaches two groups of galvanometer type micro reflection scanning mirrors through the aperture and passes through one vertical alignment (f θ   Lens) to reach the tube area of the table that can be excited, and then burn micro blind holes one by one.

After being positioned by fast electron beam in the small tube area of one inch square, the blind hole of 0.15mm can be drilled by three shots. The pulse width of the first shot is about 15 μ s. At this time, energy is provided to achieve the purpose of pore forming. Then the gun is used to clean the residue on the hole wall and at the bottom of the hole and correct the hole.

SEM cross section and 45 degree full view of 0.15mm micro blind hole with good laser energy control. This open window hole forming process method is difficult to align when large typesetting or second-order blind hole is required when the bottom pad (target disk) is small.

(2) Process method for opening 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 slightly careless, the position of the opened window will deviate, resulting in the displacement of the position of the blind hole, resulting in misalignment with the center of the bottom pad. The deviation of the copper window may be related to the expansion and contraction of the substrate material and the negative deformation used in image transfer. Therefore, the process method of opening a large copper window is to expand the diameter of the copper window to about 0.05mm larger than 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 about 0.25mm and the diameter of the large window should be 0.30mm), and then conduct excitation drilling to burn out the position and accurately align the micro blind hole of the bottom pad. Its main feature is that it has a large degree of freedom. When laser drilling, you can select another program according to the inner bottom pad to form the hole. This effectively avoids the deviation caused by the same diameter of the copper window and the hole diameter, which makes the laser point unable to align the window, so that many incomplete half holes or residual holes will appear on the surface of large batch of large splicing plates.

(3) Direct pore forming process on resin surface

There are several types of laser drilling methods:

A. For the substrate, the resin copper foil is pressed on the upper layer of the inner plate, and then all the copper foil is etched and removed. Then, CO2 laser can be used to directly form holes on the exposed resin surface, and then continue to carry out pore treatment according to the plating hole process.

B. The substrate is made of FR-4 semi cured sheet and copper foil instead of resin coated copper foil.

C. Process for coating photosensitive resin and subsequent laminating copper foil.

D. The dry film is used as the dielectric layer and copper foil.

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

4) direct ablation of ultra-thin copper foil

After the resin copper foil is pressed and coated on both sides of the inner core plate, the thickness of the copper foil can be reduced to 5 microns by "semi etching method", and then black oxidation treatment can be carried out, and CO2 laser can be used to form holes.

The basic principle is that the oxidized black surface will strongly absorb light, and holes can be formed directly on the surface of ultra-thin copper foil and resin on the premise of improving the beam energy of CO2 laser. But the most difficult thing is how to ensure that the "semi etching method" can obtain a copper layer with uniform thickness, so we should pay special attention to it. Of course, back copper tearing material (UTC) can be used. The copper foil is 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 thickness difference of the dielectric layer is 510 μ M. Because only by ensuring the uniformity of medium thickness of resin coated copper foil substrate and under the action of the same laser energy, can we ensure the accuracy of hole pattern and the cleanness of hole bottom. At the same time, it is also necessary to adopt the best drilling pollution removal process conditions in the subsequent processes to ensure that the bottom of the blind hole is clean and free of residues after laser drilling. It will have a good effect on the quality of electroless plating and electroplating layer of blind hole.

Three Nd: YAG laser drilling process

Nd: YAG is neodymium and yttrium aluminum garnet. UV laser excited by two solid crystals. Recently, diode pulse excited laser beam is widely used, which can be made into an effective laser sealing system without water cooling. The third harmonic wavelength of the laser is 355 nm and the fourth harmonic wavelength is 266 nm. The wavelength is modulated by optical crystals.

The biggest feature of this type of laser drilling is that it belongs to the ultraviolet (UV) spectrum area, and the copper foil and glass fiber composed of copper-clad laminate have strong absorbance in the ultraviolet region. In addition, the light spot of this kind of laser is small and the energy is large, so it can strongly penetrate the copper foil and glass cloth to form holes directly. Because the heat of the above type of laser is small, carbon slag will not be generated after CO2 laser drilling, which provides a good surface treatment for the subsequent process of hole wall.

Nd: YAG laser technology is used to process blind holes and through holes on many kinds of materials. Among them, through holes are drilled on polyimide copper-clad laminate with a minimum pore diameter of 25 microns. From the analysis of manufacturing cost, the most economical diameter is 25125 microns. The drilling speed is 10000 holes / min. Direct laser punching process can be adopted, with a maximum aperture of 50 microns. The inner surface of the formed hole is clean without carbonization, and it is easy to electroplate. Similarly, through holes can also be drilled in PTFE copper-clad laminate. The minimum hole diameter is 25 microns, and the most economical diameter is 25125 microns. The drilling speed is 4500 holes / min. There is no need to pre etch the window. The formed hole is very clean and does not require additional special treatment process requirements. There are other materials, such as forming hole processing. The following process methods can be adopted in specific processing:

(1) According to the speed of the two types of laser drilling, two methods are adopted

The basic operation method is to first ablate the copper foil on the upper surface of the hole with YAG, and then directly ablate the resin with CO2 laser faster than YAG drilling.

IV. quality problems in actual production

In the process of laser drilling, there are many quality problems, so we are not going to talk about them comprehensively, but only put forward the most prone quality problems for peer reference.

(1) Misalignment between CO2 laser drilling position and bottom target position by copper window method

In laser drilling, beam positioning system is very important for the accuracy of aperture forming. Although the accurate positioning of the beam positioning system is adopted, the lack of flame with hole shape deformation often occurs due to the influence of other factors. The causes of quality problems during production are analyzed as follows:

1. Make the negative film of inner core plate pad and wire pattern and the negative film for opening the window after adding the layer of resin coated copper foil (RCC), because both of them will be the potential factor of size increase and decrease due to the influence of humidity and temperature.

2. When the core plate is used to make the wire pad pattern, the size of the substrate itself increases and decreases, and after the high-temperature pressure coating resin copper foil (RCC) is added, the size of the inner and outer substrate materials increases and decreases.

3. The size and position of the copper window opened by etching will also produce errors.

4. The error caused by the between the light spot of the laser machine itself and the table displacement.

5. The second-order blind hole alignment is more difficult, which is more likely to cause position error.

According to the above cause analysis, relevant technical data mastered by the production and experience in the actual operation process, the main process countermeasures are as follows:

1. Reduce the typesetting size. Most manufacturers make multi-layer boards, and the typesetting is 450 × 600 or 525 × 600(mm)。 However, for the mobile phone board with processing wire width of 0.10mm and blind hole aperture of 0.15mm, it is best to use typesetting size of 350 × 450 (mm) upper limit.

2. Increase the laser diameter: the purpose is to increase the coverage of the copper window. The specific method is "beam diameter = hole diameter + 90 ~ 100" μ m。 When the energy density is insufficient, you can shoot one or two more shots to solve it.

3. Adopt the process method of opening a large copper window: at this time, only the size of the copper window becomes larger, but the aperture has not been changed. Therefore, the diameter of the laser formed hole is no longer completely determined by the window position, so that the hole position can burn the hole directly according to the target position of the bottom pad on the core plate.

4. Change the photochemical imaging and etching window opening method to YAG laser window opening method: first open the window according to the reference hole of the core plate by using the YAG laser spot, and then burn the hole according to the window position by CO2 laser to solve the error caused by imaging.

5. The second-order micro blind hole method: when a layer of resin coated copper foil (RCC) is accumulated on both sides of the core plate, if it is necessary to accumulate RCC again and make the second-order blind hole (i.e. accumulate two), the alignment of the "accumulate two" blind hole must be formed according to the aiming "accumulate one". The original target of the core plate can no longer be used. That is, when the "product one" is formed into a hole and pad, the plate edge will also make a target. Therefore, after the RCC of "Ji'er" is pressed and pasted, another four mechanical reference holes of "Ji'er" can be drilled through the X-ray machine for the target on "ji'i", and then the holes and lines can be formed. This method can make "Ji'er" align with "ji'i" as much as possible.

2. Incorrect hole type

According to the accumulated production experience for many times, it is mainly due to the quality problems of the adopted base material molding. The main quality problem is that the thickness of the dielectric layer is inevitably different after the resin coated copper foil is pressed. Under the same drilling energy, the bottom pad of the thinner part of the dielectric layer will not only bear more energy, but also reflect more energy, Therefore, the hole wall is made into an outward expanding pot shape. This will have a great impact on the quality of electrical interconnection between laminates.

Due to the incorrect pass pattern, a series of technical problems will be brought to the reliability of the high-density interconnection structure of multilayer printed circuit board.

Therefore, technological measures must be adopted to control and solve. The following process methods are mainly adopted:

(1) The thickness difference of dielectric layer during resin coated copper foil pressing shall be strictly controlled within 510 μ M.

(2) By changing the energy density and pulse number (gun number) of laser, the process conditions of mass production can be found out through experimental methods.

(3) The removal of rubber slag at the hole bottom and slag breaking at the hole wall is poor.

This kind of quality problem is the most likely to occur, because a little improper control will produce this type of problem. Especially for the laminated board with porous type on the large plate, it is impossible to ensure that there are no quality problems. This is because there are too many micro blind holes on the processed large row of plates (about 60000 ~ 90000 holes on average), and the thickness of the dielectric layer is different. When laser drilling with the same energy is adopted, the thickness of the residual rubber residue on the bottom pad is also different. After drilling dirt removal, it is impossible to ensure that all residues are completely clean. In addition, the inspection means are poor. Once there are defects, it will often cause the bonding force between the subsequent copper plating layer and the bottom pad and the hole wall.