Laser drilling technology and solutions to common problems of HDI products in the PCB industry

Due to the rapid development of microelectronics technology and the widespread application of large-scale integrated circuits, the manufacturing of printed circuit boards has developed in the direction of lamination and multi-function, making the linear shape of printed circuit graphics fine and the spacing of micro-holes smaller. Therefore, the mechanical method used in the processing process can no longer meet the requirements, and the rapidly developing micro-hole processing method, namely laser drilling technology, is one of them.

Principle of laser drilling:

When the "light" is stimulated by the outside world, the laser will produce a very high-intensity beam, which will produce infrared and visible light when the energy is increased, while ultraviolet light has optical energy. After this type of light is shot into the surface of the workpiece, three phenomena will occur: reflection, absorption, and penetration.

The laser light spot that hits the substrate through the selection of light has many ways of formation, and the reaction with the illuminated point can be divided into three types.

Its main function is to quickly remove the processed substrate, mainly relying on photothermal ablation and photochemical ablation or resection.

(1) Photothermal ablation: refers to the principle that the treated material absorbs high-energy laser, heats to melting in a very short time, and then evaporates to form pores. In the process method of the present invention, when the substrate is subjected to high energy, there will be burnt black carbonized residues on the pore walls formed, which must be removed before pore formation can be carried out.

(2) Photochemical ablation: It is due to the generation of high photon energy in the ultraviolet band (electron volts above 2eV), and high-energy photons are generated at laser wavelengths above 400 nanometers. This type of high-energy photons will destroy the long molecular chains of organic materials and turn them into smaller particles, while high-energy photons will be larger than the original molecules and will be released under the action of external forces, causing the substrate material to quickly fall off and form micropores. Therefore, different process methods do not contain thermal burning, and will not produce carbonization. Therefore, cleaning before pore formation is very simple.

The above is the basic principle of laser drilling technology. Among the existing laser drilling methods, CO2 gas laser and UV solid-state Nd:YAG laser drilling, which are mainly used for printed circuit board drilling, are the most common.

In terms of substrate absorption: the success rate of laser is directly related to the absorption rate of substrate material. Printed circuit boards are composed of copper foil, glass cloth and resin. The absorbance of these three materials is also different. Among them, copper foil and glass cloth have higher absorbance below 0.3m渭 in ultraviolet light, but after entering the visible light IR and infrared spectrum, the absorbance decreases significantly. Organic resin materials can maintain high absorbance in all three spectrums. This characteristic of resin materials is the basis for the current laser production process.

There are many methods for CO2 laser drilling technology:

There are two main drilling methods for CO2 laser drilling: direct drilling method and membrane drilling method. The direct hole-forming process is adopted, that is, the light beam is modulated to the same aperture as the processed printed circuit board through the main control system of the equipment, and the hole is formed directly on the surface of the insulating medium without using copper foil as the insulating medium. The cladding method is to cover the surface of the printed circuit board with a special mask, and then use conventional process methods to remove the cladding window formed by the copper foil surface on the surface of the cladding hole through exposure/development/etching process. Then, a laser beam larger than the aperture is used to irradiate these holes to remove the resin of the exposed dielectric layer. The following is our respective introduction:

(1) Copper window method:

First, RCC (copper foil layer coated with resin) is pressed on the inner layer board, and a window is made by photochemical method. Then, the resin is exposed by etching, and then the substrate material in the window is burned by laser to form micro blind holes:

After enhancement, the light beam passes through the aperture to reach two sets of galvanometer-type micro-reflection scanning mirrors, and after a vertical alignment (F-胃 lens), it reaches the tube area on the surface of the excitation table, and then burns into micro blind holes one by one.

After this positioning using electron fast beam, three holes can be made for 0.15 mm blind holes in a small area of 1 square inch. In this case, the pulse width of the first wave is about 15 microseconds, providing energy to achieve the purpose of hole formation. The gun is also used to remove residues at the bottom of the hole wall and correct holes.

In the 0.15 mm micro blind hole, laser energy is used to control the SEM cross section and 45-degree full image. This window opening method uses a large layout or second-order blind hole when the base pad (target disk) is not large. This method is difficult to implement.

(2) Opening large window program method:

The diameter of the former hole is the same as the diameter of the copper window. If a little pressure is applied, the position of the window will deviate, causing the position of the blind hole to move, resulting in the problem of inconsistency with the center of the bottom pad. The reason for this copper window offset is likely related to the expansion and contraction of the substrate and the deformation of the film used for image transmission. Therefore, a process method is used to open the copper window, that is, to increase the copper window diameter to about 0.05mm, which is larger than the base pad (usually determined by the aperture size). When the aperture is 0.15mm, the base pad diameter should be about 0.25mm, and its large window diameter is 0.30mm). Then laser irradiation is performed to burn out micro blind holes with accurate positions, which can be used to make precise copper window base pads. Its main feature is that it has a large degree of freedom of choice. When laser irradiation is performed, it can be selected to punch holes according to the procedure of the inner base pad. This method effectively avoids the offset caused by the copper window diameter being the same as the aperture, so that the laser point cannot be aligned with the positive window, resulting in a large number of large-size puzzle surfaces with many half holes or residual holes.

(3) Direct hole forming technology on resin surface:

There are several types of process methods that can be used for laser hole forming:

The substrate is coated with resin copper foil on the inner layer, and then the copper foil is completely etched away. Then, CO2 laser can be used to directly punch holes on the resin surface, and then continue to punch holes according to the plating process.

The process method of using FR-4 semi-cured sheet and copper foil instead of resin-coated copper foil is similar to that of copper foil.

The process method of coating photosensitive resin and subsequent lamination of copper foil.

Dry film is used as the dielectric layer and is prepared by pressing process together with copper foil.

Used in the process of making other types of warm film and copper foil lamination.

(4) Process of direct ablation of ultra-thin copper foil:

After laminating the inner core board with resin copper foil on both sides, the copper foil thickness of 17 渭m can be thinned to 5 渭m by "half-etching method" and then subjected to black oxidation treatment to obtain CO2 laser drilling.

The basic principle of this method is that the black surface after oxidation treatment has strong light absorption to CO2 laser. Under the premise of increasing the laser beam energy, holes can be directly punched on the ultra-thin copper foil and resin surface. However, the most difficult part is how to ensure that the "half-etching method" can obtain a copper layer of uniform thickness, so special attention should be paid during production. Of course, copper-backed tearable materials (UTC) can be used, and the copper foil is equivalent to 5 microns.

For this type of plate processing, the following methods are currently adopted in terms of technology:

This mainly requires strict quality and technical indicators to material suppliers to ensure that the dielectric layer thickness is between 510 microns. Only under the same laser energy can the uniformity of the dielectric thickness of the resin-coated copper foil substrate be guaranteed, and the accuracy of the hole shape and the cleanliness of the hole bottom be guaranteed. In the next step of processing, the best drilling and decontamination process conditions should be selected to ensure that the bottom of the blind hole is clean and free of residue after laser drilling. It can achieve good results for chemical plating and electroplating of blind holes.

YAG laser process method:

YAG is neodymium and yttrium aluminum garnet. Ultraviolet laser is excited by two crystals. The laser beam excited by diode pulse can be made into an efficient laser sealing system without water cooling. The third harmonic wavelength in this laser is 355nm and the fourth harmonic wavelength is 266nm. This is a laser made of optical crystal.

The biggest feature of this type of laser is that it belongs to the ultraviolet (UV) band. The copper foil and glass fiber composed of the copper-clad laminate have strong light absorption in the ultraviolet band. In addition, the light spot of this type of laser is small and the energy is large, so it can strongly penetrate the copper foil and glass cloth to make it directly perforated. Another laser heat source is smaller, unlike CO2 lasers that produce carbon slag, which provides a good surface treatment for the subsequent processes of the hole wall.

YAG laser technology can process micro-blind holes and through holes on a variety of materials. The minimum diameter of the via hole of the polyimide copper-clad laminate is 25 microns. Through cost analysis, the most economical diameter is 25125 microns. The high speed can reach 10,000 holes/second. Direct laser punching can reach a maximum aperture of 50 microns. The inner surface of the formed hole is clean and carbonized, and it is easy to pressurize. The same can be done on PTFE copper foil laminates, with a minimum hole diameter of 25 microns and the most economical diameter of 25125 microns. The high speed can reach 4500 holes/second. No need to etch the window in advance. The processed holes are very clean and do not require additional processing technology. There are also mold hole processing for other materials. In specific processing, the following process methods can be used:

Two parallel process methods are used according to the speed of the two types of lasers.

The basic working method is to first use YAG to ablate the copper foil on the upper surface of the hole, and then use CO2 laser, which is faster than YAG, to directly ablate the resin to form holes.

Quality problems in the production process.

There are many quality problems in the production process of lasers, which are not fully elaborated. Only the most likely quality problems are proposed for reference by peers.

The copper window method is used to measure the misalignment between the position of the CO2 laser and the bottom target position.

The beam positioning system is a key technology that is extremely critical to the accuracy of beam forming. Although the beam positioning system is used for precise positioning, due to the influence of other factors, the phenomenon of flame loss due to void deformation often occurs. The reasons for the quality problems in production are analyzed as follows:

(1) The film of the inner core plate welding plate is made, and the film is used on the window after the resin-coated copper foil (RCC) layer is added, because these two films will become larger or smaller due to the influence of humidity and temperature.

(2) When making the wire pad pattern on the core board, the expansion and contraction rate of the substrate itself, and the expansion and contraction rate of the inner and outer layer material size after high temperature pressing and pasting the resin coated copper foil (RCC).

(3) The size and position of the etched copper window will also cause errors.

(4) The error caused by the displacement of the light spot and the table of the isotope machine itself.

(5) Second-order blind holes are more difficult and more likely to cause positioning errors.

On this basis, combined with the relevant technical information and actual operation experience mastered in production practice, the following process countermeasures were mainly adopted:

(1) In order to reduce the layout, most manufacturers use 450脳600 or 525脳600 (mm) multilayer boards to make the layout. However, for mobile phone boards with a processing wire width of 0.10mm and a blind hole diameter of 0.15mm, it is best to use a printing size upper limit of 350脳450 (mm).

(2) Increase the laser diameter: Its purpose is to expand the coverage range of the copper window and make it larger. Its specific approach is "beam diameter = aperture + 90~100渭m". When the energy density is insufficient, it can be solved with one or two shots.

(3) Adopting the copper window enlargement process: In this case, only the size of the copper window is enlarged, but the aperture does not change. Therefore, the diameter of the laser hole is no longer completely determined by the position of the window, so that the hole position can be directly determined according to the position of the target plate on the core board.

(4) Changing the photochemical imaging and etching window opening method to the YAG laser window opening method: that is, using the YAG laser light spot to first open a window on the core board, and then using the CO2 laser to cut its window position to solve the imaging error.

(5) Lamination and secondary reprocessing of the second-order micro-blind method: After a layer of resin copper foil is laminated on both sides of the core board, it is necessary to laminate once more for RCC and second-order micro-blind processing. The "second layer" of the blind hole corresponding to the "second layer", that is, the hole corresponding to the "first layer", must be processed according to the aiming point "first layer". And the original target of the core board can no longer be used. That is, when the "Ji Yi" is made into holes and pads, the side of its plate will also be made into targets. Therefore, after the RCC of "Ji Er" is pressed and pasted, the X-ray machine can be used to align the target on "Ji Yi", drill "Ji Er" on the four mechanical reference holes of "Ji Er", and then make holes and lines. In this way, "Ji Er" can be made as close to "Ji Yi" as possible.

Hole type error:

Through the accumulation of many production experiences, it is mainly due to the quality problems existing when using substrate molding. The main quality problem is that the thickness of the dielectric layer of the resin-coated copper foil is bound to be different. Under the same pressure, the bottom plate of the thin dielectric layer can not only withstand more energy, but also reflect more energy, thereby punching the hole wall into a pot shape that expands outward. It has a great impact on the quality of electrical interconnection between layers of the build-up-multilayer board.

Due to the hole type error, the reliability of the high-density interconnection structure of the built-up multilayer printed circuit board will cause a series of technical problems.

Therefore, process measures must be taken to control and solve them. This article mainly adopts the following process methods:

(1) Strictly control the thickness difference of the dielectric layer when coating the resin copper foil to 510渭m.

(2) Changing the energy density and pulse number (number of guns) of the laser can experimentally determine the process conditions for mass production.

(3) The glue residue at the bottom of the hole and the debris on the hole wall are not cleaned well.

This type of quality problem is the most likely to occur, because this type of problem will occur if it is slightly controlled. Especially when dealing with porous laminates on large panels, it is impossible to guarantee 100% that there will be no quality problems. The reason is that there are a large number of micro blind holes (about 60,000 to 90,000 small holes on average) on the processed large-format boards, and the thickness of the dielectric layer is different, and the thickness of the glue residue left on the bottom pad is different when the laser drilling with the same energy is used. In the case of defects, if the drilling pollution is not treated, it cannot be guaranteed to be completely clean. In addition, the inspection means are poor, which often leads to the subsequent copper plating layer and the bottom pad and the hole wall.