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

- Aug 18, 2018-

With the rapid development of microelectronics technology, large scale and wide application of very large scale integrated circuit, micro assembly technology progress, make the printed circuit board manufacturing toward the horizon, muti_function change direction, graphics printed circuit conductor is fine, micro Kong Hua narrow spacing, used for the processing way of mechanical drilling technology has developed quickly and can't meet the requirements of a new type of micro hole machining method, laser drilling technology. 

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.Three phenomena occur when this type of light hits the surface of the workpiece: 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) photothermal ablation: the pore-forming principle that the processed material absorbs high-energy laser and is heated to melt and evaporate in a very short time.Under the action of high energy on the substrate material, the black charred residue is formed on the hole wall, which must be cleaned before the pore is formed. 

(2) photochemical ablation: it is the result of the action of high-photon energy (over 2eV electron volts) in the ultraviolet region and high-energy photons whose laser wavelength is over 400 nanometers.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.Therefore, this kind of technological method does not contain heat burning, which will not produce carbonization.Therefore, it is very simple to clear before the hole. 

  

These are the basic principles of laser drilling.At present, there are two most commonly used laser drilling methods: the laser used for PCB drilling mainly includes CO2 gas laser excited by RF and UV solid Nd: YAG laser. 

(3) regarding substrate absorbance: the success rate of laser is directly related to the absorbance of substrate material.Printed circuit boards are made of a combination of copper foil, glass cloth and resin. The absorbance of these three materials varies according to the wavelength, but copper foil and glass cloth are in the ultraviolet light 0.The absorptivity was higher in the region below 3m, but it dropped off significantly after entering visible light and IR.The organic resin material can maintain a high absorption rate in the three spectra.This is the characteristic that resin material has, it is the basis of laser drilling technology popular. 

Different process methods for laser drilling of CO2

There are two drilling methods for CO2 laser hole formation: direct hole formation and mask hole formation.The so-called direct hole forming process is to adjust the diameter of the laser beam through the main control system of the equipment to the same diameter as the hole on the printed circuit board.The method of coating mask is to apply a special mask on the surface of the printing plate.A laser beam larger than the aperture is then applied to these holes to remove the exposed medium layer resin.Respectively introduced as follows:

(1) copper window opening:

First, a layer of RCC(resin coated copper foil) is laminated 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 a micro-blind hole:

When the beam is enhanced, it passes through the aperture to reach two sets of galvanometer scanning micromirror and reaches the tube area of the exciting mesa through a vertical alignment (F colored lens), and then burns the microblind hole one by one.

Three rounds of holes can be made for the 0.15mm blind hole after the electron fast beam is positioned inside the 1-inch square tube.The pulse width of the first gun is about 15 pa, at which time energy is provided for the purpose of hole formation.After the gun is used to clear the hole wall bottom debris and correction hole. 

The SEM cross section of 0.15mm micro-blind hole with good laser energy control and the full picture of 45 degrees. Such a hole forming process in the open window is more difficult when the underlay (target disc) is small and needs large typesetting or second order blind hole. 

(2) open large window process:

The diameter of the hole formed in the former process is the same as the copper window opened. If the operation is careless, the position of the window opened will be deviated, resulting in the position of the blind hole formed in the hole, resulting in the misalignment with the center of the bottom cushion.The reason for the deviation of the copper window may be that the shrinkage of substrate material is related to the film deformation used in image transfer.Therefore, the process method of opening large copper Windows is adopted, which is to expand the diameter of the copper window to around 0.05mm larger than the bottom cushion (usually determined according to the size of the aperture, when the aperture is 0.15mm, the diameter of the bottom cushion should be at 0.About 25mm, with a large window diameter of 0.30mm) and then the laser drilling can be fired to precisely align the small blind hole of the bottom cushion.Its main feature is that the degree of freedom is large, when the laser drilling can choose another program according to the inner cushion to form holes.This effectively prevents the laser point from aligning the window due to the deviation caused by the copper window diameter and the hole diameter at the same time, resulting in the phenomenon of incomplete half-hole or residual hole on the large volume of the stencil surface. 

(3) direct hole formation process of resin surface

There are several types of laser drilling techniques:

A.The substrate is coated with resin copper foil by the upper layer of the inner plate, and then the copper foil is etched and removed. Then, CO2 laser can be used to directly form holes on the surface of the bare resin, and then the pores can be processed according to the hole plating process. 

B.The substrate is a similar fabrication process using fr-4 semi-cured sheets and copper foil in place of resin coated copper foil. 

C.The process of coating the subsequent laminated copper foil with photographic resin. 

D.Using dry film as medium layer and copper foil lamination process. 

E. Process method for coating other types of warm film and copper foil. 

  

4) direct ablation process using ultra-thin copper foil

After the inner core plate is pressed on both sides and coated with resin copper foil, the copper foil thickness of 17m can be reduced to 5 microns after etching by the "half-etching method", and then the black oxidation treatment can be carried out. The CO2 laser can be used to form holes. 

The basic principle is that the surface treated by oxidation to black will absorb light strongly, and the laser beam energy of CO2 laser can be improved, and then the ultra-thin copper foil and resin surface can be formed into holes.But the most difficult thing is to make sure that the "semi-etch method" can obtain copper layers with uniform thickness, so it needs special attention.The backing copper tear material (UTC) is of course available, and the copper foil is about 5 microns in size. 

According to this type of plate processing, the following aspects are mainly adopted in the process at present:

This is mainly to propose strict quality and technical indexes for material suppliers, and to ensure that the thickness difference of the dielectric layer is between 510 mm.Only by ensuring the uniformity of the thickness of the dielectric applied to the resin copper foil substrate, the accuracy of the groove and the clean bottom of the hole can be ensured under the same laser energy.At the same time, it is also necessary to adopt the best drilling and decontamination process conditions in the subsequent processes to ensure that the bottom of the blind hole is clean without residue after laser drilling.It has good effect on the quality of blind hole electroplating and electroplating. 

3 Nd: YAG laser drilling process

Nd: YAG is neodymium and yttrium aluminum garnet.A UV laser produced by two solid crystals.The most recent use of diode pulsed laser beams can be made into an effective laser seal system without water cooling.The laser's third harmonic wavelength is 355 nanometers and fourth harmonic wavelength is 266 nanometers, and the wavelength is modulated by optical crystals. 

The biggest characteristic of this type of laser drilling is that it belongs to the ultraviolet (UV) spectrum region. The copper foil and glass fiber composed of copper foil laminate have strong absorbance in the ultraviolet region.Due to the low heat of the laser of the above type, the carbon residue will not be generated after the CO2 laser drilling, which provides a good treatment surface for the subsequent process of the hole wall. 

Nd: YAG laser technology is used to process the blind and through holes of various materials.A pilot hole is drilled on a polyimide clad copper foil laminate with a minimum aperture of 25 microns.From the production cost analysis, the most economical used diameter is 25125 microns.Drilling rate is 10000 holes/min.Direct laser punching process can be used, with a maximum aperture of 50 microns.The inner surface of the formed hole is clean without carbonization and easy to electroplate.It is also possible to drill a conducting hole in a ptfe clad laminate with a minimum diameter of 25 microns and the most economical diameter of 25125 microns.Drilling rate is 4500 holes/min.No need to pre-etch the window.The holes are clean and do not require special processing requirements.Still have other material to shape hole to process.The following process methods can be used in the specific processing:

(1) adopt two combined process methods according to the speed of two types of laser drilling

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