PCB Technical - Laser technology of HDI PCB circuit board

With the rapid development of micro-electronics technology, the wide application of large-scale and ultra-large-scale integrated circuits, and the progress of micro-assembly technology, the HDI PCB circuit board manufacturing is developing toward layered and multifunctional direction, and the PCB circuit board graphic wires are thin and micro-porous with narrow spacing. The mechanical drilling technology used in the processing can no longer meet the requirements and has rapidly developed a new micro-hole processing method, namely laser drilling technology.

Principle of Laser Hole Formation for HDI PCB

Laser is a powerful beam of light that is stimulated by an external stimulus to increase energy, in which infrared and visible light have thermal energy and ultraviolet light have optical energy. There are three phenomena that occur when this type of light strikes the surface of the workpiece: reflection, absorption and penetration.

By striking a laser spot on a substrate through another piece of optics, there are multiple modes of composition, and there are three reactions 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 so-called excision.

1. Photothermal ablation: The principle of holes formed by a material being processed that absorbs high-energy laser light and is heated to melt and evaporate in a very short time. Under the action of high energy, the black charred residue on the pore wall formed by this process must be cleaned up before porization.

2. Photochemical ablation: refers to the high photon energy in the ultraviolet region exceeding 2eV electron volts. The result of high-energy photons with laser wavelength over 400 nanometers. This high-energy photon destroys the long molecular chain of organic materials and becomes smaller particles, but its energy is larger than the original molecule, and it is forced to escape from it, so that the substrate material is quickly removed and micro-pores are formed under the external pinch.

Therefore, this type of process does not contain boiling and no carbonization will occur. Therefore, pre-porosization cleaning is very simple.

These are the basic principles of laser pore formation. At present, there are two most commonly used laser drilling methods: the laser used for drilling holes in printed circuit boards mainly consists of RF-excited CO2 gas laser and UV solid-state Nd:YAG laser.

3. About the absorbance of the base plate: The success rate of laser has a direct relationship with the absorbance of the base material. The printed circuit board is a combination of copper foil, glass cloth and resin. The absorbance of these three materials varies according to the wavelength, but the copper foil and glass cloth are 0.3m in ultraviolet light. μ The following regions have higher absorption rates, but fall off sharply after entering visible light and IR. Organic resin materials maintain fairly high absorption rates in all three spectral segments. This is the characteristic of resin materials and the basis for the popularity of laser drilling technology.

Different HDI PCB Processes for CO2 Laser Hole Formation

There are two main drilling methods for CO2 laser drilling, direct drilling and dressing mask drilling. The so-called direct hole-forming technology is to modulate the laser beam diameter through the main control system of the device to the same diameter as the hole on the printed circuit board, and directly process the hole on the dielectric surface without copper foil. The coating mask process is to coat the surface of the printed board with a special mask, and remove the copper foil from the hole surface by exposure/development/etching with the conventional process. These holes are then irradiated with a laser beam larger than the aperture to remove the exposed dielectric layer resin. The following are described separately:

1. Bronze window opening method:

First press a layer of RCC coated resin copper foil on the inner panel. A window is made by photochemistry, then the resin is etched to expose it, and then the substrate material inside the window is ablated by laser to form a micro-blind hole:

When the beam is enhanced, it passes through the aperture to reach two groups of galvanometer-type micro-reflective scanners, which are vertically aligned once to the positive F θ Lens. The tube area is reached where the exciting countertop can be made, and then the micro-blind holes are burned one by one.

Once positioned by an electron fast beam in an inch-square tubular area, the 0.15mm blind hole can be punched with three shots. The pulse width of the first gun is about 15 μ S, which provides energy for hole formation. After that, the gun can be used to clean the residue at the bottom of the hole wall and to correct the hole.

SEM cross section and 45 degree full view of 0.15mm micro-blind hole with good laser energy control. This window-opening process is used as a backing target plate. When large typesetting or second-order blind holes are not needed very often, their alignment is difficult.

2. Opening window process method:

The diameter of holes formed by the former process is the same as that of the open copper window. A slight mistake in operation may cause the position of the open window to deviate, resulting in the misalignment of the blind hole position with the center of the base pad. The deviation of the copper window may be due to the expansion and contraction of the matrix material and the distortion of the negative used for image transfer. So the process of opening large copper window is to enlarge the diameter of the copper window to 0.05mm larger than that of the base pad. Usually, the size of the hole is determined by the size of the hole. When the hole is 0.15mm, the diameter of the base pad should be about 0.25mm, and the diameter of the large window should be 0.30mm. Then the laser drilling can be done to align the burnout position precisely to the micro-blind hole of the base pad. Its main feature is that it has a large degree of freedom to choose. When drilling laser holes, you can choose to press the program of the inner base pad to make holes. This effectively avoids the misalignment caused by the same diameter of the copper window as that of the holes, which prevents the laser point from pointing at the front window, and causes many incomplete half-holes or residual holes to appear on large batches of slabs.

3. Direct Pore Forming Process on HDI PCB Resin Surface

There are several types of laser drilling methods using HDI PCB laser to drill holes:

A. The base plate is coated with resin-coated copper foil on the inner layer, and then etched off all the copper foil, CO2 laser can be used to directly form holes on the bare resin surface, and then the holes can be further treated according to the plating process.

B. The base plate is a similar process using FR-4 semi-cured sheet and copper foil instead of resin coated copper foil.

C. Process for subsequent lamination of copper foil with coated photosensitive resin.

D. Made by dry film as dielectric layer and copper foil pressing process.

E. The process of coating other types of warm film with copper foil.

4. Direct ablation of ultra-thin copper foil

After the resin copper foil is pressed on both sides of the inner core plate, the copper foil thickness of 17m can be reduced to 5micron by "half etching" method, and then treated by black oxidation, the holes can be formed by CO2 laser.

The basic principle is that the oxidized black surface will absorb light intensively, so that the super thin copper foil and the resin surface can be directly pored on the premise of increasing the beam energy of CO2 laser. However, the most difficult thing is to ensure that the "half etching method" can obtain uniformly thick copper layers, so special attention should be paid to their fabrication. Of course, the copper-backed tearable material UTC. Copper foil is equivalent to a Book of about 5um.

According to this type of plate processing, the following main aspects are currently adopted in the process:

This mainly sets strict quality and technical criteria for material suppliers to ensure that the thickness difference of the dielectric layer is 510 μ M to M. Because only the uniformity of the dielectric thickness of the resin-coated copper foil can be guaranteed, the accuracy of the pass and the cleanness of the bottom of the hole can be guaranteed with the same laser energy. At the same time, it is necessary to adopt the best process conditions to remove the drilling dirt in the subsequent operation to ensure that the bottom of the blind hole is clean and free of residue after laser drilling. It has a good effect on the quality of blind hole electroless plating and electroplating.

Nd:YAG Laser Drilling HDI PCB Process

Nd:YAG is neodymium and yttrium aluminium garnet. UV laser emitted by two solid crystals together. Recently, most commonly used laser beams excited by a diode pulse can be used to make an effective laser sealing system without water cooling. The third-harmonic wavelength of this laser is 355 nanometers. The fourth harmonic wavelength is 266 nanometers. The wavelength is modulated by an optical crystal.

The biggest feature of this type of laser drilling is ultraviolet UV. Spectrum region, while copper foil and glass fibers consisting of copper-clad laminates absorb strongly in the ultraviolet region, together with the small energy of such laser light points, they can penetrate strongly through the copper foil and glass cloth and form holes directly. Because the laser heat of the above type is small, it does not produce charcoal slag after CO2 laser drilling, which provides a good surface for subsequent hole wall operations.

Nd:YAG laser technology processes logo blind holes and passes through holes on a variety of materials. It drills through holes in the polyimide copper clad laminate with a minimum diameter of 25 microns. From the cost analysis, the most economical diameter used is 25125 microns. The drilling speed is 10000 holes/min. Direct laser punching can be used with a maximum aperture of 50 microns. The inner surface of the formed hole is clean and free of carbonization and easy to electroplate. It can also drill through holes in PTFE copper clad laminate with a minimum diameter of 25 microns and a most economical diameter of 25125 microns. The drilling speed is 4500 holes/min. No window pre-etching is required. The holes are clean and do not require special processing requirements. There are other materials such as shaping hole processing. The following processes can be used in specific processing: 1. Two combined processes are adopted according to the speed of two types of laser drilling

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

Quality Problems in Actual HDI PCB manufacturing

In the process of laser drilling, there are many quality problems, which are not ready to be fully described. Only the most common quality problems are put forward for peer reference.

A. Misalignment between CO_2 laser drilling position and bottom target position in open-window method

In HDI PCB laser drilling, beam positioning system is very important for the accuracy of aperture shaping. Although the beam positioning system is used for precise positioning, flames with hole shape are often produced due to other factors. The quality problems arising in the production process are analyzed as follows:

1. Make negative for inner core plate weld pad and wire graphics, and RCC with resin-coated copper foil. Negative for window opening after layer addition, since both are potential factors for size increase and decrease due to humidity and temperature.

2. The size of the base material increases or decreases when the core plate is used to make the pattern of traverse bonding pad, and the RCC of resin-coated copper foil is pressed at high temperature. After the addition of layers, the factors of dimension expansion and contraction of the inner and outer baseboard materials exist.

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

4. Error caused by the light point of the laser itself and the displacement of the surface.

5. The alignment of second-order blind holes is more difficult and can easily cause position errors.

Based on the above reasons, according to the relevant technical data obtained in production and the actual operation experience, the main technological strategies adopted are as follows:

1. To reduce typesetting size, most HDI PCB manufacturers use 450 for multi-layer typesetting * 600 or 525 * 600mm. However, for mobile phone panels with 0.10 mm wire width and 0.15 mm blind hole diameter, the best layout size is 350. * 450mm. Upper limit.

2. Increase the laser diameter: the purpose is to increase the area covered by the copper window. The specific method is "beam diameter = hole diameter + 90~100" μ M. When the energy density is low, shoot one or two more more shots to solve the problem.

3. Take the process of opening a large copper window: at this time, only the size of the copper window becomes larger and the aperture is not changed, so the diameter of laser holes is no longer completely determined by the position of the window, so that the hole position can be directly based on the position of the base target on the core plate.

4. Changing the window opening method from photochemistry imaging and etching to YAG laser: using YAG laser light points to open the window according to the base hole of the core plate first, then using CO2 laser to burn out the hole on its window position, to solve the error caused by imaging.

5. Layer twice to make second-order micro-blind hole method: when the core plate is coated with resin copper foil RCC on both sides. After that, if RCC is to be accumulated once more and a second-order blind hole is made, the second-order blind hole will be accumulated. In the case of "two" blind hole alignment, holes must be formed by aiming at "one by one". The original target of the core plate cannot be reused. That is, when "accumulate one" into holes and pads, its board edges will also make targets. Therefore, after pressing the RCC of "Ji 2", four additional mechanical datum holes of "Ji 2" can be drilled through the X-ray machine to target on "Ji 1", and then the holes are lined. This method can make "Ji 2" align with "Ji 1" as far as possible.

B. Incorrect pass

Based on many production experiences, mainly because of the quality problems in the base material shaping, the main quality problem is that the thickness of the dielectric layer after resin-coated copper foil pressing will inevitably differ. With the same energy of drilling holes, the cushion of the thinner part of the dielectric layer will not only bear more energy, but also reflect more energy. Thus, the hole wall is beaten into an outward-expanding kettle. This will have a significant impact on the quality of electrical interconnection between layers of multilayer layers.

Due to the incorrect pore size, the reliability of the high density interconnect structure of the layered multilayer printed circuit board (MLPCB) will pose a series of technical problems.

Therefore, process measures must be adopted to control and solve the problem. The following processes are mainly used:

1. Strictly control the dielectric layer thickness difference between resin coated copper foil and laminated copper foil at 510 μ M to M.

2. Change the energy density of the laser and the number of pulsed guns. The process conditions for mass production can be found through test methods.

3.The slag at the bottom of the hole and the broken slag at the wall of the hole are not removed properly.

This type of HDI PCB quality problem is most likely to occur due to slight improper control. Especially for the laminates with porous type on big plate, it is impossible to guarantee 100% quality without quality problems. This is because the number of micro-blind holes on the processed large stencil is too large, averaging about 60-90,000 holes. The thickness of the media layer varies, so does the thickness of the residue left on the base when laser drilling with the same energy is used. It is not possible to ensure that all residues are thoroughly clean after the treatment of drilling dirt. In addition, poor inspection methods often result in subsequent HDI PCB copper plating and the bond between the bottom pad and the hole wall when there is a defect.