Analysis of UV Laser Drilling Technology

1. Application Status of laser in Micropore Processing

With the development of electronic products in portable and miniaturized directions, more and more demands are raised for circuit board miniaturization. For example, a circuit board of a modern mobile phone and a digital camera is installed with approximately 1200 cables per square centimeter. The key to improving the circuit board miniaturization level is the increasingly narrow line width and the smaller the line between different layers. For micro-pass holes, in order to effectively ensure the electrical connection between each layer and the fixing of external devices, designers always hope that the smaller the pass holes, the better, in the high-speed and high-density PCB design, in this way, the Board can not only have more wiring space, but also the smaller the passing hole, the more suitable for high-speed circuits, in this way, the micro-pass not only provides a high-speed connection between the surface installation device and the following signal panel, but also effectively reduces the area, Printed Circuit Board (PCB) gradually presents the features of high-density interconnection technology. At present, the cost of microholes usually accounts for 30%-40% of the cost of PCB boards. Traditional mechanical drilling has a minimum size of 100 μm, which is obviously unable to meet the requirements. Instead, it is replaced by a new laser micro-machining method. Currently, in the industry, CO2 lasers can be used to process holes with a diameter of 30-40 μm or up to 10 μm by UV laser. At present, laser micro-line hole UV Laser drilling equipment only accounts for 15% of the global market, but the market demand for such equipment is three times higher than that of the new CO2 laser drilling equipment. In addition, because the R & D cycle of electronic products is getting shorter and shorter, in order to meet the requirements of the R & D stage for fast circuit board and single piece, designers require the device to have both drilling, circuit graphics engraving and cutting functions. At present, foreign laser micro-wire hole devices that already have these features are quite expensive.

The unique characteristics of laser make it an ideal tool for micro processing. the laser is a non-contact zero wear tool that delivers a very high energy density to a precise processing position for drilling, cutting, and welding by focusing. The type of interaction between the two depends on the characteristics of the materials to be processed and the wavelength and energy of the laser.
Pulsed CO2 laser and infrared YAG laser are commonly used infrared laser light sources in material processing. However, many plastics and a large number of special polymers (such as polyimide) used in flexible circuit board matrix materials cannot be processed through infrared or "thermal" processing. Heat may cause deformation of plastics, resulting in carbonization damage on the cutting edge or the edge of the drilled hole, which may lead to structural weakening of the circuit board and parasitic transmission path, as a result, the subsequent processing procedures have to be added to improve the processing results. Therefore, infrared lasers are not suitable for processing some flexible circuits. In addition, even at high energy density, the wavelength of the CO2 laser cannot be absorbed by copper, which severely limits its use range.
In contrast, the output wavelength of an ultraviolet laser is less than 0.4 microns, which is suitable for processing polymer materials. Unlike infrared processing, the UV microprocessing process is not essentially a "hot" processing process. Most materials are easier to absorb ultraviolet light than infrared light. High-energy ultraviolet photon directly destroys the molecular bond on the surface of many non-metallic materials, this "cold" machined part has smooth edges and a minimum effect on carbonization.
Due to the advantages of ultraviolet light in focusing, the focal point can be smaller than sub-micron, which makes microtreatment of metal and polymer more advantageous and can be processed by small parts. Even at a low pulse energy level, high energy density can also be obtained to effectively process materials.

Laser Micro-wire holes have been widely used in the industry. There are two main methods:

1) is the use of infrared laser: material surface material heating (evaporation), in order to remove the material, this method is usually called Hot processing. CO2 (wavelength: 10.6 μm) or Nd: YAG laser (wavelength: 1.064 μm) are used ).

2) UV laser: it directly breaks down the molecular key of the material and disconnects the molecule from the processing method of the object. This method does not produce high heat, so it is called cold processing. The main use of UV-YAG laser (35nm, 266nm, micron nm) or laser.

The hot processing method of infrared laser has been widely used, but the cold processing method of ultraviolet laser is widely studied. Due to the many excellent features of UV laser, many companies in the world are or have developed UV Laser Micro-wire hole equipment. The following table compares the two processing methods. We can see that the capabilities of these two methods are different.

With the increasing demand for small electronic products and micro-electronics components, the precision processing of polymer materials has become one of the fastest growing application fields in laser industry. UV laser is an ideal tool for processing materials widely used in the Microelectronics component industry, such as plastic (such as polyimide) and metal (such as copper. The latest solid-state laser technology has promoted the development of a new generation of compact and all-solid-state uv lasers, making them more cost-effective processing methods in this field.

Comparison of infrared laser and ultraviolet laser drilling capabilities

 

 

	Infrared Laser (CO2 laser)	UV laser (UV-YAG)
Deep Diameter Ratio	0.4 ~ 0.9: 1 for blind holes	0.25 ~ 10:1 through hole and blind hole
Aperture	150 ~ 350 μm	10 ~ 150 μm
Processing speed	300 holes/min	24000 holes/min

Because UV laser can reach a diameter of 10 μm and can be drilled at a precision of 1 μm, the application of UV laser in drilling has been paid more and more attention with the trend of micro-production in the electronics industry. Some of the current printed circuit boards (PCB) have been replaced by UV-YA g drilling machines. The drilling speed is fast and accurate. In addition, many micropore filters, holes in the catheter for medical use, and so on, must also use UV laser to meet the requirements.

2. Market and technical level of laser micropore Processing

The commercial machines with laser holes can be divided into ultraviolet Nd: YAG laser machines (mainly suppliers of ESI) and infrared CO2 laser machines (lumonics first, hitachi, Mitsubishi, Sumitomo, and others are available, and there are three types of UV/IR Variant Types (such as eecellon 2002. The former is advantageous for the pores below 3 mil, but the pore forming speed is slow. Subscriber to 4 ~ 8 mil Micro Blind Hole production is the most convenient, the production speed is about 10 times that of the YAG machine, the latter is to burn the full number of holes in the copper skin with the YAG head, and then burn the substrate with the CO2 header to form a hole. For the mobile phone board of the mobile phone, the CO2 laser will burn 4 ~ The 6mil microblind hole is the most suitable for all cases, and the single side of the disease can be burned out about 6000 holes per minute during mass production. As for the high-speed YAG laser, due to the strong and concentrated energy of the UV beam, copper foil can be directly penetrated without the need for a conformal mask, it can burn copper foil and substrate at the same time to form a hole. Generally, it is commonly used in a variety of "package substrste" microholes below 4 mil, if used in the mobile phone board 4 ~ 6mil micropores seem to be less economical. The following is an introduction and discussion on the progress of laser casting.
1. Principle of Laser hole forming
Ray-emitting light is a powerful beam that is stimulated by external stimulation and increases energy. The infrared light or visible light has thermal energy, while the ultraviolet light has chemical energy. Three phenomena, including reflection and transmission, occur when shooting on the working surface. The effect of the plate on the plate is divided into two different reactions: thermal and optical:
1.1 photothermal ablation
It refers to the thermal energy clamped by a laser beam in its infrared light and visible light. After being absorbed by a sheet, it is formed by melting, gasification, and gas slurry. The principle of removing it into holes is as follows, it is called "photothermal ablation ". The side effect of this ablation is that there are black-burned carbon residue on the wall of the hole (even the Black oxide copper chips caused by a high degree of maturity on the hole edge Copper Foil ), after the Desmear Process is used, the blind copper wall can be removed.
1.2 photochemical ablation
It refers to the high photon energy in the ultraviolet field, which can interrupt the chemical bond of long key-like organic compounds, as a result, the plate is quickly removed from the hole due to the volume increase and external force suction caused by numerous fragments. This reaction does not include cold process, so the wall of the hole will not produce the carbonization residue.
1.3 plate Absorbance
It can be seen from the above that the efficiency of laser into the hole is directly related to the absorption rate of the plate. The degree of absorption of copper, glass cloth and resin in the Board varies with the wavelength. The first two have a high absorption rate in the areas below UV 0.3mu, But they slide sharply after entering visible light and IR. As a result, organic resins maintain a very good high absorption rate in the three-phase spectroscopy.
1.4 pulse energy
The practical Laser hole-forming technology is the processing of a intermittent Q-switch beam, so that each section of the light (measured in microsecond US) is in its pulse format) energy cracking plate, the energy of each pulse (which can be called a gun), has multiple modes ), for example, the energy of a Single-beam gemoo single-beam point is easier to concentrate, so it is mostly used for drilling. Multi-beam points require not only uniformity, but are not easy to concentrate into small beam points. Generally, they are used in laser direct imaging (LDI) or contact mask processes.
1.5 Precise Positioning System
1.5.1 small pipe location
Take the example of an RF/CO2 drilling machine developed by Hitachi micro-hole machinery (Hitachi via machine, recently renamed by Hitachi Seiko, the positioning method is the X of galvanometer and mirro. y. positioning, coupled with the XY table positioning of the device, and other systems. The latter is to divide the panel into many small "pipe area" (the maximum is 50mm square meters, generally for the sake of accuracy, more than 30mm square meters), the work can XY move the table to switch the pipe area. The former is in a single tube area, with two galvanometer XY micro-motion, point to the surface of the target targeted by the hole. After all the pores in the tube area are drilled, they are quickly moved to the next tube area and then continue drilling.
The so-called galvanometer is a kind of iron product that can be precisely moved less than or equal to 20 °. It is a DC motor combined with a magnet or a coil. Then it can be fitted with a mirror to make a small angle of rotation reflection, however, the laser beam is fast (2 ~ 4 ms. However, this system also has some disadvantages, such as: ① the beam on the panel may not be very vertical, and the amount of beam may be skewed, therefore, telecentric lense needs to be added to correct oblique light so that it is perpendicular to the Hole Position as much as possible. ② the current meter type mirror system does not cover much area, A maximum of 50mm * 50mm can be managed. Therefore, the XY table must be used to change the management area. The smaller the pipe area, the more accurate the positioning, but the time for mass production is sacrificed. ③ the handover of the pipe area on the large panel cannot be completely seamless, there will inevitably be "abutment errors", such as gaps or overlaps, which may lead to faults such as missed holes or inaccurate slots on high-density slabs. At this time, an automatic correction system can be installed to improve the replacement of the pipe area, or the size and shape of the tube area can be adjusted manually based on the density of the cloth hole.
1.5.2 full panel Positioning
In addition to the above-mentioned "Galvo XY" and "cell shift" positioning, the Galvo XY mirror can also be mounted on a set of "liner motor, move X of the entire panel. Do not add a linear motor to the table and only move y. This will avoid bad errors. This method is the same as that of the traditional mechanical drilling machine, where the drill shaft is moved around X, and the positioning method of Y is the same. This method can be applied to the positioning of UV/YAG beam with strong performance. If the line outside CO2 beam is weak, it is not suitable because its path is too long and its energy is not concentrated.
2. Different Processes of carbon dioxide laser into holes
2.1 opening copper window conformal mask
On the core board of the internal layer, press RCC first, open the copper window, and then burn the substrate in the window with ray light to complete the Micro Blind Hole. Details are the first FR-4 of the inner core board, so that the two sides of the line and the bottom pad (target pad), and then each piece of "back rubber copper foil" (RCC ). The copper foil in this RCC (resin coated Copper Foil) is 0.5 Oz, and the thickness of the rubber is about 80 ~ 100um (3 ~ 4mil ). It can be made into B-stage, B-stage and C-stage respectively. When the latter is pressed, the thickness of the media layer on its base pad is easier to control, but the cost is more expensive. Then the CO2 laser light is used to burn the resin in the window according to the coordinate Program of the etching copper negative, and then the blind hole can be dug to the end. This law was originally a "Hitachi manufacturing Institute" patent. If a general operator wants to ship goods to the Japanese market, he may have to be careful with legal issues.
2.2 large copper window large conformal mask
The above-mentioned pore size is the same as that of the copper window. Therefore, if the window position is deviated, the blind hole will be taken to the bottom pad due to misregistration. The deviation of these copper windows may come from the negative film problem of board increase and contraction and image transfer. It is not easy to completely solve the problem on the panel.
The "open big window" method expands the caliber to about 2 mil lower than the base cushion. Generally, if the aperture is 6 mil, the base pad should be around 10 mil, and the large window can be opened to 12 mil. Then, hand over the coordinate data of the bottom pad of the inner layer to the laser for use, and then the Micro Blind Hole with precise position targeting the bottom pad can be burned out. That is to say, there is a lot of ground in the large window, so that the hole space can obtain more elastic space. As a result, laser light is able to separate the program based on the bottom pad of the inner layer to form a hole, instead of completely following the window position to burn the hole that knows that it is already in place.
2.3 Direct pore forming on resin surface
This method can be subdivided into several different approaches, which are described as follows:
2.3.1 in accordance with the aforementioned RCC + core practices, but without opening a copper window, all copper foil bit light, if the process itself is cheap. After that, CO2 laser can be used to directly burn holes on the exposed surface of the resin, and then PTH and Copper Electro-copper are used to finish holes and cables. Due to the fact that many micro pits have been stepped on by copper foil on resin, peel strength (copper Layer Tensile Strength) is subsequently laid into a wire ), it should be much better than that of photo via by potassium permanganate on resin roughening. However, this kind of method of sacrificing copper skin and coarse hemp resin surface still does not know that the real copper foil is more powerful.
Although the advantages of this method can avoid the cost and engineering problems of image transfer, more problems must be solved in potassium permanganate "gum Removal Slag, the biggest crisis is still the lack of reliability attached to the welding pad.
2.3.2 other similar practices of replacing RCC with FR-4 film and copper foil; ② practices of sacrificing copper foil after photosensitive resin coating; ③ press paste method of medium layer and sacrifice copper foil of dry film; ④ other wet film Resin Coating and sacrifice copper foil method, etc., can all corrode copper to get the pit surface and then directly burn holes.
2.4 slim copper skin direct burning
After both sides of the inner core board are pressed with adhesive copper foil, the "Half etching" (half etching) can be used to thin the original 0.5 Oz (17um) copper to about 5 um, then the Black oxide layer and direct pore formation are performed.
Due to the strong absorption of Black and ultra-thin copper layer, and the improvement of CO2 laser beam energy, it will be able to directly wear copper and substrate as a hole like YAG laser, however, it is not easy to achieve a good "semi-Eclipse. With this impressive business opportunity, copper foil manufacturers now offer special "copper-backed ultra-thin copper skins" (for example, Japan Mitsui's tearing UTC ). The method is to press the UTC Prism on the two-sided tape layer outside the core board, and then tear off the thick supporting "Back copper layer" to obtain the HDI semi-finished product with ultra-thin copper skin (UTC. Then complete the laser blind hole on the black copper surface, and you can also listen to the black layer for PTH copper and copper. This method can not only directly complete the micropores, but also greatly improve the yield rate of copper-based ultra-thin in the production of fine lines. Of course, this copper-based detachable UTC, the price will not be cheap.

3. Technical Requirements for Ultraviolet Laser hole preparation

For a long time, the laser is dominant in the field of UV "cold processing". However, there are many inherent disadvantages of the laser technology: All the laser must use toxic gases, the storage and adjustment process of special gases is very troublesome. meanwhile, they are bulky, expensive, and expensive for operation and maintenance. The biggest problem is that the output beam of the laser is large and the space quality is poor, this severely limits the focus of the beam, making it necessary to use a mask during the microprocessing process. The step-by-step drilling of holes in the same shape and repetitive work of the laser is good (such as the machining of holes on the magnetic drum nozzle of the inkjet printer), but the overall efficiency is not high, only 1% of the pulse energy is applied to the processing surface, and about 99% of the other optical energy is lost in the template. In addition, the flexibility of the mask method is limited. If the image changes and the mask needs to be replaced, the entire processing process must be stopped.
The Application of Solid-state uv lasers has been limited by the insufficient output power, which cannot meet the processing needs. With the development of more reliable semiconductor pump solid technology and more reliable third-harmonic mechanism, the situation has changed. The new third-harmonic semiconductor pumped solid-state laser has become a competitor of the laser. The energy density level is equivalent, but the repetition frequency is higher, and the beam quality is better.
The Avia 355-1500 semiconductor-pumped solid-state laser manufactured by coherent has an output wavelength of 35nm, an average power of 1.5 W, and a maximum frequency of 100 kHz. It has good beam quality and is very suitable for microprocessing applications. Good beam quality, so that excellent focus capabilities allow you to get rid of the mask for processing, and transmit the beam to any position on the workbench through a computer-controlled scanning mirror system ), the CAD/CAM software is used to perform drilling, cutting, or cutting by means of direct engraving. When the drawing changes, the hardware does not need to be replaced. Drilling experiments show that drilling and cutting of any size and shape larger than the focal point can be performed through repeated cutting.
High Repetition Frequency is another outstanding advantage of modern DPSS lasers. Generally, the frequency of repetition is several hundred Hz, while Avia can reach 100 kHz. High Repetition rates can greatly increase the production volume in low density pore distribution applications and wiring or cutting processes. For example, it takes about 200 pulses to drill a 30 micron-diameter hole on a kaptontm PI material with a thickness of 2 mill (50 microns), and the energy density is 0.2j/cm2. Avia can work at a frequency of 50 kHz at a rate of about 250 holes per second, while it takes 1 second to output a hole with the same parameters for an laser working at a frequency of Hz repetition.