Characteristics and flexibility of the hottest UV

2022-08-15
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Characteristics and flexibility of UV laser drilling and mapping system

at present, UV laser drilling equipment accounts for only 15% of the global market, but the growth of market demand for such equipment is three times higher than that of new CO2 laser drilling equipment. The diameter of the hole is even less than 50 μ m. 1~2 multilayer through holes and smaller through holes are also the focus of the current competition. UV laser has proposed a solution for the current competition; In addition, it is also a tool for accurately stripping solder mask and generating precise circuit graphics. This paper summarizes the characteristics and flexibility of current UV laser drilling and drawing systems. The quality and output results of different types of through holes in various materials and the drawing results on various etched resist films are also given. By looking forward to the future development, this paper discusses and compares the characteristics of samples through the superposition of curves, and the limitations of UV laser

this paper also compares UV laser tools with CO2 laser tools, and clarifies in which aspects they can compete, in which aspects they cannot compete, and in which aspects they can be comprehensively applied as complementary tools

comparison between UV and CO2

UV laser tools are not only different from the wavelength of CO2, but also two different tools for processing materials, such as PCB and substrate. The light spot size is less than 10 times, the short pulse width and extremely high frequency make it necessary to use different operation methods in general drilling applications, and open up other windows for different applications

Table 1 shows the comparison of the main technical characteristics of the two laser devices commonly used in the current laser system

Figure 1

UV has high frequency and maximum peak power in a very small pulse width. The size of the light spot on the working face determines the energy density. The energy density of CO2 can reach 50~70j/cm2, while the energy density of UV laser can reach 50~200j/cm2 because the spot size is much smaller

since the UV spot size is smaller than the target hole diameter, the laser beam is focused in the target diameter of the hole in a so-called holing way

Figure 1 shows the casing method

for UV laser, the number of pulses required to drill a complete hole is between 30 and 120, while CO2 laser requires only 2 to 10 pulses. The frequency of UV laser is 5 to 15 times higher than that of CO2. After removing the top copper layer, use the second step to clean the gray area in the hole by expanding the light spot

of course, UV laser can also be used for stamping, but the size of the light spot determines the energy density, and the ablation limit value of different materials determines the minimum energy density required. In this way, according to the ablation limit of different materials, the use of UV stamping method and the maximum spot size can be derived

due to the energy of UV laser, at present, stamping is only used for soft materials with hole diameter less than 75im and extremely low ablation limit, such as TCD, or for the ablation of solder mask with small pad opening

the time to bring the necessary energy into the hole by casing depends largely on the size of the hole itself. The smaller the hole diameter, the faster the UV laser tool will drill. The switching point between CO2 and UV laser is between 75 and 50im hole diameter

co2 laser has three limitations:

first: due to the diffraction of 10im light wave at the edge of the hole, the smallest hole size needs to be considered

second: reflection of this wavelength on copper

third: residues on the bottom copper with a thickness of 1/2 of the wavelength

UV laser with much shorter wavelength and higher absorptivity on copper does not have the above three limitations. Therefore, UV laser has become an ideal tool, which can be used to drill holes in high-grade PCB and substrate coated with any kind of copper material, that is, high-density interconnection technology (HDI)

hdi glance

hdi requirements are: the pore diameter is within the range of 75im~30im; The line and its spacing are 2mil/mil~1 mil/1mil; The pad is 250im~200im; And the precision of solder mask opening should reach 15 to 10im. The new design requires not only 1-2 layers of blind holes, but also multi-layer through holes and through holes. It is also required that the shape of the hole can be plated and support the filling of the through hole. It is predicted that the development of the market requires not only reducing the value of the inverted chip substrate, but also reducing the value of mass production within 2 to 3 years

drilling method of UV laser

CO2 laser has only two main operation functions: the step operation function of the workbench between electric fields and the electric operation function between holes within the range of electric fields. The peak power will be reduced. The pulse width can be selected between 1 and 100ns, and the frequency range is between 1 and 4kHz. For different materials, only these three parameters and the number of pulses/holes can be used to describe the drilling tool

first of all, UV laser has an additional operation function, that is, the third function - electric micro focusing in the forming aperture. This kind of sleeve hole is sufficient to adjust the internal and external shape (concentric circle = shape) according to the hole diameter. Shape 8 A series of special engineering plastics with heteronaphthalene biphenyl structure: the repetition of polyarylamide, polyarylether, etc. is suitable for the thickness of the material. The size of the internal and external light spots determines the energy density to adapt to the ablation limit of the material. The pulse sequence formed by laser frequency and electric circulation speed overlaps to determine the energy of each shape

in terms of UV laser characteristics, laser frequency, pulse width and average maximum power are closely related to each other. The yellow area indicates a direct relationship, while the red area has the opposite relationship

to drill holes in a sequence of different materials, UV laser can provide so-called sequential steps, such as up to 8 different independent drilling processes. During the whole hole drilling process, all the tool parameters in Table 2 can be adjusted according to the online processes

Figure 2 shows the principle of sequential process drilling

figure 2

because the ablation limit of epoxy resin is lower than that of copper (yellow), the cleaning process (green) cannot probe into the bottom copper. The light beam shines gently, balancing the thickness and consistency tolerance of the material

develop the through-hole process of HDI through UV

a process B process C process

basically can meet the requirements of a common plastic raw material processing and plastic waste processing and molding. Process a: 4-step process, mixed with wetting and laser processes, with a mask tolerance between 50 and 70im, and the minimum hole size is generally 100 to 125im

b process: 2-step laser process and 1-step wetting process. Due to the diffraction of CO2 on the mask, the diameter of the small hole is about 60im. For specially treated copper material, the limit of copper opening thickness that CO2 can provide is 7im. This process still needs to remove drilling dirt

c process: one step laser process. UV laser has no restrictions on the drilling of inner and outer copper. UV also has an additional cleaning process, so that the drilling dirt removal process is reduced to a minimum, and it can even replace the drilling dirt removal process

uv laser has the ability to reduce the process steps of a complete hole to a single laser process, especially eliminating the need for dirt removal, and even completely eliminating this process, especially for pulse pattern electroplating. There is no need to use aggressive decontamination process. For example, for CO2 laser, the roughness, wicking and barrel distortion of hole shape are improved

other applications and quality results of UV laser

● blind hole

● double-layer through hole

● through hole

adopts flexible

the new laser system can not only implement the common focused irradiation operation hole, but also carry out complex drawing operations. It can be used to cut fine line patterns or remove solder mask after embedding. It can process almost any shape of processing area

so far, when the defects on the solder mask are only minor defects and irrelevant, only the laser ablation solder mask is used to repair some damaged pads, so that the whole panel will not be discarded. However, HDI technology requires that the opening size and positioning be more accurate. The following figure shows the circular and square solder mask openings and cross sections formed after pressure steam test and thermal cycle. The speed can reach more than 100 pads per second. For BGA and FC, the cost of 128 pads per IC is about 0.5 cents

when drawing thin lines, draw the figure by laser track engraving, as shown in the figure below, the speed of laser track can reach 1000mm/s. After laser ablation of 1IM thick tin, the width is between 15~25im. After the tin pattern is drawn, the pattern is etched, and the spacing of the laser track width and the side effects of etching are maintained. For copper with a thickness of 12im, a figure lower than 2mil/2mil can be obtained

the following figure shows the fan out of IC and MCM graphics of 2mil/2mil structure. The application of drawing thin line graphics directly is limited by the drawing speed. As shown in the figure below, fan out takes less than 1 second, while in 40 × It takes 10 to 15 seconds for a complete pattern to fan out in an area of 40mm

conclusion

uv laser system provides a supplementary solution for the existing CO2 drilling tools. For drilling, short wavelength and small light spots have greater flexibility and higher complexity. The goal of UV laser is more to meet the needs of HDI. Compared with CO2 performance, especially for macropores, there is still a gap in UV output, but with the development of high-power and high-frequency UV lasers, this difference will be smaller and smaller. The number of processing procedures for generating through holes with UV laser will be reduced to a single laser process, and the required decontamination process will be minimized

uv system can be used for direct drawing and precise ablation of solder mask in addition to its main drilling purpose. This provides added value for UV lasers

there is still enough room to improve the UV laser system for production. Smaller pulse width, high frequency, higher power and high-speed servo operation will increase productivity, and in the near future, as a perfect tool, UV laser systems will be more and more widely accepted in the market

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