At present, UV laser drilling equipment only accounts for 15% of the global market, but the market demand for such equipment is three times higher than the demand for new CO2 laser drilling equipment. The diameter of the hole is even smaller than 50 μm, and the multi-layer vias and the smaller vias of 1 to 2 are also the focus of competition. The UV laser provides a solution for the current competition; besides, it is also used for Precise stripping of solder masks and tools for generating sophisticated circuit patterns. This article outlines the characteristics and flexibility of current UV laser drilling and mapping systems. The quality and yield results for different types of vias for various materials are also presented, as well as plot results on various etch masks. This article discusses the limitations of UV lasers by looking forward to future developments.

This article also compares the UV laser tools and the CO2 laser tools, and clarifies where the two can compete, in which aspects they are not competitive, and in which aspects both can be integrated as complementary tools.

Comparison of UV and CO2

UV laser tools are not only different from CO2 wavelengths, but they are also two different tools in processing materials such as PCBs and substrates. The 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 gives a comparison of the main technical characteristics of the two laser devices commonly used in current laser systems.

figure 1

UV has high frequency and great peak power within a very small pulse width. The spot size determines the energy density on the work surface. The CO2 energy density reaches 50~70J/cm2, and the UV laser has a much smaller spot size, so the energy density can reach 50~200J/cm2.

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 sleeve hole manner.
Figure 1 shows the socket hole method.

For UV lasers, the number of pulses required to drill a complete hole is between 30 and 120, while the CO2 laser requires only 2 to 10 pulses. The UV laser frequency is 5 to 15 times higher than that of CO2. After the top copper layer has been removed, a second step can be used to clear the gray area in the holes with an enlarged light spot.

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

Because of the energy of UV lasers, currently only the punching method is used for soft materials with a hole diameter of less than 75 μm, extremely low ablation threshold, such as TCD, or solder mask ablation for small pad openings.

The time required to bring the necessary energy into the hole through the sleeve hole depends largely on the size of the hole itself. The smaller the hole diameter, the faster the UV laser tool drills. The switching point between CO2 and UV lasers is between 75 and 50 μm aperture diameter.
Three limitations of CO2 lasers:

First: Due to the diffraction of 10im light waves 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 wavelength.
UV lasers with a much shorter wavelength and higher absorption on copper do not have the above three limitations. Therefore, UV lasers are an ideal tool for coating any copper material. The high-grade PCB and substrate, namely high-density interconnect technology (HDI) drill holes.

A look at HDI
The HDI requirements are: hole diameter in the 75im~30im range; line and pitch 2mil/mil~1 mil/1mil; pad in 250im~200im; and accuracy of the solder mask opening to reach 15 to 10im. The new design not only requires blind holes of 1 to 2 layers, but also requires multiple vias and vias. The shape of the hole is also required to enable plating and to support via filling. It is predicted that the development of the market will not only reduce the value of the inverted chip substrate in 2 to 3 years, but also reduce the value of mass production.

UV laser drilling method

The CO2 laser has only two main operating functions: the stepping operation function between the electric fields and the electric operation function between the holes in the electric field. The peak power will decrease and the pulse width can be chosen between 1 and 100 ns, with a frequency range between 1 and 4 kHz. 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 lasers have one more operational function, the third function - electro-microfocusing within the shaped aperture. This sleeve hole allows adjustment of the inner and outer shape (concentric circles = shape) according to the aperture. The repetition of the shape is adapted to the thickness of the material, and the size of the light spots inside and outside the cluster determines the energy density to fit the ablation limit of the material. The sequence of pulses formed by the laser frequency and the electrical cycle speed overlap to determine the energy of each shape.

With regard to the characteristics of the UV laser, the laser frequency, the pulse width, and the average maximum power are quite closely related to each other. The yellow area shows a direct relationship and the red area has the opposite relationship.

To drill holes in a sequence of different materials, UV lasers can provide so-called sequential steps, for example up to 8 different individual drilling steps. During drilling of the entire hole, all the tool parameters in Table 2 can be adjusted according to the various steps on the line.
Figure 2 shows the principle of sequential drilling.

figure 2

Since the ablation limit of epoxy resin is lower than that of copper (yellow), the cleaning process (green) cannot penetrate into the underlying copper. The beam is gently illuminated, which equalizes the thickness of the material and the tolerance of consistency.

Development of HDI vias via UV
A process B process C process
A process: 4-step process, mixed with wetting and laser process, mask tolerance between 50 and 70im, generally the smallest hole size is 100 to 125 im.
B process: two-step laser process, one-step wetting process, due to CO2 diffraction on the mask, the diameter of the hole is about 60im. The limit of the copper opening thickness that can be provided for the specially treated copper material CO2 is 7 μm. This process still needs to remove the dirt.
C process: 1 step laser process, UV laser drilling of the inner and outer layer of copper is not limited, UV also has a more cleaning process, so that the removal of drilling process is reduced to a minimum, and even can replace the drilling process.
The UV laser has the ability to reduce a complete hole process step to a single laser step, especially eliminating the need for de-drilling or even completely eliminating this step, especially for pulse pattern plating. It does not require the use of aggressive de-drilling operations. For example, for CO2 lasers, the roughness of the hole shape, wicking, and barrel distortion are improved.
UV lasers for other applications and quality results ● Blind vias ● Double vias ● Through holes

In addition to the flexibility of the new laser system to implement the usual focus illumination operation hole, but also for complex drawing operations, it can be used to cut out the thin line pattern or buried mask after the removal of the solder mask. Almost any shape of processing area can be processed.

So far, when the defects on the solder mask are only minor defects and it is not important, only the laser ablated solder mask is used to repair some of the damaged pads, so that the whole panel will not be wasted, but HDI technology requires a more precise opening size and positioning. The following figure shows the circular and square solder mask openings and cross sections formed after pressure steam testing and thermal cycling. At speeds of up to 100 pads per second, for BGAs and FCs, the cost of 128 pads per IC is about 0.5 cents.

When drawing a thin line, the pattern is scribed by the laser track. As shown in the figure below, the speed of the laser track can reach 1000mm/s. After laser ablation of 1μm thick tin, the width is between 15~25im. After the tin pattern is drawn, the pattern is etched and the laser's track width is kept spaced along with the etching's side effects. For copper with a thickness of 12 μm, a pattern of less than 2 mil/2 mils can be obtained.

The following figure shows the fanout of IC and MCM graphics in a 2mil/2mil structure. The application of direct drawing of thin line graphics is limited by the drawing speed. The fanout shown in the figure below requires less than 1 second, while the fanout of a complete figure in the area of ​​40 x 40 mm takes 10 to 15 seconds.

in conclusion

UV laser systems provide a complementary solution to existing CO2 drilling tools. For drilling, short wavelengths and small spots have greater flexibility and higher complexity. The goal of UV lasers is more to meet the needs of HDI. Compared with CO2 performance, especially for large pores, there is still a gap in UV production, but with the development of high power and high frequency UV lasers, the difference will be smaller and smaller. The number of processing steps to create vias with UV lasers will be reduced to a single laser step, and the required de-drilling steps will be minimized.

In addition to the main drilling applications, UV systems can also be used for direct drawing and precise ablation of solder masks. This provides additional value for UV lasers.

There is still enough room for improving the UV laser system. Smaller pulse widths, higher frequencies, higher power and high-speed servo operation all increase productivity, and in the near future, the market will become more and more widely accepted as a complete tool for UV laser systems.

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