The laser is the tool of choice when it comes to drilling a large number of similar holes next to each other. Here is an overview of applications outside electronics production - where there is also a need for an extremely large number of fine and precise holes. However, there are also important findings for PCB drilling.
Fig. 2: ... and percussion OTF laser drillingTheFraunhofer Institute for Laser Technology ILT in Aachen has been developing and testing the technology for such processes for decades. They can say which laser system is the fastest and which drilling process is the most suitable for which application.
The pandemic has reduced air travel for the time being. Nevertheless, aircraft manufacturers are still under considerable pressure to reduce fuel consumption. The principle of 'Hybrid Laminar Flow Control' offers an opportunity for this: air flows around an aircraft wing with less resistance if its surface has many small holes. Fuel savings of up to 10 % are possible in this way.
The situation is similar for aircraft turbines. Here, small holes help to dampen engine noise. A third example is filter technology. Here, metal foils with holes in the micrometer range can efficiently filter microplastics out of wastewater.
These three examples show quite well that there are many holes to be drilled in very different areas. Turbine construction, paper production or plastics recycling are areas with great potential.
Drilling with the laser
Lasers have now been in industrial use for several decades. The applications are correspondingly diverse. In addition to marking, welding and cutting, drilling is also a common process. From a scientific point of view, it is a non-chipping thermal cutting process. Figure 3 shows four different ways of drilling holes with a laser. The balance between high speed and high precision plays a decisive role in the selection.
Fig. 3: Different laser drilling processes can be classified according to precision and drilling speed
Fig. 4: View of the micro holes with transmitted light (left) and longitudinal section of the holes (right)The fastest method is of course to 'shoot' the holes through the material with individual pulses. Helical drilling takes the longest, usually even requires special optics, but also offers high precision. In percussion drilling, several pulses are placed at the same point to laser a hole through the material. Trepanning is when the hole is cut out after the through-hole drilling by tracing the contour of the hole.
The precision of the hole and the smoothness of the hole walls naturally also depend on the material and the type of laser radiation. Copper, for example, absorbs green and blue radiation much better than the usual infrared. If printed circuit boards are drilled, the laser system must be selected and adjusted accordingly.
Pulse duration and pulse energy also influence the result. This is where ultrashort pulse (USP) lasers are something very special: they apply the laser energy in an extremely short time, converting the material into a plasma almost instantaneously. As a result, USP lasers can process practically any material, they offer excellent surface quality, but also take the longest time to process.
All of these processes have been studied and optimized at the ILT for years. The result is highly productive drilling processes that can produce several 10 to 100 holes per second. The major challenge here was to maintain low tolerances of the bore diameters and a high surface quality even at a high productivity (drilling rate). The processes used here are the familiar 'on-the-fly' drilling (OTF) with individual pulses and the OTF percussion drilling developed at the Fraunhofer ILT.
If small holes in the front part of the aircraft wing absorb the air, the formation of turbulence is reduced and less fuel is consumed. The 80 µm holes are drilled into the wing section 'on the fly' at a feed rate of around 150 mm/s
Example 1: Single-pulse micro drilling with the laser
Ultra-short laser pulses (right) produce much better surfaces than short laser pulses (left)The most productive drilling method in the above list is drilling with single pulses. It should always be noted that the speed of the process and the drill hole quality must be balanced. If the optics move too quickly over the surface, the hole will become elongated. The quality of the drill hole can be assessed according to various parameters:
- Roundness, i.e. how far the drill hole deviates from an ideal circular shape. It is influenced by the laser and the traversing speed
- Conicity is the extent to which the diameter of the hole changes with depth
- Surface quality of the hole, which is influenced by the intensity of the laser beam
At the ILT, the process was optimized so that 200 holes per second could be drilled in 1 mm thick titanium sheet. A single mode laser was used for this, with which a focus diameter of just 12 µm can be achieved to produce holes with a diameter of just under 80 µm.
The holes were drilled 'on-the-fly', i.e. with a constant feed rate of the optics in relation to the workpiece. With the optimized process parameters, a 2 m long 3D-formed demonstrator of an aircraft wing was successfully machined on a 6-axis system. At a speed of 200 holes per second, around 2 million holes per square meter were drilled on an area of around 2m2 in under three hours. The diameter of the holes was 80 µm. It was also important to precisely control the distance between the optics and the workpiece. OCT (optical coherence tomography) was used for this, as it is not affected by plasma or spatter and achieves a measurement accuracy of just a few micrometers.
Example 2: OTF percussion drilling
Ultra-short pulse drilling on turbine bladesNotall holes can be drilled with a laser pulse. Higher aspect ratios, higher requirements for hole quality or an inclination of the hole can be better achieved with percussion drilling. Larger hole diameters are another application for OTF percussion drilling. Several laser pulses are shot into the same hole. It is obvious that the feed rate plays an even greater role here: The hole must be finished before the optics have moved on, otherwise the hole will be crooked, or the laser will not be able to penetrate the material at all.
The duration of a hole depends on the number of laser pulses required and the repetition rate of the laser. The drilling process itself is more complex than with a single pulse. The individual laser pulses must be strong enough to drive the material further out of the hole before it is penetrated. Depending on the process parameters, the molten material can remain in the hole and solidify, shading the laser or even closing the hole.
The Fraunhofer ILT has carried out extensive research into this and successfully developed an OTF process for a percussion drilling process. By using a new fiber laser beam source with up to 20 kW peak pulse power and 2000 Hz repetition rate, up to 30 holes per second could be produced in 2 mm thick aluminum.
Micrographs on the quality of the drill holes
Bore diameters of 500 µm were produced with a high degree of precision. The standard deviation was less than 5 % at the inlet and even less than 2.5 % at the outlet. The high peak pulse power and repetition rates of the new laser beam sources made it possible to achieve the precision of the drill holes and productivity.
If that's not enough
Laser and process technology is constantly evolving and further advances in laser drilling can be expected in the coming years. In terms of beam sources, ultrashort pulse (USP) lasers with higher outputs are on the rise. They have two major advantages: On the one hand, the drill holes are more precise, defect-free, or simply smoother after USP processing. On the other hand, USP lasers can process practically all materials. Up to now, this has only been offset by a significantly slower working speed. In the Cluster of Excellence Advanced Photon Sources CAPS, experts from several Fraunhofer Institutes are currently developing beam sources with an output of well over 10 kW as well as the necessary process technology. They should also solve the current problem of the low productivity of USP lasers.
Generated with USP: water filters for removing microplastics
Such powerful laser beam sources also enable the use of multi-beam optics. Among other things, they allow the parallel drilling of hundreds or thousands of holes. In the SimConDrill project, filter plates for wastewater filters with millions of 10 µm holes have already been drilled in this way. With such small holes, the filters can be used in public wastewater treatment plants to trap microplastics down to the sub-10 µm range.
The multi-beam optics can be configured in different ways: Large numbers of parallel, identical partial beams can be achieved using diffractive optical elements. The distribution of the partial beams can be defined almost arbitrarily using special liquid crystal modulators. With acousto-optical modulators, individual beams can also be switched on and off.
Overall, laser drilling technology is highly dynamic. New processes are being developed and ever more powerful lasers are opening up new possibilities in terms of achievable drilling geometries and productivity. In particular, the major advances in the further development of USP beam sources will make many new laser drilling applications possible in the coming years. At the Fraunhofer ILT, the know-how is being developed from basic research to industrial application and system development.
At this year's Laser World of Photonics in Munich (April 26-29, 2022), the ILT will be presenting the range of applications and potential of laser drilling with USP technology (Fraunhofer joint booth A6.441).
Dipl.-Phys. Martin Reininghaus, Group Manager Micro- and Nanostructuring at Fraunhofer ILT
Dennis Haasler, Micro- and Nanostructuring Group at Fraunhofer ILT
Technical contact
M.Sc. Dennis Haasler, Micro- and Nanostructuring Group,
Phone +49 241 8906-8321,
Fraunhofer Institute for Laser Technology ILT,
Steinbachstraße 15,
52074 Aachen
