Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)21

Microdrilling Technology using Short Pulsed-laser

KIYOTAKA NAKAGAWA*1

TSUGUMARU YAMASHITA*2YOSHIHITO FUJITA*3HARUHIKO NIITANI*4

In recent years, laser machining is highly anticipated as a technology to replace machineprocessing and electrical discharge machining. The reason is because this type of non-contact
machining involves no tool wear and can be applied to metals and a wide range of other materials
including nonconductive crustaceous materials and resins. There is a great need for theapplication of short-pulse laser machining to various industrial fields, as well as types of parts
processing since a finer and higher-quality machined surface is obtained. But efficiency andquality can hardly be reconciled, resulting in an obstacle to wide penetration. Mitsubishi HeavyIndustries, Ltd. (MHI), having proved that one method of short-pulse laser machining significantly
differs from another in machining ability, reports here examples of laser-applied micro drilling.

|1. Introduction
Laser machining is being tried in a variety of fields for application since it can be widely
applied not only to metals, but also to brittle and other nonmetal materials on the condition that an
appropriate source of light is selected for the material to be machined. In particular, in the recent
situation of growing demand for high-accuracy and high-grade micro drilling, conventional laser
drilling could no longer meet quality requirements such as for hole diameter accuracy and surface
integrity due to its significant thermal effects on the material, but the use of picosecond and othershort pulses has enabled ablation (evaporation/sublimation) processing-based, high-accuracy andhigh-grade machining. Between higher-efficiency and higher-accuracy/grade, however, theproblem of antinomy has been pointed out, and a solution to this problem is required for adaptation
to production. In an effort to solve this problem, MHI has developed a micro-laser process machine
where the helical drilling process is employed (1).
This paper compares helical drilling with percussion machining as a typical method of micro
drilling for machining ability, as well as for machining quality, to prove the former’s superiority. Itthen introduces superior method-based examples of micro drilling together with the “ABLASER”
micro-laser process machine developed by MHI.

|2. Comparison of short-pulse laser machining methods
2.1

Characteristics and problems with machining methods

Figures 1 and 2 outline different laser machining methods and Table 1 indicates theircharacteristics. Percussion machining is a method to drill holes by irradiating a pulsed laser beamin a fixed place without allowing the beam to move or rotate (Figure 1). At present, it is used as amethod of micro drilling, but since the expanse due to light concentration by a lens is transcribed,*1

Solution Technology Center, Machine Tool Division, Machinery, Equipment & Infrastructure*2

Chief Staff Manager, Solution Technology Center, Machine Tool Division, Machinery, Equipment & Infrastructure*3

Chief Staff Manager, Engineering Department, Machine Tool Division, Machinery, Equipment & Infrastructure*4

General Manager, E Solution Technology Center, Machine Tool Division, Machinery, Equipment & InfrastructureMitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
22the cut surface always assumes a tapered shape. On the other hand, helical drilling for machining
irradiates a pulsed laser beam while allowing the beam to rotate at a high speed after being adjusted
to the desired hole diameter (Figure 2).

Figure 1

Percussion machining method

Percussion machining drills holes by irradiating
a pulsed laser beam in a fixed place without
allowing the beam to move or rotate.

Table 1
Hole diameter accuracy
Tapered shape control
Machining efficiency
Machining quality

Figure 2

Helical drilling method

Helical drilling adjusts the laser beam to the desired holediameter and irradiates a pulse laser while allowing it torotate at a high speed.

Machining method features
Percussion machining
Inferior
Unavailable
LowInferior

Helical drilling
Superior
Available
HighSuperior

If laser beams have the same amount of energy per unit area, the machining efficiency perpulse is considered constant and it has so far been conjectured that there is no difference between
percussion machining and helical drilling. At the same time, there are few cases where superiority
in terms of machining quality has been quantitatively compared, and therefore it was difficult to
examine the optimum machining method.

2.2

Comparison between percussion machining and helical drilling

Percussion machining was compared with helical drilling for machining efficiency andmachining quality. Two different types of material were used for machining; one was SUS420 as ametal material and the other was single crystal silicon as a brittle material.
(1) Machining efficiency evaluation test
Machining efficiency tests were conducted by irradiating a laser beam on the material to
be machined as shown in Figure 3(a), with the amount of energy per pulse fixed and thenumber of pulses per unit time altered, and the volumes removed were thereby compared. The
per-pulse energy was set at a value that can realize less thermally-affected ablation machining.
For helical drilling, the beam’s turning radius was adjusted so that the per-unit area volume ofenergy should become the same as that for percussion machining. Figure 4 shows the test
results. The vertical axis represents the volume removed per unit time as machining efficiency
and the horizontal represents the per-unit time number of pulses. Helical drilling was found tobe six times higher for SUS420 and 1.8 times higher for single crystal silicon than percussion
machining in terms of efficiency. In helical drilling of both SUS420 and single crystal silicon,
the machining efficiency increases with the increasing number of pulses per unit time while, atand over a certain number of pulses, it decreases. This is supposedly because ablation
processing remained as such until it came to a certain domain, but when the amount of energy
irradiated became too large, dissemination in the form of heat took place, worsening
abrasion-derived machining efficiency. It was found from the above that short pulse laser-basedhelical drilling has the optimum conditions for machining. Since the optimum conditions formachining depend upon the material to be machined, it is important in practicing
high-efficiency helical drilling to accumulate expertise for machining.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
23

Figure 3 Schematic diagram of short-pulse laser machining method comparison test(a) In machining efficiency comparison tests, the volume removed by laser irradiation was compared asmachining efficiency.
(b) In machining quality comparison tests, the laser-cut surface was observed under SEM for comparison.

Figure 4

Machining efficiency test results

In helical drilling, machining efficiency is 6 times higher for (a) SUS420 and 1.8 times higher for (b) silicon.

(2)

Machining quality evaluation test
The feed rate was applied to percussion machining and helical drilling for cutting in order
to compare the machining quality on the cut surface (Figure 3(b)). Cutting conditions including
the energy of the laser being irradiated and the pulse frequencies were the same for both
percussion machining and helical drilling.
Figure 5 shows the results of cut surface observation under SEM for each process. BothSUS420 and single crystal silicon obtained from helical drilling a cut surface that was better interms of surface integrity. The cut surface observation found that some matter that had
re-melted supposedly due to the thermal effects of the laser had been left on thepercussion-machined surface, while helical drilling had a smooth cut surface free fromre-melted matter, proving favorable ablation processing. Like the results of the aforementioned
machining efficiency tests, it was clear that helical drilling was less likely to result in thermal
fusion of the target material than percussion machining, even under the same conditions formachining.
(3) Comparison test results
The test results mentioned so far indicated that helical drilling is superior to percussion
machining in realizing high-efficiency, high-accuracy micro drilling with a short-pulsed laser
beam. It further became known that high-efficiency, high-quality machining of key metal/brittle
materials would require each target material to be optimally conditioned so that it could bemachined with the greatest efficiency.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
24

Figure 5 SEM observation results for the cut surface
machined to the same machining conditions|3. Example of micro machining by helical drilling
3.1

Example of micro drilling (target material: SUS420/silicon wafer)

Figure 6 shows an example of φ0.12mm-hole drilling on a SUS420 plate (t0.3mm). Smoothmachined inner surfaces and sharp edge sections indicate that there was little thermal effect on thedrilling due to ablation. Figure 7 show an example of φ0.15mm-hole drilling on a silicon wafer(t0.5mm). Like SUS420, silicon as a brittle material could be machined with little thermal effect.

Figure 6

Micro drilling on SUS420

Material: SUS420, thickness: t0.3mm, holediameter: φ0.12mm

3.2

Figure 7

Micro drilling on silicon water

Material: silicon wafer, thickness: t0.5mm, holediameter: φ0.15mm

Example of different hole shape machining (target material: SCM420tempered)A straight, a reverse-tapered, and a drum-shaped hole with the narrowest part of φ0.1mm
were machined on a tempered SCM420 material (t0.8mm) and Figure 8 shows an SEM
observation photo of each machined section and roughness measurement results.

Figure 8

Example of different hole shape machining

SCM420 tempered, thickness: t0.8 mm, narrowest hole diameter: φ0.1 mm.
Can be machined to any desired tapered shape and the machined surface roughness is Ra0.1μm or less.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)25

If the beam’s angle of incidence is controlled, the desired tapered shape can be machined andthe machined surface roughness is Ra0.1µm or les in all cases. In straight and reverse-tapered
holes, adhesion of foreign matter was observed, but this occurred only after machining, and bydevising a cleaning method, etc., drum (-shaped) hole machining achieved a smooth machined
surface free from any adhesion. Figure 9 shows an SEM observation photo of different holemachining. Not only confined to drilling, the process can also be used for cutting the material into
any desired shape with few thermal effects on the cut surface.

Figure 9

Example of different-hole machining

SCM420 tempered, thickness: t0.8mm|4. The “ABLASER” short pulsed-laser process machine
MHI developed the “ABLASER” short pulsed-laser process machine, in which a helicaldrilling method was employed, and sent it to “The 27th Japan International Machine Tool Fair(JIMTOF2014)” for referential display. Figure 10 shows its external appearance and Table 2
indicates its equipment specifications. Granite used for main structures and a precisionscale-equipped positioning mechanism achieved machining accuracy (with an error of ±0.002 mmor less) higher than that of a conventional laser process machine. A 5-axis control device ismounted on the machine, enabling curved surfaces and complex shapes to be machined.
Table 2Specifications for ABLASER
equipment

X axis (mm)Y axis (mm)Z axis (mm)C axis (°)
A axis (°)
Positioning accuracy (mm)NC device
Laser optical system
Taper hole control
Assist gas

Figure 10 External appearance ofABLASER

Width x depth x height(mm)

100 or more
100 or more
100 or more
360
-20－1200.001
FANUC30iLB
Made by MHI
Taper/reverse-taper
Dependent upon material
to be machined
1,850 x 1,850 x 1,900|5. Conclusion
In this report, we proved helical drilling’s predominance in the high-efficiency andhigh-grade performance of micro drilling with a short-pulsed laser beam, and presented examplesof less thermally-affected high-grade micro drilling. From now on, MHI intends to further promote
the development of such technologies for application in a variety of prospective industrial fields,
not confined only to engine fuel injection nozzles or semiconductor processes, and to continue topropose optimal solutions for laser machining that meet customer needs.

Reference
1. Watanabe, T. et al., High Speed & Quality Laser Drilling Technology by Using Prism Rotator, Proceedingsof the 82nd Laser Materials Processing Conference (2015-1) p. 79