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PDPM IIITDM JABALPUR
LASER Beam Machining Advancements
ME 306
ADVANCED MANUFACTURING PROCESSES
Submitted To Dr. TVK Gupta
Submitted By: G11
Sandeep Singh 2009105
Santosh Kr. Maurya 2009106
Satyendra Singh 2009107
Saurabh Rathi 2009108
1
ACKNOWLEDGEMENTS
We express our sincere thanks to Dr. TVK Gupta for his immense help and guidance during completion
of this term paper.
Sandeep Singh 2009105
Santosh Kr. Maurya 2009106
Satyendra Singh 2009107
Saurabh Rathi 2009108
2
INDEX
S.No. TOPIC PAGE NO.
1. Abstract 3
2. Introduction 3
3. Experimental Setup 4
4. Mechanism of Material Removal 5
5. Material Removal 6
6. Improvisations and Advancements in LBM Process 7
7. Applications of LBM 9
8. Conclusions 11
9. References 11
3
ABSTRACT:
The high intensity which can be obtained by focusing the pulsed energy emitted by a LASER can offer
potential as a tool for nearly forceless machining. The method can be used on any material, regardless
of thermal properties, which can be evaporated without decomposition, including almost all ceramics
and metals.
With most substances, almost all of the material removed by LASER machining leaves in the liquid state.
Only a small fraction is vaporized, and the high rate of the vaporization exerts forces which expel the
liquid metal.
All features of LASER beam machining improve with increased intensity. The higher the intensity, the
less heat is resonant in the uncut material, an important consideration with materials which are
sensitive to heat shock, and the more efficient the process is in terms of volume of material removed
per unit of energy. The intensities which are available with the LASER are high enough so that the heat
affected zone (HAZ) on a cut surface is too small to be detected and there is no solidified liquid film
residue on the cut surface.
INTRODUCTION:
LASER BEAM MACHINING (LBM) is a valuable tool for drilling, cutting and milling of almost any material.
The mechanism by which a LASER beam removes material from the surface being worked usually
involves a combination of melting and evaporation, although with some materials, such as carbon and
certain ceramics, the mechanism is purely one of evaporation. Any solid material which can be melted
without decomposition can be cut with the LASER beam.
Advances in nanotechnology motivate the extension of LASER machining of microstructures to the
smaller dimensions of interest. Optical LASERs such as RUBY LASERs and CO2 LASERs are widely used for
micro-milling and micro-hole drilling over a wide range of materials. The size of the smallest features
that can be created focusing intense LASER beams onto materials is limited mainly by the LASER
wavelength and by the diffusion of heat.
A variety of different techniques have been developed to overcome the limitations imposed by the
diffraction limit in order to produce ablation craters of sub-wavelength size using optical and UV-LASERs.
Nowadays, there have been several experiments over a wide range of LASER applications for material
removal and cutting in which UV-LASERs and femto-second LASERs are the most popular for industrial
use. Also efforts have been made to minimize the tapers and HAZ which result due to high temperature
of the LASER beam.
4
EXPERIMENTAL SETUP:
Shown below is the experimental setup of Excimer LASER beam system.
Figure 1: Experimental setup of an Excimer LASER beam system.
(1) LASER unit (2) LASER beam
(3) LASER shutter (4) Attenuator
(5) & (6) LV1, LV2 (vertical lens) (7) Mirror 1
(8) & (10) LH1, LH2 (horizontal lens) (9) Mirror 2
(11) Scanning system (12) Mirror 3
(13) Field lens (14) Mask plane
(15) Projection lens (16) Photo diode detector
(17) Diode LASER (18) Z-axis
(19) X-axis (20) Y-axis.
An Excimer LASER operates at 248 nm with 400 mJ maximum output pulse energy, an average power of
100 Watt and 200 Hz maximum repetition rate. The beam exiting the LASER is rectangular in shape and
not of homogeneous intensity. To correct the beam, the optics train made of cylindrical lenses (LV1-LV2
and LH1-LH2) force the beam and makes it parallel with the square cross section in the vertical and
horizontal directions. Mirror 3 scans the beam across the mask plane to make it homogeneous. The
beam further passes through the mask plane/ aperture with a maximum area of 15X15 mm2 before
finally going through a projection lens that gives 15 times linear reduction at the work piece. By
changing the mask aperture, beam spots of different size and shape can be generated at the work piece.
Automated or manual focus control is achieved using a diode LASER beam reflected from the work piece
surface and a photo-diode array detector to provide positional measurement.
5
MECHANISM OF MATERIAL REMOVAL:
The material removal by LBM process and vaporized energies are shown in the figure below.
When LASER hits the material surface, it will have some recoil force. It can drive the liquid away from
the sides. Short pulsed LASERs generate higher recoil and it results in farther liquid removal.
UV LASER will generate high temperature on material, and removed material gets ionized. This will form
plasma in the hole. Plasma can absorb further incoming LASER energy. Part of it gets reemitted in wide
spectrum and wide angle. It help the LASER energy coupling to material and also resulting in a larger
"heat affected zone".
Figure 3: LASER induced effects in the LBM process.
6
LASER beam machining is a thermal process with emphasis laid on heat requirements and heat
utilization. It is also important to determine physical properties of the work piece material and their
relationship to the operating characteristics of optical LASERs.
The following factors have to be taken into account while LASER machining:
1. Part of the energy (Large part in case of highly reflective metal surfaces) is reflected and lost.
2. Most of the energy which is not reflected is used for material removal.
3. A very small part of the energy is used to evaporate the liquid material.
4. Another small part of energy is conducted into the converted base material.
The relative magnitudes of these four avenues of heat consumption depend strongly upon the thermal
and optical properties of the material being worked and the intensity and pulse duration of the LASER
beam. Time distribution of energy also plays an important role.
The most prominent misconception in LBM is that the entire material being removed is evaporated. But
the large quantity of energy which would be required for this to happen be not actually consumed which
substantiates the argument. Most of the material leaves the work piece surface in the liquid state and
relatively high velocity.
MATERIAL REMOVAL:
The basic assumptions to analyze the material removal process are:
1. The intensity of LASER beam does not vary with time.
2. LASER beam is uniform over the entire area of the hotspot.
3. The material being removed is both melting and evaporating.
4. The steady state ablation is characterized by constant rate of material removal and by the
establishment of a steady temperature distribution.
According to the above assumptions, the steady temperature distribution is given by,
(T – To)/ (Tm – To) = e-Vx/α
(1)
Where,
T = temperature at distance x below the ablating surface,
To = initial uniform temperature of the work piece,
Tm = melting point of the work piece
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V = steady ablation velocity,
a = thermal diffusivity of work piece, i.e., (K/ Cp
K, , Cp= thermal conductivity, density, and specific heat, respectively, of the work piece.
It can be seen that the exponential distribution represented by (1) confirms with boundary conditions
that T= To when x is very large and T = Tm when x=0.
The depth at which heat penetrates the ablating surface is of considerable practical importance. It is
reflected in the depth of the HAZ which will be left when the ablation process is over. It is desirable to
keep the HAZ as shallow as possible.
A simple way to identify the depth of heated layer is to define a characteristic depth x:
xc
(2)
The characteristic depth Xc is the depth during steady ablation which has experienced a temperature
rise 1/2.718 of the way from To to Tm. The characteristic heated depth X. decreases with increasing
ablation velocity and increases with increasing thermal diffusivity.
During the initial transient period when ablation is just beginning, part of the heat delivered to the work
surface is being used to establish the temperature distribution within the solid. Once steady conditions
are obtained, the heat contained in the solid does not increase any further, and the value of this steady
heat content is given by:
(Q/A0)= ∫
p(T-To) dx = K(Tm – To)/V (3)
After steady ablation is realized, the relationship between the intensity, exposure time, thickness of
material which has been removed, and thermal properties of the material is:
t= K(Tm – To )ρH/ + ρHd (4)
Where, t is the exposure time.
IMPROVISATIONS AND ADVANCEMENTS IN LBM PROCESS:
Over the years there have been many research and advancements to improve various parameters of
LBM process like Material Removal Rate (MRR), Reducing HAZ, Improving accuracy, Thermal Effect
Characterization, analysis of ablation rate etc. A few of them have been discussed below.
One of the major areas of research in the past few years has been Nano-Machining through LASERs.
Today LBM finds its applications in electronic equipments, micro-machine devices, and also biochemical,
medical and chemical fields.
8
LASER beams used in such fields are short wave length and ultra-short pulsed beams. Beam quality is
very important in micro-machining.
So in one of the experiments, space filtering of the beam was executed using a beam expander and the
diameter of focused beam was able to be minimized. LASER power irradiated onto specimens was
controlled by the attenuator. Debris attaches on the surface of specimen by irradiation of LASER beams
with very high power density. So, the machining was carried out in flowing water to avoid attachment of
debris. Arbitrary shapes were machined using developed LASER machining system.
While, in another experiment, a UV-LASER was used but it was implemented using different
technologies and the results examined for finish. Technologies which were used are:
Microvia Drilling:
The microvia technology brings about multiple benefits. It improves routing density of the buildup
layers, and also reduces layer count and chip spacing which leads to significant cost reduction. It also
improves electrical performance to meet the demand for high frequency applications.
In via drilling, the photochemical process leads to remarkably clean via walls free of carbonized debris or
heat affected zones. The other materials, including copper, glass and other inorganic materials,
generally interact with UV photons through photo thermal process. In such a case, materials are
removed in the mixed form of overheated melt and vapor. In order to obtain a perfectly clean feature,
such as a microvia, free of debris, it is expected that the materials are all driven rapidly through the melt
phase and into the vapor phase prior to expulsion from the interaction zone by the gas dynamic effects.
Direct Copper Structuring:
Since UV LASER couples with copper very well, the UV LASER drill systems can be also used for direct
copper structuring to make fine patterns. The beam positioners accurately move the LASER beam based
on electronic CAD layout data. After light copper etching and cleaning, the well-defined features will
remain present. This process leads to remarkably clean features free of carbonized debris or heat
affected zones.
Efforts have been made to develop a LASER processing technology for High Thermal Radiation
Multilayer Module.
As short pulse LASER (pulse width: femto-nano seconds) is suited to decrease the thermal damage of the
material. The experiment was done using a DPSS UV LASER (λ = 355nm, pulse width 15nsec). The LASER
beam diameter on the specimen is changed by a metal mask, on which there are pinholes (0.45, 0.65,
0.75mm). Pulse energy is adjusted by the diode current of the oscillator.
9
Figure 4: The configuration of the experimental setup.
Shown below are the processing results when resin with aluminum filler was irradiated with a UV LASER
using a metal mask with pinhole sizes of 0.45, 0.65 and 0.75 mm. In this graph, the X-axis indicates the
LASER fluency [J/cm2] and Y-axis is the diameter of via hole.
Next, we studied the causes that influence the fluency and beam diameter threshold. The figure above
(Right) shows the relationship between fluency and ablation depth per pulse about sintered aluminum
oxide. The horizontal axis indicates fluency with a logarithmic scale and the vertical one the ablation
speed. The relationship of these satisfies the following formula.
Lp = Alog (F/Fth) (5)
APPLICATIONS OF LBM:
A great advantage of LASER machining is capability to machine any kind of material, not necessarily
conductive, depending on LASER intensity and interaction time. In contrast to some other processes,
LASER operates using high energy photons therefore there is not a typical tool as the LASER beam
directly targets the work pieces and machines breaking the work piece chemical bonds. LASER ablation
mechanism makes it possible to introduce the desired shape geometry of the work piee without any
10
prior preparations. This feature makes LASER machining particularly feasible for wide range of
applications.
Figure 6: LASER beam machining processes in relationship to LASER intensity and interaction time.
Above figure shows the various processes for which LBM can be used.
The various range of traditional LASER applications are shown in the below figure showing its uses in
production of an automobile. The reasons of LASER use are relatively high processing time compared to
conventional processes, high flexibility that enables easy automation for example using robot arms.
11
CONCLUSIONS:
After studying various papers, it can be concluded that in a LBM operation:
1. When optimal focus position is centered in the work piece, there is an optimal interaction
between the number of required scans, the diameter on the LASER beam input as well as on the
output side, and the associated flank angle. The further off-centered the focus position is, the
more scans are required for a full cut. The diameter on the LASER beam output side wanes, the
deeper the focus is positioned in the work piece.
2. The optimal feed rate obtained after conducting various experiments, amounts to 8 mm/s. This
results in a pulse overlap of 97.7 %. Even if higher pulse overlap values reduce the required
number of scans, they are not usable.
3. By analyzing the influence of the pulse overlap to the diameter on the LASER beam output as
well as on the input side, there was no dependency discovered.
4. The investigation of the track overlap found that the best value for the track overlap amounts to
14.3 %.
5. To enlarge the kerf width the number of tracks need to be increased. The more nested circles
into each other, the less number of scans are required. However, it is essential here to take into
account the increasing required manufacturing time. The optimal value of concentric circles is
two. More than two circles are not an efficient operation.
6. If a closed configuration on the LASER beam output side is required only low wobbling
frequencies are usable.
REFERENCES:
1. Femto second Micro Machining, Analysis of Ablation Rates A. Borowiec and H.K. Haugen,
Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada.
2. UV LASER solutions for Electronic Interconnect and Packaging Weisheng Lei Sudhakar Raman,
Electro Scientific Industry Inc. 13900 NW Science Park Dr., Portland, Oregon 97229, USA
3. Development of a LASER processing technology for high thermal radiation Multilayer Module
Makoto Murai, Atsuhiro Nishida, Ryosuke Usui, Hideki Mizuhara, Takaya Kusabe, Takeshi
Nakamura, Nobuhisa Takakusaki, Yusuke Igarashi, and Yasunori Inoue SANYO Electric Co., Ltd.
180 Ohmori, Anpachi-Cho, Anpachi-Gun, Gifu, Japan
4. LASER Micromachining of Polycrystalline Alumina and Aluminium Nitride to Enable
CompactOptoelectronic Interconnects Owain Williams, Martin Williams, Dr Changqing Liu, Dr
Patrick Webb, Paul Firth Loughborough University, *Oclaro plc Mechanical and Manufacturing
Engineering, Loughborough University
5. Thermal Effect Characterization of LASER-Ablated Silicon-Through Interconnect Yu-Hua Chen,
Wei-Chung Lo, Tzu-Ying Kuo Industrial Technology Research Institute (ITRI), Taiwan.