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

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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

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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

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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.

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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.

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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.

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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.

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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.

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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

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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.

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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.