Laser material processing

Laser material processing


Laser material processing is often applied to novel manufacturing techniques, and is being developed to fulfil the steady increased requirement of productivity, reliability and quality of new product.

The proper interconnections of different electrical components i.e. electrical contacts between different components on board is a necessary step to develop / manufacture any new device in every industry including automotive, electrical and electronics devices. The methods used to develop these interconnections are defined by cost, joint strength and volume requirements. As the technology is enters the micro and nano electronics era the conventional joining techniques crimping, soldering, and brazing are reaching their limitation point. Whereas welding esp. Laser welding which provides excellent joint integrity, longevity, and conduction performance, is gaining attentions from all leading manufacturing firms and research institutes [12].

Lasers are frequently used now days for precision spot / micro spot welding particularly in electronics industry. Issues related to reliability of laser spot welding of electronics micro parts esp. copper are still main area of concern and extensive research is on to find a reliable technique for Laser spot welding of electronics micro parts. Laser Spot welding of copper is not easy as Copper is highly reflective to infrared light and posses high thermal conductivity. The only suitable option for Laser spot Welding of copper is to use PULSED Nd- YAG Laser that can deliver a ‘spiked ‘Pulse that has a very high initial power [7].

In this work I will report on direct laser spot welding of components on PCB board using GSI Lumonics JK (700-701) Pulsed Nd-YAG Laser. With help of Material processing and reliability lab technicians, the welded joint will be tested for reliability.

Chapter 1


Laser welding is now been widely used in field of manufacturing as the cost of lasers have decreased and capabilities have increased. The radiation of high power laser CO2 or Nd-YAG can be focused by mirrors to a focal spot with diameter as small as 50 µm. Due to this property of generating very small focal size, only very small area must be heated till melting point and as result of less / low heat input is needed for welding as compare to conventional joining / welding technique. Also because of less heat input thermal stress is reduced and deformation of work piece is minimized. In recent years the emergence of high repetition rate [up to 40 pulses per second (pps)] high power, and more efficient pulsed Nd-YAG lasers,This makes way to laser welding of micro parts at high production rates previously carried out by conventional welding techniques [9].

Possibly the significance aspect that has led the rising use of lasers for welding is the ability to make spot welds. A laser beam focused down to a spot can heat, melt, and solidify metals in a matter of milliseconds with minimal disturbance to adjoining volume of material and components [8]. With the growing applications, there has been substantial advancement in laser power supply capabilities.

Spot welding of high reflective metals such as copper and aluminium has always been a very testing application for lasers particularly at the infra red wavelength. The difficulty in carrying our spot welding of highly reflective material (reflectivity >= 90%) is because of the highly non - linear light material interaction resulting in small process window and hence poor reliability [5].

This chapter is committed to talk about some earlier researches and open questions in the area, present the goal and contribution of my research, and describe the outline of the rest of this thesis


Lasers are unique source that generate or deliver amplified highly intense coherent radiations (coherent: waves of one wavelength all in phase) at frequencies ranging from infrared - visible to ultra violet regions of EM spectrum. The Laser light is quite different from the incoherent light of sun or any other natural source of light, which radiates in all direction from its source. The highly organised, parallel, single wave length, coherent light rays which makes a laser beam carries considerably high power and can be sharply focused to a much smaller spot size than randomly radiates rays from natural light source.

Laser actually uses the principle that was basically invented at micro-wave frequencies, and was known as microwave amplification by stimulated emission of radiation, or MASER action, the principle was extended to optical frequencies the name changes to light amplification by stimulated emission of radiation, or LASER. The Laser / Maser principle is now been used in a plenty of devices working in many regions of EM spectrum. In particular Laser devices are widely used in almost industry and sectors from defence to automobile and Electronics to Mechanical engineering .The fundamental description of what a typical Laser made of and how it works is presented in following paragraph.

Basically all Lasers are optical amplifiers which work by activating / pumping / exciting an active medium placed between two mirrors, one of which is partially transmitting (M2), figure 1. In general the essential elements of a laser device are from figure 1; (i) Laser medium or Amplifying medium which contains collections of atoms, molecules, ions or in some cases a semiconducting crystal and can be in solid, liquid or gas form and will produce radiation i.e. photons when activated by pumping action; (ii) a mechanism to activate the medium into excite the atom or ions present into higher energy levels and lastly (iii) Feedback element that allow a beam to either pass once through the laser medium (laser amplifier) or bounce back and forth continuously through the medium (Laser oscillator).[20,21]

Figure 1.1: The basic elements of a laser: amplifying medium, pumping energy source and resonator M1M2

The active mediums contains billions of atoms, molecules or ions and in whatever state solid, liquid or gas the atoms or ions are in they absorb energy when pumped and due to this extra energy the activated atoms or ions moves up in energy state level. Pumping of solids and liquid are carried out by flooding them with light from highly intense flash lamp and gases are activated / pumped using an electric discharge. The activated ions or atoms holds the absorb energy for very short but random period of time, after a particular time they come to their original energy level until activated again and release the extra energy in the form photons / Light. The excited atom spontaneously comes back to its ground state energy level (E1) from the higher level (E2) by releasing the difference in energy as a Light or Photon of frequency (v):

v = (E2-E1) / h, ---------- (1)

Where, h - plank's constant. This process is known as spontaneous emission figure2 the part of spectrum at frequency ν, where hν = (E2-E1), can induce absorption and stimulated emission transitions. We write the spectral energy density of the light at frequency ν as u (ν). Figure 2 shows transitions occurring.

To make the situation simpler to understand, Einstein introduced his A and B coefficients. A coefficient determines the rates of spontaneous transitions. The introduction of the B coefficient extends the treatment to include absorption and stimulated emission. Referring to the diagram above figure 3, the transition rates for three processes are:

Spontaneous emission (2→ 1) dN2/dt = − dN1/dt = − A21 N2

Stimulated emission (2→ 1) dN2/dt = − dN1/dt = − B21 N2 u(í)

Absorption (1→ 2) dN1/dt = − dN2/dt = − B12 N1 u(í).

The released photons move freely in all direction in relation to optical axis of laser. The freely moving photons collides with another excited atom or ion and force it to release the photons prematurely and the two will now move along in perfect phase until the next collision occurs, the process continue and hence make a stream of photons of increasing density. This process of generating highly in phase stream of photon is known as Stimulated emission (Figure 2). [20, 21]

Laser action will not occur just by exciting atoms, but a condition of population inversion is needed, figure 3 (population inversion- A condition in which more atoms are activated into some higher state than are in lower energy state in the medium).The photons travelling along the optics axis collide with large number of atoms or ions, stimulate them and by this process get amplified. These photons are reflected back and forth by the mirrors at resonator and pass through the pumping medium creating more photons. Each time a particular percentage of these photons are allowed to exit through the partially transmitting mirror as intense laser beam figure 1. [11, 19, 20, 21]

1.2 Journey of Laser:-


Einstein's treatment of stimulated emission.


Development of the maser by C.H. Townes.

The maser is basically the same idea as the laser, only it works at microwave frequencies.


Proposal by C.H. Townes and A.L. Schawlow that the maser concept could be extended to optical frequencies.


T.H. Maiman at Hughes Laboratories reports the first laser: the pulsed ruby laser.


The first continuous wave laser is reported (the helium neon laser).




The CO2 laser was invented by C.K.N.Patel at Bell Laboratories.

The Nd- YAG Laser was developed at Bell Laboratories.

Nicolay Basov, Charlie Townes and Aleksandr Prokhorov get the Nobel prize for “fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle.”


The Excimer laser was invented.


Art Schalow and Nicolaas Bloembergen get the Nobel Prize for “their contribution to the development of laser spectroscopy.”


Steven Chu, Claude Cohen-Tannoudji and William D. Phillips get the Nobel Prize for the “development of methods to cool and trap atoms with laser light.”


John Hall and Theodor Hänsch receive the Nobel Prize for “their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb. [3,11]

1.4 Industrial Applications of Lasers:-

The laser beam is the heat source in laser materials processing. Even though the laser is normally considered to be a light source, it is also a form of energy and as such can be a useful source of intense heat when concentrated by focusing. Lasers are able to produce high energy concentrations because of their monochromatic, coherent, and low divergence properties compared to an ordinary light source. Laser for manufacturing application have developed into reliable, proven, tools that compete on an economic and productivity basis with the more traditional technologies [4]. As a result, laser has wide applications from very mundane (bar code scanner) to most sophisticated (3-dimensional holography), mere commercial (audio recording) to purely scientific (spectroscopy), routine (printer) to futuristic (optical computer), and life saving (surgery) to life threatening (weapons/guide). Laser is useful in metrology (length/velocity/ roughness measurement), entertainment (laser light show), medical diagnostics and surgery/therapy and optical communication/computation. From printer to pointer, surgery to spectroscopy, isotope separation to invisible surveillance and medical to material treatment, laser finds a ubiquitous presence mainly for some unique combination of properties. These important properties that justify the use of laser in such a wide spectrum of applications are (a) spatial and temporal coherence (i.e., phase and amplitude are unique), (b) low divergence (parallel to the optical axis), (c) high continuous or pulsed power density, and (d) monochromaticity [11]. Figure 3 presents a brief overview of the application of laser in different fields with diverse objective [11]. Though the list is not exhaustive, it serves to show the diversity of application of laser.

Table 1 summarises commercially available lasers and their main areas of application. Depending on the type of laser and wavelength desired, the laser medium is solid, liquid or gaseous [11]. The typical commercially available lasers for material processing are (a) solid state crystal or glass laser - Nd:YAG, Ruby, (b) semiconductor laser - AlGaAs, GaAsSb and GaAlSb lasers, (c) dye or liquid laserssolutions of dyes in water/alcohol and other solvents, (d) neutral or atomic gas lasers - He-Ne laser, Cu or Au vapour laser, (e) ionized gas lasers or ion lasers - argon .ArC/ and krypton .KrC/ ion lasers, (f) molecular gas lasers - CO2 or CO laser, and (g) excimer laser - XeCl, KrF, etc. Wavelengths of presently available lasers cover the entire spectral range from the far-infrared to the soft X-ray[11].

The increasing demand of laser in material processing can be attributed to several unique advantages of laser namely, high productivity, automation worthiness, non-contact processing, elimination of finishing operation, reduced processing cost, improved product quality, greater material utilization and minimum heat affected zone. Laser material processing is often applied to novel manufacturing techniques. From the true application point of view, laser material processing can be broadly divided into four major categories, namely, forming (manufacturing of near net-shape or finished products), joining (welding, brazing, etc.), machining (cutting, drilling, etc.) and surface engineering (processing confined only to the near-surface region) [11]. The Most important joining process is Welding.

1.5 Laser Welding:-

Laser welding is finding growing acceptance in field of manufacturing as price of lasers have decreased and capabilities have increased. Laser welding is unique since it offers non-contact autogenous welding process that is not affected by the electrical conductivity or magnetic properties of the materials being welded [8]. Broadly laser welding can be divided into two categories CW (Continuous welding) and pulsed welding. Seam welding can be performed using both CW CO2 laser or overlapping pulsed with Nd-YAG laser. Whereas pulsed welding is done mainly by Nd-YAG Laser or by CO2 Laser. Currently continuous seam welding is performed mainly by CO2 laser with power level 500 W or more [2]. Pulsed laser welds are like resistance spot welding and CW laser welds are similar to arc welding. CW is mainly used for larger structural welds and is analogous to arc welding process; whereas pulsed welding is common in micro welds in smaller components like sensors and other electronics devices and chips. CO2 laser can be used for micro welding of components but are mainly used for spot welding of larger parts. Pulsed spot welding with CO2 lasers can be done by electronics pulsing or by just opening and closing of shutter of CW laser. Although Pulsed welding can be performed using CO2 laser but majority of Pulsed spot is carried out by Nd-YAG lasers. Nd-YAG glass lasers are well-matched for low repetition rate spot welding on small parts such as wires to terminal and other electronics components [4].

The use of laser spot welding has many advantages.

  1. There is no physical contact between welding tool and work piece; this removes the need for maintenance of arc welding electrodes.
  2. Welds some metals difficult to weld by other welding techniques esp. dissimilar metals.
  3. Small heat affected zone (HAZ) because of rapid cooling.
  4. Welding can be done in air or in vacuum.
  5. Very small weld size possible.
  6. Welds can be done in areas which are not reachable by other welding equipments and techniques.
  7. Laser beam can be controlled by computer, by optical tracing or by other control devices.

Perhaps the most important aspect that has led to the growing use of lasers for welding is the ability to make spot welds. A laser beam focused down to a spot can heat, melt, and solidify metals in a matter of milli-seconds with minimal disturbance to adjoining volume of material and components [8]. Consequently, laser spot welding is finding ever increasing applications in all segments of manufacturing including medical devices, sensors, batteries, and microwave enclosures. Along with growth in applications, there has been substantial improvement in laser power supply capabilities including closed-loop feedback and pulse shaping.

Although there has been significant improvement in laser welding technology in last decade and is used in almost all major manufacturing sectors. Another important development in the area of laser welding is the emergence of high repetition rate [up to 40 pulses per second (pps)] high power, and more efficient pulsed Nd-YAG lasers. This makes possible laser welding of small components at high production rates hitherto done by conventional welding techniques, such as resistance and capacitance discharge welding [9].

1.6 Laser Spot Welding of Highly Reflective Metals:-

The Laser Micro Welding of highly reflective materials is the new trend in the area of laser welding. Although the idea of laser spot welding of electronics micro parts is there for quite a few years now, but until last two - three years there was not much of good literature work talking about spot welding of copper. Majority of research work in last decade is focused on stainless steel processing on CO2 and Pulsed Nd-YAG laser [5].

Plenty of good quality literature is present about welding of stainless steels and aluminium, where as for copper in terms of quality just a few good papers are in stock. Seam welding of nickel - plated copper using ND-YAG laser has been studied by Hashimoto et al. Seam welding of Al with copper with a ND-YAG laser was presented by Sarady et al. A detailed study about seam welding of copper steel with CO2 laser was done by Gopinathan et al. lastly, a theoretical model on the interaction of CO2 laser radiation with copper had been described by Xie and kar[5].

Spot welding copper has always been a very challenging application for lasers especially at the near infra red wavelength. Nd-YAG laser spot welding of copper (reflectivity > 90%) and other high reflectivity metals is quite challenging and difficult because of the highly non - linear light material interaction resulting in small process window and hence poor reliability. Another problem is the lack of quality literature on and about spot welding of copper; this may be due to highly non - linear radiation material interaction that results in non reproducible welding result

A good understanding and technique about how to perform an industrial reliable procedure is still missing, therefore in this thesis/work we are only focus on investigating procedure for performing reliable Laser Spot welding of copper.

1.7 Thesis Objective:

The goal of this thesis project is to investigate possible Laser System development for fast, accurate, reliable and reproducible spot welding of copper. In order to be able to reach the proposed objective, we should first answer the following questions:

  1. What are the advantages of Laser micro spot welding environment when comparing with other conventional welding technique?
  2. How to design efficient architecture i.e. choosing pulse width, Pulse energy, Repetition rate and other Laser parameter for effective Laser spot welding?
  3. How to effectively calculate the beam diameter and spot size.

1.8 Project Contribution:-

This thesis address the problems discussed in the previous section and present the detailed design methodology and implementation results of our project. The main contribution of the project can be summarized as follows:

  • I showed it is possible to spot weld copper using Nd-Yag laser, by choosing correct laser parameters.
  • I described a method to effectively calculate the beam diameter and spot size.
  • I presented the method for direct spot of copper i.e. without the use of any sensor or photo detector to control the process.
  • I showed a comparison between Laser spot welding and other welding techniques.

1.9 Thesis Organisation:-

The rest of this thesis will be organized as follow.

  • Chapter 2 presents the definition, characteristics and parameters of Laser welding and Laser spot welding of Highly Reflective Materials.
  • Chapter 3 Beam Divergence and Spot Size measurement.
  • Chapter 4 presents experimental results and discussion.
  • Chapter 5 present a summary of my work, conclusions, and future directions for the continuation of this project.

  • Finally, in appendix Laser safety issues are discussed in details and a risk assessment form is filled and attached.

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