COMSOL Multiphyisics® based simulation of DPSS laser dicing in wide-gap thin substrate

We demonstrate the possibility of the crack-free laser dicing of borosilicate glass. The effect of a polymer layer is analyzed on the laser (diode-pumped solid-state, 355 nm, nanosecond) dicing capability of a commercial glass (Borofloat®33) for backend packaging. The main drawbacks concerned the brittleness of glass and photothermal ablation which induces thermal stresses responsible for chipping or cracks. It was assumed that a thick enough polymer would enhance the energy coupling between the beam and the target. The effect of pulse energy and speed on the scribing depth is provided in the present article. A numerical simulation using COMSOL® Multiphysics was performed in order to analyze the volume temperature distribution. It revealed that the temperature remained higher than the working point of glass (1,500 K) between two pulses. The viscosity is thus low between two pulses, and thermal stresses are diminished compared to the processing of raw glass samples.

In recent years, laser micromachining of semiconductor materials such as silicon and sapphire has attracted more and more attention. High power Diode Pumped Solid State (DPSS) lasers have been gained wider application in microelectronics, flat panel display, and solar cell manufacturing industries because it can obtain smaller feature sizes, higher throughput and lower costs. The works on typical applications of 355nm DPSS UV laser micro-processing new energy source materials have been introduced in this paper, including microprocessing carried out on fluorine-doped tin oxide (FTO) thin film and polyimide membrane to investigate the etching accuracy and depth with different laser processing modes and parameters in order to meet the technologic requirement of laser micromachining. Another important work introduced in this paper is laser selectively sintering TiO2 films for dye-sensitized solar cells (DSCs). The effects of parameters (average power, repetition rate, scanning speed and scan spacing) on the on the micro-processing quality and precision have been investigated and analysed. The incident photon-to-electron conversion efficiency and the current-voltage characteristic of the DSC were also determined.In recent years, laser micromachining of semiconductor materials such as silicon and sapphire has attracted more and more attention. High power Diode Pumped Solid State (DPSS) lasers have been gained wider application in microelectronics, flat panel display, and solar cell manufacturing industries because it can obtain smaller feature sizes, higher throughput and lower costs. The works on typical applications of 355nm DPSS UV laser micro-processing new energy source materials have been introduced in this paper, including microprocessing carried out on fluorine-doped tin oxide (FTO) thin film and polyimide membrane to investigate the etching accuracy and depth with different laser processing modes and parameters in order to meet the technologic requirement of laser micromachining. Another important work introduced in this paper is laser selectively sintering TiO2 films for dye-sensitized solar cells (DSCs). The effects of parameters (average power, repetition rate, scanning speed and scan spacing) on the o...

Increasing demand for creating fine features with high accuracy in manufacturing of electronic mobile devices has fueled growth for lasers in manufacturing. High power, high repetition rate ultraviolet (UV) lasers provide an opportunity to implement a cost effective high quality, high throughput micromachining process in a 24/7 manufacturing environment. The energy available per pulse and the pulse repetition frequency (PRF) of diode pumped solid state (DPSS) nanosecond UV lasers have increased steadily over the years. Efficient use of the available energy from a laser is important to generate accurate fine features at a high speed with high quality. To achieve maximum material removal and minimal thermal damage for any laser micromachining application, use of the optimal process parameters including energy density or fluence (J/cm²), pulse width, and repetition rate is important. In this study we present a new high power, high PRF QuasarR 355-40 laser from Spectra-Physics with TimeShift^TM technology for unique software adjustable pulse width, pulse splitting, and pulse shaping capabilities. The benefits of these features for micromachining include improved throughput and quality. Specific example and results of silicon scribing are described to demonstrate the processing benefits of the Quasar’s available power, PRF, and TimeShift technology.

Investigations on laser actuation and life cycle characteristics of NiTi shape memory alloy bimorph for non-contact functional applications

During the last decade, diode-pumped solid state lasers (DPSSL) have gained wider application in industry, of which most of the lasers’ wavelengths are in the ranges 1064nm and 532nm. With the development of DPSSL, shorter wavelength laser such as UV laser is needed to etch the materials in order to get the higher etching depth or precision. However, there are few reports about the comparison for micro machining between UV laser and the infrared laser. In this paper, a 355nm Nd:YVO4 laser with output power of 8W, repetition rates of 100kHz, pulse duration of 25ns is used for etching the copper clad laminate (CCL) and flexible printed circuit (FPC). The effect of the laser processing parameters such as laser power, single pulse energy, repetition rate and laser scanning speed on the microstructure are studied in detail. By comparing 1064nm laser and 355nm UV laser, the experimental results demonstrated that UV laser processing has higher precision and causes less thermal effect, as copper, PI and epoxy has higher absorptivity for this laser. Nevertheless, the significant benefit for IR laser is that it brings about less damage to PI or epoxy substrates, which makes it fit for the ablation of the copper layer. On the other hand, the 355nm UV laser is fit for the cutting process of the PCB boards because it can make the facile separation of thick polymer substrates quickly.During the last decade, diode-pumped solid state lasers (DPSSL) have gained wider application in industry, of which most of the lasers’ wavelengths are in the ranges 1064nm and 532nm. With the development of DPSSL, shorter wavelength laser such as UV laser is needed to etch the materials in order to get the higher etching depth or precision. However, there are few reports about the comparison for micro machining between UV laser and the infrared laser. In this paper, a 355nm Nd:YVO4 laser with output power of 8W, repetition rates of 100kHz, pulse duration of 25ns is used for etching the copper clad laminate (CCL) and flexible printed circuit (FPC). The effect of the laser processing parameters such as laser power, single pulse energy, repetition rate and laser scanning speed on the microstructure are studied in detail. By comparing 1064nm laser and 355nm UV laser, the experimental results demonstrated that UV laser processing has higher precision and causes less thermal effect, as copper, PI and epoxy has...

In order to improve the work efficiency and save resources in the process of laser cladding on the H13 steel surface, based on COMSOL, by combining computer simulation and experiment, a plane continuous heat source model was used to simulate and analyze the temperature and stress field. The optimal power and scanning speed were obtained. It is found in the simulation process that the thermal sampling points stress increases with the increase of laser power and scanning speed. Because of the existence of solid–liquid phase variation in the laser cladding process, there are two peaks in the maximum thermal stress cycle curve of the sample points located in the molten pool, and the starting and ending time of each sample point’s peak value is basically the same. When the sample point is outside the molten pool, because the metal at the corresponding location is not melted, so there is no obvious peak value in the thermal stress cycle curve. With the increase of cladding layer depth corresponding to each sample point, the variation range of the two alternating thermal stress peaks increases first and then decreases, while the duration increases. According to the peak value of alternating thermal stress at the sampling point, the molten pool depth can be predicted. The residual stress analysis of the cladding layer is carried out according to the analysis results of temperature field and stress field. Through the actual cladding experiment, it is found that the depth of molten pool in the simulation results is basically consistent with the experimental results. All simulation results are verified through actual cladding experiments.

Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) are parent Additive Manufacturing processes using a laser to solidify metallic, ceramic, polymer or composite powders. During the process, the object is built layer by layer. A laser source is responsible for the consolidation, by local heating. The light is deviated by a scanning head according to the instructions of an STL file. Then partial fusion of the particles takes place, followed by a solidification of the liquid created. Kinetics of these steps are very high and play an important role in the final microstructure (rearrangement of the particles, pore creation, residual stresses). In the case of polymers and composites the technique is now well understood and widely used, but for metals and ceramics it suffers from a lack of precision, surface roughness and poor mechanical properties. The goal of this Ph.D. work is to understand the effect of the thermal gradients on the consolidation process, using different laser parameters (power, pulse frequency, scan speed,…). In SLS, microscopic thermal gradients arise, due to the fact that the laser is pulsed and that only the exterior of the powder particles is molten. A thermal model to describe the interaction between a laser beam and a (spherical) grain is proposed. This model allows for the incorporation of the latent heat of fusion and for a realistic surrounding. The absorbed laser intensity is modeled by means of the Mie theory. Experiments where two particles of powder are isolated and illuminated by the laser are carried out in order to measure the interparticular necks and the volume of liquid formed for different repetition rates. The thermal model leads to good predictions of the particles final sintering state. Fluid flow models are investigated in order to determine the dynamic of the molten liquid. The main issue is to explain the capillary flow mechanisms leading from the molten material to the neck formation. Two models are derived to simulate the fluid flow between the particles. The first one simulates a capillary flow between two parallel plates. The second one deals with energetic considerations arising from Frenkel's principle. In both cases, the final output is the liquid life-time necessary to get the interparticular neck lengths experimentally observed. We point out that the two models predict the same liquid lifetime, although they are derived with different hypotheses. In SLM, continuous lasers are used and one has to deal with macroscopic thermal gradients, since the particles are completely molten. A very high laser power is used and the negative thermal effects (like thermal stresses or balling effects) are important. A solution to control them is to adjust the scanning strategy. Four scanning strategies are investigated for material with different thermal conductivities. The three-dimensional model used to describe the laser-matter interactions and the temperature evolution of the scanned powder bed allows for a finite latent heat (Stefan-problem) and for conductivity modifications due to the consolidation. We show that this finite element thermal model can be efficiently used to anticipate most of the problems (like cracks or balling) arising in practice. The benefit of avoiding thermal gradients is shown, in particular by EBSD analysis. Finally, applications of the SLS/SLM technique on different pieces built during this work are shown. A new way to build support structure is also proposed.

As a third-generation wide-bandgap semiconductor material, Silicon Carbide (SiC) boasts broad application prospects in power electronics, radio frequency (RF) devices, and other fields. Wafer thinning is a crucial process for improving the performance of SiC power devices. This study takes fine grinding thickness as the core control parameter to investigate its influence on the surface quality of thinned SiC wafers and optimize the process. Experimental results show that when the fine grinding thickness is increased from 10μm to 20μm, the surface quality of the wafers is significantly improved: the Total Thickness Variation (TTV) is reduced to 5.18μm, a decrease of 22.5% compared with that before Optimization; the surface roughness is reduced to 0.6407 nm, with a reduction rate of 18.7%; the warpage is suppressed to 405.93μm; and the internal stress is decreased to 24.634 MPa, a substantial reduction of 35.3%. The core mechanism lies in the reasonable increase of fine grinding thickness, which can thoroughly eliminate the damaged layer and subsurface cracks remaining from rough grinding, disperse the contact pressure of abrasive grains, and synchronously release mechanical stress and thermal stress, forming a positive Optimization cycle. This study verifies the feasibility and effectiveness of optimizing the processing quality of SiC wafers by adjusting the fine grinding thickness, providing an effective technical reference for improving the quality of subsequent processes, such as laser annealing and dicing, as well as enhancing the electrical performance and reliability of the final devices.

Investigation of laser and process parameters for Selective Laser Erosion

In order to study the mechanism of microcrack generation in the process of pulsed laser polishing of zirconia ceramics and the influence of laser polishing process parameters on the surface temperature and surface stress, this paper establishes a finite element computational model of pulsed laser polishing of zirconia ceramics based on the COMSOL Multiphysics multi-physics field simulation software. Firstly, in the process of establishing the finite element model, the temperature field and stress field coupling is used to analyze the temperature field and stress field changes during the laser polishing process, which reveals the microcrack generation mechanism and size characteristics of zirconia ceramics in the process of pulsed laser polishing. Secondly, through parameterized scanning, the variation rules of surface temperature and surface stress were studied under different process parameters of laser power, scanning speed, pulse frequency and pulse width. Finally, the validity of the finite element calculation model is verified by the pulsed laser polishing zirconia ceramics experiment. The results show that, in a certain energy range, the high-energy laser beam can effectively reduce the surface roughness of the material, and with the increase in the time of laser action on the surface layer of the material, it will cause the temperature and thermal stress of the surface layer of the material to continue to accumulate, and when the stress value exceeds the yield limit of the material, cracks will form in the surface layer of the material; because the laser power, scanning speed, pulse frequency and pulse width are used to affect the laser energy density, and then, the pulse width will be affected by the process parameters of the laser energy density, and thus the surface temperature and thermal stress of the surface layer of the material. Because the laser power, scanning speed, pulse frequency and pulse width all affect the thermal stress on the material surface by influencing the laser energy density acting on the material surface, the laser energy density is the main influencing factor of the dimensional characteristics of the microcracks. In addition, the microcrack width and depth will increase when the laser energy density acting on the material surface layer increases.

A study on residual stresses in laser surface hardening of a medium carbon steel

A low-cost, high-efficiency and high-flexibility surface modification technology for a black bisphenol A polycarbonate board

Dimples on Al2O3 ceramic plates were created with a CO2 laser using different laser parameters. The effects of the laser parameters used on the dimple geometry were investigated and the necessary laser parameters were optimized to obtain the desired dimple geometry. Taguchi method was used in the optimization process. The effects of laser power, scan speed and laser frequency from laser parameters were investigated. Optimum laser parameters were determined as a result of the Taguchi Optimization method. In addition, the laser parameter with the highest effect on the result was determined. Optimum laser parameters were obtained as 60 W for laser power, 35 s for laser exposure duration and 50 kHz for laser frequency.

This research aimed to predict the residual stress of additively manufactured rectangular specimen using Selective Laser Melting (SLM) by means of non-linear numerical computation based on Thermo-mechanical method (TMM). The procedure starts with the geometrical and material modelling of rectangular specimen with regards to Austenitic Stainless Steel 316L (SS316L) in which the temperature dependent material data properties such as Young’s Modulus, Thermal Expansion Coefficient, Specific Heat Capacity and Thermal Conductivity are taken into account. The next phase consists of numerical computation procedure which is where the specimen is position 60° of inclination angle from the substrate plate. The support structure is to be generated within the lower surface of the specimen in order to avoid material to collapse during printing process. Laser heat source is to be modelled based on the laser beam width, power, efficiency and scanning speed in order for the numerical computation to accurately predict the thermal problem in SLM process. Furthermore, layer parameters used to fabricate the specimen such as hatch distance, hatch scan width and layer thickness are taken into TMM consideration. Similar set-up from numerical computation by means of laser and layer parameters to fabricate SS316L rectangular specimen is utilized in real fabrication process using SLM machine, Renishaw RenAM 500E. After fabrication of the specimen, electropolishing as for the sample preparation for X-Ray Diffraction (XRD) measurement are to be conducted by means of various depth on both sides of the specimen. For the validation procedure, residual stress on every depth is to be analysed and compared with the result from numerical computation. In conclusion, TMM simulation forecasting an acceptable residual stress on SLM product with relative error up to 14% and the computational time taken to predict the residual stress is only 56 minutes. This exploratory research using TMM simulation to predict residual stress on SLM product could benefit metal additive manufacturing (MAM) production as a whole by neglecting expensive trial and error approach.

Photoacoustic and photothermal (PT) methods in live cells studies allow to realize direct monitoring as they do not require cell pretreatment with chemicals and because the sources of output signal are cell chromophores. The origin of PT signals from single cells that are irradiated with short pump laser pulse was studied experimentally under different rates of cell metabolism. The cells in different metabolic status were found to produce PT response of specific shape that occurs in specific pump energy region for each pump wavelength. This type of PT response is associated with secondary phenomena that are induced in cells by thermal impact. An ability of live cell to produce such PT response was found to depend upon the redox state of cell respiratory chain (RC) components that absorb pump laser radiation. The results of PT studies of in vitro modulation of hepatocyte metabolic state are presented as function of RC state (reduced/oxidized) and laser parameters (wavelength and energy). Obtained results show that several parameters of laser-cell interaction depend upon RC redox state: shape and amplitude of PT response from single cell, energy threshold for laser damage to cell.