Hans Wilhelm Bergmann

Experimental investigations in laser microsoldering

Up to now, lasers have been well established in the field of materials processing for cutting, welding, and surface treatments. Recently, lasers in the medium power range have been of increasing interest in the production of electronic components. Higher integration density of electronic circuits demands improved mounting technology. Due to the reduced contact area of modern surface mounted packages, more sophisticated soldering systems are required to ensure product quality. Standard reflow soldering techniques may damage thermally sensitive devices, and mechanical tensions in the solder joints will occur, due to different thermal expansion coefficients. These problems can be avoided using a laser, as the amount of heat induced into the component is very small. Another advantage is the step-wise heat input resulting in a minimal overall thermal loading of the device and the possibility to control individually the heatflow for each solderjoint. In some applications lasers are the only reasonable tool, e.g., repairing printed circuit boards (PCB) produced in surface mounting technology or soldering of three dimensional PCBs. To improve quality and productivity of laser soldering tools, the time required for melting and wetting has to be minimized in the same manner as defective solder joints should be detected online. There are some commercial laser soldering systems available, using different types of process control, e.g., pyrometrical temperature measurement, detection of the reflected laser beam energy, or evaluating the sound emission while melting the solder with a pulsed laser. To obtain certain time- temperature curves, an analogous regulation of the beam power is required. Therefore a pyrometer offers the best approach to get optimal thermal input, even if the measurement is difficult due to complex geometry and unknown emissivity of the surface. This paper outlines the behavior of the solder paste under irradiation of different wavelengths and the possibility of controlling the solder process via the above mentioned setup.

XeCl-excimer-laser MOPA chain for shock hardening

An excimer laser in master oscillator power amplifier (MOPA) configuration (E equals 2.3 J, tau equals 25 - 50 ns, f equals 10 Hz) was used for shock treatments of metallic samples. The process was studied and optimized using physical investigations concerning the pressure development. The effect of the laser treatment on the microstructure of the material could be demonstrated by micrographs of irradiated samples and could be quantified by roughness and hardness measurements as well as by residual stress analysis.

Influence of processing speed on cut quality in laser beam gas cutting of copper

During the last few years technical innovations in the field of laser beam sources have led to almost a complete equalization of the process impeding properties of copper for laser beam cutting. The present work was performed to outline the potential of laser beam gas cutting of copper for generating quality cuts under practice-orientated conditions. With regard to the importance of cutting speed on both cut quality and economic efficiency of processing the focus of interest was put on the influence of cutting speed on application relevant features. For this purpose a series of speed varying cutting experiments on copper sheets of 0.5 to 15 mm thickness was carried out. The tests were arranged to two commercially available cw-CO2-laser beam cutting machines and maximum power ratings of 3 kW and 18 kW. Cutting speed exercises an influence on all features examined. The most susceptible features are deviation from right angle and height of dross. Deviation from right angle can be improved for 100% to several 100%. Dross height can be reduced for between 40 and 80%. On the other hand the features kerf width and surface roughness are less susceptible to cutting speed. For the examined range of cutting speeds and sheet thicknesses width of kerf varies between 5 and 15%, surface roughness between 10 and 30%. Sheets in the range of 1 to 10 mm thickness are possible to be cut with an accuracy better than 50 micrometers , sheets thicker than 10 mm with an accuracy of 150 to 200 micrometers . With inclination angles of 0.5 to 2 degree(s) kerfs can said to be parallel sided. For sheets of thicknesses below 1 mm roughness values of about 25 micrometers are achievable. Sheets of thicknesses of between 1 and 5 mm show roughnesses of approximately 50 micrometers . For sheets of thicknesses above 10 mm roughnesses in the region of 100 micrometers can be obtained.

Contribution to the beam plasma material interactions during material processing with TEA CO2 laser radiation

The TEA-CO2-laser (transversely excited atmospheric pressure) is a tool for the pulsed processing of materials with peak power densities up to 1010 W/cm2 and a FWHM of 70 ns. The interaction between the laser beam, the surface of the work piece and the surrounding atmosphere as well as gas pressure and the formation of an induced plasma influences the response of the target. It was found that depending on the power density and the atmosphere the response can take two forms. (1) No target modification due to optical break through of the atmosphere and therefore shielding of the target (air pressure above 10 mbar, depending on the material). (2) Processing of materials (air pressure below 10 mbar, depending on the material) with melting of metallic surfaces (power density above 0.5 109 W/cm2), hole formation (power density of 5 109 W/cm2) and shock hardening (power density of 3.5 1010 W/cm2). All those phenomena are usually linked with the occurrence of laser supported combustion waves and laser supported detonation waves, respectively for which the mechanism is still not completely understood. The present paper shows how short time photography and spatial and temporal resolved spectroscopy can be used to better understand the various processes that occur during laser beam interaction. The spectra of titanium and aluminum are observed and correlated with the modification of the target. If the power density is high enough and the gas pressure above a material and gas composition specific threshold, the plasma radiation shows only spectral lines of the background atmosphere. If the gas pressure is below this threshold, a modification of the target surface (melting, evaporation and solid state transformation) with TEA-CO2- laser pulses is possible and the material specific spectra is observed. In some cases spatial and temporal resolved spectroscopy of a plasma allows the calculation of electron temperatures by comparison of two spectral lines.

Process diagnostics and control for excimer laser processing

The present contribution focuses on fundamental investigations and possible methods for quality and process control mechanisms, especially for the removal of thin hard films and deformation layers from metallic substrates. Extended fundamental investigations including short time photography and plasma emission spectroscopy were carried out to characterize the plasma formation and propagation during excimer laser treatment. The influence of both the laser and process parameters (wavelength, energy density, number of pulses, ambient gas type and pressure) on the process and the plasma properties is determined. The investigated plasma emission spectra is strongly correlated to the surface modifications achieved. It will be outlined how these signals can be used for process control in excimer laser assisted processing.

Deposition of diamondlike carbon films using XeCl excimer laser

Diamond-like carbon (DLC) films were deposited using the laser-assisted physical vapor deposition (LPVD) method. This work is dedicated to an XeCl-excimer laser PVD procedure. The influences of the power density, working distance, and substrate temperature on the DLC-deposition are discussed. The propagation velocity of the laser ignited plasma was determined using short-time photography and time-of-flight (TOF) method.

Copper vapour laser MOPA chain for highly efficient and precise material removal

Copper vapor lasers in a MOPA-chain (MOPA, master-oscillator- power-amplifier) configuration with low divergence can be used for the high precision machining of metals and ceramics. The fundamental interaction phenomena, ablation process and possible industrial applications are presented. The following paper relates the results and experiences in the operation of a copper vapor laser MOPA chain, consisting of an oscillator and up to three amplifiers, with the triggering points for these lasers exactly variable through a master-timing-system. In principle, a low-divergent laser beam is generated (511 and 578 nm wavelengths) via an off-axis unstable resonator scheme, with precise synchronization of the amplifiers producing average powers of over 140 W. Due to the excellent beam focusability, peak power densities of some 1010W/cm2 are achievable in a 50 ns pulse duration, which provides almost material-independent precision machining at high velocities. Beginning from the principles of beam-target reciprocation, the removing and cutting of metallic as well as non-metallic materials with copper vapor lasers is described. Additionally, the potential of copper vapor lasers for industrial applications is illustrated through precision machining examples.

Industrial aspects of precision machining with copper vapor lasers

The applications of conventional infrared lasers running cw or quasi-sw for drilling, cutting and shaping are limited in the precision achievable due to the long interaction time which leads to heat affected zones. The necessity to use a gas jet to blow the molten material out of the cut kerf will damage fragile workpieces like thin foils. Short laser pulses of sufficient intensity remove the material directly by evaporation and minimize the amount of heat transferred into the solid. Classical infrared laser sources generate a shielding air plasma within some ns at power densities above some 107W/cm2. The optical breakdown threshold value in air can be shifted to higher intensities by using visible light as well as reducing the focal diameter. An alternative way is to shorten the pulse duration to less than 10 ps that a plasma is generated only after the pulse. Thus, the material removal process begins after the deposition of the pulse energy into the material. But such short pulses will generate a pressure wave due to the sudden thermal expansion and can damage or destroy microscopic components. For industrial production the productivity is a further aspect. Hence, a certain mean power is required in order to obtain the desired production rate. Considering the above aspects, copper vapor lasers (CVLs) with ns pulse duration are well suited for precision machining of metals and ceramics. Processing with CVLs is an advantage in that its wavelength is highly absorbed by metallic targets and the probability for the optical breakdown in air is low. CVLs in an oscillator-amplifier-setup incorporate diffraction limited beam quality and high average power. The present paper outlines the potential of the CVL for the industrial use regarding high processing speed and precision. Under these aspects the limiting mechanisms on the material removal process and the necessary processing strategies for scaling up the productivity are shown. The relevant laser parameters for increasing the working speed and the relationship to the achievable precision are given. The design aspects of a copper vapor laser system with high mean output power and repetition rate are outlined. To conclude, several typical machining tasks, e.g. cutting of green foils, drilling of scimmer holes for thermal analysis are presented.

Excimer-laser-assisted deposition of diamondlike carbon hard coatings

Diamond-like carbon (DLC) films were deposited using the excimer laser assisted physical vapor deposition at room temperature. The films deposited at high vacuum (10-5 mbar) revealed more diamond-like character than under other atmospheres of argon and hydrogen. DLC- films can be deposited with a thickness more than 1 micrometers with the help of either an additional Ti-buffer layer or an in-situ laser treatment during the deposition. The adhesion of the films was qualitatively determined by using the indentation and bending tests. Additionally, the adhesion was found to be dependent on the power densities for the target ablation (IT) and for the in-situ laser treatment (IS), as well as, on the applied buffer layer. The roughness was found to be proportional to the film thickness at various surface morphologies of the substrate. The friction coefficient of DLC-films against steel (100Cr6) was found to be approximately 0.1 and the wear loss of the films was dependent on the properties of substrate material.

Micromachining with copper vapor lasers

When high quality laser beam are combined with suitable imaging optics and manipulation systems, laser micro machining offers excellent solutions to industrial needs. In this context the presented work was performed to demonstrate the potential of the copper vapor laser for upcoming applications. Therefore, a systematic series of laser beam cutting, drilling and milling experiments were carried out using copper sheets of thickness between 0.01 mm and 0.250 mm. In a first step zero-dimensional, 1D and 2D structures were generated with depth/width ratios varying from 1:100 to 10:1 using different processing strategies. The results have been characterized in terms of the minimal geometrical deviations and processing speeds achievable.