Numerical and Experimental Study on flexural Strength of Laser Microstructured Alumina

Transcription

1 doi: /acse Numerical and Experimental Study on flexural Strength of Laser Microstructured Alumina B. Adelmann *1, R. Hellmann 2 Applied laser and photonics group, University of Applied Sciences Aschaffenburg, WuerzburgerStrasse 45, Aschaffenburg, Germany *1 benedikt.adelmann@h-ab.de; 2 ralf.hellmann@h-ab.de Abstract We report on a comparative study between simulated and experimental determined flexural strength of alumina ceramic substrates that are micro-structured by pulsed laser. A 1 mj pulsed fiber laser is used to ablate 0.63 mm thick ceramic in a multipass process with the ablation depth increasing linearly with the number of passes. The ablation is further characterized by a surface roughness of 1.5 µm and an ablation rate of 24 mm³/h. Ceramic substrates are ablated with the ablation depth being increased stepwise by 0.1 mm up to complete puncture and the flexural strength is measured with a 3-point bending test. Using a finite element simulation, the bending test for the ceramics is modelled with the mechanical properties as being linear elastic. As a result, the simulated strength reveals excellent agreement with the measured strength. We therefore conclude that the behaviour of the flexural strength can be completely explained by the ablated geometry and laser process itself has no significant influence on the strength. In addition, it turns out that ablation in zones with compressive stress has only little influence on the strength while ablation in zones with tensile stress have significant influence, reducing the overall strength by about two third. From an application point of view, these results imply that ablation using near infrared, pulsed fiber laser is a feasible process for high quality micromachining of alumina ceramics with unaltered mechanical strength. From a fundamental view, these results indicate that simulating the mechanical strength of laser micro-structured alumina can be based solely on the material properties without including complex laser-material interactions. Keywords Alumina Ceramic; Laser Ablation; Fiber Laser; Flexural Strength; FEM Simulation Introduction Alumina ceramic is the most important material for electronic substrates, especially in hybrid circuits used for automotive, telecommunication and power electronics applications that require efficient thermal dissipation, high operation temperatures or low loss factors, which are not fulfilled by the widely used FR4 glass fiber reinforced plastic [1][2]. With respect to micromachining alumina for electrical, micromechanical and MEMS applications, laser processing has been developed as a valuable alternative technology, since the material exhibits a high hardness, brittleness and high chemical inertness, which in turn hampers the commonly employed mechanically and chemically machining [3][4]. For laser ablation of ceramics, usually infrared lasers are industrially employed, whereas the application of various laser types with emission wavelengths ranging from ultraviolet to infrared has been studied to improve quality and efficiency of the laser micromachining. These different laser sources lead to different surface modifications, such as altering surface roughness and chemical composition, which in turn may change the mechanical stability. Sciti et al. [5] used a 25 ns pulsed KrF UV excimer laser to ablate Al2O3, achieving a surface roughness between 0.8 µm and 1.0 µm at laser fluences between 1.8 J/cm² and 7.5 J/cm². Also with a KrF laser, Oliveira et al. [6] structured Al2O3-TiC (fluence between 2 J/cm² and 8 J/cm²), reporting an increase of the surface roughness with increasing pulse number. Preusch et al. [7] ablated Al2O3 and AlN with a 20 W pulsed fiber laser creating 3D structures with a surface roughness of 1.5 µm. Weichenhain et al. [8] employed a frequency tripled solid-state laser (355 nm) at fluences between 1.9 J/cm² and 16.2 J/cm² to process SiC and Si3N4 achieving for SiC a minimum surface 28

2 Advances in Ceramic Science and Engineering (ACSE) Volume 5, roughness of 0.4 µm and observing chemical decomposition in the irradiated area. Hirayama et al. [9] ablated AlN with CO2, KrF and Ti:Sapphire lasers, detecting a decomposition of AlN to aluminum and gaseous nitrogen. However, a detailed analysis of the influence of the laser process on the mechanical strength has yet not been reported. In contrast, laser cutting the influence of the laser process on the flexural strength has been investigated by several groups. Zhang et al. [10] cut 3 mm thick silicon nitride composite ceramic with a pulsed 120 W Nd:YAG laser and found a flexural strength reduction by 60 % after the laser cutting process. Black and Chua [11] cut 9.2 mm thick alumina with a 530 W CO2 laser, reporting significant problems with cracks adjacent to sharp corners. Cracks have also been observed by Hong et al. [12] upon laser cutting of silicon nitride with a 250 W Q-switched CO2 laser showing that less crack formation is achieved at higher velocities and multi pass processes. Adelmann et al. [13] determined a strength reduction between 3 % and 25 % depending on the laser parameter combination, when cutting 0.63 mm alumina using continuous wave fiber laser. By comparing experimental results with simulations, no additional damage by the laser process was found for cutting complex structures and corners [14]. According to calculations by Lee and Ahn[15], cracks in ceramics mainly depend on the laser cutting front angle while simulations from Yilbas et al. [16] attribute cracks mainly to the re-solidification of molten material in the cut kerf. As per particular given above, the influence of laser cutting on the ceramic strength has been intensively studied and proven. Quite the contrary, for laser ablation, this influence has not been examined in detail. In this paper, we, therefore, investigate the impact of laser ablation on the flexural strength of alumina ceramic by comparing experimental and simulated results. Experimental Laser System For the experiments, a pulsed 20 W fiber laser is used (YLP Series IPG Photonics), which is characterized by a pulse length of 100 ns, a beam quality of M² = 1.6 and an emission wavelength of 1064 nm. The repetition rate can be varied between 2 khz and 80 khz with a nominal average power of 20 W at 20 khz. The maximum pulse energy is 1 mj and the maximum fluence is 64 J/cm² at a 44 µm spot diameter, respectively. At fixed repetition rate, the output power can get adjusted between 10 % and 100 % of the nominal power. The laser beam is positioned over the specimen by a galvo scanner (RaylaseSuperScan) with a focal length of 163 mm. The scanner deflects the laser spot over a square of 110x110 mm² with a maximum positioning speed of larger than 7 m/s. Ceramic Material The substrate material used in the experiments is alumina with a thickness of 0.63 mm, one of the most commonly used thickness in electronic industry. The material has a purity of more than 96 % and a density of 3.5 g/cm³. To estimate the absorption of the material at the employed laser wavelength, we measured its reflectivity and transmission at room temperature with a spectrophotometer consisting of a broad band light source, an integrating sphere and an optical spectrometer (detailed description in [17]). The ration of the reflected and transmitted radiation of a 0.63 mm thick alumina substrate in the spectral range between 350 nm and 1240 nm is shown in figure 1 (the vertical line indicates the laser emission wavelength of 1070 nm). It is obvious that alumina has an almost constant reflection and transmission between 400 nm and 1240 nm (visible and near infrared region). In detail, the optical reflection at the fiber laser wavelength of 1070 nm is determined to 81 % which is comparable to 79 % reported by Liu et. al[18]. The room temperature transmission is determined to 15 % at 1070 nm. Applying these values, the absorption coefficient is calculated with the Beer Lambert law to 369 1/m. It is worthwhile to note that the reflected and transmitted radiation for these ceramic substrates is strongly scattered rather than directed, i.e. an integration sphere is indispensable for this characterization. In addition, please note that with respect to the high temperatures during the laser process, Zhang and Modest [19] measured that the absorption of alumina at 1064 nm significantly 29

3 increases to more than 90 % at temperatures exceeding its melting point. % Reflection Transmission FIG. 1 REFLECTION AND TRANSMISSION SPECTRA OF AL2O3 WITH 0.63 MM THICKNESS, THE VERTICAL BAR ILLUSTRATES THE LASER WAVELENGTH The studied specimen have dimensions of 22 mm x 3 mm, the ablated area are rectangular in shape with dimensions of 2 mm x 1mm being located in the middle of the specimen. The ablated depth is increased stepwise by 0.1 mm up to complete puncture by a multi-pass process. Flexural Strength Measurement F d b FIG. 2 SCHEMATIC ILLUSTRATION OF THE 3-POINT BENDING TEST To measure the flexural strength, a 3-point bending test machine is used with 20 mm distance between the two contact points. Figure 2 shows the schematic diagram of the 3-point bending test including the relevant dimensions. The ablated rectangular is located in the middle of the sample either on the top or on the bottom side of the specimen. The flexural strength is calculated with formula (1) from the maximum force F which the sample withstands before breaking. Here,l is the distance between the two contact points, b stands for the width of the sample and d expresses the sample thickness. The velocity of the bending test is set to 1 mm/min as recommended by Bengisu[2]. l (1) Simulation To evaluate whether the observed strength is caused by the laser process or the chosen geometry itself, we simulated the 3-point bending test using COMSOL Multiphysics Structural Mechanics Module. In a stationary model, the principal stress in the samples (length of 22 mm, width of 3 mm and a thickness of 0.63 mm) is simulated during bending. In the samples also the ablated volume with different ablation depth and the location being on top or on the bottom is included. Because alumina is a brittle ceramic which is not able to deform plastically, the samples are treated as a linear elastic material following Hooke s law (formula 2). In this formula, represents the mechanical stress, E the Young s modulus and the strain. (2) 30

4 Advances in Ceramic Science and Engineering (ACSE) Volume 5, For the bending test, two round bars with 20 mm distance are constructed under the sample (length of the bars is identical with the sample width). The samples are not fixed to these bars in order to allow them a horizontal movement. In the middle of the specimen on the entire width, a line force with totally 10 N is applied. The force s magnitude is chosen with respect to the typical measured breakup forces of the ceramic samples. The mechanical stress in a 3 dimensional material is described by the stress tensor according to formula 3containing the normal stresses in all spatial directions on the matrix main diagonal and the shear stresses in two spatial directions perpendicular to each normal stress (elements outside the diagonal). y z S ( y z y yz zy ) z (3) The eigenvaluesof the stress tensor calculated with formula 4 using the unit matrix I are the 3 principal stresses [20]. Geometrically, this effect can be explained by rotating a volume element of the stress tensor in order to eliminate the shear strength. det(s Hi I) 0 (4) Because of its brittleness alumina which breaks up at once when the maximum of the principal stresses exceeds the material strength [21]. Therefore, the maximum principal stress is calculated to locate the area in the sample which in practice would initiate the mechanical breakup. Results Ablation characteristics FIG. 3 ABLATION DEPTH AS A FUNCTION OF THE NUMBER OF PASSES Before strength measurement and simulation, the key ablation characteristics, i.e., ablation threshold, ablation depth, surface roughness and mass removal rate are determined. The ablation threshold for the employed laser wavelength is determined with the method of Liu [22], i.e. measuring the diameter of the ablated crater generated by a fixed number of laser pulses (here 1000 pulses) as a function of the fluencies. For the material under study, we find the ablation threshold to be 30 J/cm². To ablate a defined depth, a multi-pass process is chosen to adjust the depth by varying the number of passes. The parameter combination for a meander type scanning consists of a pulse repetition rate of 10 khz (pulse energy of 1 mj) and a scan velocity of 200 mm/s (pulse overlap of 56 %). As we have shown in a previous study, this parameter combination is used because this settings result in high contour accuracy, good surface quality and high ablation depth accuracy[7]. The ablation depth as a function of the number of passes is given in figure 3 revealing a linear dependence. With an ablation of 6 µm per pass, the ablation depth can be adjusted easily. With the afore mentioned laser parameters, an ablation of 30 µm depth is characterized by a roughness Ra of 1.50 µm and the mass removal rate is determined to 24 mm³/h. 31

5 Simulation results For the simulation, a series of samples is modeled with ablated rectangular (size 1x2 mm²) on the top and bottom side during bending. Starting with a non-ablated reference specimen, the ablation depth is stepwise increaseg in the simulation by 100 µm until the ceramic platelet is pierced reaching an ablation depth of 0.63 mm. During the simulation of the bending test, the principal stress with a load of 10 N for each ablation depth is calculated. FIG. 4 PRINCIPAL STRESS ON THE TOP SIDE (ABLATED ON THE TOP WITH 300 µm DEPTH) DURING BENDING TEST (LEFT) AND THE PRINCIPAL STRESS ON THE BOTTOM SIDE (ABLATED ON THE BOTTOM SIDE WITH 100 µm DEPTH) OF THE SAMPLE (RIGHT). RED AREAS INDICATE TENSILE STRESS AND BLUE AREAS INDICATE COMPRESSIVE STRESS. THE WIRE FRAME MODEL REPRESENTS THE GEOMETRY OF THE UNSTRESSED SAMPLE WITH NO DEFLECTION Figure 4 shows two exemplarily chosen surface stress distributions with positive values (red and orange regions) which indicates tensile stress while negative values (blue) indicating compressive stress. The left part of the figure depicts an ablation (300 µm depth) on the top side which is in the region of compressive stress. In the ablated region, no stress peaks are detected and the stress maximum is located on the bottom side of the sample (tensile stress). As the stress maximum is on the bottom side and ceramics typically have an about ten times higher compressive strength than tensile strength [23], the breakup should be initiated from the bottom side of the sample. As a consequence, the ablated rectangular on the top side has only minor influence and the flexural strength is expected to be unaltered by the ablation. When the ablated area is located on the bottom side of the sample during bending (right part of figure 4) its position coincides with an area of tensile stress. Even for small ablation depths around 10% of the entire substrate thickness, distinctive stress peaks arise at the corners of the ablated rectangular. As a result of these stress peaks, combined with a general trend to break at corners and a comparatively low tensile strength, ablation from the bottom of the substrate significantly influences the flexural strength. Strength Comparison To compare the simulated mechanical stress with the measured flexural strength, the simulation stress has to be converted into strength. This conversion is possible due to the linear elastic behavior of ceramic. A high simulated maximum stress at constant load means that only a low load can be applied before the maximum stress exceeds the critical stress of mechanical breakup. On the other hand, a low maximum stress at constant load results in a high possible load before the maximum stress reaches the critical stress of breakup [21]. Because alumina ceramic is linear elastic, the reciprocal of the maximum stress is a measure of the mechanical strength. To calculate a comparable value to the measured strength, the reciprocal of the maximum simulated stress is normalized to the strength of the reference sample without ablation. Figure 5 compares these simulated strengths with the measured strengths. The standard error included in the diagram results from the measurement of a series of six samples per ablated depth, revealing remarkable agreement between simulated and experimental results. Please note that due to the Weibull-type distribution of the flexural strength in ceramic material, the standard error of the strength typically increases with the absolute value of the strength, which is also seen in figure 5 for the strength of top and bottom ablated specimen. It is also worthwhile to mention that for the simulation of punctured samples (complete ablation), the top and bottom side ablation exhibit identical strengths. 32

6 Advances in Ceramic Science and Engineering (ACSE) Volume 5, Since the simulation is entirely based on the geometry, the remarkable agreement between simulated and measured results allows the conclusion that the strength reduction induced by ablation can be entirely explained by the generated geometry, i.e. the laser micromachining process and any potential damage induced by it, e.g. cracks, cause no supplementary strength reduction. FIG. 5 COMPARISON OF THE MEASURED AND SIMULATED FLEXURAL STRENGTH ABLATED ON THE TOP AND BOTTOM SIDE The trend of the flexural strength versus the ablation depths for top side and bottom side micromachining can be explained as follows. For an ablation on the top side, the slight decrease of the flexural strength with increasing ablation depth can be attributed to the reduction of the stiffness due to the missing volume and the associated increase of the deflection which in turn reduces the strength. When the ceramic substrate is punctured, the strength is strongly reduced by about two third of the unprocessed ceramic though only one third of the width is removed. Besides, the material is removed within the tensile stress region, this can be assigned to the generation of sensitive edges in the stressed region (stress peaks). The ablation on the bottom side is characterized by a significant reduction of the flexural strength as compared with the reference even for small ablation depths of 0.1 mm. This can be assigned to the stress peaks at the edges of the ablated region (see right part of figure 4) which reduce the strength. In addition, the removal of material in the tensile stress region increases the stress in adjacent areas. For ablation depths between 0.1 mm and 0.3 mm, the strength is further decreased because by removing material out of the tensile zone, the stiffness is further reduced. For a further increase of the ablation depth, the strength remains almost constant. Conclusion In this comparative study, we find an excellent agreement between simulated and experimentally determined flexural strength of multi-pass pulsed fiber laser ablated alumina substrates having a thickness of 0.63 mm. For the simulation, a finite element approach with a linear elastic model is employed, whereas the experimental strength measurements are performed using a three point bending-test. Based on the ascertained agreement, we conclude that the observed flexural strength reduction induced by ablation is solely governed by the ablated geometry and not driven by the laser process itself. In addition, our results imply that ablation in regions of compressive stress does not significantly alter the flexural strength, while ablation in regions of tensile stress reduces the flexural strength by about two third. ACKNOWLEDGEMENT We thank F. Preusch for technical support during the experiments. 33

7 REFERENCES [1] "Typial Properties of Technical Ceramics," Information Center Technical Ceramic, [Online]. Available: [2] M. Bengisu, Engineering Ceramics, Berlin: Springer, [3] A. Samant and N. Dahotre, "Laser machining of structural ceramics A review," vol. 29, pp , [4] K. Li and P. Sheng, "Plane Stress Model for Fracture of Caramics during Laser Cutting," International Jorunal of Machine Tools & Manufacture, vol. 35, no. 11, pp , [5] D. Sciti, C. Melandri and A. Bellosi, Excimer laser-induced microstructural changes of alumina and silicon carbide, Journal of Materials Science, vol. 35, no. 15, pp , [6] V. Oliveira, R. Vilar and O. Conde, Excimer laser ablation of Al2O3-TiC ceramics: laser induced modifications of surface topography and structure, Applied Surface Science, vol. 127, pp , [7] F. Preusch, B. Adelmann and R. Hellmann, Micromachining of AlN and Al2O3 Using Fiber Laser, Micromachines, vol. 5, pp , [8] R. Weichenhain, A. Horn and E. Kreutz, Three dimensional microfabrication in ceramics by solid state lasers, Applied Physics A, vol. 69, no. 1, pp , [9] Y. Hirayama, H. Yabe and M. Obara, Selective ablation of AlN ceramic using femtosecond, nanosecond, and microsecond pulsed laser, Journal of Applied Physics, vol. 89, no. 5, pp , [10] J. Zhang, T. Lee, X. Ai and W. Lau, Investigation of the surface integrity of laser-cut ceramic, Journal of Materials Processing Technology, vol. 57, pp , [11] I. Black and K. Chua, "Laser cutting of thick ceramic tile," Optics & Laser Technology, vol. 29, no. 4, pp , [12] L. Hong, L. Lic and C. Ju, "A study of laser cutting engineering ceramics," Optics & Laser Technology, vol. 31, pp , [13] B. Adelmann and R. Hellmann, Investigation on Flexural Strength Changes of Alumina Caused by Cutting using Fiber Laser, Journal of Laser Micro/Nanoengineering, vol. 9, no. 2, pp , [14] B. Adelmann and R. Hellmann, Comparative study of experimentally determined and simulated mechanical strength of fiber laser cut alumina substrates, in Proceedings of LAMP the 7th International Congress on Laser Advanced Materials Processing, [15] S. Lee S and S. Ahn, "A Probabilistic Model for Crack Formation in Laser Cutting of Ceramics," JSME International Journal Series, vol. 46, no. 3, pp , [16] B. Yilbas, S. Akhtar and C. Karatas, "Laser straight cutting of alumina tiles: thermal stress analysis," International Journal of Advanced Manufacturing Technology, vol. 58, pp , [17] B. Adelmann and R. Hellmann, Rapid micro hole laser drilling in ceramic substrates using single mode fiber laser, Journal of Materials Processing Technology, vol. 221, pp , [18] W. Liu, W. Du and J. Liao, Application of fiber laser used in the field of stent cutting and micro-machining, Lasers in Material Processing and Manufacturing II, vol. 5629, pp , [19] Z. Zhang and M. Modest, Temperature-Dependent Absorptances of Ceramics for {Nd:YAG} and {CO2} Laser Processing Applications, In Proceedings of the Materials Processing Cutting Symposium ICALEO 96, vol. 81C, pp , [20] V. Läpple, Introduction into strength of materials (german), Vieweg, [21] D. Gross and T. Seelig, Fracture Mechanics, F. Ling, Ed., Springer, [22] J. M. Liu, Simple technique for measurements of pulsed Gaussian-beam spot sizes, Optics Letters, vol. 7, no. 5, pp , May [23] H. Schaumburg, Ceramic (german: Keramik), Stuttgart: Teubner,