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Influence of glass substrate thickness in laser scribing of glass
Precision Engineering 34 (2010) 55–61
Contents lists available at ScienceDirect
Precision Engineering journal homepage: www.elsevier.com/locate/precision
Influence of glass substrate thickness in laser scribing of glass Koji Yamamoto a,∗ , Noboru Hasaka b , Hideki Morita b , Etsuji Ohmura a a b
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, Japan Mitsuboshi Diamond Industrial Co., Ltd., 1-4-37 Minami-kaneden, Suita, Osaka, Japan
a r t i c l e
i n f o
Article history: Received 18 August 2008 Received in revised form 20 February 2009 Accepted 11 March 2009 Available online 27 June 2009 Keywords: CO2 laser Glass Laser scribing Thermal stress Thickness
a b s t r a c t The thickness of the glass substrate used in liquid crystal displays continues to be decreased from its original thickness of 1.1 mm for the purpose reducing size and weight. The aim of this study was to clarify the influence of the glass substrate thickness during laser scribing with crack propagation caused by laser heating followed by quick quenching. The laser scribe conditions for soda-lime glass substrates with thickness equal to or less than 1.1 mm were obtained in laser irradiation experiments. Two-dimensional thermal elasticity analysis was conducted with a finite element method based on the scribable conditions obtained in the experiment. The laser scribable conditions can then be estimated by the upper limit of the maximum surface temperature, Tmax , and the lower limit of the maximum tensile stress, tmax , in the cooling area, regardless of the glass substrate thickness. There is a substrate thickness with which the maximum tensile stress tmax becomes the largest under each scribe condition. The substrate thickness with which tmax becomes the largest is obtained at a faster scribe velocity for thinner glass substrate and at slower scribe velocity for thicker glass substrate. Owing to these relations, the crack depth also has almost the same tendency as tmax . © 2009 Elsevier Inc. All rights reserved.
1. Introduction Today the glass substrate thickness of liquid crystal displays (LCDs) used in laptop computers and flat-screen televisions has been reduced to 0.6 or 0.7 mm from the original 1.1 mm. The glass substrate thickness of small LCDs for use in devices such as mobile phones is becoming even thinner, down to 0.4 mm or less, from the desire for smaller size and less weight. There is concern that such very small LCD glass substrates are more susceptible to breakage by shocks and drops as the glass substrate becomes thinner. A scribe method exists as a way of separating glass by quenching after CO2 laser irradiation that advances a fissure in the quenching area, as in Fig. 1(a). This method is called laser scribing. The glass edge strength increases with laser scribing [1,2] compared with the use of a mechanical glass separation method [3–5]. The laser scribing is therefore an effective method for separating thin glass. The authors [6,7] conducted thermal elasticity analysis with a finite element method based on a laser scribe experiment using soda-lime glass with thickness of 0.7 mm, and proposed the following laser scribe mechanism. The glass surface is heated by laser irradiation and the heat is transferred from the surface to the interior of the glass (Fig. 1(b)). The surface is quenched by a water jet immediately after the laser heat (Fig. 1(c)). In this way, tensile stress
∗ Corresponding author. Tel.: +81 6 6378 3843; fax: +81 6 6378 3550. E-mail address: [email protected] (K. Yamamoto). 0141-6359/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.precisioneng.2009.03.007
is generated in the surface layer and a crack proceeds. A high temperature region remains inside as only the surface layer is quenched. This becomes a compressive stress field and aids the generation of tensile stress in the surface layer (Fig. 1(c) and (d)). In other words, tensile stress is generated in the surface layer from the temperature distribution developed in the quenching region, and the laser scribe crack proceeds. Inferring from this laser scribe mechanism, we reasoned that the glass substrate thickness would influence the laser scribe characteristics as it becomes thinner. In this study, considering today’s thinning LCD glass substrates, the laser scribable condition was obtained in a laser irradiation experiment using glass with thickness equal to or less than 1.1 mm. Next, two-dimensional thermal elasticity analysis was conducted with a finite element method based on the experimental results in order to clarify the influence of the glass thickness during laser scribing. 2. Experimental Method and results 2.1. Experimental method The same apparatus as in a previous study [6,7] was used. The glass substrate was fixed on vacuum suction stage. An initial crack was notched at the glass edge with a glass cutting wheel as the origin point for scribing, and a CO2 laser formed a beam that made an elliptic shape on the glass surface. The laser beam was directed onto the glass surface to heat the planned scribe line and a relative
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K. Yamamoto et al. / Precision Engineering 34 (2010) 55–61 Table 1 Typical conditions for experiment. Glass size Glass substrate thickness, h Scribe velocity, v Laser power, P Minor axis of heating area, 2x0 Major axis of heating area, 2y0 Cooling point distance, d Minor axis of cooling area, 2xc Major axis of cooling area, 2yc
300 mm × 400 mm 0.4, 0.55, 1.1 mm 200 mm/s 58.7 W 2.1 mm 22.0 mm 10 mm 2.0 mm 3.0 mm
ical values are shown for the scribe velocity and the laser power. The size of the heating area is shown using 1/e2 on the major axis and 1/e2 on the minor axis, and the size of the cooling area is indicated by the diameter determined by the water jet nozzle diameter, the dispersing angle and the distance between the nozzle and the glass surface. The beam shape, the cooling condition and the cooling point distance d were fixed, even as the glass substrate thickness was changed. Under these laser irradiation conditions, a 300 mm side of the glass substrate was scribed. Crack depth is unstable at the glass edge compared to the central area, and it was judged as scribable when the crack proceeded in a range from 50 to 250 mm and as unscribable when the crack progress was arrested in that range. Since glass edge strength decreases if thermal damage [8] remains in the glass substrate, also in such a case, it was judged as unscribable. After scribing, the glass substrate was separated manually and the crack depth Dc was measured with an optical microscope. 2.2. Experimental results
Fig. 1. Schematic of laser scribe mechanism. (a) Top view: temperature distribution on glass surface. (b) Section view (at A–A): compressive stress induced by heat. (c) Section view (at B–B): tensile stress over compressive stress field. (d) Stress distribution along the central axis at B–B.
velocity was produced between the laser beam and the glass substrate. The distal end of the laser beam was quenched by a water jet. With this method, a median crack was formed and moved in the scribe direction, and a laser scribe line was produced. Fig. 2 shows the positional relationship between the area heated by the laser beam and the cooling area quenched by the water jet, and the respective distances. Soda-lime glass with thicknesses of 0.4, 0.55, and 1.1 mm and dimensions of 300 mm × 400 mm were used for specimens. Table 1 shows the laser scribe conditions. Typ-
Fig. 2. Definitions and variables of geometry used for heating area, cooling area and respective distances.
Scribable velocity with respect to laser power was obtained as the laser scribable condition for glass substrates with thicknesses of 0.4, 0.55, and 1.1 mm. The results and crack depth Dc in each condition are shown in Fig. 3. For the glass thickness 0.7 mm, the experimental results from previous study [6,7] were shown again. For the glass thickness of 0.4 mm in Fig. 3(d) (i), the specimens were scribable at maximum scan velocity 500 mm/s of this experimental equipment. In Fig. 3(a)–(d) (i), the “×” marks on the higher velocity side show the condition in which the laser scribe crack progress was arrested, and the “×” marks on the lower velocity side show the condition with residual thermal damage in the glass substrate. For the crack depth Dc related to each laser power in Fig. 3(a)–(d) (ii), the deeper Dc corresponds to lower velocity and the shallower Dc corresponds to higher velocity. For each glass thickness, scribable velocity tends to become higher as the laser power increases. Moreover, the scribe conditions that generated the thermal damage (“×” marks in lower velocity side) were almost the same regardless of the glass thickness. Under the conditions in which crack progress was arrested (higher side “×” marks), the scribable velocity range with a glass thickness of 1.1 mm narrows at the side with higher laser power, whereas the scribable velocity range widens at the lower power side, and scribing becomes impossible at scribe velocities equal to or more than v = 350 mm/s. On the other hand, the scribable velocity range narrows at the lower power side with a glass thickness of 0.4 mm, and scribing becomes impossible at every scribe velocity with laser power of P = 30.4 W. On the higher power side, the scribable velocity range widens and scribing is possible even at a scribe velocity of v = 500 mm/s. Ultimately, scribing tends to become difficult at higher velocity as the glass thickness increases, and at lower velocity as the glass thickness decreases. The crack depth Dc in the five scribe conditions that did not generate thermal damage, and the results were arranged with respect to the glass thickness, are shown in Fig. 4. The “×” marks show the
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Fig. 3. Domain of laser scribable conditions and crack depth for glass substrate with thicknesses of 0.4, 0.55, 0.7, and 1.1 mm. (a) Plots of scribe conditions for glass substrate with thickness of 1.1 mm. (b) Plots of scribe conditions for glass substrate with thickness of 0.7 mm [6,7]. (c) Plots of scribe conditions for glass substrate with thickness of 0.55 mm. (d) Plots of scribe conditions for glass substrate with thickness of 0.4 mm. (i) Scribable velocity versus laser power; (ii) crack depth versus laser power. (i) “×” marks at higher velocity represent conditions in which crack progress was arrested and “×” marks at lower velocity represent conditions in which glass surface was damaged by laser heating. (ii) Deeper crack depth corresponds to the lower velocity condition.
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Fig. 4. Relation between crack depth and scribe condition for glass substrates with thicknesses 0.4, 0.55, 0.7, and 1.1 mm.
Fig. 5. Mesh geometry for FEM analysis.
condition in which the crack progress was arrested. Under the conditions of laser power P = 71.0 W and scribe velocity v = 320 mm/s, scribing is impossible with a glass thickness of 1.1 mm, and under the conditions of P = 30.4 W and v = 80 mm/s, scribing is impossible with a glass thickness of 0.4 mm. The crack depth Dc decreases individually regardless the glass substrate thickness as scribe velocity decreases. 3. Thermal stress analysis Based on the scribe conditions obtained in the experiment, the following two-dimensional thermal elasticity analysis was conducted with a finite element method. The laser scribable condition and the influence of glass substrate thickness in laser scribing are discussed in next chapter. The x–y coordinates were obtained on the laser irradiation surface, with scribe direction on the y-axis and glass thickness direction on the z-axis. Fig. 5 shows the element division using FEM analysis with a thickness of 0.4 mm as an example. Considering symmetry, the target for analysis was focused in the range of 0.4 mm × 30 mm. The minimum division value in the beam width direction (x-axis direction) was set as 3.7 m, and the minimum value of division in the thickness direction (z-axis direction) was Table 2 Physical properties of soda-lime glass. Density [9] Specific heat [9] Thermal conductivity [9] Thermal expansion coefficient [10] Young’s modulus [11] Poisson’s ratio [11] Softening temperature [11] Bending fracture strength [11]
2520 kg/m3 800 J/(kg K) 1.03 W/(m K) 8.7 × 10−6 K−1 71.6 GPa 0.23 720–730 ◦ C 49 MPa
Fig. 6. Analysis results of maximum surface temperature Tmax and maximum tensile stress tmax for each glass substrate with thickness of 0.4, 0.55, 0.7, or 1.1 mm. “×” marks at higher velocity correspond the conditions for arrest of laser scribe crack progress and “×” marks at lower velocity correspond the thermal damage residual conditions where the glass surface was damaged by laser heating. (a)–(d) correspond to Fig. 3(a)–(d) (i).
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Fig. 7. Maximum surface temperature Tmax and maximum tensile stress tmax for each glass substrate with thickness from 0.4 to 1.1 mm.
2.7 m. The total number of nodes was 622 and the total number of elements was 668. The values in Table 2 were used as physical properties [9–11] of each specimen. The time step was 0.25 mm/v[s], which is the time when 0.25 mm was divided by scribe velocity v. The values shown in Table 1 were used for the size of the heating area and the cooling area, and these intensities were assumed to have Gaussian distributions. In two-dimensional heat conduction analysis on the x–z plane, the center of the laser beam was scanned from the position of y = −15 mm in the y-axis direction (farther to the front on the paper), and both the heating and the cooling conditions were changed with time. Since the optical attenuation rate was calculated to be 0.9793%, the heat absorption was set as 0.798P [W] considering that the measured reflection ratio was 18.5%. The heat transfer coefficient1 ˛0 at the collision point was calculated [12] from the flow rate of the water jet (0.8 ml/min in this case). The water temperature was controlled at 20 ◦ C. ˛0 was constant at 100 076 W/(m2 K) since the water jet flow was unchanged, even though laser power and scribe velocity were changed in the experiment. Next, two-dimensional thermal stress analysis was conducted on the x–z plane for issue of plane stress, which was assumed to be yy = yx = yz = 0 using the obtained temperature distribution. The laser irradiation side-end of the FEM analysis model was restrained in the x-axis direction, and the other side-end in x-axis and z-axis direction. As in a previous study [6,7], maximum temperature on the glass substrate surface during the laser scribing was denoted as Tmax , and maximum tensile stress of xx on the cooling area was denoted as tmax . 4. Discussion of experimental results 4.1. Laser scribable condition Previous study [6,7] showed that the scribable condition for 0.7 mm soda-lime glass can be estimated from the maximum surface temperature Tmax on the heating area and the maximum tensile stress tmax in the cooling area after heating. Similarly, thermal elasticity analysis was conducted in this study under the experimental conditions (Fig. 3(a)–(d) (i)) of laser power and scribe velocity with which glass thicknesses of 0.4, 0.55, and 1.1 mm were scribable. The results are shown in Fig. 6. Fig. 6(b) shows again the analysis results for glass thickness of 0.7 mm to compare the scribable conditions
1 Amendment: ˛0 = heat transfer coefficient of a collision point, ˛0 = / the average heat transfer coefficient of the cooling area, and ˛0 = 100 076 W/m2 K, / 10 076 W/m2 K in Ref. [7]. ˛0 =
Fig. 8. Temperature distributions on x–z plane and deformation at the time when tmax is generated for each glass substrate with thickness of (i) 1.1 mm, (ii) 0.7 mm, (iii) 0.55 mm, or (iv) 0.4 mm. (a) P = 71.0 W, v = 320 mm/s; (b) P = 52.8 W, v = 160 mm/s; (c) P = 30.4 W, v = 80 mm/s.
with different glass thicknesses. The upper side of the plots in these figures shows the maximum surface temperature Tmax (right vertical axis), and lower side of the plots shows the maximum tensile stress tmax (left vertical axis). The “×” marks on the higher velocity side correspond to the conditions in which the laser scribe crack progress was arrested, and the “×” marks on lower velocity side correspond to the conditions determined to be unscribable as a result of the thermal damage generation in Fig. 3(a)–(d) (i). The upper limit of Tmax was nearly stable at every laser power and independent of scribe velocity, and Tmax increases as scribe velocity decreases when laser power is constant for the glass thicknesses of 0.4, 0.55, and 1.1 mm. Therefore, the thermal damage would not affect the glass at or below this upper limit of Tmax . On the other hand, tmax decreases as scribe velocity increases when laser power is constant. This is the reason that the crack depth becomes shallower on the higher velocity side in Fig. 3(a)–(d) (ii). At every laser power, the lower limit of tmax is nearly stable and independent of scribe velocity. Therefore, the laser scribe crack would progress when it is equal to or above the lower limit of this tmax . Thus, the upper limit of Tmax and the lower limit of tmax also exist respectively for glass thicknesses of 0.4, 0.55, and 1.1 mm. Fig. 7 summarizes the upper limit of maximum surface temperature Tmax and the lower limit of maximum tensile stress tmax of the glass thicknesses of 0.4, 0.55, 0.7, and 1.1 mm. The upper limit of Tmax was nearly stable at approximately 500 ◦ C for every glass thickness. The lower limit of tmax was also nearly stable, at a value of approximately 65 MPa. Thus, the laser scribable condition that is obtainable experimentally can be predicted from the upper limit of maximum surface temperature Tmax and the lower limit of maximum tensile stress tmax , by conducting thermal stress analysis regardless of the glass thickness when the glass thickness is 0.4–1.1 mm.
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face side. Next, Fig. 9 shows the stress distribution corresponding to Fig. 8. As in Fig. 1(d), tensile stress is generated on the surface layer and the high temperature region inside the glass becomes a compressive field. Fig. 10 summarizes tmax the five conditions corresponding to Fig. 4 with regard to the glass thickness. Under each condition, tmax becomes convex on the top, and the glass thickness that maximizes tmax tends to shift to the thinner side as scribe velocity increases, and tends to shift to the thicker side as velocity decreases. By this tendency, under the condition of P = 71.0 W and v = 320 mm/s, the glass thickness at which tmax becomes maximum is near 0.4 mm. Consequently tmax with the glass thickness 1.1 mm is smaller than the lower limit of tmax . Because of this, scribing is not possible with glass thickness of 1.1 mm. Under the condition of P= 30.4 W and v = 80 mm/s, the glass thickness when tmax is max-
Fig. 9. Stress distributions on x–z plane and deformation at the time when tmax is generated for each glass substrate with thickness of (i) 1.1 mm, (ii) 0.7 mm, (iii) 0.55 mm, or (iv) 0.4 mm. (a) P = 71.0 W, v = 320 mm/s; (b) P = 52.8 W, v = 160 mm/s; (c) P = 30.4 W, v = 80 mm/s.
4.2. Influence of glass thickness during laser scribing Fig. 8 shows the temperature distribution and the substrate distortion state on the x–z plane at the time when the maximum tensile stress tmax is generated. Three conditions (P = 30.4 W, v = 80 mm/s; P = 52.8 W, v = 160 mm/s; P = 71.0 W, v = 320 mm/s) were selected from the five conditions in Fig. 4. In the temperature distribution on the x–z plane, the high temperature region inside the glass is just below the quenching region, as in Fig. 1(c). The heat is transferred deep inside as the scribe velocity decreases. Under the condition of P = 30.4 W and v = 80 mm/s for the glass thickness of 0.4 mm, the back side heats as high as 200 ◦ C. At this time, hollows form in the back side of the substrate. Convex distortions also form on the sur-
Fig. 10. Maximum tensile stress tmax of each laser scribe condition for each glass substrate with thickness of 0.4, 0.55, 0.7, or 1.1 mm.
Fig. 11. Stress distributions of xx along the z-axis at the time when tmax is generated for each glass substrate with thickness of 0.4, 0.55, 0.7, or 1.1 mm. (a) P = 71.0 W, v = 320 mm/s; (b) P = 52.8 W, v = 160 mm/s; (c) P = 30.4 W, v = 80 mm/s.
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imum is 1.1 mm of more. As a result, tmax with glass thickness of 0.4 mm becomes smaller than the lower limit of tmax , and scribing is not possible. The tendency of maximum tensile stress tmax with respect to glass thickness in Fig. 10 roughly matches the tendency of crack depth Dc with respect to glass thickness in Fig. 4. However, in Fig. 10, tmax is smaller under the conditions of P = 30.4 W, v = 80 mm/s and P = 39.5 W, v = 120 mm/s than it is under P = 52.8 W, v = 160 mm/s. Despite this, crack depth Dc is deeper in Fig. 4. This means that tmax alone cannot explain the crack depth Dc in the experimental results. In previous studies [6,7], it was demonstrated that the crack was likely to propagated when the inner compressive stress field is fairly small and appropriately deep, whereas when a large inner compressive stress field exists in a comparatively shallow position crack progress is inhibited and the crack bends. Based on this, the relations between the scribe condition, the inner compressive stress field, and the field’s depth was investigated. The value of z divided by h is set as relative depth z/h. The relation between the relative depth and stress xx is shown in Fig. 11. The inner compressive stress decreases with each glass thickness as scribe velocity slows from Fig. 11(a)–(c). The depth of the inner compressive stress field also tends to become deeper. Therefore, it can be considered that the crack depth Dc under the conditions of P = 30.4 W, v = 80 mm/s and P = 39.5 W, v = 120 mm/s became deeper even though tmax is smaller than under conditions of P = 52.8 W, v = 160 mm/s. In Fig. 11, the relative depth of the inner compressive stress field in the glass with thickness 0.4 mm under conditions of P= 30.4 W and v= 80 mm/s was the deepest of all the conditions. The relative depth z/h is approximately 0.4. This is one of the causes of the concaves in the surface in Figs. 8 (c) (iv) and 9 (c) (iv). 5. Conclusion Two-dimensional thermal elasticity analysis with the finite element method was conducted under the laser scribable condition for soda-lime glass substrate with thicknesses of 0.4, 0.55, and 1.1 mm. The conclusions from this study are as follows.
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The upper limit of the maximum surface temperature Tmax and the lower limit of the maximum tensile stress tmax in the cooling area are nearly stable and independent of the laser power and scribe velocity for the glass thicknesses of 0.4, 0.55, and 1.1 mm. This is the same as for the glass thickness of 0.7 mm in a previous study [6,7]. Those values are also independent of the glass thickness. Therefore, if thermal elasticity analysis is conducted, the laser scribable condition can be predicted from the combination of the upper limit value of the maximum surface temperature Tmax and the lower limit value of the maximum tensile stress tmax , regardless of glass thickness for glass thicknesses from 0.4 to 1.1 mm. Under each scribe condition, there exists a glass thickness that maximizes the maximum surface tensile stress tmax . This glass thickness that maximizes tmax tends to become thinner as scribe velocity increases and to become thicker as scribe velocity decreases. As a result, crack depth has nearly the same tendency as tmax . References [1] Miyake Y. Separation technology for FPD glass. J Jpn Soc Abrasive Tech 2001;45(7):342–7 [in Japanese]. [2] Hermanns C. Laser separation of flat glass. In: Proceedings 63rd Laser Mater. Process Conf. 2005. p. 105–10. [3] Swain MV. Median crack initiation and propagation beneath a disc glass cutter. Glass Tech 1981;22(5):222–9. [4] Ono T, Teng O, Pai G. Breakless cutting EagleXGTM using standard scoring wheel. In: Proceedings of the 14th international display workshops, FMC2-2. 2007. [5] Wang S-C, Yeh L-Y, Lin C-C, Chen MS, Gan FY. Glass-strength dependence of cutting conditions in thin laminated TFT-LCD. In: Proceedings of the 14th international display workshops, FMCp-1. 2007. p. 66. [6] Yamamoto K, Hasaka N, Morita H, Ohmura E. Thermal stress analysis on laser scribing of glass. J Laser Appl 2008;20(4):193–200. [7] Yamamoto K, Hasaka N, Morita H, Ohmura E. Three-dimensional thermal stress analysis on laser scribing of glass. Precision Eng 2008;32:301–8. [8] Rolo A, Coelho J, Pires M. Marking glass with continuous and pulsed CO2 laser radiation. In: Proceedings of the ICALEO’05, P506. 2005. [9] JSME Data Book, Heat transfer, 4th ed. 1984 [in Japanese]. [10] Shand EB. Glass engineering handbook. 2nd ed. McGraw-Hill; 1958. [11] Watanabe N. Glass engineering handbook. Asakura Shoten; 1999 [in Japanese]. [12] Yamamoto A. On the heat transfer properties of cutting fluids (Part 2). J Jpn Soc Precision Eng 1960;26:17–25 [in Japanese].
Contents lists available at ScienceDirect
Precision Engineering journal homepage: www.elsevier.com/locate/precision
Influence of glass substrate thickness in laser scribing of glass Koji Yamamoto a,∗ , Noboru Hasaka b , Hideki Morita b , Etsuji Ohmura a a b
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, Japan Mitsuboshi Diamond Industrial Co., Ltd., 1-4-37 Minami-kaneden, Suita, Osaka, Japan
a r t i c l e
i n f o
Article history: Received 18 August 2008 Received in revised form 20 February 2009 Accepted 11 March 2009 Available online 27 June 2009 Keywords: CO2 laser Glass Laser scribing Thermal stress Thickness
a b s t r a c t The thickness of the glass substrate used in liquid crystal displays continues to be decreased from its original thickness of 1.1 mm for the purpose reducing size and weight. The aim of this study was to clarify the influence of the glass substrate thickness during laser scribing with crack propagation caused by laser heating followed by quick quenching. The laser scribe conditions for soda-lime glass substrates with thickness equal to or less than 1.1 mm were obtained in laser irradiation experiments. Two-dimensional thermal elasticity analysis was conducted with a finite element method based on the scribable conditions obtained in the experiment. The laser scribable conditions can then be estimated by the upper limit of the maximum surface temperature, Tmax , and the lower limit of the maximum tensile stress, tmax , in the cooling area, regardless of the glass substrate thickness. There is a substrate thickness with which the maximum tensile stress tmax becomes the largest under each scribe condition. The substrate thickness with which tmax becomes the largest is obtained at a faster scribe velocity for thinner glass substrate and at slower scribe velocity for thicker glass substrate. Owing to these relations, the crack depth also has almost the same tendency as tmax . © 2009 Elsevier Inc. All rights reserved.
1. Introduction Today the glass substrate thickness of liquid crystal displays (LCDs) used in laptop computers and flat-screen televisions has been reduced to 0.6 or 0.7 mm from the original 1.1 mm. The glass substrate thickness of small LCDs for use in devices such as mobile phones is becoming even thinner, down to 0.4 mm or less, from the desire for smaller size and less weight. There is concern that such very small LCD glass substrates are more susceptible to breakage by shocks and drops as the glass substrate becomes thinner. A scribe method exists as a way of separating glass by quenching after CO2 laser irradiation that advances a fissure in the quenching area, as in Fig. 1(a). This method is called laser scribing. The glass edge strength increases with laser scribing [1,2] compared with the use of a mechanical glass separation method [3–5]. The laser scribing is therefore an effective method for separating thin glass. The authors [6,7] conducted thermal elasticity analysis with a finite element method based on a laser scribe experiment using soda-lime glass with thickness of 0.7 mm, and proposed the following laser scribe mechanism. The glass surface is heated by laser irradiation and the heat is transferred from the surface to the interior of the glass (Fig. 1(b)). The surface is quenched by a water jet immediately after the laser heat (Fig. 1(c)). In this way, tensile stress
∗ Corresponding author. Tel.: +81 6 6378 3843; fax: +81 6 6378 3550. E-mail address: [email protected] (K. Yamamoto). 0141-6359/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.precisioneng.2009.03.007
is generated in the surface layer and a crack proceeds. A high temperature region remains inside as only the surface layer is quenched. This becomes a compressive stress field and aids the generation of tensile stress in the surface layer (Fig. 1(c) and (d)). In other words, tensile stress is generated in the surface layer from the temperature distribution developed in the quenching region, and the laser scribe crack proceeds. Inferring from this laser scribe mechanism, we reasoned that the glass substrate thickness would influence the laser scribe characteristics as it becomes thinner. In this study, considering today’s thinning LCD glass substrates, the laser scribable condition was obtained in a laser irradiation experiment using glass with thickness equal to or less than 1.1 mm. Next, two-dimensional thermal elasticity analysis was conducted with a finite element method based on the experimental results in order to clarify the influence of the glass thickness during laser scribing. 2. Experimental Method and results 2.1. Experimental method The same apparatus as in a previous study [6,7] was used. The glass substrate was fixed on vacuum suction stage. An initial crack was notched at the glass edge with a glass cutting wheel as the origin point for scribing, and a CO2 laser formed a beam that made an elliptic shape on the glass surface. The laser beam was directed onto the glass surface to heat the planned scribe line and a relative
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K. Yamamoto et al. / Precision Engineering 34 (2010) 55–61 Table 1 Typical conditions for experiment. Glass size Glass substrate thickness, h Scribe velocity, v Laser power, P Minor axis of heating area, 2x0 Major axis of heating area, 2y0 Cooling point distance, d Minor axis of cooling area, 2xc Major axis of cooling area, 2yc
300 mm × 400 mm 0.4, 0.55, 1.1 mm 200 mm/s 58.7 W 2.1 mm 22.0 mm 10 mm 2.0 mm 3.0 mm
ical values are shown for the scribe velocity and the laser power. The size of the heating area is shown using 1/e2 on the major axis and 1/e2 on the minor axis, and the size of the cooling area is indicated by the diameter determined by the water jet nozzle diameter, the dispersing angle and the distance between the nozzle and the glass surface. The beam shape, the cooling condition and the cooling point distance d were fixed, even as the glass substrate thickness was changed. Under these laser irradiation conditions, a 300 mm side of the glass substrate was scribed. Crack depth is unstable at the glass edge compared to the central area, and it was judged as scribable when the crack proceeded in a range from 50 to 250 mm and as unscribable when the crack progress was arrested in that range. Since glass edge strength decreases if thermal damage [8] remains in the glass substrate, also in such a case, it was judged as unscribable. After scribing, the glass substrate was separated manually and the crack depth Dc was measured with an optical microscope. 2.2. Experimental results
Fig. 1. Schematic of laser scribe mechanism. (a) Top view: temperature distribution on glass surface. (b) Section view (at A–A): compressive stress induced by heat. (c) Section view (at B–B): tensile stress over compressive stress field. (d) Stress distribution along the central axis at B–B.
velocity was produced between the laser beam and the glass substrate. The distal end of the laser beam was quenched by a water jet. With this method, a median crack was formed and moved in the scribe direction, and a laser scribe line was produced. Fig. 2 shows the positional relationship between the area heated by the laser beam and the cooling area quenched by the water jet, and the respective distances. Soda-lime glass with thicknesses of 0.4, 0.55, and 1.1 mm and dimensions of 300 mm × 400 mm were used for specimens. Table 1 shows the laser scribe conditions. Typ-
Fig. 2. Definitions and variables of geometry used for heating area, cooling area and respective distances.
Scribable velocity with respect to laser power was obtained as the laser scribable condition for glass substrates with thicknesses of 0.4, 0.55, and 1.1 mm. The results and crack depth Dc in each condition are shown in Fig. 3. For the glass thickness 0.7 mm, the experimental results from previous study [6,7] were shown again. For the glass thickness of 0.4 mm in Fig. 3(d) (i), the specimens were scribable at maximum scan velocity 500 mm/s of this experimental equipment. In Fig. 3(a)–(d) (i), the “×” marks on the higher velocity side show the condition in which the laser scribe crack progress was arrested, and the “×” marks on the lower velocity side show the condition with residual thermal damage in the glass substrate. For the crack depth Dc related to each laser power in Fig. 3(a)–(d) (ii), the deeper Dc corresponds to lower velocity and the shallower Dc corresponds to higher velocity. For each glass thickness, scribable velocity tends to become higher as the laser power increases. Moreover, the scribe conditions that generated the thermal damage (“×” marks in lower velocity side) were almost the same regardless of the glass thickness. Under the conditions in which crack progress was arrested (higher side “×” marks), the scribable velocity range with a glass thickness of 1.1 mm narrows at the side with higher laser power, whereas the scribable velocity range widens at the lower power side, and scribing becomes impossible at scribe velocities equal to or more than v = 350 mm/s. On the other hand, the scribable velocity range narrows at the lower power side with a glass thickness of 0.4 mm, and scribing becomes impossible at every scribe velocity with laser power of P = 30.4 W. On the higher power side, the scribable velocity range widens and scribing is possible even at a scribe velocity of v = 500 mm/s. Ultimately, scribing tends to become difficult at higher velocity as the glass thickness increases, and at lower velocity as the glass thickness decreases. The crack depth Dc in the five scribe conditions that did not generate thermal damage, and the results were arranged with respect to the glass thickness, are shown in Fig. 4. The “×” marks show the
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Fig. 3. Domain of laser scribable conditions and crack depth for glass substrate with thicknesses of 0.4, 0.55, 0.7, and 1.1 mm. (a) Plots of scribe conditions for glass substrate with thickness of 1.1 mm. (b) Plots of scribe conditions for glass substrate with thickness of 0.7 mm [6,7]. (c) Plots of scribe conditions for glass substrate with thickness of 0.55 mm. (d) Plots of scribe conditions for glass substrate with thickness of 0.4 mm. (i) Scribable velocity versus laser power; (ii) crack depth versus laser power. (i) “×” marks at higher velocity represent conditions in which crack progress was arrested and “×” marks at lower velocity represent conditions in which glass surface was damaged by laser heating. (ii) Deeper crack depth corresponds to the lower velocity condition.
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Fig. 4. Relation between crack depth and scribe condition for glass substrates with thicknesses 0.4, 0.55, 0.7, and 1.1 mm.
Fig. 5. Mesh geometry for FEM analysis.
condition in which the crack progress was arrested. Under the conditions of laser power P = 71.0 W and scribe velocity v = 320 mm/s, scribing is impossible with a glass thickness of 1.1 mm, and under the conditions of P = 30.4 W and v = 80 mm/s, scribing is impossible with a glass thickness of 0.4 mm. The crack depth Dc decreases individually regardless the glass substrate thickness as scribe velocity decreases. 3. Thermal stress analysis Based on the scribe conditions obtained in the experiment, the following two-dimensional thermal elasticity analysis was conducted with a finite element method. The laser scribable condition and the influence of glass substrate thickness in laser scribing are discussed in next chapter. The x–y coordinates were obtained on the laser irradiation surface, with scribe direction on the y-axis and glass thickness direction on the z-axis. Fig. 5 shows the element division using FEM analysis with a thickness of 0.4 mm as an example. Considering symmetry, the target for analysis was focused in the range of 0.4 mm × 30 mm. The minimum division value in the beam width direction (x-axis direction) was set as 3.7 m, and the minimum value of division in the thickness direction (z-axis direction) was Table 2 Physical properties of soda-lime glass. Density [9] Specific heat [9] Thermal conductivity [9] Thermal expansion coefficient [10] Young’s modulus [11] Poisson’s ratio [11] Softening temperature [11] Bending fracture strength [11]
2520 kg/m3 800 J/(kg K) 1.03 W/(m K) 8.7 × 10−6 K−1 71.6 GPa 0.23 720–730 ◦ C 49 MPa
Fig. 6. Analysis results of maximum surface temperature Tmax and maximum tensile stress tmax for each glass substrate with thickness of 0.4, 0.55, 0.7, or 1.1 mm. “×” marks at higher velocity correspond the conditions for arrest of laser scribe crack progress and “×” marks at lower velocity correspond the thermal damage residual conditions where the glass surface was damaged by laser heating. (a)–(d) correspond to Fig. 3(a)–(d) (i).
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Fig. 7. Maximum surface temperature Tmax and maximum tensile stress tmax for each glass substrate with thickness from 0.4 to 1.1 mm.
2.7 m. The total number of nodes was 622 and the total number of elements was 668. The values in Table 2 were used as physical properties [9–11] of each specimen. The time step was 0.25 mm/v[s], which is the time when 0.25 mm was divided by scribe velocity v. The values shown in Table 1 were used for the size of the heating area and the cooling area, and these intensities were assumed to have Gaussian distributions. In two-dimensional heat conduction analysis on the x–z plane, the center of the laser beam was scanned from the position of y = −15 mm in the y-axis direction (farther to the front on the paper), and both the heating and the cooling conditions were changed with time. Since the optical attenuation rate was calculated to be 0.9793%, the heat absorption was set as 0.798P [W] considering that the measured reflection ratio was 18.5%. The heat transfer coefficient1 ˛0 at the collision point was calculated [12] from the flow rate of the water jet (0.8 ml/min in this case). The water temperature was controlled at 20 ◦ C. ˛0 was constant at 100 076 W/(m2 K) since the water jet flow was unchanged, even though laser power and scribe velocity were changed in the experiment. Next, two-dimensional thermal stress analysis was conducted on the x–z plane for issue of plane stress, which was assumed to be yy = yx = yz = 0 using the obtained temperature distribution. The laser irradiation side-end of the FEM analysis model was restrained in the x-axis direction, and the other side-end in x-axis and z-axis direction. As in a previous study [6,7], maximum temperature on the glass substrate surface during the laser scribing was denoted as Tmax , and maximum tensile stress of xx on the cooling area was denoted as tmax . 4. Discussion of experimental results 4.1. Laser scribable condition Previous study [6,7] showed that the scribable condition for 0.7 mm soda-lime glass can be estimated from the maximum surface temperature Tmax on the heating area and the maximum tensile stress tmax in the cooling area after heating. Similarly, thermal elasticity analysis was conducted in this study under the experimental conditions (Fig. 3(a)–(d) (i)) of laser power and scribe velocity with which glass thicknesses of 0.4, 0.55, and 1.1 mm were scribable. The results are shown in Fig. 6. Fig. 6(b) shows again the analysis results for glass thickness of 0.7 mm to compare the scribable conditions
1 Amendment: ˛0 = heat transfer coefficient of a collision point, ˛0 = / the average heat transfer coefficient of the cooling area, and ˛0 = 100 076 W/m2 K, / 10 076 W/m2 K in Ref. [7]. ˛0 =
Fig. 8. Temperature distributions on x–z plane and deformation at the time when tmax is generated for each glass substrate with thickness of (i) 1.1 mm, (ii) 0.7 mm, (iii) 0.55 mm, or (iv) 0.4 mm. (a) P = 71.0 W, v = 320 mm/s; (b) P = 52.8 W, v = 160 mm/s; (c) P = 30.4 W, v = 80 mm/s.
with different glass thicknesses. The upper side of the plots in these figures shows the maximum surface temperature Tmax (right vertical axis), and lower side of the plots shows the maximum tensile stress tmax (left vertical axis). The “×” marks on the higher velocity side correspond to the conditions in which the laser scribe crack progress was arrested, and the “×” marks on lower velocity side correspond to the conditions determined to be unscribable as a result of the thermal damage generation in Fig. 3(a)–(d) (i). The upper limit of Tmax was nearly stable at every laser power and independent of scribe velocity, and Tmax increases as scribe velocity decreases when laser power is constant for the glass thicknesses of 0.4, 0.55, and 1.1 mm. Therefore, the thermal damage would not affect the glass at or below this upper limit of Tmax . On the other hand, tmax decreases as scribe velocity increases when laser power is constant. This is the reason that the crack depth becomes shallower on the higher velocity side in Fig. 3(a)–(d) (ii). At every laser power, the lower limit of tmax is nearly stable and independent of scribe velocity. Therefore, the laser scribe crack would progress when it is equal to or above the lower limit of this tmax . Thus, the upper limit of Tmax and the lower limit of tmax also exist respectively for glass thicknesses of 0.4, 0.55, and 1.1 mm. Fig. 7 summarizes the upper limit of maximum surface temperature Tmax and the lower limit of maximum tensile stress tmax of the glass thicknesses of 0.4, 0.55, 0.7, and 1.1 mm. The upper limit of Tmax was nearly stable at approximately 500 ◦ C for every glass thickness. The lower limit of tmax was also nearly stable, at a value of approximately 65 MPa. Thus, the laser scribable condition that is obtainable experimentally can be predicted from the upper limit of maximum surface temperature Tmax and the lower limit of maximum tensile stress tmax , by conducting thermal stress analysis regardless of the glass thickness when the glass thickness is 0.4–1.1 mm.
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face side. Next, Fig. 9 shows the stress distribution corresponding to Fig. 8. As in Fig. 1(d), tensile stress is generated on the surface layer and the high temperature region inside the glass becomes a compressive field. Fig. 10 summarizes tmax the five conditions corresponding to Fig. 4 with regard to the glass thickness. Under each condition, tmax becomes convex on the top, and the glass thickness that maximizes tmax tends to shift to the thinner side as scribe velocity increases, and tends to shift to the thicker side as velocity decreases. By this tendency, under the condition of P = 71.0 W and v = 320 mm/s, the glass thickness at which tmax becomes maximum is near 0.4 mm. Consequently tmax with the glass thickness 1.1 mm is smaller than the lower limit of tmax . Because of this, scribing is not possible with glass thickness of 1.1 mm. Under the condition of P= 30.4 W and v = 80 mm/s, the glass thickness when tmax is max-
Fig. 9. Stress distributions on x–z plane and deformation at the time when tmax is generated for each glass substrate with thickness of (i) 1.1 mm, (ii) 0.7 mm, (iii) 0.55 mm, or (iv) 0.4 mm. (a) P = 71.0 W, v = 320 mm/s; (b) P = 52.8 W, v = 160 mm/s; (c) P = 30.4 W, v = 80 mm/s.
4.2. Influence of glass thickness during laser scribing Fig. 8 shows the temperature distribution and the substrate distortion state on the x–z plane at the time when the maximum tensile stress tmax is generated. Three conditions (P = 30.4 W, v = 80 mm/s; P = 52.8 W, v = 160 mm/s; P = 71.0 W, v = 320 mm/s) were selected from the five conditions in Fig. 4. In the temperature distribution on the x–z plane, the high temperature region inside the glass is just below the quenching region, as in Fig. 1(c). The heat is transferred deep inside as the scribe velocity decreases. Under the condition of P = 30.4 W and v = 80 mm/s for the glass thickness of 0.4 mm, the back side heats as high as 200 ◦ C. At this time, hollows form in the back side of the substrate. Convex distortions also form on the sur-
Fig. 10. Maximum tensile stress tmax of each laser scribe condition for each glass substrate with thickness of 0.4, 0.55, 0.7, or 1.1 mm.
Fig. 11. Stress distributions of xx along the z-axis at the time when tmax is generated for each glass substrate with thickness of 0.4, 0.55, 0.7, or 1.1 mm. (a) P = 71.0 W, v = 320 mm/s; (b) P = 52.8 W, v = 160 mm/s; (c) P = 30.4 W, v = 80 mm/s.
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imum is 1.1 mm of more. As a result, tmax with glass thickness of 0.4 mm becomes smaller than the lower limit of tmax , and scribing is not possible. The tendency of maximum tensile stress tmax with respect to glass thickness in Fig. 10 roughly matches the tendency of crack depth Dc with respect to glass thickness in Fig. 4. However, in Fig. 10, tmax is smaller under the conditions of P = 30.4 W, v = 80 mm/s and P = 39.5 W, v = 120 mm/s than it is under P = 52.8 W, v = 160 mm/s. Despite this, crack depth Dc is deeper in Fig. 4. This means that tmax alone cannot explain the crack depth Dc in the experimental results. In previous studies [6,7], it was demonstrated that the crack was likely to propagated when the inner compressive stress field is fairly small and appropriately deep, whereas when a large inner compressive stress field exists in a comparatively shallow position crack progress is inhibited and the crack bends. Based on this, the relations between the scribe condition, the inner compressive stress field, and the field’s depth was investigated. The value of z divided by h is set as relative depth z/h. The relation between the relative depth and stress xx is shown in Fig. 11. The inner compressive stress decreases with each glass thickness as scribe velocity slows from Fig. 11(a)–(c). The depth of the inner compressive stress field also tends to become deeper. Therefore, it can be considered that the crack depth Dc under the conditions of P = 30.4 W, v = 80 mm/s and P = 39.5 W, v = 120 mm/s became deeper even though tmax is smaller than under conditions of P = 52.8 W, v = 160 mm/s. In Fig. 11, the relative depth of the inner compressive stress field in the glass with thickness 0.4 mm under conditions of P= 30.4 W and v= 80 mm/s was the deepest of all the conditions. The relative depth z/h is approximately 0.4. This is one of the causes of the concaves in the surface in Figs. 8 (c) (iv) and 9 (c) (iv). 5. Conclusion Two-dimensional thermal elasticity analysis with the finite element method was conducted under the laser scribable condition for soda-lime glass substrate with thicknesses of 0.4, 0.55, and 1.1 mm. The conclusions from this study are as follows.
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The upper limit of the maximum surface temperature Tmax and the lower limit of the maximum tensile stress tmax in the cooling area are nearly stable and independent of the laser power and scribe velocity for the glass thicknesses of 0.4, 0.55, and 1.1 mm. This is the same as for the glass thickness of 0.7 mm in a previous study [6,7]. Those values are also independent of the glass thickness. Therefore, if thermal elasticity analysis is conducted, the laser scribable condition can be predicted from the combination of the upper limit value of the maximum surface temperature Tmax and the lower limit value of the maximum tensile stress tmax , regardless of glass thickness for glass thicknesses from 0.4 to 1.1 mm. Under each scribe condition, there exists a glass thickness that maximizes the maximum surface tensile stress tmax . This glass thickness that maximizes tmax tends to become thinner as scribe velocity increases and to become thicker as scribe velocity decreases. As a result, crack depth has nearly the same tendency as tmax . References [1] Miyake Y. Separation technology for FPD glass. J Jpn Soc Abrasive Tech 2001;45(7):342–7 [in Japanese]. [2] Hermanns C. Laser separation of flat glass. In: Proceedings 63rd Laser Mater. Process Conf. 2005. p. 105–10. [3] Swain MV. Median crack initiation and propagation beneath a disc glass cutter. Glass Tech 1981;22(5):222–9. [4] Ono T, Teng O, Pai G. Breakless cutting EagleXGTM using standard scoring wheel. In: Proceedings of the 14th international display workshops, FMC2-2. 2007. [5] Wang S-C, Yeh L-Y, Lin C-C, Chen MS, Gan FY. Glass-strength dependence of cutting conditions in thin laminated TFT-LCD. In: Proceedings of the 14th international display workshops, FMCp-1. 2007. p. 66. [6] Yamamoto K, Hasaka N, Morita H, Ohmura E. Thermal stress analysis on laser scribing of glass. J Laser Appl 2008;20(4):193–200. [7] Yamamoto K, Hasaka N, Morita H, Ohmura E. Three-dimensional thermal stress analysis on laser scribing of glass. Precision Eng 2008;32:301–8. [8] Rolo A, Coelho J, Pires M. Marking glass with continuous and pulsed CO2 laser radiation. In: Proceedings of the ICALEO’05, P506. 2005. [9] JSME Data Book, Heat transfer, 4th ed. 1984 [in Japanese]. [10] Shand EB. Glass engineering handbook. 2nd ed. McGraw-Hill; 1958. [11] Watanabe N. Glass engineering handbook. Asakura Shoten; 1999 [in Japanese]. [12] Yamamoto A. On the heat transfer properties of cutting fluids (Part 2). J Jpn Soc Precision Eng 1960;26:17–25 [in Japanese].