- Email: [email protected]
Frequency-tripled Nd:YAG laser ablation in laser structuring process
ARTICLE IN PRESS
Optics and Lasers in Engineering 44 (2006) 815–825
Frequency-tripled Nd:YAG laser ablation in laser structuring process Bin Zhang, K.C. Yung Department of Industrial and Systems Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong Received 1 November 2004; accepted 4 June 2005 Available online 9 November 2005
Abstract With the increasing demand for finer lines/spaces on PCB boards, a new technology—laser structuring—has emerged in recent years. In this research, the frequency-tripled Nd:YAG laser is selected as the laser source in laser structuring; this laser is often used in miniaturization machining. This paper describes in detail the processing parameters’ influences, such as laser power, numbers of repetition, repetition rate and bite size, on laser structuring results. From the research results, it can be concluded that the line width and depth are increased with increases in the laser power and numbers of repetition. Repetition rate, bite size and velocity are related to one another. When the bite size is fixed, the velocity increases with the repetition rate and the depth of the line is decreased at the same time. When the repetition rate is fixed, velocity increases with the bite size. r 2005 Elsevier Ltd. All rights reserved. Keywords: Frequency tripled Nd:YAG laser; Laser structuring; Laser ablation
1. Introduction Global trends and challenges that affect the electronics industry have a substantial influence on PCB manufacturers because of the growing demand for portable Corresponding author. EF403, ISE department, Hong Kong Polytechnic University, Hung Hom, Kowloon, HongKong. Tel: +85227664597; fax: +85223629787 E-mail address: [email protected] (B. Zhang).
0143-8166/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2005.06.008
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products such as mobile phones, notebook computers, and personal digital assistants (PDAs). The PCB industry is also regarded as following the semiconductor (IC) industry and the latter continues to demonstrate the famous Moore’s Law (chip density doubling every 18 months). In addition, the PCB industry is experiencing a tremendous increase in demand for the multilayer boards and high-density boards that will allow new IC component types such as Ball Grid Arrays (BGAs), Chip Scale Pckages (CSPs) and flip chips to be interconnected [1–3]. Considering that it is much cheaper to produce smaller features than additional layers, the trend is towards finer lines and pads, and not to increased layer counts [4]. Today’s electronics industry is, in a word, demanding. Not only do many electronics products require smaller, faster, lighter, less expensive, more complex and reliable printed circuit boards, but PCB makers must also be constantly adapting to new product design [5]. The need for finer lines and spaces will only increase in the future, and new fabrication approaches such as laser direct imaging (LDI) and laser structuring (LS) will have a wide and ready market. Laser structuring can expect a prosperous future because it eliminates many complex steps and requires less time and money to be spent, especially on the production of small amounts. Laser structuring offers an answer to the enormous microelectronics push for further miniaturization and cheaper packaging, on chip substrate as well as on PCB level. Laser structuring bridges the gap between chip-sized packages and chip-sized electronics. The laser has become the most important manufacturing tool in the electronics industry for chips, packages, connectors, PCBs and micro electromechanical systems (MEMS). The movement of the laser beam is controlled by a high-speed controller based on electronic CAD-layout data during LS. A UV laser with a wavelength of 355 or 532 nm bite size of 25 mm is used as the main laser source. This allows the LS process to achieve 50 mm lines and spaces—or even smaller—without any need for clean facilities, high yields and acceptable processing times [6]. As a promising technology in PCB manufacturing, laser structuring needs further research on the controlling parameters during the fabrication process. In this paper, laser structuring parameters such as laser power, numbers of repetition, repetition rate and bite size are selected in order to see their influences on the structuring results.
2. Laser parameters In general, a laser beam of circular cross-section is focused on the target surface by a lens, so that the spatial distribution of laser irradiance Eðr; tÞ at the focal plane can be expressed as (Fig. 1), Iðr; tÞ ¼ Ið0; tÞ GðrÞ.
(1)
The laser beam of Nd:YAG is considered Gaussian spatially [7], so GðrÞ ¼ expð2r2 =w2 Þ,
(2)
ARTICLE IN PRESS B. Zhang, K.C. Yung / Optics and Lasers in Engineering 44 (2006) 815–825
irradiance at time t
1.2
817
Spacial Profile
1 0.8 0.6 G (r) 0.4 0.2 0 w Radius of focal spot r Fig. 1. Laser spatial profile.
where r is the distance in the direction transverse to the direction of propagation, Ið0; tÞ is the laser irradiance at the centre of the focal spot, and w is the focal spot radius for a Gaussian beam. In this research, Q-switched pulsed lasers are used so that the temporal profile of the irradiance at the centre of the focal spot can be considered as Ið0; tÞ ¼ I 0 F ðtÞ,
(3)
where I 0 is the maximum irradiance at the focal centre point during the laser pulse duration t; t represents time, and F ðtÞ is the time-dependent function. The laser beam irradiance Iðr; tÞ can be obtained in terms of the spatial and temporal profile as Iðr; tÞ ¼ I 0 F ðtÞ GðrÞ. Then the pulse energy Q can be expressed as Z tZ 1 Q ¼ I0 F ðtÞGðrÞ2pr dr dt: 0
(4)
(5)
0
In this research, I0 is obtained by controlling the process parameters of the laser. I0 ¼
P=Rrate , pw2 t
(6)
P is the average power of the laser, Rrate is the repetition rate, w is the radius of the laser focal spot, and t is the pulse duration. Therefore, the pulse energy can be written as Z Z P=Rrate t 1 Q¼ F ðtÞGðrÞ2pr dr dt: (7) pw2 t 0 0 The pulse energy absorbed by a material is related to the reflectivity parameters of this material at a certain wavelength. If the reflectivity parameter is known as R, then
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the energy Q0 absorbed by the material is Z Z ð1 RÞP=Rrate t 1 Q0 ¼ F ðtÞGðrÞ2pr dr dt: pw2 t 0 0
(8)
3. Experimental procedures Laser direct structuring processes ultra-fine lines directly into the upper layer of a PCB. Laser structuring of etch resist is an advanced technology used in addition or even as an alternative to photolithography in order to produce lines and spaces on a circuit board that reach pitches smaller than 50 mm. A very thin etch-resist like immersion tin is much thinner than most conventional resists that could fulfill this function. The thickness of the small layer of tin is usually plated from 1 to 2 mm [1]. The laser beam only contours the track or pad layout and evaporates the thin tin layer and a few microns of the copper layer. This exposed copper can then subsequently be etched. Fig. 2 shows the processes of laser structuring. An ESI 5100 laser processing system (Model 5100) is used in the experiments. The Nd:YAG laser is frequency tripled and operated at 355 nm, equipped with an acoustic-optical Q-switch. The diameter of the focal spot is fixed at 25 mm. The structure of the printed circuit board used in this paper is shown in Fig. 3. Epoxy resin is selected as the base material and is coated with two layers of 12 mm copper and one layer of 2 mm tin on each side. The laser writes directly on the tin layer, and a small layer of copper is also etched at the same time. The parameter of the depth of the line written by the laser is very important because it affects the copper etching process and the final profile of the cross-section of the lines. Therefore, in this paper, the depth of the line after laser structuring is taken as the aim parameter to test its relations with the processing parameters. The parameters of copper and tin are shown in Table 1.
Copper layercleaning
ESI programming
Tinplating
Laser structuring Copper etching Tin stripping Fig. 2. The processes of laser structuring.
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Fig. 3. The structure of the printed circuit board. Table 1 Materials’ parameters of tin and copper Tin
Copper
r (g cm3) Tm (1C) Tv (1C) Lm (J g1) Lv (J g1) k (W cm1 1K1)
7.30 231.9 2270 60.7 1945 293(K) 373 473 505 573 673 773 1273
Cp (J g1 1K1)
293(K) 373 473 505 573 673 773 1273
8.96 1083 2595 212 4770 293(K) 373 473 773 1273 1356 1373 1473 1673 1873 293(K) 373 473 773 1273 1356 1373 1473 1673 1873 0.36 0.38
R355 R248
0.70 0.68
0.65 0.63 0.60 0.30 0.314 0.334 0.354 —
0.222 0.239 0.260 0.250 0.242 0.241 0.240 0.260
3.94 3.94 3.89 3.41 2.44 1.656 1.661 1.701 1.763 1.804 0.385 0.389 0.402 0.427 0.473 0.495 0.495 0.495 0.495 0.495
r, the specific weight, Tm, the melting temperature, Tv, the evaporation temperature, Lm, the melting heat, Lv, the evaporation heat, k, the heat conductivity, c, the specific heat, and R, the spectral and intensitydependent reflectivity for a defined surface.
4. Results and discussions Laser parameters directly influence the profile and depth of lines/spaces in laser structuring. Some frequently used laser parameters are therefore discussed here, including laser power, numbers of repetition, repetition rate (RR) and laser velocity (v). As the depths of spaces written by laser in laser structuring are several
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micrometers, the laser power is always set at a small value. In this research, the range of laser power is set at between 0.1 and 0.4 W. In addition to increasing laser power, adding numbers of repetition can also increase the depth of lines/spaces during laser structuring. However, with the increasing of numbers of repetition, consumption time is also increased, which is not favored in industry. Besides these two factors, the repetition rate, the velocity of laser structuring and the bite size of the laser are also very important factors. 4.1. The influences of laser power From Fig. 4, it is clear that the line depth increases with each increase in laser power. As shown in equation function (8), when the laser power is increased, the material absorbs more energy at the same time. The heat-affected zone is also enlarged by the increase in material-absorbed energy. The trends of line depths with increasing laser power are shown in Fig. 4. The depths of laser structuring with the increase of laser power are almost at the same line as shown in Fig. 4, so the polynomial formula is taken to fit the experimental results. Table 2 lists the results for the sum of squares due to error
Fig. 4. Curve fit of the experiments of power & power depth.
Table 2 Accuracy of the power & power depth curve fit Date set
Function type
SSE
R-square
Adjusted R-square
Power depth vs. power
Polynomial
0.88973
0.97709
0.97556
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(SSE), a statistic measuring the deviation of the responses from their fitted values, Rsquare (the coefficient of multiple determination, measuring how successful the fit is in explaining the variation of the data) and adjusted R-square (the degree of freedom-adjusted R-square, where a value closer to 1 indicates a better fit). 4.2. The influences of numbers of repetition In order to see the influence of numbers of repetition on the depths of laser structuring, some experiments were carried out in which the numbers of repetition were changed and other parameters were fixed at certain values. A set of experiments is shown below with laser power ¼ 0:1 W, RR ¼ 10, v ¼ 50 mm=s, Bite size ¼ 5 mm and different numbers of repetition to see the influence of numbers of repetition on line depths. A low value was selected for the laser power because this would permit a larger range for the increase of numbers of repetition. The experiment results and the curve fit are shown in Fig. 5. The function type of the curve fit is polynomial. The SSE and adjusted R-square results present the accuracy of the curve fit as shown in Table 3. When the laser moving velocities are high, the problem of the splashing of materials becomes more serious with increasing numbers of repetition. The cross sections of laser structuring are shown in Fig. 6, with power ¼ 0:1 W, repetition rate ¼ 10, velocity ¼ 50 mm=s, bite size ¼ 5 mm and numbers of repetition ¼ 2, 3 and 5 times. It can be seen from Fig. 6 that copper splashing increases with increasing numbers of repetition. In the miniaturization industry, neat patterns are required with limited heataffected zones. The splashing problem should therefore be avoided during laser
Fig. 5. Curve fit of the experiments of numbers of repetition & repetition depth.
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Fig. 6. Cross section of laser structuring with (a) numbers of repetition ¼ 2 and (b) numbers of repetition ¼ 3, and (c) numbers of repetition ¼ 5, at laser power ¼ 0:1 W, RR ¼ 10, v ¼ 50 mm=s, Bite size ¼ 5 mm, and amplify rate ¼ 50.
structuring. On the other hand, because a higher velocity is equal to higher efficiency, which is preferred in the industry, then the numbers of repetition should be reduced to the minimum. In this research, the number of repetition is selected as one. If it is necessary to increase the depth of the spaces, increasing the laser power or decreasing the repetition rate will be considered as better methods. 4.3. The influences of repetition rate In the laser structuring process, besides laser power and numbers of repetition, the influences of the parameters of repetition rate, bite size and velocity also need to be considered. Repetition rate, bite size and velocity have a relationship that velocity is equal to the multiplication value of repetition rate and bite size. Thus, when considering the repetition rate, we also need to consider its relations with velocity and bite size. In this paper, bite size is fixed at a certain value and velocity changes with variations in the repetition rate. In order to know the trends of depth with repetition rate, a set of experiments is shown below in which the laser power ¼ 0:1 W, number of repetition ¼ 1, and bite
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Fig. 7. Curve fit of the experiments of repetition rate & repetition rate depth.
Table 3 Accuracy of the numbers of repetition & repetition depth curve fit Date set
Function type
SSE
R-square
Adjusted R-square
Repetitions depth vs. numbers of repetition RR depth vs. RR
Polynomial Exponential
0.04841 3.78469
0.99692 0.99587
0.99538 0.9938
size ¼ 5 mm. With the increase in the laser repetition rate (and the bite size set as constant), the peak power of the laser pulse decreases as shown in Eq. (8), and the laser velocity is increased as mentioned above. From Fig. 7, we can see when the laser repetition rate is 1 or 2 kHz, the tin and copper layers are penetrated by the laser (the thickness of the tin and copper layer is 14 mm). In order to achieve a higher velocity and protect the polymer layer, which is under the copper layer, the repetition rate should be set above a certain level. For example, in this experiment, it should be set at equal to or higher than 3 kHz. With the number of repetition reduced to one, the acceptable repetition rate should be higher than 2 kHz to avoid the destruction of the polymer layer. The relationship between space depth and repetition rate is shown in Fig. 7. The relationship between the depth and repetition rate can be expressed with the exponential function. The SSE and adjusted R-square values are shown in Table 3. 4.4. The influences of bite size As we mentioned above, velocity is equal to the multiplication value of repetition rate and bite size. If it is necessary to increase the laser structuring velocity, besides
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Fig. 8. Laser structuring with (a) laser v ¼ 30, Bite size ¼ 6 mm and (b) laser v ¼ 45, Bite size ¼ 9 mm and (c) laser v ¼ 60, Bite size ¼ 12 mm and (d) laser v ¼ 75, Bite size ¼ 15 mm, at laser power ¼ 0:1 W, numbers of repetition ¼ 3, RR ¼ 5, and amplify rate ¼ 50.
the method of increasing the repetition rate, increasing the bite size can also have the same function. As shown in Fig. 7, when the repetition rate is above a certain value, the depths of laser structuring are very shallow, which may affect the copper etching process. Thus, at this time, the bite size can be changed to increase the structuring velocity. With the increasing of the laser bite size (setting the repetition rate as constant), the velocity of the laser is increased at the same time. Achieving a faster fabrication progress is what we wanted, but the continuation of the line should be considered at the same time. As shown in Fig. 8, when the bite size is more than 12 mm, the line consists of obvious holes, and this is unacceptable in line fabrication.
5. Conclusions From the investigation of the effect of parameters on laser structuring, it can be concluded that:
The line width and depth are increased with increases in the laser power and numbers of repetition.
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825
Repetition rate, bite size and velocity are related to one another. When the bite size is fixed, the velocity increases with the repetition rate and the depth of the line is decreased at the same time. When the repetition rate is fixed, velocity increases with the bite size, but the bite size should not be set at a large number because of the consideration of the quality of the line.
Acknowledgement The work described in this study is supported by a Central Research Grant from The Hong Kong Polytechnic University (RG9N). References [1] Krause J. High-precision working, PC FAB; March 2000. p. 24–8. [2] Taff I. Direct imaging-major trends and impact on PCB manufacturers, printed circuits EXPO; 2001. S04-1-1. [3] Partha A, Zemel M, Jain K. A comparison of PCB imaging technologies, printed circuits EXPO; 2001. S15-2-1. [4] Dr. Kimpfel, Fine line production: the issues for PCB manufacturers, printed circuit Europe; May–June 1999. p. 12–4. [5] Vaucher C, Jaquet R. Laser direct imaging and structuring; August 2002, www.circuitree.com. [6] Toelants E. Laser direct structuring as an innovative alternative for traditional lithography, Proceedings of the technical conference; March 2002. S03-3. [7] Bauerle D. Laser processing and chemistry. third ed. Berlin, Heidelberg, New York: Springer.
Optics and Lasers in Engineering 44 (2006) 815–825
Frequency-tripled Nd:YAG laser ablation in laser structuring process Bin Zhang, K.C. Yung Department of Industrial and Systems Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong Received 1 November 2004; accepted 4 June 2005 Available online 9 November 2005
Abstract With the increasing demand for finer lines/spaces on PCB boards, a new technology—laser structuring—has emerged in recent years. In this research, the frequency-tripled Nd:YAG laser is selected as the laser source in laser structuring; this laser is often used in miniaturization machining. This paper describes in detail the processing parameters’ influences, such as laser power, numbers of repetition, repetition rate and bite size, on laser structuring results. From the research results, it can be concluded that the line width and depth are increased with increases in the laser power and numbers of repetition. Repetition rate, bite size and velocity are related to one another. When the bite size is fixed, the velocity increases with the repetition rate and the depth of the line is decreased at the same time. When the repetition rate is fixed, velocity increases with the bite size. r 2005 Elsevier Ltd. All rights reserved. Keywords: Frequency tripled Nd:YAG laser; Laser structuring; Laser ablation
1. Introduction Global trends and challenges that affect the electronics industry have a substantial influence on PCB manufacturers because of the growing demand for portable Corresponding author. EF403, ISE department, Hong Kong Polytechnic University, Hung Hom, Kowloon, HongKong. Tel: +85227664597; fax: +85223629787 E-mail address: [email protected] (B. Zhang).
0143-8166/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2005.06.008
ARTICLE IN PRESS 816
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products such as mobile phones, notebook computers, and personal digital assistants (PDAs). The PCB industry is also regarded as following the semiconductor (IC) industry and the latter continues to demonstrate the famous Moore’s Law (chip density doubling every 18 months). In addition, the PCB industry is experiencing a tremendous increase in demand for the multilayer boards and high-density boards that will allow new IC component types such as Ball Grid Arrays (BGAs), Chip Scale Pckages (CSPs) and flip chips to be interconnected [1–3]. Considering that it is much cheaper to produce smaller features than additional layers, the trend is towards finer lines and pads, and not to increased layer counts [4]. Today’s electronics industry is, in a word, demanding. Not only do many electronics products require smaller, faster, lighter, less expensive, more complex and reliable printed circuit boards, but PCB makers must also be constantly adapting to new product design [5]. The need for finer lines and spaces will only increase in the future, and new fabrication approaches such as laser direct imaging (LDI) and laser structuring (LS) will have a wide and ready market. Laser structuring can expect a prosperous future because it eliminates many complex steps and requires less time and money to be spent, especially on the production of small amounts. Laser structuring offers an answer to the enormous microelectronics push for further miniaturization and cheaper packaging, on chip substrate as well as on PCB level. Laser structuring bridges the gap between chip-sized packages and chip-sized electronics. The laser has become the most important manufacturing tool in the electronics industry for chips, packages, connectors, PCBs and micro electromechanical systems (MEMS). The movement of the laser beam is controlled by a high-speed controller based on electronic CAD-layout data during LS. A UV laser with a wavelength of 355 or 532 nm bite size of 25 mm is used as the main laser source. This allows the LS process to achieve 50 mm lines and spaces—or even smaller—without any need for clean facilities, high yields and acceptable processing times [6]. As a promising technology in PCB manufacturing, laser structuring needs further research on the controlling parameters during the fabrication process. In this paper, laser structuring parameters such as laser power, numbers of repetition, repetition rate and bite size are selected in order to see their influences on the structuring results.
2. Laser parameters In general, a laser beam of circular cross-section is focused on the target surface by a lens, so that the spatial distribution of laser irradiance Eðr; tÞ at the focal plane can be expressed as (Fig. 1), Iðr; tÞ ¼ Ið0; tÞ GðrÞ.
(1)
The laser beam of Nd:YAG is considered Gaussian spatially [7], so GðrÞ ¼ expð2r2 =w2 Þ,
(2)
ARTICLE IN PRESS B. Zhang, K.C. Yung / Optics and Lasers in Engineering 44 (2006) 815–825
irradiance at time t
1.2
817
Spacial Profile
1 0.8 0.6 G (r) 0.4 0.2 0 w Radius of focal spot r Fig. 1. Laser spatial profile.
where r is the distance in the direction transverse to the direction of propagation, Ið0; tÞ is the laser irradiance at the centre of the focal spot, and w is the focal spot radius for a Gaussian beam. In this research, Q-switched pulsed lasers are used so that the temporal profile of the irradiance at the centre of the focal spot can be considered as Ið0; tÞ ¼ I 0 F ðtÞ,
(3)
where I 0 is the maximum irradiance at the focal centre point during the laser pulse duration t; t represents time, and F ðtÞ is the time-dependent function. The laser beam irradiance Iðr; tÞ can be obtained in terms of the spatial and temporal profile as Iðr; tÞ ¼ I 0 F ðtÞ GðrÞ. Then the pulse energy Q can be expressed as Z tZ 1 Q ¼ I0 F ðtÞGðrÞ2pr dr dt: 0
(4)
(5)
0
In this research, I0 is obtained by controlling the process parameters of the laser. I0 ¼
P=Rrate , pw2 t
(6)
P is the average power of the laser, Rrate is the repetition rate, w is the radius of the laser focal spot, and t is the pulse duration. Therefore, the pulse energy can be written as Z Z P=Rrate t 1 Q¼ F ðtÞGðrÞ2pr dr dt: (7) pw2 t 0 0 The pulse energy absorbed by a material is related to the reflectivity parameters of this material at a certain wavelength. If the reflectivity parameter is known as R, then
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the energy Q0 absorbed by the material is Z Z ð1 RÞP=Rrate t 1 Q0 ¼ F ðtÞGðrÞ2pr dr dt: pw2 t 0 0
(8)
3. Experimental procedures Laser direct structuring processes ultra-fine lines directly into the upper layer of a PCB. Laser structuring of etch resist is an advanced technology used in addition or even as an alternative to photolithography in order to produce lines and spaces on a circuit board that reach pitches smaller than 50 mm. A very thin etch-resist like immersion tin is much thinner than most conventional resists that could fulfill this function. The thickness of the small layer of tin is usually plated from 1 to 2 mm [1]. The laser beam only contours the track or pad layout and evaporates the thin tin layer and a few microns of the copper layer. This exposed copper can then subsequently be etched. Fig. 2 shows the processes of laser structuring. An ESI 5100 laser processing system (Model 5100) is used in the experiments. The Nd:YAG laser is frequency tripled and operated at 355 nm, equipped with an acoustic-optical Q-switch. The diameter of the focal spot is fixed at 25 mm. The structure of the printed circuit board used in this paper is shown in Fig. 3. Epoxy resin is selected as the base material and is coated with two layers of 12 mm copper and one layer of 2 mm tin on each side. The laser writes directly on the tin layer, and a small layer of copper is also etched at the same time. The parameter of the depth of the line written by the laser is very important because it affects the copper etching process and the final profile of the cross-section of the lines. Therefore, in this paper, the depth of the line after laser structuring is taken as the aim parameter to test its relations with the processing parameters. The parameters of copper and tin are shown in Table 1.
Copper layercleaning
ESI programming
Tinplating
Laser structuring Copper etching Tin stripping Fig. 2. The processes of laser structuring.
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Fig. 3. The structure of the printed circuit board. Table 1 Materials’ parameters of tin and copper Tin
Copper
r (g cm3) Tm (1C) Tv (1C) Lm (J g1) Lv (J g1) k (W cm1 1K1)
7.30 231.9 2270 60.7 1945 293(K) 373 473 505 573 673 773 1273
Cp (J g1 1K1)
293(K) 373 473 505 573 673 773 1273
8.96 1083 2595 212 4770 293(K) 373 473 773 1273 1356 1373 1473 1673 1873 293(K) 373 473 773 1273 1356 1373 1473 1673 1873 0.36 0.38
R355 R248
0.70 0.68
0.65 0.63 0.60 0.30 0.314 0.334 0.354 —
0.222 0.239 0.260 0.250 0.242 0.241 0.240 0.260
3.94 3.94 3.89 3.41 2.44 1.656 1.661 1.701 1.763 1.804 0.385 0.389 0.402 0.427 0.473 0.495 0.495 0.495 0.495 0.495
r, the specific weight, Tm, the melting temperature, Tv, the evaporation temperature, Lm, the melting heat, Lv, the evaporation heat, k, the heat conductivity, c, the specific heat, and R, the spectral and intensitydependent reflectivity for a defined surface.
4. Results and discussions Laser parameters directly influence the profile and depth of lines/spaces in laser structuring. Some frequently used laser parameters are therefore discussed here, including laser power, numbers of repetition, repetition rate (RR) and laser velocity (v). As the depths of spaces written by laser in laser structuring are several
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micrometers, the laser power is always set at a small value. In this research, the range of laser power is set at between 0.1 and 0.4 W. In addition to increasing laser power, adding numbers of repetition can also increase the depth of lines/spaces during laser structuring. However, with the increasing of numbers of repetition, consumption time is also increased, which is not favored in industry. Besides these two factors, the repetition rate, the velocity of laser structuring and the bite size of the laser are also very important factors. 4.1. The influences of laser power From Fig. 4, it is clear that the line depth increases with each increase in laser power. As shown in equation function (8), when the laser power is increased, the material absorbs more energy at the same time. The heat-affected zone is also enlarged by the increase in material-absorbed energy. The trends of line depths with increasing laser power are shown in Fig. 4. The depths of laser structuring with the increase of laser power are almost at the same line as shown in Fig. 4, so the polynomial formula is taken to fit the experimental results. Table 2 lists the results for the sum of squares due to error
Fig. 4. Curve fit of the experiments of power & power depth.
Table 2 Accuracy of the power & power depth curve fit Date set
Function type
SSE
R-square
Adjusted R-square
Power depth vs. power
Polynomial
0.88973
0.97709
0.97556
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(SSE), a statistic measuring the deviation of the responses from their fitted values, Rsquare (the coefficient of multiple determination, measuring how successful the fit is in explaining the variation of the data) and adjusted R-square (the degree of freedom-adjusted R-square, where a value closer to 1 indicates a better fit). 4.2. The influences of numbers of repetition In order to see the influence of numbers of repetition on the depths of laser structuring, some experiments were carried out in which the numbers of repetition were changed and other parameters were fixed at certain values. A set of experiments is shown below with laser power ¼ 0:1 W, RR ¼ 10, v ¼ 50 mm=s, Bite size ¼ 5 mm and different numbers of repetition to see the influence of numbers of repetition on line depths. A low value was selected for the laser power because this would permit a larger range for the increase of numbers of repetition. The experiment results and the curve fit are shown in Fig. 5. The function type of the curve fit is polynomial. The SSE and adjusted R-square results present the accuracy of the curve fit as shown in Table 3. When the laser moving velocities are high, the problem of the splashing of materials becomes more serious with increasing numbers of repetition. The cross sections of laser structuring are shown in Fig. 6, with power ¼ 0:1 W, repetition rate ¼ 10, velocity ¼ 50 mm=s, bite size ¼ 5 mm and numbers of repetition ¼ 2, 3 and 5 times. It can be seen from Fig. 6 that copper splashing increases with increasing numbers of repetition. In the miniaturization industry, neat patterns are required with limited heataffected zones. The splashing problem should therefore be avoided during laser
Fig. 5. Curve fit of the experiments of numbers of repetition & repetition depth.
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Fig. 6. Cross section of laser structuring with (a) numbers of repetition ¼ 2 and (b) numbers of repetition ¼ 3, and (c) numbers of repetition ¼ 5, at laser power ¼ 0:1 W, RR ¼ 10, v ¼ 50 mm=s, Bite size ¼ 5 mm, and amplify rate ¼ 50.
structuring. On the other hand, because a higher velocity is equal to higher efficiency, which is preferred in the industry, then the numbers of repetition should be reduced to the minimum. In this research, the number of repetition is selected as one. If it is necessary to increase the depth of the spaces, increasing the laser power or decreasing the repetition rate will be considered as better methods. 4.3. The influences of repetition rate In the laser structuring process, besides laser power and numbers of repetition, the influences of the parameters of repetition rate, bite size and velocity also need to be considered. Repetition rate, bite size and velocity have a relationship that velocity is equal to the multiplication value of repetition rate and bite size. Thus, when considering the repetition rate, we also need to consider its relations with velocity and bite size. In this paper, bite size is fixed at a certain value and velocity changes with variations in the repetition rate. In order to know the trends of depth with repetition rate, a set of experiments is shown below in which the laser power ¼ 0:1 W, number of repetition ¼ 1, and bite
ARTICLE IN PRESS B. Zhang, K.C. Yung / Optics and Lasers in Engineering 44 (2006) 815–825
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Fig. 7. Curve fit of the experiments of repetition rate & repetition rate depth.
Table 3 Accuracy of the numbers of repetition & repetition depth curve fit Date set
Function type
SSE
R-square
Adjusted R-square
Repetitions depth vs. numbers of repetition RR depth vs. RR
Polynomial Exponential
0.04841 3.78469
0.99692 0.99587
0.99538 0.9938
size ¼ 5 mm. With the increase in the laser repetition rate (and the bite size set as constant), the peak power of the laser pulse decreases as shown in Eq. (8), and the laser velocity is increased as mentioned above. From Fig. 7, we can see when the laser repetition rate is 1 or 2 kHz, the tin and copper layers are penetrated by the laser (the thickness of the tin and copper layer is 14 mm). In order to achieve a higher velocity and protect the polymer layer, which is under the copper layer, the repetition rate should be set above a certain level. For example, in this experiment, it should be set at equal to or higher than 3 kHz. With the number of repetition reduced to one, the acceptable repetition rate should be higher than 2 kHz to avoid the destruction of the polymer layer. The relationship between space depth and repetition rate is shown in Fig. 7. The relationship between the depth and repetition rate can be expressed with the exponential function. The SSE and adjusted R-square values are shown in Table 3. 4.4. The influences of bite size As we mentioned above, velocity is equal to the multiplication value of repetition rate and bite size. If it is necessary to increase the laser structuring velocity, besides
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Fig. 8. Laser structuring with (a) laser v ¼ 30, Bite size ¼ 6 mm and (b) laser v ¼ 45, Bite size ¼ 9 mm and (c) laser v ¼ 60, Bite size ¼ 12 mm and (d) laser v ¼ 75, Bite size ¼ 15 mm, at laser power ¼ 0:1 W, numbers of repetition ¼ 3, RR ¼ 5, and amplify rate ¼ 50.
the method of increasing the repetition rate, increasing the bite size can also have the same function. As shown in Fig. 7, when the repetition rate is above a certain value, the depths of laser structuring are very shallow, which may affect the copper etching process. Thus, at this time, the bite size can be changed to increase the structuring velocity. With the increasing of the laser bite size (setting the repetition rate as constant), the velocity of the laser is increased at the same time. Achieving a faster fabrication progress is what we wanted, but the continuation of the line should be considered at the same time. As shown in Fig. 8, when the bite size is more than 12 mm, the line consists of obvious holes, and this is unacceptable in line fabrication.
5. Conclusions From the investigation of the effect of parameters on laser structuring, it can be concluded that:
The line width and depth are increased with increases in the laser power and numbers of repetition.
ARTICLE IN PRESS B. Zhang, K.C. Yung / Optics and Lasers in Engineering 44 (2006) 815–825
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Repetition rate, bite size and velocity are related to one another. When the bite size is fixed, the velocity increases with the repetition rate and the depth of the line is decreased at the same time. When the repetition rate is fixed, velocity increases with the bite size, but the bite size should not be set at a large number because of the consideration of the quality of the line.
Acknowledgement The work described in this study is supported by a Central Research Grant from The Hong Kong Polytechnic University (RG9N). References [1] Krause J. High-precision working, PC FAB; March 2000. p. 24–8. [2] Taff I. Direct imaging-major trends and impact on PCB manufacturers, printed circuits EXPO; 2001. S04-1-1. [3] Partha A, Zemel M, Jain K. A comparison of PCB imaging technologies, printed circuits EXPO; 2001. S15-2-1. [4] Dr. Kimpfel, Fine line production: the issues for PCB manufacturers, printed circuit Europe; May–June 1999. p. 12–4. [5] Vaucher C, Jaquet R. Laser direct imaging and structuring; August 2002, www.circuitree.com. [6] Toelants E. Laser direct structuring as an innovative alternative for traditional lithography, Proceedings of the technical conference; March 2002. S03-3. [7] Bauerle D. Laser processing and chemistry. third ed. Berlin, Heidelberg, New York: Springer.