- Email: [email protected]
A study of the heat-affected zone in the UV YAG laser drilling of GFRP materials
Journal of Materials Processing Technology 122 (2002) 278–285
A study of the heat-affected zone in the UV YAG laser drilling of GFRP materials K.C. Yung*, S.M. Mei, T.M. Yue Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Received 9 July 2001
Abstract This paper describes the characteristics of the heat-affected zone (HAZ) of a UV YAG laser-drilled hole in glass fiber reinforced plastic (GFRP) printed circuit boards. The structures of the HAZ produced by different laser parameters were analyzed. When drilled with lower power and repetition rate, a clear hole wall with very little black charred material was obtained. On the other hand, when drilled with high power and repetition rate, matrix recession and fiber protrusion were observed, also a loose coating was found covering the protruded glass fibers. The results also show that for a given repetition rate, the size of the HAZ increases with increase in average laser power. As for the effect of the pulse repetition rate, a peak value of HAZ width is reached at about 7 kHz for each given laser power, and beyond which the HAZ width decreases. A novel evaluation parameter, defined as the quotient of total area of the HAZ and the profile length, is suggested to quantify the size of the laser-induced HAZ in fiber reinforced composites. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Heat-affected zone; UV YAG laser; GFRP materials
1. Introduction The miniaturization of electronic products for information and communication, such as notebook computers and mobile phones, has led to increase in the circuit density of printed circuit boards (PCBs). This means that the holes which connect the isolated circuit layers in a multi-layered PCB, called via holes, have decreased in diameter from 0.3 to 0.1 mm or less. Mechanical drilling of these small holes in GRFP PCB becomes extremely difficult for many reasons. Firstly, conventional drilling results in excessive tool wear, tool breakage, and mechanically induced materials damage because of the abrasiveness associated with fiber reinforcement. Secondly, it is difficult to manufacture mechanical drills with a diameter of less than 0.2 mm. On the other hand, mechanical drilling systems can generate through-holes smaller than 0.2 mm, but at a cost that could be prohibitive [1]. It is also inapplicable to drill a great number of microvias (holes less than 0.05 mm in diameter) in high density PCB with mechanical drilling in view of PCB efficiency and reliability. Even if only one hole in production has failed to be drilled because of tool breakage, the entire PCB has to be scrapped. Moreover, tool breakage is one of the most common failures in small-hole drilling [2].
*
Corresponding author. Tel.: þ852-2766-6599; fax: þ852-2362-5267.
Being a non-contact process, laser drilling offers several advantages over mechanical drilling, such as the absence of tool wear, tool breakage and cutting-force-induced problems. Thus it has been used extensively to fabricate micro-vias in PCB fabrication. Examples of lasers used in industry are CO2 and Nd:YAG lasers, which are operated at infrared wavelength of 10.6 and 1.06 mm, respectively. However, the major drawback of laser drilling GFRP materials with these infrared lasers is the existence of a heataffected zone (HAZ) that is formed around the laser-drilled hole. This is because the material-removal mechanism with infrared lasers is a thermal mechanism (melting, evaporation, or vaporization). The differences in temperatures of vaporization and decomposition between glass fibers and epoxy resin in GFRP materials result in the presence of an HAZ, which always shows itself as recession and decomposition of the matrix material [3]. The formation of an HAZ in drilled micro-vias causes difficulty in copper plating and reduces the quality and reliability of the PCB. Researchers have made great efforts to reduce the thermal effect and hence the HAZ size in the laser drilling of GFRP [4,5]. One of the most important methods to reduce the thermal effects is to use UV lasers in the drilling of micro-vias. The high energy UV photons can directly atomize material in a process known as photo-ablation, which is associated with a photochemical mechanism rather than with a photo-thermal mechanism. Although there is enough evidence to
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 1 7 7 - 3
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
confirm that the thermal mechanism still accompanies the photochemical mechanism, photo-ablation does enhance the ability of UV lasers to produce micro-vias with good quality [6–8]. This paper aims to characterize the HAZ of UV YAG laser-drilled GFRP PCB material. Three aspects related to the HAZ are addressed in this paper: (i) the dependence of the HAZ on different laser parameters; (ii) the structure and forming mechanism of the HAZ; (iii) an evaluation method, named equivalent width, is defined to quantify the size of HAZ in fiber reinforced composites.
2. Experimental method The specimens used in the experiments were made of 1080 type GFRP PCB material. They were 1.6 mm thick laminated glass/epoxy composites clad on both sides with 18 mm copper foil. The specimens were cut into 25 mm 4 mm rectangles, and the copper cladding on the laser entrance side was removed to enable the study of laminated glass/epoxy composite materials independently of the copper cladding. The core of the 1080 PCB material was laminated with 8-layer woven fabrics of glass fiber and epoxy resin. The reinforcement was E-glass fiber with a diameter of 10 mm, which is composed of SiO2 (54 wt.%), Al2O3 (14 wt.%) and CaO (18 wt.%). An ESI 5150 laser drilling system was used in this study. The laser light was generated by a pulsed and lamp-pumped Nd:YAG laser with third harmonics generation, and was
279
operated at 355 nm. The output beam had a TEM00 (Gaussian) energy distribution, and the focused spot was of 25 mm diameter. The maximum average power of the 355 nm radiation is 1.2 W. All the laser drilled holes, having a diameter of 500 mm, were drilled using a spiral technique with an inner diameter of 25 mm, a radial pitch of 5 mm and a repetition of 50 times. Constant bite size, fixed at 3 mm, between the spots was used to enable all of the holes to be drilled with same number of laser pulses. All the experiments were carried out without any offset from the focal plane. In the experiments, the pulse repetition rate varied from 0.5 to 20 kHz, and the laser power varied from 0.1 W to the maximum average power allowed at that repetition rate. Cross-sections of the drilled holes were examined by an optical microscope and scanning electronic microscope (SEM). The drilled holes were filled with epoxy resin to prevent the section edges from subsiding during polishing. Electronic dispersive X-ray (EDX) was used for the analysis the components of the recast and char layer. An image analyzer was used to measure the size of the HAZ.
3. Structure of HAZ The experimental results indicate that the structure of the HAZ is influenced strongly by the laser parameters, such as the average laser power and the pulse repetition rate. Fig. 1 shows the morphology of the holes drilled with different laser parameters. It can be seen that charred material, shows
Fig. 1. SEM photographs of holes drilled with different laser parameters: (a) 0.3 W, 1 kHz; (b) 0.3 W, 6 kHz; (c) 1.0 W, 6 kHz.
280
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
Fig. 2. SEM photograph of a hole drilled with light parameters (0.3 W, 1 kHz).
itself as black color under the optical microscope, can be found at the hole wall when drilled with relatively high laser power or high pulse repetition rate. An HAZ with an irregular shape can also be observed around the hole when drilled with high power and high repetition rate, see Fig. 1(c), whereas, neither charred material nor a visible HAZ can be seen around the hole drilled with relatively low laser power and low pulse repetition rate, see Fig. 1(a).
Fig. 2 shows a higher magnification of Fig. 1(a). The wall of the drilled hole appears to be clear. Some of the glass fiber ends had melted and were fused with each other. The surface of epoxy resin is coated with a thin layer of some melted material, in which there are some unmelted solid grains. Fig. 3 shows a higher magnification of Fig. 1(b). It was found that a layer of charred material sticks on the ends of the glass fibers and the surface of the resin matrix, but the hole profile
Fig. 3. SEM photograph of a hole drilled with medium parameters (0.3 W, 6 kHz).
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
281
Fig. 4. SEM photograph of a hole drilled with heavy parameters (0.3 W, 6 kHz).
is almost straight, and no matrix recession and fiber protrusion can be found. Fig. 4 shows a higher magnification of Fig. 1(c). It is obvious that an irregular profile exists around the drilled hole. It can also been observed that the epoxy resin is absent from the HAZ, but glass fibers are left. Another notable feature observed in Fig. 1(c) and Fig. 4 is that the fibers protruding out of the epoxy resin are covered by charred material, which forms a black ‘‘shell’’ supported by the protruding fibers. Regions A and B in Fig. 4 are shown in Fig. 5(A) and (B), respectively. Resin matrix recession and fibers protrusion, the characteristics of HAZ in laser machining of GFRP, can be found in Fig. 5(a). From Fig. 5(a) and (b), caves are formed between the recessing epoxy and the ‘‘shell’’. This is because the vaporization temperature of epoxy resin is about 600 8C, at which glass fibers can only be melted. Therefore, the epoxy resin is vaporized into gas and decomposed to carbon or graphite, while the glass fibers are only melted. The vaporized or decomposed epoxy resin jets out of the hole. At the same time, the melted glass fiber material is expelled from the machined area together with the gas jet. Some of the melted material sticks on to the fibers near to the hole, while the graphite also is deposited on to the fibers, hence the ‘‘shell’’ is formed. The existence of graphite makes the ‘‘shell’’ and the coating on the hole wall appears to be black. As shown in Fig. 5(b), the ‘‘shell’’ has a poriferous and loose structure. Fig. 6 shows the EDX test results of glass fiber and ‘‘shell’’ material. The results of EDX tests show that the composition of the ‘‘shell’’ material is similar to a glassy phase, but rich in carbon atoms, which supports the formation mechanism of the ‘‘shell’’ suggested above.
4. HAZ evaluation Fig. 7 shows a cross-section of a laser-drilled hole of GFRP PCB material. The hole and cavities are filled with epoxy resin that shows up a little darker than the original epoxy, the boundary of the HAZ being easily identified. In order to evaluate the degree of heat damage, it is important to have a reasonable evaluation method to assess the size of the HAZ. The most common parameters to characterize the size of HAZ in the laser drilling of fiber reinforced composites are the width, thickness or depth [2,9–12]. The length of fibers protruding from the matrix is also used to assess the HAZ [13]. However, it is difficult and improper to assess the size of HAZs with irregular profiles, such as that shown in Fig. 7. Another parameter, called the section area, has been used to characterize the size of the HAZ [5,14]. While the section area provides an integral and average method to assess HAZs conveniently, it has problems when assessing the HAZs, as shown in Fig. 8. When taking into account only the integrated effect, the HAZ area of the three holes shown in Fig. 8 would be identical, although it may be argued that the HAZ of hole A is the same as that of hole B, but certainly not to that of hole C. Here a new evaluation parameter is defined to assess the size of the HAZ: We ¼
A L
(1)
where A is the total section area of the HAZ, and L the length of the profile of the section hole. Since We has the dimension of length, it can be defined as the equivalent width of the HAZ. This evaluation parameter can be used to quantify the
282
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
Fig. 5. SEM photographs of the HAZ with higher magnification: (a) region A in Fig. 4; (b) the region B in Fig. 4.
size of an HAZ in laser machining. The equivalent width of the HAZ in Fig. 7 can be calculated as We ¼
A1 þ A2 L1 þ L2
(2)
where A1 and A2 are the areas of the HAZs, and L1 and L2 are the profile lengths of the drilled hole shown in Fig. 7.
5. Dependence of the HAZ on laser parameters Due to the fact that HAZ is directly caused by overheating, all factors that may influence the amount of heat import to and export from the processing region will affect the size of the HAZ. The dependence of the HAZ on laser parameters is analyzed from these two points of view.
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
Fig. 6. EDX results of: (a) glass fibre; (b) ‘‘shell’’ material.
283
284
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
Fig. 7. SEM photograph of a drilled hole filled with epoxy resin.
Fig. 8. A sketch of holes with same section areas of HAZ.
Fig. 9. Effect of laser power on the equivalent width of the HAZ for different repetition rates.
Fig. 9 shows the effect of laser power on the equivalent width of HAZ for different pulse repetition rates. It can be observed that for a given pulse repetition rate, the equivalent width of the HAZ, We, increases as the average laser power. This is obvious because as more energy goes into the processing region, a higher processing temperature results, which accelerates the charring of the epoxy and the melting of the glass fibers. Comparing curves with different repetition rates, it is found that a high repetition rate results in a larger We.
Fig. 10. Effect of pulse repetition rate on the equivalent width of the HAZ for different laser powers.
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
The higher the repetition rate, the shorter time-lapse between the laser pulses and the shorter would be the cooling time for the processing surface. However, this trend exists only below 7 kHz. Fig. 10 shows the effect of pulse repetition rate on the size of the HAZ. Regardless of the laser power, the equivalent width is found to increase with an increase in repetition rate up to a threshold of 7 kHz, beyond which a decrease in HAZ width is found. This is believed to be due to a reduction in pulse energy as the repetition rate is increased, as a result the size of the HAZ being reduced. Furthermore, a higher repetition rate results in a longer pulse duration and a decrease in the peak power of the laser pulse. This drop of peak power will weaken the pulse and reduce the temperature on the machining surface. In a word, increasing the pulse repetition rate can increase the heat damage, and also can decrease it, depending on its relative value relative to a specific threshold of the repetition rate.
6. Conclusions 1. The structure of the HAZ is influenced strongly by the average laser power and the pulse repetition rate. When drilled with low power and low pulse repetition rate, the hole wall is found to be clear, and very little black charred material can be observed at the wall. On the other hand, when drilled with high laser power and high repetition rate, epoxy resin is vaporized and glass fibers are melted. Some of the melted material sticks on to the fibers near to the hole and forms a black coating with a poriferous and loose structure. The existence of this structure may worsen the quality and the reliability of copper plating over the hole wall. 2. An evaluation parameter, defined as the quotient of the total area of HAZ to the profile length, is suggested as a better parameter to be used to quantify the size of HAZ in the machining of fiber reinforced composites with lasers. 3. Laser power and pulse repetition rate are found to be important parameters that have a significant effect on the size of the HAZ. For a given repetition rate, the equivalent width of the HAZ increases with an increase in the average laser power. For a given average laser power, the equivalent width of HAZ increases with an increase in the repetition rate up to a threshold of 7 kHz. With a further increase in pulse repetition rate, the equivalent width of HAZ starts to decrease.
285
Acknowledgements The work reported here is funded by the Industry Department of the Government of the Hong Kong Special Administrative Region. The Project No. is AF/027/96.
References [1] B. Gu, J.M. Morrison, Cost effective small via generation—the laser solution, Multi-chip Module Conference, MCMC-95, Proceedings, IEEE, pp. 212–216. [2] E. Aoyama, H. Inoue, T. Hirogaki, H. Nobe, Y. Kitahara, T. Katayama, Study on small diameter drilling in GFRP, Comp. Struct. 32 (1995) 567–573. [3] P. Sheng, G. Chryssolouris, Comparison of surface quality improvement techniques for laser grooving of composite materials, PED-Vol. 64, Manufacturing Science and Engineering, ASME, New York, 1993, pp. 795–801. [4] A.A. Cenna, P. Mathew, Evaluation of cut quality of fibre-reinforced plastics: a review, Int. J. Mach. Tools Manuf. 37 (6) (1997) 723– 736. [5] H. Hocheng, C.T. Pan, Section area of heat affected zone in laser cutting of carbon fiber-reinforced PEEK, MD-Vol. 45, PED-Vol. 66, Machining of Advanced Composites, ASME, New York, 1993, pp. 153–165. [6] R. Srinivasan, B. Braren, Ultraviolet laser ablation of organic polymers, Chem. Rev. 89 (6) (1989) 1303–1316. [7] H.K. Tonshoff, R. Butje, W. Konig, J. Trasser, Excimer laser in material processing, Ann. CIRP 37 (1988) 681–684. [8] H. Endert, M. Scaggs, D. Basting, U. Stamm, New UV lasers for materials processing in industrial applications, Section A-ICALEO, 1997, pp. 187–196. [9] S. Takeno, M. Moriyasu, M. Kuzumoto, Laser drilling of epoxy-glass printed circuit boards, Section A-ICALEO, 1997, pp. 63–72. [10] T. Hirogaki, E. Aoyama, H. Inoue, T. Katayama, A study on surface quality in laser machining of GFRP for printed wiring board temperature and damage at workpiece in drilling, J. Jpn. Soc. Mater. Sci. 46 (11) (1997) 1329–1334. [11] X. Chen, A.L. Ortiz Jr., P. Randall Staver, W.T. Lotshaw, T.J. Rockstroh, M.H. Mclaughlin, Improved hole drilling using a high peak power Nd:YAG laser at the second harmonic wavelength, J. Laser Appl. 9 (1997) 287–290. [12] C.T. Pan, H. Hocheng, Prediction of laser-induced thermal damage of fibre Mat and fibre MatUD reinforced polymers, J. Mater. Eng. Perform. 7 (6) (1998) 751–756. [13] J. Mathew, G.L. Goswami, N. Ramakrishnan, N.K. Naik, Parametric studies on pulsed Nd:YAG laser cutting of carbon fibre reinforced plastic composites, J. Mater. Process. Technol. 89–90 (1999) 198– 203. [14] C.T. Pan, H. Hocheng, The anisotropic heat-affected zone in the laser grooving of fibre-reinforced composite material, J. Mater. Process. Technol. 62 (1996) 54–60.
A study of the heat-affected zone in the UV YAG laser drilling of GFRP materials K.C. Yung*, S.M. Mei, T.M. Yue Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Received 9 July 2001
Abstract This paper describes the characteristics of the heat-affected zone (HAZ) of a UV YAG laser-drilled hole in glass fiber reinforced plastic (GFRP) printed circuit boards. The structures of the HAZ produced by different laser parameters were analyzed. When drilled with lower power and repetition rate, a clear hole wall with very little black charred material was obtained. On the other hand, when drilled with high power and repetition rate, matrix recession and fiber protrusion were observed, also a loose coating was found covering the protruded glass fibers. The results also show that for a given repetition rate, the size of the HAZ increases with increase in average laser power. As for the effect of the pulse repetition rate, a peak value of HAZ width is reached at about 7 kHz for each given laser power, and beyond which the HAZ width decreases. A novel evaluation parameter, defined as the quotient of total area of the HAZ and the profile length, is suggested to quantify the size of the laser-induced HAZ in fiber reinforced composites. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Heat-affected zone; UV YAG laser; GFRP materials
1. Introduction The miniaturization of electronic products for information and communication, such as notebook computers and mobile phones, has led to increase in the circuit density of printed circuit boards (PCBs). This means that the holes which connect the isolated circuit layers in a multi-layered PCB, called via holes, have decreased in diameter from 0.3 to 0.1 mm or less. Mechanical drilling of these small holes in GRFP PCB becomes extremely difficult for many reasons. Firstly, conventional drilling results in excessive tool wear, tool breakage, and mechanically induced materials damage because of the abrasiveness associated with fiber reinforcement. Secondly, it is difficult to manufacture mechanical drills with a diameter of less than 0.2 mm. On the other hand, mechanical drilling systems can generate through-holes smaller than 0.2 mm, but at a cost that could be prohibitive [1]. It is also inapplicable to drill a great number of microvias (holes less than 0.05 mm in diameter) in high density PCB with mechanical drilling in view of PCB efficiency and reliability. Even if only one hole in production has failed to be drilled because of tool breakage, the entire PCB has to be scrapped. Moreover, tool breakage is one of the most common failures in small-hole drilling [2].
*
Corresponding author. Tel.: þ852-2766-6599; fax: þ852-2362-5267.
Being a non-contact process, laser drilling offers several advantages over mechanical drilling, such as the absence of tool wear, tool breakage and cutting-force-induced problems. Thus it has been used extensively to fabricate micro-vias in PCB fabrication. Examples of lasers used in industry are CO2 and Nd:YAG lasers, which are operated at infrared wavelength of 10.6 and 1.06 mm, respectively. However, the major drawback of laser drilling GFRP materials with these infrared lasers is the existence of a heataffected zone (HAZ) that is formed around the laser-drilled hole. This is because the material-removal mechanism with infrared lasers is a thermal mechanism (melting, evaporation, or vaporization). The differences in temperatures of vaporization and decomposition between glass fibers and epoxy resin in GFRP materials result in the presence of an HAZ, which always shows itself as recession and decomposition of the matrix material [3]. The formation of an HAZ in drilled micro-vias causes difficulty in copper plating and reduces the quality and reliability of the PCB. Researchers have made great efforts to reduce the thermal effect and hence the HAZ size in the laser drilling of GFRP [4,5]. One of the most important methods to reduce the thermal effects is to use UV lasers in the drilling of micro-vias. The high energy UV photons can directly atomize material in a process known as photo-ablation, which is associated with a photochemical mechanism rather than with a photo-thermal mechanism. Although there is enough evidence to
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 1 7 7 - 3
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
confirm that the thermal mechanism still accompanies the photochemical mechanism, photo-ablation does enhance the ability of UV lasers to produce micro-vias with good quality [6–8]. This paper aims to characterize the HAZ of UV YAG laser-drilled GFRP PCB material. Three aspects related to the HAZ are addressed in this paper: (i) the dependence of the HAZ on different laser parameters; (ii) the structure and forming mechanism of the HAZ; (iii) an evaluation method, named equivalent width, is defined to quantify the size of HAZ in fiber reinforced composites.
2. Experimental method The specimens used in the experiments were made of 1080 type GFRP PCB material. They were 1.6 mm thick laminated glass/epoxy composites clad on both sides with 18 mm copper foil. The specimens were cut into 25 mm 4 mm rectangles, and the copper cladding on the laser entrance side was removed to enable the study of laminated glass/epoxy composite materials independently of the copper cladding. The core of the 1080 PCB material was laminated with 8-layer woven fabrics of glass fiber and epoxy resin. The reinforcement was E-glass fiber with a diameter of 10 mm, which is composed of SiO2 (54 wt.%), Al2O3 (14 wt.%) and CaO (18 wt.%). An ESI 5150 laser drilling system was used in this study. The laser light was generated by a pulsed and lamp-pumped Nd:YAG laser with third harmonics generation, and was
279
operated at 355 nm. The output beam had a TEM00 (Gaussian) energy distribution, and the focused spot was of 25 mm diameter. The maximum average power of the 355 nm radiation is 1.2 W. All the laser drilled holes, having a diameter of 500 mm, were drilled using a spiral technique with an inner diameter of 25 mm, a radial pitch of 5 mm and a repetition of 50 times. Constant bite size, fixed at 3 mm, between the spots was used to enable all of the holes to be drilled with same number of laser pulses. All the experiments were carried out without any offset from the focal plane. In the experiments, the pulse repetition rate varied from 0.5 to 20 kHz, and the laser power varied from 0.1 W to the maximum average power allowed at that repetition rate. Cross-sections of the drilled holes were examined by an optical microscope and scanning electronic microscope (SEM). The drilled holes were filled with epoxy resin to prevent the section edges from subsiding during polishing. Electronic dispersive X-ray (EDX) was used for the analysis the components of the recast and char layer. An image analyzer was used to measure the size of the HAZ.
3. Structure of HAZ The experimental results indicate that the structure of the HAZ is influenced strongly by the laser parameters, such as the average laser power and the pulse repetition rate. Fig. 1 shows the morphology of the holes drilled with different laser parameters. It can be seen that charred material, shows
Fig. 1. SEM photographs of holes drilled with different laser parameters: (a) 0.3 W, 1 kHz; (b) 0.3 W, 6 kHz; (c) 1.0 W, 6 kHz.
280
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
Fig. 2. SEM photograph of a hole drilled with light parameters (0.3 W, 1 kHz).
itself as black color under the optical microscope, can be found at the hole wall when drilled with relatively high laser power or high pulse repetition rate. An HAZ with an irregular shape can also be observed around the hole when drilled with high power and high repetition rate, see Fig. 1(c), whereas, neither charred material nor a visible HAZ can be seen around the hole drilled with relatively low laser power and low pulse repetition rate, see Fig. 1(a).
Fig. 2 shows a higher magnification of Fig. 1(a). The wall of the drilled hole appears to be clear. Some of the glass fiber ends had melted and were fused with each other. The surface of epoxy resin is coated with a thin layer of some melted material, in which there are some unmelted solid grains. Fig. 3 shows a higher magnification of Fig. 1(b). It was found that a layer of charred material sticks on the ends of the glass fibers and the surface of the resin matrix, but the hole profile
Fig. 3. SEM photograph of a hole drilled with medium parameters (0.3 W, 6 kHz).
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
281
Fig. 4. SEM photograph of a hole drilled with heavy parameters (0.3 W, 6 kHz).
is almost straight, and no matrix recession and fiber protrusion can be found. Fig. 4 shows a higher magnification of Fig. 1(c). It is obvious that an irregular profile exists around the drilled hole. It can also been observed that the epoxy resin is absent from the HAZ, but glass fibers are left. Another notable feature observed in Fig. 1(c) and Fig. 4 is that the fibers protruding out of the epoxy resin are covered by charred material, which forms a black ‘‘shell’’ supported by the protruding fibers. Regions A and B in Fig. 4 are shown in Fig. 5(A) and (B), respectively. Resin matrix recession and fibers protrusion, the characteristics of HAZ in laser machining of GFRP, can be found in Fig. 5(a). From Fig. 5(a) and (b), caves are formed between the recessing epoxy and the ‘‘shell’’. This is because the vaporization temperature of epoxy resin is about 600 8C, at which glass fibers can only be melted. Therefore, the epoxy resin is vaporized into gas and decomposed to carbon or graphite, while the glass fibers are only melted. The vaporized or decomposed epoxy resin jets out of the hole. At the same time, the melted glass fiber material is expelled from the machined area together with the gas jet. Some of the melted material sticks on to the fibers near to the hole, while the graphite also is deposited on to the fibers, hence the ‘‘shell’’ is formed. The existence of graphite makes the ‘‘shell’’ and the coating on the hole wall appears to be black. As shown in Fig. 5(b), the ‘‘shell’’ has a poriferous and loose structure. Fig. 6 shows the EDX test results of glass fiber and ‘‘shell’’ material. The results of EDX tests show that the composition of the ‘‘shell’’ material is similar to a glassy phase, but rich in carbon atoms, which supports the formation mechanism of the ‘‘shell’’ suggested above.
4. HAZ evaluation Fig. 7 shows a cross-section of a laser-drilled hole of GFRP PCB material. The hole and cavities are filled with epoxy resin that shows up a little darker than the original epoxy, the boundary of the HAZ being easily identified. In order to evaluate the degree of heat damage, it is important to have a reasonable evaluation method to assess the size of the HAZ. The most common parameters to characterize the size of HAZ in the laser drilling of fiber reinforced composites are the width, thickness or depth [2,9–12]. The length of fibers protruding from the matrix is also used to assess the HAZ [13]. However, it is difficult and improper to assess the size of HAZs with irregular profiles, such as that shown in Fig. 7. Another parameter, called the section area, has been used to characterize the size of the HAZ [5,14]. While the section area provides an integral and average method to assess HAZs conveniently, it has problems when assessing the HAZs, as shown in Fig. 8. When taking into account only the integrated effect, the HAZ area of the three holes shown in Fig. 8 would be identical, although it may be argued that the HAZ of hole A is the same as that of hole B, but certainly not to that of hole C. Here a new evaluation parameter is defined to assess the size of the HAZ: We ¼
A L
(1)
where A is the total section area of the HAZ, and L the length of the profile of the section hole. Since We has the dimension of length, it can be defined as the equivalent width of the HAZ. This evaluation parameter can be used to quantify the
282
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
Fig. 5. SEM photographs of the HAZ with higher magnification: (a) region A in Fig. 4; (b) the region B in Fig. 4.
size of an HAZ in laser machining. The equivalent width of the HAZ in Fig. 7 can be calculated as We ¼
A1 þ A2 L1 þ L2
(2)
where A1 and A2 are the areas of the HAZs, and L1 and L2 are the profile lengths of the drilled hole shown in Fig. 7.
5. Dependence of the HAZ on laser parameters Due to the fact that HAZ is directly caused by overheating, all factors that may influence the amount of heat import to and export from the processing region will affect the size of the HAZ. The dependence of the HAZ on laser parameters is analyzed from these two points of view.
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
Fig. 6. EDX results of: (a) glass fibre; (b) ‘‘shell’’ material.
283
284
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
Fig. 7. SEM photograph of a drilled hole filled with epoxy resin.
Fig. 8. A sketch of holes with same section areas of HAZ.
Fig. 9. Effect of laser power on the equivalent width of the HAZ for different repetition rates.
Fig. 9 shows the effect of laser power on the equivalent width of HAZ for different pulse repetition rates. It can be observed that for a given pulse repetition rate, the equivalent width of the HAZ, We, increases as the average laser power. This is obvious because as more energy goes into the processing region, a higher processing temperature results, which accelerates the charring of the epoxy and the melting of the glass fibers. Comparing curves with different repetition rates, it is found that a high repetition rate results in a larger We.
Fig. 10. Effect of pulse repetition rate on the equivalent width of the HAZ for different laser powers.
K.C. Yung et al. / Journal of Materials Processing Technology 122 (2002) 278–285
The higher the repetition rate, the shorter time-lapse between the laser pulses and the shorter would be the cooling time for the processing surface. However, this trend exists only below 7 kHz. Fig. 10 shows the effect of pulse repetition rate on the size of the HAZ. Regardless of the laser power, the equivalent width is found to increase with an increase in repetition rate up to a threshold of 7 kHz, beyond which a decrease in HAZ width is found. This is believed to be due to a reduction in pulse energy as the repetition rate is increased, as a result the size of the HAZ being reduced. Furthermore, a higher repetition rate results in a longer pulse duration and a decrease in the peak power of the laser pulse. This drop of peak power will weaken the pulse and reduce the temperature on the machining surface. In a word, increasing the pulse repetition rate can increase the heat damage, and also can decrease it, depending on its relative value relative to a specific threshold of the repetition rate.
6. Conclusions 1. The structure of the HAZ is influenced strongly by the average laser power and the pulse repetition rate. When drilled with low power and low pulse repetition rate, the hole wall is found to be clear, and very little black charred material can be observed at the wall. On the other hand, when drilled with high laser power and high repetition rate, epoxy resin is vaporized and glass fibers are melted. Some of the melted material sticks on to the fibers near to the hole and forms a black coating with a poriferous and loose structure. The existence of this structure may worsen the quality and the reliability of copper plating over the hole wall. 2. An evaluation parameter, defined as the quotient of the total area of HAZ to the profile length, is suggested as a better parameter to be used to quantify the size of HAZ in the machining of fiber reinforced composites with lasers. 3. Laser power and pulse repetition rate are found to be important parameters that have a significant effect on the size of the HAZ. For a given repetition rate, the equivalent width of the HAZ increases with an increase in the average laser power. For a given average laser power, the equivalent width of HAZ increases with an increase in the repetition rate up to a threshold of 7 kHz. With a further increase in pulse repetition rate, the equivalent width of HAZ starts to decrease.
285
Acknowledgements The work reported here is funded by the Industry Department of the Government of the Hong Kong Special Administrative Region. The Project No. is AF/027/96.
References [1] B. Gu, J.M. Morrison, Cost effective small via generation—the laser solution, Multi-chip Module Conference, MCMC-95, Proceedings, IEEE, pp. 212–216. [2] E. Aoyama, H. Inoue, T. Hirogaki, H. Nobe, Y. Kitahara, T. Katayama, Study on small diameter drilling in GFRP, Comp. Struct. 32 (1995) 567–573. [3] P. Sheng, G. Chryssolouris, Comparison of surface quality improvement techniques for laser grooving of composite materials, PED-Vol. 64, Manufacturing Science and Engineering, ASME, New York, 1993, pp. 795–801. [4] A.A. Cenna, P. Mathew, Evaluation of cut quality of fibre-reinforced plastics: a review, Int. J. Mach. Tools Manuf. 37 (6) (1997) 723– 736. [5] H. Hocheng, C.T. Pan, Section area of heat affected zone in laser cutting of carbon fiber-reinforced PEEK, MD-Vol. 45, PED-Vol. 66, Machining of Advanced Composites, ASME, New York, 1993, pp. 153–165. [6] R. Srinivasan, B. Braren, Ultraviolet laser ablation of organic polymers, Chem. Rev. 89 (6) (1989) 1303–1316. [7] H.K. Tonshoff, R. Butje, W. Konig, J. Trasser, Excimer laser in material processing, Ann. CIRP 37 (1988) 681–684. [8] H. Endert, M. Scaggs, D. Basting, U. Stamm, New UV lasers for materials processing in industrial applications, Section A-ICALEO, 1997, pp. 187–196. [9] S. Takeno, M. Moriyasu, M. Kuzumoto, Laser drilling of epoxy-glass printed circuit boards, Section A-ICALEO, 1997, pp. 63–72. [10] T. Hirogaki, E. Aoyama, H. Inoue, T. Katayama, A study on surface quality in laser machining of GFRP for printed wiring board temperature and damage at workpiece in drilling, J. Jpn. Soc. Mater. Sci. 46 (11) (1997) 1329–1334. [11] X. Chen, A.L. Ortiz Jr., P. Randall Staver, W.T. Lotshaw, T.J. Rockstroh, M.H. Mclaughlin, Improved hole drilling using a high peak power Nd:YAG laser at the second harmonic wavelength, J. Laser Appl. 9 (1997) 287–290. [12] C.T. Pan, H. Hocheng, Prediction of laser-induced thermal damage of fibre Mat and fibre MatUD reinforced polymers, J. Mater. Eng. Perform. 7 (6) (1998) 751–756. [13] J. Mathew, G.L. Goswami, N. Ramakrishnan, N.K. Naik, Parametric studies on pulsed Nd:YAG laser cutting of carbon fibre reinforced plastic composites, J. Mater. Process. Technol. 89–90 (1999) 198– 203. [14] C.T. Pan, H. Hocheng, The anisotropic heat-affected zone in the laser grooving of fibre-reinforced composite material, J. Mater. Process. Technol. 62 (1996) 54–60.