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Formation of microvias in epoxy-glass composites by laser ablation
Formation of microvias in epoxyglass composites by laser ablation A. N. PARGELLIS,
D. T. W. AU, T. V. LAKE, A. KESTENBAUM
Single CO2 laser pulses, of 10.6 pm wavelength, are used to form blind microvias (holes in electronic boards for through-plating conducting paths) in copper-clad epoxy glass laminates. The microvia dimensions depend on pulse energy and duration, the thicknesses of the epoxy-glass laminate and copper cladding, and the distribution of glass within the epoxy-glass laminate. The useful range of laser parameters, especially pulse energy, is primarily determined by the ability to metallize subsequently the blind microvias. Several conclusions can be drawn from the data. The pulse energy should be within f20% of the optimum value in order to form vias with a cylindrical geometry. For 300 pm thick laminates, the thickness of the copper on the bottom should be 18 ,um or more. A larger range of pulse energies could be used if the glass fibre density was more uniform and if subsequent copper metallization of the blind vias could be improved. KEYWORDS: laser ablation, copper, printed circuit boards
Introduction
The width of interconnections on printed wiring boards (PWBs) is steadily decreasing. with minimum line widths of 75 pm proJected for I990 (see Ref. 1). In many cases, multilayer PWBs are required, and thousands of blind vias (holes in the boards) need to be drilled in the outer layers of each circuit. In order to maximize the available area for interconnections, the diameter of each microvia must be as small as possible’. However, the deposition rate of copper on the walls of the holes decreases with decreasing diamete$, indicating that a useful range for microvia diameters is 75 to 150pm. Laser drilling is an attractive alternative to mechanical drilling because there are no drill bits that can break or need to be replaced, and the hole dimensions can be readily altered by changing the pulse energy’. Two major problems with laser drilling have been hole diameter variations when drilling with fixed laser parameters, and overshooting buried circuitry pads. The hole diameter, and its uniformity, is dependent on glass !ibre density. The problem of drilling down to the next circuitry layer occurs if, due to substrate misalignment. the laser misses the buried conductor pad. This creates the possibility of an electrical short between two layers of circuitry after subsequent ANP. DlWA and TVL are at AT&T Bell Laboratories, Whippany, New Jersey 07981, USA. AK is at AT&T Sell Laboratories. Princeton, New Jersey 08540, USA. Received 5 June 1989. Revised 6 November 1989.
0030-3992/90/030205-03 Optics Vol22
Et Laser Technology No 3 1990
metallization. Therefore, accurate positioning of the substrate is more critical with laser drilling than with mechanical drilling. In this study, data are presented that demonstrate the feasibility of using single. CO1 laser pulses of 10.6pm wavelength to form blind vias in copperclad epoxy-glass laminates. For a given optical spot size, the hole profiles depend primarily on the energy per pulse and, to a much lesser extent, pulse length. Experimental
methods
and results
A CO? laser was operated with the following parameters: 10.6,um wavelength; pulse length less than 1 ms; pulse energy less than 1 J; repetition rate of 50 Hz; typical focused beam diameter, IOO~m. The laser output power was observed to fluctuate less than 10%. The substrate consisted of 4.3pm of copper cladding on top of a 300 pm thick epoxyglass laminate with I8 pm of copper cladding the bottom surface. The substrate was translated at a speed of 50 mm s-‘. with the laser drilling vias every 1 mm. Fi_eure 1 shows the effect of localized glass !ibre density on hole profile. The glass content varies since it is in the form of 1OO~m diameter bundles (visible in Fig. I), that are woven into a mat. The diameter of the hole at the top of the blind via is approximately constant at 93 pm whereas the diameter at the bottom varies from 43 to 135pm, depending on the quantity of glass encountered. The 0 1990
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Ltd
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i
I
0
I
2 Pulse
b Fig. 1 Effect of glass-fibre density on hole profile. The energy . . and duratton ot each pulse was trxed and the substrate was 4.3 pm of copper on 300 ym of epoxy-glass laminate with 18 pm on the bottom: (a) 180pm of glass; (b) 250~1m of glass
energy
3 (arbitrary
4 unrtsl
Fig. 2 Hole diameter (top and bottom) versus pulse energy. Substrate: 4.3,um copper on 300pm epoxy-glass laminate. (Open symbols: diameter of the via at the top: solid symbols: diameter of the via at the bottom). The solid and dashed lines are the best fit to the standard deviations of the hole diameters at the bottom and top of the vias respectively
profile, where the diameter at the bottom of the via is bigger than at the top. At pulse energies above 3.0 au, penetration of the lower 18 pm copper layer begins to occur. Below about 1.5 au, most vias have the bottom diameter less than 75 pm. Therefore, the window about the optimum pulse energy is about kO.5 au. or &IO%. Outside of this window it is difficult to metailize these vias, especially by electroless copper deposition where hydrogen gas is evolved.
top micrograph, Fig. la, shows the bell-shaped profile that results from a minimal amount of glass, occupying about 60% of the total pathlength. The bottom micrograph, Fig. lb. shows the resultant tapered profile when there is a large amount of glass, occupying about 85?/, of the total pathlength. The epoxy matrix decomposes more easily than the reinforcing glass-fibres. creating cavities and protruding glass tibre ends. It is possible to remove chemically the fibre ends but a more uniform distribution of glass within the epoxy matrix improves the uniformity of the hole profile.
Conclusions
Fig. 2 is a plot of the hole diameter versus pulse energy. The solid symbols and lines represent the average hole diameters and the best tit to the standard deviations of the hole diameters at the bottoms of the holes. The open symbols and dashed lines are for the data obtained for the top diameters of the holes. The vertical distance between each pair of lines represents the variation of hole diameters that is obtained for a fixed pulse energy. A minimum. threshold ener_q is required to expose the copper conductor at the bottom of the blind via. Above the threshold, the microvia dimensions at both the top and bottom of the via steadily increase with energy. When drilling 3OO~m thick laminates with a cladding layer of 3.3 pm of copper on the top. the optimum pulse ener_q is 2 au (arbitrary units). Above about 2.5 au. most vias have an ‘inverted’
A CO2 laser, of 10.6pm wavelength, has been successfully used to drill microvias in epoxy glass laminates with repetition rates of up to 50 holes per second. For 300,um thick laminates, clad with 3.3pm of copper on the top and lS,~m of copper on the bottom, an optimum pulse energy exists. The pulse energy must be within t20% of this optimum, primarily because of metallization considerations. This narrow operating window could be widened if some future developments are made in copper deposition and epoxy-glass laminate construction. The lower limit of 75pm for the hole diameter could be reduced if the subsequent metallization process was improved for vias with diameters below 1OOpm. The activation step during electroless deposition appears particularly difficult with trapped hydrogen gas preventing the transport of reactants to the
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bottom of the vias. The gas evolution also affects the upper limit in pulse energy because inverted hole profiles are formed, more readily trapping the gas. The local variation in glass fibre density affects the uniformity of laser ablated vias for all pulse energies. Although the variation may be acceptable, the processing window could be widened by using epoxy-glass laminates with a more even distribution of glass, such as is obtained using glass spheres or chaff.
preliminary studies with the CO2 laser and Joe Taylor was very helpful in drilling the holes in all of the samples discussed in this work.
References I 2
Circuit World 14(3), (1988) 31-36 3 Okada, K., Asano, T. ‘High complexity
Acknowledgements Richard Borutta and John Benko assisted in the
Glantz, E.J. ‘Small hole chemistry exchange’, A’nred Circ Fab. 12(2), (1989) 60-73 Kenney III, A.L., Dally, J.W. ‘Laser drilling of very small electronic via holes in common circuit board materials’,
4
multilayered PWB with heavy copper layers for computer system’, (Printed Circuit World Convention N, Tokyo, Japan, 2-5 June 1987) Murray, J. ‘Blind buried vias’, Circ Manuf (April 1988) 62-64
Articles for inclusion in future issues of Optics & Laser Technology
include:
Photonic information storage in NaCl crystals via F-G conversion K.R. Murali, Y. V. G.S. Murti Testing of printed circuit board solder joints by speckle correlation techniques M. Muramatsu, ].J Lunazzi Determination of the linear thermal expansion coefficient of long metallic bars by Murty shearing interferometer S.N. Stivastava, MS. Tomar, R.S. Kasana Focal region isophote diagram as a calculating chart FT.S. Yu, Yajun Li Experimental analysis of the y ray ionization effects of a dc discharge CO2 laser I.B. Couceiro, R.A.D. Zanon, YK. Huang, CA. Massone Determining the absorption and scattering coefficients of suspended particles: a feasibility study D.C. Look, Jr Digital speckle pattern interferometry applied to thermal strain measurements of metal-ceramic compounds P. Aswendt, R. Htrfling, W. 7btzauer Applications of a dielectric coating to semiconductor lasers T.R. Chen, Y. Zhuang, Y.J. Xu, P. Derry, N. Bar-Chaim, A. Yariv, B. Yu, Q.Z. Wang and Y.Q. Zhou
Optics Et Laser Vol22
Technology No 3 1990
207
D. T. W. AU, T. V. LAKE, A. KESTENBAUM
Single CO2 laser pulses, of 10.6 pm wavelength, are used to form blind microvias (holes in electronic boards for through-plating conducting paths) in copper-clad epoxy glass laminates. The microvia dimensions depend on pulse energy and duration, the thicknesses of the epoxy-glass laminate and copper cladding, and the distribution of glass within the epoxy-glass laminate. The useful range of laser parameters, especially pulse energy, is primarily determined by the ability to metallize subsequently the blind microvias. Several conclusions can be drawn from the data. The pulse energy should be within f20% of the optimum value in order to form vias with a cylindrical geometry. For 300 pm thick laminates, the thickness of the copper on the bottom should be 18 ,um or more. A larger range of pulse energies could be used if the glass fibre density was more uniform and if subsequent copper metallization of the blind vias could be improved. KEYWORDS: laser ablation, copper, printed circuit boards
Introduction
The width of interconnections on printed wiring boards (PWBs) is steadily decreasing. with minimum line widths of 75 pm proJected for I990 (see Ref. 1). In many cases, multilayer PWBs are required, and thousands of blind vias (holes in the boards) need to be drilled in the outer layers of each circuit. In order to maximize the available area for interconnections, the diameter of each microvia must be as small as possible’. However, the deposition rate of copper on the walls of the holes decreases with decreasing diamete$, indicating that a useful range for microvia diameters is 75 to 150pm. Laser drilling is an attractive alternative to mechanical drilling because there are no drill bits that can break or need to be replaced, and the hole dimensions can be readily altered by changing the pulse energy’. Two major problems with laser drilling have been hole diameter variations when drilling with fixed laser parameters, and overshooting buried circuitry pads. The hole diameter, and its uniformity, is dependent on glass !ibre density. The problem of drilling down to the next circuitry layer occurs if, due to substrate misalignment. the laser misses the buried conductor pad. This creates the possibility of an electrical short between two layers of circuitry after subsequent ANP. DlWA and TVL are at AT&T Bell Laboratories, Whippany, New Jersey 07981, USA. AK is at AT&T Sell Laboratories. Princeton, New Jersey 08540, USA. Received 5 June 1989. Revised 6 November 1989.
0030-3992/90/030205-03 Optics Vol22
Et Laser Technology No 3 1990
metallization. Therefore, accurate positioning of the substrate is more critical with laser drilling than with mechanical drilling. In this study, data are presented that demonstrate the feasibility of using single. CO1 laser pulses of 10.6pm wavelength to form blind vias in copperclad epoxy-glass laminates. For a given optical spot size, the hole profiles depend primarily on the energy per pulse and, to a much lesser extent, pulse length. Experimental
methods
and results
A CO? laser was operated with the following parameters: 10.6,um wavelength; pulse length less than 1 ms; pulse energy less than 1 J; repetition rate of 50 Hz; typical focused beam diameter, IOO~m. The laser output power was observed to fluctuate less than 10%. The substrate consisted of 4.3pm of copper cladding on top of a 300 pm thick epoxyglass laminate with I8 pm of copper cladding the bottom surface. The substrate was translated at a speed of 50 mm s-‘. with the laser drilling vias every 1 mm. Fi_eure 1 shows the effect of localized glass !ibre density on hole profile. The glass content varies since it is in the form of 1OO~m diameter bundles (visible in Fig. I), that are woven into a mat. The diameter of the hole at the top of the blind via is approximately constant at 93 pm whereas the diameter at the bottom varies from 43 to 135pm, depending on the quantity of glass encountered. The 0 1990
Butterworth-Heinemann
Ltd
205
i
I
0
I
2 Pulse
b Fig. 1 Effect of glass-fibre density on hole profile. The energy . . and duratton ot each pulse was trxed and the substrate was 4.3 pm of copper on 300 ym of epoxy-glass laminate with 18 pm on the bottom: (a) 180pm of glass; (b) 250~1m of glass
energy
3 (arbitrary
4 unrtsl
Fig. 2 Hole diameter (top and bottom) versus pulse energy. Substrate: 4.3,um copper on 300pm epoxy-glass laminate. (Open symbols: diameter of the via at the top: solid symbols: diameter of the via at the bottom). The solid and dashed lines are the best fit to the standard deviations of the hole diameters at the bottom and top of the vias respectively
profile, where the diameter at the bottom of the via is bigger than at the top. At pulse energies above 3.0 au, penetration of the lower 18 pm copper layer begins to occur. Below about 1.5 au, most vias have the bottom diameter less than 75 pm. Therefore, the window about the optimum pulse energy is about kO.5 au. or &IO%. Outside of this window it is difficult to metailize these vias, especially by electroless copper deposition where hydrogen gas is evolved.
top micrograph, Fig. la, shows the bell-shaped profile that results from a minimal amount of glass, occupying about 60% of the total pathlength. The bottom micrograph, Fig. lb. shows the resultant tapered profile when there is a large amount of glass, occupying about 85?/, of the total pathlength. The epoxy matrix decomposes more easily than the reinforcing glass-fibres. creating cavities and protruding glass tibre ends. It is possible to remove chemically the fibre ends but a more uniform distribution of glass within the epoxy matrix improves the uniformity of the hole profile.
Conclusions
Fig. 2 is a plot of the hole diameter versus pulse energy. The solid symbols and lines represent the average hole diameters and the best tit to the standard deviations of the hole diameters at the bottoms of the holes. The open symbols and dashed lines are for the data obtained for the top diameters of the holes. The vertical distance between each pair of lines represents the variation of hole diameters that is obtained for a fixed pulse energy. A minimum. threshold ener_q is required to expose the copper conductor at the bottom of the blind via. Above the threshold, the microvia dimensions at both the top and bottom of the via steadily increase with energy. When drilling 3OO~m thick laminates with a cladding layer of 3.3 pm of copper on the top. the optimum pulse ener_q is 2 au (arbitrary units). Above about 2.5 au. most vias have an ‘inverted’
A CO2 laser, of 10.6pm wavelength, has been successfully used to drill microvias in epoxy glass laminates with repetition rates of up to 50 holes per second. For 300,um thick laminates, clad with 3.3pm of copper on the top and lS,~m of copper on the bottom, an optimum pulse energy exists. The pulse energy must be within t20% of this optimum, primarily because of metallization considerations. This narrow operating window could be widened if some future developments are made in copper deposition and epoxy-glass laminate construction. The lower limit of 75pm for the hole diameter could be reduced if the subsequent metallization process was improved for vias with diameters below 1OOpm. The activation step during electroless deposition appears particularly difficult with trapped hydrogen gas preventing the transport of reactants to the
206
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5
bottom of the vias. The gas evolution also affects the upper limit in pulse energy because inverted hole profiles are formed, more readily trapping the gas. The local variation in glass fibre density affects the uniformity of laser ablated vias for all pulse energies. Although the variation may be acceptable, the processing window could be widened by using epoxy-glass laminates with a more even distribution of glass, such as is obtained using glass spheres or chaff.
preliminary studies with the CO2 laser and Joe Taylor was very helpful in drilling the holes in all of the samples discussed in this work.
References I 2
Circuit World 14(3), (1988) 31-36 3 Okada, K., Asano, T. ‘High complexity
Acknowledgements Richard Borutta and John Benko assisted in the
Glantz, E.J. ‘Small hole chemistry exchange’, A’nred Circ Fab. 12(2), (1989) 60-73 Kenney III, A.L., Dally, J.W. ‘Laser drilling of very small electronic via holes in common circuit board materials’,
4
multilayered PWB with heavy copper layers for computer system’, (Printed Circuit World Convention N, Tokyo, Japan, 2-5 June 1987) Murray, J. ‘Blind buried vias’, Circ Manuf (April 1988) 62-64
Articles for inclusion in future issues of Optics & Laser Technology
include:
Photonic information storage in NaCl crystals via F-G conversion K.R. Murali, Y. V. G.S. Murti Testing of printed circuit board solder joints by speckle correlation techniques M. Muramatsu, ].J Lunazzi Determination of the linear thermal expansion coefficient of long metallic bars by Murty shearing interferometer S.N. Stivastava, MS. Tomar, R.S. Kasana Focal region isophote diagram as a calculating chart FT.S. Yu, Yajun Li Experimental analysis of the y ray ionization effects of a dc discharge CO2 laser I.B. Couceiro, R.A.D. Zanon, YK. Huang, CA. Massone Determining the absorption and scattering coefficients of suspended particles: a feasibility study D.C. Look, Jr Digital speckle pattern interferometry applied to thermal strain measurements of metal-ceramic compounds P. Aswendt, R. Htrfling, W. 7btzauer Applications of a dielectric coating to semiconductor lasers T.R. Chen, Y. Zhuang, Y.J. Xu, P. Derry, N. Bar-Chaim, A. Yariv, B. Yu, Q.Z. Wang and Y.Q. Zhou
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Technology No 3 1990
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