355 nm Nd:YAG laser ablation of polyimide and its thermal effect

Transcription

1 Journal of Materials Processing Technology 101 (2000) 306± nm Nd:YAG laser ablation of polyimide and its thermal effect Winco K.C. Yung *, J.S. Liu, H.C. Man, T.M. Yue Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Hung Hum, Kowloon, Hong Kong Received 21 June 1999 Abstract Ablation results of polyimide (PI) by a single pulse of an acoustic±optical Q-switched Nd:YAG laser at 355 nm are reported for the rst time and are compared with the results for RCC 1 (resin coated copper, trade mark of Allied Signal) and PI at other wavelengths. The difference of the ablation depth of PI from that of RCC 1 is small for a same uence ranging from 3 to 184 J cm 2, especially the ablation depth of the two materials is nearly the same for a same uence ranging from 5 to 100 J cm 2, although the ablation threshold of the former is much lower than that of the latter, which are 0.1 and 0.7 J cm 2, respectively. Comparing the ablation results of PI at 355 nm with those at shorter wavelengths, both the threshold and slope of the ablation rate against the uence are higher than those at shorter wavelengths. At the same time, melting is evident in the process. The temperature pro le and temperature change with time caused by the laser at different wavelengths and different pulse frequencies are calculated, the calculated results being in agreement with the experimental studies reported by previous authors. It is shown that the advantage of the small thermal effect occurring in the acoustic±optical Q-switched laser ablation is offset signi cantly because of a small absorption coef cient for laser energy, although the ablation rates can be increased at a relatively high power density. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Laser ablation; Polyimide; RCC (r) 1. Introduction Laser drilling microvias in high density printed circuit boards has increased rapidly in the last 4 years and is becoming the dominant technology in some countries [1]. The main laser sources used in the PCB drilling include the CO 2 laser, the excimer laser, and the 266 and 355 nm Nd:YAG laser. The main issues in the laser drilling of PCBs are the ablation rate and the thermal effect caused by the laser energy, which can result in volcano-shaped delamination at the copper±dielectric interface [2]. Some measures, including reducing the pulse width, choosing an appropriate laser wavelength and making the copper layer more massive, can reduce the delamination to some extent. For example, delamination can be avoided if the thickness of the copper is greater than 36 mm for a 1.06 mm wavelength YAG laser where the pulse width is 0.2 ms [3], or if the thickness was as small as 9 mm for a 10.6 mm wavelength CO 2 laser where the pulse width is between 65 and 250 ms [4]. However, whilst these measures improve hole quality, they also make the * Corresponding author. address: mfkcyung@inet.polyu.edu.hk (W.K.C. Yung) ablation rate and throughput decrease: a compromise has to be made to give consideration to both hole quality and productivity. Although extensive studies on the ablation of polyimide using different UV wavelength lasers have been carried out, including a 193 nm, and a 248 nm laser for a uence below 10 J cm 2 [5±8], a 308 nm for a uence below 100 J cm 2 [9,10] and a 351 nm laser for a uence below 0.16 J cm 2 [8], investigation on acoustic±optical Q-switched 355 nm YAG laser ablation of polyimide (PI) has not been reported previously. Although it is claimed that the laser drilling of polymers of PCB at 355 nm is a photochemical process, increasingly more studies [10±12] have shown that the thermal effect cannot be neglected for the UV laser ablation of polymers. Even for deep UV laser ablation of PI, which is thought to be dominated by the photochemical effect [8], the surface temperature can be as high as 1600 K [11] when the laser power is at threshold level and the laser wavelength is at 193 nm. At the same time, nearly all of the laser energy is transformed into the heat for 308 nm laser ablation of polymethyl methacrylate (PMMA) [8]. It is believed that for the 355 nm laser ablation of polymers, the thermal effect should not be ignored also /00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S (00)

2 W.K.C. Yung et al. / Journal of Materials Processing Technology 101 (2000) 306± In this paper, the morphology of the ablated crater of PI is analyzed with a conventional light microscope, a scanning electron microscope (SEM) and an atomic force microscope (AFM). The diameter and the depth of laser ablation of PI by a single acoustic±optical Q-switched pulse at 355 nm are reported for the rst time. The temperature pro les caused by laser pulses of different pulse repetition rates at the ablation threshold power density were calculated. The effects of the absorption coef cient on the temperature pro le were analyzed. 2. Experimental The experimental method is the same as that reported earlier [13]. The Nd:YAG laser is frequency-tripled and is operated at 355 nm. Because of the frequency conversion process, the output beam is oval, of about 4:5, and with small satellite spots. The energy of these `side lobes' is about a few percent of that of the main spot. Fig. 1a±c shows the SEM (magni ed 500 times) images of ablated PI by one single pulse of 13 and 494 mj, and the image (magni ed 250 times) by a pulse of 494 mj, respectively. Figs. 2 and 3 show an image of a crater that was ablated by a single-shot pulse of 494 mj taken by a conventional light microscope and an image ablated by a pulse of 211 mj taken by an AFM, respectively. The plain part at the bottom of the AFM graph indicates that the crater depth is out of the measurable range of the microscope. It was shown that the morphology is signi cantly different from that of ablated RCC 1 reported previously by Yung et al. [13]. A melting zone exists at the edge of ablated crater of the PI, whereas at the edge of the crater of RCC 1 there are stalagmitic residues. There are some ripples appearing at bottom of the craters that was ablated by a laser beam with relatively high uence. The melting residues and stalagmitic residues can be seen on the Fig. 2. Conventional light micrograph of PI ablated by a single pulse of 494 mj at 355 nm. surface of polymers ablated at other power levels. The results indicate that the thermal effect in the process of 355 nm laser drilling of PI cannot be ignored. Figs. 4 and 5 show the ablation depth and diameter of PI and RCC 1, respectively, for different levels of uence. The pulse repetition rates that were used to ablate PI and RCC 1 were and khz, and the corresponding pulse widths (full width between half maximum, FWHM) are 21 and 26 ns, respectively. It can be seen from these gures that the difference of the ablation depth between PI and RCC 1 is small when the laser uence is the same and within the range 3±184 J cm 2. Especially, when the uence ranges from 5 to 100 J cm 2, there is almost no difference between the ablation depth for the two materials. However, the ablation diameter of PI is much larger than that of RCC 1, which indicates that the ablation threshold of PI is much lower than that of RCC 1. Extrapolating the curve of ablation depth, the same conclusion (that PI has a lower ablation threshold) can be obtained. Fig. 1. SEM of PI ablated by a single pulse of: (a) 13 mj (500); (b) 494 mj (500); (c) 494 mj (250) at 355 nm. Fig. 3. AFM of PI ablated by a single pulse of 211 mj at 355 nm.

3 308 W.K.C. Yung et al. / Journal of Materials Processing Technology 101 (2000) 306±311 Fig. 4. Ablation depth of PI and RCC 1, where the PI repetition rate is 1 khz and the pulse width (FWHM) is 21 ns, and the RCC 1 repetition rate is khz and the pulse width (FWHM) is 26 ns. Table 1 gives the ablation rates and threshold uence of PI at 355 nm and other wavelengths obtained in previous studies [5±10,14,15]. It can be seen from the table that the ablation rate of polyimide at 355 nm is much higher than that at shorter wavelengths. For example, when the uence is 3 J cm 2, the ablation depths at 193, 248, 308 and 355 nm are 0.25, 0.6, 1.2 and 2 mm, respectively. Further, the ablation threshold of 0.1 J cm 2 or 4.8 MW cm 2 at 355 nm is higher than the threshold at 193, 248 and 308 nm which is from 0.02 to 0.07 J cm Temperature calculation When the laser power density is below the threshold, all of the absorbed laser energy will be converted into heat [8,16]. Experimental measurements and simulation calculations show that the peak surface temperature of PI can be as high as 1660 K when the laser wavelength is 193 nm and the threshold power density is 36 mj cm 2 [11]. To estimate the thermal effect caused by the 355 nm laser ablation of PI, the temperature eld is calculated. The properties of the polyimide are shown in Table 2 [11]. For an acoustic±optical Q-switched pulse, the full temporal width of the laser pulse is p generally less than 200 ns. The heat diffusion distance 4kt is therefore no more than Fig. 5. Ablation diameter of PI and RCC 1, where the PI repetition rate is 1 khz and the pulse width (FWHM) is 21 ns, and the RCC 1 repetition rate is khz, and the pulse width (FWHM) is 26 ns. 0.5 mm, which is approximately equal to the optical penetration depth a 1. Since both of the dimensions are much smaller than the diameter of a focused laser beam and the thickness of a polymer lm of PCB, the heat-conduction equation can be linearized and the ablated polymer can be regarded as a semi-in nite body. When the initial temperatures are a uniform eld, the temperature increment of different depths x at the time t can be given by an analytic solution [17]: DT x; t ˆ1 R Ka I 0f x; t (1) where R is the re ectivity of the polymer to the laser beam, and f(x, t) is! f x; t ˆ2a kt 1=2 x ierfc exp ax 2 kt 1=2 " # 1 2 exp ka2 t ax erfc a kt 1=2 x 2 kt 1=2 " # 1 2 exp ka2 t ax erfc a kt 1=2 x 2 kt 1=2 (2) Table 1 Comparison of ablation rate of polyimide at different wavelength l (nm) 193 [5±10] 248 [14] 308 [15] 355 Fluence (J cm 2 ) 0.02± ± ±3.0± ±184.0 Ablation depth (mm per pulse) 0±0.3 0±0.6 0±1.2±3 2±14 Threshold fluence (J cm 2 ) ± ±

4 W.K.C. Yung et al. / Journal of Materials Processing Technology 101 (2000) 306± Table 2 Properties of polyimide [11] Absorption coefficient a at 355 nm (cm 1 ) Thermal conductivity K (W cm 1 K 1 ) Thermal diffusivity k (cm 2 s 1 ) Heat capacity c (J g 1 K 1 ) Density r (g cm 3 ) Polyimide K/rc T T where ierfc is the integral of the error function erfc: ierfc y ˆ Z 1 erfc y ˆp 2 p y Z 1 erfc s ds (3) y e s2 ds (4) A temperature eld caused by a pulse with a general shape (refer to Fig. 6) can be calculated through synthesizing the pulse shape from a linear combination of step-on, step-off Heaviside functions [10]. The temperature pro le would be DT x; t ˆ t Noff ˆ1 R X N I 0 t ioff f x; t ioff Ka iˆ1 I 0 t ion f x; t ion Š; 0 < t < t (5) DT x; t ˆ1 R X N I 0 t ion f x; t t ion Ka iˆ1 I 0 t ioff f x; t t ioff Š; t < t < 1 (6) f Table 3 shows the temperature increment at xˆ0, 1, 2, 3, 4 mm, which are caused by a single pulse with threshold uence at different wavelengths of 248, 308 and 355 nm. It is necessary to note that the calculated surface temperature increment of 13558C at 248 nm is in agreement with the experimental study reported previously, which is 1660 K [11]. It can be seen from Table 2 that the temperature increment in the substrate caused by the 355 nm laser is larger than that by a shorter wavelength laser, although the surface temperature of the former is lower, the reason being that the polymer has a smaller absorption coef cient for longer wavelengths. Another effect of a small absorption coef cient of the polymer to the laser beam is that it increases the thermal damage in the process. When opaque materials with a large Fig. 6. Pulse shape synthesizes from a linear combination of step-on, stepoff Heaviside functions. absorption coef cient is ablated by a Q-switched pulse laser, the ablated surface quality can be satisfactory and little thermal damage occurs because of its short pulse width and small duty ratio. However, for those materials with a small absorption coef cient, it makes the material more dif cult to cool down before the start of the next pulse. Figs. 7±9 show that the in uences of the absorption coef cient a on the surface temperature change from the rst pulse to the beginning of the second pulse at different repetition rates of 1, 10 and 20 khz. As the repetition rate increases, the pulse width and the duty ratio increase, and the temperature at the beginning of the next pulse is greater. For an absorption coef cient of cm 1, the temperature increments at the beginning of the second pulse are 35, 108 and 149 K, respectively, when the pulse repetition rate is 1, 10 and 20 khz, and they are 13, 40 and 57 K for an absorption coef cient of cm 1. Under the action of multipulses, the polymer is repeatedly heated and cooled, i.e., a small absorption coef cient will cause an increase in the ablation threshold, the slope of ablation rate against the uence, and also in the thermal damage to the substrate. It is Table 3 Calculated temperature increment at different depths caused by a laser beam with threshold uence at different wavelengths l (mm) Fluence (mj cm 2 ) Full width of pulse (ns) a (cm 1 ) DT xˆ0 mm C DT xˆ1 mm C DT xˆ2 mm C ± ± 23.8 DT xˆ3 mm C _ ± 3.3 DT xˆ4 mm C ± ± 0.5

5 310 W.K.C. Yung et al. / Journal of Materials Processing Technology 101 (2000) 306±311 therefore the copper layer is not so massive, a big absorption coef cient is expected in order to decrease the thermal effect. However, this will make the ablation rate at relatively high laser power density range not so high. A further quantitative study is needed in order to be able to choose optimal materials properties and laser ablation parameters based on the required hole size and construction of the Cu and the dielectric. 4. Conclusions Fig. 7. Temperature changes from the beginning of the rst pulse to the beginning of the second pulse (pulse full width 43 ns; frequency 1 khz). Fig. 8. Temperature changes from the beginning of the rst pulse to the beginning of the second pulse (pulse full width 143 ns; frequency 10 khz). 1. The difference between the ablation depth of PI and that of RCC 1 is small for the same uence ranging from 3 to 184 J cm 2 : especially, they are nearly the same at the same uence ranging from 5 to 100 J cm 2, although the ablation threshold of the former is much lower than that of the latter, which are 0.1 and 0.7 J cm 2, respectively. 2. Comparing the ablation results of PI by 355 nm laser with those at shorter wavelengths, both the threshold and slope of the ablation rate against the uence are higher at 355 nm than those at shorter wavelengths. The ablation rate can be as high as 2±14 mm for a uence range from 3 to 184 J cm Evident melting occurs in the process of 355 nm laser ablation of PI, and indicates that a photothermal mechanism has contributed to the ablation process. 4. Compared with the absorption coef cient of PI at 248 nm, the absorption coef cient at 355 nm is smaller by about an order of magnitude. It is shown that the advantage of a small thermal effect occurring in the acoustic±optical Q-switched laser ablation is offset signi cantly because of the smaller absorption coef cient, although the ablation rates can be increased at a relatively high power density range. Acknowledgements The work reported in this paper is funded by the Industry Department of the Government of the Hong Kong Special Administrative Region. The project No. is AF/027/96. The authors would like to acknowledge the measurement performed by Ms. Sandy Suet To, Department of Manufacturing Engineering, Ultra-Precision Machining Centre, The Hong Kong Polytechnic University. References Fig. 9. Temperature changes from the beginning of the rst pulse to the beginning of the second pulse (pulse full width 257 ns; frequency 20 khz). [1] A. Beuhler, A. Tungare, J. Savic, Development of an HDI photovia process for portable products, Circuit World 24 (4) (1998) 36±39. [2] M. Owen, E. Roelants, J. Van Puymbroeck, Laser drilling of blind holes in FR4/glass, Circuit World 24 (1) (1997) 45±49. [3] M.N. Watson, Laser drilling of printed circuit boards, Circuit World 11(1) (1984) 13±17, 29.

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