INVESTIGATING PCB PROCESSING USING Q-SWITCHED DPSS NANOSECOND GREEN LASER Paper# M1306 Ashwini Tamhankar and Rajesh Patel Spectra-Physics Lasers, Newport Corporation, 3635 Peterson Way, Santa Clara, CA 95054, USA Abstract Diode pumped solid state (DPSS) Q-switched nanosecond pulsed lasers are becoming the laser of choice for a wide variety of processes in printed circuit board (PCB) manufacturing. Lasers are being used today in PCB manufacturing for via drilling, depaneling, profiling (cutting), laser direct imaging (LDI), repair, trimming, marking, skiving and other processes. The majority of these processes are practiced today using a 10.6 μm CO 2 or 355nm ultraviolet (UV) laser. In this study we have investigated the use of our new generation of Q- switched DPSS nanosecond pulsed 532 nm green laser for PCB processing. We demonstrate this green laser is a good candidate for processing some thin PCBs. Introduction The growing trend towards miniaturization and cost reduction in the production of electronic products and devices has led to the ever increasing demand for laser micromachining applications. With innovations in laser technology as nanosecond pulsed Q- switched laser sources become more compact, robust, reliable and affordable more micromachining applications have become commercially viable. One such example is the use of lasers in the microelectronics industry, especially for printed circuit board (PCB) manufacturing processes. The microelectronics industry is in constant search for innovative laser sources that are compact, lightweight, and cost effective solutions for manufacturing of advanced electronic packages. The driving factor for the use of laser technology as a solution is its ability to process small, high-precision features in a non-contact manner at high speed in a cost effective way. Lasers are routinely used today in a variety of PCB manufacturing processes including via drilling, depaneling, profiling (cutting), laser direct imaging (LDI), repair, trimming, marking, and skiving processes. a) b) Figure 1: Example of rigid PCB, a) Rigid printed circuit board, b) Schematic cross-section of rigid PCB showing 1) blind via, 2) through via a) b) Copper Solder mask 1) Copper 1) 2) Epoxy or glass fiber Figure 2: Example of flexible PCB, a) Flexible circuit board, b) Schematic cross-section of flexible PCB showing 1) blind via, 2) through via Figures 1 and 2 show the two primary types of PCB structures used in industry today rigid and flexible. Rigid PCB s usually have laminated epoxy or glass fiber material in between copper layers whereas 2) Polyimide
flexible PCB s have polyimide or some other softer dielectric material in between copper layers. Considering the variation in PCB materials, types, and thicknesses, there are various criteria involved in selecting a laser for these processes [1]. A laser wavelength that is efficiently absorbed in the material results in cleaner machining quality [2]. PCB material processing has been investigated and practiced with various laser sources such as, CO 2, excimer, and DPSS Q-switched lasers with wavelengths in infrared (IR) and ultraviolet (UV) region [1-9]. The choice of laser wavelength is primarily determined by the material to be processed, size of the features to be machined, and the quality requirements. In general, UV lasers offer various advantages over IR lasers such as creating smaller features and cleaner processing with less thermal degradation. As a result, 355 nm DPSS nanosecond lasers are being commonly used in various higher density PCB micromachining laser systems. With continued pressure to reduce manufacturing cost of PCB s, cheaper alternatives to current processes are always desirable. While the use of UV wavelength lasers for PCB processing has met the need for creating smaller features with high quality, a high performance 532 nm wavelength laser might in some cases provide sufficiently high quality results at a lower cost. The limited research conducted so far on PCB material processing using 532 nm DPSS nanosecond lasers has shown promising results on thin flexible PCB materials. However, research has been somewhat limited to the case of a thin (~40 μm) stack of copper, polyimide, copper, PCB material [9]. In an effort to more fully understand the potential of 532- nm Q-switched DPSS lasers for PCB applications, we have extended the research to include various rigid and flexible PCB types that are made of copper, polyimide, epoxy, and glass fiber materials of various thicknesses. For these materials, we have explored a variety of PCB manufacturing processes including via drilling, profiling, cutting, and depaneling. experiments. Several key specifications of this laser are outlined in Table 1. Table 1. Specifications of the Spectra Physics laser system used in the experiments. Parameters Mosaic 532-11 Wavelength 532 nm Average Power >11 W at 50 khz Peak Power ~22 kw Pulse Repetition Frequency 0-500 khz Pulse Width <15 ns M 2 <1.2 The Mosaic 532-11 laser is an innovative new product platform and is unique compared to a typical 532 nm DPSS Q-switched laser. It has an integrated design in which both the laser head and power supply are combined into a single package, making it a very compact laser that is simple to integrate into any machine tool. Another attribute of the Mosaic 532-11 laser is that it outputs a high average power over a very large range of pulse repetition frequencies (PRFs), while at the same time maintaining a low pulse energy variation (typically below 3%, 1σ) and near diffraction limited beam quality. This capability of the Mosaic laser to deliver high output power and precise energy pulses even at a high PRF was helpful in exploring the optimized parameters for different PCB material types. Optical Setup Galvo scan head Laser Experimental Details The sample materials used in this study consisted of several types of thin-rigid and flexible PCB materials with thicknesses in the range of 100 μm to 300 μm that are commonly used in manufacturing of microelectronics devices. Laser System A Spectra-Physics Mosaic TM 532-11 DPSS Q- switched pulsed green laser was used in the Figure 3: Experimental optical setup for PCB micromachining. A schematic representation of the optical setup used for the experiments is shown in Figure 3. It consists of a scanning galvanometer system (SCANLAB HurrySCAN II 10) with a 163 mm focal length f- theta lens to generate ~10 to 15 µm (1/e 2 ) theoretically calculated focal spot on sample. The
galvo scanner is integrated with Newport Corporation s linear motorized XYZ positioning stage system (Newport Corporation IMS series stages and XPS motion controller). The motion system facilitates sample positioning under the galvo scanner for step and scan processing; and the highprecision Z-axis stage allows stable and precise focal plane positioning onto the material s surface. Experimental Procedure The PCB material was secured to a flat surface of the motion stages to maintain the focal plane over the scanning area. Laser parameters such as focus spot size, PRF, pulse energy, scan speed, and number of scans was varied to achieve optimum process quality on a variety of PCB materials. Compressed air assist gas was used to facilitate debris removal from the work piece during the processing of some materials. Post processing techniques such as acetone swab and/or ultrasonic water bath cleaning were also sometimes used. of PCB. The metal plated vias form electrical connections between the layers on PCB materials and electronic components such as chips, transistors, resistors, etc. mounted on PCB. In our experiment, the sample drilled was a 180 μm thick FPCB consisting of polyimide-copperpolyimide with acrylic adhesive in between. The laser was operated at 20 khz, 50 khz and 100 khz PRFs where the average power and pulse-to-pulse energy stability remains constant, while the pulse energy decreases with the increase in PRF. Percussion drilling technique was used with multiple pulses to drill ~50 μm target diameter through vias in the material as shown in Figure 4. Analysis of the processed samples including depth measurements was performed using an optical microscope integrated with a camera system. Results and Discussion The primary goal of the study was to determine the suitability of the 532-nm laser for various PCB processes and materials. Two different types of PCB materials were processed flexible PCBs and thinrigid PCBs. Flexible PCB (FPCB) Today FPCBs are used as a packaging solution in addition to being used as an interconnect device from one circuit board to another. Depending on the applications, FPCB is available in a variety of material compositions to form single-sided, doublesided, and rigid-flex circuits. Most common FPCB structures consist of a copper conductor and a polyimide insulator, along with adhesive binding layers. In this study we have explored two different FPCB materials for drilling and profiling/cutting processes. Laser Drilling The multilayered PCB requires the ability to connect one layer of metal to another. This is achieved by drilling micron-size holes ( vias ) through PCB layers. As shown in Figures 1 and 2, vias drilled through all the layers of a PCB are called through vias; whereas those drilled through selective layers of PCB are referred to as blind vias. The walls of vias are then copper plated to connect metal layers (c) Figure 4: Microscope picture shows 50 μm target diameter through vias drilled in 180 μm thick polyimide-copper-polyimide FPCB sample at various PRFs 20 khz (385 μj) at 1.75 ms/hole 50 khz (204 μj) at 0.9 ms/hole (c) 100 khz (102 μj) at 0.65 ms/hole
For this particular FPCB material combination, the copper layer strongly absorbs 532 nm wavelength and couples the energy more efficiently into the neighboring polyimide material resulting in a good quality through vias. The data shows that hole size and the drill rate achieved depends on the pulse energy and corresponding PRF. At 100 khz PRF, increased HAZ is observed, perhaps due to heat accumulation at the higher PRF, the relatively longer pulse duration that is coincident with the higher PRF, or some combination thereof. A good compromise between hole quality and throughput is achieved at 50 khz PRF with ~200 μj energy. With these settings, 65 μm entrance and 35 μm exit holes were drilled at the rate of <1 ms/hole. Small amount of splatter observed around the entrance side of the holes can be easily wiped clean with acetone swab followed by an ultrasonic water bath cleaning techniques. Pre-cleaned and postcleaned entrance holes quality is shown in Figure 5 below. Figure 5: Microscope picture showing 65 μm via from Figure 4 b) at 50 khz PRF, Pre-cleaned, Post-cleaned with acetone swab and ultrasonic water bath techniques Laser Profiling/Cutting Profiling is the final step in PCB manufacturing where individual PCBs are singulated from the large production panels. Laser micromachining is one of the technologies used to cut around a predetermined pattern on the printed circuit board to separate individual PCBs. To explore the feasibility of Mosaic 532 nm laser for profiling/cutting process, two different PCB materials were used: A) 180 μm thick FPCB layered structure of polyimide-copper polyimide with acrylic adhesive between layers, and B) 160 μm thick all-insulator FPCB structure of polyimide-polyimide with acrylic adhesive in between. Figure 6: Microscope picture showing 5 mm circular trepan cuts in FPCB processed at 20 khz PRF, Entrance side Exit side Using the Mosaic laser, 5 mm circles were trepan cut in both FPCB materials using multiple passes with an average (i.e. "processing ) speed of 11.5 mm/s. The laser was operated at low PRF of 20 khz, chosen as optimized setting, where ~390 μj pulse energy was available at the work piece. Debris generated at the entrance and exit sides of the cuts were easily cleaned using acetone swab. As seen from Figure 6, good quality cuts were achieved on both FPCB materials. This result demonstrates that this 532 nm wavelength laser can be suitable for profiling/cutting of FPCB material consisting of polyimide and copper materials. Thin Rigid PCB A A 50 um B A 50 um Rigid PCBs are the easiest to handle in routine electronic assembly. The demand for light weight, compact electronic components necessitates the use of thinner rigid PCB substrates. In this paper we have used thin rigid PCBs to demonstrate depaneling and via drilling processes. Laser Depaneling Depaneling is a process used in high-volume electronics assembly production where a large PCB panel is cut apart or depanelled into individual PCBs. B B In recent years, laser technology is increasingly preferred over traditional mechanical router technology for the depaneling process. However,
laser interaction with the PCB materials like resins or glass fibers can have a tendency to generate high levels of undesirable carbonization. The amount of carbonization primarily depends on the wavelength and pulse width of laser used for processing. IR, green or UV wavelength lasers are often used for this application, depending on quality requirements. In general, IR lasers have higher average power and can result in faster speeds. However, IR laser processing can leave behind heavily carbonized edges. UV lasers have comparatively lower average powers, hence slower speeds but result in better cut quality with reduced carbonization. A 532 nm laser may be a good compromise in terms of power and throughput. If 532 nm laser processing could result in acceptable quality cuts, then they would be an attractive option for thin rigid PCB depaneling applications. copper or stiffening material. Operating the laser at high PRF of 150 khz with 64 μj energy was found to be optimal to achieve good quality cuts and low carbonization. Multiple scans were used to achieve higher throughput, resulting in an average cutting speed of 50 mm/s. Top view of the depaneled portion of PCB in Figure 7 c) shows high cut quality without HAZ. Cross-sectional view in Figure 7 d) shows smoother sides with minimal localized carbonation occurring mostly at the resin material location. Laser Drilling High density interconnect (HDI) PCB s involve laser drilling of fine microvias with diameters <40 μm. HDI PCB s increase the functionality of PCB s while using the same or less amount of area. This allows miniaturization of components onto a circuitry. The thin rigid PCB sample tested for through via drilling was 420 μm thick copper, epoxy/glass fiber, copper material. The Mosaic laser was operated at 50 khz PRF, where 200 µj pulse energy was available for processing. Percussion drilling technique was used to drill through vias using 280 pulses. As shown in Figure 8, the entrance side holes quality shows minimum HAZ with discoloration visible around the holes. The exit side of the via holes shows delamination or lifting of the copper layer. In this particular material, the top layer of copper efficiently absorbs 532 nm wavelength, whereas epoxy/glass fiber is mostly transparent at 532 nm. This results in internal heating at the epoxy/glass fiber and copper interface causing separation of the bottom copper layer from epoxy/glass fiber. Based on these results, we can conclude that 532 nm wavelength laser may not be suitable for machining of PCBs with epoxy / glass fiber constituent materials. 35 um 25 um Delamination (c) (d) Figure 7: Microscope picture showing, 300 μm thin rigid PCB panel depanelled individual PCB (c) Entrance side cut quality (d) Cross-section of a cut The thin rigid PCB sample used for the experiments consisted of 300 μm thick PCB resin material without Figure 8: Microscope picture showing 35 μm vias processed in 420 μm thick copper, epoxy/glass fiber, copper type thin rigid PCB at 50 khz PRF, Entrance side Exit side
Conclusions We have investigated the feasibility of a high performance 532 nm laser wavelength for 4 PCB material combinations which included 2 FPCBs and 2 thin rigid PCBs of different thicknesses. A new allin-one platform Mosaic 532-11 laser was used to demonstrate various PCB manufacturing processes such as via drilling, profiling (cutting) and depaneling. Through vias were drilled in 180 μm thick FPCB sample with polyimide-copper-polyimide (acrylic adhesive binder) materials using Mosaic 532-11 laser. Good quality vias were achieved using 200 μj energy at 50 khz PRF at the rate of >1000 holes per second. Profiling or cutting was demonstrated on FPCB sample consisting of two combinations of material A) 180 μm thick FPCB with layers of polyimide, acrylic adhesive, copper, acrylic adhesive, and polyimide and B) 160 μm thick FPCB with layers of polyimide, acrylic adhesive, and polyimide. Good quality cuts were demonstrated with ~390 μj pulse energy at 20 khz PRF at an average scan speed of 11.5 mm/s. Depaneling of 300 μm thin rigid PCB resin material without copper or stiffening material showed reasonably good quality cuts. Laser was operated at high PRF of 150 khz with 64 μj energy to achieve higher throughput, with an average speed of 50 mm/s. Through vias demonstrated using the percussion drilling technique in 420 μm thick copper, epoxy / glass fiber, copper thin rigid PCB material showed less promising results. The entrance hole quality appears reasonable, but exit-side holes exhibit intermittent delamination of the bottom copper layer. For this type of material a high power UV laser such as Pulseo 355-20 offered by us is a more suitable laser for processing. Results achieved demonstrate that high performance 532 nm wavelength lasers can be suitable for processing thin (up to 160 to 180 μm) FPCB materials consisting of copper and polyimide, as well as thin (up to 300 μm) rigid PCBs consisting of resin materials. References [1] Meier, D.J., Schmidt, S.H. (2002) PCB laser technology for rigid and flex HDI: via formation, structuring, and routing (October), CircuiTree. [2] Illyefalvi-Vitez, Z. (2001) Laser processing for microelectronics packaging applications, Journal of Microelectronics Reliability 41, 563-570. [3] Rumsby, P., Harvey, E., Thomas, D. & Rizvi, N. (1997) Excimer laser patterning of thick and thin films for high density packaging, in Proceedings of the SPIE, Vol. 3184. [4] Deak, J., Hertel, L. (2001) Laser via formation in flexible substrates for high density electronic assembly, in Proceedings of 24 th Annual Spring Seminar on Electronics Technology, Calimanesti Caciulata, Romania, 163-166. [5] Venkat, S. (2001) Cutting-edge of flex-processing laser technology, The Board Authority (March). [6] Burgess, L. W. Blind microvia technology by laser, in Proceedings of NEPCON West 99. [7] Schaeffer, R. (2001) CO 2 lasers for microvia drilling and other PCB and flex applications (September), CircuiTree, p. 90. [8] Lange, B. (2005) PCB Machining and repair via laser (February), OnBoard Technology. [9] Henry, M., Harrison, P., Wendland, J. & Parsons- Karavassilis, D. (2005) Cutting flexible printed circuit board with a 532 nm Q-switched diode pumped solid state laser, in Proceedings of ICALEO, Paper M804. Meet the Authors Ashwini Tamhankar has M.S. in Physics from San Jose State University and is currently working as a senior laser applications engineer at Spectra Physics, a division of Newport Corporation. She has extensive experience in laser material processing for various applications in microelectronics, semiconductor, biomedical, and photonics industry using nsec and psec pulsed laser sources. Her professional interests are in areas of laser technology development, optical system design, laser beam shaping techniques, laser application development and product marketing. She also has published and presented technical papers, authored journal publications and magazine articles. Rajesh (Raj) S. Patel has accumulated 23 years of experience in the laser material processing field. He is currently a Director of Strategic Marketing and Applications at Spectra Physics, a division of Newport Corporation. Prior to working at Spectra Physics he has worked in various engineering and senior management positions at IBM, Aradigm, and IMRA. He received his Ph.D. degree from the University of Illinois at Urbana-Champaign. He has worked with various lasers for developing applications in microelectronics, semiconductor, bio-
medical, medical device, photovoltaic, and photonic industry. He has authored 26 U.S. patents and published and presented more than 70 technical papers and articles related to laser processing, optics, and mask technology. He is an active member of Laser Institute of America (LIA), SPIE, and OSA and was elected to serve as a President of LIA for year 2009.