Recent years have introduced products which continue on a trend toward smaller,

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1 Alexander Holiat MSE 542 I Flexible Electronics Term Paper May 11,2006 I I Laser Drilling for Electrical Interconnections in Flexible Electronics Recent years have introduced products which continue on a trend toward smaller, lighter, and more powerful. These include cell phones, PDA's, digital cameras, MP3 players, and medical electronics. These products are requiring an increased number of chips per product with an even greater number of 110 signals per chip in a smaller package. To incorporate a greater degree of electronic function into a smaller volume, circuit traces and the holes used to connect them must have smaller physical dimensions [I]. The use of flexible circuits and substrates are also becoming more important to decrease the volume and weight of the product. Industry has been on a continual struggle to keep pace and to reliably and inexpensively manufacture these products. I Since the invention of printed wiring boards (PWBs) in the 1950's, vias have been drilled with high-speed mechanical drills [2]. The process has since then become very sophisticated, fast, and accurate. The vias that are usually made with the mechanical drills are in the range of pm. As technology pushes for better performance and smaller footprints, the sizes of the vias continues to shrink. I Around 1995, the production of circuit boards with microvias started in Japan and was referred to as high density interconnect (HDI) [3]. Microvias are holes with a diameter less than 150 pm. Other advances such as the use of blind vias and the use of two HDI layers on each side helped continue the technology trends. Though holes of 150

2 p are achievable with mechanical drills, the cost effectiveness of (his process under 200 p is no longer practical 121. Mechanical drills are also inapt for the formation of blind vias. -3 Future technologies are also introducing the application of polymeric materials as >. P.? the substrate which would produce flexible substrates. Polyimides are also being used as : protective and insulating layers of the interconnect substrates which is advantageous for the cost and performance of the circuit module [4]. Current technology offers a wide range of materials for circuit boards, from polymers to ceramics. They are primarily made with a dielectric between two layers of copper. PCB substrates are either diglycidyl ether of tetrabromobisphenol (FR4)- or bismaleimide triazine(bt)-based with glass fiber reinforcement for rigid boards, polyimide for flexible circuits, or alumina for high-performance devices [5]. V urn Blind Via 100 pm Through Via Cu (18 pm) I Epoxy (60 pm) I Cu (18 pm) Slasfi-Ulxir-reinforeed mumlayei Figure 1 -Holes in various PCB materials [I] The new technology to replace mechanical drilling must be able to drill through a variety of materials and produce blind vias, not to mention to maintain the usual cost and throughput variables. Three technologies began to compete to take over microvia drilling: plasma etching, photo via formation/etching, and lasers. [l] Any of these processes must also fulfill the following requirements for vias to have reliable

3 interconnections, they include: clean vias without residue, no delamination of copper and substrate, tapered sidewalk without undercut, big land diameter for robust interconnection to the inner layer, and no perforation of the inner layer [6]. Plasma etching requires a very large capital investment. It is also limited to certain substrates which makes it not very attractive to small businesses and would only be feasible for very high volume production Photo via forming process is limited to photo imageable dielectrics and does not have a very high yield. However, these first two methods are able to form vias in each layer simultaneously while lasers drill each hole one at a time. Lasers, however, are very versatile and provide the user with many options. The wide varieties of lasers such as C02 Laser, solid state UV Lasers, and excimer lasers all have different strengths and weaknesses. Laser drilling is also the easiest and cheapest option to implement as it provides the user with the ability to implement it with established techniques and substrates, as well as providing future benefits to drill other materials [5]. Other laser advantages include: direct large amounts of energy to specific locations, localized heating, high heating and cooling rates (10"-10~' Klsec), low thermal budget useful for plasticlflexible substrates, patterning, and ability for large scale manufacturing (roll to roll). It is no surprise that they have become the most popular method of microvia formation. The term used when lasers drill holes is ablation. Ablation is the removal of material using light. It can be done either by vaporization of the material due to heat, or

4 [7]. The laser energy per unit area is called the fluence. There are many key parameters of interest when dealing with ablation in laser drilling. One must take into account several laser beam and processing parameters. The first is wavelength. Figure 2 shows the spectrum of light with the various laser types at different wavelengths. The choice of wavelength is very important depending on the absorption spectra of the copper, epoxy, polyimide, or other materials. You want the material of choice to absorb highly at the laser wavelength being used. Having a small absorption depth is also important to ensure a high energy deposition in a small volume for rapid and complete ablation. Figure 2 -Laser Type vs. Wavelength [I] The second parameter is pulse duration and energy per pulse. It is ideal to maximize peak power and to minimize heat transfer to the surrounding material. The result may induce thermal degradation of the surrounding material known as the Heat Affected Zone (HAZ) [S]. The third parameter is the pulse repetition rate. If the rate is too low, ablation will not be as efficient because the material will cool. If the rate is just right, the material will ablate more efficiently because more of the previous energy will be available with the next pulse. The fourth parameter is the beam quality. Beam quality is measured by

5 the energy, beam spot size, the focusabiity, and the homogeneity. Lenses, beam shapers and expanders, and masks are used to control the beam [4]. If the beam is not controlled, the ablation will not be as efficient and the shape of the hole will not be as expected. There is also the interaction between the laser beam and target material with the beam focus depth [S]. Each of these variables must be optimized in the processing environment in order to obtain the correct properties desired for the given application. The most common types of lasers are CO; and UV. They all have their advantages and disadvantages which are studied below. The differences between the beam properties, quality of the laser types, and the various material interactions strongly determine their applicability fields. [4] COi Lasers CO; lasers emit infrared light with a wavelength 10.6 (un and the beam is focused using lenses. This long wavelength limits the minimum focus diameter which reaches its physical limit around 75 (un. The short depth of focus associated with this laser also limits via diameter. However, the wavelength is ideal to ablate organic materials such as polyimide and epoxies. COz lasers can not effectively ablate metals due to the absorption of a few percent of the beam and the reflection of the rest [4]. However, if one treats the surface of copper typically with black oxide and has the optics optimized for the process, CO; has been able to ablate copper for 100 (un holes [9].

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7 very clean ablation with no side thermal damage. However, they are very big, have a high cost of ownership, and are inefficient. They lose approximately 95% of the output power due to masking [12]. They have a very low repetition rate (several hundred pulses per second), and do not remove metals as well as solid state lasers. [I] The advantage of excimer lasers over solid state lasers is that they selectively remove material over large areas uniformly, as opposed to solid state lasers which rely on focusing the beam to a very small area to achieve sufficient fluences. New advances in optics and new materials are continually making excimer lasers more attractive, however solid state lasers dominate over excimer lasers for microvia applications [13]. Solid State Lasers W solid state lasers are primarily third-harmonic Nd-doped designs (YAG and Vanadate) at 355 nm, though other harmonics are available 121. The light is emitted in very short high power pulses that can be focused to very small spots. The size of the focus spot is directly proportional to the wavelength of the light. To achieve smaller diameters, smaller wavelengths must be used. Solid state lasers can create microvias that are less than 25 pn in diameter. Unlike excimer lasers, solid state lasers exhibit repetition rates ranging from 1-100,000 pulses per second without losing beam quality and are capable of ablating metals [I]. Other advantages include high reliability, minimal service downtime, lower operating costs, and greater simplicity [12].

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9 ig There has been a steady progression of in power and pulse repetition frequency through the years. As this trend continues, UV solid state lasers will soon become dominant in the manufacturing of circuit boiads. An increase in throughput can also come from advancements in beam positio determined by the time spent drilling each vim [2]. "t technology since the processing speed is and the time required to move between CO2 vs. UV - Hybria Using C@ is essentially a thermal process in which the intense beam melts and vaporizes the material, either by conformal mask drilling or large window drilling [14]. This usually results in thermal degradation of the surrounding material known as the Heat Affected Zone, HAZ. The HAZ reduces precision and can create problems in circuits by

10 creating conductive paths between circuits that are supposed to be isolated. In contrast, UV lasers interact with materials primarily through a photochemical process in which the high energy photons actually break molecular bonds. The result is a clean cut with minimal effect on the surrounding material [15]. Figure 4 illustrates the different hole profiles by drilling at different wavelengths in the same material. Harmonics YAG CO, of YAG Figure 4 - Som 2 HAZ using higher wavelengths in polyimide material [7] The limit of CO; laser via size is 75 pn while solid state UV at 355 nrn can go to 25 pn. Another comparison is the greater absorption of many materials with UV, especially copper which is mostly reflective in the IR. UV lasers also eliminate the need of using masks or large windows on the surface since the beam defines the shape and size of the hole. There are also fewer alignment problems than CO; lasers because vias are aligned...; with respect to internal fiducials [14]. CO; lasers are known for their high average power. '2 and high drilling throughput, while UV lasers can drill through metals easily and make the smallest vias. + : In PCB manufacturing, hybrid lasers have been developed that integrate the two lasers together; using each laser's strengths to increase throughput. The copper is drilling

11 with the W laser and the dielectric with the C@ laser in parallel processing [14]. This hybrid laser provides manutkdwers with the flexibility of ddling through any variety of circuit board materials or configurations. Lasers have already become a standard in manufacturing of PCB. In 2006,0.4 mm chip scale packages on telecom products will be in volume production. This dictates design parameters of stacked microvias with 2-3 HDI layers on each side, lineslspaces below 60 urn, and three times the amount of laser vias with the diameter down to 70 pn [9]. The laser drilling technology could switch completely to UV when the next generation of 0.3 mm chip scale packages with thinner dielectrics and 50 pn diameter microvias are used [9]. This will only be possible with the continuing trend of high power UV solid state lasers that can provide higher throughputs and will provide longer lifetimes. Also, if a company invests on UV laser systems for a specific product, it will most likely be able to use the same system on different PCB materials for future products. r..

12 References 1. Egitto, Frank: Laser drilling for electrical interconnection in advanced flexible electronics applications. (Endicott Interconnect, 2006). 2. Dunsky, Corey: High-speed Microvia Formation with UV Solid-state Lasers. Proceedings of the ZEEE, 90, 1670, (2002). 3. Moser, David: Lasers tool up for via formation. Laser Focus World, 37,213, (2001). 4. P. Gordon and R. Berenyi: Laser processing of flexible substrates. (Department of Electronics Technology, Budapest University of Technology). 5. J. Paulus and M. Petti: Microvia materials: enablers for high-density interconnects. Proc. ZPC Printed Circuits Expo 1997, San Jose, CA, S Lange, B.: PCB laser technology for rigid and flex HDI -via formation, structuring, routing. LPKFLaser & Electronics AG, (2002). 7. Sun, Y.: Micro-via drilling on polymeric with laser pulses. (Electro Scientific Industries, Inc.). 8. Schmidt, H.: Ultraviolet laser ablation of polymers: spot size, pulse duration, and plume attenuation effects explained..i Appl. Phys., 83,5458, (1998). 9. Stahr, Hannes: The laser drilling horse race. Printed Circuit Design & Manufacture, 23,28 (2006). 10. Savage, Neil: UV lasers make their mark. OE Mag., 1,46, (2001) Kincade, Kathy: Excimers take more refined approach to micrornaching. Laser Focus World, 41,69, (2005). 12. M. Keirstead and P. Somerville: UV lasers are workhorses. Laser Focus World, 37,197, (2001). 13. A. Masters and T. Geiiking: Beam-shaping optics expand excimer laser applications. Laser Focus World, 41,99 (2005). 14. Justino, Pierre: A comparative review of laser manufacturing processes. PC FAB, 25,44, (2002). 15. R. Schaeffer and T. Hannon: UV solid-state lasers exhibit precision and dependability. Laser Focus World, 37,115, (2001).