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PCB-Aug2017

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56 The PCB Magazine • August 2017 steps, no catalyst for electroless plating, and no copper etching steps: • Rather than undertaking photolitho- graphic steps, the pattern of wires, pads and vias is directly etched by laser abla- tion into the dielectric material. The line width and spacing of the features is lim- ited by the processing laser, process pa- rameters, and the physics of the laser-ma- terial interaction. Rather than developing an entirely new chemical/material set for further advancing the miniaturization of PCB features, advances in laser technolo- gy, pulse shaping, and beam positioning can drive this trend. • Tin and palladium chemistry are removed from the electroless plating process. Cleaning, conditioning, microetching, catalyst pre-dip, catalyst activation, and acceleration steps are all eliminated from the electroless plating process line. Haz- ardous and costly chemicals are removed from the process stream. • There is no copper etching required. The copper that is deposited represents all the copper in that layer of the board. It is apparent then that this process repre- sents a "green chemistry" approach (i.e., an approach that aims to minimize both the use and disposal of hazardous materials: the best green chemistry approaches are those that avoid hazardous materials altogether). The process also presents the opportunity for sub- stantial base material savings, with glass being up to 100-fold more affordable than current high frequency dielectrics, and energy savings through the absence of thermal lamination processes. The best analysis would be a total life cycle analysis for an all-glass or glass core PCB to ex- plore the process rates, throughput, and energy and material requirements for the laser seeding process compared to those of typical PCB fabri- cation. An effort is currently underway to make this analysis, but the biggest difference between the two processes is readily apparent: there are currently no methods in place for incorporating glass dielectrics into traditional PCB fabrication lines, and the methods described in this paper offer a pathway to make this possible. Conclusions This paper details a new methodology for the plating of conductive features onto glass di- electrics. A laser is used to ablate material from a glass substrate in the desired pattern, and cop- per is "seeded" into these features using laser- induced forward transfer of a copper foil. The copper droplets, or seeds, that are deposited act as sites for the growth of copper in electroless plating processes. Using thin borosilicate glass substrates, we demonstrated this process for the preparation of conductive thin wires, through- holes, blind vias, and multiple-layer architec- tures. The process can be applied toward dielec- trics other than glass as well. Material costs, haz- ardous waste, and waste recycling streams are significantly reduced compared to traditional PCB fabrication processes. Future work will be aimed at better understanding the throughput and yield of this process, especially with com- parison to typical PCB fabrication. PCB References 1. Cui, X. Electroless metallization of glass for electrical interconnect applications. (Loughbor- ough University, 2010). 2. Lampe-Önnerud, C., Jansson, U., Hårsta, A. & Carlsson, J.-O. Chemical vapour deposition of copper on Si(111) and SiO 2 substrates. J. Cryst. Growth 121, 223–234 (1992). 3. Pulker, H. K. in Coatings on Glass (Second Edition) 429–510 (Elsevier, 1999). 4. Hidai, H. & Tokura, H. Direct laser writing of aluminum and copper on glass surfaces from metal powder. Appl. Surf. Sci. 174, 118–124 (2001). 5. Bohandy, J., Kim, B. F. & Adrian, F. J. Metal deposition from a supported metal film using an excimer laser. J. Appl. Phys. 60, 1538–1539 (1986). 6. Nagel, M. & Lippert, T. in Nanomaterials: Processing and Characterization with Lasers 255– 316 (Wiley-VCH Verlag, 2012). 7. Arnold, C. B., Serra, P. & Pique, A. Laser Direct-Write Techniques for Printing of Complex Materials. MRS Bull. 32, 23–31 (2007). 8. Serra, P., Duocastella, M., Fernández-Pra - das, J. M. & Morenza, J. L. in Cell and Organ Print- ing (eds. Ringeisen, R. B., Spargo, J. B. & Wu, K. P.) 53–80 (Springer Netherlands, 2010). LASER PATTERNING AND METALLIZATION TO REDUCE PROCESS STEPS FOR PCB MANUFACTURING

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