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22 The PCB Magazine • June 2016 3. Transmission Line Control To hold a characteristic impedance as the line width gets smaller requires thinner di- electrics to the reference plane. For a 50 ohm transmission line, a 0.004 inch (.10 mm) trace on FR-4 needs a 0.00315 inch dielectric, but at 0.003 inch (.075 mm) the thickness is only 0.0027 inch (0.069 mm). The dimensional sta- bility of <0.003 inch material is significantly re- duced. 4. Impedance Tolerances It may be more difficult for a fabricator to hold a +/- 10% tolerance on a transmission line when the trace width is reduced to 0.003 inch (0.075 mm) or smaller. A +/- 10% tolerance on 50 ohms with a 0.004 inch trace is +/- 0.0008 inch (0.0203 mm), but at 0.003 inch trace, it is +/- 0.0006 inch (0.0152 mm). Landless vias have all but dropped out of sight. The last publication showing landless vias was in 2002 (Figure 10). But since the tech- nology is real and has been used for many years, there is nothing holding it back from a reap- pearance and renaissance. PCB References 1. Hirai, Osamu, "The Development Of Ad- vanced Cu-Plated Through Hole PWB With Landless Vias," Printed Circuit World Conven- tion IV, Session 58, June 5, 1987, Tokyo, Japan. 2. Goosey, M. & Wilkinson, G., "Manufac- ture of Advanced Interconnects," Future Cir- cuits International-Vol. 5, 1999, pp. 181-185. 3. Hertlein, W.G., Tornkvist, K., & Smith, K., "ED Photoresist Technology Becomes a Manu- facturing Process," Circuit World, Vol.27-Iss 3, 2001, pp. 6-14. 4. Goldman, Patricia & Schmidt, Tim, "New Design Applications Made Possible by Utilizing a Positive Acting Electrophoretic Photoresist," IPC's 2002 Designers Learning Symposiums, Dallas, TX (May) and Toronto (October). 5. Jalonen, P. & Tuominen, A., "The Applica- bility of Electrodeposited Photoresist in Produc- ing Ultra-Fine Lines Using Sputtered Seeding Lay- ers," Circuit World, Vol.28-Iss 2, 2002, pp. 11-13. 6. Holden, H, "Design for Manufacturing & Assembly," I-Connect007 Daily Newsletter, (to be published on or about June 29, 2016). 7. Coombs, C.F. Jr., Holden, H, Printed Cir- cuit Handbook, 7 th Edition, McGraw-Hill Educa- tion, 2016, NY, NY. Happy Holden has worked in print- ed circuit technology since 1970 with Hewlett-Packard, NanYa/Westwood, Merix, Foxconn and Gentex. Cur- rently, he is the co-editor, with Clyde Coombs, of the Printed Cir cuit Hand- book, 7th Ed. To contact Holden, click here. Figure 10: Landless via technology in Japan, circa 2002. Further publications have not been found. AGAINST THE DENSITY WALL: LANDLESS VIAS MIGHT BE THE ANSWER