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18 The PCB Magazine • June 2016 exposure and developing, its resistance to plat- ing (especially Ni-Au) and etching, its fine-line resolution (down to 10 micron [0.4 mil]) and its insensitivity to dust and clean-room debris. The coating process is relatively simple: • Clean panel • Coat for 60–90 sec at ~60–100V (voltage will determine finished thickness) • Rinse • Bake dry The panels are now ready for exposure or transportation. A finished via and trace are shown in Figure 6. Perfect Registration Now with the advent of direct digital imag- ing (DDI) and board scanning, it is possible to have perfect registration on both sides of a board. The DDI unit seen in Figure 7 has nine cameras that scan a panel as it enters the machine from both sides. Then, as it starts to expose each side, the artwork is modified on-the-fly so that each land is perfectly registered to its hole. Without high-speed computers, scanners and the new software, this was impossible before. Novel Russian Technique At a recent PCB West conference, I had a conversation with a Russian PCB fabricator. It seems the Russians also used landless vias after they saw it on Japanese computers. The Russian process was novel and I wish I had thought it up! They used the property of dry film photore- sist's sensitivity to oxygen. When they laminate the dry film to the panel, they have a shroud around the unit filled with oxygen. Thus oxy- gen is trapped in the holes by the dry film. They expose the panel to their normal artwork (but there are no lands for traces that will intercon- nect to the landless vias). They then do a short 100°C bake of the panels, remove the cover sheet on the dry film and develop normally. The oxygen and heat has polymerized the dry film from the inside at exactly the same diam- eter as the hole. Thus when developed, the via has perfect dry film registration and is landless. Issues with Smaller Traces and Annular Rings Very small traces and spaces are very much alive in the IC packaging arena. When the line length is under the critical length created by the rise-time of the signals, transmission lines AGAINST THE DENSITY WALL: LANDLESS VIAS MIGHT BE THE ANSWER Figure 6: Landless via (0.010 in [0.25 mm] and .004 in [0.1 mm] traces) created with the liquid, electrophoretic, positive photoresist from PPG. Figure 7: The new generation of DDI production exposure units using DLP, solid-state LEDs and computer scanners to insure registration and high productivity.