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June 2016 • The PCB Magazine 15 cantly reduce the via's reliability. Therefore, land- less is an excellent way to achieve higher density without reducing trace widths or spacings. Because the number of through-holes on a multilayer block numerous routing channels all through the board, the use of blind and buried vias can significantly increase a layer's routing density, on the order of 2X to 4X without their use. This is also measured by layout efficiency (L.E.), the amount of space used for routing as compared to the entire area on the signal layer. L.E. is also enhanced by blind and buried vias, to the order of 2X to 4X. The L.E. for a through- hole multilayer is 8% to 10%; 16% with TH and blind-vias; to 24% for TH/with two-sided blind vias and multiple build-up layers. Some of the landless vias are seen in Figures 4, 5 and 6. Landless Via Processes There are numerous patent applications about landless vias. All require laser sculpting of the via or pinpoint laser exposure for the imaging. None of these have ever entered pro- duction. The two that have been used in high- volume production are listed here, with two additional techniques that appear to be very practical. HP Process Learned from the Japanese The Japanese process for making landless vias is very simple, but anyone I talked to never figured it out until I explained the process. It is a true example of thinking outside the box: Whatever your registration tolerances are, then reduce the artwork land opening size by AGAINST THE DENSITY WALL: LANDLESS VIAS MIGHT BE THE ANSWER Figure 3: The maximum number of traces available for various routing channel widths as a function of via diameters when all the vias are landless. Figure 4: Landless via supplemented with the same diameter as the trace width, in this case 0.004 in. (0.1 mm). These are referred to as invisible vias. Table 1: PCB design rules effect on routing density on one layer.