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20 The PCB Magazine • June 2016 are not required. This is the ideal case for trace width of 5–10 microns or larger. But in general board routing, there are a number of issues with reducing trace widths and annular rings on multilayers. Here are four: 1. Reliability of Very Small Annular Rings HP Lab's scientist discovered the reason that landless vias were so reliable. As the laminate expands when it is heated, the copper hole does not expand as much. This creates stress on the copper plating in the hole and is concen- trated in the corners. Depending on the duc- tility of the copper plating, the resulting land rise will initiate a crack in the corner (called corner cracking). The smaller the annular ring, the higher the angle of the land edge, and this results in increased corner cracking with the smaller annular rings. When there is no an- nular ring (landless), then there is no corner crack. This is different from the barrel cracking defect that is related to the total thickness of copper in the barrel of the via. Figure 8a and 8b show this phenomena. An excellent explana- tion is written by Dr. Reza Ghaffarian from the JPL in Chapter 60 in the new PCB Handbook, 7 th edition [7] . 2. Copper Losses The energy loss in copper is fairly constant until you start to get above 2 MHz signals. As seen in Figure 9, the copper losses at 1 GHz for a 0.004 inch (0.10 mm) trace are around 30 ohms per meter or about 6 db/m. But at 0.003 inch (0.075 mm) the losses are above 60 ohms/m or a loss of 8 db/m. AGAINST THE DENSITY WALL: LANDLESS VIAS MIGHT BE THE ANSWER Figure 9: Copper losses for 1 oz. copper foil as a function of frequency for different trace widths. Figure 8: On temperature elevation to lead-free reflow temperatures, a via pad will tilt. a) The angle is a function of the annular ring diameter—stresses introduced will create a crack failure in the corner. b) The number of these cycles-to-failure as a function of the copper ductility. The horizontal line is the bar- rel and also a landless via pad .[7] .