Design007 Magazine

PCBD-Nov2015

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54 The PCB Design Magazine • November 2015 Ask any group of PCB designers what they think of autorouters and the majority will say that they do not use them because they do not work. I have been battling this mindset for over 20 years now and it still persists to- day, even with the dramatic advances in rout- ing technology. This way of thinking generally comes from those designers who use the entry- level tools that have limited routing capability. But even the most primitive autorouter may have some useful features. It's all about chang- ing that mindset of the designer and having a crack at it. I started laying out boards back in the Bish- op Graphics days where layout began with a pencil sketch, on graph paper. Then, donuts and fine black tape were stuck to clear film, at twice the actual size, to produce the required connectivity. The 12 mil tape, which we re- ferred to as "spiderweb," was the thinnest trace width (6 mils finished) manufacturable at that time. It was really a matter of just connecting the dots. Double-sided layouts were sometimes stuck to the same film to improve registration, using red and blue colors to photographically distinguish the layers. But routing has come a long way since then. The first computer-based PCB design tools that emerged in the late 1970s were grid-based, ran on DOS or UNIX operating systems, and were very basic. Again it was still just connect- ing the dots, with a graphic trace from point-to- point to build up the layout, and then drawing the circuit on an XY plotter. Basic, but it was ef- fective for the construction on single- and dou- ble-sided boards. The next step was to include a netlist for connectivity and then to draw the schematic graphically and extract the netlist to column by Barry Olney iN-CirCuiT DESiGN PTY lTD AuSTrAliA BEYOND DESIGN Why Autorouters Don't Work: The Mindset! Figure 1: DDr3 memory fly-by address/clock and point-to-point data/strobe tuning.

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