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60 The PCB Design Magazine • November 2015 options, using a simple and fast methodology. Figure 2 shows sketch routing using multi-drop topology for address/clock and point-to-point for data/strobe on DDR3 memory. Once the ba- sic routes are connected, the matched length traces are then tuned as in Figure 1. So why don't autorouters work? I guess you will have to ask yourself that question! They certainly do for me. The trick is to control the router with constraints, cross-probe with the schematics, check as you go, tune and fix criti- cal traces and allow the autorouter to do all the hard work. Believe me, once you know how to control your router, it will definitely save a great deal of time and frustration. PCB layout is a means to combine your artistic side and your creative skills with the power of automation, but you need to uncover the right mix to make it work. Points to Remember • Even the most primitive autorouter may have some useful features. It's all about changing that mindset of the designer. • The first computer based PCB design tools that emerged in the late 1970s were grid- based, ran on DOS or UNIX operating sys- tems and were very basic. • PCB routers were developed using either the grid-based, gridless, shape-based, or geometrical approaches. • A shape-based autorouter does not have to work at a particular resolution, so routing of high density or fine pitch boards is not significantly slower than for lower density work. • The first autorouters were not very capa- ble, limited by computing power and lack of memory. They added too many vias, wasted space due to the strict X/Y bias and the quality was poor compared to manual routing. • The most popular shape-based router, 20 years ago, was CCT's Specctra router. • Xpedition is arguably the best routing technology still available today. • The design constraints need to be estab- lished before an attempt to route is initi- ated. • The router needs rules to determine the most effective path, but too many rules can also bog it down. • To obtain a high route-completion rate, component placement is extremely impor- tant. If the board is difficult to route, it may just be the result of poor placement. beyond design WHY AuTOROuTERS DON'T WORk: THE MINDSET! Figure 3: Sketch routing of DDr3 memory.

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