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44 The PCB Design Magazine • August 2014 routing to flow in the most efficient path, with- out X/Y bias, through the channels available between components. Often the routing of one set of traces needs to be done while recognizing that one or more additional sets of traces need to be in the same area. Controlling the routing location, while leaving enough room for other routes, is essential. Advanced design tools such as Mentor Graphics' Xpedition Layout use a freeform or angled line sketch path that enables the designer to maintain this kind of control (Figure 2). Constraints Routing needs to be controlled in ways that enable the designer to fulfill the signal integrity and fabrication requirements. This is accom- plished by following the constraints that are setup prior to routing. For example, setting ap- propriate physical constraints between objects enable higher yield during fabrication. Trace widths and clearances also allow managing impedance and avoiding crosstalk. The sketch routing environment follows the physical rules set in the constrain manager, which has a hi- erarchical structure to define general and very specific rules. The signal integrity rules are also very detailed along with the means to define pad entries and Z-axis clearances. A hierarchi- cal matched length definition, including phase matching, enables control of the tuning algo- rithms. There is one particular object in PCB rout- ing that needs to be restricted and avoided: the through-via. Designers need the ability to mini- mize the use of vias. Vias used to be considered undesirable primarily due to fabrication costs— reduce the number of vias, reduce the cost. While that is still true, it is a less significant factor compared to the negative impact that vias have on signal integrity. There are enough problems created by the through-via that a pa- per could be written just on that subject. Since I don't want to bore your with through-via con- demnation in this article, can we simply agree that through-vias should be avoided with high- speed signals? A successful routing methodol- ogy must give the designer the ability to pre- sketch routing Figure 2: Meandering path location described by a sketch and after routing. PART 1: TAKING CoNTRoL continues