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PCBD-Oct2014

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42 The PCB Design Magazine • October 2014 This is the second of three articles that dis- cusses the core elements of effective PCB rout- ing (control, quality, and performance). I recom- mend reading my last column on taking control, because it gives some background information that will help when reading this column. PCB designers care about route quality and it is not just a matter of being artistic. Route qual- ity is objective and subjective—both are essential to successful routing. Objective Quality Objective quality can be defined as fulfilling the fabrication and signal integrity requirements as defined. The intent of route quality for fabrica - tion is to enable high board yields, high reliabil- ity and low cost. It is important to understand the critical elements required by the fabricator, then set up and follow the physical constraints that will satisfy the desired objectives. Interac- tive and automatic routing must follow these physical rules, otherwise the time spent prepar- ing any moderately complex design for fabrica- tion will be excessive. Signal integrity requirements are more com- plex. The timing, impedance management, and noise control requirements can be expressed through constraints. However, sometimes it is necessary to compromise in order to build a board that meets the fabrication goals. There is a tendency to over-constrain for signal integrity purposes. The thinking is if the constraints are very conservative, the design should perform as desired. Unfortunately, over-constraining for sig - nal integrity purposes can lead to lower yields, poorer reliability and higher fabrication cost. The right balance between the fabrication and signal integrity requirements must be discovered and applied. The routing environment needs to provide the ability to constrain, route, tune, and ana - lyze both single-ended and differential signals in a manner that satisfies these objective quality aspects of design. If you're manually routing a SkETCh ROUTING column by Charles Pfeil MEnTOr GraPHICS This is Sketch Routing, Part 2: Quality Figure 1: Objective quality for differential pair pad entry.

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