Sketch Routing, Part 2: Quality


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This is the second of three articles that discusses the core elements of effective PCB routing (control, quality, and performance). I recommend 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 quality 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 fabrication is to enable high board yields, high reliability, 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. Interactive and automatic routing must follow these physical rules, otherwise the time spent preparing any moderately complex design for fabrication will be excessive.

Signal integrity requirements are more complex. 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 signal 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 analyze both single-ended and differential signals in a manner that satisfies these objective quality aspects of design. If you’re manually routing a design and the constraints are set up properly, fulfilling the objective quality metrics will be accomplished--the only problem is that it may take a long time. Tuning the nets in matched sets or tuning for phase matching is clearly a painstaking process if it all has to be done manually. The most effective routing environment for objective quality is to have high levels of automation under the control of the designer.Read the full column here.Editor's Note: This column originally appeared in the October 2014 issue of The PCB Design Magazine.

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