Assumptions made in simulation may not reflect live board conditions.
As simulation and analysis continue to mature, the evolution is from a mechanism for finding problems to a methodology for preventing them. Many design teams have advanced beyond checking for violations and now use the tools to make informed design decisions early in product development. This results in better products reaching production floors despite condensed schedules.
Chasing correlation. With increasing reliance on simulation and analysis comes the responsibility to understand the capabilities and limitations of these tools. While able to calculate remarkably accurate results, they do rely on some basic assumptions. Not long ago, the “via” was assumed to be a plated hole extending the full thickness of the circuit board, resembling a coaxial cable, and could be sufficiently modeled as such. The drive for miniaturization and advances in manufacturing revolutionized this structure, introducing laser drilling, buried and blind, nested, and stacked constructs that quickly invalidated the coaxial cable model. Now they can be modeled in detail using 3-D solvers. Design teams, likely aided by simulation, recognized the via as the single structure in the signal path that presented both the greatest threat to signal integrity and the largest contributor to discrepancies between simulation and measurement. Had we not challenged the assumptions made regarding the via, simulation and measurement would continue to diverge as complex via structures became mainstream. Instead, traditional simulation, aided where necessary with 3-D electromagnetic field solvers, continues to produce results predictive of the actual measured product.
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