Steve wrote:
>This message is pointed to those of you involved in RF circuitry I think. 
>
>The IPC-6012 specification for trace with tolerance of ±20% is coming under
>fire here. Typically what can most PCB fabricators hold for trace width
>tolerance regardless of what IPC says. How do other RF designers specify
>trace width tolerances in critical circuits.
>
>Steve Collins
>PCB Design Supervisor 

Steve,

we're a RF PCB fabricator and that is a tough question.
Etching tolerance depends on several things. Best is to 
look into IPC-D-316 "Design Guide for Microwave Circuit
Boards Utilizing Soft Substrates".
Normally we get a drawing of the critical part of the pattern
where a trace width is defined including tolerance. The
important width for a RF pattern is that at the bottom
of the conductor.
A normal tolerance for RF traces is +-30mikrons (1.2 mil) for
about 50mikrons (2mil) final copper thickness.

As a rule of thumb you can say:
normal tolerance = final copper thickness x 0.75
special tolerance = final copper thickness x 0.6
very special tolerance = final copper thickness x 0.5

Hope this helps.
Simon Huetter

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