BOOK EXCERPT: The Printed Circuit Designer's Guide to... DFM Essentials, Chapter 3

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The Printed Circuit Designer’s Guide to... DFM Essentials
By Anaya Vardya, American Standard Circuits / ASC Sunstone Circuits

Chapter 3: Controlled Impedance

Printed circuit boards that contain controlled impedance traces require specific constructions and tighter manufacturing process controls. The fabricator needs to tailor the construction for PCBs requiring impedance to precisely match the required nominal impedance values.

The fabrication drawing should specify the required nominal impedance and tolerance, and
the fabricator will create a construction to meet the intended design impedance
requirements. Routing preference on internal layers for controlled impedance traces must
consider the difference in structure and Dk value for the environment (air) above the trace
on an outer layer and absence of reference plane above the trace for a microstrip/embedded microstrip compared to the uniform structure of stripline/dual stripline.

Primary Impedance Factors

• Trace width
• Copper thickness
• Dielectric spacing
• Dielectric constant (Dk)/relative permittivity (εr)
• Indicated “Reference Only” (See drawing/specification impedance requirements)
• Material requirements

Fabricators use some form of impedance modeling software to determine the specific PCB
construction required to produce the specified impedance. The PCB drawing should specify
the nominal impedance, tolerance, and nominal line width. This will allow for the creation of
the most cost-effective PCB material construction.

Multiple Impedance Considerations
Some PCBs require multiple impedance values on the same signal layers. Your PCB
fabricator must have the ability to create impedance coupons to reflect the appropriate
model for each impedance requirement. However, testing multiple impedance values on a
given signal layer could require coupon redesign and may cause the coupon to be wider
than normal and reduce the panel utilization.

There are two types of impedance structure models that are generally specified: single-
ended and differential.

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