Engineering-Controlled Impedance

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Last Modified: 09/15/2006

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Controlled Impedance has become a major manufacturing aspect in recent years. Design are requiring tighter tolerance in line widths and dielectrics to ensure the proper impedance results. Discontinuity related to board fabrication is a prime focus and the introduction of more stable dielectric material and predictable manufacturing guidelines helps ensure DCS can meet tomorrows impedance goals.

Why Controlled Impedance

As higher frequency digital designs have become more popular, these nets become transmissive and exhibit characteristics that are more common with RF and Analog designs. Controlled impedance now becomes more important to prevent signal loss or mismatch related to the PCB, Cable, Connector and component. 2D Field Solver Data Circuit Systems uses a 2D Field Solver from Applied Simulations Technology. This allows DCS the flexibility to optimize the impedance calculation to ensure the most accurate models in the industry. Why use a Field Solver over more popular formula based systems? The text book formulas developed over the years do not account for all aspects of today's designs.

Smaller trace width and dielectric ratios
Solder Mask Conformal Coating
Off Centered Stripeline and uneven dielectrics
Different Dielectric properties
Guard Band Traces and Coplanar geometries
Differential Impedance

There are newer formulas available today to account for past limitations, but a true field solver has become the industry standard for calculating impedance values. It provides more flexibility and more accuracy do to unique flexibility.

Independent Dielectric Specifications to allow for Hybrid builds or different electrical property values for each substrate used.
Trapezoidal Trace Shapes allows a more realistic representation of the etch line width characteristics.
Skin Effect based on higher frequency nets.
True differential modeling and Ground Shield modeling
Unlimited Geometry and Stack-up capabilities. Each model is a unique creation based on the individual need of the design.

Material Material selection can be a major aspect of a control impedance design. With this in mind, a lower loss material can reduce dielectric spacing or increase line width geometry to help with cost and mechanical limitations. DCS provides a full list of materials for your reference. You can visit the Material section of this site or download At-A-Glance reference below.

Documenting Impedance Requirements

Clear and detailed call outs on prints will ensure that the impedance values are matched. Even when a design has documented values, DCS will model the geometry to ensure that manufacturing matrix information and customer design requirement are achieved. If any discrepancy is found a DFM Engineer will contact the customer to resolve all potential issues. For examples of Fabrication note samples you can visit the Data & Documentation section of this sight or review the Sample Fabrication Notes.

Testing

The most common device used to test impedance on a finished fabrication is the TDR (Time Domain Reflectometer). This device transmits a fast step pulse signal, discontinuity is reflected back and read. The magnitude of the reflection is relational to the discontinuity experienced. Most machines are designed to use custom test coupons built on the panel. These coupons are designed to represent the unique characteristics of the board design, without potential limitations being included in the measurement. Traces can be measured within the board if the design criteria meets with the test spec required by DCS.

Reference Document	Description	Size
Full Material Cross Reference Sheet	This document will cross reference many different types of material. This can be used to get a quick general view of the information contained on the individual material data sheets.	68K
Brief Material Cross Reference Sheet	This document is a scaled down version of the one above. It contains the most commonly asked for values.	15.5K