-
- News
- Books
Featured Books
- design007 Magazine
Latest Issues
Current IssueMechatronics
Our expert contributors discuss the advent of mechatronics in PCB design, the challenges and opportunities this creates for circuit board designers, and the benefits—to the employee and the company—of becoming a mechatronics engineer.
Creating a Culture of Collaboration
PCB designers could learn quite a bit from NASA and the private companies that develop spacecraft: Every one of these vehicles is a testament to the value of collaboration among disparate stakeholders. Without a collaborative culture, the rocket might never get off the ground.
Breaking High-speed Material Constraints
Do you need specialty materials for your high-speed designs? Maybe not. Improvements in resins mean designers of high-speed boards can sometimes use traditional laminate systems. Learn more in this issue.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - design007 Magazine
Estimated reading time: 1 minute
The Do’s and Don’ts of Signal Routing for Controlled Impedance
In this column, we will once again be focusing on controlled impedance structures, both from the layout side and the simulation side. I will break them down into the sub-categories of the models they represent and the important points to remember when using the various models. I will also be asking questions such as, “Why would a fabricator ask for a larger impedance tolerance?” and “Where does the fabricator draw the line for controlling various structures?”
Later, I will break down my Top 10 do’s and don’ts of signal routing.
A Few Rules of Thumb
Let’s start with single-ended structures, both co-planar and those in “free space,” i.e., not coupled to any adjacent copper pour.
For external single-ended structures starting with quarter or half-ounce copper, the trace width is typically about twice the dielectric needed between the impedance signal and its reference plane.
Example: A 4.25 mil trace needs about a .0026”–.0028” dielectric to be a reference plane for 50 ohms on half-ounce starting copper (1.5 ounces after plating).
Keep the copper pour that resides on the impedance layer a minimum of 3x the chosen trace width for impedance; this ensures no unwanted co-planar coupling occurs. At larger trace widths upwards of .012” this distance can be as little as 2x the trace width.
To read this column by Mark Thompson which appeared in the May 2015 issue of The PCB Design Magazine, click here.
More Columns from The Bare (Board) Truth
The Bare (Board) Truth: My Top Six Design ChallengesThe Bare (Board) Truth: Via Basics
The Bare (Board) Truth: 5 Questions About Improving Thermal Management
The Bare (Board) Truth: Teaching the Next Generation—An Overview of Today’s University Courses
The Bare (Board) Truth: Fabrication Starts With Solid Design Practices
Board Negotiations: Design Rules and Tolerances
The Bare (Board) Truth: Eliminate Confusion
The Bare (Board) Truth: Getting on the Same Page—A Data Story