Reading time ( words)
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.
I-Connect007 Editorial Team
When we decided to cover the future of PCB packaging, we knew we would have to interview Charles Bauer, Ph.D., owner of TechLead Corporation. Chuck recently spoke with Happy Holden, Andy Shaughnessy and Barry Matties about current trends in packaging, the need for product designers and manufacturers to communicate, and why no matter how cool the technology is, cost is still king.
Dave Lackey and Anaya Vardya, American Standard Circuits
The design process is arguably the most important part of the flex circuit procurement process. The decisions made in the design process will have a lasting impact, for better or worse, throughout the manufacturing cycle. In advance of providing important details about the actual construction of the flex circuit, it is of value to provide some sort of understanding of the expected use environment for the finished product.
Dan Beaulieu, D.B. Management Group
Mark Thompson wants to help PCB designers. He’s seen it all in CAM support at Prototron Circuits: the incomplete or inaccurate data packages, boards that are unnecessarily complex or over-constrained, and so much more. Mark just returned to writing his popular Design007 Magazine column, The Bare (Board) Truth, which addresses questions such as, “What happens to your design at CAM?” I asked Mark to explain why it’s so important for designers to communicate with their fabricators, and why they need to get out of the office and visit a board shop every now and then.