Printed Circuit Design & Fab Online Magazine - Current Issue

Current Issue

Bill Hargin Part three of our series on Dk and Df characterization looks at stripline methods.

This is part three in a series that examines how the industry at-large characterizes laminates. It’s true such things are of little interest below 1GHz, but I suspect the relative comfort of the sub-GHz world has slipped into the distant past for many of us.

Part I summarized the “anarchy” that permeates this part of the PCB design world. Part II, last month’s column, pointed out a dozen different “standards” can be used to characterize Dk and Df. In fact, there’s little evidence to suggest the standardization process has led the industry to true standards. To me, “standard” means that if I measure something in Taipei, Tokyo, Toulouse or Toronto, I’m going to get pretty close to the same result if I follow a “standard” test method. That’s not what we have. Here in Part III, we’ll focus primarily on the electric-field (E-field) orientation of the measurement equipment.

Read more: Dk and Df Characterization Methods for PCB Laminates – Part III

The background of the measurement specs and tests.

Read more: Dk and Df Characterization Methods for PCB Laminates – Part II

Bill Hargin Determining tradeoffs among various laminates.

Read more: Dk and Df Characterization Methods for PCB Laminates – Part I

Bill Hargin Should you use wider trace widths? And how will you know?

An engineer recently asked me about the relationship between trace width and insertion loss, while adjusting dielectric height to maintain a 50Ω single-ended impedance.

At a high level, five variables are at work here, including trace width, copper weight, dielectric height, Dk and Df. Include frequency and resin content, and we’re really talking about seven variables. (Then there are stripline vs. microstrip configurations, which change things a bit, as well as percent-copper, which impacts prepreg thickness, and copper roughness.) We’ll keep things simple, for discussion’s sake, and address some of these factors in future columns.

Lossy transmission-line effects become significant signal integrity concerns at clock frequencies above roughly 1GHz and for interconnect lengths that exceed 12 inches. Assuming we’re talking striplines and grabbing data from a stackup I was looking at [Megtron 6 (G)], we’ll use an insertion-loss comparison between two different trace width/dielectric height combinations. (Dks are similar, but a little bit different, depending on resin content.)

Read more: Trace Width’s Effect on High-Frequency Insertion Loss