Material Selection for Digital Design


Reading time ( words)

In a previous column, Material Selection for SERDES Design, I pointed out that materials used for the fabrication of the multilayer PCB absorb high frequencies and reduce edge rates and that loss, in the transmission lines, is a major cause of signal integrity issues. But we are not all designing cutting-edge boards, and sometimes we tend to over-specify requirements that can lead to inflated production costs.

In this column, I will look at what types of materials are commonly used for digital design, and how to select an adequate material to minimize costs. Of course, selecting the best possible material will not hurt, but it may blow out the costs.

Olney 1.jpg

Figure 1: Simulated signal propagating through a curved waveguide.

Signals propagate in a vacuum or air at approximately the speed of light. But, as the electromagnetic energy is enveloped in a dielectric material, sandwiched between planes in the PCB medium, it slows down. Figure 1 illustrates a signal propagating through a curved waveguide. This is representative of a typical stripline configuration of a PCB. What needs to be understood is that the signal traces on a PCB simply guide the signal wave, as the electromagnetic energy propagates in the surrounding dielectric material. It is the dielectric material that determines the velocity (v) of propagation of the electromagnetic energy:

 Olney Equation.jpg

Keep in mind that c is the speed of light (in free space) and Er is the dielectric constant of the material (FR-4 is ~4.0). By contrast, the Er of air is 1. Therefore, the velocity of propagation in FR-4 is about half the speed of light or 6 inches per ns. The important concept is that it is the electromagnetic energy that propagates down the transmission line--not electron flow. Electrons flow at about 0.4 inches per second, a snail’s pace by comparison.

Read the full column here.


Editor's Note: This column originally appeared in the August 2014 issue of The PCB Design Magazine.

Share

Print


Suggested Items

Words of Advice: The OEM Systems Designer

11/07/2019 | Andy Shaughnessy, Design007 Magazine
In a recent survey, we asked the following question: What advice would you give an OEM systems designer? Here are a few of the answers, edited slightly for clarity.

Dugan Karnazes Discusses His New Startup

11/07/2019 | Andy Shaughnessy, Design007 Magazine
I caught up with Dugan Karnazes again this year and discussed his new startup, Velocity Research, which is a one-stop shop for design made up of technical creatives. The Grand Rapids company is already doing design work for a variety of customers, from individuals to multinational companies.

Design For Excellence: Karen McConnell on Standards

10/31/2019 | Andy Shaughnessy, Design007 Magazine
During the IPC Summer Meetings, I spoke with Karen McConnell, senior staff CAD CAM engineer with Northrop Grumman Mission Systems. Karen is a veteran PCB designer as well as an IPC committee chair and mentor. She offered an update on some of the IPC committees she chairs, the need for more mentor programs in this industry, and why you can call something a ham sandwich if you define it correctly in the standard.



Copyright © 2019 I-Connect007. All rights reserved.