Surface Finishes for High-Speed PCBs


Reading time ( words)

The Nickel Doesn’t Make Cents!

PCB surface finishes vary in type, price, availability, shelf life, assembly process, and reliability. While each treatment has its own merits, electroless nickel immersion gold (ENIG) finish has traditionally been the best fine pitch (flat) surface and lead-free option for SMT boards over recent years. But unfortunately, nickel is a poor conductor with only one third the conductivity of copper. Also, nickel has a ferromagnetic property that can adversely affect electromagnetic fields in the high-frequency domain.  

The PCB industry has addressed the issue of the ferromagnetic properties of nickel by introducing a nickel/gold (NiAu) alloy. Gold is slightly less conductive than copper, and has no ferromagnetic properties, so it has relatively little impact on the conductor's loss characteristics at high frequencies.

Microstrip (outer) layers of a multilayer PCB suffer from wide variations in both trace width and thickness. This is due to the additional fabrication process of electroplating the through-holes. Copper barrel thickness is generally specified as a minimum of 1 mil (25.4 µm), and so extra copper plating is applied to the surface in order to produce the correct barrel wall thickness. This, unfortunately, is also added to the traces. But as the thickness and width varies, so does the impedance. This is one of the reasons why routing controlled impedance signals, on the microstrip layers, should be avoided.

It is also very important not to pour copper fills on the signal layers of the board, as these will dramatically change the impedance of the traces rendering the impedance control ineffective.

Read the full column here.


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

Share




Suggested Items

Pulsonix Collision Avoidance to Bring Mechanical Capabilities Into ECAD

05/19/2022 | I-Connect007 Editorial Team
The I-Connect Editorial Team recently spoke with Bob Williams, managing director of Pulsonix. He discussed some of the new features in the upcoming version of the Pulsonix PCB design tool, Version 12, including collision avoidance and other 3D options that allow certain MCAD functions within the ECAD environment.

A Textbook Look: Signal Integrity and Impedance

05/18/2022 | Pete Starkey, I-Connect007
Believing that I knew a bit about signal integrity and controlled impedance, I was pleased to take the opportunity to connect with an educational webinar that I hoped would extend my knowledge. In the event I was surprised at how little I actually knew, and the webinar was an excellent learning opportunity. The webinar was introduced and expertly moderated by Anna Brockman of Phoenix Contact in Germany.

Designing in a Vacuum Q&A: Carl Schattke

05/11/2022 | Andy Shaughnessy, Design007 Magazine
Not long ago, I caught up with Carl Schattke, CEO of PCB Product Development LLC and a longtime PCB designer, for his thoughts on “designing in a vacuum.” As Carl points out, if you follow PCB design best practices, knowing the identity of your fabricator is not a “must-have.” He also offers some communication tips for discovering the information you do need, including one old-fashioned technique—just asking for it.



Copyright © 2022 I-Connect007. All rights reserved.