Fundamentals

Editor’s Note: Due to the overwhelming amount of positive feedback from the September 2013 issue of In Compliance (pp. 42-48), a follow-up article with additional lab testing by the author was necessary to respond to an aerospace prime.

Thank you for your informative article on “Pin Holes & Staples Lead to Diminished Performance in Metallized Static Shielding Bags.” Have you considered what happens if there is an ESD discharge to the staple in a bag?

As early as 1985, the author recalls re-occurring discussions of the effects of puncture holes from component leads and stapling of static shielding bags.

In July 2013, the ESD Experts page on LinkedIn® started a discussion by a USA computer manufacturing company that generated participation from end users, suppliers and consultants both here and abroad. In years past, some held opinions that pin holes do not greatly affect static shielding of metallized bags. There is, however, minimal published data to fall back upon regarding this subject matter.

“I’ve all ready read the books on EMC and visited a lot of home pages... But all these references did not mention anything about the physical phenomenon that causes common mode currents... Are common mode emissions inherent in any physical system? Can I model them?” Overheard on the ‘Net

It’s by no means a trivial question. And, in spite of decades of hand waving by authors and consultants, the principal mechanism by which common mode currents are created in digital devices was not well understood until the decade of the 90s. In this article, we’ll explore the physics behind the creation of common mode currents, and perform some experiments to verify our understanding.