Last year we ran an article on the "air gap shielding" benefits of three dimensional packages such as clamshells or other thermoformable style packages. After we ran the article, one ESD Guru at the EOS/ESD Symposium asked the question, "What would happen if a voltage of 100,000 volts were moved slowly by a pallet load of these packages?"

His idea was that when clamshell packaged items were packed in corrugated outer boxes, stacked on a pallet and had been recently stretch wrapped, they might be subjected to slow moving high static fields when a forklift moved another stretched wrapped pallet next to the first. It was suggested the fields could reach 100,000 volts, be within 6 inches of the clamshells and be moving at approximately 5 miles per hour (the speed of a forklift). Good question!

When VPI Mirrex heard the question, they asked Fowler Associates to perform tests to determine the actual ESD hazards of such a condition.

We set up a moving box situation with a curved electrode on which we applied a high voltage. We attempted 100,000 volts at six (6) inches from a corrugated box but could not maintain that voltage due to the discharge characteristics of the test set up.

This probably also means that 100,000 volts in this situation could not last long enough to be a threat. We could hold a 50,000 volt field at 3 inches.

 

This is in fact probably a much larger voltage field at the clamshell than the 100,000 volts at 6 inches. (maybe as high as 200kV) Therefore we believe that the test set up we had faithfully reproduced the question.

 

 

 

In this test set up we had clamshells with ESD sensitive MOSFET's (2N4351's - 150 volt HBM) as well as actual circuit cards from a major communications company.

The clamshells with circuit cards and MOSFET's were positioned at the outside wall of the corrugated box. The curved probe with 50,000 volts applied was moved at 5 miles per hour past each box surface at a distance of 3 inches This movement was performed 10 times per side. The MOSFET's were tested before and after exposure to the high voltage field at Fowler Associates.

 

The actual communications circuits were tested before and after exposure by the manufacturing company.

 

Conclusions:

Our tests show that even in this extreme situation, there were no failures of either the ESD sensitive MOSFET's or any of the acutal communication circuit cards. These results confirm the tests performed in the previous study and confirm the merit of "Air Gap" shielding.

As we stated in the previous paper, this does not diminish the benefits of static shielding bags. It only allows the packaging engineer to have more opitions in chosing ESD protective packages. This means the engineer may take such parameters into account such as transparency and physical protection.

 

50,000 Volt @ 3 inches / 5 mphTests
10 Passes of Each Polarity per Side of Corrugated Box

Communications Circuit Boards 4 Circuit cards 10 passes each at +50kV and -50kV	all passed QC Checks at manufacturer
2N4351 MOSFET's 12 MOSFET's exposed at various locations in clamshells and subjected to 10 passes each at +50kV and -50kV	All passed curve tracer tests