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!
Mirrex heard the question, they asked Fowler Associates
to perform tests to determine the actual ESD hazards of such a
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
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
The actual communications circuits
were tested before and after exposure by the manufacturing company.
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
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
12 MOSFET's exposed at various locations in clamshells and
subjected to 10 passes each at +50kV and -50kV
All passed curve tracer tests