Metal-in Versus Metal-out
Editorial by ESD Journal Editorial Staff
A recent high profile trade magazine published
an article on shielding bags. The article was very useful for
the most part but did have some incorrect statements about the
two types of shielding bags. Our editorial staff reviewed the
article and believes some corrections are in order.
1. The article stated that metal-out bags "
typically outperform the charge-dissipation capability of metal-in
bags because the metal layers are closer to the surface and the
surface- resistance levels are lower." It also stated
that, "the average charge-dissipation rate for metal-in bags
can be 1 min for static charges from 1 kV to 100 kV but <2s
for metal-out types." No metal-in bag should
dissipate a charge in greater than 2 seconds from 5 kV to 50 Volts
by the definition of static dissipative materials. Charge
dissipation is certainly a function of the resistance. If the
resistance is less than 1 E11 Ohms by ESD S11.11 then the dissipation
can not be as large as stated for either type of bag. In
fact the metal-out bag will dissipate a charge on it's outer surface
in a few milliseconds or in as fast as a few nonoseconds if there
occurs a breakdown of the coating on the metal layer - which is
common. A metal-in bag will dissipate a charge on its surface
in a few tenths of a second up to 2 seconds depending on the surface
resistance. The late Dan Anderson in his good humored wisdom
always made the statement, " ESD is created by a spark, so
don't let it happen." Metal-out shielding bags have
a highly conductive outer layer which promotes a more rapid discharge
in static fields thereby creating the event which the metal layer
must then attempt to attenuate. Metal-in shielding bags
have a dissipative outer layer which inherently dampens the discharge
thereby lessening very damaging electrostatic events. The
shielding capabilities of the actual metal layer is a function
of its conductivity not its position in the bag.
2. Metal-in and metal-out bags provide similar
static shielding with little measurable differences in most of
the present test methods. The article implied that a metal-out
bag may allow 25 volts inside with 1000 volts applied and a metal-in
bag might allow 100 volts inside. This is not true.
Both bags should be below the 30 Volt level using
the EIA 541 static shielding test method which is required by
Mil-B-81705C. The first military approved shielding
material was metal-in. Many metal-in bags out perform metal-out
bags in actual use due to the difficulty in manufacturing the
metal-out bags with the metal so close to the surface.
This difficulty as well as the difference in the quantity of metal-out
bags used makes the metal-out version of static shielding bags
generally more expensive. Scratching of the surface causes
many metal-out bags to be less effective than their metal-in counterparts.
The article did state that manufacturing quality is very
important. However in actual use when a well made metal-out bag
and a well made metal-in bag are compared, the results are indistinguishable.
Where rf field sensitivity is important, the rapid
discharge of an electrostatic event to the highly conductive outer
layer of a metal-out bag may cause a higher radiated field which
may cause concern for objects inside the bag or near the bag.
The article gave good information on the sources
of bags from several major suppliers. Our comments are not
intended to take away from this service. The end user of
static shielding bags can be comfortable in chosing which type
of bag to use knowing there is little or no difference between
the types in the actual shielding capablities.