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Fowler Associates Labs

 

 

Static Fire Stories Articles & Technical Papers Current News

Metal-in Versus Metal-out Shielding Bags

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.

 

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