FAQ on Shielding Bags    

 

 

Q:     Antistatic shielding bags, how do I determine when to discard them?

A:    There are many types of shielding bags some reusable, perhaps the ones you mention, and of course the so call single-use that must be discarded after a return trip to the field. The latter are here recommended as once the bag is discarded you will know that the new one to use will be ok, hence you will save yourself the problem of having to manage their full life spans, particularly if the company is large and short on manpower resources for this "menial" task.

 

Q:    What is a polymer bag with carbon?

A:    These bags are seldom used anymore,  they are made with carbon on some 50% proportion, the intention is to protect from static discharges as it gives a a more-or-less low resistivity, somewhere between 10kΩ and a 10MΩ. The problem with them is they do not allow see-trough and also electronic devices protected gets some carbon dust deposition that may damage some delicate circuitry. 

 

Q:    What is being used to cushion ESD sensitive devices when packing?

A:    Different types of foams made of a conductive polyethylene material and a dissipative polyurethane used in different concentrations and combinations by manufacturers as to achieve ESD protection and/or ESC inhibiting of charge generation. 

 

Q:    Why some polymer bags are coloured?

A:    Manufacturers stain them as to facilitate identification. The antistatic compound is imbedded in the polymer. Even though they are transparent to the eye which is good but they have other shortcomings as their surface reacts with ambient humidity making them inefficient as they loose their antistatic qualities.

 

Q:    What are these pink poly antistatic bags?

A:    They are made of polyethylene and free of amine. They are used for packing highly sensitive devices which are to be protected from tribocharging. 

 

Q:    How are made and how good are  those metallic looking bags?

A:    These polymer bags can be metal-in or metal-out. The metal-in bags have an outer and inner surfaces resistance of a dissipative material (outer polyester and inner polyethylene) under 100GΩ and sandwiched in between there is a very thin layer (~20 Angstroms) metallic layer (usually aluminium) with a resistance of a few tens of ohms. The metal-out however are coated with a very thin layer of a few nm  layer of metal (usually nickel or aluminium), the outer metallic layer would be protected by some abrasion resistant compound. These bags are of good quality and about 40% to 50% of light is let through they are to be used in demanding applications, their external resistivity is in the vicinity of 10kΩ and should be able to dissipate some 5,000V of charges to zero in some 50us. They may have a tough layer of polyester as to make them more resilient to punctures and polyethylene to reduce tribocharging . They are also internally coated with a dissipative compound. Their thickness is of the order of around 3mils.

 

Q:    What about packaging materials other than shielding bags?

A:    There are the insulating, dissipative and metallic types of packagings. The insulating type material once touched by a charged body will not trigger an ESD event as there is no path for charges to move through into an earthed surface. Consequently because of the absence of a spark there is assumed no damage to the electronic device transported inside. There is however danger of the device having created its own charges inside by bouncing around during transport and the device may get ESD damaged at the time of opening the the package due to the external or internal charges. The dissipative type however can be ESD zapped when touched by a charged body but it will control de passing of charges to-and-fro over its surface, helping to control the ESC charges movement process.  The metallic type is good for protecting an electronic device inside however are not see through and they cost more. 

 

Q:    What are these finger cots and what are they used for?

A:    They are used on the fingers and thumb of the hand and are about 1.5inch long and there intention is to protect against ESD discharges (antistatic type) and the dissipative type to allow for charges from charged objects to leak to earth under control.

 

Q:    Some bags are offered by manufacturers with a zipper, isn't this likely to introduce charges due to the tribocharging effect?

A:    In theory yes, however manufacturers use special low charging materials for the zipper eg polyethylene.

 

Q:    I noticed that most Hard Disk in the PC industry are delivered within clamshell transparent packages, how good are they?

A:    They are also referred to as 3-D or thermoformed packages. There are many articles in the ESD industry where these packages have been analysed and discussed. Most of their recommendations to use them are favourable, particularly in the telecommunications industry where technicians are required to carry and exchange panels in the field.  The packages are rigid and achieve components' protection due to their double walled structure where air is sandwiched in between. It is said that in addition to their protecting properties they are physically resilient to mistreatment.