ESD packaging and bags must be one of the most frequently misunderstood aspects of an ESD protection program. Today I have been writing a workshop on the subject for the Electrostatics 2005 Conference.
Three main types of bag are commonly used - pink polythene, black polythene, and metalised shielding bags. Pink polythene is cheap - but gives little or no ESD protection to a susceptible electronic component or board. It is best used for other purposes within an ESD Protected Area (EPA), for example enclosing non-ESD sensitive parts or documentation. Black polythene ESD bags are quite conductive and can give good shielding against electrostatic fields. However, because of their high conductivity, a direct discharge to the bag can be conducted through the bag to damage a component inside.
Shielding bags are a multilayer structure, with low charging and dissipative outer layers. They include a conductive metalisation layer that acts as a shield to electrostatic fields. They also include an insulating layer to stop direct ESD current flow through to the bag's contents. The 61340-5-1 standard requires this shielding performance to protect any ESD susceptible parts outside an EPA. Bags are tested by subjecting them to a simulated Human Body Model ESD event - a sensor inside the bag picks up the residual ESD signal, and the energy is measured. Shielding bags pass this test if the residual energy detected from a 1 kV HBM event is reduced to less than 50 nJ.
Surfing the web while writing my presentation I came across quite a useful article on the subject of Choosing the right ESD bag.