Monday, June 13, 2005

Are capacitors ESD suceptible?

>> What is the likely device sensitivity of a thru-hole ceramic capacitor?
>> My general understanding is that passive devices are not ESD sensitive.

Many types of component can be damaged by ESD - including passives. Whether damage is likely to occur depends on the ESD withstand of the component and the strength of the ESD. (Taking an extreme example, lightning is a form of ESD!)

In the case of capacitors, damage can occur if the capacitor is charged up enough to exceed the dielectric breakdown voltage. This can occur if sufficient charge is dumped into the component in an ESD event. Low value low voltage types will require less charge to exceed the breakdown voltage and so it is normally these that might be susceptible to damage. Low value MOS capacitors may be particularly prone to damage.

Many higher value capacitors are reasonably rugged from an ESD view and can largely be considered not susceptible to ESD under ordinary circumstances - especially if the y have high breakdown voltage.

Some types of resistor, such as some film resistors, my also be susceptible to ESD damage.

Tuesday, June 07, 2005

ESD bags

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.