devices

In Part 2 of this series we indicated that a key element in a successful static control program was the identification of those items (components, assemblies and finished products) that are sensitive to ESD and the level of their sensitivity. Damage to an ESDS device by the ESD event is determined by the device’s ability to dissipate the energy of the discharge or withstand the current levels involved. This is known as device “ESD sensitivity” or “ESD susceptibility.”

Perhaps the most recognized industry standard in North America for surge protectors may be UL 1449, the Standard for Safety for Surge Protective Devices ranging in applications from surge arresters (1000v ac max) to service entrance and distribution panels to plug-in surge strips. The standard was officially adopted as an ANSI standard in May 2010 and a supplement specific to the evaluation of Photovoltaic applications is expected in 2011. However, it is important to keep in mind that UL 1449 is first and foremost a safety standard. For more in-depth information regarding characterization of the surge environment, test methods and the application of these devices, the IEEE C62 standards are the definitive source in North America.

Many sources recently have reported that electrical failures to components previously classified as EOS (Electrical Overstress) are instead the result of ESD (Electrostatic Discharge) failures due to charged-board events (CBE) [1,2]. A charged printed circuit board assembly stores substantially more charge than a discrete device as its capacitance is larger. A subsequent discharge of the board assembly results in increased current for that event - versus that of the discrete component. Consequently, a device’s CDM (charged device model) rating is lowered when mounted in a printed circuit board (PCB). In an attempt to get a feel for just how much it is lowered, we conducted CDM stress tests on components in discrete form, and again after insertion into larger and larger sized pc boards. We found that the CDM ratings are lowered dramatically!