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Avoiding electrical overstress in electronics

Posted: 30 Jan 2014  Print Version  Bookmark and Share

Keywords:Electrical overstress  EOS  electrostatic discharge  power supply  electromagnetic interference 

One of the most typical causes of failure among electronic products is electrical overstress (EOS). While a good instructional manual prevents users from using devices at non-prescribed power levels, unintentional EOS is always possible. Overstress could be an outcome of numerous scenarios, including supply surges and application of over-voltage due to incorrect input supplies. Having effective EOS protection is a primary requirement for product durability.

EOS is used to describe the thermally-induced damage that may occur when an electronic device is subjected to a current (figure 1) or voltage that is beyond the specified limitations of the device (figure 2). The high current that can occur during an EOS event can generate a localized high temperature that causes damage to the materials used in the device's construction. An EOS event can be a momentary event lasting only milliseconds or can last as long as the conditions persist. EOS can be the result of a single, non-recurring event or the result of ongoing periodic or non-periodic events.

Both EOS and electrostatic discharge (ESD) are voltage overstress conditions, but as shown in table 1, they differ in the energy involved and time span of the event.

Table 1: Comparison of EOS and ESD.

A typical failure symptom of EOS involves excess supply current being drawn, low resistance between the supply and ground pins, a pin short between either supply and ground, and/or functional failure of devices.

In some failure scenarios, visual damage to the device is evident. This may be in form of bulge or physical hole in component package, burn/discolour/crack in component package. In majority of the cases, the damage may be internal in form of melted/burned metal, melted/open bond wires.

Figure 1: EOS external damage.

Figure 2: EOS internal damage.

EOS causes
The primary cause of EOS failure is a voltage surge on the power supply. Overshooting or undershooting during I/O switching, voltage spikes due to internal switching, or an external connection (coupling) can also lead to EOS. Additional designs issues like poor return path due to improper grounding (leading to excessive noise on the ground plane) can lead to EOS of devices. For systems operating in noisy environments, poor shielding can make the system vulnerable to electromagnetic interference (EMI), in turn leading to EOS failures. Devices weakened due to events like ESD will be more susceptible to future EOS events. Latch-up can lead to EOS if the current is not limited or if the event occurs over a long time period. An improper input provided by a user can also lead to EOS on power or I/O lines.

Preventing EOS at the product level
The ideal scenario is to have clean power supplies and controlled ramping at power up and power down. Having proper de-coupling capacitors helps the suppression of any noise on power domains. Additionally, having low resistance conduction paths for power and ground in the printed circuit board help avoid unwanted noise from reaching the device I/O or power lines.

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