Sometimes when reaching for a doorknob you may feel a static discharge. Surprisingly that voltage has to be over 3 kV to be felt! However, some semiconductors and sensitive circuits can be damaged by electrostatic discharges with less than a hundred volts. This is a concern for the component manufacturers and production technicians who must manufacture, ship, store, and install ESD sensitive components. The damage may already have occurred, with no indication of a discharge apparent.
MOSFETs are well known to have static sensitivity, as are some integrated circuits. LEDs are also sensitive, especially blue LEDs. To help protect these devices, internal ESD protection is built in. Devices which are sometimes overlooked are precision thick film resistors and some capacitors. Also, it has been incorrectly assumed that an assembled circuit board is not sensitive to ESD, however this is not the case. ESD damage can occur to each of these devices including full circuit boards from several sources. And finally latent failures, a failure which does not occur immediately but over time due to damage and deterioration of a component, is a common issue with ESD.
Understanding ESD Issues
First, understand that a direct electrostatic discharge event is an amazingly fast current discharge which most models have voltage rise times of less than 1 ns, and pulse durations less than 150 ns. When the discharge is directly to a device or circuit board, the fast transfer of charge can cause overheating in semiconductors or on some small traces. If the energy is applied to a pin or wire, a current surge and subsequent voltage transient can cause damage.
The most common source of ESD is from human contact. Unless properly protected and grounded, a person may become charged for many reasons. Reasons to include walking across a carpet, having static generating clothing, or interaction with chairs and seats and many other items with which a person can come in contact. The transfer of the static charge may be through direct contact, or by picking up a tool such as a screwdriver and touching the equipment. Handling connectors can place a charge on the connector shell or directly on open circuit wires.
There are three models used to approximate the energy from ESD – the Human Body Model (HBM); the Charged Device Model (CDM), and the Machine Model (MM). HBM has the highest amount of impedance, or series resistance associated with it, 1500 ? from a 100-pF capacitance. For CDM, when a device becomes charged and is then placed on a grounded surface, the device can experience ESD. In this case the capacitance is small, 4 – 30 pF, but the series impedance is also very small, with the common model having 1 ? in series. The Machine Mode is the most difficult to meet, since it requires a larger capacitance, no series resistance, and possibly some series inductance. It simulates what might occur if a person holds a screwdriver and discharges to the device, or by machines, metal carts and cabinets, or other items capable of holding large charges and discharging without series resistance.
How To Filter Out Transient Response
The elimination of these issues requires that the persons are grounded, as are the equipment and devices used to manufacture, move, and store the part. However, discharges continue to occur in spite of our best efforts to avoid it. During installation, if the equipment is connected to a cable and the installer is highly charged, it is possible for the installer to place a significant charge on the connector or wiring itself. When the cable is connected to the equipment, there may be a discharge on contact. If the equipment is not properly designed, damage (either immediate or latent) may occur.
Protection from field related ESD damage such as this, if not designed into the product, may be retrofitted into the connector. Transient voltage suppression diodes (TVS), varistors, or in some cases larger values of capacitors, can protect the circuit board from ESD and other voltage pulses. By shunting the energy to chassis, the damaging current can be diverted to safety ground.
When placed on signal lines with high-speed data, such as USB or ethernet, TVS diodes are often the best solution. These devices have very low capacitance and should not filter the data as much as a capacitor would.
Equipment should be protected from ESD, either through the initial design or through retrofit. This can avoid potential latent and field failures which may be costly.