Gtech Accutherm - ESD-Induced Failure Modes; Typical Symptoms of ESD Damage; Common False Assumptions Concerning ESD

Handling Precautions
Service Manual for Accutherm Ultra Printer A-iii
ESD-Induced Failure Modes
Radiated Electromagnetic Fields
Radiated electromagnetic fields induce low-level voltages in unshielded signal
conductors. These can cause intermittent unit halts from which the operator may
recover. Older products are more sensitive to these fields. Products manufactured
today are designed with covers and shielded external cables to protect them from
most induced voltages.
Conducted Charges
Conducted charges (usually at points where the operator touches the unit) may
transfer directly to components and result in either intermittent or permanent
failures.
Typical Symptoms of ESD Damage
Hard failures such as blown semiconductor junctions, cracked oxide layers, fused
metallization or bond wires can result from ESD, however, intermittent failures
are the most common result of ESD. The device becomes temperature sensitive,
input thresholds shift, output levels and drive ability degrade, etc.
Increased failure rates are also typical. Normal stresses such as temperature
swings, power surges, or another “zap” could permanently disable a device
previously exposed to ESD, even if no symptoms existed from the first exposure!
Common False Assumptions Concerning ESD
• MYTH: Only MOS devices are ESD sensitive.
FACT: All semiconductor materials are sensitive to ESD. Some devices
are just more sensitive than others.
• MYTH: A component cannot be damaged once it is installed in a board.
FACT: It may be even more susceptible to induced fields due to the
antenna effect of the etch or wire connected to it.
• MYTH: If the device works after I replaced it, I got lucky and did not
damage it.
FACT: Most failures are not catastrophic and only reveal themselves as
intermittent or latent failures.
• MYTH: A grounded metal table top is a good anti-static work surface.
FACT: A much better way to dissipate electrostatic fields is to use an anti-
static mat and a 1-Megohm discharge current limiting resistor connected to
earth ground so that the charge is drained in a controlled manner.