ESD events occur naturally when two materials with different electrical potentials attempt to balance out. Human skin is one of the most easily charged materials second only to air. The charge is derived from an excess of electrons being stored. When we come into contact, or proximity, with a device or substance with a fewer number of electrons the balancing process can be seen as an arc. These static events can range in voltage from a few millivolts to several thousand volts. This voltage is not necessarily responsible for curcuit damage rather the current and how quickly the current passes defines the extent of the damage done.
Large amounts of current can cause complete component failure or a partial failure that gets worse over time. Complete failure is fairly easy to diagnose. Partial failure can cause odd component reactions intermittently which are never easy to diagnose. See photos below courtesy of SRI, Source Research Inc.
As indicated by the photos, an ESD event has damaged a board trace and a capacitor. The trace should fail more quickly than the pit showing on the cap. This could cause a slow failure over time.
Components can be damaged even before they are populated on the board. Proper ESD prevention can reduce the number and severity of these events. Never take a part from someone without touching them first to equal the potential voltage between you. Preferrably, touch the hand that does not contain the part, or use and anti-static bag or foam to transport parts. Grounding bracelets, mats and anti-static bags are the most popular preventative devices.
ESD can be destructive to high speed digital circuits even if you can't feel a shock. Keeping this in mind will keep you from adding more problems than you are trying to fix.
As indicated by the photos, an ESD event has damaged a board trace and a capacitor. The trace should fail more quickly than the pit showing on the cap. This could cause a slow failure over time.
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