All devices experience stresses resulting from their environment, which can change
according to the application and state of operation. For example, all electronic packages
experience thermal cycle stress simply from turning the device on and off. Because
these stresses lead to failure, it is important to understand how the product responds
to environmental stress. In order to gain that understanding quickly, tests are run
at extreme (accelerated) conditions. Once a failure is observed, a full physical characterization
is performed in order to relate the failure to physical features of the product. A
few common failure types are discussed below.
Thermal Cycle Fatigue: Perhaps this is the most common cause of failure owing to the continued thermal cycles
that products experience. Products fail because the differential expansion/contraction
resulting from mismatch of the coefficients of thermal expansion of the various materials.
This occurs even in thermally controlled settings, such as an office or lab. Thermal
cycle or thermal shock chambers afford a means of systematically evaluating failure
under thermal cycling.
Dendrite Growth: When operating in a moist environment, biased circuits tend to form fine filaments
that can bridge wires and cause glitches or even short circuits. Such growth can be
forced in a temperature and humidity chamber which typically operates at 85C and 85%
relative humidity (RH). To achieve an even high stress, parts can be stressed in a
pressurized Highly Accelerated Stress Test (HAST) chamber; typical conditions are
130C and 85% RH. Bias in excess of the normal application condition is applied.
Corrosion: With the exception of gold and platinum, metals used in electronic circuits are susceptible
to water enhanced oxidation. This particularly troublesome at junctions of dissimilar
metals where galvanic action is part of the corrosion process. This type of failure
is conveniently forced in a temperature and humidity chamber or a HAST chamber. Since,
bias is not part of the normal corrosion mechanism, no potential is applied to specimens
under stress.
Shock and Vibration: Depending on the end user, mechanical shock and vibration may be a significant contributor
to wear out of the product. Vibration of an automobile or airplane in transit mechanically
loosens connections. Consumer electronics are prone to shock induced failure; dropping
a cell phone is a common occurrence. A vibration table and a shock tower provide the
means to assess quantitatively the effect of mechanical shock (e.g. 500G with a pulse
width of 1ms) and mechanical vibration (vibration sweep from 20 Hz to 2 KHz).
Electromigration: With sufficiently high current density, the electron motion is able to induce movement
of the atoms in the conductor and interconnections, giving rise to voids. Accumulation
of the voids restricts the current, and ultimately may cause complete fracture of
the conductor/interconnection. Interfaces are particularly prone to electromigration
failure. Assessment is done by forcing high current through the structures to be evaluated.
To further increase the stress, the samples are maintained at the elevated temperature
throughout the test. A simple test can be conducted with a current regulated power
supply and a well-controlled oven.
