Factors Affecting Reliability

[As of April, 2011]

Operating Environment Factors

Environmental Factors

The factors that affect reliability and should be taken into consideration as described above include the above-described failure factors that exist within a product as well as external stresses that can accelerate those factors, i.e., the operating environment. Table 1 shows the operating environment factors that affect reliability.
Some of these factors operate independently. Generally, however, external factors affecting reliability are intricately interrelated. For example, corrosion breakage is caused by a combination of temperature and humidity.
Stresses are broadly divided into those deriving from the natural environment and those related to human factors.
Stresses deriving from the natural environment include temperature, humidity, atmospheric pressure, salinity, overvoltage surges due to lightning, and the cosmic rays. Of these, temperature and humidity are the most critical factors.
In general, a rise in temperature speeds up chemical reactions and accelerates changes in materials. This in turn accelerates failure mechanisms which cause failure. Therefore, temperature must be monitored carefully. During actual use, increases in temperature due to the environment as well as self-generated heat resulting from power consumption must be taken into account.
A change in temperature produces distortion stress on the junction between two materials with different expansion rates. If this occurs repeatedly, material fatigue arises, causing failures such as hermetic seal damage, die bond adhesion damage and bonding wire opens. In addition, if the device is used with improper connections, the heat generated from the equipment or element can accelerate the temperature change and affect the product in an accelerated manner.
Humidity primarily causes condensation to adhere to the surface of the device and, consequently, increases the electric conductivity of the material surface. This increases leakage current, which in turn leads to defective characteristics and defective operation. Humidity can also accelerate chemical and electrical reactions, producing metal corrosion.
Resin encapsulated devices especially have an inherent moisture permeation problem. However, great strides have been made to improve resin materials, resulting in improved resin capsulation that, when a comparison in actual operating environments is made, is by no way inferior to hermetic sealing.
Atmospheric pressure affects devices used in mountainous regions or in aerospace applications. Low atmospheric pressure induces a corona discharge between electrodes and reduces the package’s heat radiation rate. This results in internal thermal generation, accelerating the rise in chip temperature.
Salinity greatly affects devices used in coastal regions, ships and marine applications. Salt adhering to the element surface deteriorates the insulation between electrodes and increases the change of damage caused by metal corrosion.
Lightning readily affects outdoor applications, such as traffic signaling equipment. Special protective measures must be taken to increase the ability of devices to withstand voltage surges caused by lightning.
Other natural environment factors include soft errors caused by the alpha rays of radioactive isotopes in packaging materials which affect high-integration memory, and damage or malfunction caused by radiation in certain applications, such as nuclear reactors and the aerospace.
Human factors affecting reliability include vibration during transport and in vehicle applications (Table 2); shock during handling by industrial robots or caused by dropping (Table 3); heating during printed circuit board soldering; voltage surges during the opening and closing of switches; noise from poor relay contacts or motor devices; electrostatic damage caused by use in low humidity environments; malfunction due to strong electromagnetic waves from a nearby transmitter or communicator; and ultrasonic vibration during printed circuit board cleaning after soldering.

Factors that Arise During Equipment or System Use

In addition to physical factors caused by the natural environment or human error, operating conditions imposed by semiconductor use in a device or system also affect reliability. This occurs when a product is used irrespective of the maximum ratings defined in specifications. Examples are device breakdown due to use at a voltage higher than the rated voltage, malfunction due to use at low voltage, breakdown due to excessive loads, and malfunction or breakdown due to use based on an operation timing other than that specified.

Table 1 Operating Environment Stress Factors Affecting Reliability

Stress Factor		Potential Time/Location	Failure Mode
Temp-erature	High	Tropics, deserts, space, cars, other special environments	Characteristic failure, unstable operation
	Low	Cold regions, high altitude, space, aircraft, other special environments	Characteristic failure, unstable operation
Temperature change		When operated intermittently	Die cracks, degraded die bonding, characteristic failure, unstable operation
Humi- dity	High	Tropics, tunnels, cars, other special environments	Rusting, improper contact, wire corrosion breakage, characteristic failure
	Low	Deserts, in low-humidity weather	Electrostatic damage
Atmo-spheric pressure	Low	High altitude, mountainous regions, aircraft	Corona discharge, low heat dissipation, characteristic failure
	High vacuum	Space	Corona discharge, low heat dissipation, operation failure
Salinity		Coastal regions, on the sea, ships, marine facilities	Rusting, improper contact, lead damage
Vibration		During product transport, machine tools, aircraft equipment	Bonding wire open (hermetic seal), lead damage (board-mounted device), package damage
Impact, dropping		During product transport, vehicle-mounted equipment, machine tools, aircraft equipment	Package damage, lead deformation
Acceleration		Aircraft equipment, rockets, other special-purpose equipment	Bonding wire open (hermetic seal), package damage
Heating		Assembly process (soldering process, etc.)	Characteristic failure, abnormal package shape
Overvoltage, surge		Switches, relay switching, capacitive loads, motors	Characteristic failure, short
Noise		Motors, improper contact	Malfunction, characteristic failure, short
Static electricity		When handling in low humidity, near strong field generating equipment, during transport	Short, characteristic failure
Strong electromagnetic field		Near a transmitter or signal generator	Malfunction
Ultrasonic wave		When cleaning circuit boards after soldering	Bonding wire open (hermetic seal) , erased marking
Radiation		Nuclear power related facilities, space (satellite)	Characteristic failure, malfunction, destruction, memory soft error
Misuse	Over-voltage	When used with invalid operation supply voltage	Destruction, degraded breakdown voltage, short
	Overload	When used with invalid drive capacity	Destruction, short
	Other	When used at invalid operating timings, etc.	Malfunction, latch-up

Table 2 Main Environments with Vibration

Type		Description
Land transportation	Road	1 to 3 Hz up to 29.4 m/s2, 15 to 40 Hz at 9.81 m/s2
	Rail	14.7 m/s2, an amplitude of ±0.05 mm at 2 to 100 Hz
Sea		Frequency: 1 to 50 Hz, Amplitude: 2.5 to 0.075 mm
Air		Frequency: 3 to 500 Hz, Amplitude 3 to 0.025 mm

Table 3 Main Environments with Shock

Type		Description
Land transportation	Road	58.9 m/s2 peak, 5 to 40 mm/s shock width
	Rail	When coupling and decoupling rolling stock: 196 m/s2, Speed change: 5.4 m/s
Sea		-
Air		Shock on arrival and departure: Up to 36.3 m/s2

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