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Bathtub Curve
When you look at large numbers of electronic components, the rate at which they fail falls into a statistical pattern called a "bathtub curve," so-called because of its shape. A batch of parts will have some early failures; these are mostly due to tiny structural defects that happen in the manufacturing process. These problems form a high point at the beginning of the statistical curve; manufacturing specialists call these early failures "infant mortality." After this early period, the parts that function will continue to work for a long time. Minimal operating problems give this part of the curve a low, flat shape, forming the "bottom" of the bathtub. After several years, cumulative stresses and aging effects cause parts to fail at rates that increase with time; the rising curve forms the right side of the bathtub shape.
Thermal Stress
Virtually all electronic parts are rated for a specific range of temperatures; outside these temperatures, the components fail or their operating parameters change. At extremely high temperatures, plastic parts melt and burn and solder connections come loose. Transistors, diodes and integrated circuits are very sensitive to heat; their lifetimes become markedly shorter as temperatures rise. For decades, PCs have needed cooling fans to keep expensive parts from overheating and burning out.
Electrical Stress
All electronic components tolerate a limited range of voltage and current; electrical extremes cause significant stress to parts and can cause catastrophic failure. An incorrectly-wired diode or transistor, for example, can explode; some types of capacitors burst when exposed to too much voltage, emitting puffs of pungent black smoke.Even copper wires--the simplest electrical components--melt when a circuit carries excessive current. Complementary metal-oxide semiconductors, which make up most computer chips, are highly sensitive to voltage; the tiny static build-up in your body from walking across a carpet can destroy these components instantly; these parts require special anti-static tools during equipment assembly.
Mechanical Stress
Although most solid-state components can withstand extremes of vibration, jars and other mechanical stress, the connections between them become fatigued over time and break. Engineers rate mechanical stress in terms of "g forces," where 1 g is the force exerted on a component by the Earth's gravity, an acceleration of 32 feet per second squared. Components that are solid and rigid, such as integrated circuits, are highly resistant to vibration stress. On the other hand, cables, connectors and other "floppy" parts can break from fatigue stress when exposed to mechanical shock and vibration. A tumble from a desk can expose an electronic device to thousands of gs of impact force from a concrete floor; most consumer electronics can withstand a few such mishaps, but repeated abuse will break the device.