burn-in[分享]

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What is Burn-in?
Burn-in is the application of thermal and electrical stresses for the purpose of inducing the failure of "marginal (microelectronic) devices, those with inherent defects or defects resulting from manufacturing aberrations which cause time and stress dependent failures."

Who uses Burn-in?
Burn-in is normally performed by semiconductor manufacturers or independent test labs (ITL's); however it is not unusual for end users or contract manufacturers to use burn-in. Sample units of new devices or new lots are burned in to provide reliability data for the remaining devices. ICs for military and aerospace applications are burned in to ensure the highest level of quality.

How is Burn-in accomplished?
Devices are loaded into high temperature sockets which make temporary electrical contact with the device leads, and are mounted on high temperature circuit boards with circuitry to provide the proper voltages and stimuli to the devices. The devices are isolated from one another with passive components which limit the current each device can draw, and filter noise from voltage busses. The boards containing the devices (called Burn-in boards, or BIBs) are then loaded into a convection oven which elevates the temperature of the devices and provides an electrical interconnect to the power supplies and signal generators. (The oven, power supplies and signal generators comprise the Burn-in system.) The devices remain in the oven for an amount of time determined to induce failure in the marginal devices.

The rate that these devices fail is called the Infant Mortality Rate. If this rate is plotted using a line graph, as shown in figure #1, it is called the Bathtub Curve.

How does Burn-in work?
Most ICs consist of ion junctions implanted on a silicon substrate. The junctions are connected with small traces of conductive oxides (aluminum oxide being one). In burn-in, the ambient heat and the heat caused by current flow causes the junction temperature to rise. This causes the ion 'clouds' which surround the junctions to dissipate from their original locations. In addition, all of the elements which comprise the chip experience acceleration as the heat causes their subatomic particles to begin to move more quickly. These stresses can lead to premature failure of weaker devices. They can also lead to the failure of non-marginal devices, if the junction temperature exceeds the manufacturers' maximum rating. At the "end of operational life" shown in Figure #1, the heat of burn-in has caused the ions to dissipate to the point where the devices no longer function. Eventually, all of the ions will dissipate completely.