Lucas, 

Hybrid manufacturers, as well as semiconductor manufacturers, frequently
"burn-in" devices to weed out the infant mortalities - the weak devices in
the wafer that otherwise appear to be in great shape.  The characteristic
'bath-tub' curve indicates a relatively large number of infant (initial)
mortalities (failures) followed by a long, low failure rate lifetime and
once again ending with a large failure rate at end of life.

Considering that is characteristic of semiconductors to fail quicker at
elevated temperatures (hence the reason for heat sinks on heat generating
devices to keep them cool), we hybrid manufactures use burn-in to assure
that the products we manufacture - will likely continue to function for a
long time to come.  

This is especially handy for life sustaining devices like heart pacers,
pumps and hard to repair items for military & space applications.

I am not sure whether the semiconductor manufacturers originated the
concept, or the Military (Mil-Std-883), but it has been used for many, many
years.

Typically, the device is sustained at a junction temperature of 125°C.
Great care is given to maintaining the junction temperature.  

Passive burn-in is essentially a device with DC bias applied (may be forward
or reverse bias - whichever is most likely to induce problems quicker) while
the junction is maintained at the desired temperature.

Active burn-in incorporates the applied DC bias as well as active signal
processing, such as the clock for a microprocessor as well as data being
sequenced through the memory, registers, etc.  Since functioning devices
frequently generate additional heat, the junction temperature must be monitored.

Even commercial hybrid products sometimes go through burn-in of sorts -
usually dropping the temperature to around 80°C, or so.

In summary, burn-in still seems to have its application for high rel
products where there really is no other way to assure that the assembly
remains reliable for its projected lifetime.  The good news is that the
semiconductor manufacturers are doing a far better job than they were 20
years ago.  Catastrophic burn-in failures are reasonable few.  Most problems
result from "minor" parametric shifts in devices which cause the overall
assembly to fail (sometimes this is the result of an overly optimistic design).

Steven Creswick
Mgr Advanced Process Development
CTS Microelectronics

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