Microelectronics Burn In
Burn-in is designed to weed-out infant mortalities during the manufacturing process.
It is particularly important for high-reliability chip & wire circuits, where the bare die have only undergone dc probe checks. After initial electrical test, the circuits are powered-up in a burn-in board and loaded, either actively or passively. The board is then placed in an oven at +125°C for 100 hours. They are then re-tested and if good, they are lid-sealed and re-submitted to a further 68 hours at +125°C.
This process is effectively an accelerated life test to get the circuits through the higher failure rate which occurs at the start of a device's lifecycle.