Comparing accelerated ageing with "real" ageing
"How is an electronic board that spent 100 hours in an oven at 90°C "similarly aged" than the same board that spent 10 years under operation ?"

It is not 'similarly aged'. There are two types of failure at work here, one is use-related and one is and underlying weakness that needs to be forced to fail. The purpose of 'burn-in' is to force any weak spot to fail and not to 'age' the electronics. Most electronics do not follow an age degradation profile (with the known exception of some components such as electrolytic capacitors) and therefore do not have a wear-out life. In fact according to Nowlan and Heap only 2% of equipments failures (mechanical and electrical) follow the tradition 'bathub' curve and a total of 14% of equipment failures conform to a wear-out pattern. The concept of an increase in probability of instantaneous failure with time has largely been discounted. These curves have been investigated time and again and form the basis for the RCM 'industry'. It is dealt with in great detail in Mowbray's RCM II and MIL-STD-3034 (US) and DS 02-45 (UK).

Most of the burn-in of boards and equipment is a calculated mix of thermal cycling, electrical load cycling and shock &amp; vibration cycling. This is designed to cause manufacturing weaknesses to fail such as poor quality solder joints that flex under thermal and vibration cycling and then fatigue and crack. It will also cause poorer quality electronic components to fail. In theory these boards, once the failed components have been replaced, will be more reliable in the field.