I am trying to develop a very high reliability product, with a MTBF exceeding 100,000 hours, hopefully in the 250,000 hours range. Yep, I know - stupidly high reliability, at least 10 years.
Most LED's I've seen have a reliability rating of only 20,000 hours. What happens after this? Do they reduce in output too much? Are they usable as power indicators past this point?
Must say I've always been highly sceptical of the meaningfulness of MTBF figures. LEDs will gradually reduce brightness over time, but it will be many times their stated life before they cease to be useable as a power indicator. Running at well below the specc'd current will increase lifetime many-fold. And would failure of a power-on LED be counted as a failure of the product?
I think you are utterly doomed trying to get to MTBF of 250000 hours on any usefully complex product, but then you haven't said what it is, so it might be very simple.
LEDs tend to age faster if they're hotter / driven at higher currents, and the failure mechanism is different for white leds (which are fluorescent) and natural-coloured LEDs. But generally they get dimmer as they age.
I would expect that you would get very long life if you significantly under-drove the LED (modern traditional-colour (red, etc)) LEDs are incredibly bright and tend to need very little current to act as a useful indicator, and if you weren't too fussy about the specific output you could play faster-and-looser about the failure condition.
But if I was trying to make a fantastically reliable product, one way I'd do that is to dispense with absolutely every component I possibly could. I expect the power-on LED would fail to make the cut in that regard.
Update: Well, clearly I know even less about reliability than I thought I did - it seems lots of diskdrives claim MTBFs well over 250k hours - so good luck with your product!
MTBF is based off of actual run time. So at a goal of 250,000 hours you are hoping your product can run 24/7/365 for almost 30 years.
A MTBF of 20,000 gives you just over 2.25 years of constant run time at recommended max power. This time is pretty good for most commercial products right now. If your LED is on for 8 hours a day, you are looking at a life of almost 7 years. Now if you decrease the power your putting into the LED you can even extend this life longer.
Now with all of that said, if it is just a power indicator you wont be switching it often so you will put less wear on the device.
I know I haven't directly answered your question, but I am hoping you are starting to realize that MTBF isn't something you should worry about too much. I realize you want your device to last awhile, which is a good thing, but you will probably find that if you always go with the highest life items they will generally be more expensive which will cause your product to be inflated in price, resulting in very few wanting to buy it. What good is a device that lasts for 30 years if no one buys it?
MTBF is assurance that reasonable derating guidelines are met. Nothing more. An MTBF of 250k hours does not mean any single piece of equipment will last 30 years. It's possible to get that sort of MTBF on equipment that uses electrolytic capacitors, which almost certainly will dry out in much less time than that.
Drive the LED with the minimum amount of current that provides a useful amount of 'signal' (i.e. is viewable under whatever conditions you deem inportant.) The LED manufacturer should have datasheet information about operating current / temperature rise / ambient temperature, so you can judge how 'hard' you're driving it and make your own decision about reliability.
