Steven.D.Ligett@mac.dartmouth.edu (05/26/90)
There was recently a question about the reliability of the tantalum capacitors. I just got a new tantalum cap catalog from AVX. Here's a summary of the reliability info. Reliability is dependent on three things 1. Vapplied/Vrated 2. Temerature 3. Series resistance. The caps I'm ordering are rated at 1% failures/1000 hours, @85 (degrees)C, with 0.1ohm series resistance. Factor 1: All the caps will be rated at 20 volts or higher. For the 12 volt supply, this gives a failure factor of 0.03. For the 5 volt supply, this gives a failure factor of 0.002. Factor 2: If you cool your system to 50 C, this gives a failure factor of 0.09. If you cool your system to 40C, the factor is 0.05. Factor 3: For a series resistance of 0.2 ohms/volt, the factor is 0.6; for 3 ohms/volt, it's 0.07. ------------- So, calculating the failure rate of a 20 volt cap on the 12 volt line, in a system at 50C, with a series resistance of 0.2 ohms/volt, gives 0.03 x 0.09 x 0.6 x 1%/1000 hours = 0.00162%/1000 hours That sounds pretty good to me. Perhaps someone with a flair for statistics can tell us when we'll likely see the first failure of that cap in the 100 systems that are being built. Final note - I can buy 25 volt caps and halve that failure rate. If you want it, I'll do it.