David, The main problem with tantalum caps are, to assure reliable/safe operation, the voltage rating must have at least a 2X derating. Many designers, fresh from school, are not aware of this practice and still derate caps by only 0.5x. I've heard of a 2X+5 rule that is also recommended by reliability consultants. The derating helps insure the cap can dissipate heat from large or repetitive di/dt transient surges, or simple inrush current, quickly enough to prevent the catastrohpic failure of the cap.
As a design example, to filter a five volt supply, the cap should be rated at least 10 volts, 15 volts if feasible. You are also correct about the failure mode. While electrolytics tend to ooze quietly when they fail, tantalums are impressively loud when they go. This mode of failure can lead to greater damage to the circuit board than an electrolytic might do, and the somewhat violent scattering of part fragments. So tantalum caps are a hazard to the people at manufacturing test stations that power up the devices without protective eyewear, or electronics where the user could be operating the product with a cover or access panel removed. (A common assembly error is to install polarized capacitors backwards, this way tantalum caps often explode when powered up for the first time at a test station.) Regards, Eric Lifsey Compliance Engineer/Manager National Instruments
