Sigh... aluminum electrolytics are not leaky! They will not dry out unless they are going bad. They are hermetically sealed.
No, Aluminum Electrolytics are NOT hermetically sealed. Their seals MUST allow some venting to occur. As the electrolyte is consumed, hydrogen gas builds up. That gas must be allowed to diffuse through either the case or seal, otherwise it would explode. By definition, Aluminum Electrolytics are not hermetic.
Don't be temped by the new solid electrolyte aluminum capacitors. The lifetime goes down -much- faster with elevated temperature than it does for standard damp electrolytic aluminum capacitors.
While it is true solid organic aluminum capacitors do have reduced lifetime at elevated temperatures (like all components), that loss of life does not occur until well beyond 85C.
Many people mistake the "qualification" of solid aluminums to be the same as "endurance" for wet electrolytics. This is absolutely not the case. Solid Organic Polymer capacitors do not have a wear out mechanism like the "wets". The Polymer is the actual cathode material, not an electrolyte. (This is not the case with OS-CONs which use a solid salt AS an electrolyte.)
Also, don't compare Solid Aluminums from a non-name Asian company. Tier 1 Solid Aluminums are excellent components.
It seems solid tant capacitors are now becoming the cap of choice in high reliability applications.
Solid Tantalum-[u]Polymer[/u] capacitors are becoming the choice for high reliability applications.
Here is a paper from Raytheon about why they are switching to Polymer-Tantalum in their applications: http://ecadigitallibrary.com/detail.php?cid=27&pid=1965
Traditional Tantalum-MnO2 are the ones that ignite on failure. While it is possible to make a highly reliable MnO2, the cost goes from 10s of cents to 100s of dollars (seriously.)
a) I believe the common failure mode of a tant cap is short, as opposed to open with an electrolytic
All caps can fail short. The most common for tantalum is short. Most common for electrolytic is also short. Ceramic can fail short. Film very rarely fail short, but can. The key difference with wet-aluminum electrolytic caps is that their wearout mechanism is to fail open.
Keep in mind that "reliability" and "lifetime" are very different things. Tantalums have lifetimes on the order of 1000s of years. However, their most likely failure is during power-on. Fortunately they self-heal which makes them more robust over time. I explain more in this posting: http://www.baldengineer.com/blog/2013/09/13/ouch-the-arduino-gsm-shield-has-a-pretty-serious-design-flaw-with-its-capacitors/
I suppose I could use a Polyfuse or similar in series with the bulk tant caps to prevent shorts IF failure occurs
It depends on the application, but some times a fuse can be used. You have to watch picking a fuse value that would limit the cap's ability to charge or discharge itself.
It would be a very good idea to consider a Polymer-Tantalum and de-rate the voltage by 10-20%. If you de-rate 50% its failure rate drops significantly. It is very important you properly de-rate the applied voltage of a Tantalum (MnO2 or Polymer). De-rating does not improve lifetime (well it does, but not to an order anyone cares). Instead de-rating improves the reliability by reducing the chance for a dielectric breakdown to occur during power-on.
with electrolytic caps extend their life by preventing the caps from drying out?
You really don't want to put an aluminum electrolytic. As it ages, it releases hydrogen gas.
EDIT - http://www.vishay.com/docs/40126/ds04053.pdf It would seem good quality solid tants have internal fuses in them already!
Tantalum with a built-in fuse is a "specialty" product. If the data sheet doesn't say it has a fuse, it doesn't. Again, I would recommend using Polymer-Tantalum over any variation of MnO2. I don't know of anyone putting fuses in Polymer-Tantalums because of their improved reliability.
Polymers-Tantalums have a lower failure rate, lower ESR, and in the event they do fail, are unable to ignite.