http://www.eevblog.com/forum/projects/whenwhy-(not)-to-use-tantalum-capacitors-pros-cons-alternatives/

A few weeks ago I worked through the Altium SMPS design course from FEDEVEL academy and that little project left me with an unresolved question. Somehow I got the idea in the past few years that tantalum capacitors are to be avoided, never really knowing why. Yet try to find a 330uF, 6.3V capacitor with ~4mOhm ESR  and you’ll find that tantalum is basically the only option. So I wanted to start a discussion about tantalum caps with the main questions being:

-   When and why to use tantalum capacitors?
-   Why avoid using tantalum capacitors?
-   Alternatives to tantalum capacitors with pros and cons.


I did some quick research already and here’s summary of what I’ve found so far

Why and when to use tantalum capacitors?
Tantalum is used to create small sized capacitors with ‘large’ capacitance. Compared to other materials the oxide layer can be quite thin. So for all applications where pcb space is limited (e.g. mobile phones) they are the to go to type of capacitor when ceramic doesn’t cut it anymore. 

Also tantalum capacitors can be created with quite small ESR. This is why they are used a lot in (local) switched power supplies as bulk capacitor. Ceramics have even lower ESR, but in power supply regulators that might be too low for loop stability.

Why avoid using tantalum capacitors?
From what I can find these are the main reasons to avoid tantalum capacitors in your design:

Financial:
Tantalum capacitors are relatively expensive even when supply and demand are balanced. Not too long ago prices went sky high due to a shortage in raw materials. 

Reliability:
There are numerous reports of tantalum capacitors spontaneously combusting.  I haven’t read into this any further, but I take it some forum members can elaborate on this.

Moral:
Since some of the major tantalum ore (columbite-tantalite / coltan) mines are located in conflict zones (e.g. Democratic Republic of Congo) so one can argue that tantalum is a conflict mineral. I’m not sure though to what extent that still counts today as there are major mines elsewhere in the world too. 
Tantalum is a heavy metal and as such toxic and not really well for the environment. I did find a paper by AVX that discussed ‘green’ tantalum technology though.

Alternatives to tantalum capacitors and pros and cons?
Niobium caps
Apart from being mentioned I haven’t been able to find useful information yet, apart from that there is more Niobium ore available which should make Niobium caps cheaper. On the other hand Coltan is also used for mining Niobium so maybe the conflict argument could apply here too?

(Wet) Aluminum Electrolytic caps (Elco)
Specific low-ESR Elcos can be used. Larger values, larger ripple current rating, larger voltage ratings are some benefits. I must admit though that I realized that I haven’t seen SMT Elcos a lot outside consumer gear. Did I miss something?

Polymer (Solid) Aluminum caps
Used as in- or output bypass caps in SMPSs. Larger values only available in small voltage ratings. I don’t know by how much these must be derated though. Please tune in if you know more about this kind of caps.

Monolytic Ceramic caps
Larger voltage ratings, smaller derating, and higher ripple current ratings are among its benefits compared to tantalum caps. Since the ESR is much lower a small external resistance may be needed for loop stability in SMPS designs. 

Sources 
Here is some of the info I found. There is a lot to be found, and you'd have sift to get the info you want too. I included the year of publishing when available, since a few years in our industry means lots of changes.

AVX. (n.d.). Comparison of Multilayer Ceramic and Tantalum Capacitors

AVX. (n.d.). “Green” Environmentally Friendly Technology For Tantalum And Niobium Oxide Capacitors

Digikey. (n.d.). Tantalum Alternative Solutions by Panasonic

EETimes. (2001). Tantalum capacitor options weighed

Kemet. (2008). Comparison of Ceramic and Tantalum Capacitors

NIC Components. (1999). Alternates to surface mount tantalum electrolytic capacitors

Wikipedia. (n.d.) Coltan


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