Why Do They Even Make Tantalum Capacitors?

Someone asked me the other day what is the purpose of tantalum capacitors, especially ones with lower values.  In the 90s I used them all the time.  We liked them because you could get large values in modest package sizes, and they had lower ESR and better reliability than aluminum electrolytics.  When the tantalum shortage happened in 2000, I was perplexed by an order to use no tants on a design.  A colleague said that’s like asking us to design boards with one hand tied behind our backs. 

Without any publicized revolution in capacitor technology, over the next decade the maximum capacitance per package size of ceramic caps grew dramatically.  I would do a search for a 10uF cap and be surprised to find there was a ceramic option in an SMT package.  More than once I saw a switching power regulator datasheet say that designers using ceramics need to put a resistor in series because their part was designed to work with higher-ESR tants and aluminum electrolytics. 

So apart from special cases where you want at least some ESR, what’s the point of tants?  It’s not price:

2.2uF - 0402

2.2uF - 0805

100uF -1210

There is a good app note from Kemet on comparing tantalums and ceramics.  It explains the tradeoffs and makes it clear why we still need tants. 

Here is quick summary of the merits of tantalum caps:

1. Ceramic values decay over time.  Soldering them recovers their full capacitance, which can cause them to exceed their rated capacitance for around day.  After this they can lose around 10% of their capacitance over a few years.
2. Ceramics’ capacitance is a function of bias voltage.  Capacitance can decay nearly in half from 0 to rated voltage.
3. Piezo electric effect - Signals in the capacitor can create audible vibrations.  Vibrations of the capacitor can create electrical noise.
4. Vulnerable to failure due to mechanical strain.
5. Capacitance is lower at DC than AC. (An AVX source listed this as a drawback that could cause instability when ceramics are used for AC coupling, but I question if in practice this ever impacts designs.)

Most people will be more familiar with the drawbacks of tantalum caps:

1. Polar
2. Can catch fire when they fail
3. The need to avoid using tants at over half their rated voltage (I had wondered if it was engineering folklore, but the AVX app note says this rules is related to changes that can take place during soldering.)
4. Lower capacitance at higher frequencies
5. Higher leakage
6. Higher ESR than ceramic (although lower than aluminum)

The issue on this list that stands out the most is the capacitance change over time.  I try to avoid using any large value caps for timing or high-tolerance applications.  In cases where I need moderately large values and tight tolerances, I will consider tants.

I would love to hear stories about how the introduction of high-value small-package ceramics has affected other engineers.