In addition to exploding, Tantalum capacitors almost always fail to a short if you reverse their polarity or exceed their working voltage.
A few weeks ago, I found myself in need of a single 22µF Tantalum capacitor for use with a 3.3V voltage regulator. Since I didn't require any other components, I felt a bit silly placing an order with Digi-Key, so I emailed a couple of friends to ask them if they had an unused device skulking around at the bottom of their spare parts drawers.
In response, my friend Rick Curl emailed me to say that he's largely stopped using Tantalum capacitors in his designs. Intrigued, I asked him why this was, and he responded as follows:
For a long time, I've been in the habit of generously sprinkling Tantalum bypass capacitors between the supply and ground buses of my analog and digital PCB designs. It's good engineering practice to prevent noise and spikes on these buses. However, back in 2011, a couple of things happened that caused me to rethink my choice of Tantalum capacitors.
I had received a hand-assembled prototype PCB containing a bunch of surface mount parts. As I powered it up, I was thinking that I would probably need to make a few minor tweaks before releasing the design to production. About that time, there was a loud BANG... BANG... BANG, and I noticed the bench supply that was initially supplying 5V at about 0.25A was now reading ~0.5V, while the current was ~2A. I quickly shut the supply down and took the PCB outside, leaving a trail of smoke and a nasty odor behind.
Max here; I'm assuming that the nasty odor was from the charred circuit board and not Rick himself, but we digress...
I stood beside the road out front and examined the board. There were little charred holes in the PCB where some of the bypass capacitors had been. Looking at the remaining capacitors that had not yet failed, I realized that they'd all been installed backwards. It turned out we had somehow acquired a different brand of capacitor than the ones with which the assembler was accustomed, and she had misinterpreted the polarity markings. My first thought was to get more of the original brand of capacitor, but I soon discovered that the price of Tantalum had more than tripled almost overnight, thereby driving prices through the roof. Combined with the fact that Tantalum capacitors almost always fail to a short if you reverse their polarity or exceed their working voltage, I decided that it was time to find some other alternative. I also decided that it would be really nice if I could drop the new components on the same surface mount footprint.
Aluminum electrolytics were not an option. Their ESR (equivalent series resistance) is too high for them to be effective as bypass capacitors, they're too big, and -- like Tantalum's -- they're also polarity sensitive. Film capacitors might work, but it's hard to find physically small ones with values above a microfarad or so. Eventually I started looking at Multi-Layer Ceramic Capacitors (MLCCs). It used to be that these were only available in lower capacitance values, but a quick search of Digi-Key's website turned up values over 100 microfarads, even in a 1206 footprint! These are not polarized, so installing them backwards is not a problem. Could the solution really be as simple as dropping a ceramic capacitor in the footprint where I used to have a Tantalum? I started doing some research to see if there were any hidden "Gotcha's," and -- as you might expect -- there were. I found this technical article from Maxim Integrated to be particularly helpful.
What I discovered is that ceramic capacitors are generally split into two types, or classes. Class 1 is the stable, low-capacitance type that is often used in tuned circuits. Type 2 is available in much higher capacitance values, but with a couple of major shortcomings that we must keep in mind. The first of these shortcomings is the temperature coefficient. The chart below shows the temperature ranges and the capacitance variation over temperature. I usually use X7R, which is specified from -55 to +125° C, where the capacitance is allowed to vary 15% over this range.
Class 2 ceramic capacitors: Code system regarding to EIA RS-198 for some temperature ranges and inherent change of capacitance (Source: Wikipedia)
It's important to make sure that the capacitance remains within an acceptable range over the expected temperature range of the product.
Now that's out of the way, we need to look at voltage coefficients. This is actually the bigger, but more obscure, issue. Take a look at a typical Y5V capacitor at half of its rated voltage as illustrated below.
(Source: NIC Components)
As we see, the capacitance has dropped from 1.0µF to 0.3µF. That's a HUGE loss! There are a few things you can do about this. One is to go to a larger case size. Notice that the above chart is for an 0805 part. Changing to a 1206 causes the capacitance to drop much less as voltage is applied. You can also select a higher voltage part or find a part that uses a different dielectric.
It is hard to measure the actual capacitance value while voltage is applied to a capacitor. Empirical testing of the actual in-circuit parts over a range of temperatures and voltages can help to understand if the capacitors are performing as expected. While the capacitor's performance over temperature is not that difficult to predict, information about capacitance change as voltage is applied can be a bit more elusive (Murata has this very nice online tool that will allow you to see the effects of temperature, voltage and other characteristics for their capacitors).
By the way, the part about the voltage coefficient is apparently not well known at all. When I first started reading about it, I called the Murata FAE in Huntsville and asked him if I was reading the graph correctly where it indicated that I would lose 70% of the capacitance value at 50% of the rated voltage. He initially replied "Surely not -- nobody would want capacitors that had that problem." But he called me back the next day to say he had checked with the factory and that the chart was indeed correct. (Just to clarify, this voltage coefficient issue mainly occurs in with Class 2 / Bypass ceramic capacitors; it is much less pronounced in tantalum, aluminum electrolytic, and film capacitors.)
In closing, the discussions above are certainly not meant to be a thorough comparison of capacitor types. They're also not intended to imply that Tantalum capacitors are bad. They are not. There is much more in-depth information available about the comparison between capacitor type, such as this technical article.
It's my goal to share my experiences with you to keep you from learning some lessons the hard way (like I did). I was able to put ceramic capacitors on the same footprints as Tantalum ones on those boards, but only after careful research. I continue to use ceramic MLCCs on almost all my board designs today.
Wow. As you may recall, all I wanted was a single 22µF Tantalum capacitor for use with a 3.3V voltage regulator. Happily, while I was wading through Rick's email, my chum Duane Benson rooted one out and dropped it in the post. Having said this, whenever a circuit calls for a Tantalum capacitor in the future, I shall certainly consider using a Multi-Layer Ceramic equivalent.
What say you? Do you have any capacitor-related experiences you'd care to share?
— Max Maxfield, Editor of All Things Fun & Interesting