|SMD electrolytic capacitors|
Message #5 Posted by Ellis Easley on 18 June 2003, 10:57 a.m.,
in response to message #3 by Vieira, Luiz C. (Brazil)
First, I want to say that tantalum capacitors are also electrolytic - there are aluminum electrolytic capacitors and tantalum electrolytic capacitors (and probably some other types - for instance, some other chemistry might be at work in the "supercaps" that go up to 1 farad and more in a small, low voltage package, they were intended to provide low current for memory backup as an alternative to batteries - I never actually saw them in any equipment, though). Anyway, I understand that in normal discussions, "electrolytic" means aluminum electrolytic and "tantalum" means tantalum electrolytic.
There are SMD (aluminum) electrolytic capacitors. They consist of a cylindrical can mounted on an SMD base. They are not as compact as the same value tantalum capacitor and are usually tall, since they are basically cylindrical cans with a seal at the bottom. I think aluminum electrolytics need to be built this way because they need a pressure relief valve (built into the rubber seal). Aluminum electrolytics have higher leakage and tend to "form" to the voltage they operate at: if they are used at a voltage much lower than the rated voltage, the dielectric tends to re-form in a thinner layer causing the capacitance to increase. If the applied voltage is increased, the thinner dielectric leaks more for a time until a thicker dielectric layer is built up - through "electrolysis" (that's why they are called electrolytic - there is no dielectric layer when they are first made, it is built up by application of voltage and current at the factory). As the dielectric layer becomes thicker, the capacitance decreases. If the voltage is increased too much-too fast, the leakage is so high that gas builds up to a high pressure in the can and the relief valve must vent it to the atmosphere.
Since tantalum capacitors are also electrolytic, they must have a dielectric layer formed by electrolysis also. One plate of the capacitor consists of a porous slug of powdered tantalum metal joined by heat and pressure (sintered) (BTW tantalum has a high melting point, ~3000 degrees C, it is so named because it tantalized metallurgists who tried to extract it from its ore). The other plate is the conductive electrolyte that saturates the slug and is in electrical contact with the metal container. Tantalum capacitors have much lower leakage and can be made with a glass seal (no pressure relief valve) so that's why they can be put in molded SMD packages and dipped wire-leaded packages.
Because of the sintered slug, all parts of the plate are electrically close together, compared to an aluminum electrolytic, where the plate is a roll of foil so the ends of the foil strip are electrically far from the terminal. This is why tantalum capacitors have lower inductance and lower ESR (equivalent series resistance - the resistance presented to an AC current).
There are aluminum electrolytic capacitors with extra-low ESR, these are important for use as filters in high frequency switching power supplies. I think they achieve the lower ESR by having multiple connections along the length of the foil strip to the terminal.