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These days more and more electronic equipment uses tiny tantalum capacitors, with capacitance values that were impossible in such small volumes only a few years ago. This is the story of how they are made. The how, when, where and why of a Tantalum Capacitor By PETER HOLTHAM 14  Silicon Chip www.siliconchip.com.au J ust as silicon chips pack more and more function into less and less space, other electronic components have also shrunk. Tiny surface mount resistors replace the wire-ended components of just a few years ago. Capacitors used to be bulky items –even the low voltage types. But like resistors, they too have shrunk to minuscule proportions. Few people realise that the key to making some of these very tiny capacitors is found deep underground in Western Australia. It is the rare mineral tantalite, a complex oxide of iron, manganese and tantalum, and the principal source of tantalum metal. Two mines in the state supply more than a quarter of the world’s annual tantalum requirements. One is outside the small town of Greenbushes, 250km south of Perth. The other is at Wodgina in the remote Pilbara region, 1500km north of Perth. Australian gold mining company Sons of Gwalia owns both and together they form the world’s largest known tantalum resource. Fifty eight million kilograms of tantalum (as tantalum pentoxide) has been found, enough to give both mines at least 25 years more life. The tantalum bearing ore is mined from huge open pits by drilling and blasting. Every tonne mined Wodgina requires the remov(Tantalum) al of nearly seven tonnes of waste rock. The ore trucked out of the pit contains just over 200 grams of tantalite mineral per tonne (or 200 parts per million), far too Greenbushes (Tantalum/ Lithium) little to be saleable. So the trucks These two mines in Western dump their loads Australia produce more than 25% at processing plants of the world’s tantalum requirements. close to the mines. And yes, we know Tassie is missing! Here, crushers followed by grinding Sons of Gwalia sells all its tantalite to mills pulverise the ore to a powder. two customers, Cabot Corporation in This allows the few specks of denser the USA and the German company, tantalite to be separated from the great H.C Starck. bulk of lower density waste minerals. These companies extract tantalum A final clean-up using electrostatic metal from tantalite by chemical separators and high intensity magnets means rather than smelting. The produces a saleable concentrate con- tant-alite is dissolved in hydrofluoric taining up to 40% tantalum pentoxide. and sulphuric acid and then extractLast year these two West Australian ed into a solvent leaving impurities mines produced just 500 tonnes of behind in the acid solution. tantalite from 2.4 million tonnes of ore. Tantalum is stripped from the sol- It starts deep underground as the rare mineral tantalite, a complex oxide of iron, manganese and tantalum. There are just 200g of tantalite in every tonne of ore mined! This is the Wodgina open-cut mine in the Pilbara, N-W Western Australia. www.siliconchip.com.au August 2002  15 The tantalum processing plant at Greenbushes, in southern West Australia. This plant also produces lithium. vent in the form of tantalum fluoride. is why you don’t find air-spaced 1µF Finally, sodium reduction of the flu- capacitors! oride produces powdered tantalum It is clear from the equation there metal. are two things you can do to decrease More than half the tantalum goes the plate area: increase the dielectric into the manufacture of capacitors, constant (K) or decrease the plate twenty-five billion of them in 2000, spacing (d). up from 13 billion in 1995. Some capacitors use mica, another So why is tantalum used? What’s so mineral, as the dielectric material special about it that allows a tantalum between the plates. Mica has a diecapacitor to pack so many microfarads lectric constant of seven (Table 1). So into such a small volume? A look at a 1µF capacitor with one millimetre what a capacitor is and how it works thickness of mica dielectric will be provides the answer. seven times smaller than the air spaced A capacitor is basically two con- version. In fact, mica occurs naturalductors separated by an insulator or ly in very thin sheets. So the plate dielectric. In an air-spaced capacitor, spacing (d) could be much less than the conductors are metal plates and Uses of Tantalum Metal Uses of Tantalum Metal the dielectric is air. The value of a Chemicals 10% Chemicals 10% Metal working 15% capacitor, C, depends on the area A Metal working 15% of the plates, the dielectric constant K of the insulation between them, and its thickness d. Here is the equation: one millimetre, making the capacitor even smaller. In tantalum capacitors the dielectric is tantalum pentoxide, Ta2O5, which has a K of 26. Despite the relatively modest K compared with the very large values of some ceramics, capacitor manufacturers use tantalum for a number of reasons. Firstly, and most importantly, it is a ‘valve’ metal (another is aluminium), meaning it forms a uniform stable oxide on its surface. It is easy to make tantalum pentoxide layers less than 15µm (one millionth of a metre) thick. It is this thinness of the dielectric layer that more than compensates for the comparatively low value of K. At the same time, the layer has a high dielectric strength, meaning it is able to withstand the large electric fields that occur in the capacitor. Secondly, tantalum can be made extremely pure. It melts at 2996°C and any impurities present evaporate off at much lower temperatures. High purity of the metal substrate guarantees high quality oxide films. Finally, tantalum is easy to work. It can be produced as a powder, rolled into sheets and drawn into wires. It is almost immune to corrosion by acids and is stable with respect to temperature. Temperature stability translates into excellent temperature performance in the finished capacitors. They are capable of working from -55 to +125°C Electronics 55% Electronics 55% C = E0K(A/d) E0 is the dielectric constant of free space; it has a value of 8.85 x 10-12 farads per metre. The key to using this equation correctly is to get the units right. K is a ratio and has no units, area A must be in square metres and dielectric thickness d is in metres. The capacitance is then in farads. The equation can be turned around to find the area of the plates for a particular value capacitor. For example, if you tried to make a 1µF capacitor with two plates separated by one mm of air (K for air is one), the plate area would be nearly 113 square metres. Which 16  Silicon Chip Fig.1 (left): uses of tantalum metal. Electronics, partic-ularly tantalum capacitors, takes the lion’s share of world-wide tanatalum production. Special alloys 20% Special alloys 20% Material Air or vacuum Table 1: the dielectric of many common (and some less common) materials. While not up there with most ceramics it is significantly higher than many other materials traditionally used for capacitor production. Dielectric Constant K 1 Paper 2-6 Plastic 2-6 Glass 5-8 Mica 7 Aluminium oxide 8 Tantalum pentoxide 26 Ceramic Variable 12-30,000 www.siliconchip.com.au High power microscope pics of two types of tantalum powder: nodular (left) and flake (right). with little variation in electrical properties. Capacitor manufacture starts with powdered tantalum metal. The typical particle size for a high voltage capacitor is 10µm. Because the dielectric layer eats into the particle, the thicker layers needed for a high voltage capacitor might consume the entire particle if it were any smaller. As the equation above shows, capacitance is proportional to surface area. In the past 10 years, tantalum powder manufacturers have been able to change the shape of the particles from simple spheres through flakes to complex coral structures. Each change in shape has increased the capacitance-voltage product (CV) of the powder. CV is a measure of the volumetric efficiency of a capacitor, or the number of microfarads (µF) in a given volume. Values have increased from 8000µFV/gram for simple particles to 27,000µFV/gram for coral structured particles. What this means is simply that tantalum capacitors have steadily become smaller. Surface-mount tantalum capacitors are now available in the 0402 format; that’s 0.04 by 0.02 inches, or 1mm by 0.5mm. The powder is mixed with a binder and compressed under high pressure around a tantalum wire to make a small ‘slug’. The wire will eventually become the anode of the capacitor. Heating the slug of powder and binder under vacuum at high temperature (1500-2000°C) fuses the individual particles together. They form a strong porous sponge, with a huge internal surface area. Connecting the slug to a positive voltage and dipping it into an acid bath allows a small current to pass through it (see Fig.2). This electrolytic process creates the dielectric layer of tantalum pentoxide on all the exposed tantalum surfaces of the sponge. The applied voltage sets the thickness of the layer. The higher the voltage, the thicker the oxide layer. As you can see from the first equation, a thicker layer gives a lower value of capacitance. But it also means a higher voltage rating for the finished capacitor. Typically, the layer is around 0.25µm thick. What does this mean for a typical 25µF 25VW tantalum bead capacitor two or three millimetres in size? Putting the values for C, K and d into the second equation shows that the surface area inside the capacitor is around 209cm2. That’s about one third the area of this page. Now look at the dielectric strength of the layer and the electric field it has to withstand in operation. The dielectric strength is simply the working voltage (25) divided by the layer thickness (0.25m), in this case an amazing 125kV per millimetre. So far we have half the capacitor –one electrode of tantalum metal sponge and the dielectric of tantalum pentoxide. Now the second electrode is added. The slug is dipped into manganese nitrate solution which fills up all the pores in the sponge. Heating the slug drives off the water and decomposes the nitrate to manganese dioxide, which now becomes the second electrode (Fig.3). The manganese dioxide cathode layer provides the capacitor with a unique ‘self-healing’ mechanism. If there is a localised imperfection in the dielectric, a heavy current will flow in this region. Resistance of the manganese dioxide causes it to heat up and change to a more resistive form, plugging the imperfection. Once the manganese dioxide layer is in place, a cathode wire is glued on using a combination of graphite and silver loaded epoxy. Welding a wire to the stub of tantalum wire in the slug creates the anode lead. Fig.3 shows the layers of the finished capacitor. All that remains to be done is to decide on the packaging method. Tantalum capacitors come as either surface mount-chips or wire-ended beads, with chips outnumbering beads by four to one in recent years. The body is coded with its capacitance value and voltage rating, and then if it tests OK, the capacitor is ready to leave the factory. So next time you casually reach for a tiny surface-mount tantalum capacitor, spare a thought about how it was made and where the raw material came from. It may well have started life deep underground in Western Australia. SC Acknowledgement: Our thanks to Suzanna Hughes and Kevin O'Keefe, Sons of Gwalia Ltd, and John Gill, AVX Ltd Tantalum Divi-sion, for their assistance with this feature. .     Cathode wire Acid bath Tantalum slug DC Volts Fig.2 (above): the making of a tantalum capacitor. An electrolytic process deposits a very fine layer of tantalum pentoxide – the dielectric, on a tantalum metal slug. The coated slug is then dipped in manganese nitrate and heated, which creates the cathode of manganese dioxide. The finished capacitor is shown in graphical form in Fig.3 (right). www.siliconchip.com.au Manganese dioxide (cathode) Tantalum pentoxide (dielectric) Tantalum metal (anode) Tantalum wire stub Anode wire August 2002  17