Tantalum Capacitors: Types and Identification

Ⅰ. Types of tantalum capacitors

Tantalum capacitors are available in solid chip capacitors, porous anode/solid electrolytic capacitors, porous anode/liquid electrolytic capacitors, and tantalum foil capacitors.

1. Solid chip capacitor

In the 1980s, in order to meet the needs of surface mounting on printed circuit boards, a new generation of tantalum capacitors, namely solid chip capacitors, was developed. It cancels the lead wire of the capacitor, and directly assembles (solders) the anode and the cathode on the circuit board with two pieces of metal. In this way, the density of components is increased to 30%~60%, the installation cost is reduced by 20%~40%, and the installation speed is as high as 1500~5000 pieces per hour. The chipping of tantalum capacitors has greatly promoted the high-density assembly of integrated circuit boards, making it possible to miniaturize electronic devices. Solid chip capacitors are especially suitable for applications in computers, mobile phones, cameras, telecommunication equipment, etc. Now a mobile phone printed circuit board is usually installed with 32 chip tantalum capacitors. Coupled with the heat resistance advantages of chip capacitors, it has gradually replaced traditional aluminum capacitors. This kind of capacitor is mainly for civilian use, so it is produced in the largest amount in Japan.

2. Porous Anode/Liquid Electrolyte Capacitor

Porous anode/liquid electrolyte capacitors consist of an anode, an electrolyte consisting of sulfuric acid or lithium chloride, and a silver shell. The insulator is an oxide film formed on the porous anode. Tantalum capacitors before the 1950s were liquid capacitors. The working voltage of this kind of capacitor is generally limited to 6~15V, and the specific capacitor is generally 4000~10000μF·V/g. Low-voltage capacitors are usually made directly from sodium reduction powder; high-voltage capacitors are usually made by electron beam melting tantalum ingot hydrogenation powder. Due to the use of liquid electrolyte, the capacitance of this capacitor varies greatly with temperature, and the loss is not stable. Moreover, the liquid electrolyte has the risk of solidification, leakage and dryness, which can easily cause the capacitor to fail. Therefore its use is subject to certain restrictions.

3. Porous anode/solid electrolyte capacitor

Porous anode/solid electrolyte capacitors were developed in the mid-1950s. It not only eliminates the shortcomings of liquid capacitors, but also has the best temperature characteristics and frequency characteristics. It is currently the most used type. Solid capacitors are covered with a layer of manganese dioxide (solid electrolyte) in the pores of the tantalum anode. Capacitor cases are encapsulated in metal or epoxy (called resin-impregnated capacitors). It has the highest specific capacitance of all tantalum capacitors and can operate over a wide temperature range. It has excellent anti-leakage current performance, long storage performance and high reliable working lifespan, and this type of capacitor is often used in the military. The largest production volume of this type of capacitor is in the United States.

4. Tantalum foil capacitor

Tantalum foil capacitors consist of two tantalum foils separated by a liner paper impregnated with electrolyte. The tantalum foil is pressed and sintered from tantalum powder, and the anode tantalum foil is etched to increase the surface area, and then oxidized into an insulating layer. It is processed with a voltage above 400V during production, so it can work at a high voltage of 250V. But its capacitance is the lowest among the four kinds of capacitors, and it is also the least produced. It is only used in high working voltage occasions and occasions where a considerable reverse voltage is applied to the capacitor when the circuit is running.

Ⅱ. SMD tantalum capacitor identification

1. SMD electrolytic capacitor: material electrolyte. It has silk screen printing on the surface and is polarized. It has an aluminum casing visible on the outside. There is a black silk screen on the surface of the electrolytic capacitor to indicate the negative electrode of the electrolytic capacitor, and the capacitance value and working voltage are marked on the silk screen. Most manufacturers also add some tracking marks on the silk screen. The basic unit of chip electrolytic capacitor: μF

2. SMD capacitors: common type, made of porcelain. It has a single appearance, no silk screen on the surface, and no polarity. It has brown, gray, lavender and other colors. The basic unit of ordinary chip capacitors: pF

3. Chip tantalum capacitor: material tantalum dielectric. It has silk screen printing on the surface and is polarized. There are mainly black, yellow and other colors. There is a white silk screen on the surface of the tantalum capacitor to indicate the positive electrode of the tantalum capacitor, and the capacitance value and working voltage are marked on the silk screen. Most manufacturers also add some tracking marks on the silk screen. The basic unit of chip tantalum capacitor: μF

4. SMD paper multilayer capacitor: common type, made of paper. The components of some manufacturers on the surface have silk screen printing, and the shapes are mainly oval and square. Its appearance is generally silvery white with a metallic luster in the oval shape, and brown in the square shape, and the layering of the paper medium can be seen from the side. This capacitor has no polarity. Sizes come in a variety of sizes, but are generally larger in size. The basic unit of ordinary chip capacitors: pF. But this capacitance is generally larger at the μF level.

Ⅲ. How to choose a tantalum capacitor?

Tantalum capacitor selection mainly includes the following aspects:

1. If there is a large ripple current in the circuit, please choose a tantalum capacitor with a smaller ESR or a larger case size.

2. Tantalum capacitor is a kind of electrolytic capacitor, which has positive and negative poles. When using, we need to distinguish between positive and negative poles.

3. After determining the withstand voltage, we need to select the capacity according to the actual needs of the circuit. If you choose a product with a larger capacitance, its filtering characteristics will be better, and its anti-surge capability will be higher, so you can choose a product with a slightly larger capacity in the power circuit. However, if the capacity is too large, it will affect the signal response speed, so a capacitor with a smaller capacity should be selected on the signal transmission power supply.

4. Tantalum capacitors have higher requirements on the environment. For example, the operating temperature of tantalum capacitors needs to be between -25°C and +125°C. Tantalum capacitors cannot be stored in a strong acid and alkali environment for a long time.

5.When selecting a tantalum capacitor, the withstand voltage of the tantalum capacitor must be selected according to the actual circuit. Generally, the voltage at which tantalum capacitors can work stably is two-thirds of the withstand voltage of tantalum capacitors. If it is a power circuit or a low-impedance path, it needs to be derated to one-third; if it works at 85°C~125°C, it needs to be derated again on the basis of the previous derating.

Ⅳ. When to choose tantalum capacitors?

Although tantalum capacitors account for a relatively small proportion of capacitors and are not used in a wide range of scenarios, they have their own very distinctive characteristics. In some extreme working environments, tantalum capacitors are even the only choice.

1. Scenarios with particularly high requirements for stability and precision: the temperature stability of the capacitance of tantalum capacitors is relatively good. Therefore, in some coupling and filtering scenarios, if the phase and filtering frequency characteristics and capacity accuracy are relatively high, we will choose non-polar tantalum capacitors, such as audio circuit designs that require high sound quality.

2. Long-life scenario: Tantalum capacitors have the characteristics of automatically repairing or isolating oxide film defects, so that the oxide film medium can be strengthened and restored at any time without causing continuous cumulative damage. This unique self-healing property guarantees its advantages of long life and reliability. At this point, the life of aluminum electrolytic capacitors is shorter due to the dryness of the electrolyte.

3. Scenarios that require a relatively wide temperature range: Tantalum capacitors are suitable for working at high temperatures because there is no electrolyte inside. Tantalum capacitors can work normally from minus 25 degrees to 125 degrees, and the operating temperature range is very wide.

4. Scenarios with relatively high integration: the working medium of tantalum capacitors is an extremely thin layer of tantalum pentoxide film formed on the surface of tantalum metal. This layer of oxide film is integrated with one end of the capacitor and cannot exist alone. Therefore, the tantalum capacitor has a very high working electric field strength and a particularly large capacitance per unit volume. Therefore, it is especially suitable for scenarios with high miniaturization and high integration, such as the situation where the area occupied by aluminum electrolytic capacitors is relatively large and the capacity of ceramic capacitors is not enough.

5. Other specific scenarios: Tantalum capacitors have excellent performance, small size and large capacitance. It has few competitors in power filter, AC bypass, etc. For example, in the case of large capacity but low ESL, we choose tantalum capacitors.

Ⅴ. The difference between tantalum capacitors and ceramic capacitors

Tantalum capacitors are used in a wide variety of circuits, although they often require an external fault protection system to prevent problems caused by their failure modes. Personal computers, laptops, medical equipment, audio amplifiers, automotive circuits, mobile phones and other surface mount devices (SMD) are just a few examples. Tantalum electrolytics are a common replacement for aluminum electrolytics in military applications because it does not dry out or change capacitance over time.

Ceramic capacitors are used in a wide variety of applications, the most popular of which are personal electronic devices. MLCCs are the most widely used capacitors, with approximately 1 billion electronic devices installed each year. Printed circuit boards (PCBs), induction furnaces, DC-DC converters, and power circuit breakers are some examples of applications. Because ceramic capacitors are non-polarized and come in a wide variety of capacitances, voltage ratings, and sizes, they are often used as general-purpose capacitors.

While tantalum and ceramic capacitors serve similar functions, their construction methods, materials, and performance are quite different. Tantalum and ceramic capacitors vary in several major ways in terms of performance:

1. Temperature response

Tantalum capacitors have a linear capacitance change with temperature, while ceramic capacitors have a non-linear response. On the other hand, ceramic capacitors can achieve linear trend by reducing the operating temperature range and considering the temperature response in the design stage.

2. Voltage response

The capacitance of tantalum capacitors varies significantly with applied voltage, while ceramic capacitors do not. The dielectric constant inside a ceramic capacitor shrinks as the applied voltage increases, causing a change in capacitance. While the capacitance change of most ceramic capacitors is linear and easily explained, some high-k dielectrics lose 70% of their initial capacitance when operated at rated voltage.

3. Polarization

Most tantalum capacitors are polarized. This means they can only be connected to a DC power source while maintaining proper terminal polarity. Non-polarized ceramic capacitors, on the other hand, can be safely connected to AC mains. Ceramic capacitors have a higher frequency response because they are not polarized.

4. Aging

When it comes to capacitors, aging is the logarithmic decrease in capacitance over time. Tantalum capacitors do not age while ceramic capacitors do. There is no known wear mechanism for tantalum capacitors.

Ⅵ. Will ceramic capacitors completely replace tantalum capacitors?

No. Ceramic capacitors have almost no rivals in high-frequency circuits due to their ultra-small high-frequency impedance, high ripple resistance, and non-polarity. Moreover, with the continuous improvement of process technology, ceramic capacitors have also occupied part of the market for tantalum capacitors. However, it is wrong to assert that ceramic capacitors will replace tantalum capacitors, especially in low-to-medium frequency circuits, ceramic capacitors will not be the best choice.

First of all, if the circuit has high requirements on the temperature characteristics of components, the discomfort of ceramic capacitors to temperature will make the whole circuit unable to work.

Secondly, in terms of failure rate, the life test is also carried out at 125°C and rated voltage, and the failure rate of ceramic capacitors is also an order of magnitude higher than that of tantalum capacitors.

Finally, in terms of materials, the main component of ceramic capacitors is barium titanate. One of the more obvious characteristics of this material is the existence of piezoelectric effect. When there is a current in the audio frequency range, the ceramic capacitor uses the PCB board as the sounding board to act as a horn to spread the sound, making the whole machine noisy. In addition, multilayer ceramic capacitors also have cracks after soldering. It is not easy to be detected in the early stage, and in the later stage, as the gas enters the gap, a short circuit will be formed and the circuit will fail.

Ⅶ. Common failure phenomena of tantalum capacitors

For tantalum capacitors, failure is the same as other types of capacitors, and there are three types of failures: electrical parameter change failure, short circuit failure and open circuit failure. However, due to the stable electrical performance and unique "self-healing" characteristics of tantalum capacitors, there are few failures caused by changes in electrical parameters, and open-circuit failures are also rare. Most of the failures are caused by insufficient circuit derating, reverse voltage, and excessive power consumption. The main failure mode is short circuit. Common failure phenomena are as follows:

1. Thermally induced failure of solid tantalum

Solid tantalum capacitor dielectric oxide film (Ta2O5) has unidirectional conductivity. When a large current is charged and passed through the dielectric oxide film, it will cause heat failure. The microscopic ions of the dielectric oxide film are gradually transformed from a random and disorderly arrangement of a fixed row structure to an ordered row structure, and micro "crystallization". The scattered structure of the Ta2O5 dielectric oxide film leads to the deterioration of the performance of the tantalum capacitor until the breakdown failure.

2. Field-induced failure of solid tantalum

Solid tantalum capacitors are applied with a high voltage, and a high electric field is formed inside, which is prone to local breakdown. Therefore, in order to improve reliability, voltage derating must be used.

3. Failure caused by high ESR

High ESR values can easily lead to failure of tantalum capacitors. Poor connections inside the capacitor or increased material resistivity cause high ESR. Presence of protective coatings or fluxes on solder joints, unsuitable bonding solder, oxidation of terminal solder joints, and unsuitable probes can all cause ESR values to be higher than they actually are.

Tantalum capacitors are exposed to thermal stress during pick and place, placement, reflow, and use, which can affect external or internal connections, resulting in high ESR values for the capacitor. In a high temperature and high humidity environment, since the tantalum capacitor is not hermetically sealed, this will oxidize the external leads, resulting in a high ESR value. When moisture enters the interior, it will also cause the oxidation of the lead terminals, and the electrode layer will be cracked and delaminated.

4. Tantalum capacitor high leakage current failure

As a polarized element, it can withstand a small amount of reverse bias voltage for a short period of time. However, the continuous reverse polarity voltage makes its inherent resistance generate a large amount of high temperature, which causes the leakage current of the tantalum capacitor to increase, and then fails. The instability of the circuit causes high surge voltage and high surge current to damage the tantalum dielectric, thus causing failure.

Chip type tantalum capacitors are not hermetically sealed, so they tend to absorb moisture. During the fabrication process by reflow soldering, the moisture generated inside causes the molded epoxy to crack, introducing more moisture. During the use of capacitors, the internal conductive substances undergo chemical migration, resulting in high leakage currents and product failure.