An overview of tantalum capacitors
30 April 2020
There are many trends and dynamics at play within the modern electronics world, but one of the most common is getting more out of something within a confined space.
There are many examples of this – such as accessing more computing/processing or data storage capacity, or possibly elevating power density levels. And as Mark Patrick, Technical Marketing Manager at Mouser Electronics tells us, the same basic trend applies within passive components: the ability to provide more capacitance in a smaller volume is one of the key reasons that tantalum capacitors originally came into existence, and has been pivotal to their incredible popularity.
What is a tantalum capacitor?
Tantalum capacitors are fundamentally electrolytic capacitors, albeit a very specific type. When most people think of electrolytic capacitors, they are thinking of the very common aluminium electrolytics that typically come in a cylindrical format. Tantalum capacitors have a similar internal construction and share some characteristics with aluminium electrolytics, but offer very different performance.
The primary advantage of tantalum capacitors is that they offer a high level of capacitance for a given volume and weight – which is a significant benefit to designers of modern systems, as they are almost always space-limited. Tantalum capacitors also usually have a lower equivalent series resistance (ESR), which is especially beneficial in power applications – where ESR equates to unwanted losses and reduced efficiency. Tantalum capacitors are also able to work at higher temperatures. This is another useful characteristic for contemporary systems that are often densely packed, with relatively little cooling available.
Construction, materials & types
Being electrolytic components, tantalum capacitors are polarized and consist of an anode and a cathode, along with an electrolyte. The anode is formed from a very small pellet of tantalum material that is ground into a fine powder and sintered at high temperatures. As a result, it has a very porous structure, with a large surface area that leads to high capacitance values. The anode is covered by a very thin insulating oxide layer through a carefully controlled anodization process. This oxide acts as the dielectric between the anode and the conductive cathode. As the dielectric is very thin, high levels of capacitance can be created in a relatively small volume. The oxide is also very stable over time and when subjected to different operating frequencies.
For wet-type tantalum capacitors, the oxidized anode is simply placed within a liquid electrolyte. The electrolyte for solid tantalum capacitors is added using a process known as pyrolysis – whereby the anode is dipped in a solution and then baked to form a manganese dioxide (MnO2) coating that is finally dipped in graphite and silver to provide a good connection to the cathode.
Tantalum capacitors are often associated with small epoxy-coated components that have two leads. These were the original type of tantalum capacitor and were often bright in colour – with their ratings, and the all-important polarity information, printed directly onto the epoxy. Their unique shape earned them the nickname “tantalum beads.” At one time almost all capacitors were leaded devices and one of the major challenges was that the popular aluminium electrolytics struggled with the temperatures associated with surface-mount reflow. Consequently, tantalum capacitors became the most common choice when a cost-effective, high-value capacitor in a small package was needed.
It should be noted that tantalum is one of four minerals (along with tungsten, tin and gold) covered by international conflict mineral regulations. All manufacturers of products available from Mouser containing these minerals are compliant with the relevant legislation, and detailed statements are available on the Mouser website.
Design considerations with tantalum
While they offer a number of benefits, tantalum capacitors are fairly sensitive components with a number of failure modes that can potentially catch out the unwary. The very thin oxide dielectric layer that gives them many of their key advantages is not hugely robust, and design engineers must take care to adhere to specification limits.
As polarized devices, they must be oriented correctly on the PCB. This was more of a challenge in the past, when manual assembly techniques were employed, with errors all too frequently leading to failure during testing. The automated production processes associated with surface-mount devices mean that errors of this kind are now extremely rare.
Provided they are used within their rated limits, tantalum capacitors are very reliable and give good service. Many design guidelines and reliability standards recommend derating the working voltage by as much as 50% to ensure there is sufficient margin to prevent damage, although some manufacturers have qualified their products to be reliable with derating as low as 10%.
Uses for tantalum capacitors
Many applications can benefit from the ability of tantalum capacitors to deliver high capacity with low leakage current and long-term stability. They are commonly used for filtering of power supply rails, particularly in space-restricted applications – such as smartphones and tablets. They are also specified for sensitive analogue applications, where the low leakage current allows sample-and-hold circuits to retain signal levels for longer. Their prolonged stability makes them popular in medical applications, as well as high-end audio amplifiers. High-grade solid tantalum capacitors are sometimes employed in military and other high-reliability hardware (including automotive), principally as they are stable over time and have no electrolyte to dry out – unlike aluminium electrolytics.
Latest trends & devices
There are many general-purpose tantalum products available, such as Panasonic’s POSCAP tantalum-polymer solid capacitors, that achieve stable capacitance at high frequency and temperature, as well as low ESR values. Several series of POSCAP devices exist – covering the range 3.9 to 1500µF, in case sizes from 2.0mm x 1.25mm to 7.3mm x 4.3mm. Given their strong frequency performance, POSCAP capacitors are often used in high-frequency digital devices.
Figure 1: Example of AVX’s F9H series.
Many of the recent tantalum capacitors to reach the market have focused on high-reliability applications. With the ever-increasing electrification of vehicles, in terms of the motive power, plus enhanced technology in infotainment, driver assistance and actuation, there is a huge market for electronic components in this sector. AVX’s F9H solid tantalum capacitors are highly reliable resin-moulded devices that offer AEC-Q200 compliance. These surface-mount devices operate over a temperature range from -55°C to +150°C with a 105°C rated temperature. Available with rated voltages of 10V and 16V, capacitance values in this series range from 10 to 47µF. Also from AVX, and intended for automotive applications, is the TCQ series. These robust AEC-Q200-qualified devices have a 10 to 470µF range. Each features a conductive polymer electrode, and also offers a benign failure mode when used in line with datasheet recommendations. They are suitable for body electronics, infotainment and cabin controls.
Figure 2: One of the T543 series from KEMET.
KEMET’s T543 series of organic capacitors are components with a tantalum anode and a Ta2O5 dielectric that results in very low ESR (15mO), improved capacitance at high frequencies and industry-leading ripple current handling. Available as commercial off-the-shelf (COTS) products with a -55°C to +105°C temperature range, the T543 series is intended to merge the best features of MLCCs (low ESR), aluminium electrolytics (higher capacitance, benign failure mode) and solid tantalums (volumetric efficiency, SMD package, long life).
Among the toughest applications are those found in the drilling industry, where data is captured way below the surface at the drill head. Designed specifically for measurement while drilling (MWD) applications is KEMET’s T502 series of MnO2 tantalum capacitors. These highly reliable devices are qualified for 1,000 hours at 230°C and include 3-sigma screening for iL, DF and ESR. The moulded cases are constructed from a unique high-temperature material that protects against the extreme shock and vibration forces experienced in MWD applications.
Tantalum capacitors meet many of the needs of modern electronic designs, especially with regard to their volumetric efficiency – providing high levels of capacitance in small, easy-to-handle surface-mount package formats. While they must be used within the recommended datasheet parameters to avoid failure, if properly derated they are highly reliable devices that exhibit exceptional long-term stability.
Increasingly, tantalum capacitors are being used in applications that require harsh environmental resilience, and leading manufacturers such as AVX and KEMET have products that are specifically designed to provide reliable service in automotive, industrial and down-hole applications.
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